| 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. |
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