There is considered the calculation of the average value of the overall heat-transfer coefficient for counterflow between parallel plates or in concentric tubes when the flows are laminar and the thermal entry lengths for one or both streams extend over a significant part of the exchanger length. It is shown that if this length is at least less than the total length for one of the fluids, the average value of the overall coefficient can be calculated with adequate accuracy from the average values of the individual coefficients by means of the usual definition of the overall coefficient. When both thermal entry lengths exceed the exchanger length, then a special calculation is needed to obtain suitable average overall values. This result is given in graphical form and supported by more exact results from the literature and as obtained here.

This content is only available via PDF.