Accuracy of Harbeck Diagram for Forced Evaporation

The Harbeck Diagram, a method for estimating the forced evaporation from a cooling pond, gives the fraction of heat load dissipated by evaporation as a function of windspeed and natural (unloaded) water surface temperature. The derivation of the diagram is presented, emphasizing the approximations that permit suppression of the temperature excess as an independent variable. The error in the Diagram results from these approximations, increasing as the temperature excess increases, and from inaccuracy in the estimate of the natural surface temperature. The magnitude of the error is presented as a function of wind speed and surface temperature. The accuracy of the Diagram is shown to be satisfactory for ordinary engineering purposes. However, in order to relate the evaporative heat-flux fraction to forced evaporation (by multiplying by the power-plant heat load) the condition is required that the heat load be dissipated completely to the atmosphere, thus that the thermal structure be equilibrated.