US20070289320A1 - Vapor compression AC system with evaporative cooler assisted evaporator - Google Patents

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
• 1. Field of the Invention
• The subject invention relates generally to the conditioning of air, and, more specifically, to conditioning air using evaporative cooling.
• 2. Description of the Prior Art
• 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.
• 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.
• There is a need for an improved vapor compression HVAC system that overcomes these and other disadvantages.
SUMMARY OF THE INVENTION AND ADVANTAGES
• 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
• $T_o=T_i-\left[\left(\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}\right)\left(\frac{\stackrel{.}{m_s}}{{\stackrel{.}{m}}_p}\right)\left({\omega }_s-{\omega }_i\right)\right].$
• 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
• $T_o=T_i-\left[\left(\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}\right)\left(\frac{\stackrel{.}{m_s}}{{\stackrel{.}{m}}_p}\right)\left({\omega }_s-{\omega }_i\right)\right].$
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 a heating and ventilating and air conditioning system according to the present invention;
• FIG. 2 is a psychrometric chart demonstrating the manner in which a system according to the present invention operates to cool air;
• FIG. 3 is an isometric view of an evaporative cooler according to a first exemplary embodiment the present invention;
• 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;
• FIG. 5 is an isometric view of an evaporative cooler showing an alternative aspect of the first exemplary embodiment;
• FIG. 6 is an isometric view of an evaporative cooler according to a second exemplary embodiment of the present invention; and
• FIG. 7 is a block diagram of a method of conditioning air according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
• 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.
• 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:
• $\begin{array}{cc}{T}_o=T_i-\left[\left(\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}\right)\left(\frac{\stackrel{.}{m_s}}{{\stackrel{.}{m}}_p}\right)\left({\omega }_s-{\omega }_i\right)\right]& \left(1\right)\end{array}$
• 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):
• $\begin{array}{cc}\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}=\{\begin{array}{c}4\text{,}364.6°R\mathrm{at}T_i=560°R\\ 4\text{,}297.5°R\mathrm{at}T_i=585°R\end{array}& \left(2\right)\end{array}$
• However, the difference in absolute humidity between the primary and secondary airstreams is largely dependent upon the incoming air temperature:
• $\begin{array}{cc}{\omega }_s-{\omega }_i=\{\begin{array}{c}0.0270{\mathrm{lb}}_mH_2O/{\mathrm{lb}}_m\mathrm{air}\mathrm{at}T_i=560°R\\ 0.0800{\mathrm{lb}}_mH_2O/{\mathrm{lb}}_m\mathrm{air}\mathrm{at}T_i=585°R\end{array}\hspace{1em}& \left(3\right)\\ T_o=T_i-\{\begin{array}{c}117.84\left(\frac{{\stackrel{.}{m}}_s}{{\stackrel{.}{m}}_p}\right)\mathrm{at}T_i=560°R\\ 343.80\left(\frac{{\stackrel{.}{m}}_s}{{\stackrel{.}{m}}_p}\right)\mathrm{at}T_i=585°R\end{array}\hspace{1em}& \left(4\right)\end{array}$
• 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.
• 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.
• 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.
• 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.
• 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 (17)

1. A method of conditioning air comprising the steps of;

providing intake air having a specific heat c_pa and an initial temperature T_i and an initial absolute humidity ω_i,

dividing the intake air into a primary airstream flowing at a primary mass flow rate {dot over (m)}_p and a secondary airstream,

flowing the secondary airstream transversely to the primary airstream at a secondary mass flow rate {dot over (m)}_s,

providing a liquid having a latent heat of evaporation h_fg to the secondary airstream,

extracting heat from the primary airstream,

transferring the extracted heat to the secondary airstream to evaporate the liquid into the secondary airstream having a secondary absolute humidity ω_s, and

varying the ratio of the secondary airstream to the primary airstream from the intake air to maintain an output temperature T_o of the primary airstream according to the equation

$T_o=T_i-\left[\left(\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}\right)\left(\frac{\stackrel{.}{m_s}}{{\stackrel{.}{m}}_p}\right)\left({\omega }_s-{\omega }_i\right)\right].$

2. A method as set forth in claim 1 wherein said varying the ratio is further defined as varying the airflow through a plurality of orifices fluidly connecting the primary and secondary airstreams.

3. A method as set forth in claim 2 wherein said varying the ratio is further defined as moving a cover to selectively diminish and increase the flow through said plurality of orifices.

4. A heating and ventilating and air conditioning (HVAC) system comprising;

an evaporative cooler for receiving a supply of intake air at a temperature T_i and a specific heat c_pa and an absolute humidity ω_i,

said evaporative cooler defining a plurality of dry channels for establishing a primary airstream having a primary mass flow rate {dot over (m)}_p,

said evaporative cooler defining a plurality of wet channels extending transversely to said dry channels for establishing a secondary airstream having an absolute humidity ω_s and a secondary mass flow rate {dot over (m)}_s,

a flow divider for dividing said intake air into said primary airstream and said secondary airstream,

a tank for providing a liquid having a latent heat of evaporation h_fg to said wet channels for evaporation into the secondary airstream, and

a controller for controlling said flow divider for varying the ratio of the secondary airstream to the primary airstream from the intake air to maintain an output temperature T_o of said primary airstream according to the equation

$T_o=T_i-\left[\left(\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}\right)\left(\frac{\stackrel{.}{m_s}}{{\stackrel{.}{m}}_p}\right)\left({\omega }_s-{\omega }_i\right)\right].$

5. A system as set forth in claim 4 wherein said wet channels are lined with a wicking material for drawing liquid from said tank into said wet channels.

6. A system as set forth in claim 5 wherein said flow divider includes a plurality of orifices fluidly connecting said dry and wet channels.

7. A system as set forth in claim 6 wherein said flow divider includes a cover movable by said controller for selectively diminishing and increasing the flow through said orifices.

8. A system as set forth in claim 7 wherein said controller includes an actuator engaged with said cover for moving said cover.

9. A system as set forth in claim 6 wherein said flow divider includes a slide cover movable by said controller for selectively obstructing and opening said orifices.

10. A system as set forth in claim 6 wherein said flow divider includes a hinge cover pivotably connected to said evaporative cooler vertically above said wet channels for selectively diminishing or increasing the flow through said wet channels.

11. A heating and ventilating and air conditioning (HVAC) system comprising:

an evaporative cooler for receiving a supply of intake air having an initial temperature of T_i and an absolute humidity ω_iand a specific heat c_pa,

said evaporative cooler defining a plurality of horizontal dry channels for establishing a primary airstream at a primary mass flow rate {dot over (m)}_p,

said evaporative cooler defining a plurality of wet channels extending transversely to said dry channels for establishing a secondary airstream having an absolute humidity of ω_s at a secondary mass flow rate {dot over (m)}_s,

a flow divider for dividing said intake air into said primary airstream and said secondary airstream,

an evaporator core downstream of said evaporative cooler for receiving the primary airstream at an output temperature T_o from said dry channels, a tank disposed under said wet channels for containing liquid having a latent heat of evaporation h_fg,

a wicking material disposed in said wet channels for distributing liquid from said tank into said wet channels,

a reservoir for collecting liquid condensate from said evaporator core, and

a controller for controlling said flow divider for varying the ratio of the secondary airstream to the primary airstream from the intake air to maintain the temperature T_o according to the equation

$T_o=T_i-\left[\left(\frac{h_{\mathrm{fg}}}{c_{\mathrm{pa}}}\right)\left(\frac{\stackrel{.}{m_s}}{{\stackrel{.}{m}}_p}\right)\left({\omega }_s-{\omega }_i\right)\right].$

12. A system as set forth in claim 11 wherein said flow divider comprises a plurality of orifices fluidly connecting said dry and wet channels .

13. A system as set forth in claim 12 wherein said flow divider includes a cover movable by said controller for selectively diminishing and increasing the flow through said orifices.

14. A system as set forth in claim 13 wherein said controller includes an actuator engaged with said cover for moving said cover.

15. A system as set forth in claim 12 wherein said flow divider includes a slide cover movable by said controller for selectively obstructing and opening said orifices.

16. A system as set forth in claim 12 wherein said flow divider includes a hinge cover pivotably connected to said evaporative cooler above said wet channels for selectively diminishing and increasing the flow through said wet channels.

17. A system as set forth in claim 16 wherein said flow divider includes at least one hinge disposed along an upper surface of said evaporative cooler and connected to said hinge cover.

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