【正文】 英文 文獻(xiàn) Air Conditioning Systems Air conditioning has rapidly grown over the past 50 years, from a luxury to a standard system included in most residential and mercial buildings. In 1970, 36% of residences in the . were either fully air conditioned or utilized a room air conditioner for cooling (Blue, et al., 1979). By 1997, this number had more than doubled to 77%, and that year also marked the first time that over half (%) of residences in the . had central air conditioners (Census Bureau, 1999). An estimated 83% of all new homes constructed in 1998 had central air conditioners (Census Bureau, 1999). Air conditioning has also grown rapidly in mercial buildings. From 1970 to 1995, the percentage of mercial buildings with air conditioning increased from 54 to 73% (Jackson and Johnson, 1978, and DOE, 1998). Air conditioning in buildings is usually acplished with the use of mechanical or heatactivated equipment. In most applications, the air conditioner must provide both cooling and dehumidification to maintain fort in the building. Air conditioning systems are also used in other applications, such as automobiles, trucks, aircraft, ships, and industrial facilities. However, the description of equipment in this chapter is limited to those monly used in mercial and residential buildings. Commercial buildings range from large highrise office buildings to the corner convenience store. Because of the range in size and types of buildings in the mercial sector, there is a wide variety of equipment applied in these buildings. For larger buildings, the air conditioning equipment is part of a total system design that includes items such as a piping system, air distribution system, and cooling tower. Proper design of these systems requires a qualified engineer. The residential building sector is dominated by single family homes and lowrise apartments/condominiums. The cooling equipment applied in these buildings es in standard ―packages‖ that are often both sized and installed by the air conditioning contractor. The chapter starts with a general discussion of the vapor pression refrigeration cycle then moves to refrigerants and their selection, followed by packaged Chilled Water Systems。 2 Vapor Compression Cycle Even though there is a large range in sizes and variety of air conditioning systems used in buildings, most systems utilize the vapor pression cycle to produce the desired cooling and dehumidification. This cycle is also used for refrigerating and freezing foods and for automotive air conditioning. The first patent on a mechanically driven refrigeration system was issued to Jacob Perkins in 1834 in London, and the first viable mercial system was produced in 1857 by James Harrison and . vapor pression, there are two less mon methods used to produce cooling in buildings: the absorption cycle and evaporative cooling. These are described later in the chapter. With the vapor 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。C (150176。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 rel