Abstract:
A humidity control system. The system comprises a housing containing one or more hygroscopic monoliths to provide, and passively sustain, the capacity to control the humidity on an intermittent basis. The present invention may be employed either independently or as part of a larger climate control system.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to humidity control systems. More particularly, the present invention relates to a dehumidifier comprising a housing containing one or more hygroscopic monoliths that is capable of providing and passively sustaining the capacity to dehumidify air. The present invention may be employed either independently or as part of a larger or more extensive climate control system. 
     Typical atmospheric desiccant dehumidifiers employ solid or liquid desiccant to dry working gases, such as removing water vapor from air. Typical solid desiccant dehumidifiers employ desiccant impregnated wheels, columns packed with randomly dispersed desiccants, horizontal desiccant bed trays or multiple vertical beds. Before a desiccant dehumidifier can dry a process air stream, water must be removed from the desiccant by drying or activation induced by, for example, a drying air stream. The drying air stream may employ heat to raise the vapor pressure on the surface of the desiccant causing desorption so that the evolved water vapor can be carried away. Alternatively, the drying stream can comprise dry air having a low water vapor pressure relative to the surface of the desiccant such that desorption is promoted. The activated desiccant must then either be used to modify a process air stream or be isolated by some means, typically with valves or other control mechanisms, to prevent significant reduction in its drying capacity due to exposure to potentially damp ambient conditions. Desiccant dehumidifiers that incorporate desiccant wheels, horizontal bed trays or multiple vertical beds typically work continuously without the ability to retain their drying capacity for intermittent use. Dehumidifiers using columns packed with randomly dispersed desiccant can isolate the activated desiccant with valves affixed at either end of the column to allow intermittent use, but such designs are limited by the pressure drop induced by turbulence created while moving air through the packed column. Further limitations are imposed by the expense of valves and other control components necessary for large air flows as well as the support and retention components required in the packed desiccant columns. 
     The present invention overcomes limitations inherent to existing systems by incorporating, as a desiccant, one or more hygroscopic monoliths shaped and arranged in such a manner that air is permitted to flow through the desiccant housing without the relatively high pressure resistance or turbulence induced by existing packed columns. The unique shape and arrangement of the hygroscopic monoliths of the present invention also induce a “chimney” effect which allows the dehumidification capacity of the desiccant to be passively preserved, not by the use of valves or control mechanisms, but by allowing the vapor pressure at the surface of the monoliths to approach that of the surrounding air, at which time the drying or adsorption process slows, or even stops, as a state of equilibrium is achieved. In this way, the dehumidification capacity permitted by the present invention can be controlled simply by regulating the airflow through the system without the need to physically isolate the desiccant during non-use. The present system may alternatively operate as a humidifier with the minor modifications described herein. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention provides a solid desiccant humidity control system that permits flow across a desiccant of, alternatively, a drying air stream or a process air stream with relatively little resistance and pressure drop due to the shape and arrangement of the desiccant. One or more hygroscopic monoliths are incorporated, as a desiccant, into the present invention in a manner that induces a “chimney” effect within and around the monoliths, thereby permitting the dehumidification capacity of the desiccant to be passively preserved without the use of valves or control mechanisms by allowing the vapor pressure at the surface of the monoliths to approach that of the surrounding air, at which time the drying or adsorption process slows, or even stops, as a state of equilibrium is achieved. Because of this “chimney” effect and the state of equilibrium it encourages, the dehumidification capacity of the desiccant employed in the present invention can be preserved simply by regulating the airflow through the system without the need to physically isolate the desiccant during periods of non-use. 
     The drying air stream may be air at an elevated temperature relative to the process stream or an air stream with a water vapor pressure lower than that of the process stream. Thus, the present invention provides a dehumidifier that can be utilized in a wide variety of settings, including incorporation into a wide variety of climate control systems, due to the ease of flow through the unit and its ability to operate intermittently in response to dehumidification demand or the availability of a drying air stream that reactivates the desiccant. By using a charging stream in lieu of a drying stream, the present invention may alternatively serve as a humidifier. The present invention comprises a housing having a cavity containing a hygroscopic material, such as one or more hygroscopic monoliths. In an exemplary embodiment of the invention, the housing defines a cavity with inlet and outlet ports located in the base of the housing. Except for the inlet and outlet ports, the housing is sealed at its top, its bottom and along all sides, although optional, sealed access may be provided for sensors, gauges and the like if desired. Disposed or positioned within the housing is an array of hygroscopic monoliths, such as the kind shaped to facilitate circulation and drying by means of a “chimney” effect as disclosed in U.S. patent application Ser. No. 09/157,208, filed Sep. 18, 1998, entitled “Hygroscopic Monolith Having a Channel Therethrough for Modifying a Gas Therein by Adsorption or Desorption, and Processes Therefore,” the disclosure of which is incorporated by reference as if set forth herein in its entirety. Such application discloses a variety of shapes useful for achieving the beneficial effects imparted by the present invention. In addition, hygroscopic monoliths shaped as cubes having a single cylindrical channel, as disclosed herein, may be utilized in the present invention. 
     The hygroscopic monoliths may be composed of, for example, zeolite, or any similar hygroscopic material. The hygroscopic monoliths are situated within the housing such that a drying air stream can, with relatively low resistance or pressure drop, flow across the external surfaces of the monoliths as well as through channels disposed among the monoliths. Elastomeric Hi gaskets create passages within the housing to direct the drying stream, and alternatively the process stream, to and from the monoliths. A drying air stream may be obtained from a wide variety of sources, including solar thermal, geothermal, waste or vented heat, combustion, electrical resistance, heat pipes, heat pumps or other dehumidifiers. 
     After undergoing activation, the hygroscopic monoliths gradually dry the air contained within the housing and the drier air, being more buoyant, tends to rise within the housing. The activated monoliths therefore promote air circulation within the housing, generally filling the cavity from the top down with the driest portion of the contained air. The monoliths will continue to promote circulation of the air contained within the housing until the vapor pressure at the surface of the monoliths approaches that of the surrounding air, at which time the drying or adsorption process slows, or even stops, as a state of equilibrium is achieved. In this way, the present invention possesses the ability to retain its drying capability by isolating the monoliths from the process air stream and ambient air external to the housing through passive means during cooling or inactivity between periods of intermittent use. 
     A dehumidifier according to the present invention may have various configurations of enclosure surfaces, port locations, passageway orientations and hygroscopic monolith shapes to permit selective utilization of the buoyant forces within the enclosure, the available drying stream and any other performance-modifying aspects attributable to a particular setting. Airflow through the ports can be induced by suction, positive air pressure or a combination of both suction and pressure. Airflow may also be induced or assisted by orienting the housing such that buoyant forces expel drier air out of the housing while drawing relatively heavier, water-vapor-laden ambient air into the housing. 
     The present invention may alternatively be utilized as a humidifier if ports are located at the top of the housing and, in lieu of a drying stream, a charging air stream having a lower temperature or higher water vapor pressure than the process stream flows across the desiccant, thereby promoting adsorption of water vapor upon contact of the charging stream with the monoliths. In the absence of a charging or process air stream, the air within the housing interacts with the charged monoliths and, in so doing, becomes progressively more laden with water vapor. This moisture-laden air, being heavier relative to drier air, generally tends to settle around the monoliths until the vapor pressure at the surface of the monoliths approaches that of the surrounding air and desorption slows, or even stops, as a state of equilibrium is achieved. In this way, a system comprising the present invention possesses the ability to retain its humidification capability by isolating the monoliths from the process air stream through passive means during cooling or inactivity between periods of intermittent use. 
     The dehumidifying capability provided by the present invention may be employed either independently or as a component of a larger unit. Similarly, the humidifying capability provided by the present invention may also be employed either independently or as a component of a larger unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a system according to the invention. 
     FIGS. 2 a ,  2   b  and  2   c  present a composite view of the front, side and bottom, respectively, of a system according to the present invention. 
     FIGS. 3 a ,  3   b  and  3   c  present a composite view of the top, front and side, respectively, of one embodiment of the hygroscopic monoliths utilized in the present invention. 
     FIG. 4 is a cross-sectional view of the system depicted in FIG.  1 . 
     FIG. 5 is a cross-sectional view of the system depicted in FIG. 1 illustrating airflow in the activation mode. 
     FIG. 6 is a view depicting a portion of the system depicted in FIG. 5 with exaggerated dimensions showing airflow through and across the hygroscopic monoliths utilized in the present invention. 
     FIG. 7 is a cross-sectional view of the system depicted in FIG. 1 illustrating airflow in the inactive mode. 
     FIG. 8 is a cross-sectional view of the system depicted in FIG. 1 illustrating airflow in the working mode. 
     FIG. 9 is a cross-sectional view of the system according to the present invention illustrating airflow in the working mode upon reorientation and with passive isolation. 
     FIG. 10 is a cross-sectional view of the system depicted in FIG. 1 illustrating airflow in the activation mode in the absence of a drying stream and with a heat source for activating the monoliths. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an embodiment of the present invention is shown as a dehumidifier  100  comprising a housing  112  constructed of aluminum sheeting 0.03 inches thick. Housing  112  comprises a front panel  121 , a back panel  123  opposing front panel  121 , a first side panel  120 , a second side panel  122  opposing first side panel  120 , a bottom panel  126 , and a top panel  124  opposing bottom panel  126 . Front panel  121  and back panel  123  each measure 12 inches wide by 12 inches high. First side panel  120  and second side panel  122  each measure 12 inches high by 1 inch wide. Bottom panel  126  and top panel  124  each measure 12 inches in length by 1 inch wide. Bottom panel  126  includes ports  114 ,  116 , and  118 , each being 0.5 inches in diameter. 
     Referring to FIG. 2 a , front panel  121  of dehumidifier  100  is shown. Referring to FIG. 2 b , first side panel  120  of dehumidifier  100  is shown. 
     Referring now to FIG. 2 c , a line from  2   c ′ to  2   c ″ bisects the width of bottom panel  126 . Port  116  is centered in bottom panel  126  such that the center of port  116  bisects both the length and the width of bottom panel  126 . Port  114  is positioned in bottom panel  126  such that port  114  is equidistant from port  116  and the juncture of bottom panel  126  with second side panel  122  while the center of port  114  bisects the width of bottom panel  126 . Port  118  is positioned in bottom panel  126  such that port  118  is equidistant from port  116  and the juncture of bottom panel  126  with first side panel  120  while the center of port  118  bisects the width of bottom panel  126 . 
     Referring to FIGS. 3 a ,  3   b  and  3   c , a single hygroscopic monolith  128  of the preferred embodiment is shown in three views. FIG. 3 a  presents a top view of hygroscopic monolith  128 . FIG. 3 b  presents a front view of hygroscopic monolith  128 . FIG. 3 c  presents a side view of hygroscopic monolith  128 . Hygroscopic monolith  128  is comprised of a hygroscopic material shaped into cubes with each of its three axes being 1 inch in length. Extending through hygroscopic monolith  128  is cylindrical passageway  130 . Cylindrical passageway  130  has a diameter of 0.25 inches centered laterally within the cube and extending the fall height of hygroscopic monolith  128 . 
     Referring to FIG. 4, a plurality of hygroscopic monoliths  128  are disposed within housing  112 . The plurality of hygroscopic monoliths  128  are arranged top-to-bottom in each of ten columns so that, in a given column, cylindrical passageway  130  of each hygroscopic monolith  128  is aligned with every other cylindrical passageway  130  of the remaining hygroscopic monoliths  128  to form a continuous, elongated cylindrical passageway. Each hygroscopic monolith  128  is placed in contact with the interior of front panel  121  and the interior of the back panel  123  at opposing sides. A distance of 0.125 inches separates each column from the next, except that a channel  136  separates the first five columns from the last five columns by a span of 0.75 inches from column-to-column. The 0.125 inch separation between each set of adjacent columns creates a channel that permits airflow over the external surfaces of hygroscopic monoliths  128 . 
     Referring again to FIG. 4, bottom panel  126  has opposing ends, one of which joins first side panel  120  at juncture  127  while the other joins second side panel  122  at juncture  125 . An elastomeric gasket  148  covers the entire interior surface of bottom panel  126  except that holes through elastomeric gasket  148  are aligned with, and have the same diameter as, each of ports  114 ,  116 , and  118 . An elastomeric gasket  150  covers the entire interior surface of first side panel  120 . An elastomeric gasket  152  covers the entire interior surface of second side panel  122 . An elastomeric gasket  154  is disposed between channel  136  and an adjacent column such that it covers the entire side of the column nearest elastomeric gasket  154  while extending to contact bottom panel  126 . Similarly, an elastomeric gasket  156  is disposed between channel  136  and the other column adjacent to channel  136  such that it covers the entire side of the column nearest elastomeric gasket  1 , 56  while extending to contact bottom panel  126 . Each elastomeric gasket  148 ,  150 ,  152 ,  154  and  156  is 0.125 inches thick and 1 inch wide such that each elastomeric gasket simultaneously contacts both front panel  121  and back panel  123 . 
     Referring again to FIG. 4, a channel  138  extends across the upper portion of housing  112  left unoccupied by the plurality of hygroscopic monoliths  128 , spanning from elastomeric gasket  152  to elastomeric gasket  150 . A channel  140  extends across the lower left portion of housing  112  from left unoccupied by the hygroscopic monoliths  128 , spanning from elastomeric gasket  152  to elastomeric gasket  156 . A channel  142  extends across the lower right portion of housing  112  from left unoccupied by the hygroscopic monoliths  128 , spanning from elastomeric gasket  150  to elastomeric gasket  154 . Each channel  138 ,  140  and  142  has an approximate height of 1 inch and spans from the interior surface of front panel  121  to the interior surface of back panel  123 . 
     Referring now to FIG. 5, FIG. 6, FIG.  7  and FIG. 8, an exemplary operation of dehumidifier  100  will be described. In an exemplary operation, dehumidifier  100  runs sequentially through three modes. In the first such mode (activation mode), the plurality of hygroscopic monoliths  128  are dried or activated by a drying air stream  160  (exemplified in FIG. 5 by outlined arrows). In the second such mode (passive cooling or inactive mode), the system is at rest and permitted to achieve equilibrium. In the third such mode (working mode), a process air stream  170  is dried by the plurality of hygroscopic monoliths  128 , progressively becoming product air stream  180  as moisture is adsorbed or desorbed from process air stream  170 . The period of time during which the dehumidifier  100  operates in any of the three modes depends selectively upon either dehumidification demand, product air stream  180  demand or the availability of drying air stream  160 . The period of time during which the dehumidifier  100  operates in any of the three modes is not critical to functionality, but will determine the relative performance of the system, which will also vary due to ambient conditions, monolith saturation, airflow rates and other similar factors. The order of occurrence of the modes may also vary according to demand. For instance, the system permits selection of the inactive mode and the working mode on an intermittent basis as desired. 
     Referring again to FIG. 5, when dehumidifier  100  operates in activation mode, drying air stream  160  enters housing  112  at port  116  and then travels through channel  136 . After exiting the uppermost portion of channel  136 , drying air stream  160  splits into substantially equal portions to flow in either direction through channel  138 . After flowing through channel  138 , drying air stream  160  flows downward between the plurality of hygroscopic monoliths  128  as well as through each elongated cylindrical passageway formed by a plurality of cylindrical passageways  130  while maintaining fluid contact therewith, thereby heating the monoliths and thus raising the water vapor pressure at the surface of the monoliths and causing water vapor to be desorbed from the monoliths into drying air stream  160 . Drying air stream  160 , as it flows through and across the plurality of hygroscopic monoliths  128 , becomes progressively laden with water vapor until flowing into either channel  140  or channel  142 , respectively, before being purged through ports  114  and  118 , respectively. The air stream purged from housing  112  is depicted by solid arrows  162 . 
     Referring again to FIG. 6, drying air stream  160  is shown flowing through the elongated cylindrical passageways formed by the plurality of cylindrical passageways  130  and through the channels separating the columns of hygroscopic monoliths  128 . As drying air stream  160  flows in this manner, water vapor is desorbed from hygroscopic monoliths  128 , becoming progressively laden with moisture as depicted by hatched arrows  161 . 
     Referring again to FIG. 7, when dehumidifier  100  operates in passive mode, the plurality of hygroscopic monoliths  128  promote air circulation by way of a “chimney” effect which causes the lighter, drier air  164  (exemplified in FIG. 7 as outlined arrows) within housing  112  to rise toward the upper portion of housing  112  while the relatively heavier, damper air  166  (exemplified in FIG. 7 as solid arrows) within housing  112  accumulates in the lower portion of housing  112 . The circulation resulting from the interaction of the air within housing  112  with the plurality of hygroscopic monoliths  128  surrounds the monoliths with progressively drier air, and as it ensues, the vapor pressure near the surface of the monoliths approaches that of the surrounding air at which point the adsorption of water vapor from the air within housing  112  slows, or even stops, as equilibrium is achieved. Thus, the remaining drying or dehumidification capacity of the dehumidifier is retained for future use as desired. The passive mode described herein may dually serve as a cooling mode wherein heat accumulated within the dehumidifier is conducted through, and then dissipated by, the aluminum comprising housing I  12 . 
     Referring again to FIG. 8, when the dehumidifier operates in working mode, a negative pressure is applied to port  116  inducing the flow of a process air stream flow  170  (exemplified in FIG. 8 as solid arrows) while collecting a product air stream  180  (exemplified in FIG. 8 as outlined arrows). Process air stream flow  170  enters in approximately equal portions through ports  114  and  118  and flows through channels  140  and  142  respectively before passing through and across the plurality of hygroscopic monoliths  128  where it is progressively dried to become product air stream  180 . Product air stream  180  then flows, in separate portions, into channels  134  and  138 , respectively, before combining at channel  136  and then being collected through port  116 . 
     Referring now to FIG. 9, a working mode of dehumidifier  100  is illustrated wherein the dehumidifier has been previously activated and reoriented by rotating it so that ports  114 ,  116 , and  118  are at the top of housing  112 . In this manner, passage of air through the system provides buoyant forces or a “chimney” effect that induces the flow of a product air stream  190  (exemplified in FIG. 9 as outlined arrows) and the flow of a process air stream  195  (exemplified in FIG. 9 as solid arrows) that persist until the water vapor pressure at the surface of monoliths nears equilibrium with that of the process stream. 
     An alternative use of the present invention is also provided wherein the activation mode is performed using a separate heat source in lieu of a drying air stream. A variety of heat sources can be utilized for this purpose, including solar insolation, radiant thermal energy from heating elements or components in inefficient thermal systems and devices, or by contact heating with heating elements, heat exchangers or heat pipes. Referring then to FIG. 10, in the usage described herein, heat is supplied to the exterior surface of the aluminum comprising housing  112 . Heat is distributed across, and conducted through, housing  112  to the plurality of hygroscopic monoliths  128 , resulting in drying or activation of the monoliths. As the thermal energy raises the water vapor pressure in the plurality of hygroscopic monoliths  128 , some water vapor is adsorbed or desorbed into the portion of air surrounding the monoliths, thus making the air heavier and inducing a discharge through ports  114  and  118 . Thus, a flow or “chimney” effect is established with the heavier, damper portion of air  186  (exemplified in FIG. 10 as solid arrows) exiting housing  112  through ports  114  and  118 , while replacement air  188  (exemplified in FIG. 10 as outlined arrows) flows into housing  112  through port  116 . The induced flow is maintained as long as heat is supplied to housing  112  and continues until the water vapor pressure near the surface of the plurality of hygroscopic monoliths  128  approaches that of the surrounding air within housing  112 . 
     Accordingly, a system according to the present invention allows the implementation of a method for controlling the humidity of a working gas with passive means for intermittent use as desired. The method of controlling humidity of a gas comprises the following steps: (a) drawing gas into a housing having a cavity therein; (b) channeling the gas to a hygroscopic material, having one or more channels, positioned within the cavity, said hygroscopic material being shaped to facilitate airflow by a chimney effect, whereby the gas is in fluid communication with said hygroscopic material and at least some of which is adsorbed or desorbed; and (c) collecting the gas from the housing. The hygroscopic monoliths may be activated or dried, once or repeatedly, as desired. This method for controlling humidity may be employed in isolation or, alternatively, in conjunction with other climate control operations. 
     While the present invention has been described in connection with the control of water vapor, it should be understood that the present invention may be utilized to remove other constituents from a working gas, such as volatile hydrocarbons, ammonia, mercaptains, hydrogen sulfide, organics or other components. To do so, a system according to the present invention would incorporate monoliths selected for their ability to adsorb or desorb the targeted constituent. It should also be understood that multiple constituents could be targeted by employing an appropriate variety of monoliths within a single housing. 
     While the humidity control system of the present invention has been described in connection with an exemplary embodiment, it is not intended to limit the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.