【正文】 rheated when it reaches the suction of the pressor. 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 3 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 quality, twophase fluid to a superheated vapor under normal operating conditions in the evaporator. The vapor formed must be removed by the pressor 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 condenser. In many instances, this heat energy is rejected to the environment directly to the air in the condenser 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 65176。C (–47176。C (12176。 thus, R410A cannot be used as a dropin refrigerant for R22. R410A systems utilize pressors, expansion valves, and heat exchangers designed specifically for use with that refrigerant. Ammonia is widely used in industrial refrigeration applications and in ammonia water absorption chillers. It is moderately flammable and has a class B toxicity rating but has had limited applications in mercial 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 and high thermal conductivity. Its enthalpy of vaporization is typically 6 to 8 times higher than that of the monly used halocarbons, and it provides higher heat transfer pared to halocarbons. It can be used in both reciprocating and centrifugal pressors. Research is underway to investigate the use of natural refrigerants, such as carbon dioxide (R744) 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 requires operation 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 pressors and have been used in industrial applications. R290, propane, has operating pressures close to R22 and has been proposed as a replacement for R22 (Kramer, 1991). Currently, there are no mercial systems sold in the . for building operations that use either carbon dioxide or flammable refrigerants. Chilled Water Systems Chilled water systems were used in less than 4% of mercial buildings in the . in 1995. However, because chillers are usually installed in larger buildings, chillers cooled over 28% of the . mercial building floor space that same year (DOE, 1998). Five types of chillers are monly applied to mercial buildings: reciprocating, screw, scroll, 6 centrifugal, and absorption. The first four utilize the vapor pression cycle to produce chilled water. They differ primarily in the type of pressor used. Absorption chillers utilize thermal energy (typically steam or bustion source) in an absorption cycle with either an ammoniawater or waterlithium bromide solution to produce chilled water. Overall System An estimated 86% of chillers are applied in multiple chiller arrangements like that shown in the figure (Bitondo and Tozzi, 1999). In chilled water systems, return water from the building is circulated through each chiller evaporator where it is cooled to an acceptable temperature (typically 4 to 7176。 5000 tons).Chillers can utilize either an HCFC (R22 and R123) or HFC (R134a) refrigerant. The steady state efficiency of chillers is often stated as a ratio of the power input (in kW) to the chilling capacity (in tons). A capacity rating of one ton is equal to kW or 12,000 btu/h. With this measure of efficiency, the smaller number is better. centrifugal chillers are the most efficient。中央空調在商業(yè)建筑物中也得到了快速的發(fā)展，從 1970年到 1995年，有空調的商業(yè)建筑物的百 分比從 54%增加到 73%(杰克森和詹森 ,1978)。居住的建筑物（即研究對象）被劃分成單獨的家庭或共有式公寓，應用于這些建筑物的冷卻設備通常都是標準化組裝的，由空調廠家進行設計尺寸和安裝。 13 每個蒸汽壓縮制冷系統(tǒng)中都有四大部件，它們是壓縮機、冷凝器、節(jié)流裝置和蒸發(fā)器。膨脹閥是一個節(jié)流的裝置，例如毛細管或有孔的短管，或一個活動的裝置，例如熱力膨脹閥或電子膨脹閥，膨脹閥的作用是到蒸發(fā)器中分流制冷劑以便當它到壓縮物吸入口的時候 , 制冷劑處于過熱狀 態(tài)，在膨脹閥的出口，制冷劑的溫度在介質 (空氣或水 ) 的溫度以下。在一些應用中，利用這些廢熱向建筑物提供熱量是可能的，回收這些最高溫度為 65℃ (150176。 美國社會的供暖、制冷和空調工程師學會 (ASHRAE)有一個標準的限制系統(tǒng) (表 )用來區(qū)分制冷劑，許多流行的氟氯碳化物，氟氯碳氫化物和氟碳化物的制冷劑是在甲烷和乙烷的制冷劑系列中，因為鹵素元素的存在他們被叫作碳化鹵或鹵化的碳化氫，例如氟或氯。 F)和一個凝結點 (露點 )是 – 37176。 F)，一個 azeotropic 混合物的性能像單獨成份制冷劑那樣，它在不變的壓力下蒸發(fā)或冷凝它們的飽和溫度不會有少許變化。 A1組合是不燃燒的和最沒有毒的，而 B3組是易燃的和最有毒的，以空氣為媒介的制冷 劑最高安全限制是毒性，如果制冷劑在少于每百萬分之 400是無毒的，它是一個 A級制冷劑，如果對泄露少于每百萬分之 400是有毒的 ,那么該物質被稱 B級制冷劑，這幾個級別表示制冷劑的易燃性，表 的最后一欄列出了常用的制冷劑的毒性和易燃的等級。在對氟氯碳化物的制造的禁令頒布之前 , R11和 R12已經是冷卻設備的首選制冷劑，在已存在的系統(tǒng)維護中，現(xiàn)在這兩種制冷劑的使用已經被限制，現(xiàn)在， R123 和 R134a都廣泛的用在新的冷卻設備中。從 2020開始， HCFCs的制造將會受到限制。然而，用R407C來替換 R22應該在和設備制造者商議之后才能進行，至少潤滑油和膨脹裝置將需要更換。作為制冷劑，氨有許多良好的品質，例如，它有較高的比熱和高的導熱率，它的蒸發(fā)焓通常比那普遍使用的鹵化碳高 6到 8倍，而且氨和鹵化碳比較來看，它能提供更高的熱交換量，而且它能用在往復式和離心式壓縮機中。目前，在美國沒有用二氧化碳或可燃的制冷劑的商業(yè)系統(tǒng)用于建筑部門。前四種利用蒸汽壓縮式循環(huán)來制得冷凍水。在冷凍水系統(tǒng)中，建筑物的回水通過每個蒸發(fā)器循環(huán)流動，在蒸發(fā)器中，回水被冷卻到合意的溫度（典型的為 4～ 7℃－）（ 39～ 45℉）。 制冷機組是冷水機組。冷凝器也可以是空冷式的。在設計中， 這種系統(tǒng)所使用的壓縮機也有往復式、螺桿式、旋渦式和離心式。在所需制冷量小于 1000kw（ 280tons）時，使用往復式或螺桿式制冷機組較合適。各種系統(tǒng)的制冷能力范圍從最小的旋渦式（ 30kw， 8tons）到最大的離心式（ 18000kw， 5000tons）。用這種方法衡量效率，其數(shù)值越小越好。只有在建筑物的最高熱負荷時，制冷機才在額定制冷量附近運行。相反地，帶葉片控制的離心式的效率在負 荷為額定負荷的 60％以后是基本不變的 ,它的 kw/ton值隨百分