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1、Air Conditioning SystemsAir conditioning has rapidly grown over the past 50 years, from a luxury to a standard system included in most residential and commercial buildings. In 1970, 36% of residences in the U.S. were either fully air conditioned or utilized a room air conditioner for cooling (Blue,
2、et al., 1979). By 1997, this number had more than doubled to 77%, and that year also marked the first time that over half (50.9%) of residences in the U.S. had central air conditioners (Census Bureau, 1999). An estimated 83% of all new homes constructed in 1998 had central air conditioners (Census B
3、ureau, 1999). Air conditioning has also grown rapidly in commercial buildings. From 1970 to 1995, the percentage of commercial buildings with air conditioning increased from 54 to 73% (Jackson and Johnson, 1978, and DOE, 1998).Air conditioning in buildings is usually accomplished with the use of mec
4、hanical or heat-activated equipment. In most applications, the air conditioner must provide both cooling and dehumidification to maintain comfort in the building. Air conditioning systems are also used in other applications, such as automobiles, trucks, aircraft, ships, and industrial facilities. Ho
5、wever, the description of equipment in this chapter is limited to those commonly used in commercial and residential buildings.Commercial buildings range from large high-rise office buildings to the corner convenience store. Because of the range in size and types of buildings in the commercial sector
6、, 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 engine
7、er. The residential building sector is dominated by single family homes and low-rise apartments/condominiums. The cooling equipment applied in these buildings comes in standard “packages” that are often both sized a by the air conditioning contractor.The chapter starts with a general discussion of t
8、he vapor compression refrigeration cycle then moves to refrigerants and their selection, followed by packaged Chilled Water System%1.1 Vapor Compression CycleEven though there is a large range in sizes and variety of air conditioning systems used in buildings, most systems utilize the vapor compress
9、ion 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 commerci
10、al system was produced in 1857 by James Harrison and D.E. Siebe.Besides vapor compression, there are two less common methods used to produce cooling in buildings: the absorption cycle and evaporative cooling. These are described later in the chapter. With the vapor compression cycle, a working fluid
11、, which is called the refrigerant, evaporates and condenses at suitable pressures for practical equipment designs.The four basic components in every vapor compression refrigeration system are the compressor, condenser, expansion device, and evaporator. The compressor raises the pressure of the refri
12、gerant vapor so that the refrigerant saturation temperature is slightly above the temperature of the cooling medium used in the condenser. The type of compressor used depends on the application of the system. Large electric chillers typically use a centrifugal compressor while small residential equi
13、pment uses a reciprocating or scroll compressor.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 residentia
14、l-sized 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,
15、 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 compressor.At the exit of the expansion valve, the refrige
16、rant 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 sys tfswater is circulated through the evaporator, it is called a chille in either case, the refrigerant does not make direct contact with the air or water in the evapor
