Compensating for End Effects in Plate-and-Frame Heat Exchangers

Because fluid flowing through the end channels of plate exchangers only passes heat through one of the channel walls (as opposed to core channels where heat is transferred through both walls), the heat transfer for this element of fluid is less than that for the bulk. The result is that the temperature profile along the end channel can be substantially different from that in the center of the exchanger. This distortion affects each channel in turn. Thus, the temperature profile in the exchanger is not uniform. What may come as a surprise to many is that the end effects can still be significant when the exchanger has many plates. Correction factors accounting for this effect have been proposed by Shah & Focke [1]. The correction factors are dependent upon the ratio of the heat capacity flow rates of two streams, the required effectiveness, and the number of plates used. The results are given in the form of tables relating the correction factor to discrete values of each of these variables. These correction factors are applied to the log mean temperature difference in order to obtain an effective value. Consequently, the procedure treats the exchanger area as a continuous variable. Because exchanger plates come in a range of fixed sizes and plate count is an integer, the exchanger area is a discrete variable. A simple bypass model is presented here that allows the designer to determine and adjust for the end effects while treating the area as a discrete variable. The predictions of the model compare favorably with the tabulated corrections. Using the model, it is possible to determine the end effects for any combination of duty and exchanger size. The implications of the end effects upon the design are briefly considered.