空调系统毕业论文外文翻译.doc

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1、 外文翻译来源:外文文献一Air Conditioning SystemsDennis L. ONeal and John A. BryantAir 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 util

2、ized 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 newhomes constructed

3、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 buildin

4、gs 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,

5、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

6、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 desi

7、gn of these systems requires a qualified engineer. The residential building sector is dominatedby 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 co

8、ntractor.The chapter starts with a general discussion of the vapor compression refrigeration cycle then moves to refrigerants and their selection. Chillers and their auxiliary systems are then covered, followed by packaged air conditioning equipment.4.2.1 Vapor Compression CycleEven though there is

9、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 o

10、n 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 (Thevenot 1979).Besides vapor compression, there are two less common methods used to produce cooling in buildings:

11、the absorption cycle and evaporative cooling. These are described later in the chapter. With the vaporcompression cycle, a working fluid, which is called the refrigerant, evaporates and condenses at suitable pressures for practical equipment designs.The four basic components (Figure 4.2.1) in every

12、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 o

13、f 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 r

14、efrigerant 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 refr

15、igerant 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

16、 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

17、. 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 contac

18、t 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 evaporato

19、r 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

20、 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 65C (150F) can be used to reduce costs for space heating.Capacities of air conditioning are often expressed in either to

21、ns 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 elec

22、trical power (also expressed in kW) required to produce the cooling effect.4.2.2 Refrigerants Use and SelectionUp 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 refri

23、gerants 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

24、, 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 c

25、hoice 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 stan

26、dard numbering system (Table 4.2.1) 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 o

27、r 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 (bu

28、bble) point of 44C (47F) and a condensation (dew) point of 37C (35F), which gives it a temperature glide of 7C (12F). 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

29、-410A has a small enough temperature glide (less than 5.5C, 10F) that it is considered a near-azeotropic refrigerant mixture.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

30、. 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. T

31、he 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 a

32、nd 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

33、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,

34、 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-41

35、0A 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 r

36、efrigeration 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 refrige

37、rant, 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 recipr

38、ocating 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

39、 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 ope

40、rating 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.外文翻译空调系统过去 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

41、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

42、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，然后介绍冷却设备及附属系

43、统，最后介绍组合式空调机组。4.2.1 蒸汽压缩循环待添加的隐藏文字内容1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Perkins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Harrison和D.E. Siebe生产出第一个有活力的商业系统(Thevenot 1979)，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

44、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

45、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

46、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(150F)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。4.2.2 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

47、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，

48、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37C(35F), 产生了7C的温度移动 (12F)，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

49、小的足够温度滑动 (少于5.5 C,10F)，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