暖通空调专业外文翻译--空调系统

英文文献

Air Conditioning Systems

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 increased from 54 to 73% (Jackson and Johnson, 1978, and DOE, 1998).

Air conditioning in buildings is usually accomplished with the use 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 used in other applications, such as automobiles, trucks, aircraft, ships, and industrial facilities. However, the description of equipment in this chapter is limited to those commonly used in commercial and residential buildings.

Commercial buildings range from large high-rise office buildings to the corner convenience store. Because of the range in size and types of buildings in the commercial sector, there is a wide variety of equipment applied in these 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 these systems requires a qualified engineer. The residential building sector is dominated by single family homes and low-rise apartments/condominiums. The cooling equipment applied in these buildings comes in standard ―packages‖ that 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 selection, followed by packaged Chilled Water Systems。

1.1 Vapor Compression Cycle

Even though there is a large range in sizes and variety of air conditioning systems used in buildings, most systems utilize the vapor compression cycle to produce the desired cooling and dehumidification. This cycle is also used for refrigerating and freezing foods and for automotive air conditioning. 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.Besides vapor compression, there are two less common methods used to produce cooling in buildings: the absorption cycle and evaporative cooling. These are described later in the chapter. With the vapor

compression cycle, a working fluid, which is called the refrigerant, evaporates and condenses at suitable pressures for practical equipment designs.

The four basic components in every vapor compression refrigeration system are the compressor, condenser, expansion device, and evaporator. The compressor raises the pressure of the refrigerant vapor so that the refrigerant saturation temperature is slightly above the temperature of the cooling medium used in the condenser. The type of compressor used depends on the application of the system. Large electric chillers typically use a centrifugal compressor while small residential equipment uses a reciprocating or scroll compressor.

The condenser is a heat exchanger used to reject heat from the refrigerant to a cooling medium. The refrigerant enters the condenser and usually leaves as a subcooled liquid. Typical cooling mediums used in condensers are air and water. Most residential-sized equipment uses air as the cooling medium in the condenser, while many larger chillers use water. After leaving the condenser, 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 purpose 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 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 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 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 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 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 confused with the amount of electrical power (also expressed in kW) required to produce the cooling effect.

2.1 Refrigerants Use and Selection

Up until the mid-1980s, refrigerant selection was not an issue in most building air conditioning applications because there were no regulations on the use of refrigerants. Many of the refrigerants historically used 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) have placed restrictions on the production and use of CFCs and HCFCs. Hydrofluorocarbons (HFCs) are now being used in some applications where CFCs and HCFCs were used. 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 section discusses the different refrigerants used in or proposed for building air conditioning applications and the regulations affecting their use.

The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has a standard numbering system,for identifying refrigerants (ASHRAE, 1992). Many popular CFC, HCFC, and HFC refrigerants are in the methane and ethane series of refrigerants. They are called halocarbons, or halogenated hydrocarbons, because of the presence of halogen elements such as fluorine or chlorine (King, 1986).

Zeotropes and azeotropes are mixtures of two or more different refrigerants. A zeotropic mixture changes saturation temperatures as it evaporates (or condenses) 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 condenses 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.

ASHRAE groups refrigerants by their toxicity and flammability (ASHRAE, 1994).Group A1 is nonflammable and least toxic, while Group B3 is flammable and most toxic. Toxicity is based 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 used in many of the same applications, and is still the refrigerant of choice in many reciprocating and screw chillers as well as small commercial and residential packaged equipment. It operates at a much higher pressure than either R-11 or R-12. Restrictions on the production of HCFCs will start in 2004. In 2010, R-22 cannot be used in new air conditioning equipment. R-22 cannot be produced after 2020 (EPA, 1993b).

R-407C and R-410A are both mixtures of HFCs. Both are considered replacements for R-22. R-407C is expected to be a drop-in replacement refrigerant for R-22. Its evaporating and condensing pressures for air conditioning applications are close to those of R-22 (Table 4.2.3). However, replacement of R-22 with 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 conditioning equipment using R-410A was introduced in the U.S. in 1998. Systems using R-410A operate at approximately 50% higher pressure than R-22 (Table 4.2.3); thus, R-410A cannot be used as a drop-in refrigerant for R-22. R-410A systems utilize compressors, 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 commercial 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 commonly used halocarbons, and it provides higher heat transfer compared to halocarbons. It can be used 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 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 compressors and have been used in industrial applications. R-290, propane, has operating pressures close to R-22 and has been proposed as a replacement for R-22 (Kramer, 1991). Currently, there are no commercial systems sold in the U.S. for building operations that use either carbon dioxide or flammable refrigerants.

3.1 Chilled Water Systems

Chilled water systems were used in less than 4% of commercial buildings in the U.S. in 1995. However, because chillers are usually installed in larger buildings, chillers cooled over 28% of the U.S. commercial building floor space that same year (DOE, 1998). Five types of chillers are commonly applied to commercial buildings: reciprocating, screw, scroll, centrifugal, and absorption. The first four utilize the vapor compression cycle to produce chilled water. They differ primarily in the type of compressor used. Absorption chillers utilize thermal energy (typically steam or combustion source) in an absorption cycle with either an ammonia-water or water-lithium bromide solution to produce chilled water.

3.2 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 7°C) (39 to 45°F). The chilled water is then distributed to water-to-air heat exchangers spread throughout the facility. In these heat exchangers, air is cooled and dehumidified by the cold water. During the process, the chilled water increases in temperature and must be returned to the chiller(s).

The chillers are water-cooled chillers. Water is circulated through the condenser of each chiller where it absorbs heat energy rejected from the high pressure refrigerant. The water is then pumped to a cooling tower where the water is cooled through an evaporation process. Cooling towers are described in a later section. Chillers can also be air cooled. In this configuration, the condenserwould be a refrigerant-to-air heat exchanger with air absorbing the heat energy rejected by the high pressure refrigerant.

Chillers nominally range in capacities from 30 to 18,000 kW (8 to 5100 ton). Most chillers sold in the U.S. are electric and utilize vapor compression refrigeration to produce chilled water. Compressors for these systems are either reciprocating, screw, scroll, or centrifugal in design. A small number of centrifugal

chillers are sold that use either an internal combustion engine or steam drive instead of an electric motor to drive the compressor.

The type of chiller used in a building depends on the application. For large office buildings or in

chiller plants serving multiple buildings, centrifugal compressors are often used. In applications under 1000 kW (280 tons) cooling capacities, reciprocating or screw chillers may be more appropriate. In smaller applications, below 100 kW (30 tons), reciprocating or scroll chillers are typically used.

3.3 Vapor Compression Chillers

The nominal capacity ranges for the four types of electrically driven vapor compression chillers. Each chiller derives its name from the type of compressor used in the chiller. The systems range in capacities from the smallest scroll (30 kW; 8 tons) to the largest centrifugal (18,000 kW; 5000 tons).Chillers can utilize either an HCFC (R-22 and R-123) or HFC (R-134a) 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 3.52 kW or 12,000 btu/h. With this measure of efficiency, the smaller number is better. centrifugal chillers are the most efficient; whereas, reciprocating chillers have the worst efficiency of the four types. The efficiency numbers provided in the table are the steady state full-load 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 0.06 to 0.31 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 compressor. 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 4.2.3 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 0.12, 0.45, 0.42, and 0.01, respectively. The equation to determine IPLV is:

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 compressors used in vapor compression chillers are each briefly described below. While centrifugal and screw compressors are primarily used in chiller applications, reciprocating and scroll compressors are also used in smaller unitary packaged air conditioners and heat pumps.

3.4 Reciprocating Compressors

The reciprocating compressor is a positive displacement compressor. On the intake stroke of the piston, a fixed amount of gas is pulled into the cylinder. On the compression stroke, the gas is compressed until the discharge valve opens. The quantity of gas compressed on each stroke is equal to the displacement of the cylinder. Compressors used in chillers have multiple cylinders, depending on the capacity of the

compressor. Reciprocating compressors use refrigerants with low specific volumes and relatively high pressures. Most reciprocating chillers used in building applications currently employ R-22.

Modern high-speed reciprocating compressors are generally limited to a pressure ratio of

approximately nine. The reciprocating compressor is basically a constant-volume variable-head machine. It handles various

discharge pressures with relatively small changes in inlet-volume flow rate as shown by the heavy line (labeled 16 cylinders).Condenser operation in many chillers is related to ambient conditions, for example, through cooling towers, so that on cooler days the condenser pressure can be reduced. When the air conditioning load is lowered, less refrigerant circulation is required. The resulting load characteristic is

represented by the solid line that runs from the upper right to lower left.

The compressor must be capable of matching the pressure and flow requirements imposed by the system. The reciprocating compressor matches the imposed discharge pressure at any level up to its limiting pressure ratio. Varying capacity requirements can be met by providing devices that unload individual or multiple cylinders. This unloading is accomplished by blocking the suction or discharge valves that open either manually or automatically. Capacity can also be controlled through the use of variable speed or multi-speed motors. When capacity control is implemented on a compressor, other factors at part-load conditions need to considered, such as (a) effect on compressor vibration and sound when unloaders are used, (b) the need for good oil return because of lower refrigerant velocities, and (c) proper functioning of expansion devices at the lower capacities.

With most reciprocating compressors, oil is pumped into the refrigeration system from the compressor during normal operation. Systems must be designed carefully to return oil to the compressor crankcase to provide for continuous lubrication and also to avoid contaminating heat-exchanger surfaces.

Reciprocating compressors usually are arranged to start unloaded so that normal torque motors are adequate for starting. When gas engines are used for reciprocating compressor drives, careful matching of the torque requirements of the compressor and engine must be considered.

3.5 Screw Compressors

Screw compressors, first introduced in 1958 (Thevenot, 1979), are positive displacement compressors. They are available in the capacity ranges that overlap with reciprocating compressors and small centrifugal compressors. Both twin-screw and single-screw compressors are used in chillers. The

twin-screw compressor is also called the helical rotary compressor. A cutaway of a twin-screw compressor design. There are two main rotors (screws). One is designated male and the other female .

The compression process is accomplished by reducing the volume of the refrigerant with the rotary motion of screws. At the low pressure side of the compressor, a void is created when the rotors begin to unmesh. Low pressure gas is drawn into the void between the rotors. As the rotors continue to turn, the gas is progressively compressed as it moves toward the discharge port. Once reaching a predetermined volume ratio, the discharge port is uncovered and the gas is discharged into the high pressure side of the system. At a rotation speed of 3600 rpm, a screw compressor has over 14,000 discharges per minute (ASHRAE, 1996).

Fixed suction and discharge ports are used with screw compressors instead of valves, as used in

reciprocating compressors. These set the built-in volume ratio — the ratio of the volume of fluid space in the meshing rotors at the beginning of the compression process to the volume in the rotors as the discharge port is first exposed. Associated with the built-in volume ratio is a pressure ratio that depends on the properties of the refrigerant being compressed. Screw compressors have the capability to operate at pressure ratios of above 20:1 (ASHRAE, 1996). Peak efficiency is obtained if the discharge pressure

imposed by the system matches the pressure developed by the rotors when the discharge port is exposed. If the interlobe pressure in the screws is greater or less than discharge pressure, energy losses occur but no harm is done to the compressor.

Capacity modulation is accomplished by slide valves that provide a variable suction bypass or delayed suction port closing, reducing the volume of refrigerant compressed. Continuously variable capacity control is most common, but stepped capacity control is offered in some manufacturers’ machines. Variable discharge porting is available on some machines to allow control of the built-in volume ratio during operation.

Oil is used in screw compressors to seal the extensive clearance spaces between the rotors, to cool the machines, to provide lubrication, and to serve as hydraulic fluid for the capacity controls. An oil separator is required for the compressor discharge flow to remove the oil from the high-pressure refrigerant so that performance of system heat exchangers will not be penalized and the oil can be returned for reinjection in the compressor.

Screw compressors can be direct driven at two-pole motor speeds (50 or 60 Hz). Their rotary motion makes these machines smooth running and quiet. Reliability is high when the machines are applied properly. Screw compressors are compact so they can be changed out readily for replacement or maintenance. The efficiency of the best screw compressors matches or exceeds that of the best

reciprocating compressors at full load. High isentropic and volumetric efficiencies can be achieved with screw compressors because there are no suction or discharge valves and small clearance volumes. Screw compressors for building applications generally use either R-134a or R-22.

中文译文

空调系统

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表明，在美国有超过一半的住宅安装了中央空调 (人口普查局, 1999)。在1998年，83%的新建住宅安装了中央空调 ( 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的百分比从54%增加到 73%(杰克森和詹森,1978)。

建筑物中的空气调节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，仅说明空调在商业和住宅建筑中的应用。

商业的建筑物从比较大的多层的办公大楼到街角的便利商店，占地面积和类型差别很大，因此应用于这类建筑的设备类型比较多样，对于比较大型的建筑物，空调设备设计是总系统设计的一部分，这部分包括如下项目：例如一个管道系统设计，空气分配系统设计，和冷却塔设计等。这些系统的正确设计需要一个有资质的工程师才能完成。居住的建筑物（即研究对象）被划分成单独的家庭或共有式公寓，应用于这些建筑物的冷却设备通常都是标准化组装的，由空调厂家进行设计尺寸和安装。

本章节首先对蒸汽压缩制冷循环作一个概述，接着介绍制冷剂及制冷剂的选择，最后介绍冷水机组。

1.1 蒸汽压缩循环

虽然空调系统应用在建筑物中有较大的尺寸和多样性,大多数的系统利用蒸汽压缩循环来制取需要的冷量和除湿，这个循环也用于制冷和冰冻食物和汽车的空调，在1834年，一个名叫帕金斯的人在伦敦获得了机械制冷系统的第一专利权，在1857年，詹姆士和赛博生产出第一个有活力的商业系统，除了蒸汽压缩循环之外 , 有两种不常用的制冷方法在建筑物中被应用: 吸收式循环和蒸发式冷却，这些将在后面的章节中讲到。对于蒸汽压缩制冷循环，有一种叫制冷剂的工作液体，它能在适当的工艺设备设计压力下蒸发和冷凝。

每个蒸汽压缩制冷系统中都有四大部件，它们是压缩机、冷凝器、节流装置和蒸发器。压缩机提升制冷剂的蒸汽压力以便使制冷剂的饱和温度微高于在冷凝器中冷却介质温度，使用的压缩机类型和系统的设备有关，比较大的电冷却设备使用一个离心式的压缩机而小的住宅设备使用的是一种往复或漩涡式压缩机。

冷凝器是一个热交换器，用于将制冷剂的热量传递到冷却介质中，制冷剂进入冷凝器变成过冷液体，用于冷凝器中的典型冷却介质是空气和水，大多数住宅建筑的冷凝器中使用空气作为冷却介质，而大型系统的冷凝器中采用水作为冷却介质。

液体制冷剂在离开冷凝器之后，在膨胀阀中节流到一个更低的压力。膨胀阀是一个节流的装置，例如毛细管或有孔的短管，或一个活动的装置，例如热力膨胀阀或电子膨胀阀，膨胀阀的作用是到蒸发器中分流制冷剂以便当它到压缩物吸入口的时候, 制冷剂处于过热状态，在膨胀阀的出口，制冷剂的温度在介质(空气或水) 的温度以下。之后制冷剂经过一个热交换器叫做蒸发器，它吸收通过蒸发器的空气或水的热量，如果空气经过蒸发器在流通,该系统叫做一个直接膨胀式系统，如果水经过蒸发器在流通,它叫做冷却设备，在任何情况下，在蒸发器中的制冷剂不直接和空气或水接触，在蒸发器中，制冷剂从一个低品位的两相液体转换成在正常的工艺条件下过热的蒸汽。蒸汽的形成要以一定的足够速度被压缩机排出以维持在蒸发器中低压和保持循环进行。

所有在生产中的机械冷却产生的热量必须经过冷凝器散发，在许多例子中，在冷凝器中这个热能被直接散发到环境的空气中或间接地散发到一个冷却塔的水中。在一些应用中，利用这些废热向建筑物提供热量是可能的，回收这些最高温度为65℃(150°F)的废热可以减少建筑物中采暖的费用。

空调的制冷能力常用冷吨或千瓦 (千瓦) 来表示，冷吨是一个度量单位，它与制冰厂在 24小时内使1吨 (907 公斤)的水结冰的能力有关，其值是3.51千瓦 (12,000 Btu/hr)，空调的冷却能力不要和产生冷量所需的电能相互混淆。

2.1 制冷剂的使用和选择

直到20世纪80年代中叶，制冷剂的选择在大多数的建筑物空调设备中不是一个问题，因为在制冷剂的使用上还没有统一的的标准，在以前，用于建筑物空调设备的大多数制冷剂是氟氯碳化物和氟氯碳氢化物，且大多数的制冷剂是无毒的和不可燃的，然而，最近的美国联邦的标准 (环保署 1993a；环保署 1993b) 和国际的协议 (UNEP,1987) 已经限制了氟氯碳化物和氟氯碳氢化物的制造和使用，现在，氟氯碳化物和氟氯碳氢化物在一些场合依然被使用，对制冷剂的理解能帮助建筑物拥有者或者工程师更好的了解关于为特定的设备下如何选择制冷剂，这里将讨论不同制冷剂的使用并给出影响它们使用的建筑空调设备和标准。

美国社会的供暖、制冷和空调工程师学会(ASHRAE)有一个标准的限制系统 (表 4.2.1)用来区分制冷剂，许多流行的氟氯碳化物，氟氯碳氢化物和氟碳化物的制冷剂是在甲烷和乙烷的制冷剂系列中，因为卤素元素的存在他们被叫作碳化卤或卤化的碳化氢，例如氟或氯。

Zeotropes 和 azeotropes 是混合二种或更多不同的制冷剂，一种zeotropic混合物能改变饱和温度在它在不变的压力蒸发 ( 或冷凝)。这种现象被称温度的移动，在大气压力下，R-407 C的沸点

(沸腾)是–44 °C(– 47° F)和一个凝结点 (露点)是–37°C(–35°F), 产生了7°C的温度移动 (12°F)，一个 azeotropic 混合物的性能像单独成份制冷剂那样，它在不变的压力下蒸发或冷凝它们的饱和温度不会有少许变化。R-410有微小的足够温度滑动 (少于5.5 C,10°F)，可以认为接近azeotropic混合制冷剂。

ASHRAE组制冷剂(表4.2.2)根据它们的毒性和易燃性(ASHRAE,1994)划分的。A1组合是不燃烧的和最没有毒的，而B3组是易燃的和最有毒的，以空气为媒介的制冷剂最高安全限制是毒性，如果制冷剂在少于每百万分之400是无毒的，它是一个A级制冷剂，如果对泄露少于每百万分之400是有毒的,那么该物质被称B级制冷剂，这几个级别表示制冷剂的易燃性，表 4.2.1 的最后一栏列出了常用的制冷剂的毒性和易燃的等级。因为他们是无毒的和不燃烧的 , 所以在A1组中制冷剂通常作为理想的制冷剂能基本满足舒适性空调的需求。在A1中的制冷剂通常用在建筑空调设备方面的，包括 R-11，R-12，R-22，R-134a,和R-410A。R-11，R-12，R-123和R-134a是普遍用在离心式的冷却设备的制冷剂，R-11，氟氯碳化物, 和R-123, HCFC, 都有低压高容积特性，是用在离心式压缩机上的理想制冷剂。在对氟氯碳化物的制造的禁令颁布之前, R-11和R-12已经是冷却设备的首选制冷剂，在已存在的系统维护中，现在这两种制冷剂的使用已经被限制，现在，R-123 和 R-134a都广泛的用在新的冷却设备中。R-123拥有的效率优势在 R-134a之上 (表 4.2.3)。然而，R-123有 B1安全等级，这就意谓它有一个比较低的毒性而胜于R-134a，如果一个使用R-123冷却设备在一栋建筑物中被用，当使用这些或任何其他有毒的或易燃的制冷剂时候，标准 15(ASHRAE,1992) 提供安全预防的指导方针。

制冷剂22 属于HCFC，在多数的相同设备中被用，也是在多数往复和螺旋式冷却设备和小型商业和住宅的集中式设备中的首选制冷剂，它可以在一个更高的压力下运行，这一点要优于R-11或R-12中的任何一个。从2004开始，HCFCs的制造将会受到限制。在2010年，R-22不能在新的空调设备中被使用。 2020年之后，R-22不允许生产(环保署,1993b)。

R-407C和R-410A是 HFCs的两种混合物，两者都是R-22的替代品，|R-407C预期将很快地替换R-22，在空调设备中，它的蒸发和冷凝压力接近R-22 (表格4.2.3)。然而，用R-407C来替换R-22应该在和设备制造者商议之后才能进行，至少润滑油和膨胀装置将需要更换。在1998年，第一个使用R-410A的空调设备的住宅在美国出现。使用R-410A的系统运作中，压力大约比R-22高50% (表

4.2.3)；因此，R-410A不能够用于当作速冻制冷剂来替代 R-22。R-410A系统利用特定的压缩机，膨胀阀和热交换器来利用该制冷剂.

氨广泛地被在工业的冷却设备和氨水吸收式制冷中用，它具有可燃性并且分毒性等级为B，因此在商业建筑物中使用受到限制，除非冷却设备的制造工厂独立于被冷却的建筑物之外。作为制冷剂，氨有许多良好的品质，例如，它有较高的比热和高的导热率，它的蒸发焓通常比那普遍使用的卤化

碳高6到8倍，而且氨和卤化碳比较来看，它能提供更高的热交换量，而且它能用在往复式和离心式压缩机中。

天然制冷剂的使用, 例如二氧化碳 (R-744) 和碳化氢在空调和制冷系统中的使用正在研究之中，二氧化碳能在高于传统的HCFCs或HFCs的压力下工作和在超过临界点的典型的空调设备中工作。人们通常认为碳化氢制冷剂易燃且比较危险，但它在传统的压缩机中和有的工业设备中都可以被使用。R-290, 丙烷, 都有接近R-22的工作压力，并被推荐来替代R-22 (Kramer, 1991)。目前，在美国没有用二氧化碳或可燃的制冷剂的商业系统用于建筑部门。

3.1冷水机组

1995年，在美国，冷水机组应用在至少4％的商用建筑中。而且，由于制冷机组通常安装在较大的建筑中，在同一年里，制冷机组冷却了多于28％的商用建筑的地板空间（DOE，1998）。在商用建筑中普遍采用五种型式的制冷机：往复式、螺杆式、旋涡式、离心式和吸收式。前四种利用蒸汽压缩式循环来制得冷冻水。它们的不同主要在于使用的压缩机种类的不同。吸收式制冷机在吸收循环中利用热能（典型的是来自蒸汽或燃料燃烧）并利用氨－水或水－锂溴化物制得冷冻水。

3.2总的系统

大约86％的制冷机和表所示的一样用在多台制冷机系统中（Bitondo和Tozzi，1999）。在冷冻水系统中，建筑物的回水通过每个蒸发器循环流动，在蒸发器中，回水被冷却到合意的温度（典型的为4～7℃－）（39～45℉）。然后，冷冻水通过各设备传送到水－空气换热器。在换热器中，空气被冷冻水冷却和加湿。在这个过程中，冷水的温度升高，然后必须回送到蒸发器中。

制冷机组是冷水机组。水通过每个机组的冷凝器循环，在冷凝器中，水吸收了来自高压制冷剂的热量。接着，水用水泵打到冷却塔中，水通过蒸发而降温。冷却塔将在后一部分讲述。冷凝器也可以是空冷式的。在这种循环中，冷凝器应是制冷剂－空气热交换器，空气吸收来自高压制冷剂的热量。

制冷机组名义制冷量为30～18000kw（8～5100tons）。在美国，出售的大部分制冷机组是用电的，利用蒸汽压缩制冷循环来制得冷冻水。在设计中，这种系统所使用的压缩机也有往复式、螺杆式、旋涡式和离心式。一小部分的离心式制冷机利用内燃机或蒸汽机代替电来启动压缩机。

在建筑中所使用的制冷机组类型根据应用场所来确定。对于大的办公室建筑或制冷机组需服务于多个建筑时，通常使用离心式压缩机。在所需制冷量小于1000kw（280tons）时，使用往复式或螺杆式制冷机组较合适。在小的应用场合，若低于100kw（30tons）时，使用往复式或旋涡式制冷机组。

3.3蒸汽压缩式制冷机

四种电启动的蒸汽压缩式制冷机组的名义制冷量范围。每种制冷机以所使用的压缩机类型来命名。各种系统的制冷能力范围从最小的旋涡式（30kw，8tons）到最大的离心式（18000kw，5000tons）。制冷机可使用HCFCs（R22，R123）或HFCs（R－134a）制冷剂。制冷机的效率通常用输入功（用kw表示）与制冷量（用tons表示）的比值表示。1tons的制冷量等于3.52kw或1200btu／h。用这种方法衡量效率，其数值越小越好。离心式制冷机的效率最高。而往复式是这四种类型中效率最低的。表中所提供的效率是根据ASHRAE Standard30（ASHRAE，1995）在稳定状态下测得满负荷时的效率，这些效率中不包括辅助设备的能耗，比如泵，冷却塔的风机，而这些设备可以增加0.06～0.31kw/ton

制冷机组在大部分时候是在部分负荷下运行的。只有在建筑物的最高热负荷时，制冷机才在额定制冷量附近运行。知道制冷机在部分负荷下效率是怎样变化的，这是很重要的。图4.2.3给出了往复式、螺杆式、旋涡式、带叶片控制的离心式制冷机组、压缩机频繁启动的制冷机组在满负荷时的百分比下相应的效率（用kw/ton表示）。往复式制冷机在占满负荷较小的百分比运行时，效率增加。相反地，带叶片控制的离心式的效率在负荷为额定负荷的60％以后是基本不变的,它的kw/ton值随百分数的减小而增加到满负荷时的两倍.

1998年，空调制冷学会提出了一项新的标准，用来划归在部分负荷下制冷机组的运行情况。部分负荷时的效率用综合部分负荷值（IPLV）这个简单的数值来表示。IPLV在数值上和图4.2.3相似。用25%，50%，75%，100%负荷时的效率来计算这个简单的综合效率。在这些负荷下的度量值分别为0.12，0.45，0.42，0.01。IPLV的计算公式为

IPLV=1/（0.01/A+0.42/B+0.45/C+0.12/D）

其中A——100%负荷时的效率

B——75%负荷时的效率

C——50%负荷时的效率

D——25 %负荷时的效率

大多数的IPLV由满负荷的50%，75%时的效率决定的，根据要求，制造商部分负荷时的效率，还会提供IPLV值。

以下对使用在蒸汽压缩式制冷机中的四种压缩机做简要的讲述。离心式和螺杆式压缩机主要应用在制冷机组上。往复式和旋涡式压缩机应用在整体式空调和热泵中。

3.4往复式压缩机

往复式压缩机是一种有确定排量的压缩机。在活塞的进气冲程时，一定量的气体被吸进气缸。

在压缩冲程时，气体被压缩直到排气阀打开。在每个冲程被压缩的气体数量等于气缸的体积。在制冷机中使用的压缩机根据压缩机制冷能力不同有不同个气缸的。往复式压缩机使用的制冷剂具有较小体积和相对较高的压力。使用在建筑上的往复式制冷机组目前大多采用R22。

现在高速往复式压缩机所限制的压力比大约为9。往复式压缩机基本上是具有固定容积可变压力的机器。从图4.2.4所示的最粗的一条线（16个气缸）可以看出：内容积流量发生较小的变化，压缩机的排气压力会发生各种变化。在一些制冷机中的冷凝器的运行情况与周围环境有关。比如，在制冷时，通过冷却塔后，冷凝压力会降低。当空调负荷降低时，所需的循环制冷剂流量会减少，这种结果—负荷特性用实线表示了，从右上角指向左下角。

压缩机必须要和系统压力和所需的流量相匹配。往复式压缩机在任何水平时会让排气压力达到它限定的压力比。不同制冷能力的需求可以通过卸载一个或多个气缸来实现。这种卸载又可通过阻止手动或自动开启的吸气和排气阀门来完成。制冷能力也可通过使用变速或多速电动机来控制。当控制好了压缩机的制冷能力，在部分负荷时的其他影响因素也应考虑，比如（a）压缩机震动的影响和卸载装置运行时的噪声；（b）较低的制冷剂流速需要较好的回油；（c）在较低制冷能力时膨胀装置正确的使用。

在大多数往复式压缩机中，在正常运行时油从压缩机被打到制冷系统中。系统必须仔细设计使油能回到压缩机曲轴箱，以便连续润滑，同时也能避免对热交换器表面的污染。

往复式压缩机通常是轻载启动的。一般转矩的电动机也能适用于启动。当蒸汽机用于往复式压缩机启动时，压缩机所需的转矩和蒸汽机的匹配问题必须仔细考虑。

3.5螺杆式压缩机

螺杆式压缩机，在1958年（Thevenot，1979年）第一次被提出，是一种有固定容积的压缩机。它的制冷能力是可变的，其范围与部分的往复式压缩机及较小的离心式压缩机一致。双螺杆和单螺杆这两种都有在制冷机中使用。双螺杆制冷压缩机也叫螺旋式压缩机。图4.2.5所示为双螺杆压缩机剖面图。它有两个转子（即螺杆），一个叫阳转子，另一个叫阴转子。

压缩过程是通过螺杆的旋转运动减少制冷剂的体积来完成的。在压缩机的低压一侧，当转子开始不啮合时，形成一个空间。低压气体进入转子间的这个空间。随着转子继续旋转，气体被逐渐地压缩直到移向排气口。一旦达到预定的体积比，排气口打开，气体被排到系统的高压侧。在3600rpm的旋转速度下，螺杆式压缩机有每分钟多于14000的排量（ASHRAE，1996）。

在螺杆式压缩机中，用吸气口、排气口来代替使用在往复式压缩机上的阀门。它有一个内容积比---指在开始压缩前啮合转子间的液体空间的体积和排气口第一次打开时转子间的体积之比。内容积比与由被压缩的制冷剂的特性而定的压力比相联系的。螺杆式压缩机能够在大于20：1的压力比

下运行（ASHRAE，1996）。假如系统的排气压力和排气口打开时转子间的压力相匹配时，可以达到最高的效率。当内部压力大于或小于排气压力时，就会产生能量的损失，但这对压缩机没有害处。

制冷量的调节靠接有各种变化的吸气旁通管的滑板阀门或延迟吸气口的关闭来减少被压缩的制冷剂的体积来实现。连续可变的制冷量控制是最普遍的。但是阶梯式制冷量的控制在一些制造商的机器中也被提供。各种形状的排气口适用在一些机器中，用来控制运行时的内容积比。

油在螺杆式压缩机上可解决转子间宽大的间隙、冷却机器、润滑和充当制冷量控制时水力上的液体。在压缩机的排气口需安装一个油分离器，以分离高压制冷剂中的油，这样不会对系统中的热交换器的运行造成不利的影响，而且油能回流到压缩机中。

利用单相电动机（50或60Hz）能直接启动螺杆式压缩机。它们的旋转部件能使机器平滑安静地运行。当机器正确运行时可靠性是很高的。螺杆式压缩机是紧凑的，这使得它能被替代和维修。最好的螺杆式制冷压缩机的效率等于甚至超过最好的往复式制冷压缩机在满负荷时的效率。由于螺杆式压缩机没有吸气或排气阀门和只有较小的余隙容积，因此能达到较高的等熵容积效率。建筑上使用的螺杆式压缩机大体上使用R134a或R22。