Evaporative cooler and desiccant assisted vapor compression AC system

Abstract

An air conditioning system includes an evaporator assembly and a condenser assembly. A first portion of the exhaust airstream leaves the condenser and enters a primary channel of an evaporative cooler assembly. Water evaporates from the evaporative cooler tubes and creates a moisture-laden airstream. A plurality of apertures in the evaporative cooler tubes bleeds the moisture-laden airstream into a secondary channel defined within the evaporative cooler tubes. The heat drawn from the air in the primary channel produces an evaporatively cooled airstream that enters a desiccant wheel. A solid desiccant material within the desiccant wheel absorbs moisture from the evaporatively cooled airstream to produce a dehumidified airstream that enters the evaporator assembly. A second portion of the exhaust airstream is directed through a heater and then into the desiccant wheel to provide heat for regeneration of the solid desiccant material.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The subject invention relates generally to an air conditioning system.

2. Description of the Prior Art

Known air conditioning systems include an evaporator assembly and a condenser assembly cooperating to cyclically evaporate and condense a refrigerant. The evaporator assembly includes a plurality of evaporator tubes carrying a refrigerant and an evaporator fan for moving ambient air across the evaporator tubes. Heat transfers from the air to the refrigerant, thereby evaporating the refrigerant and producing a conditioned airstream. The condenser assembly includes a plurality of condenser tubes in fluid communication with the evaporator tubes. A condenser fan moves ambient air over the condenser tubes, and heat transfers from the refrigerant to the ambient air, thereby condensing the refrigerant and producing an exhaust airstream. The exhaust airstream is typically rejected to the atmosphere. The system also includes a compressor for compressing the refrigerant into a superheated vapor prior to entering the condenser, and an expansion device for reducing pressure on the refrigerant to produce a sub-cooled liquid prior to entering the evaporator.

This system requires a significant amount of energy input. The greater the difference between the ambient air and the desired temperature of the conditioned air, the faster the refrigerant must cycle through the system, in order to continue exchanging the heat. Much of the energy required is used in the compressor. Efforts have been made to reduce the load on this system. The two types of cooling load on an air conditioning system are the sensible load, and the latent load. The sensible load is the energy required to reduce the dry bulb temperature of the conditioned air. Sensible load is so named because the temperature difference can be sensed, or detected, by an observer (e.g. a thermometer, or a person occupying the cooled space). The latent load is the energy required to condense water vapor in the ambient air onto the evaporator surface. As the water vapor condenses on the cold evaporator surface, it releases thermal energy, which is absorbed by the refrigerant inside the evaporator tubes.

U.S. Pat. No. 6,776,001 to Maisotsenko, et al., teaches reducing the latent load by using a desiccant wheel. However, the reaction that occurs in a desiccant wheel gives off heat, so that in many cases the desiccant wheel simply trades latent load for sensible load. U.S. patent application Ser. No. 11/453,721, assigned to the assignee of the present invention, teaches using an evaporative cooler to reduce the sensible load of the air entering the evaporator. However, this does nothing to reduce the latent load of the incoming air, as the evaporative cooler does not reduce humidity of the ambient air.

SUMMARY OF THE INVENTION AND ADVANTAGES

The invention provides for an air conditioning system including an evaporator assembly. The evaporator assembly include a plurality of evaporator tubes for carrying a refrigerant, and a condenser assembly including a plurality of condenser tubes in fluid communication with the evaporator tubes. A condenser fan moves ambient air over the condenser tubes, and heat transfers from the refrigerant to the ambient air to condense the refrigerant and to produce an exhaust airstream. An evaporative cooler assembly defines a primary channel for receiving an incoming airstream and for producing an evaporatively cooled airstream. A desiccant wheel has a solid desiccant material and a housing supporting the solid desiccant material. A first air inlet is in airflow communication with the evaporative cooler assembly for receiving the evaporatively cooled airstream and for directing the evaporatively cooled airstream through a first sector of the housing. An exothermic reaction between the solid desiccant material and the evaporatively cooled airstream occurs to dry the evaporatively cooled airstream to produce a dehumidified airstream. A first outlet is in airflow communication with the evaporator assembly for directing the dehumidified airstream over the evaporator tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:

FIG. 1 is a schematic of an air conditioning system according to an exemplary embodiment of the present invention;

FIG. 2 is a perspective view of a desiccant wheel according to the exemplary embodiment of the present invention;

FIG. 3 is a perspective view of an evaporative cooler according to the exemplary embodiment of the present invention; and

FIG. 4 is a psychrometric chart demonstrating the state of the air as it cycles through the air conditioning system according to the exemplary embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, an air conditioning system is generally shown at 20. Referring first to FIG. 1, the system 20 includes an evaporator assembly 22 having a plurality of evaporator tubes 24 spaced apart from one another. An evaporator air passage is defined between the evaporator tubes 24 to receive a dehumidified airstream flowing over the evaporator tubes 24. The source of the dehumidified airstream will be explained in more detail below. The evaporator tubes 24 carry a sub-cooled liquid refrigerant, and an evaporator fan 26 is used to move the dehumidified airstream across the evaporator tubes 24. Heat transfers from the air to the refrigerant and evaporates the liquid refrigerant into a vapor. The air is cooled to produce a conditioned airstream and can then be directed as desired.

The evaporated refrigerant leaves the evaporator tubes 24 and heads into a compressor 28, which compresses the evaporated refrigerant to produce a superheated vapor. A condenser assembly 30 includes a plurality of condenser tubes 32 spaced apart from one another and in fluid communication with the compressor 28 to receive the superheated vapor. A condenser air passage is defined between the condenser tubes 32. A condenser fan 33 moves ambient air through the condenser air passage over the condenser tubes 32. Heat is transferred from the superheated vapor to the ambient air, to condense the refrigerant into a liquid. The ambient air is heated to produce an exhaust airstream leaving the condenser air passage.

To complete the refrigeration cycle, an expansion device 34 is in fluid communication between the condenser tubes 32 and the evaporator tubes 24. The expansion device 34 decreases the pressure on the liquid to produce a sub-cooled liquid refrigerant for supply back to the evaporator tubes 24.

Referring to FIG. 3, an evaporative cooler assembly 36 is provided. The evaporative cooler assembly 36 includes a plurality of evaporative cooler tubes 38 extending vertically and spaced apart from one another. A plurality of fins 40 extends back and forth between the evaporative cooler tubes 38 to define a primary channel extending along the fins 40 and perpendicularly to the evaporative cooler tubes 38. A plurality of internal dividers 42 extends within the evaporative cooler tubes 38 to define a secondary channel extending perpendicularly to the primary channel. A water tank 44 is disposed about an end of the evaporative cooler tubes 38, and a wicking coating 46 extends along the evaporative cooler tubes 38 for wicking water by capillary action from the water tank 44 into contact with the secondary channel.

Referring to FIGS. 1 and 3, a first conduit 48 connects the condenser air passage in airflow communication with the primary channel of the evaporative cooler assembly 36. A first portion of the exhaust airstream flows through the first conduit 48 to the primary channel. A plurality of apertures 50 are disposed along the evaporative cooler tubes 38 for splitting the airstream between the primary and secondary channels. Therefore, a fraction of the first portion of the exhaust airstream enters the apertures 50 and flows into the secondary channel and flows over the wet surfaces. The air in the secondary channel evaporates the water along the sides of the evaporative cooler tubes 38 to produce a moisture-laden airstream. The evaporation draws the latent heat of vaporization away from the first portion of the exhaust airstream to produce an evaporatively cooled airstream flowing through the primary channel and having a lower dry bulb temperature than the ambient air. This cooler airstream lowers the sensible load on the evaporator assembly 22, thereby reducing the cooling load exerted by the system 20. The moisture-laden airstream flowing through the secondary channel can simply exit the evaporative cooler assembly 36 through the openings of the evaporative cooler tubes 38.

Referring to FIGS. 1 and 2, a desiccant wheel 52 is provided. The desiccant wheel 52 includes a housing 54 having a pair of end plates 56 spaced apart from one another. A plurality of desiccant tubes 58 extend between the end plates 56, and a pair of trunnions 60 each extend from one of the end plates 56 to define an axis Z. A solid desiccant material extends within each of the desiccant tubes 58. The housing 54 of the desiccant wheel 52 includes a first air inlet 62 in airflow communication with the primary channel of the evaporative cooler for receiving the evaporatively cooled airstream. The first air inlet 62 directs the evaporatively cooled airstream through a first sector of the desiccant tube 58 to pass over the solid desiccant material. The presence of the evaporatively cooled airstream causes an exothermic reaction with the solid desiccant material. As a result, moisture from the evaporatively cooled airstream is adsorbed by the solid desiccant material, thereby producing the dehumidified airstream. The reaction is governed according to equation (1), wherein M refers to the solid desiccant material, and the reaction adsorbs n molecules of water vapor nH₂O into the solid desiccant material, forming a complex M.nH₂O and liberating heat Q_o. This liberated heat can raise the temperature of air flowing through the first section of the desiccant wheel 52.

$\begin{array}{cc}M\left(s\right)+{nH}_2O\stackrel{\mathrm{exothermic}\mathrm{adsorption}}{}M·{nH}_2O+Q_o& \left(1\right)\end{array}$

A first air outlet 64 is provided in airflow communication with the evaporator air passage for directing the dehumidified airstream over the evaporator tubes 24. This reaction leaves less water vapor in the airstream that will condense in the evaporator assembly 22, thereby reducing the latent load on the evaporator and further reducing the overall cooling load on the system 20.

However, once the solid desiccant material absorbs the water vapor, it must be regenerated. Therefore, a second air inlet 66 is in airflow communication with the condenser air passage to receive a second portion of the exhaust airstream. A second conduit 68 connects the condenser air passage with second air inlet 66 for directing the second portion of the exhaust airstream to the desiccant tubes 58. An exhaust flow divider 70 connects the condenser air passage in airflow communication with the first and second conduits 48, 68 for dividing the exhaust airstream into the first portion and the second portion. According to the exemplary embodiment, the exhaust flow divider 70 is a Y-shaped conduit that connects the first and second conduits 48, 68 with the condenser air passage. A heater 72 is provided in airflow communication between the condenser air passage and the second air inlet 66 of the desiccant wheel 52 for adding heat to the second portion of the exhaust airstream.

The second portion of the exhaust airstream is directed from the second air inlet 66 through a second sector of the desiccant tubes 58. When the warm air from the heater 72 comes into contact with the solid desiccant material, an endothermic reaction results, which removes the water vapor molecules from the solid desiccant material. This reaction is governed according to equation (2), where the heat from the second portion of the exhaust airstream replaces the heat Q_o liberated during the exothermic reaction:

$\begin{array}{cc}M\left(s\right)+{nH}_2O\underset{\mathrm{endothermic}\mathrm{desportion}}{}M·{nH}_2O+Q_o& \left(2\right)\end{array}$

A second air outlet 74 is provided to discharge the second portion of the exhaust airstream after the endothermic reaction.

The desiccant wheel 52 is supported by the trunnions 60 for rotation about the axis Z to alternately move the solid desiccant material between the first and second sectors to successively expose the solid desiccant material to the second portion of the exhaust airstream and to the evaporatively cooled airstream. This cycle allows the solid desiccant material to be continually used and regenerated. According to the exemplary embodiment, the desiccant wheel 52 rotates at a speed of about 5-6 RPM. In addition, the amount of moisture that can be absorbed by the desiccant wheel 52 is proportional to the rotational speed, so the humidity of the dehumidified airstream can be controlled simply by altering the speed.

Referring to FIG. 4, the psychrometric properties of air flowing through the system 20 are shown. The letters designating points throughout the psychrometric chart of FIG. 4 correspond to positions within the system 20 of FIG. 1. To wit, ambient air enters the system 20 at point A, having an ambient temperature T_i and an absolute humidity ω_i. The ambient air is heated in the condenser air passage, and leaves as exhaust airstream at point B, having the same absolute humidity, and an increased temperature T_s, which increases the water vapor uptake capacity of air both in the evaporative cooler assembly 36 and the desiccant wheel 52. The first portion of the exhaust airstream then flows into the evaporative cooler and exits as the evaporatively cooled airstream at point C, having the same absolute humidity and reduced temperature T_p. The moisture-laden airstream exits the evaporative cooler assembly 36 through the secondary channel at point D, having a slightly lower temperature T_SE, due to direct evaporative cooling in the secondary channels, and increased absolute humidity ω_s. The evaporatively cooled airstream in the primary channels then flows into the desiccant wheel 52 and exits as the dehumidified airstream at point E with an increased temperature that does not exceed the ambient temperature T_i, and a lower absolute humidity ω_o. The dehumidified airstream then enters the evaporator air passage and is further cooled to point F, having the same absolute humidity and lower temperature T_e. The second portion of the exhaust airstream leaves the condenser air passage and enters the heater 72. The second portion of the exhaust airstream exits the heater 72 at point G, having the same absolute humidity ω_i, and an increased temperature T_d. The second portion of the exhaust airstream then enters the desiccant wheel 52 to dry the solid desiccant material, receives the moisture from the solid desiccant material, and exits the desiccant wheel 52 at point H, having substantially the same temperature T_d, and higher absolute humidity ω_d.

The air conditioning system 20 utilizes the waste heat from the condenser airstream to increase the water vapor uptake capacity of air. Additionally, the condenser fan 33 performs multiple functions, including directing air through the condenser assembly 30, evaporative cooler assembly 36, and the desiccant wheel 52, rather than using a separate fan for each assembly.

Within the evaporative cooler assembly 36, the higher water vapor uptake capacity of air flowing through the first conduit 48 and into the evaporative cooler assembly 36 increases the evaporation rate of water in the secondary channel. A higher evaporation rate translates into a more efficient evaporative cooling process. Within the desiccant wheel 52 the higher water vapor uptake capacity of air flowing through the second conduit 68 increases the desiccant material regeneration rate by absorbing a greater amount of moisture from the solid desiccant material. Both of these factors increase the overall efficiency of the air conditioning system 20. The evaporatively cooled airstream leaving the primary channel of the evaporative cooler assembly 36 absorbs the heat of the exothermic reaction occurring within the desiccant wheel 52 thereby delivering the dehumidified air to the evaporator assembly 22 at or below ambient air temperature T_i. Thus, the evaporative cooler assembly 36 operating in conjunction with the desiccant wheel 52 completely handles the latent load of the evaporator assembly 22 with the expenditure of waste heat only, thereby increasing the overall efficiency of the air conditioning system 20. The load reduction achieved by the use of this waste heat could be substantial, typically amounting to about 40% of the total load.

While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.

Claims

1. An air conditioning system comprising; an evaporator assembly including a plurality of evaporator tubes for carrying a refrigerant and an evaporator fan for moving a dehumidified airstream across said evaporator tubes for transferring heat from the dehumidified airstream to the refrigerant to evaporate the refrigerant and to produce a conditioned airstream,a condenser assembly including a plurality of condenser tubes in fluid communication with said evaporator tubes and a condenser fan for moving ambient air over said condenser tubes for transferring heat from the refrigerant to the ambient air to condense the refrigerant and to produce an exhaust airstream,an evaporative cooler assembly defining a primary channel for receiving an incoming airstream and for producing an evaporatively cooled airstream, anda desiccant wheel including a solid desiccant material and a housing supporting said solid desiccant material and a first air inlet in airflow communication with said evaporative cooler assembly for receiving the evaporatively cooled airstream and for directing the evaporatively cooled airstream through a first sector of said housing to cause an exothermic reaction with said solid desiccant material to dry the evaporatively cooled airstream to produce the dehumidified airstream and including a first outlet in airflow communication with said evaporator assembly for directing the dehumidified airstream over said evaporator tubes.

2. A system as set forth in claim 1 wherein said condenser tubes are spaced apart from oneanother defining a condenser air passage therebetween for receiving the ambient air and for discharging the exhaust airstream and said evaporative cooler assembly is in airflow communication with said condenser air passage for receiving at least a first portion of the exhaust airstream and for cooling the first portion of the exhaust airstream to produce the evaporatively cooled airstream.

3. A system as set forth in claim 2 including a first conduit connecting said condenser air passage to said evaporative cooler assembly in airflow communication for directing at least the first portion of the exhaust airstream to the primary channel of the evaporative cooler.

4. A system as set forth in claim 3 wherein said evaporative cooler assembly includes; a plurality of evaporative cooler tubes extending vertically defining a secondary channel therein and spaced apart from one another defining said primary channel extending perpendicularly therebetween for receiving the first fraction of the exhaust airstream,a water tank disposed about an end of said evaporative cooler tubes and said evaporative cooler tubes extending upwardly from said water tank and including a wicking coating extending thereon for wicking water by capillary action from said water tank into contact with said primary channel for evaporation into a moisture-laden airstream to draw the latent heat of vaporization from the first fraction of the exhaust airstream to produce the evaporatively cooled airstream carried by said primary channel, anda plurality of apertures disposed along said evaporative cooler tubes for bleeding the moisture-laden airstream from said primary channel into said secondary channel for discharge from said evaporative cooler.

5. A system as set forth in claim 4 wherein said evaporative cooler assembly includes a plurality of fins extending back and forth between said tubes extending parallel with said primary channel.

6. A system as set forth in claim 2 including a second conduit in airflow communication with said condenser air passage and said desiccant wheel including a second air inlet connected with said second conduit for airflow communication with said condenser air passage for directing at least a second portion of the exhaust airstream through a second sector of said housing to cause an endothermic reaction with said solid desiccant material to dry said solid desiccant material and said desiccant wheel including a second air outlet for discharging the second fraction of the exhaust airstream.

7. A system as set forth in claim 6 including a heater disposed along said second conduit between said condenser air passage and said second air inlet for heating the second portion of the exhaust airstream.

8. A system as set forth in claim 6 wherein said housing of said desiccant wheel includes a pair of end plates spaced apart and a plurality of desiccant tubes extending therebetween supporting said solid desiccant material therewithin and a pair of trunnions each extending from one of said end plates defining an axis and supporting said desiccant wheel for rotation about said axis to alternately move said solid desiccant material between said first and second sectors to successively expose said solid desiccant material to the second portion of the exhaust airstream and to the evaporatively cooled airstream.

9. An air conditioning system comprising; an evaporator assembly including a plurality of evaporator tubes for carrying a refrigerant therein and said evaporator tubes being spaced apart from oneanother defining a evaporator air passage therebetween for receiving a dehumidified airstream to flow over said evaporator tubes for transferring heat from the dehumidified airstream to the refrigerant to evaporate the refrigerant within said evaporator tubes and to produce a conditioned airstream within said evaporator air passage,a condenser assembly including a plurality of condenser tubes in fluid communication with said evaporator tubes and spaced apart from oneanother defining a condenser air passage therebetween and a condenser fan for moving ambient air through said condenser air passage over said condenser tubes for transferring heat from the refrigerant to the ambient air to condense the refrigerant within said condenser tubes and to produce an exhaust airstream within said condenser air passage,an evaporative cooler assembly including a plurality of evaporative cooler tubes extending vertically defining a secondary channel therein and spaced apart from one another defining a primary channel therebetween for receiving a first portion of the exhaust airstream and for producing an evaporatively cooled airstream and a moisture-laden airstream and said evaporative cooler tubes including a plurality of apertures for bleeding the moisture-laden airstream from said primary channel into said secondary channel,a first conduit in airflow communication with said condenser air passage and said, primary channel of said evaporative cooler for directing the first portion of the exhaust airstream to said primary channel,a desiccant wheel including a solid desiccant material and a housing supporting said solid desiccant material and a first air inlet in airflow communication with said primary channel of said evaporative cooler assembly for receiving the evaporatively cooled airstream and for directing the evaporatively cooled airstream through a first sector of said housing to cause an exothermic reaction with said solid desiccant material to dry the evaporatively cooled airstream to produce the dehumidified airstream and including a first outlet in airflow communication with said evaporator assembly for directing the dehumidified airstream over said evaporator tubes,said desiccant wheel including a second air inlet for receiving a second portion of the exhaust airstream and for directing the second portion of the exhaust airstream through a second sector of said housing to cause an endothermic reaction with said solid desiccant material to dry said solid desiccant material and said desiccant wheel including a second air outlet for discharging the second fraction of the exhaust airstream,a second conduit in airflow communication with said condenser air passage and said second air inlet for directing the second portion of the exhaust airstream to said second sector of said housing, andan exhaust flow divider connected in airflow communication between said condenser air passage and said first and second conduits for dividing the exhaust airstream into the first and second portions.

10. An air conditioning system as set forth in claim 9 including a heater disposed along said second conduit in airflow communication between said condenser air passage and said second air inlet for heating the second portion of the exhaust airstream.

11. An air conditioning system as set forth in claim 9 including a water tank disposed about an end of said evaporative cooler tubes and said evaporative cooler tubes including a wicking coating extending thereon for wicking water by capillary action from said water tank into contact with said primary channel for evaporation into the moisture-laden airstream to draw the latent heat of vaporization from the first fraction of the exhaust airstream to produce the evaporatively cooled airstream carried by said primary channel.

12. An air conditioning system as set forth in claim 9 wherein said housing of said desiccant wheel is rotatably supported about an axis for rotation about said axis to alternately move said solid desiccant material between said first and second sectors to successively expose said solid desiccant material to the second portion of the exhaust airstream and to the evaporatively cooled airstream.

13. A system as set forth in claim 9 including an evaporator fan for moving the dehumidified airstream through said evaporator air passage over said evpaorator tubes.

14. An air conditioning system comprising; an evaporator assembly including a plurality of evaporator tubes for carrying a refrigerant and an evaporator fan for moving a dehumidified airstream across said evaporator tubes for transferring heat from the dehumidified airstream to the refrigerant to evaporate the refrigerant and to produce a conditioned airstream,a compressor in fluid communication with said evaporator tubes for compressing the evaporated refrigerant to produce a superheated vapor,a condenser assembly including a plurality of condenser tubes in fluid communication with said compressor and spaced apart from oneanother defining a condenser air passage therebetween and a condenser fan for moving ambient air through said condenser air passage over said condenser tubes for transferring heat from the superheated vapor to the ambient air to condense the superheated vapor into a liquid refrigerant and to produce an exhaust airstream,an expansion device in fluid communication with said condenser tubes and with said evaporator tubes for decreasing the pressure on the liquid to produce a sub-cooled liquid refrigerant for supply back to said evaporator tubes,an evaporative cooler assembly including a plurality of evaporative cooler tubes extending vertically and spaced apart from one another and including a plurality of fins extending back and forth between said evaporative cooler tubes defining a primary channel extending perpendicularly between said evaporative cooler tubes and being in airflow communication with said condenser air passage for receiving a first portion of the exhaust airstream for producing an evaporatively cooled airstream,said evaporative cooler tubes including a plurality of internal dividers extending within said tubes defining a secondary channel extending perpendicularly to said primary channel,said evaporative cooler assembly including a water tank disposed about an end of said evaporative cooler tubes,said evaporative cooler tubes extending upwardly from said water tank and including a wicking coating extending thereon for wicking water by capillary action from said water tank into contact with said primary channel for evaporation into a moisture-laden airstream to draw the latent heat of vaporization from the first fraction of the exhaust airstream to produce the evaporatively cooled airstream carried by said primary channel,said evaporative cooler tubes including a plurality of apertures for bleeding the moisture-laden airstream from said primary channel into said secondary channel for discharge from said evaporative cooler,a desiccant wheel including a housing having a pair of end plates spaced apart and a plurality of desiccant tubes extending therebetween and a pair of trunnions each extending from one of said end plates defining an axis,said desiccant wheel including a solid desiccant material extending within each of said desiccant tubes,said desiccant wheel including a first air inlet in airflow communication with said primary channel of said evaporative cooler for receiving the evaporatively cooled airstream and for directing the evaporatively cooled airstream through a first sector of said desiccant tubes to cause an exothermic reaction with said solid desiccant material to dry the evaporatively cooled airstream to produce the dehumidified airstream and including a first air outlet in airflow communication with said evaporator assembly for directing the dehumidified airstream over said evaporator tubes,said desiccant wheel including a second air inlet in airflow communication with said condenser air passage for receiving a second portion of the exhaust airstream and for directing the second portion of the exhaust airstream through a second sector of said desiccant tubes to cause an endothermic reaction with said solid desiccant material to dry said solid desiccant material and including a second air outlet for discharging second portion of the exhaust airstream,said desiccant wheel being supported by said trunnions for rotation about said axis to alternately move said solid desiccant material between said first and second sectors to successively expose said solid desiccant material to said desiccant regenerating airstream and to said ambient air,a first conduit in airflow communication with said condenser air passage and said primary channel of said evaporative cooler for directing the first portion of the exhaust airstream to said primary channel,a second conduit in airflow communication with said condenser air passage and second air inlet for directing the second portion of the exhaust airstream to said second sector of said desiccant tubes,an exhaust flow divider connected in airflow communication between said condenser air passage and said first and second conduits for dividing the exhaust airstream into the first portion and the second portion, anda heater in airflow communication between said exhaust flow divider and said second air inlet of said desiccant wheel for receiving the second portion of the exhaust airstream and for adding heat to the second portion of the exhaust airstream.