Page 408 - Handbook of Energy Engineering Calculations

measure of the total heat being transferred. Figure 2 shows how the
temperature curves vary with the amount of heat-transfer area for counterflow
and parallel-flow exchangers when the fluid inlet temperatures are kept
constant. As Fig. 2 shows, the counterflow arrangement is superior.

FIGURE 2 For certain conditions, the area between the temperature
curves measures the amount of heat being transferred.

If enough heating surface is provided, in a counterflow exchanger, the
leaving cold-fluid temperature can be raised above the leaving hot-fluid
temperature. This cannot be done in a parallel-flow exchanger, where the
temperatures can only approach each other regardless of how much surface is
used. The counterflow arrangement transfers more heat for given conditions

and usually proves more economical to use.

TEMPERATURE DETERMINATION IN HEAT-EXCHANGER