【正文】 r pression cycle, a working fluid, which is called the refrigerant, evaporates and condenses at suitable pressures for practical equipment designs. The four basic ponents in every vapor pression refrigeration system are the pressor, condenser, expansion device, and evaporator. The pressor raises the pressure of the refrigerant vapor so that the refrigerant saturation temperature is slightly above the temperature of the cooling medium used in the condenser. The type of pressor used depends on the application of the system. Large electric chillers typically use a centrifugal pressor while small residential equipment uses a reciprocating or scroll pressor. The condenser is a heat exchanger used to reject heat from the refrigerant to a cooling medium. The refrigerant enters the condenser and usually leaves as a subcooled liquid. Typical cooling mediums used in condensers are air and water. Most residentialsized equipment uses air as the cooling medium in the condenser, while many larger chillers use water. After leaving the condenser, the liquid refrigerant expands to a lower pressure in the expansion valve. The expansion valve can be a passive device, such as a capillary tube or short tube orifice, or an active device, such as a thermal expansion valve or electronic expansion valve. The purpose of the valve is toregulate the flow of refrigerant to the evaporator so that the refrigerant is superheated when it reaches the suction of the pressor. At the exit of the expansion valve, the refrigerant is at a temperature below that of the medium (air or water) to be cooled. The refrigerant travels through a heat exchanger called the evaporator. It absorbs energy from the air or water circulated through the evaporator. If air 3 is circulated through the evaporator, the system is called a direct expansion system. If water is circulated through the evaporator, it is called a chiller. In either case, the refrigerant does not make direct contact with the air or water in the evaporator. The refrigerant is converted from a low quality, twophase fluid to a superheated vapor under normal operating conditions in the evaporator. The vapor formed must be removed by the pressor at a sufficient rate to maintain the low pressure in the evaporator and keep the cycle operating. All mechanical cooling results in the production of heat energy that must be rejected through the condenser. In many instances, this heat energy is rejected to the environment directly to the air in the condenser or indirectly to water where it is rejected in a cooling tower. With some applications, it is possible to utilize this waste heat energy to provide simultaneous heating to the building. Recovery of this waste heat at temperatures up to 65176。F) can be used to reduce costs for space heating. Capacities of air conditioning are often expressed in either tons or kilowatts (kW) of cooling. The ton is a unit of measure related to the ability of an ice plant to freeze one short ton (907 kg) of ice in 24 hr. Its value is kW (12,000 Btu/hr). The kW of thermal cooling capacity produced by the air conditioner must not be confused with the amount of electrical power (also expressed in kW) required to produce the cooling effect. Refrigerants Use and Selection Up until the mid1980s, refrigerant selection was not an issue in most building air conditioning applications because there were no regulations on the use of refrigerants. Many of the refrigerants historically used for building air conditioning applications have been chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Most of these refrigerants are nontoxic and nonflammable. However, recent . federal regulations (EPA 1993a。C (–47176。C (–35176。C (12176。C, 10176。 thus, R410A cannot be used as a dropin refrigerant for R22. R410A systems utilize pressors, expansion valves, and heat exchangers designed specifically for use with that refrigerant. Ammonia is widely used in industrial refrigeration applications and in ammonia water absorption chillers. It is moderately flammable and has a class B toxicity rating but has had limited applications in mercial buildings unless the chiller plant can be isolated from the building being cooled (Toth, 1994, Stoecker, 1994). As a refrigerant, ammonia has many desirable qualities. It has a high specific heat and high thermal conductivity. Its enthalpy of vaporization is typically 6 to 8 times higher than that of the monly used halocarbons, and it provides higher heat transfer pared to halocarbons. It can be used in both reciprocating and centrifugal pressors. Research is underway to investigate the use of natural refrigerants, such as carbon dioxide (R744) and hydrocarbons in air conditioning and refrigeration systems (Bullock, 1997, and Kramer, 1991). Carbon dioxide operates at much higher pressures than conventional HCFCs or HFCs and requires operation above the critical point in typical air conditioning applications. Hydrocarbon refrigerants, often thought of as too hazardous because of flammability, can be used in conventional pressors and have been used in industrial applications. R290, propane, has operating pressures close to R22 and has been proposed as a replacement for R22 (Kramer, 1991). Currently, there are no mercial systems sold in the . for building operations that use either carbon dioxide or flammable refrigerants. Chilled Water Systems Chilled water systems were used in less than 4% of mercial buildings in the . in 1995. However, because chillers are usually installed in larger buildings, chillers cooled over 28% of the . mercial building floor space that same year (DOE, 1998). Five types of chillers are monly applied to mercial buildings: reciprocating, screw, scroll, 6 centrifugal, and absorption. The first four utilize the vapor pression cycle to produce chilled water. They