The Preparation of a Common Technique for Selecting Counter Flow and Crossflow Cooling Towers

Abstract

The industrial cooling tower is a device for cooling water by energy transfer into a passing air stream, Such a device is mechanically simple but analysis and selection are relatively difficult because of the many possible combinations of operating conditions. For these latter reasons a comprehensive selection chart is fundamental to the correct sizing of cooling towers. Ideally it is the purpose of the selection chart to represent all probable cooling duties whilst remaining easy to use. In practice more than one chart is necessary to satisfy these requirements. Unfortunately, few detailed selection techniques have been published and no standard method of presentation formulated, although most attempts have been based upon the Merkel Method. Some Authors have used the development of the Merkel Method in the form of the Merkel Diagram and its derivatives to plot selection curves. Others, mainly within the cooling tower industry use the Merkel Method in combination with their own techniques of presentation. For evaluation of the Merkel Method, knowledge of the bulk air enthalpy and the equilibrium enthalpy temperature relationship is required, It is the object of this work to consider the published cooling tower theory and selection techniques, to explore the theoretical analysis of cooling towers and ultimately to propose easily used common techniques for presenting selection data for counter flow and crossflow cooling towers. Each method is considered for its accuracy, scope and usefulness with particular reference to its day-to-day use in industry. A short section of the work has been devoted to surveying the enthalpy temperature relationships which appear in the literature as well as developing several new ones. The relationships were considered for accuracy over the desired temperature range and the time required for computation and as a result one of the available relationships and also one of the newly developed ones were selected for use in the computer program. To obtain the large amount of information necessary to plot the selection charts, computing methods were used to evaluate the enthalpy temperature relationships, the energy balance and driving force equations and hence the Number of Diffusion Units (NDU).