The Use of the Axial Heat Conduction of the Wall for Increasing the Effectiveness of Heat Exchangers

The effect of heat transfer in the wall in the direction of the flow of heat carriers on the effectiveness of heat exchangers is considered for the cases when the wall ends are not thermally insulated from the environment. Use is made of the analytic solution to the problem of parallel-plane flow of heat carriers, which has been considered earlier. The solution depends on the following parameters: ratio of coefficients α of heat transfer from heat carriers; ratio of thermal equivalents β of heat carriers (it is assumed that α = β = 1); number Ntu of heat-transfer units; parameter C_A characterizing the axial heat conduction of the wall; the Biot number determining the relative heat transfer from the wall ends to the environment, and the temperatures of the media contacting the wall ends. The effect of the heat conduction the wall increases with decreasing C_A. The following two cases are considered: (case I) the temperatures of the environment are assumed to be equal to the inlet and outlet temperatures of the hot heat carrier; (case II) these temperatures are equal to the inlet and outlet temperatures of the cold heat carrier. It is shown that for large Biot numbers (Bi > 1), the effectiveness of using of one of heat carriers can be elevated due to the effect of the axial heat conduction of the wall; in this case, the temperature of the other heat carrier during its flow in the heat exchanger varies insignificantly. The boundaries of the beginning of the effect of the axial heat conduction of the wall on the effectiveness of the heat exchanger are determined.