Abstract:
A heating and ventilating and air conditioning system includes an evaporative cooler positioned upstream from an evaporator. The evaporative cooler defines a primary airstream and a secondary airstream flowing transversely to the primary airstream. The output temperature T o  of the primary airstream is maintained by controlling the mass flow rate ratio of the secondary airstream to the primary airstream. By controlling the mass flow rate ratio, cooler air from the primary airstream is provided to the evaporator, thereby improving the efficiency of the overall system.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The subject invention relates generally to the conditioning of air, and, more specifically, to conditioning air using evaporative cooling. 
         [0003]    2. Description of the Prior Art 
         [0004]    Vapor compression air conditioning systems are generally known in the art to cool air in an evaporator. Essentially, the air is passed over cooling tubes containing refrigerant, usually in liquid form. As the air passes over the tubes, heat is transferred from the air to the refrigerant, causing it to vaporize. The refrigerant is then compressed by means of a compressor into superheated vapor. The refrigerant next passes through a condenser where heat is rejected to the atmosphere and refrigerant is condensed back into a liquid. The condensed refrigerant flows through an expansion device and then back into the evaporator. 
         [0005]    To improve the efficiency of the above described system, it is known to pre-cool the air before it enters the evaporator in order to reduce the air conditioning load on the system. One such apparatus is found in patent application Ser. No. 11/333,904 (Attorney Docket # DP-312627), which is assigned to the assignee of the present invention. In the prior system, an evaporative cooler is provided upstream of the evaporator. The evaporative cooler extracts heat from the primary airstream by evaporating water into a secondary airstream. The secondary airstream is discarded while the primary airstream is then fed into the evaporator and is conditioned according to the rest of the HVAC system. However, the prior system is limited in that once it is implemented, the temperature of the primary airstream is only affected by the ambient conditions. Thus, the apparatus is constrained in its ability to improve the cooling efficiency of the HVAC system, and cannot be varied to account for changes in ambient air temperature and humidity. 
         [0006]    There is a need for an improved vapor compression HVAC system that overcomes these and other disadvantages. 
       SUMMARY OF THE INVENTION AND ADVANTAGES 
       [0007]    The invention conditions air by providing intake air having a specific heat c pa , an initial temperature T i , and an initial absolute humidity ω i . The intake air is divided into a primary airstream and a secondary airstream. The primary airstream flows at a primary mass flow rate {dot over (m)} p , while the secondary airstream flows transversely to the primary airstream at a secondary mass flow rate {dot over (m)} s . Heat is extracted from the primary airstream and transferred to the secondary airstream. A liquid having a latent heat of evaporation h fg  is evaporated by the heat transferred to the secondary airstream. The secondary airstream acquires an absolute humidity ω s . The ratio of the secondary airstream to the primary airstream from the intake air is varied to maintain an output temperature T o  of the primary airstream according to the equation 
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         [0008]    The invention may be implemented in a heating and ventilating and air conditioning system including an evaporative cooler which receives intake air at a temperature T o , a specific heat c pa , and an absolute humidity ω i . The evaporative cooler defines a plurality of dry channels for establishing a primary airstream, and a plurality of wet channels for establishing a secondary airstream. The primary airstream flows at a primary mass flow rate {dot over (m)} p , while the secondary airstream flows at a secondary mass flow rate {dot over (m)} s . A flow divider divides the intake air into the respective primary and secondary airstreams. A tank provides a liquid to the wet channels for evaporation into the secondary airstream. The liquid has a latent heat of evaporation h fg , and provides an absolute humidity of the secondary airstream ω s . A controller controls the flow divider to vary the ratio of the secondary airstream to the primary airstream from the intake air. This maintains an output temperature T o  of the primary airstream according to the equation 
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       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    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: 
           [0010]      FIG. 1  is a schematic of a heating and ventilating and air conditioning system according to the present invention; 
           [0011]      FIG. 2  is a psychrometric chart demonstrating the manner in which a system according to the present invention operates to cool air; 
           [0012]      FIG. 3  is an isometric view of an evaporative cooler according to a first exemplary embodiment the present invention; 
           [0013]      FIG. 4  is a magnified view of a portion of an evaporative cooler showing a valve cover for selectively closing and revealing a plurality of orifices; 
           [0014]      FIG. 5  is an isometric view of an evaporative cooler showing an alternative aspect of the first exemplary embodiment; 
           [0015]      FIG. 6  is an isometric view of an evaporative cooler according to a second exemplary embodiment of the present invention; and 
           [0016]      FIG. 7  is a block diagram of a method of conditioning air according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Referring to the Figures, wherein like numerals indicate corresponding parts throughout the several views, a heating and ventilating and air conditioning (HVAC) system is shown generally at  20 . The HVAC system  20  includes a blower  22  for providing a supply of intake air. The intake air has an initial temperature T i , an absolute humidity ω i , a relative humidity Φ i , and a specific heat c pa . The air flows into an evaporative cooler  24 , which is shown in more detail in  FIG. 3 . The evaporative cooler  24  includes a plurality of dry channels  26  and a plurality of wet channels  28  extending transversely to the dry channels  26 . A primary airstream flows through the dry channels  26  at a primary mass flow rate {dot over (m)} p . A secondary airstream flows through the wet channels  28  at a secondary mass flow rate {dot over (m)} s . A flow divider  30  separates the dry and wet channels  26 ,  28  and divides the intake air into the respective primary and secondary airstreams. A tank  32  is placed beneath the wet channels  28 . A wicking material  34  is applied to the wet channels  28  to draw liquid from the tank  32 . 
         [0018]    According to a first exemplary embodiment of the present invention, a  20  liquid, such as water, has a latent heat of evaporation h fg . The wicking material  34  draws the liquid from tank  32  for distributing water to the wet channels  28  by surface tension effect. As the secondary airstream flows through the wet channels  28 , the water is evaporated resulting in an absolute humidity of the secondary airstream ω s . An evaporator core  36  is positioned downstream of the evaporative cooler  24  for receiving the primary airstream at an output temperature T o  from the dry channels  26 . As the primary airstream flows over the cold surface of the evaporator core  36 , water vapor in the primary airstream condenses. A reservoir  38  collects this condensate from the evaporator core  36  and provides it to the tank  32  of the evaporative cooler  24 . A controller  40  maintains the temperature T o  at a desired value by controlling the flow divider  30 . The controller  40  varies the ratio of the secondary airstream to the primary airstream from the intake air. Accordingly, the output temperature is determined by the following equation: 
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         [0019]    It can be appreciated from the equation that the ratio of latent heat of evaporation of water to the specific heat of air is sensibly constant. For example, consider the dry bulb air temperature between 100° F.(560° R) and 125° F.(585° R): 
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         [0000]    However, the difference in absolute humidity between the primary and secondary airstreams is largely dependent upon the incoming air temperature: 
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         [0020]    Hence, the ratio of the secondary mass flow rate to the primary mass flow rate has a significant impact on the output temperature of the primary airstream entering the evaporator core  36 . 
         [0021]    According to the first exemplary embodiment, the flow divider  30  comprises a plurality of orifices  42  having variable area. The ratio is controlled with the flow divider  30  by selectively increasing or decreasing the area of the orifices  42  to respectively increase or decrease the secondary mass flow rate {dot over (m)} s  relative to the primary mass flow rate {dot over (m)} p . According to an aspect of the present invention, shown specifically in  FIGS. 3-7 , the flow divider  30  includes a cover  46 ,  44  connected to an actuator  48 . The actuator  48  allows the cover  46 ,  44  to be movable by the controller  40  for selectively increasing and diminishing the flow through the orifices  42 . Referring specifically to  FIG. 3 , a linear actuator  48  is used to move a slide cover  44  fore and aft to selectively obstruct and open the orifices  42 . Alternatively, referring to  FIG. 5 , a rotary actuator  48  is used to accomplish the same fore and aft motion. 
         [0022]    According to a second exemplary embodiment, shown in  FIG. 6 , a hinge cover  46  is shown along the top of the evaporative cooler  24 . The hinge cover  46  pivots about the plurality of hinges  50  to selectively diminish and increase the flow through the wet channels  28 . By activating the actuator  48  to partially obstruct the wet channels  28 , the secondary mass flow rate {dot over (m)} s  decreases. By activating the actuator  48  to open the wet channels  28 , the secondary mass flow rate {dot over (m)} s  increases. 
         [0023]    Accordingly, the invention includes a method of conditioning air described with reference to the psychrometric chart in  FIG. 2  and the flow chart in  FIG. 7 . First, intake air is provided with the specific heat c pa , the initial temperature T i , and the initial absolute humidity ω i . This is indicated at point A in  FIG. 2 , which corresponds to location A in  FIG. 1 . The intake air is divided into the primary airstream and the secondary airstream. The primary airstream flows at the primary mass flow rate {dot over (m)} p , and the secondary airstream flows transversely to the primary airstream at the secondary mass flow rate {dot over (m)} s . A liquid, such as water, is provided to the secondary airstream by the capillary action of the wicking material  34 . The liquid has a latent heat of evaporation h fg . Heat is extracted from the primary airstream, lowering its temperature to the output temperature T o , and transferred to the secondary airstream. The temperature of the primary airstream is indicated at point B in  FIG. 2  which corresponds to location B in  FIG. 1 . All of this heat is used to evaporate the liquid, giving the secondary airstream a secondary absolute humidity ω s , but leaving its temperature unchanged. This is shown at point C in  FIG. 2 , which corresponds to location C in  FIG. 1 . As can be appreciated from the psychrometric chart, the airstreams have moved from a relative humidity of the intake air Φ i , to a new relative humidity ω=1, which indicates that the air is fully saturated. The output temperature T o  is maintained by varying the ratio of the secondary airstream to the primary airstream from the intake air according to equations 1-4, above. The primary airstream can then be introduced into the evaporator core  36 , where its relative humidity Φ o  will remain constant, but its temperature and absolute humidity will further decrease to T e  and ω e . This is represented at point D in  FIG. 2 , which corresponds to location D in  FIG. 1 . 
         [0024]    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.