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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 commercial buildings. In 1970, 36% of residences in the U.S. were either fully 查四级成绩air conditioned or utilized a room air conditioner for cooling (Blue, et al., 1979). By 1997, this numbergkn 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 Bureau, 1999). Air conditioning has also grown rapidly in commercial buildings. From 1970 to 1995, the percentage of commercial buildings with workoutair conditioning incread from 54 to 73% (Jackson and Johnson, 1978, and DOE, 1998).
Air conditioning in buildings is usually accomplished with the u of mechanical or heat-acti
vated 监督的拼音equipment. In most applications, the air conditioner must provide both cooling and dehumidification to maintain comfort in the building. Air conditioning systems are also ud in other applications, such as automobiles, trucks, aircraft, ships, and industrial facilities. However, the description of equipment in this chapter is limited to tho commonly ud in commercial and residential buildings.
Commercial buildings range from large high-ri office buildings to the corner convenience store. Becau of the range in size and types of buildings in the commercial ctor, there is a wide variety of equipment applied in the 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 the systems requires a qualified engineer. The residential building ctor is dominated
by single family homes and low-ri apartments/condominiums. The cooling equipment applied in the buildings comes in standard “packages” that are often both sized and installed by the air conditioning 在线英语广播contractor.
The chapter starts with a general discussion of the vapor compression refrigeration cycle then moves to refrigerants and their lection, followed by packaged Chilled Water Systems。
1.1 Vapor Compression Cycle
Even though there is a large range in sizes and variety of air conditioning systems ud in buildings, most systems utilize the vapor compression cycle to produce the desired cooling and dehumidification. This cycle is also ud 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 commercial system was produced in 1857 by James Harrison and D.E. Siebe.Besides vapor compression, there are two less common methods ud to produce cooling in buildings:struggle什么意思 简介翻译the absorption cycle and evaporative cooling. The are described later in the chapter. With the vapor
compression cycle, a working fluid, which is called the refrigerant, evaporates and conde
ns at suitable翻译官结局 pressures for practical equipment designs.
The four basic components in every vapor compression refrigeration system are the compressor, condenr, expansion device, and evaporator. The compressor rais the pressure of the refrigerant vapor so that the refrigerant saturation temperature is slightly above the temperature of the cooling medium ud in the condenr. The type of compressor ud depends on the application of the system. Large electric chillers typically u a centrifugal compressor while small residential equipment us a reciprocating or scroll compressor.
The condenr is a heat exchanger ud to reject heat from the refrigerant to a cooling medium. The refrigerant enters the condenr and usually leaves as a subcooled liquid. Typical cooling mediums ud in condenrs are air and water. Most residential-sized equipment us air as the cooling medium in the condenr, while many larger chillers u water. After leaving the condenr, 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 purpo 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 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 ca, the refrigerant does not make direct contact with the air or water in the evaporator.
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