Patent Publication Number: US-2019176084-A1

Title: Systems and methods for multi-stage air dehumidification and cooling

Description:
This application is a continuation of U.S. patent application Ser. No. 14/170,186, entitled “SYSTEMS AND METHODS FOR MULTI-STAGE AIR DEHUMIDIFICATION AND COOLING”, which was filed Jan. 31, 2014, and is herein incorporated by reference in its entirety for all purposes. 
    
    
     BACKGROUND 
     Heating, ventilating, and air conditioning (HVAC) systems often have dehumidification systems integrated into the cooling apparatus for dehumidifying the air being conditioned by such systems. When cooling is required in warm to hot environments, the air being cooled and dehumidified will usually have a humidity ratio above approximately 0.009 (pounds of H 2 O per pounds of dry air). In these environments, the HVAC systems traditionally use refrigerant compressors for sensible cooling of the air and removal of latent energy (i.e., humidity). The air is typically cooled to about 55° F., which condenses H 2 O out of the air until the air is about 100% saturated (i.e., relative humidity at about 100%). The 55° F. temperature lowers the humidity ratio to about 0.009 pounds of H 2 O per pound of dry air, which is the water vapor saturation point at 55° F., resulting in a relative humidity of almost 100%. When this air warms to about 75° F., the humidity ratio remains approximately the same, and the relative humidity drops to approximately 50%. This traditional method of dehumidification requires the air to be cooled to about 55° F., and can usually achieve a coefficient of performance (COP) of approximately 3-5. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of embodiments of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a schematic diagram of an HVAC system having a dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 2A  is a perspective view of the dehumidification unit of  FIG. 1  having multiple parallel air channels and water vapor channels in accordance with an embodiment of the present disclosure; 
         FIG. 2B  is a perspective view of the dehumidification unit of  FIG. 1  having a single air channel located inside a single water vapor channel in accordance with an embodiment of the present disclosure; 
         FIG. 3  is a plan view of an air channel and adjacent water vapor channels of the dehumidification unit of  FIGS. 1, 2A, and 2B  in accordance with an embodiment of the present disclosure; 
         FIG. 4  is a perspective view of a separation module formed using a membrane that may be used as a water vapor channel of the dehumidification unit of  FIGS. 1-3  in accordance with an embodiment of the present disclosure; 
         FIG. 5  is a psychrometric chart of the temperature and the humidity ratio of the moist air flowing through the dehumidification unit of  FIGS. 1-3  in accordance with an embodiment of the present disclosure; 
         FIG. 6  is a schematic diagram of the HVAC system and the dehumidification unit of  FIG. 1  having a vacuum pump for removing noncondensable components from the water vapor in the water vapor extraction chamber of the dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 7  is a schematic diagram of the HVAC system and the dehumidification unit of  FIG. 6  having a control system for controlling various operating conditions of the HVAC system and the dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 8  is a schematic diagram of an HVAC system having a plurality of dehumidification units arranged in series in accordance with an embodiment of the present disclosure; 
         FIG. 9  is a schematic diagram of an HVAC system having a plurality of dehumidification units arranged in parallel in accordance with an embodiment of the present disclosure; 
         FIG. 10  is a schematic diagram of an HVAC system having a first plurality of dehumidification units arranged in series, and a second plurality of dehumidification units also arranged in series, with the first and second plurality of dehumidification units arranged in parallel in accordance with an embodiment of the present disclosure; 
         FIG. 11  is a schematic diagram of an HVAC system having an evaporative cooling unit disposed upstream of the dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 12A  is a psychrometric chart of the temperature and the humidity ratio of the air flowing through a direct evaporative cooling unit and the dehumidification unit of  FIG. 11  in accordance with an embodiment of the present disclosure; 
         FIG. 12B  is a psychrometric chart of the temperature and the humidity ratio of the air flowing through an indirect evaporative cooling unit and the dehumidification unit of  FIG. 11  in accordance with an embodiment of the present disclosure; 
         FIG. 13  is a schematic diagram of an HVAC system having the evaporative cooling unit disposed downstream of the dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 14A  is a psychrometric chart of the temperature and the humidity ratio of the air flowing through the dehumidification unit and a direct evaporative cooling unit of  FIG. 13  in accordance with an embodiment of the present disclosure; 
         FIG. 14B  is a psychrometric chart of the temperature and the humidity ratio of the air flowing through the dehumidification unit and an indirect evaporative cooling unit of  FIG. 13  in accordance with an embodiment of the present disclosure; 
         FIG. 15A  is a psychrometric chart of the temperature and the humidity ratio of the air flowing through a plurality of dehumidification units and a plurality of direct evaporative cooling units in accordance with an embodiment of the present disclosure; 
         FIG. 15B  is a psychrometric chart of the temperature and the humidity ratio of the air flowing through a plurality of dehumidification units and a plurality of indirect evaporative cooling units in accordance with an embodiment of the present disclosure; 
         FIG. 16  is a schematic diagram of an HVAC system having a mechanical cooling unit disposed downstream of the dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 17  is a schematic diagram of an HVAC system having the mechanical cooling unit of  FIG. 16  disposed upstream of the dehumidification unit in accordance with an embodiment of the present disclosure; 
         FIG. 18  is a schematic diagram of an HVAC system using mini-dehumidification units in accordance with an embodiment of the present disclosure; 
         FIG. 19  is a schematic diagram of an HVAC system using multiple cooling and dehumidification stages disposed in series, in accordance with an embodiment of the present disclosure; 
         FIG. 20  is a schematic diagram of the HVAC system of  FIG. 19 , including a control system; 
         FIG. 21  is a schematic diagram of an HVAC system using multiple cooling and dehumidification stages disposed in parallel and in series, in accordance with an embodiment of the present disclosure; 
         FIG. 22  is a schematic diagram of an HVAC system using multiple dehumidification units disposed in series and fluidly coupled to a cooling system disposed downstream of the multiple dehumidification units, in accordance with an embodiment of the present disclosure; 
         FIG. 23  is a schematic diagram of an HVAC system using multiple dehumidification units disposed in series and fluidly coupled to a cooling system disposed upstream of the multiple dehumidification units, in accordance with an embodiment of the present disclosure; 
         FIG. 24  is a schematic diagram of an HVAC system using multiple dehumidification units disposed in parallel and fluidly coupled to a cooling system disposed downstream of the multiple dehumidification units, in accordance with an embodiment of the present disclosure; 
         FIG. 25  is a schematic diagram of an HVAC system using multiple dehumidification units disposed in parallel and fluidly coupled to a cooling system disposed upstream of the multiple dehumidification units, in accordance with an embodiment of the present disclosure; and 
         FIG. 26  is a schematic diagram of an HVAC system using multiple dehumidification units disposed in parallel and in series and fluidly coupled to a cooling system disposed downstream of the multiple dehumidification units, in accordance with an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
     Specific embodiments of the present disclosure will be described herein. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements. 
     The subject matter disclosed herein relates to dehumidification systems and, more specifically, to systems and methods capable of dehumidifying air without initial condensation by establishing a humidity gradient in a dehumidification unit. In one embodiment, a water vapor permeable material (i.e., a water vapor permeable membrane) is used along at least one boundary separating an air channel from a secondary channel or chamber to facilitate the removal of water vapor from the air passing through the air channel. The secondary channel or chamber separated from the air channel by the water vapor permeable material may receive water vapor extracted from the air channel via the water vapor permeable material. 
     In operation, the water vapor permeable material allows the flow of H 2 O (which may refer to H 2 O as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, or combinations thereof) through the water vapor permeable material from the air channel to the secondary channel or chamber, while substantially blocking the flow of other components of the air flowing through the air channel from passing through the water vapor permeable material. As such, the water vapor permeable material reduces the humidity of the air flowing through the air channel by removing primarily only water vapor from the air. Correspondingly, the secondary channel or chamber is filled with primarily water vapor. It should be noted that the passage of H 2 O through the water vapor permeable material may be facilitated by a pressure differential. Indeed, a lower partial pressure of water vapor (i.e., a partial pressure less than the partial pressure of water vapor in the air channel) may be created in the secondary channel or chamber to further facilitate passage of the H 2 O through the water vapor permeable material. Accordingly, the side of the water vapor permeable material opposite the air channel may be referred to as the suction side of the water vapor permeable material. 
     Once the H 2 O has been passed through the water vapor permeable material, a vacuum pump is used to increase the partial pressure of the water vapor on the suction side of the water vapor permeable material to a minimal saturation pressure used to enable condensation of the water vapor by a condenser. That is, the vacuum pump compresses the water vapor to a pressure in a range suitable for condensing the water vapor into liquid water (e.g., a range of approximately 0.25-1.1 pounds per square inch absolute (psia), with the higher value applying to embodiments using multiple dehumidification units in series), depending on desired conditions for condensation. The condenser then condenses the water vapor into a liquid state, and the resulting liquid water is then pressurized to approximately atmospheric pressure, such that the liquid water may be rejected at ambient atmospheric conditions. By condensing the water vapor to a liquid state prior to expelling it, certain efficiencies are provided. For example, pressurizing liquid water to atmospheric pressure uses less energy than pressurizing water vapor to atmospheric pressure. Alternatively, the water vapor may be rejected to ambient conditions through a membrane water vapor rejection unit. It should also be noted that the dehumidification unit described herein in general uses significantly less energy than conventional systems. 
     While the embodiments described herein are primarily presented as enabling the removal of water vapor from air, other embodiments may enable the removal of other H 2 O components from air. For example, in certain embodiments, instead of a water vapor permeable material, an H 2 O permeable material may be used. As such, the H 2 O permeable material may allow the flow of one, all, or any combination of H 2 O components (i.e., water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth) through the H 2 O permeable material from the air channel to the secondary channel or chamber, while substantially blocking the flow of other components of the air flowing through the air channel from passing through the H 2 O permeable material. In other words, the disclosed embodiments are not limited to the removal of water vapor from air, but rather to the removal of H 2 O (i.e., in any of its states) from air. However, for conciseness, the embodiments described herein are primarily focused on the removal of water vapor from air. 
     In certain embodiments, as described in more detail below with respect to  FIGS. 19-26 , one or more of the aforementioned dehumidification units may be combined with one or more cooling systems, such as evaporative cooler systems. In one example, multiple stages, each stage including one evaporative or mechanical cooler and one dehumidification unit, may be combined in series and/or in parallel. Outside air may enter a first stage of the multiple stages, and be subsequently directed through multiple stages, exiting a final stage as cooler, drier air. That is, each subsequent stage may cool and dry the air from the previous stage. In one embodiment, a multi-stage vacuum pump may be used to create a low pressure side, providing a partial pressure differential suitable for enabling the outside air to move through the multiple stages. In other embodiments, multiple pumps may be used alternative or additional to the multi-stage pump. The low pressure side may also include a purge unit useful in removing certain components of the air, such as noncondensable components (e.g., oxygen, nitrogen, and other atmospheric gas components). A condenser may also be provided, suitable for condensing water vapor which may then be directed into a liquid receiver. A pump may then discharge the liquid from the receiver. Controller systems may be communicatively coupled to the various components of the multiple stages (e.g., pumps, valves, condensers, evaporative coolers) and used to more efficiently control the drying and cooling of the air. 
     By providing for the aforementioned multiple stages, each stage including an evaporator or mechanical cooler and a dehumidification system, a drier, cooler air may be produced in a more efficient manner, when compared to using a single stage. Additionally, including the multiple stages may enhance reliability and provide for redundancy. For example, bypass valves may be used to bypass certain stages in the event of an unexpected maintenance event. Indeed, maintenance, including the complete removal of one or more stages, may be performed, for example, by using the bypass valves, while the remaining stages may continue drying and/or cooling operations. Further, each stage may be provided at different producing capacities (e.g., drying, cooling capacity), thus enabling an HVAC system suitable for use in a variety of conditions. 
     With the foregoing in mind, it may be useful to describe certain systems and methods, such as an HVAC system  10  depicted in  FIG. 1 . More specifically,  FIG. 1  is a schematic diagram of an HVAC system  10  having a dehumidification unit  12  in accordance with an embodiment of the present disclosure. As illustrated, the dehumidification unit  12  may receive inlet air  14 A having a relatively high humidity and expel outlet air  14 B having a relatively low humidity. In particular, the dehumidification unit  12  may include one or more air channels  16  through which the air  14  (i.e., the inlet air  14 A and the outlet air  14 B) flows. In addition, the dehumidification unit  12  may include one or more water vapor channels  18  adjacent to the one or more air channels  16 . As illustrated in  FIG. 1 , the air  14  does not flow through the water vapor channels  18 . Rather, the embodiments described herein enable the passage of water vapor from the air  14  in the air channels  16  to the water vapor channels  18 , thus dehumidifying the air  14  and accumulating water vapor in the water vapor channels  18 . In particular, water vapor from the air  14  in the air channels  16  may be allowed to flow through an interface  20  (i.e., a barrier or membrane) between adjacent air channels  16  and water vapor channels  18 , while the other components (e.g., nitrogen, oxygen, carbon dioxide, and so forth) of the air  14  are blocked from flowing through the interface  20 . In general, the water vapor channels  18  are sealed to create the low pressure that pulls the water vapor from the air  14  in the air channels  16  through the interfaces  20  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). 
     As such, a humidity gradient is established between the air channels  16  and adjacent water vapor channels  18 . The humidity gradient is generated by a pressure gradient between the air channels  16  and adjacent water vapor channels  18 . In particular, the partial pressure of water vapor in the water vapor channels  18  is maintained at a level lower than the partial pressure of water vapor in the air channels  16 , such that the water vapor in the air  14  flowing through the air channels  16  tends toward the suction side (i.e., the water vapor channels  18  having a lower partial pressure of water vapor) of the interfaces  20 . 
     Components of air other than H 2 O may be substantially blocked from passing through the interfaces  20  in accordance with present embodiments. In other words, in certain embodiments, approximately 95% or more, approximately 96% or more, approximately 97% or more, approximately 98% or more, or approximately 99% or more of components of the air  14  other than H 2 O (e.g., nitrogen, oxygen, carbon dioxide, and so forth) may be blocked from passing through the interfaces  20 . When compared to an ideal interface  20  that blocks 100% of components other than H 2 O, an interface  20  that blocks 99.5% of components other than H 2 O will experience a reduction in efficiency of approximately 2-4%. As such, the components other than H 2 O may be periodically purged to minimize these adverse effects on efficiency. 
       FIG. 2A  is a perspective view of the dehumidification unit  12  of  FIG. 1  having multiple parallel air channels  16  and water vapor channels  18  in accordance with an embodiment of the present disclosure. In the embodiment illustrated in  FIG. 2A , the air channels  16  and the water vapor channels  18  are generally rectilinear channels, which provide a substantial amount of surface area of the interfaces  20  between adjacent air channels  16  and water vapor channels  18 . Further, the generally rectilinear channels  16 ,  18  enable the water vapor  26 A to be removed along the path of the air channels  16  before the air  14  exits the air channels  16 . In other words, the relatively humid inlet air  14 A (e.g., air with a dew point of 55° F. or higher such that the air is appropriate for air conditioning) passes straight through the air channels  16  and exits as relatively dry outlet air  14 B, because moisture has been removed as the air  14  traverses along the atmospheric pressure side of the interfaces  20  (i.e., the side of the interfaces  20  in the air channels  16 ). In an embodiment where a single unit is dehumidifying to a 60° F. saturation pressure or below, the suction side of the interfaces  20  (i.e., the side of the interfaces  20  in the water vapor channels  18 ) will generally be maintained at a partial pressure of water vapor that is lower than the partial pressure of water vapor on the atmospheric pressure side of the interfaces  20 . 
     As illustrated in  FIG. 2A , each of the water vapor channels  18  is connected with a water vapor channel outlet  22  through which the water vapor in the water vapor channels  18  is removed. As illustrated in  FIG. 2A , in certain embodiments, the water vapor channel outlets  22  may be connected via a water vapor outlet manifold  24 , wherein the water vapor  26 A from all of the water vapor channels  18  is combined in a single water vapor vacuum volume  28 , such as a tube or a chamber. Other configurations of the air channels  16  and the water vapor channels  18  may also be implemented. As another example,  FIG. 2B  is a perspective view of the dehumidification unit  12  of  FIG. 1  having a single air channel  16  located inside a single water vapor channel  18  in accordance with an embodiment of the present disclosure. As illustrated, the air channel  16  may be a cylindrical air channel located within a larger concentric cylindrical water vapor channel  18 . The embodiments illustrated in  FIGS. 2A and 2B  are merely exemplary and are not intended to be limiting. 
       FIG. 3  is a plan view of an air channel  16  and adjacent water vapor channels  18  of the dehumidification unit  12  of  FIGS. 1, 2A, and 2B  in accordance with an embodiment of the present disclosure. In  FIG. 3 , a depiction of the water vapor  26  is exaggerated for illustration purposes. In particular, the water vapor  26  from the air  14  is shown flowing through the interfaces  20  between the air channel  16  and the adjacent water vapor channels  18  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). Conversely, other components  30  (e.g., nitrogen, oxygen, carbon dioxide, and so forth) of the air  14  are illustrated as being blocked from flowing through the interfaces  20  between the air channel  16  and the adjacent water vapor channels  18 . 
     In certain embodiments, the interfaces  20  may include membranes that are water vapor permeable and allow the flow of H 2 O through permeable volumes of the membranes while blocking the flow of the other components  30 . Again, it should be noted that when the H 2 O passes through the interfaces  20 , it may actually pass as one, all, or any combination of states of water (e.g., as water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth) through the interfaces  20 . For example, in one embodiment, the interfaces  20  may adsorb/desorb water molecules. In another example, the interfaces  20  may adsorb/desorb water molecules and enable passage of water vapor. In other embodiments, the interfaces  20  may facilitate the passage of water in other combinations of states. The interfaces  20  extend along the flow path of the air  14 . As such, the water vapor  26  is continuously removed from one side of the interface  20  as the relatively humid inlet air  14 A flows through the air channel  16 . Therefore, dehumidification of the air  14  flowing through the air channel  16  is accomplished by separating the water vapor  26  from the other components  30  of the air  14  incrementally as it progresses along the flow path of the air channel  16  and continuously contacts the interfaces  20  adjacent to the air channel  16  from the inlet air  14 A location to the outlet air  14 B location. 
     In certain embodiments, the water vapor channels  18  are evacuated before use of the dehumidification unit  12 , such that a lower partial pressure of the water vapor  26  (i.e., a partial pressure less than the partial pressure of water vapor in the air channels  16 ) is created in the water vapor channels  18 . For example, the partial pressure of the water vapor  26  in the water vapor channels  18  may be in the range of approximately 0.10-0.25 psia during normal operation, which corresponds to dehumidifying to a 60° F. saturation pressure or below. In this example, an initial pressure of approximately 0.01 psia may be used to remove other air components (e.g., noncondensables such as oxygen, nitrogen, and carbon dioxide), whereas the partial pressure of water vapor in the air channels  16  may be in the range of approximately 0.2-1.0 psia. However, at certain times, the pressure differential between the partial pressure of the water vapor in the water vapor channels  18  and the air channels  16  may be as low as (or lower than) approximately 0.01 psia. The lower partial pressure of water vapor in the water vapor channels  18  further facilitates the flow of water vapor  26  from the air channels  16  to the water vapor channels  18 , because the air  14  flowing through the air channels  16  is at local atmospheric pressure (i.e., approximately 14.7 psia at sea level). Since the partial pressure of water vapor in the air  14  in the air channels  16  is greater than the partial pressure of the water vapor  26  in the water vapor channels  18 , a pressure gradient is created from the air channels  16  to the water vapor channels  18 . As described previously, the interfaces  20  between adjacent air channels  16  and water vapor channels  18  provide a barrier, and allow substantially only water vapor  26  to flow from the air  14  in the air channels  16  into the water vapor channels  18 . As such, the air  14  flowing through the air channels  16  will generally decrease in humidity from the inlet air  14 A to the outlet air  14 B. 
     The use of water vapor permeable membranes as the interfaces  20  between the air channels  16  and the water vapor channels  18  has many advantages. In particular, in some embodiments, no additional energy is used to generate the humidity gradient from the air channels  16  to the water vapor channels  18 . In addition, in some embodiments, no regeneration is involved and no environmental emissions (e.g., solids, liquids, or gases) are generated. Indeed, in accordance with one embodiment, separation of the water vapor  26  from the other components  30  of the air  14  via water permeable membranes (i.e., the interfaces  20 ) can be accomplished at energy efficiencies much greater than compressor technology used to condense water directly from the airstream. 
     Because water vapor permeable membranes are highly permeable to water vapor, the costs of operating the dehumidification unit  12  may be minimized because the air  14  flowing through the air channels  16  does not have to be significantly pressurized to facilitate the passage of H 2 O through the interfaces  20 . Water vapor permeable membranes are also highly selective to the permeation of the water vapor from the air  14 . In other words, water vapor permeable membranes are very efficient at blocking components  30  of the air  14  other than water vapor from entering the water vapor channels  18 . This is advantageous because the H 2 O passes through the interfaces  20  due to a pressure gradient (i.e., due to the lower partial pressures of water vapor in the water vapor channels  18 ) and any permeation or leakage of air  14  into the water vapor channels  18  will increase the power consumption of the vacuum pump used to evacuate the water vapor channels  18 . In addition, water vapor permeable membranes are rugged enough to be resistant to air contamination, biological degradation, and mechanical erosion of the air channels  16  and the water vapor channels  18 . Water vapor permeable membranes may also be resistant to bacteria attachment and growth in hot, humid air environments in accordance with one embodiment. 
     One example of a material used for the water vapor permeable membranes (i.e., the interfaces  20 ) is zeolite supported on thin, porous metal sheets. In particular, in certain embodiments, an ultrathin (e.g., less than approximately 2 μm), zeolite membrane film may be deposited on an approximately 50 μm thick porous metal sheet. The resulting membrane sheets may be packaged into a membrane separation module to be used in the dehumidification unit  12 .  FIG. 4  is a perspective view of a separation module  32  formed using a membrane that may be used as a water vapor channel  18  of the dehumidification unit  12  of  FIGS. 1-3  in accordance with an embodiment of the present disclosure. Two membrane sheets  34 ,  36  may be folded and attached together into a generally rectangular shape with a channel for the water vapor having a width w msm  of approximately 5 mm The separation module  32  may be positioned within the dehumidification unit  12  such that the membrane coating surface is exposed to the air  14 . The thinness of the metal support sheet reduces the weight and cost of the raw metal material and also minimizes resistance to the H 2 O diffusing through the water vapor permeable membrane film deposited on the membrane sheets  34 ,  36 . The metallic nature of the sheets  34 ,  36  provides mechanical strength and flexibility for packaging such that the separation module  32  can withstand a pressure gradient of greater than approximately 60 psi (i.e., approximately 4 times atmospheric pressure). 
     Separation of water vapor from the other components  30  of the air  14  may create a water vapor permeation flux of approximately 1.0 kg/m 2 /h (e.g., in a range of approximately 0.5-2.0 kg/m 2 /h), and a water vapor-to-air selectivity range of approximately 5-200+. As such, the efficiency of the dehumidification unit  12  is relatively high compared to other conventional dehumidification techniques with a relatively low cost of production. As an example, approximately 7-10 m 2  of membrane area of the interfaces  20  may be used to dehumidify 1 ton of air cooling load under ambient conditions. In order to handle such an air cooling load, in certain embodiments, 17-20 separation modules  32  having a height h msm  of approximately 450 mm, a length  1 msm of approximately 450 mm, and a width w msm  of approximately 5 mm may be used. These separation modules  32  may be assembled side-by-side in the dehumidification unit  12 , leaving approximately 2 mm gaps between the separation modules  32 . These gaps define the air channels  16  through which the air  14  flows. The measurements described in this example are merely exemplary and not intended to be limiting. 
       FIG. 5  is a psychrometric chart  38  of the temperature and the humidity ratio of the moist air  14  flowing through the dehumidification unit  12  of  FIGS. 1-3  in accordance with an embodiment of the present disclosure. In particular, the x-axis  40  of the psychrometric chart  38  corresponds to the temperature of the air  14  flowing through the air channels  16  of  FIG. 1 , the y-axis  42  of the psychrometric chart  38  corresponds to the humidity ratio of the air  14  flowing through the air channels  16 , and the curve  44  represents the water vapor saturation curve of the air  14  flowing through the air channels  16 . As illustrated by line  46 , because water vapor is removed from the air  14  flowing through the air channels  16 , the humidity ratio of the outlet air  14 B (i.e., point  48 ) from the dehumidification unit  12  of  FIGS. 1-3  is lower than the humidity ratio of the inlet air  14 A (i.e., point  50 ) into the dehumidification unit  12  of  FIGS. 1-3 , while the temperature of the outlet air  14 B and the inlet air  14 A are substantially the same. 
     Returning now to  FIG. 1 , as described previously, a lower partial pressure of the water vapor  26  (i.e., a partial pressure less than the partial pressure of water vapor in the air channels  16 ) is created in the water vapor channels  18  of the dehumidification unit  12  to further facilitate the passage of H 2 O through the interfaces  20  from the air channels  16  to the water vapor channels  18 . In certain embodiments, the water vapor channels  18  may initially be evacuated using a vacuum pump  52 . In particular, the vacuum pump  52  may evacuate the water vapor channels  18  and the water vapor vacuum volume  28 , as well as the water vapor outlets  22  and the water vapor manifold  24  of  FIG. 2A . However, in other embodiments, a pump separate from the vacuum pump  52  may be used to evacuate the water vapor channels  18 , water vapor vacuum volume  28 , water vapor outlets  22 , and water vapor manifold  24 . As illustrated in  FIG. 1 , the water vapor  26  removed from the air  14  in the dehumidification unit  12  may be distinguished between the water vapor  26 A in the water vapor vacuum volume  28  (i.e., the suction side of the vacuum pump  52 ) and the water vapor  26 B expelled from an exhaust side (i.e., an outlet) of the vacuum pump  52  (i.e., the water vapor  26 B delivered to a condensation unit). In general, the water vapor  26 B expelled from the vacuum pump  52  will have a slightly higher pressure and a higher temperature than the water vapor  26 A in the water vapor vacuum volume  28 . The vacuum pump  52  may be a compressor or any other suitable pressure increasing device capable of maintaining a lower pressure on the suction side of the vacuum pump  52  than the partial pressure of water vapor in the humid air  14 . 
     For example, the lower partial pressure of water vapor  26 A maintained in the water vapor vacuum volume  28  may be in the range of approximately 0.15-0.25 psia, which corresponds to saturation temperatures of approximately 45° F. to 60° F., with the water vapor  26 A in the range of approximately 65-75° F. However, in other embodiments, the water vapor  26 A in the water vapor vacuum volume  28  may be maintained at a partial pressure of water vapor in the range of approximately 0.01-0.25 psia and a temperature in the range of approximately 55° F. up to the highest ambient air temperature. A specific embodiment may be designed to lower the partial pressure in the water vapor vacuum volume  28  to the range of 0.01 psia to increase the capacity for removing water vapor from the air  14  to enable an evaporative cooler to process the entire air conditioning load when atmospheric conditions permit this mode of operation. 
     In certain embodiments, the vacuum pump  52  is a low-pressure pump configured to decrease the pressure of the water vapor  26 A in the water vapor vacuum volume  28  to a lower partial pressure than the partial pressure of water vapor on the atmospheric side of the interfaces  20  (i.e., the partial pressure of the air  14  in the air channels  16 ). On the exhaust side of the vacuum pump  52 , the partial pressure of the water vapor  26 B has been increased just high enough to facilitate condensation of the water vapor (i.e., in a condensation unit  54 ). Indeed, the vacuum pump  52  is configured to increase the pressure such that the water vapor  26 B in the condensation unit  54  is at a pressure proximate to a minimal saturation pressure in the condensation unit  54 . Alternatively, the condensation unit  54  and subsequent components may be replaced by a membrane water vapor rejection unit. 
     As an example operation of the HVAC system  10 , the air  14  may enter the system at a partial pressure of water vapor of 0.32 psia, which corresponds to a humidity ratio of approximately 0.014 pounds of H 2 O per pounds of dry air. The system may be set to remove approximately 0.005 pounds of H 2 O per pounds of dry air from the air  14 . Pressure differentials across the interfaces  20  may be used to create a flow of H 2 O through the interfaces  20 . For example, the partial pressure of water vapor in the water vapor vacuum volume  28  may be set to approximately 0.1 psia. The pressure of the water vapor  26 B is increased by the vacuum pump  52  in a primarily adiabatic process, and as the pressure of the water vapor  26 B increases, the temperature increases as well (in contrast to the relatively negligible temperature differential across the interfaces  20 ). As such, if for example the pressure of the water vapor  26 B is increased in the vacuum pump  52  by approximately 0.3 psi (i.e., to approximately 0.4 psia), the condensation unit  54  is then capable of condensing the water vapor  26 B at a temperature of approximately 72-73° F., and the temperature of the water vapor  26 B increases to a temperature substantially higher than the condenser temperature. The system may continually monitor the pressure and temperature conditions of both the upstream water vapor  26 A and the downstream water vapor  26 B to ensure that the water vapor  26 B expelled from the vacuum pump  52  has a partial pressure of water vapor just high enough to facilitate condensation in the condensation unit  54 . It should be noted that the pressure and temperature values presented in this scenario are merely exemplary and are not intended to be limiting. 
     As the pressure difference from the water vapor  26 A entering the vacuum pump  52  to the water vapor  26 B exiting the vacuum pump  52  increases, the efficiency of the dehumidification unit  12  decreases. For example, in one embodiment, the vacuum pump  52  may be set to adjust the pressure of the water vapor  26 B in the condensation unit  54  slightly above the saturation pressure at the lowest ambient temperature of the cooling media (i.e., air or water) used by the condensation unit  54  to condense the water vapor  26 B. In another embodiment, the temperature of the water vapor  26 B may be used to control the pressure in the condensation unit  54 . The temperature of the water vapor  26 B expelled from the vacuum pump  52  may be substantially warmer than the humid air  14 A (e.g., this temperature could reach approximately 200° F. or above depending on a variety of factors). Because the vacuum pump  52  may only increases the pressure of the water vapor  26 B to a point where condensation of the water vapor  26 B is facilitated (i.e., approximately the saturation pressure), the power requirements of the vacuum pump  52  are relatively small, thereby obtaining a high efficiency from the dehumidification unit  12 . 
     Once the water vapor  26 B has been slightly pressurized (i.e., compressed) by the vacuum pump  52 , the water vapor  26 B is directed into the condensation unit  54 , wherein the water vapor  26 B is condensed into a liquid state. In certain embodiments, the condensation unit  54  may include a condensation coil  56 , a pipe/tube condenser, a flat plate condenser, or any other suitable system for achieving a temperature below the condensation point of the water vapor  26 B. The condensation unit  54  may either be air cooled or water cooled. For example, in certain embodiments, the condensation unit  54  may be cooled by ambient air or water from a cooling tower. As such, the costs of operating the condensation unit  54  may be relatively low, inasmuch as both ambient air and cooling tower water are in relatively limitless supply. 
     Once the water vapor  26 B has been condensed into a liquid state, in certain embodiments, the liquid water from the condensation unit  54  may be directed into a reservoir  58  for temporary storage of saturated vapor and liquid water. However, in other embodiments, no reservoir  58  may be used. In either case, the liquid water from the condensation unit  54  may be directed into a liquid pump  60  (i.e., a water transport device), within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. As such, the liquid pump  60  may be sized just large enough to increase the pressure of the liquid water from the condensation unit  54  to approximately atmospheric pressure. Therefore, the costs of operating the liquid pump  60  may be relatively low. In addition, the liquid water from the liquid pump  60  may be at a slightly elevated temperature due to the increase in the pressure of the liquid water. As such, in certain embodiments, the heated liquid water may be transported for use as domestic hot water for use in the home, further increasing the efficiency of the system by recapturing the heat transferred into the liquid water. 
     Although the interfaces  20  between the air channels  16  and the water vapor channels  18  as described previously generally allow only H 2 O to pass from the air channels  16  to the water vapor channels  18 , in certain embodiments, very minimal amounts (e.g., less than 1% of the oxygen (O 2 ), nitrogen (N 2 ), or other noncondensable components) of the other components  30  of the air  14  may be allowed to pass through the interfaces  20  from the air channels  16  to the water vapor channels  18 . Over time, the amount of the other components  30  may build up in the water vapor channels  18  (as well as in the water vapor vacuum volume  28 , the water vapor outlets  22 , and the water vapor manifold  24  of  FIG. 2A ). In general, these other components  30  are noncondensable at the condenser temperature ranges used in the condensation unit  54 . As such, the components  30  may adversely affect the performance of the vacuum pump  52  and all other equipment downstream of the vacuum pump  52  (in particular, the condensation unit  54 ). 
     Accordingly, in certain embodiments, a second vacuum pump, such as a pump  62  shown in  FIG. 6 , may be used to periodically purge the other components  30  from the water vapor vacuum volume  28 .  FIG. 6  is a schematic diagram of the HVAC system  10  and the dehumidification unit  12  of  FIG. 1  having the vacuum pump  62  for removing noncondensable components  30  from the water vapor  26 A in the water vapor vacuum volume  28  of the dehumidification unit  12  in accordance with an embodiment of the present disclosure. The vacuum pump  62  may, in certain embodiments, be the same pump used to evacuate the water vapor vacuum volume  28  (as well as the water vapor channels  18 , the water vapor outlets  22 , and the water vapor manifold  24 ) to create the lower partial pressure of water vapor described previously that facilitates the passage of the H 2 O through the interfaces  20  from the air channels  16  to the water vapor channels  18 . However, in other embodiments, the vacuum pump  62  may be different from the pump used to evacuate the water vapor vacuum volume  28  to create the lower partial pressure of water vapor. 
     The dehumidification unit  12  described herein may also be controlled between various operating states, and modulated based on operating conditions of the dehumidification unit  12 . For example,  FIG. 7  is a schematic diagram of the HVAC system  10  and the dehumidification unit  12  of  FIG. 6  having a control system  64  for controlling various operating conditions of the HVAC system  10  and the dehumidification unit  12  in accordance with an embodiment of the present disclosure. The control system  64  may include one or more processors  66 , for example, one or more “general-purpose” microprocessors, one or more special-purpose microprocessors and/or ASICS (application-specific integrated circuits), or some combination of such processing components. The processors  66  may use input/output (I/O) devices  68  to, for example, receive signals from and issue control signals to the components of the dehumidification unit  12  (i.e., the vacuum pumps  52 ,  62 , the condensation unit  54 , the reservoir  58 , the liquid pump  60 , other equipment such as a fan blowing the inlet air  14 A through the dehumidification unit  12 , sensors configured to generate signals related to characteristics of the inlet and outlet air  14 A,  14 B, and so forth). The processors  66  may take these signals as inputs and calculate how to control the functionality of these components of the dehumidification unit  12  to most efficiently remove the water vapor  26  from the air  14  flowing through the dehumidification unit  12 . The control system  64  may also include a nontransitory computer-readable medium (i.e., a memory  70 ) which, for example, may store instructions or data to be processed by the one or more processors  66  of the control system  64 . 
     For example, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26 A from the water vapor vacuum volume  28  of the dehumidification unit  12  by turning the vacuum pump  62  on or off, or by modulating the rate at which the vacuum pump  62  removes the noncondensable components  30  of the water vapor  26 A. More specifically, in certain embodiments, the control system  64  may receive signals from a sensor in the water vapor vacuum volume  28  that detects when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volume  28 . This process of noncondensable component removal may operate in a cyclical manner In “normal” operation of removing the water vapor  26  from the air  14 , the vacuum pump  62  may not be in operation. As the noncondensable components  30  build up in the water vapor vacuum volume  28 , the internal pressure in the water vapor vacuum volume  28  eventually reaches a setpoint. At this point in time, the vacuum pump  62  turns on and removes all components (i.e., both the noncondensable components  30  as well as H 2 O, including the water vapor) until the internal pressure in the water vapor vacuum volume  28  reaches another setpoint (e.g., lower than the starting vacuum pressure). Then, the vacuum pump  62  shuts off and the dehumidification unit  12  returns to the normal operational mode. Setpoints may either be preset or dynamically determined. One method will be to have the vacuum pump  62  only operating in the purge mode intermittently. 
     Another example of the type of control that may be accomplished by the control system  64  is modulating the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  (as well as the water vapor channels  18 , the water vapor outlets  22 , and the water vapor manifold  24 ) to modify the water vapor removal capacity and efficiency ratio of the dehumidification unit  12 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volume  28 , the water vapor channels  18 , the water vapor outlets  22 , and/or the water vapor manifold  24 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the inlet and outlet air  14 A,  14 B, among other things. The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A (e.g., with respect to the partial pressure of water vapor in the air  14  flowing through the air channels  16 ) to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20 . 
     For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be increased. Furthermore, in certain embodiments, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification unit  12 . More specifically, under certain operating conditions, the dehumidification unit  12  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification unit  12  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification unit  12  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. In addition, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification unit  12 . 
     The dehumidification unit  12  may be designed and operated in many various modes, and at varying operating conditions. In general, the dehumidification unit  12  operates with the water vapor vacuum volume  28  (as well as the water vapor channels  18 , the water vapor outlets  22 , and the water vapor manifold  24 ) at a water vapor partial pressure below the water vapor partial pressure of the air  14  flowing through the air channels  16 . In certain embodiments, the dehumidification unit  12  may be optimized for dedicated outside air system (DOAS) use, wherein the air  14  may have a temperature in the range of approximately 55-100° F., and a relative humidity in the range of approximately 55-100%. In other embodiments, the dehumidification unit  12  may be optimized for residential use for recirculated air having a temperature in the range of approximately 70-85° F., and a relative humidity in the range of approximately 55-65%. Similarly, in certain embodiments, the dehumidification unit  12  may be optimized for dehumidifying outside air in commercial building recirculated air systems, which dehumidifies the inlet air  14 A having a temperature in the range of approximately 55-110° F., and a relative humidity in the range of approximately 55-100%. The outlet air  14 B has less humidity and about the same temperature as the inlet air  14 A, unless cooling is performed on the outlet air  14 B. 
     The dehumidification unit  12  described herein uses less operating power than conventional dehumidification systems because of the relatively low pressures that are used to dehumidify the air  14 A. This is due at least in part to the ability of the interfaces  20  (i.e., water vapor permeable membranes) to remove the water vapor  26  from the air  14  efficiently without requiring excessive pressures to force the water vapor  26  through the interfaces  20 . For example, in one embodiment, the minimal power used to operate the dehumidification unit  12  includes only the fan power used to move the air  14  through the dehumidification unit  12 , the compressive power of the vacuum pump  52  to compress the water vapor  26  to approximately the saturation pressure (for example, to approximately 1.0 psia, or to a saturation pressure that corresponds to a given condensation temperature, for example, approximately 100° F.), the pumping and/or fan power of the condensation unit  54  (e.g., depending on whether cooling tower water or ambient air is used as the cooling medium), the pumping power of the liquid pump  60  to reject the liquid water from the condensation unit  54  at ambient conditions, and the power of the vacuum pump  62  to purge noncondensable components  30  that leak into the water vapor vacuum volume  28  of the dehumidification unit  12 . As such, the only relatively major power component used to operate the dehumidification unit  12  is the compressive power of the vacuum pump  52  to compress the water vapor  26  to approximately the saturation pressure (for example, only to approximately 1.0 psia, or to a saturation pressure that corresponds to a given condensation temperature, for example, approximately 100° F.). As mentioned previously, this power is relatively low and, therefore, operating the dehumidification unit  12  is relatively inexpensive as opposed to conventional refrigeration compression dehumidification systems. Moreover, calculations for an embodiment indicate that the dehumidification unit  12  has a coefficient of performance (COP) at least twice as high (or even up to five times as high, depending on operating conditions) as these conventional dehumidification systems. In addition, the dehumidification unit  12  enables the dehumidification of air without reducing the temperature of the air below the temperature at which the air is needed, as is often done in conventional dehumidification systems. 
     In certain embodiments, multiple instances of the dehumidification unit  12  described previously with respect to  FIGS. 1 through 7  may be used in a single HVAC system. For example,  FIG. 8  is a schematic diagram of an HVAC system  72  having a plurality of dehumidification units  12  (i.e., a first dehumidification unit  74 , a second dehumidification unit  76 , and a third dehumidification unit  78 ) arranged in series in accordance with an embodiment of the present disclosure. Although illustrated as having three dehumidification units  74 ,  76 ,  78  in series, any number of dehumidification units  12  may indeed be used in series in the HVAC system  72 . For example, in other embodiments, 2, 4, 5, 6, 7, 8, 9, 10, or even more dehumidification units  12  may be used in series in the HVAC system  72 . 
     The HVAC system  72  of  FIG. 8  generally functions the same as the HVAC system  10  of  FIGS. 1, 6, and 7 . More specifically, as illustrated in  FIG. 8 , the HVAC system  72  receives the inlet air  14 A having a relatively high humidity. However, the relatively dry air  14 B from the first dehumidification unit  74  is not expelled into the atmosphere. Rather, as illustrated in  FIG. 8 , the air  14 B expelled from the first dehumidification unit  74  is directed into the second dehumidification unit  76  via a first duct  80 . Similarly, air  14 C expelled from the second dehumidification unit  76  is directed into the third dehumidification unit  78  via a second duct  82 . Outlet air  14 D from the third dehumidification unit  78  is then expelled into the conditioned space. Because the dehumidification units  74 ,  76 ,  78  of the HVAC system  72  are arranged in series, each successive airstream will be relatively dryer than the upstream airstreams. For example, outlet air  14 D is dryer than air  14 C, which is dryer than air  14 B, which is dryer than inlet air  14 A. 
     As illustrated, many of the components of the HVAC system  72  of  FIG. 8  may be considered identical to the components of the HVAC system  10  of  FIGS. 1, 6 , and  7 . For example, as described previously, the dehumidification units  74 ,  76 ,  78  of the HVAC system  72  of  FIG. 8  may be considered identical to the dehumidification units  12  of  FIGS. 1, 6, and 7 . In addition, the HVAC system  72  of  FIG. 8  also includes the condensation unit  54  that receives water vapor  26 B having a partial pressure just high enough to facilitate condensation, as described previously. In certain embodiments, the HVAC system  72  of  FIG. 8  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. 
     As illustrated in  FIG. 8 , in certain embodiments, each dehumidification unit  74 ,  76 ,  78  may be associated with a respective vacuum pump  84 ,  86 ,  88 , each of which is similar in functionality to the vacuum pump  52  of  FIGS. 1, 6, and 7 . However, because water vapor is removed from each successive dehumidification unit  74 ,  76 ,  78 , the partial pressure of water vapor in the air  14  may be gradually reduced at each successive dehumidification unit  74 ,  76 ,  78 . For example, as described previously, the partial pressure of water vapor in the inlet air  14 A may be in the range of approximately 0.2-1.0 psia; the partial pressure of water vapor in the air  14 B from the first dehumidification unit  74  may be in the range of approximately 0.17-0.75 psia (accomplishing approximately ⅓ of the drop); the partial pressure of water vapor in the air  14 C from the second dehumidification unit  76  may be in the range of approximately 0.14-0.54 psia (accomplishing approximately the next ⅓ of the drop); and the partial pressure of water vapor in the outlet air  14 D from the third dehumidification unit  78  may be in the range of approximately 0.10-0.25 psia, which is consistent with a 60° F. saturation temperature or lower. The very low values may be used to increase capacity for occasional use. 
     As such, in certain embodiments, the partial pressure of water vapor in the water vapor vacuum volumes  90 ,  92 ,  94  (e.g., that are similar in functionality to the water vapor vacuum volume  28  described previously) associated with each respective vacuum pump  84 ,  86 ,  88  may be modulated to ensure an optimal flow of water vapor  26  from each respective dehumidification unit  74 ,  76 ,  78 . For example, the partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  described previously may be maintained in a range of approximately 0.15-0.25 psia. However, in the HVAC system  72  of  FIG. 8 , the partial pressure of the water vapor  26 A in the first water vapor vacuum volume  90  may be maintained in a range of approximately 0.15-0.7 psia, the partial pressure of the water vapor  26 A in the second water vapor vacuum volume  92  may be maintained in a range of approximately 0.12-0.49 psia, and the partial pressure of the water vapor  26 A in the third water vapor vacuum volume  94  may be maintained in a range of approximately 0.09-0.24 psia. Regardless, it may be expected that less water vapor  26  is removed in each successive dehumidification unit  74 ,  76 ,  78 , and is generally be optimized to minimize energy used to operate the system. 
     In certain embodiments, each of the vacuum pumps  84 ,  86 ,  88  may compress the water vapor  26  and direct it into a common manifold  96  having a substantially constant partial pressure of water vapor (i.e., just high enough to facilitate condensation in the condensation unit  54 ) such that the water vapor  26  flows in a direction opposite to the flow of the air  14 . In other embodiments, the water vapor  26  extracted from each successive dehumidification unit  74 ,  76 ,  78  may be compressed by its respective vacuum pump  84 ,  86 ,  88  and then combined with the water vapor  26  extracted from the next upstream dehumidification unit  74 ,  76 ,  78 . For example, in other embodiments, the water vapor  26  from the third dehumidification unit  78  may be compressed by the third vacuum pump  88  and then combined with the water vapor  26  from the second dehumidification unit  76  in the second water vapor vacuum volume  92 . Similarly, the water vapor  26  compressed by the second vacuum pump  86  may be combined with the water vapor  26  from the first dehumidification unit  74  in the first water vapor vacuum volume  90 . In this embodiment, the exhaust side of each successive vacuum pump  84 ,  86 ,  88  increases the partial pressure of the water vapor  26  only to the operating pressure of the next upstream vacuum pump  84 ,  86 ,  88 . For example, the third vacuum pump  88  may only increase the pressure of the water vapor  26  to approximately 0.2 psia if the partial pressure of water vapor in the second water vapor vacuum volume  92  is approximately 0.2 psia. Similarly, the second vacuum pump  86  may only increase the pressure of the water vapor  26  to approximately 0.35 psia if the partial pressure of water vapor in the first water vapor vacuum volume  90  is approximately 0.35 psia. In this embodiment, the water vapor  26  compressed by the first vacuum pump  84  is directed into the condensation unit  54  at a partial pressure of water vapor just high enough to facilitate condensation. 
     It should be noted that the specific embodiment illustrated in  FIG. 8  having a plurality of dehumidification units  74 ,  76 ,  78  arranged in series may be configured in various ways not illustrated in  FIG. 8 . For example, although illustrated as using a respective vacuum pump  84 ,  86 ,  88  with each dehumidification unit  74 ,  76 ,  78 , in certain embodiments, a single vacuum pump  52  may be used with multiple inlet ports connected to the first, second, and third water vapor vacuum volumes  90 ,  92 ,  94 . In addition, although illustrated as using a single condensation unit  54 , reservoir  58 , and liquid pump  60  to condense the water vapor  26 B into a liquid state, and store and/or transport the liquid water from the HVAC system  72 , in other embodiments, each set of dehumidification units  74 ,  76 ,  78  and vacuum pumps  84 ,  86 ,  88  may be operated independently and be associated with their own respective condensation units  54 , reservoirs  58 , and liquid pumps  60 . 
     In addition, the control system  64  of  FIG. 7  may also be used in the HVAC system  72  of  FIG. 8  to control the operation of the HVAC system  72  in a similar manner as described previously with respect to  FIG. 7 . For example, as described previously, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26  in the water vapor vacuum volumes  90 ,  92 ,  94  by turning the vacuum pumps  84 ,  86 ,  88  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 6 and 7 ) on or off, or by modulating the rate at which the vacuum pumps  84 ,  86 ,  88  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 6 and 7 ) remove the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volumes  90 ,  92 ,  94  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volumes  90 ,  92 ,  94 . 
     In addition, the control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  92 ,  94  to modify the water vapor removal capacity and efficiency ratio of the dehumidification units  74 ,  76 ,  78 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volumes  90 ,  92 ,  94 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14 , among other things. The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  92 ,  94  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification units  74 ,  76 ,  78  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). 
     For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  92 ,  94  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  92 ,  94  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification units  74 ,  76 ,  78 . More specifically, under certain operating conditions, the dehumidification units  74 ,  76 ,  78  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification units  74 ,  76 ,  78  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification units  74 ,  76 ,  78  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. In addition, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification units  74 ,  76 ,  78 . 
     While  FIG. 8  includes a serial arrangement of multiple dehumidification units  12 , present embodiments include other ways in which multiple dehumidification units  12  may be arranged in a single HVAC system. For example,  FIG. 9  is a schematic diagram of an HVAC system  98  having a plurality of dehumidification units  12  (i.e., a first dehumidification unit  100 , a second dehumidification unit  102 , and a third dehumidification unit  104 ) arranged in parallel in accordance with an embodiment of the present disclosure. Although illustrated as having three dehumidification units  100 ,  102 ,  104  in parallel, any number of dehumidification units  12  may indeed be used in parallel in the HVAC system  98 . For example, in other embodiments, 2, 4, 5, 6, 7, 8, 9, 10, or even more dehumidification units  12  may be used in parallel in the HVAC system  98 . 
     The HVAC system  98  of  FIG. 9  generally functions the same as the HVAC system  10  of  FIGS. 1, 6, and 7  and the HVAC system  72  of  FIG. 8 . More specifically, as illustrated in  FIG. 9 , each dehumidification unit  100 ,  102 ,  104  of the HVAC system  98  receives the inlet air  14 A having a relatively high humidity and expels the outlet air  14 B having a relatively low humidity. As illustrated, many of the components of the HVAC system  98  of  FIG. 9  may be considered identical to the components of the HVAC system  10  of  FIGS. 1, 6, and 7  and the HVAC system  72  of  FIG. 8 . For example, the dehumidification units  100 ,  102 ,  104  of the HVAC system  98  of  FIG. 9  may be considered identical to the dehumidification units  12  of  FIGS. 1, 6, and 7  and the dehumidification units  74 ,  76 ,  78  of  FIG. 8 . In addition, the HVAC system  98  of  FIG. 9  also includes the condensation unit  54  that receives water vapor  26 B having a partial pressure just high enough to facilitate condensation, as described previously. In certain embodiments, the HVAC system  98  of  FIG. 9  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. 
     As illustrated in  FIG. 9 , in certain embodiments, each dehumidification unit  100 ,  102 ,  104  may be associated with a respective vacuum pump  106 ,  108 ,  110 , each of which is similar in functionality to the vacuum pump  52  of  FIGS. 1, 6, and 7  and the vacuum pumps  84 ,  86 ,  88  of  FIG. 8 . However, as opposed to the HVAC system  72  of  FIG. 8 , because the dehumidification units  100 ,  102 ,  104  and associated vacuum pumps  106 ,  108 ,  110  are arranged in parallel, the partial pressure of water vapor in the air  14  will be approximately the same in each dehumidification unit  100 ,  102 ,  104 . As such, in general, the partial pressure of water vapor in the water vapor vacuum volumes  112 ,  114 ,  116  associated with each respective vacuum pump  106 ,  108 ,  110  will also be approximately the same. For example, as described previously with respect to the HVAC system  10  of  FIGS. 1, 6, and 7 , the partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  114 ,  116  may be maintained in a range of approximately 0.10-0.25 psia. 
     As illustrated in  FIG. 9 , in certain embodiments, each of the vacuum pumps  106 ,  108 ,  110  may compress the water vapor  26  and direct it into a common manifold  118  having a substantially constant partial pressure of water vapor (i.e., just high enough to facilitate condensation in the condensation unit  54 ). In other embodiments, the water vapor  26  extracted from each successive dehumidification unit  100 ,  102 ,  104  (i.e., from top to bottom) may be compressed by its respective vacuum pump  106 ,  108 ,  110  and then combined with the water vapor  26  extracted from the next downstream (i.e., with respect to the common manifold) dehumidification unit  100 ,  102 ,  104 . For example, in other embodiments, the water vapor  26  from the first dehumidification unit  100  may be compressed by the first vacuum pump  106  and then combined with the water vapor  26  from the second dehumidification unit  102  in the second water vapor vacuum volume  114 . Similarly, the water vapor  26  compressed by the second vacuum pump  108  may be combined with the water vapor  26  from the third dehumidification unit  104  in the third water vapor vacuum volume  116 . In this embodiment, the exhaust side of each successive vacuum pump  106 ,  108 ,  110  increases the partial pressure of the water vapor  26  only to the operating pressure of the next downstream vacuum pump  106 ,  108 ,  110 . For example, the first vacuum pump  106  may only increase the pressure of the water vapor  26  to approximately 0.2 psia if the partial pressure of water vapor in the second water vapor vacuum volume  114  is approximately 0.2 psia. Similarly, the second vacuum pump  108  may only increase the pressure of the water vapor  26  to approximately 0.35 psia if the partial pressure of water vapor in the third water vapor vacuum volume  116  is approximately 0.35 psia. In this embodiment, the water vapor  26  compressed by the third vacuum pump  110  will be directed into the condensation unit  54  at a partial pressure of water vapor just high enough to facilitate condensation. 
     It should be noted that the specific embodiment illustrated in  FIG. 9  having a plurality of dehumidification units  100 ,  102 ,  104  arranged in parallel may be configured in various ways not illustrated in  FIG. 9 . For example, although illustrated as using a respective vacuum pump  106 ,  108 ,  110  with each dehumidification unit  100 ,  102 ,  104 , in certain embodiments, a single vacuum pump  52  may be used with multiple inlet ports connected to the first, second, and third water vapor vacuum volumes  112 ,  114 ,  116 . In addition, although illustrated as using a single condensation unit  54 , reservoir  58 , and liquid pump  60  to condense the water vapor  26 B into a liquid state, and store and/or transport the liquid water from the HVAC system  98 , in other embodiments, each set of dehumidification units  100 ,  102 ,  104  and vacuum pumps  106 ,  108 ,  110  may be operated independently and be associated with their own respective condensation units  54 , reservoirs  58 , and liquid pumps  60 . 
     In addition, the control system  64  of  FIG. 7  may also be used in the HVAC system  98  of  FIG. 9  to control the operation of the HVAC system  98  in a similar manner as described previously with respect to  FIG. 7 . For example, as described previously, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26 A in the water vapor vacuum volumes  112 ,  114 ,  116  by turning the vacuum pumps  106 ,  108 ,  110  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 6 and 7 ) on or off, or by modulating the rate at which the vacuum pumps  106 ,  108 ,  110  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 6 and 7 ) remove the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volumes  112 ,  114 ,  116  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volumes  112 ,  114 ,  116 . 
     In addition, the control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  114 ,  116  to modify the water vapor removal capacity and efficiency ratio of the dehumidification units  100 ,  102 ,  104 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volumes  112 ,  114 ,  116 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14 , among other things. The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  114 ,  116  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification units  100 ,  102 ,  104  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). 
     For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  114 ,  116  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  114 ,  116  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification units  100 ,  102 ,  104 . More specifically, under certain operating conditions, the dehumidification units  100 ,  102 ,  104  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification units  100 ,  102 ,  104  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification units  100 ,  102 ,  104  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. In addition, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification units  100 ,  102 ,  104 . 
     In addition to the serial arrangement of dehumidification units  12  illustrated in  FIG. 8  and the parallel arrangement of dehumidification units  12  illustrated in  FIG. 9 , multiple dehumidification units  12  may be used in other ways. Indeed, much more complex and expansive arrangements may also be used. For example,  FIG. 10  is a schematic diagram of an HVAC system  120  having a first set  122  of dehumidification units  12  (i.e., a first dehumidification unit  124  and a second dehumidification unit  126 ) arranged in series, and a second set  128  of dehumidification units  12  (i.e., a third dehumidification unit  130  and a fourth dehumidification unit  132 ) also arranged in series, with the first and second sets  122 ,  128  of dehumidification units  12  arranged in parallel in accordance with an embodiment of the present disclosure. In other words, the first set  122  of serial first and second dehumidification units  124 ,  126  are arranged in parallel with the second set  128  of serial third and fourth dehumidification units  130 ,  132 . 
     Although illustrated as having two sets  122 ,  128  of serial dehumidification units  12  arranged in parallel, any number of parallel pluralities of dehumidification units  12  may indeed be used in the HVAC system  120 . For example, in other embodiments, 3, 4, 5, 6, 7, 8, 9, 10, or even more parallel sets of dehumidification units  12  may be used in the HVAC system  120 . Similarly, although illustrated as having two dehumidification units  12  arranged in series within each set  122 ,  128  of dehumidification units  12 , any number of dehumidification units  12  may indeed be used in series within each set  122 ,  128  of dehumidification units  12  in the HVAC system  120 . For example, in other embodiments, 1, 3, 4, 5, 6, 7, 8, 9, 10, or even more dehumidification units  12  may be used in series within each set  122 ,  128  of dehumidification units  12  in the HVAC system  120 . 
     All of the operating characteristics of the HVAC system  120  of  FIG. 10  are similar to those described previously with respect to the HVAC systems  72 ,  98  of  FIGS. 8 and 9  (as well as the HVAC system  10  of  FIGS. 1, 6, and 7 ). For example, as illustrated, each of the dehumidification units  124 ,  126 ,  130 ,  132  may be associated with its own respective vacuum pump  134 ,  136 ,  138 ,  140  (e.g., similar to the vacuum pump  52  of  FIGS. 1, 6, and 7 ). However, in other embodiments, one vacuum pump  52  may be used for each set  122 ,  128  of dehumidification units  12  with multiple inlet ports connected to the respective water vapor vacuum volumes  142 ,  144 ,  146 ,  148 . Indeed, in other embodiments, all of the dehumidification units  124 ,  126 ,  130 ,  132  may be associated with a single vacuum pump  52  with multiple inlet ports connected to all of the water vapor vacuum volumes  142 ,  144 ,  146 ,  148 . 
     In addition, although illustrated as using a single condensation unit  54 , reservoir  58 , and liquid pump  60  to condense the water vapor  26 B into a liquid state, and store and/or transport the liquid water from the HVAC system  120 , in other embodiments, each set of dehumidification units  124 ,  126 ,  130 ,  132  and vacuum pumps  134 ,  136 ,  138 ,  140  may be operated independently and be associated with their own respective condensation units  54 , reservoirs  58 , and liquid pumps  60 . In addition, the control system  64  described previously may also be used in the HVAC system  120  of  FIG. 10  to control operation of the HVAC system  120  in a similar manner as described previously. 
     The embodiments described previously with respect to  FIGS. 8 through 10  are slightly more complex than the embodiments described previously with respect to  FIGS. 1 through 7  inasmuch as multiple dehumidification units  12  are used in series, parallel, or some combination thereof. As such, the control of pressures and temperatures of the HVAC systems  72 ,  98 ,  120  of  FIGS. 8 through 10  are slightly more complicated than the control of a single dehumidification unit  12 . For example, the partial pressures in the water vapor vacuum volumes may need to be closely monitored and modulated by the control system  64  to take into account variations in temperature and partial pressure of water vapor in the air  14  within the respective dehumidification units  12 , operating pressures of adjacent water vapor vacuum volumes and vacuum pumps (which may be cross-piped together as described previously to facilitate control of pressures, flows, and so forth), among other things. In certain embodiments, variable or fixed orifices may be used to control pressures and changes in pressures in and between the dehumidification units  12 . In addition, as described previously, each of the respective vacuum pumps may be controlled to adjust the partial pressures of water vapor in the water vapor vacuum volumes to account for variations between dehumidification units  12 . 
     In certain embodiments, the dehumidification unit  12  described with respect to  FIGS. 1 through 7  may be used in conjunction with one or more evaporative cooling units  12 . For example,  FIG. 11  is a schematic diagram of an HVAC system  150  having an evaporative cooling unit  152  disposed upstream of the dehumidification unit  12  in accordance with an embodiment of the present disclosure. The HVAC system  150  of  FIG. 11  generally functions the same as the HVAC system  10  of  FIGS. 1, 6, and 7 . However, as illustrated in  FIG. 11 , the HVAC system  150  specifically includes the evaporative cooling unit  152  disposed upstream of the dehumidification unit  12 . Thus, the HVAC system  150  first receives the relatively humid inlet air  14 A into the evaporative cooling unit  152 , instead of the dehumidification unit  12 . The evaporative cooling unit  152  reduces the temperature of the relatively humid inlet air  14 A and expels cooler (but still relatively humid) air  14 B, which is directed into the dehumidification unit  12  via a duct  154 . As described previously, the cooler (but still relatively humid) air  14 B is then dehumidified in the dehumidification unit  12  and expelled as relatively dry air  14 C into the conditioned space. 
     The evaporative cooling unit  152  of  FIG. 11  may either be a direct evaporative cooling unit or an indirect evaporative cooling unit. In other words, when the evaporative cooling unit  152  uses direct evaporative cooling techniques, a relatively cool and moist media  156  (e.g., relatively cool water) is directly added to the relatively humid inlet air  14 A. However, when the evaporative cooling unit  152  uses indirect evaporative cooling techniques, the relatively humid air  14 A may, for example, flow across one side of a plate of a heat exchanger while the relatively cool and moist media  156  flows across another side of the plate of the heat exchanger. In other words, generally speaking, some of the relatively cool moisture from the relatively cool and moist media  156  is indirectly added to the relatively humid air  14 A. Whether direct or indirect evaporative cooling techniques are used in the evaporative cooling unit  152  affects the rate of humidity removal and temperature reduction of the air  14  that flows through the HVAC system  150  of  FIG. 11 . In general, however, the evaporative cooling unit  152  of  FIG. 11  initially cools the air  14  to a temperature as low as possible for the particular application, and the dehumidification unit  12  lowers the humidity ratio at approximately constant temperature. 
     As illustrated, many of the components of the HVAC system  150  of  FIG. 11  may be considered identical to the components of the HVAC system  10  of  FIGS. 1, 6, and 7 . For example, as described previously, HVAC system  150  of  FIG. 11  includes the condensation unit  54  that receives water vapor  26 B having a partial pressure just high enough to facilitate condensation, as described previously. In certain embodiments, the HVAC system  150  of  FIG. 11  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. 
     In addition, the control system  64  of  FIG. 7  may also be used in the HVAC system  150  of  FIG. 11  to control the operation of the HVAC system  150  in a similar manner as described previously with respect to  FIG. 7 . For example, as described previously, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26 A in the water vapor vacuum volume  28  by turning the vacuum pump  52  (or separate vacuum pump  62 ) on or off, or by modulating the rate at which the vacuum pump  52  (or separate vacuum pump  62 ) removes the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volume  28  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volume  28 . 
     In addition, the control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  to modify the water vapor removal capacity and efficiency ratio of the dehumidification unit  12 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volume  28 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14  in the evaporative cooling unit  152 , the dehumidification unit  12 , or both, among other things. 
     The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification unit  12  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification unit  12 . More specifically, under certain operating conditions, the dehumidification unit  12  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification unit  12  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a v (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification unit  12  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. 
     Furthermore, the control system  64  may also be configured to control operation of the evaporative cooling unit  152 . For example, the control system  64  may selectively modulate how much (direct or indirect) evaporative cooling occurs in the evaporative cooling unit  152 . As an example, valves may be actuated to control the flow rate of the relatively cool and moist media  156  through the evaporative cooling unit  152 , thereby directly affecting the amount of (direct or indirect) evaporative cooling in the evaporative cooling unit  152 . In addition, operation of the evaporative cooling unit  152  and the dehumidification unit  12  may be controlled simultaneously. Furthermore, the control system  64  may be configured to control start-up and shutdown sequencing of the evaporative cooling unit  152  and the dehumidification unit  12 . 
       FIGS. 12A and 12B  are psychrometric charts  158 ,  160  of the temperature and the humidity ratio of the air  14  flowing through the evaporative cooling unit  152  and the dehumidification unit  12  of  FIG. 11  in accordance with an embodiment of the present disclosure. More specifically,  FIG. 12A  is the psychrometric chart  158  of the temperature and the humidity ratio of the air  14  flowing through a direct evaporative cooling unit  152  and the dehumidification unit  12  of  FIG. 11  in accordance with an embodiment of the present disclosure, and  FIG. 12B  is the psychrometric chart  160  of the temperature and the humidity ratio of the air  14  flowing through an indirect evaporative cooling unit  152  and the dehumidification unit  12  of  FIG. 11  in accordance with an embodiment of the present disclosure. In particular, in each chart  158 ,  160 , the x-axis  162  corresponds to the temperature of the air  14  flowing through the evaporative cooling unit  152  and the dehumidification unit  12  of  FIG. 11 , the y-axis  164  corresponds to the humidity ratio of the air  14  flowing through the evaporative cooling unit  152  and the dehumidification unit  12  of  FIG. 11 , and the curve  166  represents the water vapor saturation curve for a given relative humidity of the air  14  flowing through the evaporative cooling unit  152  and the dehumidification unit  12  of  FIG. 11 . 
     As illustrated by line  168  in  FIG. 12A , because the relatively cool and moist media  156  is directly introduced into the air  14  flowing though the direct evaporative cooling unit  152 , the humidity ratio of the air  14 B (i.e., point  170 ) out of the direct evaporative cooling unit  152  is substantially higher than the humidity ratio of the inlet air  14 A (i.e., point  172 ) into the direct evaporative cooling unit  152 . However, the temperature of the air  14 B (i.e., point  170 ) out of the direct evaporative cooling unit  152  is substantially lower than the temperature of the inlet air  14 A (i.e., point  172 ) into the evaporative cooling unit  152 . As illustrated by line  174  of  FIG. 12A , because water vapor  26  is removed from the air  14 B flowing through the dehumidification unit  12 , the humidity ratio of the outlet air  14 C (i.e., point  176 ) from the dehumidification unit  12  is lower than the humidity ratio of the air  14 B (i.e., point  170 ) into the dehumidification unit  12 , while the temperature of the outlet air  14 C and the air  14 B are substantially the same. Indeed, the direct evaporative cooling unit  152  humidifies and cools the air  14 , while the dehumidification unit  12  subsequently dehumidifies the air  14  at substantially constant temperature. 
     As illustrated by line  178  in  FIG. 12B , because the relatively cool and moist media  156  indirectly cools the air  14  flowing through the indirect evaporative cooling unit  152 , the humidity ratio of the air  14 B (i.e., point  180 ) out of the indirect evaporative cooling unit  152  is substantially the same as the humidity ratio of the inlet air  14 A (i.e., point  182 ) into the indirect evaporative cooling unit  152 . However, the temperature of the air  14 B (i.e., point  180 ) out of the indirect evaporative cooling unit  152  is substantially lower than the temperature of the inlet air  14 A (i.e., point  182 ) into the indirect evaporative cooling unit  152 . As illustrated by line  184  of  FIG. 12B , because water vapor  26  is removed from the air  14 B flowing through the dehumidification unit  12 , the humidity ratio of the outlet air  14 C (i.e., point  186 ) from the dehumidification unit  12  is lower than the humidity ratio of the air  14 B (i.e., point  180 ) into the dehumidification unit  12 , while the temperature of the outlet air  14 C and the air  14 B are substantially the same. Indeed, the indirect evaporative cooling unit  152  cools (without substantially humidifying) the air  14 , while the dehumidification unit  12  subsequently dehumidifies the air  14  at substantially constant temperature. 
     As described previously, the control system  64  of  FIG. 11  may be configured to control the operation of the evaporative cooling unit  152  and the dehumidification unit  12 . For example, the control system  64  may be configured to adjust where points  170 ,  172 ,  176  and points  180 ,  182 ,  186  of the air  14  fall in the psychrometric charts  158 ,  160  of  FIGS. 12A and 12B  when direct and indirect evaporative cooling techniques, respectively, are used in the evaporative cooling unit  152  of  FIG. 11 . 
       FIG. 13  is a schematic diagram of an HVAC system  188  having the evaporative cooling unit  152  disposed downstream of the dehumidification unit  12  in accordance with an embodiment of the present disclosure. The HVAC system  188  of  FIG. 13  generally functions the same as the HVAC system  10  of  FIGS. 1, 6, and 7  and the HVAC system  150  of  FIG. 11 . However, as illustrated in  FIG. 13 , the HVAC system  188  first receives the relatively humid inlet air  14 A into the dehumidification unit  12 . As described previously, the relatively humid inlet air  14 A is first dehumidified in the dehumidification unit  12  and expelled as relatively dry air  14 B into the duct  154 . The evaporative cooling unit  152  then reduces the temperature of the dry air  14 B and expels cooler dry air  14 C into the conditioned space. 
     As described previously with respect to  FIG. 11 , the evaporative cooling unit  152  of  FIG. 13  may either be a direct evaporative cooling unit or an indirect evaporative cooling unit. In other words, when the evaporative cooling unit  152  uses direct evaporative cooling techniques, the relatively cool and moist media  156  (e.g., relatively cool water) is directly added to the relatively dry air  14 B in the duct  154 . However, when the evaporative cooling unit  152  uses indirect evaporative cooling techniques, the relatively dry air  14 B may, for example, flow across one side of a plate of a heat exchanger while the relatively cool and moist media  156  flows across another side of the plate of the heat exchanger. In other words, generally speaking, some of the relatively cool moisture from the relatively cool and moist media  156  is indirectly added to the relatively dry air  14 B in the duct  154 . Whether direct or indirect evaporative cooling techniques are used in the evaporative cooling unit  152  affects the rate of humidity removal and temperature reduction of the air  14  that flows through the HVAC system  188  of  FIG. 13 . In general, however, the dehumidification unit  12  initially lowers the humidity ratio at approximately constant temperature, and the evaporative cooling unit  152  cools the air  14  to a temperature as low as possible for the particular application. 
     As illustrated, many of the components of the HVAC system  188  of  FIG. 13  may be considered identical to the components of the HVAC system  10  of  FIGS. 1, 6, and 7  and the HVAC system  150  of  FIG. 11 . For example, as described previously, HVAC system  188  of  FIG. 13  includes the condensation unit  54  that receives water vapor  26 B having a partial pressure just high enough to facilitate condensation, as described previously. In certain embodiments, the HVAC system  188  of  FIG. 13  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. 
     In addition, the control system  64  of  FIGS. 7 and 11  may also be used in the HVAC system  188  of  FIG. 13  to control the operation of the HVAC system  188  in a similar manner as described previously with respect to  FIGS. 7 and 11 . For example, as described previously, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26 A in the water vapor vacuum volume  28  by turning the vacuum pump  52  (or separate vacuum pump  62 ) on or off, or by modulating the rate at which the vacuum pump  52  (or separate vacuum pump  62 ) removes the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volume  28  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volume  28 . 
     In addition, the control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  to modify the water vapor removal capacity and efficiency ratio of the dehumidification unit  12 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volume  28 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14  in the dehumidification unit  12 , the evaporative cooling unit  152 , or both, among other things. 
     The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification unit  12  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification unit  12 . More specifically, under certain operating conditions, the dehumidification unit  12  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification unit  12  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification unit  12  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. 
     Furthermore, the control system  64  may also be configured to control operation of the evaporative cooling unit  152 . For example, the control system  64  may selectively modulate how much (direct or indirect) evaporative cooling occurs in the evaporating cooling unit  152 . As an example, valves may be actuated to control the flow rate of the relatively cool and moist media  156  through the evaporative cooling unit  152 , thereby directly affecting the amount of (direct or indirect) evaporative cooling in the evaporative cooling unit  152 . In addition, operation of the dehumidification unit  12  and the evaporative cooling unit  152  may be controlled simultaneously. Furthermore, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification unit  12  and the evaporative cooling unit  152 . 
       FIGS. 14A and 14B  are psychrometric charts  190 ,  192  of the temperature and the humidity ratio of the air  14  flowing through the dehumidification unit  12  and the evaporative cooling unit  152  of  FIG. 13  in accordance with an embodiment of the present disclosure. More specifically,  FIG. 14A  is the psychrometric chart  190  of the temperature and the humidity ratio of the air  14  flowing through the dehumidification unit  12  and a direct evaporative cooling unit  152  of  FIG. 13  in accordance with an embodiment of the present disclosure, and  FIG. 14B  is the psychrometric chart  192  of the temperature and the humidity ratio of the air  14  flowing through the dehumidification unit  12  and an indirect evaporative cooling unit  152  of  FIG. 13  in accordance with an embodiment of the present disclosure. In particular, as described previously with respect to  FIGS. 12A and 12B , the x-axis  162  corresponds to the temperature of the air  14  flowing through the dehumidification unit  12  and the evaporative cooling unit  152  of  FIG. 13 , the y-axis  164  corresponds to the humidity ratio of the air  14  flowing through the dehumidification unit  12  and the evaporative cooling unit  152  of  FIG. 13 , and the curve  166  represents the water vapor saturation curve for a given relative humidity of the air  14  flowing through the dehumidification unit  12  and the evaporative cooling unit  152  of  FIG. 13 . 
     As illustrated by line  194  in  FIG. 14A , because water vapor  26  is removed from the relatively humid inlet air  14 A flowing through the dehumidification unit  12 , the humidity ratio of the relatively dry air  14 B (i.e., point  196 ) from the dehumidification unit  12  is lower than the humidity ratio of the relatively humid inlet air  14 A (i.e., point  198 ) into the dehumidification unit  12 , while the temperature of the relatively dry air  14 B and the relatively humid inlet air  14 A are substantially the same. As illustrated by line  200  of  FIG. 14A , because the relatively cool and moist media  156  is directly introduced into the relatively dry air  14 B flowing through the direct evaporative cooling unit  152 , the humidity ratio of the outlet air  14 C (i.e., point  202 ) from the direct evaporative cooling unit  152  is substantially higher than the humidity ratio of the relatively dry air  14 B (i.e., point  196 ) into the direct evaporative cooling unit  152 . However, the temperature of the outlet air  14 C (i.e., point  202 ) from the direct evaporative cooling unit  152  is substantially lower than the temperature of the relatively dry air  14 B (i.e., point  196 ) into the direct evaporative cooling unit  152 . Indeed, the dehumidification unit  12  dehumidifies the air  14  at substantially constant temperature, while the direct evaporative cooling unit  152  subsequently humidifies and cools the air  14 . 
     As illustrated by line  204  in  FIG. 14B , because water vapor  26  is removed from the relatively humid inlet air  14 A flowing through the dehumidification unit  12 , the humidity ratio of the relatively dry air  14 B (i.e., point  206 ) from the dehumidification unit  12  is lower than the humidity ratio of the relatively humid inlet air  14 A (i.e., point  208 ) into the dehumidification unit  12 , while the temperature of the relatively dry air  14 B and the relatively humid inlet air  14 A are substantially the same. As illustrated by line  210  of  FIG. 14B , because the relatively cool and moist media  156  indirectly cools the relatively dry air  14 B flowing though the indirect evaporative cooling unit  152 , the humidity ratio of the outlet air  14 C (i.e., point  212 ) from the indirect evaporative cooling unit  152  is substantially the same as the humidity ratio of the relatively dry air  14 B (i.e., point  206 ) into the indirect evaporative cooling unit  152 . However, the temperature of the outlet air  14 C (i.e., point  212 ) from the indirect evaporative cooling unit  152  is substantially lower than the temperature of the relatively dry air  14 B (i.e., point  206 ) into the indirect evaporative cooling unit  152 . Indeed, the dehumidification unit  12  dehumidifies the air  14  at substantially constant temperature, while the indirect evaporative cooling unit  152  cools (without substantially humidifying) the air  14 . 
     As described previously, the control system  64  of  FIG. 13  may be configured to control the operation of the dehumidification unit  12  and the evaporative cooling unit  152 . For example, the control system  64  may be configured to adjust where points  196 ,  198 ,  202  and points  206 ,  208 ,  212  of the air  14  fall in the psychrometric charts  190 ,  192  of  FIGS. 14A and 14B  when direct and indirect evaporative cooling techniques, respectively, are used in the evaporative cooling unit  152  of  FIG. 13 . 
     The embodiments of the HVAC systems  150 ,  188  of  FIGS. 11 and 13  are not the only ways in which dehumidification units  12  may be combined with evaporative cooling units  152 . More specifically, whereas  FIGS. 11 and 13  illustrate the use of a single dehumidification unit  12  and a single evaporative cooling unit  152  in series with each other, in other embodiments, any number of dehumidification units  12  and evaporative cooling units  152  may be used in series with each other. As another example, in one embodiment, a first dehumidification unit  12  may be followed by a first evaporative cooling unit  152 , which is in turn followed by a second dehumidification unit  12 , which is in turn followed by a second evaporative cooling unit  152 , and so forth. However, any number of dehumidification units  12  and evaporative cooling units  152  may indeed be used in series with each other, wherein the air  14  exiting each unit  12 ,  152  is directed into the next downstream unit  12 ,  152  in the series (except from the last unit  12 ,  152  in the series, from which the air  14  is expelled into the conditioned space). In other words, the air  14  exiting each dehumidification unit  12  in the series is directed into a downstream evaporative cooling unit  152  (or to the conditioned space, if it is the last unit in the series), and the air  14  exiting each evaporative cooling unit  152  in the series is directed into a downstream dehumidification unit  12  (or to the conditioned space, if it is the last unit in the series). As such, the temperature of the air  14  may be successively lowered in each evaporative cooling unit  152  between dehumidification units  12  in the series, and the humidity ratio of the air  14  may be successively lowered in each dehumidification unit  12  between evaporative cooling units  152  in the series. This process may be continued within any number of dehumidification units  12  and evaporative cooling units  152  (e.g., 2, 3, 4, 5, 6, 7, 8, 9, 10, or more units  12  and/or units  152 ) until the desired final temperature and humidity ratio conditions of the air  14  are achieved. In one embodiment, each dehumidification unit  12  may be combined with a corresponding evaporative cooling unit  152 . In another embodiment, more than one dehumidification unit  12  may be combined with a single evaporative cooling unit  152 , or vice versa. The combinations may include the dehumidification unit  12  upstream of the evaporative cooling unit  152 , or downstream of the evaporative cooling unit  152 . 
       FIGS. 15A and 15B  are psychrometric charts  214 ,  216  of the temperature and the humidity ratio of the air  14  flowing through a plurality of dehumidification units  12  and a plurality of evaporative cooling units  152  in accordance with an embodiment of the present disclosure. More specifically,  FIG. 15A  is a psychrometric chart  214  of the temperature and the humidity ratio of the air  14  flowing through a plurality of dehumidification units  12  and a plurality of direct evaporative cooling units  152  in accordance with an embodiment of the present disclosure, and  FIG. 15B  is a psychrometric chart  216  of the temperature and the humidity ratio of the air  14  flowing through a plurality of dehumidification units  12  and a plurality of indirect evaporative cooling units  152  in accordance with an embodiment of the present disclosure. In particular, in each chart  214 ,  216 , the x-axis  162  corresponds to the temperature of the air  14  flowing through the plurality of dehumidification units  12  and the plurality of evaporative cooling units  152 , the y-axis  164  corresponds to the humidity ratio of the air  14  flowing through the plurality of dehumidification units  12  and the plurality of evaporative cooling units  152 , and the curve  166  represents the water vapor saturation curve for a given relative humidity of the air  14  flowing through the plurality of dehumidification units  12  and the plurality of evaporative cooling units  152 . 
     As illustrated by lines  218  in  FIG. 15A , because water vapor  26  is removed from relatively humid air  14  flowing through each of the plurality of dehumidification units  12 , the humidity ratio of the air  14  substantially decreases while the temperature of the air  14  remains substantially the same in each of the plurality of dehumidification units  12 . As illustrated by lines  220  in  FIG. 15A , because the relatively cool and moist media  156  is directly introduced into the relatively dry air  14  flowing though each of the direct evaporative cooling units  152 , the humidity ratio of the air  14  increases while the temperature of the air  14  substantially decreases in each of the plurality of direct evaporative cooling units  152 . In other words, each of the plurality of dehumidification units  12  successively dehumidifies the air  14  at substantially constant temperature, while each of the plurality of direct evaporative cooling units  152  successively humidifies and cools the air  14  until the desired final conditions of temperature and humidity ratio are achieved. More specifically, as illustrated in  FIG. 15A , the lines  218 ,  220  generally form a “step function” progression from the initial conditions of temperature and humidity ratio of the inlet air  14  (i.e., point  222 ) to the final conditions of temperature and humidity ratio of the outlet air  14  (i.e., point  224 ). 
     As illustrated by lines  226  in  FIG. 15B , because water vapor  26  is removed from relatively humid air  14  flowing through each of the plurality of dehumidification units  12 , the humidity ratio of the air  14  substantially decreases while the temperature of the air  14  remains substantially the same in each of the plurality of dehumidification units  12 . As illustrated by lines  228  in  FIG. 15B , because the relatively cool and moist media  156  indirectly interacts with the relatively dry air  14  flowing though each of the indirect evaporative cooling units  152 , the humidity ratio of the air  14  remains substantially the same while the temperature of the air  14  substantially decreases in each of the plurality of indirect evaporative cooling units  152 . In other words, each of the plurality of dehumidification units  12  successively dehumidifies the air  14  at substantially constant temperature, while each of the plurality of indirect evaporative cooling units  152  successively cools the air  14  at substantially constant humidity ratio until the desired final conditions of temperature and humidity ratio are achieved. More specifically, as illustrated in  FIG. 15B , the lines  226 ,  228  generally form a “sawtooth” progression from the initial conditions of temperature and humidity ratio of the inlet air  14  (i.e., point  230 ) to the final conditions of temperature and humidity ratio of the outlet air  14  (i.e., point  232 ). 
     Because evaporative cooling units  152  are used between dehumidification units  12 , each dehumidification unit  12  receives air  14  that is cooler and at a lower partial pressure of water vapor than the upstream dehumidification units  12 . As such, each of the dehumidification units  12  operates at substantially different operating conditions. Accordingly, the control system  64  may be used to modulate the operating parameters (e.g., the partial pressures of water vapor in the water vapor vacuum volumes  28 , among other things) of the dehumidification units  12  to take into account the variations between dehumidification units  12 . Similarly, because dehumidification units  12  are used between evaporative cooling units  152 , each evaporative cooling unit  152  also receives air  14  that is cooler and at a lower partial pressure of water vapor than the upstream evaporative cooling units  152 . As such, each of the evaporative cooling units  152  also operates at substantially different operating conditions. Accordingly, the control system  64  may also be used to modulate the operating parameters (e.g., the flow rates of the relatively cool and moist media  156 , among other things) of the evaporative cooling units  152  to take into account the variations between evaporative cooling units  152 . In addition, the control system  64  may also simultaneously coordinate operation of the plurality of dehumidification units  12  and the plurality of evaporative cooling units  152  to take the variations into account. 
     The evaporative cooling units  152  of  FIGS. 11 and 13  not only serve to lower the temperature of the air  14 , but also serve to clean the air  14  by, for example, passing the air  14  through a moist, fibrous mat. In addition, the dehumidification units  12  and the evaporative cooling units  14  may be operated at variable speeds or fixed speeds for optimal operation between different initial temperature and humidity conditions (i.e., operating points  222  and  230  in  FIGS. 15A and 15B , respectively) and the final temperature and humidity conditions (i.e., operating points  224  and  232  in  FIGS. 15A and 15B , respectively). Furthermore, the evaporative cooling units  152  are relatively low-energy units, thereby minimizing overall operating costs. 
     In addition to the embodiments described previously, in other embodiments, one or more of the dehumidification unit  12  described herein may be used in conjunction with one or more mechanical cooling units. For example,  FIG. 16  is a schematic diagram of an HVAC system  234  having a mechanical cooling unit  236  disposed downstream of the dehumidification unit  12  in accordance with an embodiment of the present disclosure, and  FIG. 17  is a schematic diagram of an HVAC system  238  having the mechanical cooling unit  236  of  FIG. 16  disposed upstream of the dehumidification unit  12  in accordance with an embodiment of the present disclosure. In each of these embodiments, the mechanical cooling unit  236  may include components typical for mechanical cooling units  236  such as a compressor  240  (e.g., a variable speed compressor), a condenser  242 , and so forth. A refrigerant is recycled through the components to cool the air received from the dehumidification unit  12  (i.e.,  FIG. 16 ) or the air delivered to the dehumidification unit (i.e.,  FIG. 17 ) to deliver non-latent, sensible compression cooling to the air. Although the embodiments illustrated in  FIGS. 16 and 17  illustrate the use of one dehumidification unit  12  and one mechanical cooling unit  236  in series, in other embodiments, any number of the dehumidification units  12  and mechanical cooling units  236  may be used in series, parallel, or some combination thereof (similar to the embodiments described previously). In certain embodiments, one or more dehumidification units  12  may be retrofitted into existing HVAC systems have mechanical cooling units  236 . 
     In addition, in certain embodiments, the dehumidification units  12  described herein may be used as distributed dehumidification units  12  that may, for example, be portable and may be retrofitted into existing HVAC systems. For example,  FIG. 18  is a schematic diagram of an HVAC system  244  using mini-dehumidification units  246  in accordance with an embodiment of the present disclosure, wherein the mini-dehumidification units  246  include all of the functionality of the dehumidification units  12  described previously. As illustrated, the mini-dehumidification units  246  may be connected to existing ducts  248  of the components  250  of the HVAC system  244  to improve the dehumidification capabilities of the HVAC system  244 . In certain embodiments, fans  252  (e.g., variable speed fans) may be used to blow air from the existing HVAC components  250  of the HVAC system  244  into the mini-dehumidification units  246 . The mini-dehumidification units  246  may be sized to facilitate coordination with standard components of existing HVAC systems. 
     In addition, in certain embodiments, the dehumidification units  12  described herein may be modified slightly to use them as enthalpy recovery ventilators (ERVs). For example, in a first ERV embodiment, relatively high humidity air and relatively low humidity air may flow in a counterflow arrangement on opposite sides of an interface  20  (e.g., a water vapor permeable membrane) as described previously. Alternatively, in a second ERV embodiment, relatively high humidity air and relatively low humidity air may flow in a parallel flow arrangement on opposite sides of an interface  20  as described previously. In both of these embodiments, the vacuum pump  52  described previously may not be used. Rather, both humidity and sensible heat may be recovered through transfer between the relatively high humidity air and the relatively low humidity air through the interface  20 . In addition, both of the ERV embodiments may have sections inserted between the interface  20  to increase heat transfer between the relatively high humidity air and the relatively low humidity air on opposite sides of the interface  20 . 
     In addition, the ERV embodiments described previously may be combined with other stages to improve the overall performance of the system. For example, in certain embodiments, a single section membrane dehumidification unit  12  with associated vacuum pump  52  and condensation unit  54  (e.g., such as the HVAC system  10  of  FIGS. 1, 6, and 7 ) may be connected upstream or downstream (or both) of one of the ERV embodiments. In other embodiments, a multistage membrane dehumidification unit  12  with associated vacuum pump  52  and condensation unit  54  (e.g., such as the HVAC systems  72 ,  98 ,  120  of  FIGS. 8 through 10 ) may be connected upstream or downstream (or both) of one of the ERV embodiments. In other embodiments, a single stage or multi-stage dehumidification unit  12  with associated vacuum pump  52 , condensation unit  54 , and one or more evaporative cooling units  152  (e.g., such as the HVAC systems  150 ,  188  of  FIGS. 11 and 13 ) may be connected upstream or downstream (or both) of one of the ERV embodiments. In other embodiments, a single stage or multi-stage membrane dehumidification unit  12  with sensible compression cooling (e.g., such as the HVAC systems  234 ,  238  of  FIGS. 16 and 17 ) may be connected upstream or downstream (or both) of one of the ERV embodiments. 
     In addition, in other embodiments, the vacuum pump  52  described previously may be a multi-stage vacuum pump. This multi-stage vacuum pump  52  will make the improved efficiency of the multi-stage HVAC systems  72 ,  98 ,  120  of  FIGS. 8 through 10  and the evaporative cooling HVAC systems  150 ,  188  of  FIGS. 11 and 13  more readily achievable in practice. In certain embodiments, the multi-stage vacuum pump  52  may be a turbine type vacuum pump that has multiple inlets, such that the multi-stage vacuum pump  52  may suction water vapor  26 A into the multi-stage vacuum pump  52  at increasing pressures in a continuous flow process. The flow rate increases as the pressure increases, because additional water vapor  26 A is sucked into the multi-stage vacuum pump  52 . The multi-stage vacuum pump  52  may be combined with the multi-stage dehumidification units  12  (e.g., dehumidification units  74 ,  76 ,  78  of  FIG. 8 , dehumidification units  100 ,  102 ,  104  of  FIG. 9 , or dehumidification units  124 ,  126 ,  130 ,  132  of  FIG. 10 ). The high pressure end of the turbine in the multi-stage vacuum pump  52  removes moisture from the highest moisture stage, while the lowest pressure stage of the turbine of the multi-stage vacuum pump  52  is coupled to the lowest moisture stage. The controller  64  described previously may be used to control the flow into the various stages. In addition, in certain embodiments, two or more turbines may operate in parallel so that the turbines can have more pressure difference between inlets than may exist between sequential stages. The multi-stage vacuum pump  52  may also be combined with the dehumidification units  12  and evaporative cooling units  152  of  FIGS. 11 and 13 . In addition, the multi-stage vacuum pump  52  may also be combined with a multi-stage dehumidifier that is followed by a compression cooler to provide sensible cooling (e.g., such as the HVAC systems  234 ,  238  of  FIGS. 16 and 17 ). 
     In addition, in certain embodiments, the condensation unit  54  described previously may be replaced with a membrane module, which includes one or more interfaces  20  (e.g., water vapor permeable membranes) similar to those used in the dehumidification units  12  described herein. In these embodiments, the water vapor  26 B from the vacuum pump  52  may be directed into the membrane module, where part of the water vapor  26 B passes through the interfaces  20  and is rejected to atmosphere, whereas other components in the water vapor  26 B are substantially blocked from flowing into a water vapor channel of the membrane module. In addition, in other embodiments, this membrane module may be used in combination with the condensation unit  54 . 
     Turning now to  FIG. 19 , the figure is a schematic diagram of an HVAC system  300  including a multi-stage vacuum pump  302  coupled to multiple cooling and dehumidification stages  304  and  306 . Although illustrated as having two stages  304  and  306  disposed in series, any number of cooling and dehumidification stages may be used. For example, in other embodiments, 2, 4, 5, 6, 7, 8, 9, 10, or even more dehumidification and cooling stages may be used in series in the HVAC system  300 . As illustrated, each stage  304  and  306  includes the evaporative cooling unit  152  disposed upstream of the dehumidification unit  12 , in accordance with an embodiment of the present disclosure. The HVAC system  300  of  FIG. 19  generally functions the same as the HVAC system  10  of  FIGS. 1, 6, and 7  and the HVAC system  150  of  FIG. 11 . However, as illustrated in  FIG. 19 , the HVAC system  300  first receives the relatively humid inlet air  14 A into the evaporative cooling unit  152 . Accordingly, the relatively humid inlet air  14 A may first be cooled. The evaporative cooling unit  152  then expels the cooler air  14 B into the duct  154 . The dehumidification unit  12  then dries the cooler air, and expels cooler dry air  14 C into the conditioned space, approximate to a section  308 . 
     As illustrated, the section  308  of the HVAC system  300  may include one more cooling and dehumidification stages, each stage including the evaporative cooling unit  152  disposed upstream of the dehumidification unit  12 . Indeed, 2, 3, 4, 5, 6, 7, 8, 9, 10 or more cooling and dehumidification stages may be disposed in the section  308  of the HVAC system. The stages found in section  308  may then further cool and dehumidify the air  14 C, resulting in a cooler, drier air  14 D. In the depicted embodiment, the air  14 D may then be further processed by the final stage  306 . That is, the air  14 D may first be cooled into a cooler air  14 E by the final stage, and then the air  14 E may be dehumidified, thus producing a cooler, low humidity air  14 F. By providing for multiple stages (each subsequent stage further cooling and dehumidifying the initial air  14 A), a cooler, low humidity air  14 F may be produced in a more efficient manner For, example, the single multi-stage vacuum pump  302  may be used to drive the conversion between the air  14 A and the air  14 F. 
     In certain embodiments, the multi-stage vacuum pump  302  includes a centrifugal pump (e.g., turbine type vacuum pump) that has multiple inlets  310  and  312 . While two inlets  310  and  312  are depicted, 3, 4, 5, 6, 7, 8, 9, 10 or more inlets may be used. As the water vapor passes through the turbine pump  302 , the turbine pump  302  can suction vapor at increasing pressures in a continuous flow process. The flow rate increases as the pressure increases because additional vapor is sucked into the turbine pump  302 . In other embodiment, the multi-stage vacuum pump may be combined with a multi-stage dehumidifier and the high pressure end of the turbine pump  302  may remove moisture from the highest moisture stage of the multi-stage dehumidifier, while the lowest pressure stage of the turbine is coupled to the lowest moisture stage of the dehumidifier. Additionally, while each of the stages  304  and  306  are depicted as having a single evaporative cooling unit  152  and a single dehumidification unit  12 , other stages may have multiple evaporative cooling units  152  and/or multiple dehumidification units  12 . Further, in other embodiments, the evaporative cooling unit  152  may be replaced or added to the mechanical cooling unit  236  described in  FIG. 16 . 
     As described previously with respect to  FIG. 13 , each evaporative cooling unit  152  of  FIG. 19  may either be a direct evaporative cooling unit or an indirect evaporative cooling unit. In other words, when the evaporative cooling unit  152  uses direct evaporative cooling techniques, the relatively cool and moist media  156  (e.g., relatively cool water) is directly added, for example, to the relatively dry air  14 B and  14 E. However, when the evaporative cooling unit  152  uses indirect evaporative cooling techniques, the relatively dry air  14 B and  14 E may, for example, flow across one side of a plate of a heat exchanger while the relatively cool and moist media  156  flows across another side of the plate of the heat exchanger. In other words, generally speaking, some of the relatively cool moisture from the relatively cool and moist media  156  is indirectly added to the relatively dry air  14 B and  14 E. Whether direct or indirect evaporative cooling techniques are used in the evaporative cooling unit  152  affects the rate of humidity removal and temperature reduction of the air  14  that flows through the HVAC system  300  of  FIG. 19 . In general, however, each of the dehumidification units  12  initially lowers the humidity ratio at approximately constant temperature, and each of the evaporative cooling unit  152  cools the air  14  to a temperature as low as possible for the particular stage. 
     As illustrated, many of the components of the HVAC system  300  of  FIG. 19  may be considered identical to the components of the HVAC system  10  of  FIGS. 1, 6, and 7 , the HVAC system  150  of  FIG. 11 , and the HVAC system  188  of  FIG. 13 . For example, as described previously, the HVAC system  300  of  FIG. 19  includes the condensation unit  54  that receives water vapor  26 A, as described previously. In certain embodiments, the HVAC system  300  of  FIG. 19  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. Additionally, or alternatively, a low pressure side may include the vacuum pumps  62  useful in purging noncondensable components. 
     In certain embodiment, the HVAC system  300  may provide for increased reliability and redundancy by using bypass conduits and valves, such as the depicted conduits  314 ,  316  (e.g., bypass ducts) and bypass valve  318 . In these embodiments, the bypass conduits  314 ,  316  and valve  318  may bypass certain cooling and dehumidification stages. For example, if it is desired to perform maintenance on the stages disposed in section  308 , the bypass valve may be actuated and air  14 C may be directed to enter the final stage  306  rather than the stages disposed in section  308 . Accordingly, components of the HVAC system  300  may be maintained or replaced without discontinuing cooling and/or dehumidification operations. The valve may be actuated manually, or by using a control system, such as the control system  64  embodiment depicted in  FIG. 20 . Additionally, the bypass valve  316  may be used to optimize the cooling and drying. For example, the bypass valve  316  may be used to reduce the number of cooling and drying stages in use by the HVAC system  300  when it is desired to lower the cooling and drying capabilities of the HVAC system  300  (e.g., in hot, dry weather). Likewise, the bypass valve  318  may be actuated open (or partially open) in warmer, more humid weather, to include use of the cooling and dehumidification stages disposed in section  308 . 
       FIG. 20  is a schematic diagram of an HVAC system  300  of  FIG. 19  including the control system  64 . The control system  64  may be communicatively coupled to various components of the HVAC system  300 , including the pumps  60 ,  62 , and  302 , the evaporative cooling units  152 , and the bypass valve  318 . In certain embodiments, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26 A in the water vapor vacuum volume  28  by turning the vacuum pumps  62  on or off, or by modulating the rate at which the multi-stage vacuum pump  302  removes the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volume  28  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volume  28 . 
     The control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  of each stage  304  and  306  to modify the water vapor removal capacity and efficiency ratio of the dehumidification units  12 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volumes  28 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14  in the dehumidification units  12 , the evaporative cooling units  152 , or both units  12  and  152 , among other components. 
     The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification units  12  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification units  12 . More specifically, under certain operating conditions, the dehumidification units  12  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification units  12  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification units  12  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. In one embodiment, modulation of the partial pressure of the water vapor  26 A may be accomplished by opening and closing (partially or fully) one or more valves (not shown) disposed on each inlet  310  and  312 . Indeed, each inlet  310  and  312  may include one or more valves suitable for controlling flow through the inlet. 
     Furthermore, the control system  64  may also be configured to control operation of the evaporative cooling units  152 . For example, the control system  64  may selectively modulate how much (direct or indirect) evaporative cooling occurs in the evaporating cooling units  152 . As an example, valves may be actuated to control the flow rate of the relatively cool and moist media  156  through the evaporative cooling units  152 , thereby directly affecting the amount of (direct or indirect) evaporative cooling in the evaporative cooling units  152 . In addition, operation of the dehumidification units  12  and the evaporative cooling units  152  may be controlled simultaneously. Furthermore, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification units  12  and the evaporative cooling units  152 . Indeed, by controlling the several cooling and dehumidification stages  304  and  306 , and the multi-stage pump  302  of the HVAC system  300 , the control system  64  may enable a more energy efficient, reliable HVAC system  300  suitable for producing cooler, lower humidity air  14 F. 
     It is to be noted that the stage  304  and/or the stage  306  may be replaced by other cooling and/or dehumidification systems. For example, rather than using an evaporative cooling unit, a mechanical cooling unit may be used. Indeed, the HVAC system  300  may include embodiments where a mechanical cooling unit, such as the mechanical cooling unit  236  described above with respect to  FIGS. 16 and 17 , may replace each of the evaporative cooling units  152  depicted in  FIG. 20 . In this embodiment, sensible compression cooling may be provided by the mechanical cooling unit  236 . Additionally or alternatively, the multi-stage vacuum pump  302  may be used with cooling and dehumidification stages  304  and  306  disposed in parallel, as described in more detail below with respect to  FIG. 21 . Further, the multi-stage pump  302  may be replaced by multiple single stage pumps (e.g., pumps  52 ). Additionally, multiple pumps  52  described in all the embodiments herein, may be replaced with a single multi-stage pump  302 , each stage of the multi-stage pump  302  corresponding to one of the pumps  52 . 
       FIG. 21  is a schematic view illustrating an embodiment of an HVAC system  320  using the multi-stage vacuum pump  302  with the cooling and dehumidification stages  304  and  306  disposed in parallel. Also depicted is a section  322  of the HVAC system  320  that may include one or more cooling and dehumidification stages also disposed in parallel. Indeed, 3, 4, 5, 6, 7, 8, 9, 10 or more cooling and dehumidification stages may be disposed in parallel and connected to the multi-stage vacuum pump  302  having multiple inlets  310  and  312 . Additionally, sections  324  and  326  may include further cooling and dehumidification stages disposed in series. Thus the HVAC system  320  may include cooling and dehumidification stages disposed in parallel and in series. In addition, the control system  64  may also be used to control the HVAC system  320 . 
     As illustrated may of the components, including but not limited to the components  152 ,  12 ,  62 ,  54 ,  58 ,  302 , and  60  may be considered identical to the components of the HVAC system  300  of  FIG. 20 . For example, as described previously with respect to the HVAC system  300  of  FIG. 20 , each stage  304  and  306  includes the evaporative cooling unit  152  of  FIGS. 19 and 20 , which may either be a direct evaporative cooling unit or an indirect evaporative cooling unit that function as described above. The evaporative cooling unit  152  may be disposed upstream of the dehumidification unit  12 . Relatively humid inlet air  14 A may enter in parallel into the evaporative cooling units  152 . The relatively humid inlet air  14 A is then first cooled in parallel in each of the evaporative cooling units  152  and expelled as cooler air  14 B into the ducts  154 . The dehumidification units  12  then reduce the humidity of the air  14 B, and expel cooler dry air  14 C into the conditioned space. Sections  324  and  326  may include multiple cooling and dehumidification stages suitable for further cooling and drying the air  14 C. 
     In the depicted embodiment, each of the dehumidification units  12  is depicted as fluidly coupled to the inlets  310  and  312  of multi-stage pump  302 . Indeed, the multi-stage pump  302  may include a stage and an inlet corresponding to each cooling and dehumidification stage. Therefore, if 2 stages are used, 2 inlets are included, if 4 stages are used, 4 inlets are included, if 10 stages are used, 10 inlets are included, and so on. In the depicted embodiment, the multi-stage pump  402  may be used, for example by the control system  64 , to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  of each stage  304  and  306  to modify the water vapor removal capacity and efficiency ratio of the dehumidification units  12 . In one example, the water vapor removal capacity and the efficiency ratio of each of the dehumidification units  12  may be approximately similar. In other examples, the water vapor removal capacity and efficiency ratio may be varied between dehumidification units  12 , for example, to provide cooler or warmer air, and for drier or more humid air. For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification units  12 . 
     The condensation unit  54  receives water vapor  26 B having a partial pressure just high enough to facilitate condensation from an outlet of the multi-stage pump  302 . In certain embodiments, the HVAC system  320  of  FIG. 21  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. Additionally, or alternatively, a low pressure side may include the vacuum pump  62  useful in purging noncondensable components  30 . 
     Additionally HVAC system&#39;s  320  flexibility may be provided by turning on or off certain number of cooling and dehumidification stages. For example, the control system  64  may turn on or off the stage  304  or the stage  306  to provide for differing cooling and/or dehumidification capacities, or for system maintenance. For example, if maintenance on the stage  304  is desired, it may be turned off while the stage  306  is allowed to continue operations. Likewise, stage  306  may be turned off while stage  304  is operating. Additionally, each stage  304  and  306  may be disposed in a different floor or room of a building, thus enabling for multi-zone cooling and dehumidification. Further, the multi-stage vacuum pump may be used to modulate cooling and dehumidification of any stage disposed in parallel, in series, or a combination thereof, thus providing for differing cooling and dehumidification for various zones. 
       FIG. 22  is a schematic view illustrating an embodiment of the HVAC system  330  including multiple dehumidification units  74  and  78  disposed in series with the mechanical cooling unit  236  disposed downstream of the dehumidification units  74  and  78 . The dehumidification units  74  and  78  are equivalent to the dehumidification unit  12  described above. Also depicted is a section  332  which may include 2, 3, 4, 5, 6, 7, 8, 9, 10 or more dehumidification units disposed in series. Because water vapor is removed from each successive dehumidification unit  74 ,  78 , the partial pressure of water vapor in the air  14  will be gradually reduced at each successive dehumidification unit  74 ,  78 . For example, as described previously, the partial pressure of water vapor in the inlet air  14 A may be in the range of approximately 0.2-1.0 psia; the partial pressure of water vapor in the air  14 B from the first dehumidification unit  74  may be in the range of approximately 0.17-0.75 psia (accomplishing approximately ⅓ of the drop); the partial pressure of water vapor in the air  14 C from a second dehumidification unit (not shown) disposed in section  332  may be in the range of approximately 0.14-0.54 psia (accomplishing approximately the next ⅓ of the drop); and the partial pressure of water vapor in the outlet air  14 D from the third dehumidification unit  78  may be in the range of approximately 0.10-0.25 psia, which is consistent with a 60° F. saturation temperature or lower. The very low values may be used to increase capacity for occasional use. 
     As such, in certain embodiments, the partial pressure of water vapor in the water vapor vacuum volumes  90 ,  94  (e.g., that are similar in functionality to the water vapor vacuum volume  28  described previously) associated with each respective vacuum pump  84 ,  88  may be modulated to ensure an optimal flow of water vapor  26 A from each respective dehumidification unit  74 ,  78 . For example, the partial pressure of the water vapor  26 A in the water vapor vacuum volume  28  described previously may be maintained in a range of approximately 0.15-0.25 psia. However, in the HVAC system  330  of  FIG. 22 , the partial pressure of the water vapor  26 A in the first water vapor vacuum volume  90  may be maintained in a range of approximately 0.15-0.7 psia, the partial pressure of the water vapor  26 A in a second water vapor vacuum volume of a single dehumidification unit (not shown) disposed in section  332  may be maintained in a range of approximately 0.12-0.49 psia, and the partial pressure of the water vapor  26 A in the third water vapor vacuum volume  94  may be maintained in a range of approximately 0.09-0.24 psia. Regardless, it may be expected that less water vapor  26  will be removed in each successive dehumidification unit  74 ,  78 , and may generally be optimized to minimize energy used to operate the system  330 . 
     In certain embodiments, each of the vacuum pumps  84 ,  88  may compress the water vapor  26  and direct it into a common manifold  96  having a substantially constant partial pressure of water vapor (i.e., just high enough to facilitate condensation in the condensation unit  54 ) such that the water vapor  26  flows in a direction opposite to the flow of the air  14 . In other embodiments, the water vapor  26  extracted from each successive dehumidification unit  74 ,  78  may be compressed by its respective vacuum pump  84 ,  88  and then combined with the water vapor  26  extracted from the next upstream dehumidification unit  74 ,  78 . For example, in other embodiments, the water vapor  26  from the dehumidification unit  78  may be compressed by the third vacuum pump  88  and then combined with the water vapor  26  from the dehumidification unit  74  in the second water vapor vacuum volume  90 . In this embodiment, the exhaust side of each successive vacuum pump  84 ,  88  increases the partial pressure of the water vapor  26  only to the operating pressure of the next upstream vacuum pump  84 ,  88 . In this embodiment, the water vapor  26  compressed by the first vacuum pump  84  will be directed into the condensation unit  54  at a partial pressure of water vapor just high enough to facilitate condensation, thus increasing efficiency. 
     It should be noted that the specific embodiment illustrated in  FIG. 22  having a plurality of dehumidification units  74 ,  78  arranged in series may be configured in various ways not illustrated in  FIG. 22 . For example, although illustrated as using a respective vacuum pump  84 ,  88  with each dehumidification unit  74 ,  78 , in certain embodiments, the single multi-stage vacuum pump  302  described above with respect to  FIGS. 19, 20, and 21  may be used with multiple inlet ports  310  and  312  connected to the first, and second water vapor vacuum volumes  90 ,  94 , respectively. In addition, although illustrated as using a single condensation unit  54 , reservoir  58 , and liquid pump  60  to condense the water vapor  26 B into a liquid state, and store and/or transport the liquid water from the HVAC system  330 , in other embodiments, each set of dehumidification units  74 ,  78  and vacuum pumps  84 ,  88  may be operated independently and be associated with their own respective condensation units  54 , reservoirs  58 , and liquid pumps  60 . 
     Additionally, the low humidity air  14 D may then be cooled by the mechanical cooling unit  236 . Alternative or additional to the mechanical cooling unit  236 , the evaporative cooling unit  152  described above may be used. In addition, the control system  64  may also be used to control the operation of the HVAC system  330  in a similar manner as described previously with respect to  FIGS. 7 and 8 . For example, as described previously, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26  in the water vapor vacuum volumes  90 ,  94  by turning the vacuum pumps  84 ,  88  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 7 and 8 ) on or off, or by modulating the rate at which the vacuum pumps  84 ,  88  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 7 and 8 ) remove the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volumes  90 ,  94  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volumes  90 ,  94 . 
     In addition, the control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  94  to modify the water vapor removal capacity and efficiency ratio of the dehumidification units  74 ,  78 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volumes  90 ,  94 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14 , among other things. The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  94  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification units  74 ,  78  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). 
     For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  94  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  90 ,  94  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification units  74 ,  78 . More specifically, under certain operating conditions, the dehumidification units  74 ,  78  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification units  74 ,  78  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification units  74 ,  78  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. Further, the control system  64  may be used to control the mechanical cooling unit  236 , for example, by actuating the compressor to increase or decrease compression and cooling. In addition, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification units  74 ,  78 , the mechanical cooling unit  236 , and the HVAC system  330 . While  FIG. 22  includes a disposition of the mechanical cooling unit  236  downstream of the dehumidification and cooling units  74 ,  78 , other arrangements are contemplated herein. For example,  FIG. 23  depicts an upstream arrangement of the mechanical cooling unit  236 . 
     More specifically,  FIG. 23  is schematic view of an embodiment of an HVAC system  334  including the mechanical cooling  236  disposed in series upstream of the dehumidification units  74 ,  78 , and the section  332 . Because the figure includes similar elements to  FIG. 22 , like numbers are used to denote like elements. In the depicted embodiment, hot, humid air  14 A enters the mechanical cooling unit  236 . The mechanical cooling unit  236  may then cool (and slightly dry) the air  14 , resulting in a cooler (and slightly drier) air  14 B. The air  14 B may then be further dried by the cooling units  74 ,  78 , and the section  332  as described above, to produce the air  14 E having a drier state when compared to the air  14 B. Additionally, the control system  64  may also be configured to control start-up and shutdown sequencing of the dehumidification units  74 ,  78 , the mechanical cooling unit  236 , and the HVAC system  334 . Additional or alternative to the mechanical cooling unit  236 , the evaporative cooling unit  152  may be provided, thus enhancing the cooling abilities of the HVAC system  334 . 
     While  FIG. 23  includes a serial arrangement of multiple dehumidification units  74 ,  78 , present embodiments include other ways in which multiple dehumidification units  74 ,  78  may be arranged in a single HVAC system. For example,  FIG. 24  depicts a parallel arrangement of the dehumidification units  100  and  104 . More specifically,  FIG. 24  is a schematic view of an embodiment of an HVAC system  336  including the dehumidification units  100 ,  104  disposed in parallel, and the mechanical cooling unit  236  disposed downstream from the dehumidification units  100 ,  104 . Each of the dehumidification units  100 ,  104  is substantially the same as the dehumidification unit  12 . Although illustrated as having two dehumidification units  100 ,  104  in parallel, any number of dehumidification units may indeed be used in parallel in the HVAC system  336 . For example, in other embodiments, 2, 4, 5, 6, 7, 8, 9, 10, or even more dehumidification units may be used in parallel in the HVAC system  336 . For example, a section  338  may be used to dispose additional dehumidification units in parallel. 
     The HVAC system  336  of  FIG. 24  generally functions the same as the HVAC system  10  of  FIGS. 1, 6, and 7  and the HVAC system  98  of  FIG. 9 , but with the addition of a single mechanical cooling unit  236 . It is to be understood that, in other embodiments, each of the dehumidification units  100 ,  104  may include a corresponding mechanical cooling unit  236 . As illustrated in  FIG. 24 , each dehumidification unit  100 ,  104  of the HVAC system  336  receives the inlet air  14 A having a relatively high humidity and expels the outlet air  14 B having a relatively low humidity. As illustrated, many of the components of the HVAC system  336  of  FIG. 24  may be considered identical to the components of the HVAC system  10  of  FIGS. 1, 6, and 7 , the HVAC system  98  of  FIG. 9 , and the HVAC system  334  of  FIG. 23 . For example, the dehumidification units  100 ,  104  of the HVAC system  336  of  FIG. 24  may be considered identical to the dehumidification units  12  of  FIGS. 1, 6, and 7 . In addition, the HVAC system  336  of  FIG. 24  also includes the condensation unit  54  that receives water vapor  26 B having a partial pressure just high enough to facilitate condensation, as described previously. In certain embodiments, the HVAC system  336  of  FIG. 24  may also include the reservoir  58  for temporary storage of saturated vapor and liquid water. However, as described previously, in other embodiments, no reservoir may be used. In either case, the liquid water from the condensation unit  54  may be directed into the liquid pump  60 , within which the pressure of the liquid water from the condensation unit  54  is increased to approximately atmospheric pressure (i.e., approximately 14.7 psia) so that the liquid water may be rejected at ambient conditions. 
     As illustrated in  FIG. 24 , in certain embodiments, each dehumidification unit  100 ,  104  may be associated with a respective vacuum pump  106 ,  110 , each of which is similar in functionality to the vacuum pump  52  of  FIGS. 1, 6, and 7 . However, as opposed to the HVAC system  334  of  FIG. 23 , because the dehumidification units  100 ,  104  and associated vacuum pumps  106 ,  110  are arranged in parallel, the partial pressure of water vapor in the air  14  will be approximately the same in each dehumidification unit  100 ,  104 . As such, in general, the partial pressure of water vapor in the water vapor vacuum volumes  112 ,  116  associated with each respective vacuum pump  106 ,  110  will also be approximately the same. For example, as described previously with respect to the HVAC system  10  of  FIGS. 1, 6, and 7 , the partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  116  may be maintained in a range of approximately 0.10-0.25 psia. 
     As illustrated in  FIG. 24 , in certain embodiments, each of the vacuum pumps  106 ,  110  may compress the water vapor  26  and direct it into a common manifold  118  having a substantially constant partial pressure of water vapor (i.e., just high enough to facilitate condensation in the condensation unit  54 ). In other embodiments, the water vapor  26  extracted from each successive dehumidification unit  100 ,  104  (i.e., from top to bottom) may be compressed by its respective vacuum pump  106 ,  110  and then combined with the water vapor  26  extracted from the next downstream (i.e., with respect to the common manifold) dehumidification unit  100 ,  104 . For example, in other embodiments, the water vapor  26  from the first dehumidification unit  100  may be compressed by the first vacuum pump  106  and then combined with the water vapor  26  from the second dehumidification unit  104  in the second water vapor vacuum volume  116 . In this embodiment, the exhaust side of each successive vacuum pump  106 ,  110  increases the partial pressure of the water vapor  26  only to the operating pressure of the next downstream vacuum pump  106 ,  110 . For example, the first vacuum pump  106  may only increase the pressure of the water vapor  26  to approximately 0.2 psia if the partial pressure of water vapor in the second water vapor vacuum volume  116  is approximately 0.2 psia. In this embodiment, the water vapor  26  compressed by the vacuum pump  110  will be directed into the condensation unit  54  at a partial pressure of water vapor just high enough to facilitate condensation. 
     It should be noted that the specific embodiment illustrated in  FIG. 24  having a plurality of dehumidification units  100 ,  104  arranged in parallel may be configured in various ways not illustrated in  FIG. 24 . For example, although illustrated as using a respective vacuum pump  106 ,  110  with each dehumidification unit  100 ,  104 , in certain embodiments, the single multi-stage vacuum pump  302  may be used with multiple inlet ports  310 ,  312  connected to the first and second water vapor vacuum volumes  112 ,  116 . In addition, although illustrated as using a single condensation unit  54 , reservoir  58 , and liquid pump  60  to condense the water vapor  26 B into a liquid state, and store and/or transport the liquid water from the HVAC system  336 , in other embodiments, each set of dehumidification units  100 ,  104  and vacuum pumps  106 ,  110  may be operated independently and be associated with their own respective condensation units  54 , reservoirs  58 , and liquid pumps  60 . 
     In addition, the control system  64  may also be used in the HVAC system  336  of  FIG. 24  to control the operation of the HVAC system  336  in a similar manner as described previously with respect to  FIG. 9 . For example, as described previously, the control system  64  may be configured to control the rate of removal of the noncondensable components  30  of the water vapor  26 A in the water vapor vacuum volumes  112 ,  116  by turning the vacuum pumps  106 ,  110  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 7 and 9 ) on or off, or by modulating the rate at which the vacuum pumps  106 ,  110  (or separate vacuum pumps  62 , as described previously with respect to  FIGS. 7 and 9 ) remove the noncondensable components  30 . More specifically, in certain embodiments, the control system  64  may receive signals from sensors in the water vapor vacuum volumes  112 ,  116  that detect when too many noncondensable components  30  are present in the water vapor  26 A contained in the water vapor vacuum volumes  112 ,  116 . 
     In addition, the control system  64  may modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  116  to modify the water vapor removal capacity and efficiency ratio of the dehumidification units  100 ,  104 . For example, the control system  64  may receive signals from pressure sensors in the water vapor vacuum volumes  112 ,  116 , the water vapor channels  18 , as well as signals generated by sensors relating to characteristics (e.g., temperature, pressure, flow rate, relative humidity, and so forth) of the air  14 , among other things. The control system  64  may use this information to determine how to modulate the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  116  to increase or decrease the rate of removal of water vapor  26  from the air channels  16  to the water vapor channels  18  through the interfaces  20  of the dehumidification units  100 ,  102 ,  104  as H 2 O (i.e., as water molecules, gaseous water vapor, liquid water, adsorbed/desorbed water molecules, absorbed/desorbed water molecules, and so forth, through the interfaces  20 ). 
     For example, if more water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  116  may be reduced and, conversely, if less water vapor removal is desired, the lower partial pressure of the water vapor  26 A in the water vapor vacuum volumes  112 ,  116  may be increased. Furthermore, as described above, the amount of dehumidification (i.e., water vapor removal) may be cycled to improve the efficiency of the dehumidification units  100 ,  104 . More specifically, under certain operating conditions, the dehumidification units  100 ,  104  may function more efficiently at higher rates of water vapor removal. As such, in certain embodiments, the dehumidification units  100 ,  104  may be cycled to remove a maximum amount of water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove relatively no water vapor from the air  14  for a period of time (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), then to remove a maximum amount of water vapor from the air  14  for a v (e.g., approximately 1 sec, 10 sec, 100 sec, 10 min), and so forth. In other words, the dehumidification units  100 ,  104  may be operated at full water vapor removal capacity for periods of time alternating with other periods of time where no water vapor is removed. In addition, the control system  64  may be configured to control start-up and shutdown sequencing of the dehumidification units  100 ,  104 , the mechanical cooling unit  236 , and the HVAC system  336 . 
     While  FIG. 24  includes a disposition of the mechanical cooling unit  236  downstream of the dehumidification and cooling units  100 ,  104 , other arrangements are contemplated herein. For example,  FIG. 25  depicts an upstream arrangement of the mechanical cooling unit  236 . More specifically,  FIG. 25  is schematic view of an embodiment of an HVAC system  340  including the mechanical cooling  236  disposed in series upstream of the dehumidification units  100 ,  104 , and the section  338 . Because the figure includes similar elements to  FIG. 24 , like numbers are used to denote like elements. In the depicted embodiment, hot, humid air  14 A enters the mechanical cooling unit  236 . The mechanical cooling unit  236  may then cool (and slightly dry) the air  14 , resulting in a cooler (and slightly drier) air  14 B. The air  14 B may then be further dried by the cooling units  100 ,  104  and the section  338  as described above, to produce the air  14 C having a drier state when compared to the air  14 B. Additionally, the control system  64  may also be configured to control start-up and shutdown sequencing of the dehumidification units  100 ,  104 , the mechanical cooling unit  236 , and the HVAC system  340 . Additional or alternative to the mechanical cooling unit  236 , the evaporative cooling unit  152  may be provided, thus enhancing the cooling abilities of the HVAC system  340 . 
     In addition to the serial arrangement of dehumidification units  74 ,  78  illustrated in  FIGS. 22 and 23 , and the parallel arrangement of dehumidification units  100 ,  104  illustrated in  FIGS. 24 and 25 , multiple dehumidification units may be used in other ways. Indeed, much more complex and expansive arrangements may also be used. For example,  FIG. 26  is a schematic diagram of an HVAC system  342  having a first set  122  of dehumidification units (i.e., a first dehumidification unit  124  and a second dehumidification unit  126 ) arranged in series, and a second set  128  of dehumidification units (i.e., a third dehumidification unit  130  and a fourth dehumidification unit  132 ) also arranged in series, with the first and second sets  122 ,  128  of dehumidification units arranged in parallel in accordance with an embodiment of the present disclosure. Additionally, a section  344  may be used to dispose further dehumidification units in series and in parallel. In other words, the first set  122  of serial first and second dehumidification units  124 ,  126  are arranged in parallel with the second set  128  of serial third and fourth dehumidification units  130 ,  132 . The dehumidification units  124 ,  126 ,  130 , and  132  are functionally equivalent to the dehumidification unit  12  described above. 
     Although illustrated as having two sets  122 ,  128  of serial dehumidification units  12  arranged in parallel, any number of parallel pluralities of dehumidification units  12  may indeed be used in the HVAC system  342 . For example, in other embodiments, 3, 4, 5, 6, 7, 8, 9, 10, or even more parallel sets of dehumidification units may be used in the HVAC system  342 . Similarly, although illustrated as having two dehumidification units arranged in series within each set  122 ,  128  of dehumidification units, any number of dehumidification units may indeed be used in series within each set  122 ,  128  of dehumidification units  12  in the HVAC system  342 . For example, in other embodiments, 1, 3, 4, 5, 6, 7, 8, 9, 10, or even more dehumidification units may be used in series within each set  122 ,  128  of dehumidification units  12  in the HVAC system  342 , such as dehumidification units disposed in sections  346  and  348 . 
     Substantially all of the operating characteristics of the HVAC system  342  of  FIG. 26  are similar to those described previously with respect to the HVAC systems described in  FIGS. 22-25 . For example, as illustrated, each of the dehumidification units  124 ,  126 ,  130 ,  132  may be associated with its own respective vacuum pump  134 ,  136 ,  138 ,  140  (e.g., similar to the vacuum pump  52  of  FIGS. 1, 6 , and  7 ). However, in other embodiments, one multi-stage vacuum pump  302  may be used for each set  122 ,  128  of dehumidification units with multiple inlet ports connected to the respective water vapor vacuum volumes  142 ,  144 ,  146 ,  148 . Indeed, in other embodiments, all of the dehumidification units  124 ,  126 ,  130 ,  132  may be associated with the single multi-stage vacuum pump  302  with multiple inlet ports connected to all of the water vapor vacuum volumes  142 ,  144 ,  146 ,  148 . 
     In addition, although illustrated as using a single condensation unit  54 , reservoir  58 , and liquid pump  60  to condense the water vapor  26 B into a liquid state, and store and/or transport the liquid water from the HVAC system  342 , in other embodiments, each set of dehumidification units  124 ,  126 ,  130 ,  132  and vacuum pumps  134 ,  136 ,  138 ,  140  may be operated independently and be associated with their own respective condensation units  54 , reservoirs  58 , and liquid pumps  60 . In addition, the control system  64  described previously may also be used in the HVAC system  342  of  FIG. 26  to control operation of the HVAC system  342  in a similar manner as described previously. 
     The embodiments described previously with respect to  FIGS. 19 through 26  are slightly more complex than the embodiments described previously with respect to  FIGS. 1 through 7  inasmuch as multiple dehumidification units are used in series, parallel, or some combination thereof. As such, the control of pressures and temperatures of the HVAC systems of  FIGS. 19 through 26  are slightly more complicated than the control of a single dehumidification unit  12 . For example, the partial pressures in the water vapor vacuum volumes may need to be closely monitored and modulated by the control system  64  to take into account variations in temperature and partial pressure of water vapor in the air  14  within the respective dehumidification units  12 , operating pressures of adjacent water vapor vacuum volumes and vacuum pumps (which may be cross-piped together as described previously to facilitate control of pressures, flows, and so forth), among other things. In certain embodiments, variable or fixed orifices may be used to control pressures and changes in pressures in and between the dehumidification units  12 . In addition, as described previously, each of the respective vacuum pumps may be controlled to adjust the partial pressures of water vapor in the water vapor vacuum volumes to account for variations between dehumidification units  12 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.