Abstract :
The paper describes the construction of charts showing the economic duty conditions of natural- and mechanical-draught cooling towers for generating stations in the British climate. The charts are drawn in terms of two factors, namely (a) the annual station load-factor, and (b) the full-load turbine heat-rejection per square foot of net exhaust area. Typical seasonal temperature and load cycles are assumed. The calculations indicate that past design practice often led to towers which were unduly large and water-temperature ranges which were unduly small. By correct design, net capitalized savings of the order of £200 000 per 240-MW station can be made in some instances. The advent of larger turbine units and the associated higher values of factor (b) will make higher water temperatures economic. With mechanical-draught towers, the water temperatures specified should generally be lower than with natural-draught units. A chart is constructed showing the difference in the overall costs of generation of stations fitted with natural and mechanical-draught towers. This chart suggests that mechanical-draught towers are economic only at low values of factor (b), such as those obtained on small turbines. Another chart shows the effect of varying climatic conditions on the comparison. Higher air temperatures favour mechanical draught. Circuits for combined river and cooling-tower schemes are examined. With these arrangements the cooling towers generally operate mainly during the summer season, and consequently mechanical-draught towers are often economic. Even with small rivers having widely variable discharges, such combined schemes can be responsible for substantial savings relative to pure cooling-tower schemes.
