大型自然通风冷却塔冬季水损及相关问题研究
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摘要
大型自然通风冷却塔以其较优越的冷却性能,广泛用于电站循环水冷却系统中。冷却塔在冬季运行时会出现很多问题,例如冷却塔水损大,冷却塔进风口结冰导致调节风门失效,凝汽器真空度过高等,由此引起巨大的经济损失或者影响机组安全性。所以研究冬季冷却塔水损等问题具有重要的现实意义。本文主要研究工作如下：
首先,基于Merkel焓差法模型,分别计算了某3500m2自然通风冷却塔在一机一塔模式和并塔模式且冷却负荷给定时的冬季水损特性,分析了环境温度、相对湿度、循环水量等因素对水损的影响。结果表明,当一机一塔模式运行时,环境温度对于蒸发水损的影响显著,在基准循环水量上下20%的范围内,循环水量对于蒸发水损的影响较小,相对湿度对于水损的影响很小；对于并塔模式,环境温度、相对湿度、循环水量等因素对水损的影响规律与一机一塔模式运行时类似,在相同条件下,与一机一塔模式相比并塔模式水损增加25%左右,但并塔模式运行对防冻结及控制凝汽器真空度有利。
其次,设计了一种冷却塔改造方案,该方案在自然通风冷却塔(原塔)进风口处添加一个空气预热器,预热进塔空气,分担冷却负荷,原塔变为干湿复合塔。建立了相应的水损计算模型,计算了旁通比对干湿复合塔水损的影响,结果表明,与原塔相比,干湿复合塔进塔风量明显减少,在某些环境温度下,进塔风温明显提高,水损在某些环境温度范围内可得到有效控制(少于原塔),验证了在适当的旁通比范围内本改造方案可实现防冻结、控制凝汽器真空度和水损的目的。
再次,分别建立了自然通风冷却塔及干湿复合塔水损三维数值模型,计算分析了某3500m2自然通风冷却塔及干湿复合塔的冬季水损特性。结果表明,无侧风且给定冷却负荷时,原塔水损随环境温度的变化规律与一维结果一致；在保证进塔水温不变的条件下,侧风对自然通风冷却塔塔内换热有明显影响,冷却负荷和水损随侧风的增大显著地减少；无侧风且保证进塔水温不变时,在一定的旁通比范围内,干湿复合塔的水损较原塔明显减小；有侧风且保证进塔水温不变时,干湿复合塔水损随侧风的增大而明显减小。
最后,综合考虑凝汽器和冷却塔对凝汽器真空度的影响,建立了冷却塔与凝汽器的耦合计算模型,分析了自然通风冷却塔在一机一塔和并塔运行工况下,环境温度、循环水量等因素对于凝汽器真空度的影响。结果表明,对原塔而言,一机一塔模式运行且冷却负荷一定时,凝汽压力随环境温度的升高而明显升高,循环水量的增加会降低凝汽压力,相对湿度对于凝汽压力的影响较小,并塔模式运行且冷却负荷一定时,凝汽压力随环境温度及循环水量的变化规律与一机一塔模式运行时类似,但凝汽压力较后者高1000-3000Pa左右；对于干湿复合塔,在冷却负荷一定时,凝汽压力随旁通比(0.1-0.4范围内)的增大而增大,随循环水量的增大而减小,验证了通过调节旁通比控制凝汽器真空度的可行性。
本文工作表明,作者所设计的自然通风冷却塔改造方案,基本可以达到防冻结、控制水损和凝汽器真空度的目标,可为冷却塔运行调节和系统改进提供参考。
Large natural draft wet cooling towers (NDWCT) are widely used to cool circulating water of power plants due to their superior performance. In winter cooling towers operate with some problems such as excessive cooling load and water loss, tower inlet freezing due to low temperature air, and high condenser vacuum degree. Such problems cause enormous economic loss, and even threaten the safety of the units. Therefore the research on such problems of cooling towers in winter is very important. The main work in this thesis is as follows.
Firstly, based on Merkel enthalpy difference model, for specified cooling load, evaporation water loss in winter of NDWCT (3500 m2) for one unit-one tower mode and two units-one tower mode are calculated and the effects of ambient temperature, relative humidity, circulating water flow rate are analyzed. The results show that for one unit-one tower mode, ambient temperature influences evaporation loss significantly. Circulating water flow rate has little effect on evaporation loss in the range of±20%fluctuation. The effect of relative humidity on evaporation loss is very small and negligible. For two units-one tower mode, the effects of ambient temperature, relative humidity and circulating water on evaporation loss are similar to that of one unit-one tower mode. Although the evaporation loss of two units-one tower mode is about 25%higher than one unit-one tower mode, it provides a way of anti-freezing and controlling condenser vacuum degree.
Secondly, a dry-wet hybrid cooling system (DWHCS) is designed based on original NDWCT. DWHCS introduces an air preheater at the inlet of NDWCT to heat the ambient air before it enters the cooling tower. A water loss calculation model is built and the effect of bypass ratio is evaluated. The results show that compared with NDWCT, air flow rate of DWHCS decreases significantly. In a specified ambient temperature range, the air temperature increased greatly and the water loss is effectively controlled (less than the original NDWCT). The conclusions confirm that it is feasible to use DWHCS to prevent freezing, control condenser vacuum degree and water loss.
Thirdly, three dimensional numerical simulation model for NDWCT (3500 m2) and DWHCS is built and water loss in winter is calculated and analyzed. For zero crosswind velocity and constant cooling load, the water loss characteristics of NDWCT agree to the one-dimensional results. When the tower inlet water temperature is specified, crosswind velocity affects cooling load and water loss of NDWCT significantly. For zero crosswind velocity and constant cooling load, water loss of DWHCS is less than that of NDWCT in suitable bypass ratio range. When the tower inlet water temperature is specified, water loss of DWHCS deceases greatly with the increasing in crosswind velocity.
Finally, the author builds a coupled model combining cooling tower and condenser to investigate the condenser vacuum degree. The results show that for one unit-one tower mode of NDWCT with specified cooling load, the condensing pressure increases greatly with the increasing in ambient temperature and the increasing in water flow rate decreases the condensing pressure while the effect of.relative humidity is small. For two units-one tower mode NDWCT with specified cooling load, the effects of ambient temperature and water flow rate are similar to one unit-one tower mode but with 1000-3000Pa higher value of condensing pressure. For DWHCS with specified cooling load, condensing pressure increases with the increasing in bypass ratio (in the range 0.1-0.4) and decreases with the increasing in water flow rate. The results confirm the feasibility of using bypass ratio to control the condenser vacuum degree.
The present study shows that using dry-wet hybrid cooling technique to prevent freezing, control the condenser vacuum degree and reduce water loss is feasible and the work in this thesis provides a good reference for improving the operation of natural draft cooling tower.
首先,基于Merkel焓差法模型,分别计算了某3500m2自然通风冷却塔在一机一塔模式和并塔模式且冷却负荷给定时的冬季水损特性,分析了环境温度、相对湿度、循环水量等因素对水损的影响。结果表明,当一机一塔模式运行时,环境温度对于蒸发水损的影响显著,在基准循环水量上下20%的范围内,循环水量对于蒸发水损的影响较小,相对湿度对于水损的影响很小；对于并塔模式,环境温度、相对湿度、循环水量等因素对水损的影响规律与一机一塔模式运行时类似,在相同条件下,与一机一塔模式相比并塔模式水损增加25%左右,但并塔模式运行对防冻结及控制凝汽器真空度有利。
其次,设计了一种冷却塔改造方案,该方案在自然通风冷却塔(原塔)进风口处添加一个空气预热器,预热进塔空气,分担冷却负荷,原塔变为干湿复合塔。建立了相应的水损计算模型,计算了旁通比对干湿复合塔水损的影响,结果表明,与原塔相比,干湿复合塔进塔风量明显减少,在某些环境温度下,进塔风温明显提高,水损在某些环境温度范围内可得到有效控制(少于原塔),验证了在适当的旁通比范围内本改造方案可实现防冻结、控制凝汽器真空度和水损的目的。
再次,分别建立了自然通风冷却塔及干湿复合塔水损三维数值模型,计算分析了某3500m2自然通风冷却塔及干湿复合塔的冬季水损特性。结果表明,无侧风且给定冷却负荷时,原塔水损随环境温度的变化规律与一维结果一致；在保证进塔水温不变的条件下,侧风对自然通风冷却塔塔内换热有明显影响,冷却负荷和水损随侧风的增大显著地减少；无侧风且保证进塔水温不变时,在一定的旁通比范围内,干湿复合塔的水损较原塔明显减小；有侧风且保证进塔水温不变时,干湿复合塔水损随侧风的增大而明显减小。
最后,综合考虑凝汽器和冷却塔对凝汽器真空度的影响,建立了冷却塔与凝汽器的耦合计算模型,分析了自然通风冷却塔在一机一塔和并塔运行工况下,环境温度、循环水量等因素对于凝汽器真空度的影响。结果表明,对原塔而言,一机一塔模式运行且冷却负荷一定时,凝汽压力随环境温度的升高而明显升高,循环水量的增加会降低凝汽压力,相对湿度对于凝汽压力的影响较小,并塔模式运行且冷却负荷一定时,凝汽压力随环境温度及循环水量的变化规律与一机一塔模式运行时类似,但凝汽压力较后者高1000-3000Pa左右；对于干湿复合塔,在冷却负荷一定时,凝汽压力随旁通比(0.1-0.4范围内)的增大而增大,随循环水量的增大而减小,验证了通过调节旁通比控制凝汽器真空度的可行性。
本文工作表明,作者所设计的自然通风冷却塔改造方案,基本可以达到防冻结、控制水损和凝汽器真空度的目标,可为冷却塔运行调节和系统改进提供参考。
Large natural draft wet cooling towers (NDWCT) are widely used to cool circulating water of power plants due to their superior performance. In winter cooling towers operate with some problems such as excessive cooling load and water loss, tower inlet freezing due to low temperature air, and high condenser vacuum degree. Such problems cause enormous economic loss, and even threaten the safety of the units. Therefore the research on such problems of cooling towers in winter is very important. The main work in this thesis is as follows.
Firstly, based on Merkel enthalpy difference model, for specified cooling load, evaporation water loss in winter of NDWCT (3500 m2) for one unit-one tower mode and two units-one tower mode are calculated and the effects of ambient temperature, relative humidity, circulating water flow rate are analyzed. The results show that for one unit-one tower mode, ambient temperature influences evaporation loss significantly. Circulating water flow rate has little effect on evaporation loss in the range of±20%fluctuation. The effect of relative humidity on evaporation loss is very small and negligible. For two units-one tower mode, the effects of ambient temperature, relative humidity and circulating water on evaporation loss are similar to that of one unit-one tower mode. Although the evaporation loss of two units-one tower mode is about 25%higher than one unit-one tower mode, it provides a way of anti-freezing and controlling condenser vacuum degree.
Secondly, a dry-wet hybrid cooling system (DWHCS) is designed based on original NDWCT. DWHCS introduces an air preheater at the inlet of NDWCT to heat the ambient air before it enters the cooling tower. A water loss calculation model is built and the effect of bypass ratio is evaluated. The results show that compared with NDWCT, air flow rate of DWHCS decreases significantly. In a specified ambient temperature range, the air temperature increased greatly and the water loss is effectively controlled (less than the original NDWCT). The conclusions confirm that it is feasible to use DWHCS to prevent freezing, control condenser vacuum degree and water loss.
Thirdly, three dimensional numerical simulation model for NDWCT (3500 m2) and DWHCS is built and water loss in winter is calculated and analyzed. For zero crosswind velocity and constant cooling load, the water loss characteristics of NDWCT agree to the one-dimensional results. When the tower inlet water temperature is specified, crosswind velocity affects cooling load and water loss of NDWCT significantly. For zero crosswind velocity and constant cooling load, water loss of DWHCS is less than that of NDWCT in suitable bypass ratio range. When the tower inlet water temperature is specified, water loss of DWHCS deceases greatly with the increasing in crosswind velocity.
Finally, the author builds a coupled model combining cooling tower and condenser to investigate the condenser vacuum degree. The results show that for one unit-one tower mode of NDWCT with specified cooling load, the condensing pressure increases greatly with the increasing in ambient temperature and the increasing in water flow rate decreases the condensing pressure while the effect of.relative humidity is small. For two units-one tower mode NDWCT with specified cooling load, the effects of ambient temperature and water flow rate are similar to one unit-one tower mode but with 1000-3000Pa higher value of condensing pressure. For DWHCS with specified cooling load, condensing pressure increases with the increasing in bypass ratio (in the range 0.1-0.4) and decreases with the increasing in water flow rate. The results confirm the feasibility of using bypass ratio to control the condenser vacuum degree.
The present study shows that using dry-wet hybrid cooling technique to prevent freezing, control the condenser vacuum degree and reduce water loss is feasible and the work in this thesis provides a good reference for improving the operation of natural draft cooling tower.
引文
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[3]江自生,韩买良．火电机组水资源利用情况及对策[J].华电技术,2008,30(6)：1-5
[4]王芳.我国火电厂耗水水平分析与节水措施[J].河北电力技术,2001,2,6-9
[5]姚梅.火力发电厂冷却塔节能分析[J].江西电力职业技术学院学报,2005,18(2)：8-9
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[7]王佩章.火电机组的干湿联合冷却方式[J].河北电力技术,1995,3,11-15
[8]王佩璋.研究各种节水技术大幅度降低火力发电取水量[J].电站辅机,2004,91(4)：30-34
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