Most catalogs of evaporative condensers show tables for selecting condensers by two different methods. One is the condenser capacity or heat-rejection method and the other is the refrigeration capacity method. The condenser capacity method is most straightforward because it deals with the actual heat-transfer rate at the condenser. The refrigeration capacity method is simply a tool to facilitate quick selections, because the user of the catalog will normally think in terms of the refrigeration capacity which then becomes the entry point of the catalog.
The condenser capacity method will be explored first. Two tables are associated with this method—one table presents the capacity factors as functions of condensing and wet-bulb temperatures and the other table is nominal heat rejection capacity of various models of condensers. Most manufacturers show the capacity factors in tabular form. The presentation in Figure 7.15 is graphical. Table 7.2 shows a excerpt of a manufacturer’s table11 of nominal capacity of a series of condensers.
The capacity factors shown in catalogs and in Fig. 7.15 may at first seem to present trends opposite of expectations. For example, the capacity factor would be expected to increase as the wet-bulb temperature drops and the condensing
temperature increases. But the main purpose of the catalog is to facilitate selection of condensers, not to analyze their performance. The consistency of the method can be demonstrated by an example.
The application of the capacity factors of Figure 7.15 in the example explains what might have appeared to be an incongruity. The capacity of a condenser is highest at high condensing temperatures and low wet-bulb temperatures, but Fig. 7.15 shows just the opposite trends. The purpose of the capacity factor is to aid in selecting the condenser and is multiplied by the specified heat-rejection rate. In so doing, when the capacity factor is low, a smaller condenser will be adequate, and the capacity factor is low when the condensing temperature is high and the wet-bulb temperature low.
If the refrigeration capacity method is chosen, a different capacity factor table and nominal rating table will be available from the manufacturer. Section 7.4 pointed out that the ratio of heat rejected at the condenser to the refrigeration capacity, the HRR, depends on the condensing temperature and the evaporating temperature. The capacity factor applicable to this method must therefore incorporate these two temperatures if the selection process is entered with the refrigerating capacity. A capacity factor table comparable to Fig. 7.15 offers the chance of incorporating the influence of the condensing temperature on the HRR, so a separate table correcting for the evaporating temperature will also be provided by the manufacturer.
Of these two selection techniques, the heat-rejection method is more powerful in accommodating system complexities, such as might occur in two-stage plants. The refrigeration capacity method is useful for quick estimates of the condenser size.