ASHRAE 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 isbased on the upper safety limit for airborne exposure to the refrigerant. If the refrigerant is nontoxicin quantities less than 400 parts per million, it is a Class A refrigerant. If exposure to less than 400 partsper million is toxic, then the substance is given the B designation. The numerical designations referto the flammability of the refrigerant. 

Refrigerant 22 is an HCFC, is used in many of the same applications, and is still the refrigerant ofchoice in many reciprocating and screw chillers as well as small commercial and residential packagedequipment. It operates at a much higher pressure than either R-11 or R-12. Restrictions on the productionof HCFCs will start in 2004. In 2010, R-22 cannot be used in new air conditioning equipment. R-22cannot be produced after 2020 (EPA, 1993b).

R-407C and R-410A are both mixtures of HFCs. Both are considered replacements for R-22. R-407Cis expected to be a drop-in replacement refrigerant for R-22. Its evaporating and condensing pressuresfor air conditioning applications are close to those of R-22 (Table 4.2.3). However, replacement of R-22with R-407C should be done only after consulting with the equipment manufacturer. At a minimum,the lubricant and expansion device will need to be replaced. The first residential-sized air conditioningequipment using R-410A was introduced in the U.S. in 1998. Systems using R-410A operate at approximately50% higher pressure than R-22 (Table 4.2.3); thus, R-410A cannot be used as a drop-in refrigerantfor R-22. R-410A systems utilize compressors, expansion valves, and heat exchangers designed specificallyfor use with that refrigerant.

Ammonia is widely used in industrial refrigeration applications and in ammonia water absorptionchillers. It is moderately flammable and has a class B toxicity rating

but has had limited applications incommercial buildings unless the chiller plant can be isolated from the building being cooled (Toth, 1994,Stoecker, 1994). As a refrigerant, ammonia has many desirable qualities. It has a high specific heat andhigh thermal conductivity. Its enthalpy of vaporization is typically 6 to 8 times higher than that of thecommonly used halocarbons, and it provides higher heat transfer compared to halocarbons. It can beused in both reciprocating and centrifugal compressors.

Research is underway to investigate the use of natural refrigerants, such as carbon dioxide (R-744)and hydrocarbons in air conditioning and refrigeration systems (Bullock, 1997, and Kramer, 1991).Carbon dioxide operates at much higher pressures than conventional HCFCs or HFCs and requiresoperation above the critical point in typical air conditioning applications. Hydrocarbon refrigerants,often thought of as too hazardous because of flammability, can be used in conventional compressors andhave been used in industrial applications. R-290, propane, has operating pressures close to R-22 and hasbeen proposed as a replacement for R-22 (Kramer, 1991). Currently, there are no commercial systemssold in the U.S. for building operations that use either carbon dioxide or flammable refrigerants.

过去50年以来，空调得到了快速的发展，从曾经的奢侈品发展到可应用于大多数住宅和商业建筑的比较标准的系统。在 1970 年的美国， 36% 的住宅不是全空气调节就是利用一个房间空调器冷却；到1997年，这一数字达到了 77%，在那年作的第一次市场调查中可以发现，在美国有超过一半的住宅安装了中央空调系统 (人口普查局, 1999)。在1998年，83%的新建住宅安装了中央空调系统 ( 人口普查局, 1999)。中央空调在商业建筑物中也得到了快速的发展，从 1970年到1995年，有空调的商业建筑物占有率从54%增加到 73%(杰克森和詹森,1978)。

建筑物中的空气调节系统通常是利用机械设备或热交换设备完成.在大多数应用中，建筑物中的空调器为维持舒适要求必须既能制冷又能除湿，空调系统也用于其他的场所，例如汽车、卡车、飞机、船和工业设备，然而，在本章中，只说明空调在商业和住宅建筑中的应用。