【文章內(nèi)容簡介】 gure recirculation of the plume air occurs in this region Because of the approximately 10 m high wind wall surrounding the array of Aframe heat exchanger bundles, a reduction in effectiveness of less that one percent is experienced under normal operating conditions in the absence of wind. The effectiveness can be determined according to equation (). : Plume air recirculating in aircooled steam condenser. 內(nèi)蒙古工業(yè)大學本科畢業(yè)論文 8 : Visualization of recirculation with smoke at the Matimba power plant. 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. 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 there 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 automatically controlled louvers that cause more or less of the hot plume air to mix with the ambient cooling air as shown in figure . Other arrangements are also possible [80RU1]. Figure : Louver controlled plume air recirculation in aircooled heat exchanger. Steam coils located immediately below the tube bundles may be required to preheat the air during startup in winter. 內(nèi)蒙古工業(yè)大學本科畢業(yè)論文 9 OF WIND ON AIRCOOLED HEAT EXCIHANGERS In 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 reduced during windy periods. Laboratory studies and field tests have shown that the output of drycooled power stations may be significantly reduced by winds. As shown in figure the wind speed and direction significantly influences the turbine output at the Wydok power plant [76SC1]. Figure : Reduction 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 aircooled condenser and power plant orientation, taking into consideration local wind patterns. The results of the tests are shown in figure . 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 experimental and numerical investigations modifications to the wind walls and cladding were implemented as shown in figure . Due to the resultant improved air flow pattern into the aircooled condenser during periods of westerly winds, no further trips were experienced and performance was significantly improved [97GO1]. 內(nèi)蒙古工業(yè)大學本科畢業(yè)論文 10 Figure : Reduction in turbine output at the Utrillas power plant due to wind. Figure : 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 significantly plicate 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 tower39。s effluent plume) characteristics. Slawson and Sullivan [81SL1] conducted experiments in a water plume to recirculation and interference for two conceptual configurations of forced draft drycooling towers, a rectangular array and a multiple round tower arrangement. The objective of the study was to investigate and make remendations on the design and arrangement of 內(nèi)蒙古工業(yè)大學本科畢業(yè)論文 11 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 arrangement. Field 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 longitudinal 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]. In 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. Duvenhage and Krosger [96DU1] numerically modelled the air flow patterns about and through, an aircooled heat exchanger during windy conditions, taking into consideration the coupled effects of both recirculation and fan performance. They consider a long heat exchanger bank as shown schematically in figure consisting of bays, each bay having two 6blade m diameter fans. The heat exchanger is subjected to winds blowing across or parallel to the longitudinal axis and having a velocity distribution as given by equation () with b = as remended by VDI 2049 [78VDI] . : Schematic of air cooled heat exchanger. 內(nèi)蒙古工業(yè)大學本科畢業(yè)論文 12 Figure : Details of bay geometry. A more detailed crosssection of the bay is shown in figure . Each bay has an effective bu