【正文】 只有在建筑物的最高熱負(fù)荷時，制冷機(jī)才在額定制冷量附近運(yùn)行。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）。在設(shè)計中， 這種系統(tǒng)所使用的壓縮機(jī)也有往復(fù)式、螺桿式、旋渦式和離心式。 制冷機(jī)組是冷水機(jī)組。前四種利用蒸汽壓縮式循環(huán)來制得冷凍水。作為制冷劑，氨有許多良好的品質(zhì)，例如，它有較高的比熱和高的導(dǎo)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(fù)式和離心式壓縮機(jī)中。從 2020開始， HCFCs的制造將會受到限制。 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(zhì)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。 F)和一個凝結(jié)點 (露點 )是 – 37176。在一些應(yīng)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。 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(jī)、冷凝器、節(jié)流裝置和蒸發(fā)器。中央空調(diào)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(diào)的商業(yè)建筑物的百 分比從 54%增加到 73%(杰克森和詹森 ,1978)。 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。C (–47176。C (150176。F). An azeotropic mixture behaves like a single ponent refrigerant in that the saturation temperature does not change appreciably as it evaporates or condenses at constant pressure. R410A has a small enough temperature glide (less than 176。 whereas, reciprocating chillers have the worst efficiency of the four types. The efficiency numbers provided in the table are the steady state fullload efficiency determined in accordance to ASHRAE Standard 30 (ASHRAE, 1995). These efficiency numbers do not include the auxiliary equipment, such as pumps and cooling tower fans that can add from to kW/ton to the numbers shown Chillers run at part load capacity most of the time. Only during the highest thermal loads in the building will a chiller operate near its rated capacity. As a consequence, it is important to know how the efficiency of the chiller varies with part load capacity. a representative data for the efficiency (in kW/ton) as a function of percentage full load capacity for a reciprocating, screw, and scroll chiller plus a centrifugal chiller with inlet vane control and one with variable frequency drive (VFD) for the pressor. The reciprocating chiller increases in efficiency as it operates at a smaller percentage of full load. In contrast, the efficiency of a centrifugal with inlet vane control is relatively constant until theload falls to about 60% of its rated capacity and its kW/ton increases to almost twice its fully loaded value. In 1998, the Air Conditioning and Refrigeration Institute (ARI) developed a new standard that incorporates into their ratings part load performance of chillers (ARI 1998c). Part load efficiency is expressed by a single number called the integrated part load value (IPLV). The IPLV takes data similar to that in Figure and weights it at the 25%, 50%, 75%, and 100% loads to produce a single integrated efficiency number. The weighting factors at these loads are , , , and , respectively. The equation to determine IPLV is: 8 Most of the IPLV is determined by the efficiency at the 50% and 75% part load values. Manufacturers will provide, on request, IPLVs as well as part load efficiencies. The four pressors used in vapor pression chillers are each briefly described below. While centrifugal and screw pressors are primarily used in chiller applications, reciprocating and scroll pressors are also used in smaller unitary packaged air conditioners and heat pumps. Reciprocating Compressors The reciprocating pressor is a positive displa