A Contribution to the Theory of Natural Draught Cooling Towers

Abstract

In recent years the mechanical draught cooling tower has had the benefit of theoretical analysis developed in Germany and in the United States, resulting in the evolution of the “total heat method” of heat transfer calculation which together with empirical data provides a sound background for thermal design. The natural draught tower contains the same elements with the difference that its performance is also dependent on the atmospheric dry bulb temperature, which introduces a fourth variable in addition to the wet bulb, the load and the water rate which apply in mechanical draught towers. Because of the greater complication it has hitherto resisted a theoretical approach. The present paper offers an extension of the theory to cope with varying air rate, establishes the relevant equations and shows how these can be solved. After deriving the basic characteristics of a tower and fill from analysis of the test data, it is possible to predict the performance of a similar tower under other conditions. Though the analysis may be rather involved for routine design work it offers a basis for the formation of logical ideas as to how the various factors influence performance and enables the effects to be visualized with the aid of diagrams. An explanation is offered to account for the observed effect of wind in increasing the draught. The issues of water loss by evaporation and carryover, the formation of the visible plume, and the avoidance of nuisance are discussed from physical principles.