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1、学校代码： 10128学 号： 031203060 本科毕业论文外文翻译（ 题 目：空冷热交换器和空冷塔 学生姓名： 学 院：电力学院 系 别：能源与动力工程 专 业：热能与动力工程 班 级：动本2003 指导教师： 二 七 年 六 月Air-cooled Heat Exchangers and Cooling TowersD.G.KROGER Sc.D. (MIT)(This text is a part of MR KROGERs book. include 8.4, 9.3, 9.4)8.4 RECIRCULATIONHeated plume air may recirculate in
2、 an air-cooled heat 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 vertically above the heat exchanger. A part of the warm plume air may
3、however be drawn back into the inlet of the tower. This phenomenon is known as recirculation. Plume recirculation is usually a variable phenomenon influenced by many factors, including heat exchanger configuration and orientation, surrounding structures and prevailing weather conditions. Because of
4、higher discharge velocities, recirculation is usually less in induced draft than in forced draft designs. Figure 8.4.1: Air-flow pattern about forced draft air-cooled heat exchanger.Lichtenstein 51LI1 defines a recirculation factor as (8.4.1)where mr is the recirculating air mass flow rate, while ma
5、 is the ambient air flow rate into the heat exchanger.Although the results of numerous studies 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. Gunte
6、r and Shipes 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 reported 81EP1. Kroger et al. investigated the problem analytically, experim
7、entally and numerically and recommend a specific equation with which the performance effectiveness of essentially two-dimensional mechanical draft heat exchangers experiencing recirculation, can be predicted 88KR1, 89KR1, 91DU1, 93DU1, 95DU1.8.4.1 RECIRCULATION ANALYSISConsider one half of a two-dim
8、ensional mechanical draft air-cooled heat exchanger in which recirculation occurs. For purposes 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). Figure 8.4.2: Flow pattern about heat exchanger.It is assumed that the
9、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 platform and decreases towards ground level). The outlet velocity, vo, is assumed to be uniform and in the vertical
10、 direction.Consider the particular streamline at the outlet of the heat exchanger that 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 it will
11、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, Bernoullis equation can be applied between 1 and 2 to give (8.4.2)It is reasonable to assume that the total pressure at I is approximately e
12、qual to the stagnation pressure of the ambient air at that elevation i.e. (8.4.3)At2 the static pressure can be expressed as (8.4.4)Furthermore, for the ambient air far from the heat exchanger (8.4.5)Substitute equations (8.4.3), (8.4.4) and (8.4.5) into equation (8.4.2) and find (8.4.6)Due to visco
13、us 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 a value that is higher than the mean inlet velocity. Examples of numerically determined inlet velocity distributions for d
14、ifferent outlet velocities and heat exchanger geometries are shown in figure 8.4.3 95DU1. Since most of the recirculation occurs in this region the velocity v2 is of importance but difficult to quantify analytically. For it will be assumed that v2 can be replaced approximately by the mean inlet velo
15、city, vi, in equation (8.4.6). Thus (8.4.7) Figure 8.4.3: Two-dimensional inlet velocity distribution for Wi/2 = 5.1 m.According to the equation of mass conservation, the flow per unit depth of the tower can be expressed as (8.4.8)if the amount of recirculation is small.According to equations (8.4.1
16、) and (8.4.8) the recirculation factor is (8.4.9)Substitute equations (8.4.7) and (8.4.8) into equation (8.4.9) and find (8.4.10)where is the Froude number based on the width of the heat exchanger.The influence of a wind wall or deep plenum can be determined approximately by considering flow conditi
17、ons between the top of the wind wall, (Hi + Hw), as shown in figure 8.4.2(b) and elevation Hi. Consider the extreme case 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
18、 the tower exit is essentially equal to the ambient stagnation pressure. With these assumptions, apply Bernoullis equation between the tower outlet at the top of the wind wall and the elevation Hi. (8.4.11)But (8.4.12)Substitute equation (8.4.12) into equation (8.4.11) and find (8.4.13)If it is assu
19、med that the recirculation decreases approximately linearly with increasing wind wall height, equation (8,4.10) may be extended as follows: (8.4.14)Since the recirculation is assumed to be essentially zero at Hw = Hwo, find a = 1.Substitute equation (8.4.13) into equation (8.4.14) and find (8.4.15)w
20、here is the densimetric Froude number based on the wind wall height.It is important to determine the effectiveness of the system when recirculation occurs. Effectiveness in this case, is defined as (8.4.16)The interrelation between the recirculation and the effectiveness is complex in a real heat ex
21、changer. Two extremes can however be evaluated analytically i.e.1. No mixingThe warm 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 .
22、Figure 8.4.4: Recirculation flow patterns.This in effect means that the part of the heat exchanger where recirculation occurs, transfers no heat. The actual heat transfer rate is thus given by (8.4.17)resulting in an effectiveness due to recirculation of (8.4.18)Substitute equation (8.4.15) into equ
23、ation (8.4.18) and find (8.4.19)2. Perfect mixingThe recirculating air mixes perfectly with the inflowing 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).If for purpose of illustration, it is assumed
24、that the temperature of the heat exchanger,is constant, it follows from equation (3.5.22) that the effectiveness under cross-flow conditions is (8.4.20)or (8.4.21)Furthermore the enthalpy entering the heat exchanger is or (8.4.22)Substitute equation (8.4.22) into equation (8.4.21) and find (8.4.23)
25、In this case the effectiveness due to recircuiation is given byFrom equation (8.4.22) and (8.4.23), substitute the values of Tir and Tor into this equation, to find the effectiveness of the heat exchanger. (8.4.24)In practice the effectiveness will be some value between that given by equation (8.4.1
26、8) 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 numerical analysis of the problem 89KR1, 95DU1. Figure 8.4.5: Heat exchanger effectiveness.Duvenhage and Kroger 95DU1 solved the
27、 recirculation problem numerically and correlated their results over a wide range of operating conditions and heat exchanger geometries by means of the following empirical equation: (8.4.25)This equation is valid in the and where. In this equation represents the effective height above the inlet to t
28、he fan platform and includes the plenum height in addition to any wind wall height.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).8.4.2 MEASURING RECIRCULATIONIn the absence of wind walls, recirculation can be sign
29、ificant 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 forced draft air-cooled heat exchanger without wind walls, is drawn directly downwards into the low pressure region created by the fans.
30、 The results of recirculation tests conducted at the Marimba power plant are reported by Conradie and Kroger 89CO1. They actually measured the vertical temperature distribution of the air entering the heat exchanger and observed a relatively higher temperature in the vicinity of the fan platform. As
31、 shown by the smoke trail in figure 8.4.7 recirculation of the plume air occurs in this region Because of the approximately 10 m high wind wall surrounding the array of A-frame heat exchanger bundles, a reduction in effectiveness of less that one percent is experienced under normal operating conditi
32、ons in the absence of wind. The effectiveness can be determined according to equation (8.4.25).Figure8.4.6: Plume air recirculating in air-cooled steam condenser.Figure8.4.7: Visualization of recirculation with smoke at the Matimba power plant.Generally less recirculation occurs in induced draft coo
33、ling systems due to the relatively high fan outlet velocity and height of diffuser if one is present.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 t
34、here 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. In such situations recirculation is employed in a system incorporating automati
35、cally controlled louvers that cause more or less of the hot plume air to mix with the ambient cooling air as shown in figure 8.4.8. Other arrangements are also possible 80RU1.Figure 8.4.8: Louver controlled plume air recirculation in air-cooled heat exchanger.Steam coils located immediately below th
36、e tube bundles may be required to preheat the air during startup in winter.9.3EFFECT OF WIND ON AIR-COOLED HEAT EXCIHANGERSIn general winds have a negative effect on the performance of mechanical draft heat exchangers. Plume air recirculation tends to increase while fan performance is usually reduce
37、d during windy periods.Laboratory studies 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 Wydok power plant 76SC1.Figure 9.3.1: Redu
38、ction in turbine output due to wind at the Wyodak power plant.Before the 160 MWe power plant at Utrillas in Spain was built, extensive 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 w
39、ind patterns. The results of the tests are shown in figure 9.3.2. 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 under extremely gusty conditions. After extensive experimen
40、tal 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 pattern into the air-cooled condenser during periods of westerly winds, no further trips were experienced and performance was significantly
41、improved 97GO1. Figure 9.3.2: Reduction in turbine output at the Utrillas power plant due to wind. Figure 9.3.3: Modifications at the Matimba power plant.From the case studies listed above it is clear that the interaction between the air cooled heat exchanger and adjacent buildings or structures can
42、 significantly complicate flow patterns and consequently reduce plant performance.Kennedy and Fordyce 74KE1 report the results of model studies to determine downwind temperature distribution, recirculation and interference (ingestion of an adjacent towers effluent plume) characteristics.Slawson and
43、Sullivan 81SL1 conducted experiments in a water plume to recirculation 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 on the desi
44、gn 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 recirculation and interference. Recirculation and interference measurements of 40 to 70 percent were found to exits for the rectangular array concept, while values of 20 to 30 percent were measured for the round tower arrangeme
