AUGMENTATION OF HEAT TRANSFER BY EVAPORATIVE COOLING TO REDUCE CONDENSING TEMPERATURES. (VOLUMES I AND II)

BRANISLAV KORENIC, Purdue University

Abstract

The overall thermal efficiency of power producing and power consuming processes increases if they operate at lower condensing temperatures. The purpose of this work is to show that reduced condensing temperatures can be achieved by augmentation of external heat transfer. Substantial improvement is obtained when the outside heat transfer surfaces are continuously wetted by recirculated water of which only a smal quantity evaporates and needs to be replenished. Such heat exchangers known as evaporatively cooled condensers use air and water simultaneously. Problems associated with the evaporative condenser theory exist because the heat and mass transfer processes within the system are not of the closed type. Differential equations which describe the process cannot be solved, since the number of unknown parameters is larger than the number of available equations. The lack of acceptable theory is compensated by the collection of experimental data and by fitting them in some empirical form. This work is directed in an entirely different way than similar reported work in literature. The study provides analytical solutions to the problem by recognizing that the heat transfer performance and the change of state of air and water along their paths in the heat exchanger can be evaluated in a step-by-step procedure. This was first performed with the aid of a Mollier i-x diagram and later simulated by an extensive computer program. A platefin tube type condenser and bare tube type condenser models were investigated. The heat and mass transfer processes in an evaporative condenser were solved by computerized graphical methods using the i-x diagram which obeys all conservation laws. Analytical results were compared with measured experimental data from a bench type apparatus. An in-line bare tube test section has been tested and its performance was compared with the predicted performance. Results are very close. Deviation of measured from predicted data are of the order of only a few percent. Results of this investigation show undoubtedly the benefit of the evaporative cooling method for augmentation of heat transfer in a condenser. Reduced condensing temperatures are commonly close to ambient temperature or even below it. Savings of energy consumption range from 10 percent for power plants to 50 percent for refrigeration systems. Wetting the condenser with water requires only a very small amount of additional power.

Degree

Ph.D.

Subject Area

Mechanical engineering

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