【正文】 ically 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 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 3 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。C (150176。F) and a condensation (dew) point of –37176。F). An azeotropic mixture behaves like a single ponent refrigerant in that the saturation temperature does not change appreciably as it evaporates or condenses at constant pressure. R410A has a small enough 4 temperature glide (less than 176。C) (39 to 45176。 whereas, reciprocating chillers have the worst efficiency of the four types. The efficiency numbers provided in the table are the steady state fullload efficiency determined in accordance to ASHRAE Standard 30 (ASHRAE, 1995). These efficiency numbers do not include the auxiliary equipment, such as pumps and cooling tower fans that can add from to kW/ton to the numbers shown Chillers run at part load capacity most of the time. Only during the highest thermal loads in the building will a chiller operate near its rated capacity. As a consequence, it is important to know how the efficiency of the chiller varies with part load capacity. a representative data for the efficiency (in kW/ton) as a function of percentage full load capacity for a reciprocating, screw, and scroll chiller plus a centrifugal chiller with inlet vane control and one with variable frequency drive (VFD) for the pressor. The reciprocating chiller increases in efficiency as it operates at a smaller percentage of full load. In contrast, the efficiency of a centrifugal with inlet vane control is relatively constant until theload falls to about 60% of its rated capacity and its kW/ton increases to almost twice its fully loaded value. In 1998, the Air Conditioning and Refrigeration Institute (ARI) developed a new standard that incorporates into their ratings part load performance of chillers (ARI 1998c). Part load efficiency is expressed by a single number called the integrated part load value (IPLV). The IPLV takes data similar to that in Figure and weights it at the 25%, 50%, 75%, and 100% loads to produce a single integrated 7 efficiency number. The weighting factors at these loads are , , , and , respectively. The equation to determine IPLV is: Most of the IPLV is determined by the efficiency at the 50% and 75% part load values. Manufacturers will provide, on request, IPLVs as well as part load efficiencies. The four pressors used in vapor pression chillers are each briefly described below. While centrifugal and screw pressors are primarily used in chiller applications, reciprocating and scroll pressors are also used in smaller unitary packaged air conditioners and heat pumps. Reciprocating Compressors The reciprocating pressor is a positive displacement pressor. On the intake stroke of the piston, a fixed amount of gas is pulled into the cylinder. On the pression stroke, the gas is pressed until the discharge valve opens. The quantity of gas pressed on each stroke is equal to the displacement of the cylinder. Compressors used in chillers have multiple cylinders, depending on the capacity of the pressor. Reciprocating pressors use refrigerants with low specific volumes and relatively high pressures. Most reciprocating chillers used in building applications currently employ R22. Modern highspeed reciprocating pressors are generally limited to a pressure ratio of approximately nine. The reciprocating pressor is basically a constantvolume variablehead machine. It handles various discharge pressures with relatively small changes in inletvolume flow rate as shown by the heavy line (labeled 16 cylinders).Condenser operation in many chillers is related to ambient conditions, for example, through cooling towers, so that on cooler days the condenser pressure can be reduced. When the air conditioning load is lowered, less refrige