外文翻译-空调系统.doc

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. Air conditioning has also grown rapidly in commercial buildings. From 1970 to 1995, the percentage of commercial buildings with

2、 air conditioning increased from 54 to 73%.Air conditioning in buildings is usually accomplished with the use of mechanical or heat-activated equipment. In most applications, the air conditioner must provide both cooling and dehumidification to maintain comfort in the building. Air conditioning syst

3、ems 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 commonly used in commercial and residential buildings. Commercial buildings range from large high-rise office bu

4、ildings to the corner convenience store. Because of the range in size and types of buildings in the commercial 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

5、piping system, air distribution system, and cooling tower. 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 and installed by the air condit

6、ioning contractor.The chapter starts with a general discussion of the vapor compression refrigeration cycle then moves to refrigerants and their selection, followed by packaged Chilled Water Systems.1. Vapor Compression CycleEven though there is a large range in sizes and variety of air conditioning

7、 systems used in buildings, most systems utilize the vapor compression 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 issue

8、d 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 used to produce cooling in buildings: the absorption cycle and evaporative cooling. These are described late

9、r in the chapter. With the vapor compression cycle, a working fluid, 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 devi

10、ce, and evaporator. The compressor 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 compressor used depends on the application of the system. Large electric chillers

11、typically use a centrifugal compressor while small residential equipment 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 co

12、oling mediums used in condensers are air and water. Most residential-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 b

13、e 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 suctio

14、n 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

15、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 qu

16、ality, two-phase fluid to a superheated vapor under normal operating conditions in the evaporator. The vapor formed must be removed by the compressor at a sufficient rate to maintain the low pressure in the evaporator and keep the cycle operating.2. Refrigerants Use and SelectionUp until the mid-1980s, 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