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托福考试官方网站Air Conditioning Systems
Dennis L. O’Neal and John A. Bryant
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 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 (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 air conditioning incread from 54 to 73% (Jackson and Johnson, 1978, and DOE, 19
98).
Air conditioning in buildings is usually accomplished with the u 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 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” tha t are often both sized and installed by the air conditioning contractor.
The chapter starts with a general discussion of the vapor compression refrigeration cycle then moves to refrigerants and their lection. Chillers and their auxiliary systems are then covered, followed by packaged air conditioning equipment.
4.2.1 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 cpeppapig
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angel darkonditioning. 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 (Thevenot 1979).
Besides vapor compression, there are two less common methods ud to produce cooling in buildings: the absorption cycle and evaporative cooling. The are described later in the chapter. With the vapor
陌路人英文版compression cycle, a working fluid, which is called the refrigerant, evaporates and condens at suitable pressures for practical equipment designs.
The four basic components (Figure 4.2.1) 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.
The refrigerant is converted from a low quality, two-pha 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.
All mechanical cooling results in the production of heat energy that must be rejected through the condenr. In many instances, this heat energy is rejected to the environment directly to the air in the condenr 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 65°C (150°F) can be ud to reduce costs for space heating.
Capacities of air conditioning are often expresd in either tons or kilowatts (kW) of cooling. The ton is a unit of measure related to the ability of an ice plant to freeze one short ton (907 kg) of ice in 24 hr. Its value is 3.51 kW (12,000 Btu/hr). The kW of thermal cooling capacity produced by the air conditioner must not be confud with the amount of electrical power (also expresd in kW) required to produce the cooling effect.
littlebear4.2.2 Refrigerants U and Selection
Up until the mid-1980s, refrigerant lection was not an issue in most building air conditioning applic
ations becau there were no regulations on the u of refrigerants. Many of the refrigerants historically ud for building air conditioning applications have been chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Most of the refrigerants are nontoxic and nonflammable. However, recent U.S. federal regulations (EPA 1993a; EPA 1993b) and international agreements (UNEP, 1987) have placed restrictions on the production and u of CFCs and HCFCs. Hydrofluorocarbons (HFCs) are now being ud in some applications where CFCs and HCFCs
were ud. Having an understanding of refrigerants can help a building owner or engineer make a more informed decision about the best choice of refrigerants for specific applications. This ction discuss the different refrigerants ud in or propod for building air conditioning applications and the regulations affecting their u.
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The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has a standard numbering system (Table 4.2.1) for identifying refrigerants (ASHRAE, 1992). Many popular CFC, HCFC, and HFC refrigerants are in the methane and ethane ries of refrigerants. They are called halocarbons, or halogenated hydrocarbons, becau of the prence of halogen elements such as fluorine or chlorine (King, 1986).
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Zeotropes and azeotropes are mixtures of two or more different refrigerants. A zeotropic mixture changes saturation temperatures as it evaporates (or condens) at constant pressure. The phenomena is called temperature glide. At atmospheric pressure, R-407C has a boiling (bubble) point of –44°C (–47°F) and a condensation (dew) point of –37°C (–35°F), which gives it a temperature glide of 7°C (12°F). An azeotropic mixture behaves like a single component refrigerant in that the saturation temperature does not change appreciably as it evaporates or condens at constant pressure. R-410A has a small enough temperature glide (less than 5.5°C, 10°F) that it is considered a near-azeotropic refrigerant mixture.
rejectsASHRAE groups refrigerants (Table 4.2.2) by their toxicity and flammability (ASHRAE, 1994).Group A1 is nonflammable and least toxic, while Group B3 is flammable and most toxic. Toxicity is bad on the upper safety limit for airborne exposure to the refrigerant. If the refrigerant is nontoxic in quantities less than 400 parts per million, it is a Class A refrigerant. If exposure to less than 400 parts per million is toxic, then the substance is given the B designation. The numerical designations refer to the flammability of the refrigerant. The last column of Table 4.2.1 shows the toxicity and flammability rating of common refrigerants.
Refrigerant 22 is an HCFC, is ud in many of the same applications, and is still the refrigerant
