The Design of Contra-Flow Heat Exchangers

The problem of contra-flow heat exchangers, for gases at turbulent flow, is treated in a general way, using the analogy between heat transfer and friction. For making this analogy more evident, another non-dimensional heat transfer coefficient is introduced. This number may be used profitably instead of the Nusselt number for describing heat transfer in tubes, and in similar problems. Thus the main dimensions of the apparatus are given in formulae based on the intended heat exchange, the total drop of temperature, the total loss of energy, and the properties of the fluid. It is shown that weight and total volume of the heat exchanger depend on the ratio of temperature drop and loss of energy on both sides of the heat exchanging surface. The conditions of minimum weight and minimum volume are derived.