Abstract:
In accordance with the present invention, a desiccant/evaporative cooling system for providing cool and dry air to an enclosed structure is disclosed. The system includes a desiccant chamber, at least one evaporative cooler, and a heat exchanger. The desiccant chamber intermittently receives and drys a first airstream from the enclosed structure to produce a second airstream. The evaporative cooler(s) receive and cool the second airstream to produce a third airstream at a temperature below a dew point for an ambient airstream. The heat exchanger uses the third airstream for absorbing heat from the ambient airstream to produce a fourth airstream before the fourth airstream is forced into the enclosed structure. Condensate is removed from the ambient airstream on the dry side surface of the heat exchanger. In another embodiment, a method for conditioning an ambient airstream in order to provide cool and dry air to an enclosed structure is disclosed. The ambient airstream is cooled to a temperature approaching the dew point of the ambient airstream to produce a first airstream which is cooler than the ambient airstream. The first airstream is forced into the enclosed structure. The second airstream is removed from the enclosed structure. The second airstream is dried to produce a third airstream. The temperature of the third airstream is reduced with one or more evaporative coolers to produce a fourth airstream. The fourth airstream has a temperature lower than the dew point of the ambient airstream. Heat and condensation is absorbed from the ambient airstream by using the fourth airstream.

Description:
FIELD OF THE INVENTION 
     The present invention relates to air conditioners, and in particular, to air conditioners which utilize both desiccants and evaporative coolers. 
     BACKGROUND INFORMATION 
     Evaporative coolers, both direct and indirect, are well known in the art. Evaporative cooling is generally considered energy efficient because fans, rather than more energy intensive pumps used in traditional vapor compression systems, power the cooling processes. In dry desert and high altitude climates, direct evaporative cooling alone provides energy efficient cooling in popular commercially available products, commonly referred to as “swamp coolers.” In humid regions, however, these direct evaporative coolers are less effective as they will not appreciably lower the conditioned air temperature and they increase room humidity when the room air is already laden with moisture. 
     Direct evaporative coolers remove heat while increasing the moisture in the air. In general terms, direct evaporative cooling dissipates the heat in air, using the heat energy in the air to evaporate water. The energy consumed to evaporate the water in evaporative coolers reduces the air temperature. In direct evaporative coolers, a fan is used to blow warm outside air through a water absorbent medium such as an open cell foam, corrugated cardboard or wood filings. When the warm air passes the water absorbent medium, the water evaporates, causing the air to become cooler, but more humid. This process is called adiabatic cooling because no energy is lost. This loss of sensible heat energy (generally expressed as temperature) causes the vapor pressure in the air (generally expressed as humidity) to rise. The energy that causes the humidity to rise is called latent heat energy and the reduction in sensible heat equals the increase in latent heat. 
     When hot, dry air enters the direct evaporative cooler, it can only be cooled to the wet-bulb temperature. As air is cooled, the relative humidity increases. When air is cooled to the point where it will hold no more water (i.e., 100 percent relative humidity) that temperature is referred to as the dew point. The wet-bulb temperature is the lowest temperature that can be measured on a water absorbent surface when air is flowing past at high velocity. The dew point temperature is often well below that of the wet-bulb temperature, particularly when the air is hot and dry. The dew point is the temperature at which liquid within the air will condense. 
     Indirect evaporative cooling extends the usefulness of evaporative cooling to more humid climates. In an indirect evaporative cooler, the evaporative cooling process is separated from the occupants of the building or enclosed structure by a heat exchanger that conducts heat. On one side (hereinafter, the “wet” side) of an indirect evaporative cooler, contained within the heat exchanger, a first stream of air blows through a water filled medium or air spray, lowering air temperature and increasing humidity in a way similar to the direct evaporative cooler. On the other side of the cooler (hereinafter, the “dry” side), a second stream of warm air blows past the heat exchanger, cooling the second stream without adding moisture to the air. In this way, humid air can pass the wet side of the heat exchanger without changing the moisture content of the air cooled on the dry side of the heat exchanger. In other words, indirect evaporative coolers work by dividing a first air flow into a first and second portion, then by directing the first portion through one or more wet internal pre-cooling stages, lowering the temperature within the last stage of the cooler to a level that approaches the dew point of the air at entry. Heat in the air on the dry side of the cooler is transferred to the wet side of the cooler, such that the temperature of the air leaving the dry side of the cooler also approaches the dew point temperature of the air at entry. 
     Indirect evaporative cooling is more effective than direct evaporative cooling in more humid climates because indirect methods do not add moisture to the air. However, as described above, relative humidity increases when air is cooled so there is a limit to the effectiveness of indirect evaporative cooling. This is especially true in more humid climates. 
     Evaporative coolers typically employ moisture absorbing surfaces that must be cleaned or replaced at regular intervals. Indirect evaporative cooling systems are deficient in providing access to such moisture absorbing surfaces for service or replacement. Many systems have a limited life span due to the deteriorating effects of water on the moisture absorbing surfaces and the lack of service access to such surfaces. Accordingly, a need exists for indirect evaporative coolers to be easily serviceable. 
     Desiccant air drying techniques, used in concert with evaporative cooling or existing vapor compression systems, are used to provide deep dehumidification in certain applications where this is desirable. Supermarkets, for example, have employed desiccant air drying as it is seen as beneficial for reducing spoilage in perishable food storage. However in practice, these systems have yet to achieve efficiencies substantially greater than existing vapor compression systems. This is largely due to the mechanical complexity of the complete system and the continuous energy demand of the desiccant and enthalpy wheels common in such systems. 
     Conventional desiccant/evaporative cooling systems also have a disadvantage in that they are relatively large and expensive, due to their mechanical complexity. For example, a typical desiccant wheel alone may be four feet in diameter and one foot in depth. This drawback limits the acceptance of conventional systems in markets where initial cost and package size is a major factor, such as residential and transportation markets. 
     Theoretical efficiencies for air conditioners which use desiccant dehumidification in combination with evaporative cooling have been estimated at up to five times that of conventional vapor-compression air conditioning systems. Additionally, desiccant/evaporative cooling systems offer an advantage over vapor-compression systems in that they continuously pass a large volume of air through an enclosed structure resulting in more fresh air being introduced into an enclosed structure. Accordingly, the air quality is improved along with efficiency. 
     A basic psychrometric cycle for desiccant/evaporative cooling systems was devised by Pennington and the cycle is known as the Pennington or ventilation cycle (see FIG.  2 ). With reference to FIGS. 1 and 2, the cycle involves drying an incoming ambient airstream  100  with a desiccant wheel  140  to produce a second airstream  104  which is dryer and hotter. The second airstream  104  is then cooled with an air to air heat exchanger  144  to produce a third airstream  108 . Typically, the air-to-air heat exchanger  144  is implemented with a rotating wheel assembly, known in the art as an enthalpy wheel. After cooling in the heat exchanger  144 , the third airstream  108  is passed through a first direct evaporative cooler  148  to cool and humidify the third airstream to produce a fourth airstream  112  which is introduced into an enclosed structure  152 . In this way, cool air is introduced into the enclosed structure  152 . 
     The fourth airstream  112  is heated by such sources as solar radiation striking the exterior of the structure and/or the presence of people and humidified by such sources as cooking, industrial activities and/or people within the enclosed structure  152  to generate a fifth airstream  116 . The fifth airstream  116  passes through a second direct evaporative cooler  156  where it is cooled and humidified to produce a sixth airstream  120 . The sixth airstream is passed though the air-to-air heat exchanger  144  which cools the second airstream  104 . In the process of cooling the second airstream  104 , the sixth airstream  120  is heated to produce a seventh airstream  124 . In a heater  160 , the seventh airstream  124  is heated to produce an eighth airstream  128 . The eighth airstream  128  is used to regenerate one side of the desiccant wheel  140  before being exhausted as a ninth airstream  132 . 
     Conventional Pennington cycle desiccant/evaporative cooling systems typically utilize a continuously-regenerated desiccant wheel  140  to remove latent heat from a confined area, together with one or more evaporative coolers  148  to remove heat and humidify. In other words, the incoming airstream is first dehumidified and then cooled which extends the area in which an evaporative cooler is practical to include more humid climates. The desiccant wheel  140  is generally divided into a first portion  168  which removes humidity from incoming ambient air and a second portion  164  which is regenerated by heating the desiccant with a warm airstream  128 . While the desiccant wheel  140  rotates, the first portion  168  is constantly changing so that it is regenerated in the second portion  164 . As those skilled in art can appreciate however, rotating the desiccant wheel  140  to constantly regenerate the desiccant consumes energy. Further, use of the enthalpy wheel as the air-to-air heat exchanger  144  consumes additional energy. Consumption of energy in this way, reduces the efficiency of desiccant/evaporative cooling systems. 
     In summary, it would be desirable to develop a desiccant/evaporative cooling system which: (1) improves upon the Pennington cycle to more efficiently cool an enclosed structure, (2) does not use desiccant or enthalpy wheels because of their excessive energy demands, (3) allows for easy serviceability of the evaporative coolers, and (4) is not mechanically complex such that applications which require less space may use the cooling system. 
     SUMMARY OF THE INVENTION 
     In accordance with a first embodiment of the present invention, a desiccant/evaporative cooling system for providing cool and dry air to an enclosed structure is disclosed. The system includes a desiccant chamber, at least one evaporative cooler, and a heat exchanger. The desiccant chamber intermittently receives and drys a first airstream from the enclosed structure to produce a second airstream. The evaporative cooler(s) receive and cool the second airstream to produce a third airstream at a temperature below a dew point for an ambient airstream. The heat exchanger uses the third airstream for absorbing heat from the ambient airstream to produce a fourth airstream before the fourth airstream is forced into the enclosed structure. Condensate is removed from the ambient airstream on the dry side surface of the heat exchanger. 
     In another embodiment, a method for conditioning an ambient airstream in order to provide cool and dry air to an enclosed structure is disclosed. The ambient airstream is cooled to a temperature approaching the dew point of the ambient airstream to produce a first airstream which is cooler than the ambient airstream. The first airstream is forced into the enclosed structure. The second airstream is removed from the enclosed structure. The second airstream is dried to produce a third airstream. The temperature of the third airstream is reduced with one or more evaporative coolers to produce a fourth airstream. The fourth airstream has a temperature lower than the dew point of the ambient airstream. Heat and condensation is absorbed from the ambient airstream by using the fourth airstream. 
     Based upon the foregoing summary, a number of important advantages of the present invention are readily discerned. A new psychrometric cycle is introduced which improves upon the Pennington cycle to closely approach theoretical efficiencies for desiccant/evaporative cooling systems. The present system does not use desiccant or enthalpy wheels because their excessive energy demands reduce efficiency. The unique design is not mechanically complex and allows for easy serviceability of the evaporative coolers. The simplicity of the design accommodates applications which require less space than consumed by convention desiccant/evaporative cooling systems. 
     Additional advantages of the present invention will become readily apparent from the following discussion, particularly when taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram which schematically depicts an a conventional Pennington cycle desiccant/evaporative cooling system; 
     FIG. 2 is a psychrometric chart showing changes to the airflow throughout the conventional desiccant/evaporative cooling system of FIG. 1; 
     FIG. 3 is a block diagram which schematically depicts an embodiment of the present invention; 
     FIG. 4 is a psychrometric chart showing changes to the airflow throughout the desiccant/evaporative cooling system of FIG. 3 while operating in a first mode which both cools and drys; 
     FIG. 5 is a psychrometric chart showing changes to the airflow throughout the desiccant/evaporative cooling system of FIG. 3 while operating in a second mode which regenerates the desiccant while cooling; 
     FIG. 6 is a psychrometric chart showing changes to the airflow throughout the desiccant/evaporative cooling system of FIG. 3 while operating in a third mode which only cools without desiccant drying; 
     FIG. 7 is a side sectional view which schematically depicts an embodiment of the desiccant/evaporative cooling system; 
     FIG. 8 is a side sectional view which schematically depicts air flow while operating in the first mode; 
     FIG. 9 is a side sectional view which schematically depicts air flow while operating in the second mode; and 
     FIG. 10 is a side sectional view which schematically depicts air flow while operating in the third mode. 
    
    
     DETAILED DESCRIPTION 
     The present invention operates on a new psychrometric control cycle. In one embodiment, a cool airstream is used to generally cool ambient air below its dew point before entry into an enclosed structure. The cool airstream is generated by using previously cooled air from the enclosed structure which is subjected to desiccant drying and two stages of evaporative cooling. Since the ambient air is cooled below its dew point, moisture condenses on the outer surface of the indirect evaporative cooler. This condensation is recycled to provide moisture to the evaporative coolers. As a result, the air entering the enclosed structure is both dryer and cooler than ambient air so that the desiccant/evaporative cooling system is suited for the most humid of climates. 
     With reference to FIG. 3, an embodiment of the current desiccant/evaporative cooling system is shown in block diagram form. The cooling system includes first through third dampers  340 ,  344 ,  348 , a control circuit  346 , a desiccant chamber  352 , a first indirect evaporative cooler  356 , a second indirect evaporative cooler  360 , a water tank  364 , a water pump  368 , a first fan  380 , and a second fan  382 . Dampers  340 ,  344 ,  348  are used to direct airstreams through the system under the management of the control circuit  346 . 
     The desiccant/evaporative cooling system can operate in at least three modes by appropriately controlling the dampers  340 ,  344 ,  348 . The third airstream  308  is divided according to the sole table amongst a fourth through seventh airstreams  310 ,  316 ,  320 ,  312  as directed by the dampers  340 ,  344 ,  348 . In a first mode, the first damper  340  passes all of a third air flow  308  into the desiccant chamber  352  and past the third damper  348 . While in a regeneration or a second mode, approximately 90% of the third airstream  308  bypasses the desiccant chamber  352  and only approximately 10% of the third airstream  308  enters the desiccant chamber  352  to remove any moisture from the heated desiccant. In a third mode, the third airstream  308  bypasses the desiccant chamber  352  altogether. Alternatively in the third mode, the third airstream  308  may be allowed to continue to pass through a saturated desiccant chamber  352  as it will have no effect on the temperature and humidity condition of the third airstream  308  because the saturated desiccant is no longer able to adsorb additional moisture. Generally, the third mode is utilized when the third airstream  308  is sufficiently dry so as to not require any drying in the desiccant chamber  352 . 
     
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 
               
             
             
               
                   
               
               
                 Division of thc Third Airstream 308 
               
             
          
           
               
                   
                 First Mode 
                 Second Mode 
                 Third Mode 
               
               
                 Air Flows 
                 (Cool/Dry) 
                 (Regeneration) 
                 (Cool Only) 
               
               
                   
               
               
                 Fourth Airstream 310 
                 100 % 
                 10 % 
                  0 % 
               
               
                 Fifth Airstream 316 
                  0 % 
                 10 % 
                  0 % 
               
               
                 Sixth Airstream 320 
                  0 % 
                 90 % 
                 100 % 
               
               
                 Seventh Airstream 312 
                 100 % 
                 90 % 
                 100 % 
               
               
                   
               
             
          
         
       
     
     To configure the dampers  340 ,  344 ,  348  for each of the modes, the control circuit  346  uses predetermined algorithms, input from the humidity sensor  350  and a desired humidity set by the user. Commonly available actuators, utilizing means such as electrical, hydraulic or pneumatically initiated action are activated by the control circuit  346  to operate the dampers. 
     The humidity sensor  350  measures the moisture content of the third airstream  308  from the enclosed structure  372 . Input from the humidity sensor  350  is used to determine when the desiccant is saturated (i.e., when the regeneration of second mode is necessary) and when drying is unnecessary (i.e., the third mode is adequate). 
     By knowing the amount of moisture in the third airstream  308  and utilizing predetermined factors such as the known moisture capacity of the desiccant, rate of moisture adsorption and airflow rate, the control circuit  346  algorithm can determine when the desiccant has become saturated so that the second mode is necessary. Alternatively, algorithms which rely only on the measurement of the rate and direction of humidity changes, as sensed by humidity sensor  350  within the enclosed structure may also be utilized to initiate the second mode. 
     The amount of moisture in the third airstream  308  may indicate to the control circuit  346  that the dryness of the air does not warrant the energy spent operating the desiccant chamber  352  such that the system may operate in the second mode. The humidity may be programmable by the user so that the desired humidity would affect the algorithms in the control circuit  346 . 
     With reference to FIGS. 3-6, the block diagram for the desiccant/evaporative cooling system and corresponding psychrometric charts are shown for each of the three modes of operation. Fresh ambient air  376  forming a first airstream  300  is drawn into the desiccant/evaporative cooling system by the first fan  380 . The first airstream  300  is passed through the dry side of the second indirect evaporative cooler  360  where the first airstream  300  is generally cooled below its dew point. It should be noted, the dry side of the second indirect evaporative cooler  360  adds no moisture to the first airstream  300  to produce a second airstream  304 . Additionally, since the wet side of the indirect evaporative cooler  360  passes air cooler than the dew point of the first airstream, moisture is removed from the first airstream  300  as condensation. In this way, a second airstream  304  is introduced into the enclosed structure which is cooler and dryer than the ambient air  376 . 
     To provide cool air to the wet side of the second indirect evaporative cooler  360 , a third airstream  308  is removed from the enclosed structure  372 . While operating in a first mode (see FIG.  4 ), both desiccants and evaporative cooling are utilized to cool the third airstream  308 . The entire third airstream  308  is forced by the second fan  382  and the dampers  340 ,  344 ,  348  through the desiccant chamber  352  to produce the seventh airstream  312 . The seventh airstream  312  is generally warmer and dryer than the third airstream  308 . The seventh airstream  312  is forced into the first indirect evaporative cooler such that approximately half enters the wet side to provide cooling to the other half which enters the dry side to produce an eighth and a ninth airstreams  324 ,  328 . The division between the wet and dry side may be optimized so that each side may receive between 20-50% of the total airflow. The eighth airstream  324 , which passed through the wet side, is exhausted after cooling the dry side of the first indirect evaporative cooler  356 . 
     The ninth airstream  328  is generally cooler than the seventh airstream  312  and has the same moisture content. All of the ninth airstream  328  is passed into the wet side of the second indirect evaporative cooler  360  to further cool the ninth airstream  328  and absorb heat from the first airstream  300  passing through the dry side of the second indirect evaporative cooler  360 . The ninth airstream  328  entering the wet side of the second indirect evaporative cooler  360  is cooled to a temperature close to the dew point of the seventh airstream  312  which is generally well below the dew point of the first airstream  300 . Because of this, condensation forms on the heat exchange surfaces of the dry side of the second indirect evaporative cooler  360 . 
     Operation in the second or regenerative mode (see FIG. 5) is required to periodically remove moisture from the desiccant chamber  352  so that the desiccant may adsorb moisture once again. To remove the moisture, the desiccant is heated by a heating source  392  to a predetermined temperature and approximately 10% of the third airstream  308  passes over the heated desiccant to exhaust the released moisture as a fifth airstream  316 . The dampers  340 ,  344 ,  348  are configured accordingly. While regeneration is taking place, the remaining approximately 90% of the third airstream  308  bypasses the desiccant chamber  352  to be cooled by the first and second indirect evaporative coolers  356 ,  360 . In this way, the desiccant may be regenerated while the enclosed structure is still being cooled. Generally, the regeneration in the second mode takes considerably less time than the time consumed in the first mode. To shorten the regeneration period, the design of the desiccant chamber could include a provision for additional desiccant surface area, increasing heat transfer to the desiccant, and/or lowering the mass of heat transfer surfaces in communication with the desiccant. 
     The third or coasting mode (see FIG. 6) is utilized when the third airstream  308  is determined by the humidity sensor  350  to be adequately dry. In some very dry climates, the ambient air  376  is so dry that additional drying in the desiccant chamber  352  is rarely necessary. The humidity sensor  350  determines the moisture content of the third airstream  308  and signals the control circuit  346  accordingly. If the desired humidity set by the user is achieved, the control circuit would command the dampers  340 ,  344 ,  348  so that substantially all the third airstream bypasses the desiccant chamber  352 . As those skilled in the art can appreciate, only using the desiccant chamber when necessary increases the energy efficiency of the desiccant/evaporative cooling system. If sources in the enclosed structure  372  raise the humidity beyond the desired humidity, the control circuit  346  will change to the first mode once again to dry the air. These sources include the presence of people, opening of windows and doors, cooking, industrial processes, etc. By cycling through the three modes of operation, the energy efficiency of the desiccant/evaporative cooling system is improved while maintaining a desired humidity. 
     The water tank  364  stores the excess water or fluid needed for operation. Gravity or pumps are used to transport water to the first and second indirect evaporative coolers  356 ,  360  and from the second evaporative cooler  360 . In the embodiment depicted in FIG. 3, water flows by gravity to a drip pan beneath the wet side of the first and second indirect evaporative coolers  356 ,  360  and water is recycled from the drip pan beneath the dry side of the second evaporative cooler  360  by way of a pump  368 . As mentioned above, condensation from the dry side of the second evaporative cooler  360  is produced because the temperature produced by the wet side is generally below the dew point of the airstream  300  entering the dry side. By recycling the condensation in this way, the filling of the water tank  364  may be performed less frequently and cooling in the first and second evaporative coolers  356 ,  360  may be increased. 
     When directed by the dampers  340 ,  344 ,  348  latent heat energy in the third airstream  308  is converted to sensible heat by the desiccant chamber  352 . The desiccant is contained in an insulated desiccant chamber  352  and is mounted by adhesive and/or mechanical means in a manner that exposes maximum of the desiccant surface area to the incoming airstream  310  with minimal restriction upon the air flow. Such methods well known in the art as parallel strips or staggered parallel strips with desiccant bonded by mechanical or adhesive means, may be used. Such a configuration is beneficial in both regeneration and dehumidification of the airstream. The desiccant may be any heat-regenerative, moisture-attracting material such as silica gel, molecular sieves and hydratable salts including desiccant liquids such as glycols and lithium chloride. Regeneration heat may be applied to the desiccant by any heat transfer method, which may including thermal conduction, radiation and/or convection. For example, the desiccant could be heated by any combination of direct thermal conduction from an electrical resistance heater, radiation from a heat lamp and/or convection from a natural gas combustion heated airflow. 
     While passing through the desiccant chamber  352 , moisture is adsorbed by the desiccant and latent heat energy in the airstream is converted to sensible heat energy, such that the air temperature of the seventh airstream  312  is significantly higher than the temperature of the fourth airstream  310  entering the desiccant chamber  352 . The configuration of the desiccant is such that the rate of adsorption during the first mode has been previously determined. The limit of the heat of adsorption is determined after considering the most demanding operating conditions to which the system may be exposed. For example, if it is determined that the air conditioner will see as its maximum, ambient air conditions which never exceed 100° F. with a humidity ratio of 240 grains/pound (34.29 grams/kilogram), it may be determined that it is desirable to limit the air drying capacity of the desiccant chamber to 80 grains/pound (11.43 grams/kilogram). According to this example, when the airstream  310  enters the desiccant chamber  352  at 100° F., with a humidity ratio of 240 grains/pound (34.29 grams/kilogram), it will leave with a temperature at 150 degrees F., with a humidity ratio of 160 grains/pound (22.86 grams/kilogram). 
     The air and liquid are propelled throughout the desiccant/evaporative cooling system by way of the pump  368  and the fans  380 ,  382 . Other embodiments could place the pumps and fans in different locations and still adequately propel the liquid and air. However, it is preferred to place the fans  380 ,  382  respectfully before and after the enclosed structure  372  to propel the second and third airstreams  304 ,  308 . Additionally, a common motor could be used to propel the fan blades in a way that reduces energy consumption. 
     To provide more or less fresh air to the enclosed structure  372 , the fan speeds may be increased or decreased accordingly. As can be appreciated by those skilled in the art, controlling the air flow rate also allows for regulation of the temperature and relative humidity. For example, increasing the flow rate will decrease the temperature difference between the airstream  304  and temperature within the enclosed structure  372 . However, increasing the air flow rate excessively will decrease the heat transfer to the airstream  304  and the amount of moisture removed by condensation. For this reason, those skilled in the art will also appreciate the importance of matching heat exchanger cooling capacity with the anticipated sensible and latent heat load of the enclosed structure. 
     With reference to FIG. 7, an embodiment of the desiccant/evaporative cooling system is schematically shown. The compact implementation of the system allows for placement of the system in a window opening or other small orifice in the enclosed structure  372 . FIG. 7 shows a condensation collection pan  704 , a precooling heat exchanger  708 , a first damper  712 , a second damper  716 , the desiccant chamber  352 , the first indirect evaporative cooler  356 , the second indirect evaporative cooler  360 , the water tank  364 , a first fan  380 , and a second fan  382 . The condensation collection pan  704  collects moisture removed from the first airstream  300 . Condensation is produced because the second indirect evaporative cooler  360  can cool the dry side below the dew point of the ambient air in the first airstream  300 . To precool the third airstream  308 , the heat exchanger  708  uses exhaust from the first indirect evaporative cooler  356  which takes advantage of the fact that the exhaust is generally cooler than the third airstream  308 , in particular during the second and third modes of operation. The first and second dampers  712 ,  716  are hinged on one end to allow redirection of the airstream passing therethrough. 
     FIGS. 8-10 depict the air flow throughout the desiccant/evaporative cooling system dictated by the position of the first and second dampers  712 ,  716 . These dampers  712 ,  716  allow operation of the system in the first through third modes respectively depicted in FIGS. 8-10. The first mode (see FIG. 8) uses both desiccant drying and evaporative cooling to condition the enclosed structure  372  by placing the first damper  712  in the horizontal position and the second damper  716  in the vertical position. To operate in the second or regeneration mode (see FIG.  9 ), the first damper  712  is positioned to allow the majority of the third airstream  308  to go directly to the first evaporative cooler  356  and avoid the desiccant chamber  352 . In this mode, the desiccant is heated to evaporate the moisture therein and a small portion of the third airstream is used to exhaust the resulting airstream  316  past the second damper  716 . Operation in the third or coasting mode (see FIG. 10) diverts all of the third airstream  308  such that the desiccant chamber  352  is avoided entirely. In the third mode, the humidity sensor  350  has indicated further drying of the air in the enclosed structure  372  is unnecessary. 
     The presently disclosed invention described herein includes provision to service or replace the moisture absorbing surfaces in the first and second indirect evaporative coolers  356 ,  360 . An advantage of the presently disclosed desiccant/evaporative cooling system is its ease of serviceability. As can be seen in FIG. 7, the moisture absorbing surfaces of each indirect evaporative cooler  356 ,  360  are accessible from the portion of the system which faces the enclosed structure  372 . This allows the occupants to easily remove and replace the absorbing surfaces without leaving their room. Conventional evaporative coolers often require climbing onto the roof of the enclosed structure  372  and further require elaborate disassembly to remove the absorbing surfaces. 
     Excess water may be provided to the moisture absorbing surfaces within the evaporative coolers  356 ,  360  such that heat transfer between the wet and dry sides of the heat exchangers is improved as the water passes, along with cooled air, through the heat exchangers. In the embodiment described in FIG. 7, excess water from the evaporative cooler  360  is recovered along with condensation in condensation collection pan  704 . The condensation is generally cool which, when recycled to the evaporative coolers  356 ,  360  improves their efficiency. 
     In one embodiment, the invention is a self-contained cooling system configured as a window unit for cooling a room. The invention can stand alone for applications such as automotive cooling systems or duct work can be attached to the unit for larger residential or industrial applications. The desiccant/evaporative cooling system can be configured in a size comparable to that of vapor compression machinery with essentially equivalent cooling output (see FIG.  7 ). This mechanical configuration is considerably less complex than conventional Pennington cycle systems, eliminating the desiccant and enthalpy wheels in conventional systems and only requiring a relatively low power motor, or motors, to drive the fans  380 ,  380  and pump  368 . 
     Contrary to the embodiment in FIG. 7, the desiccant chamber  352  and the indirect evaporative cooler components  356 ,  360  of the invention may also be constructed in separate housings that communicate through duct work, as needed for the particular application. The desiccant chamber  352  and multi-stage indirect evaporative coolers  356 ,  360  may also be configured as separate assemblies for air drying or air cooling applications or for use with other types of air conditioning equipment, such as vapor compression machinery. 
     Although the above discussion is generally limited to two stages of indirect evaporative cooling, those skilled in the art can appreciate the principles may be applied to systems with more than two stages of evaporative cooling. Further, the second airstream  304  may be additionally cooled by an indirect or direct evaporative cooler prior to entry into the enclosed structure  372 . 
     Those skilled in the art will appreciate that the first stage indirect evaporative cooler  356  serves the function of rejecting the heat of adsorption generated in desiccant chamber  352 . As such, alternative means of rejecting this heat, to include air to air heat exchangers or vapor compression equipment may be utilized in place of the first stage indirect evaporative cooler. 
     Additionally, as those skilled in the art can appreciate, embodiments of this system can provide energy efficient heating with essentially the same equipment by using the desiccant chamber  352  and heating source  392 . The heating source  392  could include, but is not limited to, solar heaters, resistive elements, gas fuel combustion, liquid fuel combustion, thermal radiation, or microwave heaters. 
     The forgoing description of the invention has been presented for the purposes of illustration and description and is not intended to limit the invention. Variations and modifications commensurate with the above description, together with the skill or knowledge of the relevant art, are within the scope of the present invention. The embodiments described herein are further intended to explain the best mode known for practicing the invention and to enable those skilled in the art to utilize the invention in such best mode or other embodiments, with the various modifications that may be required by the particular application or use of the invention. It is intended that the appended claims be construed to include alternative embodiments to the extent permitted by the prior art.