The thermal design of heat exchangers is directed to calculate an adequate surface area to handle the thermal duty for the given specifications whereas the hydraulic analysis determines the pressure drop of the fluids flowing in the system, and consequently the pumping power or fan work input necessary to maintain the flow. The most common problems in heat exchanger design are rating and sizing. The rating problem is evaluating the thermo-hydraulic performance of a fully specified exchanger. The rating program determines the heat transfer rate and the fluid outlet temperatures for prescribed fluid flow rates, inlet temperatures, and the pressure drop for an existing heat exchanger; therefore, the heat transfer surface area and the flow passage dimensions are available. The sizing problem, however, is concerned with the determination of the dimensions of the heat exchanger. In the sizing problem, an appropriate heat exchanger type is selected and the size to meet the specified hot and cold fluid inlet and outlet temperatures, flow rates, and pressure drop requirements, is determined.
