冷卻塔外文翻譯

1、<p><b>　　本科畢業(yè)設(shè)計</b></p><p><b>　　外文文獻(xiàn)及譯文</b></p><p>　　文獻(xiàn)、資料題目：Cooling Towers</p><p>　　文獻(xiàn)、資料來源：HVAC Equipment and Systems</p><p>　　文獻(xiàn)、資料發(fā)表（出版

2、）日期：</p><p>　　院 （部）： </p><p>　　專 業(yè)： </p><p>　　班 級： </p><p>　　姓 名： </p><p>　　學(xué) 號： </p><p>　　指導(dǎo)教師

3、： </p><p>　　翻譯日期： </p><p><b>　　外文資料</b></p><p>　　Cooling Towers</p><p>　　If a chiller is used to provide chilled water for building air c

4、onditioning, then the heat energy that is absorbed through that process must be rejected. The two most common ways to reject thermal energy from the vapor compression process are either directly to the air or through a c

5、ooling tower. In a cooling tower, water is recirculated and evaporatively cooled through direct contact heat transfer with the ambient air. This cooled water can then be used to absorb and reject the thermal energy f<

6、/p><p>　　Figure 4.2.14 shows the relationship between the recirculating water and air as they interact in a counterflow cooling tower. The evaporative cooling process involves simultaneous heat and mass transfe

7、r as the water comes into contact with the atmospheric air. Ideally, the water distribution system causes the water to splash or atomize into smaller droplets, increasing the surface area of water available for heat tran

8、sfer. The approach to the wet-bulb is a commonly used indicator of tower size a</p><p>　　FIGURE 4.2.14 Air/water temperature relationship in a counterflow cooling tower.</p><p>　　The range for a

9、 chiller/tower combination is determined by the condenser thermal load and the cooling water flow rate, not by the capacity of the cooling tower. The range is defined as the temperature difference between the water enter

10、ing the cooling tower and that leaving. The driver of tower performance is the ambient wet-bulb temperature. The lower the average wet-bulb temperature, the “easier” it is for the tower to attain the desired range, typic

11、ally 6°C (10°F) for HVAC applications. Thus, </p><p>　　Cooling towers are widely used because they allow designers to avoid some common problems with rejection of heat from different processes. The

12、 primary advantage of the mechanical draft cooling tower is its ability to cool water to within 3–6°C (5–10°F) of the ambient wet-bulb temperature. This means more efficient operation of the connected chilling

13、equipment because of improved (lower) head pressure operation which is a result of the lower condensing water temperatures supplied from the tower.</p><p>　　Cooling Tower Designs</p><p>　　The AS

14、HRAE Systems and Equipment Handbook (1996) describes over 10 types of cooling tower designs.Three basic cooling tower designs are used for most common HVAC applications. Based upon air and water flow direction and locati

15、on of the fans, these towers can be classified as counterflow induced draft, crossflow induced draft, and counterflow forced draft. </p><p>　　One component common to all cooling towers is the heat transfer p

16、acking material, or fill, installed below the water distribution system and in the air path. The two most common fills are splash and film.Splash fill tends to maximize the surface area of water available for heat transf

17、er by forcing water to break apart into smaller droplets and remain entrained in the air stream for a longer time. Successive layers of staggered splash bars are arranged through which the water is directed. Film fi</

18、p><p>　　Counterflow Induced Draft </p><p>　　Air in a counterflow induced draft cooling tower is drawn through the tower by a fan or fans located at the top of the tower. The air enters the tower at

19、 louvers in the base and then comes into contact with water that is distributed from basins at the top of the tower. Thus, the relative directions are counter (down for the water, up for the air) in this configuration. T

20、his arrangement is shown in Figure 4.2.15. In this configuration, the temperature of the water decreases as it falls down throu</p><p>　　FIGURE 4.2.15 Counterflow induced draft cooling tower.</p><

21、p>　　Counterflow towers generally have better performance than crossflow types because of the even air distribution through the tower fill material. These towers also eject air at higher velocities which reduces proble

22、ms with exhaust air recirculation into the tower. However, these towers are also somewhat taller than crossflow types and thus require more condenser pump head.</p><p>　　Crossflow Induced Draft</p>&l

23、t;p>　　As in the counterflow cooling tower, the fan in the crossflow tower is located at the top of the unit (Figure 4.2.16). Air enters the tower at side or end louvers and moves horizontally through the tower fill. W

24、ater is distributed from the top of the tower where it is directed into the fill and is cooled by direct contact heat transfer with the air in crossflow (air horizontal and water down). Water collected in the sump is pum

25、ped back to the chiller condenser. The increased airflow possible wit</p><p>　　Counterflow Forced Draft</p><p>　　Counterflow forced draft cooling towers have the fan mounted at or near the botto

26、m of the unit near the air intakes (Figure 4.2.17). As in the other towers, water is distributed down through the tower and its fill, and through direct contact with atmospheric air it is cooled. Thermal operation of thi

27、s tower is similar to the counterflow induced draft cooling tower. Fan vibration is not as severe for this arrangement compared to induced draft towers. There is also some additional evaporative cool</p><p>

28、<b>　　Materials</b></p><p>　　Cooling towers operate in a continuously wet condition that requires construction materials to meet challenging criteria. Besides the wet conditions, recirculating wat

29、er could have a high concentration of mineral salts due to the evaporation process. Cooling tower manufacturers build their units from a combination of materials that provide the best combination of corrosion resistance

30、and cost. Wood is a traditional material used in cooling tower construction. Redwood or fir are often used and ar</p><p>　　FIGURE 4.2.16 Crossflow induced draft cooling tower.</p><p>　　FIGURE 4.

31、2.17 Counterflow forced draft cooling tower.</p><p>　　Galvanized steel is commonly used for small- to mid-sized cooling tower structures. Hardware is usually made of brass or bronze. Critical components, suc

32、h as drive shafts, hardware mounting points, etc., may be made from 302 or 304 stainless steel. Cast iron can be found in base castings, motor housings, and fan hubs. Metals coated with plastics are finding application f

33、or special components.</p><p>　　Many manufacturers make extensive use of fiberglass-reinforced plastic (FRP) in their structure, pipe, fan blades, casing, inlet louvers, and connection components. Polyvinyl

34、chloride (PVC) is used for fill media, drift eliminators, and louvers. Fill bars and flow orifices are commonly injection molded from polypropylene and acrylonitrile butadiene styrene (ABS). </p><p>　　Concre

35、te is normally used for the water basin or sump of field erected towers. Tiles or masonry are used in specialty towers when aesthetics are important.</p><p>　　Performance</p><p>　　Rejection of t

36、he heat load produced at the chilling equipment is the primary goal of a cooling tower system. This heat rejection can be accomplished with an optimized system that minimizes the total compressor power requirements of th

37、e chiller and the tower loads such as the fans and condenser pumps. Several criteria must be determined before the designer can complete a thorough cooling tower analysis, including selection of tower range, water-to-air

38、 ratio, approach, fill type and configuration,</p><p>　　Most common HVAC applications requiring a cooling tower will use an “off the shelf” unit from a cooling tower manufacturer. Manufacturer representative

39、s are usually well informed about their products and their proper application. After the project design process has produced the information called for in Table 4.2.6, it is time to contact one or more cooling tower repr

40、esentatives and seek their input on correct tower selection.</p><p>　　Control Scheme with Chillers</p><p>　　Most cooling towers are subject to large changes in load and ambient wet-bulb temperat

41、ure during normal operations. For a typical cooling tower, the tower fan energy consumption is approximately 10% of the electric power used by the chiller compressor. The condenser pumps are about 2–5% of the compressor

42、power. Controlling the capacity of a tower to supply adequately cooled water to the condenser while minimizing energy use is a desirable operational scheme. Probably the most common control sche</p><p>　　Low

43、ering the condensing water temperature increases a chiller’s efficiency. As long as the evaporator temperature is constant, a reduced condenser temperature will yield a lower pressure difference between the evaporator an

44、d condenser and reduce the load on the compressor. However, it is important to recognize that the efficiency improvements initially gained through lower condenser temperatures are limited. Improved chiller efficiency may

45、 be offset by increased tower fan and pumping costs. Main</p><p>　　Since most modern towers use two- or three-speed fans, a near optimal control scheme can be developed as follows (Braun and Diderrich, 1990)

46、:</p><p>　　? Tower fans should be sequenced to maintain a constant approach during part load operation to minimize chiller/cooling tower energy use.</p><p>　　? The product of range and condensin

47、g water flow rate, or the heat energy rejected, should be used to determine the sequencing of the tower fans.</p><p>　　? Develop a simple relationship between tower capacity and tower fan sequencing.</p&g

48、t;<p>　　De Saulles and Pearson (1997) found that savings for a setpoint control versus the near optimal control for a cooling tower were very similar. Their control scheme called for the tower to produce water at

49、the lowest setpoint possible, but not less than the chiller manufacturer would allow, and to compare that operation to the savings obtained using near optimal control as described above. They found that the level of savi

50、ngs that could be achieved was dependent on the load profile and the method</p><p>　　The system designer should ensure that any newly installed cooling tower is tested according to ASME Standard PTC 23 (ASME

51、 1986) or CTI Standard ATC-105. These field tests ensure that the tower is performing as designed and can meet the heat rejection requirements for the connected chiller or refrigeration load.</p><p>　　Select

52、ion Criteria</p><p>　　The criteria listed in Table 4.2.6 are usually known a priori by the designer. If not known explicitly, then commonly accepted values can be used. These criteria are used to determine t

53、he tower capacity needed to reject the heat load at design conditions. Other considerations besides the tower’s capacity include economics, servicing, environmental considerations, and aesthetics. Many of these factors a

54、re interrelated, but, if possible, they should all be evaluated when selecting a particular tow</p><p>　　Because economics is an important part of the selection process, two methods are commonly used — life-

55、cycle costing and payback analysis. These procedures compare equipment on the basis of owning, operation, and maintenance costs. Other criteria can also affect final selection of a cooling tower design: building codes, s

56、tructural considerations, serviceability, availability of qualified service personnel, and operational flexibility for changing loads. In addition, noise from towers can become a</p><p>　　Applications</p&

57、gt;<p>　　Unlike chillers, pumps, and air handlers, the cooling tower must be installed in an open space with careful consideration of factors that might cause recirculation (recapture of a portion of warm and humi

58、d exhaust air by the same tower) or restrict air flow. A poor tower siting situation might lead to recirculation, a problem not restricted to wet cooling towers. Similar recirculation can occur with air-cooled condensing

59、 equipment as well. With cooling tower recirculation, performance is adverse</p><p>　　Multiple tower installations are susceptible to interference — when the exhaust air from one tower is drawn into a tower

60、located downwind. Symptoms similar to the recirculation phenomenon then plague the downwind tower. For recirculation, interference, or physically blocking air-flow to the tower the result is larger approach and range whi

61、ch contribute to higher condensing pressure at the chiller. Both recirculation and interference can be avoided through careful planning and layout.</p><p>　　Another important consideration when siting a cool

62、ing tower installation is the effect of fogging, or plume, and carryover. Fogging occurs during cooler weather when moist warm air ejected from the tower comes into contact with the cold ambient air, condenses, and forms

63、 fog. Fog from cooling towers can limit visibility and can be an architectural nuisance. Carryover is when small droplets of entrained water in the air stream are not caught by the drift eliminators and are ejected in th

64、e exhaust </p><p>　　Operation and Maintenance</p><p>　　Winter Operation </p><p>　　If chillers or refrigeration equipment are being used in cold weather, freeze protection should be

65、considered to avoid formation of ice on or in the cooling tower. Capacity control is one method that can be used to control water temperature in the tower and its components. Electric immersion heaters are usually instal

66、led in the tower sump to provide additional freeze protection. Since icing of the air intakes can be especially detrimental to tower performance, the fans can be reversed to de-ice </p><p>　　Water Treatment&

67、lt;/p><p>　　The water circulating in a cooling tower must be at an adequate quality level to help maintain tower effectiveness and prevent maintenance problems from occurring. Impurities and dissolved solids ar

68、e concentrated in tower water because of the continuous evaporation process as the water is circulated through the tower. Dirt, dust, and gases can also find their way into the tower water and either become entrained in

69、the circulating water or settle into the tower sump. To reduce the concentration o</p><p>　　Legionellosis </p><p>　　Legionellosis has been connected with evaporative condensers, cooling towers,

70、and other building hydronic components. Researchers have found that well-maintained towers with good water quality control were not usually associated with contamination by Legionella pneumophila bacteria. In a position

71、paper concerning Legionellosis, the Cooling Tower Institute (CTI, 1996) stated that cooling towers are prone to colonization by Legionella and have the potential to create and distribute aerosol droplet</p><p&

72、gt;　　The CTI proposed recommendations regarding cooling tower design and operation to minimize the presence of Legionella. They do not recommend frequent or routine testing for Legionella pneumophila bacteria because the

73、re is difficulty interpreting test results. A clean tower can quickly be reinfected, and a contaminated tower does not mean an outbreak of the disease will occur.</p><p>　　Maintenance</p><p>　　T

74、he cooling tower manufacturer usually provides operating and maintenance (O&M) manuals with a new tower installation. These manuals should include a complete list of all parts used and replaceable in the tower and al

75、so details on the routine maintenance required for the cooling tower. At a minimum, the following should also be included as part of the maintenance program for a cooling tower installation.</p><p>　　? Perio

76、dic inspection of the entire unit to ensure it is in good repair.</p><p>　　? Complete periodic draining and cleaning of all wetted surfaces in the tower. This gives the opportunity to remove accumulations of

77、 dirt, slime, scale, and areas where algae or bacteria might develop.</p><p>　　? Periodic water treatment for biological and corrosion control.</p><p>　　? Continuous documentation on operation a

78、nd maintenance of the tower. This develops the baseline for future O&M decisions and is very important for a proper maintenance policy.</p><p>　　4.2.4 Packaged Equipment</p><p>　　Central HVA

79、C systems are not always the best application for a particular cooling or heating load. Initial costs for central systems are usually much higher than unitary or packaged systems. There may also be physical constraints o

80、n the size of the mechanical components that can be installed in the building. Unitary or packaged systems come factory assembled and provide only cooling or combined heating and cooling. These systems are manufactured i

81、n a variety of configurations that allow the desi</p><p>　　Unitary systems find application in buildings up to eight stories in height, but they are more generally used in one-, two-, or three-story building

82、s that have smaller cooling loads. They are most often used for retail spaces, small office buildings, and classrooms. Unitary equipment is available only in preestablished capacity increments with set performance charac

83、teristics, such as total L/s (cfm) delivered by the unit’s air handler. Some designers combine central HVAC systems with packaged eq</p><p>　　Table 4.2.7 lists some of the advantages and disadvantages of pac

84、kaged and unitary HVAC equipment.</p><p>　　Table 4.2.8 lists energy efficiency ratings (EERs) for typical electric air- and water-cooled split and single package units with capacity greater than 19 kW (65,00

85、0 Btuh). </p><p>　　Typically, commercial buildings use unitary systems with cooling capacities greater than 18 kW (5 tons). In some cases, however, due to space requirements, physical limitations, or small a

86、dditions, residential-sized unitary systems are used. If a unitary system is 10 years or older, energy savings can be achieved by replacing unitary systems with properly sized, energy-efficient models. </p><p&

87、gt;　　a Electric air- and water-cooled split system and single package units with capacity over 19 kW(65,000 Btuh) are covered here.</p><p>　　b EER, or energy efficiency ratio, is the cooling capacity in kW (

88、Btu/h) of the unit divided by its electrical input (in watts) at standard (ARI) conditions of 35°C (95°F) for air-cooled equipment, and 29°C (85°F) entering water for water-cooled models.</p>&

89、lt;p>　　c Based on ARI 210/240 test procedure.</p><p>　　d SEER (seasonal energy efficiency ratio) is the total cooling output kW (Btu) provided by the unitduring its normal annual usage period for cooling

90、divided by the total energy input (in Wh)during the same period.</p><p>　　e Split system and single package units with total capacity under 19 kW (65,000 Btuh) are covered here. This analysis excludes window

91、 units and packaged terminal units.</p><p>　　FIGURE 4.2.18 Comparison between TXV and short-tube orifice systems capacity for a range of charging conditions and 95°F (35°C) outdoor temperature. (Fr

92、om Rodriquez et al., 1996).</p><p>　　As with any HVAC equipment, proper maintenance and operation will ensure optimum performance and life for a system. Split-system air conditioners and heat pumps are the m

93、ost common units applied in residential and small commercial applications. These units are typically shipped to the construction site as separate components; after the condenser (outdoor unit) and the evaporator (indoor

94、unit) are mounted, the refrigerant piping is connected between them. The air conditioning technician must ensu</p><p>　　The plot in Figure 4.2.18 clearly shows that for a 20% under-charge in refrigerant, a u

95、nit with a short tube orifice suffers a 30% decrease in cooling capacity. This same study also investigated the effects of return-air leakage. A common problem with new installations is improper sealing of duct connectio

96、ns at the diffusers and grills as well as around the return-air plenum. Leakage amounts as low as 5% in the return air ducts resulted in capacity and efficiency reductions of almost 20% for hig</p><p>　　FIGU

97、RE 4.2.19 Rooftop packaged heating and air conditioning unit. (Adapted from Carrier Corporation).</p><p>　　Packaged Units</p><p>　　Packaged units are complete HVAC units that are usually mounted

98、 on the exterior of a structure (roof or wall) freeing up valuable indoor floor space (Figure 4.2.19). They can also be installed on a concrete housekeeping pad at ground level. Because they are self-contained, complete