Improvement of the environmental and economic characteristics of cooling towers : Part 1 The periodic cooling tower - small scale, full scale, and surface roughening tests

摘要

This report describes research for the past year on methods to enhance the environmental and economic performance of dry cooling towers. The work has concentrated on dry cooling towers utilizing periodic water to air heat exchangers. Periodic towers have sheet metal discs partially submerged in hot, power plant condenser effluent. As the discs rotate, the discs alternately absorb heat from the effluent and discharge heat to the air. The means of heat transfer on the air side can be either evaporative and/or convective, depending upon the amount of effluent carried into the air side on the surface of the plates. An oil layer, floated on the water's surface, has been shown to be effective in eliminating water carry-over to the air side. The advantages of the periodic tower lie in the low cost of the discs and the ability to operate dry . A periodic tower should be significantly less expensive than a conventional dry tower fabricated with finned tubes. Further, since the periodic tower can also operate wet, by removing the oil film, the high capacity losses incurred during warm months by conventional dry towers can be eliminated. In the past year, tests on a 20 inch model of the periodic tower indicated that at operational speeds of the full sized disc, no noticeable churning of the oil layer ocurred. Heat transfer rates measured with the 20 inch model were 20 percent higher than predicted by an approximate analysis. Finally, silicone oils' low vapor pressure make them superior to hydrocarbon oils on the basis of evaporation rates, but their higher costs and possible ecological impact necessitates a closer look at hydrocarbon oils. Construction of the full scale test facility is complete. Shaped, rigid five foot diameter discs have been designed, fabricated, and assembled into an initial test module. Instrumentation for heat transfer tests is also complete; including instrumentation for measuring disc temperatures, and both air and water flow rates and temperatures. A parametric study of the oil film thickness on the air side of the test module has been completed. Measured thicknesses were 20 percent less than that previously predicted. This thinner oil layer means an increase in the disc heat transfer. A test apparatus to study the further enhancement of disc performance by surface roughening has been constructed, instrumented and calibrated. The apparatus has reproduced experimental friction factor and heat transfer coefficient curves for flat plates, and will be used to evaluate enhancement as a function of rib shape, size, spacings and Reynold's number. Visits to several architectural engineers across the country were made with representatives of Air Preheater Company to gage their acceptance of the rotary concept. All felt that if the reliability of the rotary concept could be demonstrated and the costs were less than equivalent conventional dry towers, the rotary desing would be the preferred choice. Based on these findings, Air Preheater is planning to do design and cost studies of the rotary concept for the coming year.