外文資料翻譯--空冷熱交換器和空冷塔

1、<p><b>　　中文5600字</b></p><p><b>　　（</b></p><p>　　二 〇 一 三 年 五 月</p><p>　　Air-cooled Heat Exchangers and Cooling Towers</p><p>　　D.G.KROGER Sc

2、.D. (MIT)</p><p>　　(This text is a part of MR KROGER's book. include 8.4, 9.3, 9.4)</p><p>　　8.4 RECIRCULATION</p><p>　　Heated plume air may recirculate in an air-cooled he

3、at exchanger, thereby reducing the cooling effectiveness of the system. Figure 8.4.1 depicts, schematically, a cross-section of an air-cooled heat exchanger. In the absence of wind, the buoyant jet or plume rises vertic

4、ally above the heat exchanger. A part of the warm plume air may however be drawn back into the inlet of the tower. This phenomenon is known as "recirculation". Plume recirculation is usually a variable phenomen

5、on influenced by m</p><p>　　Figure 8.4.1: Air-flow pattern about forced draft air-cooled heat exchanger.</p><p>　　Lichtenstein [51LI1] defines a recirculation factor as</p><p><b

6、>　　(8.4.1)</b></p><p>　　where mr is the recirculating air mass flow rate, while ma is the ambient air flow rate into the heat exchanger.</p><p>　　Although the results of numerous studie

7、s on recirculation do appear in the literature, most are experimental investigations performed on heat exchangers having specific geometries and operating under prescribed conditions e.g. [74KE1, 81SL1]. Gunter and Ship

8、es [72GUll define certain recirculation flow limits and present the results of field tests performed on air-cooled heat exchangers. Problems associated with solving recirculating flow patterns numerically have been repor

9、ted [81EP1]. Kroger </p><p>　　8.4.1 RECIRCULATION ANALYSIS</p><p>　　Consider one half of a two-dimensional mechanical draft air-cooled heat exchanger in which recirculation occurs. For purpos

10、es of analysis, the heat exchanger is represented by a straight line at an elevation Hi above ground level as shown in figure 8.4.2(a).</p><p>　　Figure 8.4.2: Flow pattern about heat exchanger.</p>&l

11、t;p>　　It is assumed that the velocity of the air entering the heat exchanger along its periphery is in the horizontal direction and has a mean value, vi (the actual inlet velocity is highest at the edge of the fan pla

12、tform and decreases towards ground level). The outlet velocity, vo, is assumed to be uniform and in the vertical direction.</p><p>　　Consider the particular streamline at the outlet of the heat exchanger tha

13、t diverges from the plume at 1 and forms the outer "boundary" of the recirculating air stream. This streamline will enter the platform at 2, some distance Hr below the heat exchanger. For purposes of analysis

14、it will be assumed that the elevation of 1 is approximately Hr above the heat exchanger. If viscous effects, mixing and heat transfer to the ambient air are neglected, Bernoulli's equation can be applied between 1 an

15、</p><p><b>　　(8.4.2)</b></p><p>　　It is reasonable to assume that the total pressure at I is approximately equal to the stagnation pressure of the ambient air at that elevation i.e.&

16、lt;/p><p><b>　　(8.4.3)</b></p><p>　　At2 the static pressure can be expressed as</p><p><b>　　(8.4.4)</b></p><p>　　Furthermore, for the ambient air f

17、ar from the heat exchanger</p><p><b>　　(8.4.5)</b></p><p>　　Substitute equations (8.4.3), (8.4.4) and (8.4.5) into equation (8.4.2) and find</p><p><b>　　(8.4.6)<

18、;/b></p><p>　　Due to viscous effects the velocity at the inlet at elevation Hi is in practice equal to zero. The Velocity gradient in this immediate region is however very steep and the velocity peaks at

19、a value that is higher than the mean inlet velocity. Examples of numerically determined inlet velocity distributions for different outlet velocities and heat exchanger geometries are shown in figure 8.4.3 [95DU1]. Since

20、most of the recirculation occurs in this region the velocity v2 is of importance but diffi</p><p><b>　　(8.4.7) </b></p><p>　　Figure 8.4.3: Two-dimensional inlet velocity distributio

21、n for Wi/2 = 5.1 m.</p><p>　　According to the equation of mass conservation, the flow per unit depth of the tower can be expressed as</p><p><b>　　(8.4.8)</b></p><p>　　if

22、 the amount of recirculation is small.</p><p>　　According to equations (8.4.1) and (8.4.8) the recirculation factor is </p><p><b>　　(8.4.9)</b></p><p>　　Substitute equat

23、ions (8.4.7) and (8.4.8) into equation (8.4.9) and find</p><p><b>　　(8.4.10)</b></p><p>　　where is the Froude number based on the width of the heat exchanger.</p><p>　　T

24、he influence of a wind wall or deep plenum can be determined approximately by considering flow conditions between the top of the wind wall, (Hi + Hw), as shown in figure 8.4.2(b) and elevation Hi. Consider the extreme ca

25、se when Hw is so large (Hw = Hwo) that no recirculation takes place and the ambient air velocity near the top of the wind wall is zero. In this particular case the static pressure at the tower exit is essentially equal t

26、o the ambient stagnation pressure. With these assumptions,</p><p><b>　　(8.4.11)</b></p><p>　　But (8.4.12)</p><p>　　Substitute

27、equation (8.4.12) into equation (8.4.11) and find</p><p><b>　　(8.4.13)</b></p><p>　　If it is assumed that the recirculation decreases approximately linearly with increasing wind wall

28、 height, equation (8,4.10) may be extended as follows:</p><p><b>　　(8.4.14)</b></p><p>　　Since the recirculation is assumed to be essentially zero at Hw = Hwo, find a = 1.</p>

29、<p>　　Substitute equation (8.4.13) into equation (8.4.14) and find</p><p><b>　　(8.4.15)</b></p><p>　　where is the densimetric Froude number based on the wind wall height.</p

30、><p>　　It is important to determine the effectiveness of the system when recirculation occurs. Effectiveness in this case, is defined as</p><p><b>　　(8.4.16)</b></p><p>　　T

31、he interrelation between the recirculation and the effectiveness is complex in a real heat exchanger. Two extremes can however be evaluated analytically i.e.</p><p>　　1. No mixing</p><p>　　The w

32、arm recirculating air does not mix at all with the cold ambient inflow, resulting in a temperature distribution as shown in figure 8.4.4(a). The recirculating stream assumes the temperature of the heat exchanger fluid .&

33、lt;/p><p>　　Figure 8.4.4: Recirculation flow patterns.</p><p>　　This in effect means that the part of the heat exchanger where recirculation occurs, transfers no heat. The actual heat transfer rate

34、 is thus given by</p><p><b>　　(8.4.17)</b></p><p>　　resulting in an effectiveness due to recirculation of</p><p><b>　　(8.4.18)</b></p><p>　　Subs

35、titute equation (8.4.15) into equation (8.4.18) and find</p><p><b>　　(8.4.19)</b></p><p>　　2. Perfect mixing</p><p>　　The recirculating air mixes perfectly with the inf

36、lowing ambient air, resulting in a uniform increase in both the effective inlet air temperature and the outlet air temperature as shown in figure 8.4.4(b).</p><p>　　If for purpose of illustration, it is assu

37、med that the temperature of the heat exchanger,,is constant, it follows from equation (3.5.22) that the effectiveness under cross-flow conditions is</p><p><b>　　(8.4.20)</b></p><p>　

38、　or (8.4.21)</p><p>　　Furthermore the enthalpy entering the heat exchanger is</p><p>　　or (8.4.22)</p><p>　　Substitute equation (8.4.22) into equ

39、ation (8.4.21) and find </p><p><b>　　(8.4.23) </b></p><p>　　In this case the effectiveness due to recircuiation is given by</p><p>　　From equation (8.4.22) and (8.4.23),

40、 substitute the values of Tir and Tor into this equation, to find the effectiveness of the heat exchanger.</p><p><b>　　(8.4.24)</b></p><p>　　In practice the effectiveness will be som

41、e value between that given by equation (8.4.18) and equation (8.4.24). Actual measurements conducted on air-cooled heat exchangers appear to suggest that relatively little mixing occurs. This tendency is confirmed by nu

42、merical analysis of the problem [89KR1, 95DU1].</p><p>　　Figure 8.4.5: Heat exchanger effectiveness.</p><p>　　Duvenhage and Kroger [95DU1] solved the recirculation problem numerically and correl

43、ated their results over a wide range of operating conditions and heat exchanger geometries by means of the following empirical equation:</p><p><b>　　(8.4.25)</b></p><p>　　This equati

44、on is valid in the and </p><p>　　where. In this equation represents the effective height above the inlet to the fan platform and includes the plenum height in addition to any wind wall height.</p>&

45、lt;p>　　Equation （8.4.25) is shown graphically in figure 8.4.5. For values of , equation (8.4.19) is in good agreement with equation(8.4.25).</p><p>　　8.4.2 MEASURING RECIRCULATION</p><p>

46、　　In the absence of wind walls, recirculation can be significant resulting in a corresponding reduction in heat transfer effectiveness. As shown in figure 8.4.6, smoke generated at the lower end outlet of an A-frame type

47、 forced draft air-cooled heat exchanger without wind walls, is drawn directly downwards into the low pressure region created by the fans. The results of recirculation tests conducted at the Marimba power plant are report

48、ed by Conradie and Kroger [89CO1]. They actually measured the </p><p>　　Figure8.4.6: Plume air recirculating in air-cooled steam condenser.</p><p>　　Figure8.4.7: Visualization of recirculation w

49、ith smoke at the Matimba power plant.</p><p>　　Generally less recirculation occurs in induced draft cooling systems due to the relatively high fan outlet velocity and height of diffuser if one is present.<

50、;/p><p>　　There are numerous situations where a minimum tube wall temperature must be maintained. For example to avoid plugging during cooling of heavy crude stocks with high pour points or in the case where th

51、ere is a danger of solidification fouling due to the deposition of ammonium salts when tube wall temperatures fall below 70~ C in an overhead condenser for a sour water stripper etc. air temperature control is essential.

52、 In such situations recirculation is employed in a system incorporating automati</p><p>　　Figure 8.4.8: Louver controlled plume air recirculation in air-cooled heat exchanger.</p><p>　　Steam coi

53、ls located immediately below the tube bundles may be required to preheat the air during startup in winter.</p><p>　　9.3EFFECT OF WIND ON AIR-COOLED HEAT EXCIHANGERS</p><p>　　In general winds hav

54、e a negative effect on the performance of mechanical draft heat exchangers. Plume air recirculation tends to increase while fan performance is usually reduced during windy periods.</p><p>　　Laboratory studie

55、s and field tests have shown that the output of dry-cooled power stations may be significantly reduced by winds. As shown in figure 9.3.I the wind speed and direction significantly influences the turbine output at the W

56、ydok power plant [76SC1].</p><p>　　Figure 9.3.1: Reduction in turbine output due to wind at the Wyodak power plant.</p><p>　　Before the 160 MWe power plant at Utrillas in Spain was built, exte

57、nsive model tests (scale 1:150) were conducted to determine the optimum position of the air-cooled condenser and power plant orientation, taking into consideration local wind patterns. The results of the tests are shown

58、in figure 9.3.2. </p><p>　　Goldshagg [93GO1] reports that turbine performance at the Matimba power plant was reduced measurably during certain windy periods and that occasional turbine trips had occurred und

59、er extremely gusty conditions. After extensive experimental and numerical investigations modifications to the wind walls and cladding were implemented as shown in figure 9.3.3. Due to the resultant improved air flow patt

60、ern into the air-cooled condenser during periods of westerly winds, no further trips were experience</p><p>　　Figure 9.3.2: Reduction in turbine output at the Utrillas power plant due to wind.</p><

61、;p>　　Figure 9.3.3: Modifications at the Matimba power plant.</p><p>　　From the case studies listed above it is clear that the interaction between the air cooled heat exchanger and adjacent buildings or

62、 structures can significantly complicate flow patterns and consequently reduce plant performance.</p><p>　　Kennedy and Fordyce [74KE1] report the results of model studies to determine downwind temperature di

63、stribution, recirculation and interference (ingestion of an adjacent tower's effluent plume) characteristics.</p><p>　　Slawson and Sullivan [81SL1] conducted experiments in a water plume to recirculation

64、 and interference for two conceptual configurations of forced draft dry-cooling towers, a rectangular array and a multiple round tower arrangement. The objective of the study was to investigate and make recommendations o

65、n the design and arrangement of cooling towers in order to provide optimum ambient air distribution to the heat transfer surfaces. Optimum air distribution is maintained by minimizing recirculatio</p><p>　　F

66、ield tests conducted by the Cooling Tower Institute (CTI) on induced mechanical draft cooling towers, clearly show a measurable increase of plume recirculation with an increase in wind speed when the wind blows in the lo

67、ngitudinal direction of the cooling, tower bank [58CT1,77CT1]. The results of numerous other experimental studies on recirculation have been reported [71GU1, 72GU1, 74KE1, 76ON1, 81SL1, 88t).11].</p><p>　　In

68、 addition to the effect of recirculation, the performance of the funs, especially in forced draft systems, are influenced during windy periods due to inlet air flow distortions.</p><p>　　Duvenhage and Krosge

69、r [96DU1] numerically modelled the air flow patterns about and through, an air-cooled heat exchanger during windy conditions, taking into consideration the coupled effects of both recirculation and fan performance. They

70、consider a long heat exchanger bank as shown schematically in figure 9.3.4 consisting of bays, each bay having two 6-blade 4.31 m diameter fans. The heat exchanger is subjected to winds blowing across or parallel to the

71、longitudinal axis and having a velocity </p><p>　　Figure9.3.4: Schematic of air cooled heat exchanger.</p><p>　　Figure 9.3.5: Details of bay geometry.</p><p>　　A more detailed cr

72、oss-section of the bay is shown in figure 9.3.5. Each bay has an effective bundle frontal area of 7.07 m x 10.2 m - 72.12 m2 and a tube bundle height of</p><p>　　0.72 m. The fans have cylindrical inlet shro

73、uds. The fan platform or inlet height Hi = 5.7 m and the plenum chamber is 3 m high. They find that with increasing wind speed the air volume flow rate through the upwind fans (Fup) is reduced due to flow distortions whi

74、le the flow through the downwind fans (Fdo) may actually increase slightly as shown ill figure 9.3.6, due to the increased kinetic energy in the air stream. The air-cooled heat exchanger performance is however reduced du

75、e to a net decre</p><p>　　Figure 9.3.6: Fan air flow rate during crosswinds for an inlet height Hi = 5.7 m.</p><p>　　The influence on performance of recirculating hot plume air in this installa

76、tion is relatively small. As shown in figure 9.3.7 the effectiveness of the heat exchanger actually increases slightly for a light wind when compared to windless conditions. This is due to the fact that recirculation at

77、the downwind side of the heat exchanger is eliminated. At higher wind speed recirculation gradually increases. This trend is in agreement with results observed by DU Toit et al. [93D153]. </p><p>　　To eval

78、uate the influence of the inlet height on air flow rate through the particular heat exchanger, Hi was varied in the numerical model while a fixed wind profile was retained with a reference velocity of Vwr - 3 m/s at a re

79、ference height of zr = 5.7 m. The corresponding changes in fan air volume flow rate and effectiveness are shown in figures 9.3.8 and 9.3.9 respectively. By increasing the height of the fan platform, the performance of th

80、e heat exchanger is improved due to the corresponding</p><p>　　Figure 9.3.7: Effectiveness due to recirculation during crosswinds for an inlet height Hi= 5.7m.</p><p>　　Figure 9.3.8: Fan air flo

81、w rate during crosswind for different fan platform heights.</p><p>　　Figure 9.3.9: Effectiveness due to recirculation during crosswinds for different fan platform height.</p><p>　　Figure 9.3.10:

82、 Recirculation for winds blowing in the direction of the longitudinal axis.</p><p>　　The influence on performance of winds blowing in the direction of the longitudinal axis are evaluated numerical/y for a fa

83、n platform height of 10.7 m with wind reference velocities of 3 m/s and 5 m/s at a reference height of 5.7 m Heat exchanger banks consisting of up to 6 bays are evaluated. In the numerical model the crosswind solutions a

84、re applied to the two fans in the first two up-wind bays while the remaining fans are assumed to operate ideally. The resultant recirculation is shown in fig</p><p>　　Recirculation clearly increases with in

85、creasing heat exchanger length and wind speed. For purposes of comparing trends, a correlation for recirculation recommended by the CTI [58CT1, 77CT1] is also shown in figure 9.3.10. It should be noted that this correlat

86、ion is applicable to induced draft cooling towers although the authors do state that they expect</p><p>　　the recirculation of a forced draft system to be double the value of the correlation shown. Duvenhage

87、 et al. [96DU2] show that the addition of a solid walkway along the periphery of the air-cooled heat exchanger (at the fan platform elevation) tends to improve</p><p>　　the mean flow rate through the fans (s

88、ee figure 93.ll). </p><p>　　Figure 9.3.1l: Walkway effect</p><p>　　According to the abovementioned findings the reduction of performance in a long forced draft air-cooled heat exchanger may gen

89、erally be ascribed primarily to a reduction in air flow through the fans along the windward side of the bank when crosswinds prevail as shown in figure 9.3.12 (a), and to recirculation of hot plume air as shown in figure

90、 9.3.12 (b) when the winds blow in the direction of the major axis of the heat exchanger. Fahlsing [95FAll observed reverse rotation of out of service fan</p><p>　　Figure 9.3.12: Flow patterns reducing perfo

91、rmance.</p><p>　　9.4 RECIRCULATION AND INTERFERENCE</p><p>　　As in the case of banks of air-cooled heat exchangers，recirculation of hot，moist plume air is known reduce the performance of rows of

92、 cooling tower units or cells [77CI1，88BS1].</p><p>　　Furthermore, when several banks of air-cooled heat exchangers or rows of cooling tower cell are located next to each other，the plume of one bank or row m

93、ay be drawn into an adjacent one．This phenomenon is referred to as interference.</p><p>　　Ribier [88RI1] conducted recirculation tests on models of induced draft cooling towers cells similar to the types sho

94、wn in figure 1.1.3, but without a diffuser. Initial tests were conducted on a row consisting of three cells with fills in counterflow and crossflow respectively. The results of these tests are shown respectively in figur

95、es 9.4.1(a) and 9.4.1(b) as a function of different wind directions and ratios of wind speed(measured 10 m aboveground level)to plume exhaust speed Vw/Vp．The perce</p><p>　　Figure 9.4.1: Recirculation in th

96、ree-cell counterflow and crossflow cooling tower.</p><p>　　A further set of tests was conducted by Ribier in which two rows of counterflow cooling towers each consisting of three ceils were first arranged en

97、d to end (six ceils) and then systematically spaced one, two and three cells apart. Of these tests the continuous row of six cells experienced most recirculation with results as shown in figure 9.4.2. Recirculation appea

98、rs to be a maximum at Vw/Vp ＝ 0.9.</p><p>　　Figure 9.4.2: Recirculation in six-cell cooling tower.</p><p>　　Figure 9.4.3: Recirculation in a counterflow cooling tower consisting of two three-c

99、ell rows, two cells apart.</p><p>　　As shown in figure 9.4.3 recirculation is considerably reduced when the two rows of three cells each are separated by a distance of two cells. Further separation does not

100、reduce recirculation much.</p><p>　　By placing two rows of three cells each side by side, recirculation is relatively high as shown in figure 9.4.4.</p><p>　　Figure 9.4.4: Recirculation in cool

101、ing tower consisting of two rows of three cells located side by side.</p><p>　　If the two rows of three cells are separated by one cell width only a relatively small reduction in maximum recirculation is exp

102、erienced as is shown in figure 9.4.5.</p><p>　　Based on these results it may be concluded that a row of induced draft cooling tower cells should be arranged in-line with the prevailing wind direction. A high