空调与制冷技术英文文献和中文翻译(2)

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The expansion valve can be a passive device, such as a capillary tube or short tube orifice, or an activedevice, 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 reachesthe suction of the compressor.

At the exit of the expansion valve, the refrigerant is at a temperature below that of the medium (air orwater) to be cooled. The refrigerant travels through a heat exchanger called the evaporator. It absorbsenergy 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

achiller. 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, two-phase fluid to a superheated vapor under normaloperating conditions in the evaporator. The vapor formed must be removed by the compressor at asufficient 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 thecondenser. In many instances, this heat energy is rejected to the environment directly to the air in thecondenser or indirectly to water where it is rejected in a cooling tower. With some applications, it ispossible to utilize this waste heat energy to provide simultaneous heating to the building. Recovery ofthis waste heat at temperatures up to 65°C (150°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 tonis 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 conditionermust not be confused with the amount of electrical power (also expressed in kW) required to producethe cooling effect.

Refrigerants Use and Selection

Up until the mid-1980s, refrigerant selection was not an issue in most building air conditioning applicationsbecause there were no regulations on the use of refrigerants. Many of the refrigerants historicallyused for building air conditioning applications have been chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons(HCFCs). Most of these refrigerants are nontoxic and nonflammable. However,recent U.S. federal regulations (EPA 1993a; EPA 1993b) and international agreements (UNEP, 1987) haveplaced restrictions on the production and use of CFCs and HCFCs. Hydrofluorocarbons (HFCs) are nowbeing used in some applications where CFCs and HCFCs were used. Having an understanding ofrefrigerants can help a building owner or engineer make a more informed decision about the best choiceof refrigerants for specific applications. This section discusses the different refrigerants used in or proposedfor building air conditioning applications and the regulations affecting their use.

The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has astandard numbering system (Table 4.2.1) for identifying refrigerants (ASHRAE, 1992). Many popularCFC, HCFC, and HFC refrigerants are in the methane and ethane series of refrigerants. They are calledhalocarbons, or halogenated hydrocarbons, because of the presence of halogen elements such as fluorineor

chlorine (King, 1986).

Zeotropes and azeotropes are mixtures of two or more different refrigerants. A zeotropic mixture changessaturation temperatures as it evaporates (or condenses) at constant pressure. The phenomena is calledtemperature glide. At atmospheric pressure, R-407C has a boiling (bubble) point of –44°C (–47°F) and acondensation (dew) point of –37°C (–35°F), which gives it a temperature glide of 7°C (12°F). An azeotropicmixture behaves like a single component refrigerant in that the saturation temperature does not changeappreciably as it evaporates or condenses at constant pressure. R-410A has a small enough temperatureglide (less than 5.5°C, 10°F) that it is considered a near-azeotropic refrigerant mixture.