空調(diào)系統(tǒng)外文翻譯

1、<p>　　Air Conditioning Systems</p><p>　　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%

2、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

3、 that over half (50.9%) of residences in the U.S. had central air conditioners (Census Bureau, 1999). An estimated 83%</p><p>　　homes constructed in 1998 had central air conditioners (Census Bureau, 1999). A

4、ir 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).</p&g

5、t;<p>　　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 mai

6、ntain 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

7、is limited to those commonly used in commercial and residential buildings. </p><p>　　Commercial buildings range from large high-rise office buildings to the corner convenience store. Because of the range in

8、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 i

9、tems such as a piping system, air distribution system, and cooling tower. Proper design of these systems requires a qualified engineer. The residential buildi</p><p>　　by single family homes and low-rise apa

10、rtments/condominiums. The cooling equipment applied in these buildings comes in standard “packages” that are often both sized and installed by the air conditioning contractor.</p><p>　　The chapter starts wit

11、h a general discussion of the vapor compression refrigeration cycle then moves to refrigerants and their selection, followed by packaged Chilled Water Systems。</p><p>　　1.1 Vapor Compression Cycle</p>

12、<p>　　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. Thi

13、s 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 v

14、iable commercial system was produced in 1857 by James Harrison and D.E. </p><p>　　compression cycle, a working fluid, which is called the refrigerant, evaporates and condenses at suitable pressures for pract

15、ical equipment designs.</p><p>　　The four basic components in every vapor compression refrigeration system are the compressor, condenser, expansion device, and evaporator. The compressor raises the pressure

16、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.

17、 Large electric chillers typically use a centrifugal compressor while small residential equipment uses a r</p><p>　　The condenser is a heat exchanger used to reject heat from the refrigerant to a cooling med

18、ium. 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 con

19、denser, while many larger chillers use water. After leaving the condenser, the liquid refrigerant expands to a lower pressure in the expansion valve.</p><p>　　The expansion valve can be a passive device, suc

20、h 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

21、the refrigerant is superheated when it reaches the suction of the compressor.</p><p>　　At the exit of the expansion valve, the refrigerant is at a temperature below that of the medium (air or water) to be co

22、oled. 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

23、 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 wi</p><p>　　The refrigerant is converted from a l

24、ow 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 evaporat

25、or and keep the cycle operating.</p><p>　　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 e

26、nvironment 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

27、the building. Recovery of this waste heat at temperatures up to 65°C (150°F) can be used to reduce costs for space </p><p>　　Capacities of air conditioning are often expressed in either tons or kil

28、owatts (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 produ

29、ced by the air conditioner must not be confused with the amount of electrical power (also expressed in kW) required to produce the cooling effect.</p><p>　　2.1 Refrigerants Use and Selection</p><p

30、>　　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

31、 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 (E

32、PA 1993a; EPA 1993b) and international agreements (UNEP, 1987) hav</p><p>　　The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) has a standard numbering system,for identify

33、ing 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 hal

34、ogen elements such as fluorine or chlorine (King, 1986). </p><p>　　Zeotropes and azeotropes are mixtures of two or more different refrigerants. A zeotropic mixture changes saturation temperatures as it e

35、vaporates (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°

36、;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 temperat</p><p>　　ASHRAE groups refrigera

37、nts 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 refrig

38、erant. 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. Th

39、e numerical designations refer to the</p><p>　　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

40、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 con

41、ditioning equipment. R-22 cannot be produced after 2020 (EPA, 1993b).</p><p>　　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 repla

42、cement 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 consult

43、ing with the equipment manufacturer. At a minimum, the lubricant and expansion device will need to be replaced. The first residential-sized air conditio</p><p>　　Ammonia is widely used in industrial refriger

44、ation 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

45、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

46、to 8 times high</p><p>　　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

47、 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 t

48、hought of as too hazardous because of flammability, can be used in conventional compressors and ha</p><p>　　3.1 Chilled Water Systems</p><p>　　Chilled water systems were used in less than 4% of

49、 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

50、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 t

51、he type of</p><p>　　3.2 Overall System</p><p>　　An estimated 86% of chillers are applied in multiple chiller arrangements like that shown in the figure (Bitondo and Tozzi, 1999). In chilled wat

52、er 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

53、to water-to-air heat exchangers spread throughout the facility. In these heat exchangers, air is cooled and dehumidified by the cold water. Durin</p><p>　　The chillers are water-cooled chillers. Water is cir

54、culated 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

55、. 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

56、 pressur</p><p>　　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

57、. 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 a

58、n electric motor to drive the compressor.</p><p>　　The type of chiller used in a building depends on the application. For large office buildings or in chiller plants serving multiple buildings, centrifugal c

59、ompressors 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 ch

60、illers are typically used.</p><p>　　3.3 Vapor Compression Chillers</p><p>　　The nominal capacity ranges for the four types of electrically driven vapor compression chillers. Each chiller derives

61、 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

62、-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</p><p>　　Chillers run at part load capacity

63、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 ca

64、pacity. 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 v

65、ariable frequency drive (</p><p>　　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).

66、 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 sin

67、gle integrated efficiency number. The weighting factors at these loads are 0.12, 0.45, 0.42, and 0.01, respe</p><p>　　Most of the IPLV is determined by the efficiency at the 50% and 75% part load values. Man

68、ufacturers will provide, on request, IPLVs as well as part load efficiencies.</p><p>　　The four compressors used in vapor compression chillers are each briefly described below. While centrifugal and screw co

69、mpressors are primarily used in chiller applications, reciprocating and scroll compressors are also used in smaller unitary packaged air conditioners and heat pumps.</p><p>　　3.4 Reciprocating Compressors&l

70、t;/p><p>　　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 compre

71、ssed 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 com

72、pressor. Reciprocating compressors use refrigerants with low specific volumes and</p><p>　　Modern high-speed reciprocating compressors are generally limited to a pressure ratio of approximately nine. The rec

73、iprocating compressor is basically a constant-volume variable-head machine. It handles various</p><p>　　discharge pressures with relatively small changes in inlet-volume flow rate as shown by the heavy line

74、(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 l

75、oad 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.</p><p>　　The compressor must be capa

76、ble 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

77、can be met by providing devices that unload</p><p>　　individual or multiple cylinders. This unloading is accomplished by blocking the suction or discharge valves that open either manually or automatically. C

78、apacity 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

79、on compressor vibration and sound when unloaders are used, (b) the need for good oil return because of lower refrigerant veloc</p><p>　　With most reciprocating compressors, oil is pumped into the refrigerati

80、on 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 sur

81、faces. </p><p>　　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, carefu

82、l matching of the torque requirements of the compressor and engine must be considered.</p><p>　　3.5 Screw Compressors</p><p>　　Screw compressors, first introduced in 1958 (Thevenot, 1979), are p

83、ositive 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 chiller

84、s. 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 .</p><p&g

85、t;　　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 p

86、ressure 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

87、port is uncovered and the gas is discharged into the high pressu</p><p>　　Fixed suction and discharge ports are used with screw compressors instead of valves, as used in reciprocating compressors. These set

88、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

89、 built-in volume ratio is a pressure ratio that depends on the properties of the refrigerant being compressed. Screw compressors have the capabilit</p><p>　　Capacity modulation is accomplished by slide valve

90、s 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 s

91、ome manufacturers’ machines. Variable discharge porting is available on some machines to allow control of the built-in volume ratio during operation.</p><p>　　Oil is used in screw compressors to seal the ext

92、ensive 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 remov

93、e 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.</p><p>　　Screw compressors can

94、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 the

95、y 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 efficie

96、ncies can be achieved with screw compr</p><p><b>　　空調(diào)系統(tǒng)</b></p><p>　　過去 50 年以來，空調(diào)得到了快速的發(fā)展，從曾經(jīng)的奢侈品發(fā)展到可應用于大多數(shù)住宅和商業(yè)建筑的比較標準的系統(tǒng)。在 1970 年的美國， 36% 的住宅不是全空氣調(diào)節(jié)就是利用一個房間空調(diào)器冷卻；到1997年，這一數(shù)字達到了 77%