The present invention relates to an air-conditioning system and more particularly relates to an air conditioning system having heat pipes which increase the dehumidification capacity or the capacity and efficiency of the system.
Air conditioning requirements differ greatly with variations in geographic location, weather conditions, building design, human occupancy, etc. In order to respond to such variations, an air conditioning system must be capable of responding to a wide range of cooling loads and widely different sensible heat ratios (SHR). In general, the heaviest loads with the highest SHR usually occur on hot, sunny days (hot and dry hours), while higher latent loads occur during cooler, more humid hours such as in the early morning and evening hours or following a rain (cool and humid hours). During these cool and humid hours, the load on the building is somewhat lower than in the hot and dry hours. However, air-conditioning is still needed to remove sufficient moisture from the air to reduce the humidity to comfortable levels.
It is known to use a single over-sized air conditioner for both hot and dry hours and for cool and humid hours. The typical system utilizes an air conditioner that is sufficiently large to provide cooling during the hot and dry hours and to provide dehumidification during cool and humid hours by over-cooling the air. However, though the resulting air is sufficiently dehumidified, it is also uncomfortably cool. The excessively cooled air must then be reheated by a heater to comfortable levels. This process is extremely inefficient since the air conditioner must be run excessively to dehumidify the air and since even more energy must be expended to reheat the over-cooled air.
It is also known that in order to obtain higher efficiencies, the most common practice in designing air conditioners is to raise the evaporation temperatures by using an oversized evaporator coil. However, higher evaporator temperatures drastically reduce the ability of the coil to condense moisture from the air. On the other hand, using a smaller evaporator operating at lower temperatures increases the moisture condensing capacity of the system, but results in a loss of efficiency and cooling capacity.
In order to increase the dehumidification capability of air-conditioning systems, passive heat-pipe heat exchangers have been proposed which increase the dehumidification capacity of the system without employing a supplemental heater to reheat the air. One such system is disclosed in U.S. Pat. No. 4,607,498, which issued to Khanh Dinh on Aug. 26, 1986, the subject matter of which is incorporated herein by reference. With reference to FIG. 1, this system includes a housing 12 having an inlet 14, an outlet 16, and a blower 20 which draws air through the housing in a direction indicated by arrow 22. A coil 24 is disposed in the housing. This coil can be either an evaporator or chilled water coil or a condenser or hot water coil depending on whether the system is being used for heating or cooling.
A heat pipe heat exchanger 26 is also provided in the housing to provide for added dehumidification during cool and humid hours. The heat pipe heat exchanger comprises an evaporator coil 28 located upstream of coil 24 and a condenser coil 30 located downstream of coil 24. Evaporator 28 has a vapor outlet 32 which is connected through tubing 34 to a vapor inlet 36 of condenser coil 30. A liquid refrigerant outlet 38 of condenser coil 30 is connected through tubing 40 to a liquid inlet 42 of evaporator 28.
In operation, when member 24 acts as an evaporator so that system functions as an air-conditioner, liquid refrigerant disposed in evaporator 28 absorbs heat from air flowing over the evaporator and vaporizes, while concurrently cooling the air. The vaporized refrigerant rises out of the evaporator 28 and enters the condenser coil 30. Air, which has been additionally cooled and dehumidified via contact with evaporator 24, removes heat from the vaporized refrigerant in condenser 30 so that the air is warmed to a more comfortable level while condensing the refrigerant in condenser 30. The liquid refrigerant then flows back into the evaporator 28 through the tube 40, where the cycle is repeated.
If the member 24 is used as a condenser so that the system functions as a heater, the air flowing into the evaporator 28 will be cooler than the air flowing out of condenser 30. Accordingly, the vaporization/condensation cycle of the heat pipe heat exchanger 26 will not occur, and the exchanger 26 will not affect the temperature of the air or the dehumidification capacity of the system.
It can thus be seen that the heat pipe heat exchanger increases the dehumidification capacity of the system during cool and humid hours by precooling the air entering the evaporator 26 so that the air exiting evaporator is further cooled and thus further dehumidified, and then reheats the over-cooled air exiting the evaporator 26 to a comfortable level. The dehumidification capacity is enlarged by lowering the operating temperature of the evaporator at a cost of a decrease in the overall capacity and efficiency of the system. The higher latent capacity and lower total capacity are desirable during cool and humid hours, but are not desirable during hot and dry hours since the maximum cooling capacity of the system is required to cool the hot air and since less dehumidification capacity is required to dehumidify the relatively dry air. It is therefore desirable to neutralize the effect of the heat-pipe heat exchanger while air-conditioning during hot and dry hours.
One possible way of neutralizing the effect of a heat pipe heat exchanger is to interrupt the flow of the working fluid to the heat pipes, for example through the use of a solenoid valve in the line connecting the condenser coil to the evaporator coil. Although this solution would work, it requires a separate valve for each circuit, thereby increasing the complexity of the cost of the system if multiple circuits are used in the system.
Moreover, since there is little appreciable pressure difference within the heat-pipe exchanger system, a special valve must be used which can be activated without a pressure differential. This type of valve is much more expensive than a conventional pilot-actuated valve which utilizes a pressure differential across the valve to aid in the switching operation.
Moreover, when the heat-pipes are deactivated in this manner, both the condenser coil and the evaporator coil are de-energized, performing no useful work for the system. If, on the other hand, the heat pipes are used as a secondary evaporator, the system could have extra evaporator capacity and can operate at a higher evaporation temperature and pressure, resulting in increased efficiency and capacity.