Effects of dissipation and temperature-dependent viscosity on the performance of plate heat exchangers

The fluid temperature profiles in a multi-passage plate heat exchanger and its effectiveness were calculated with a model which includes dissipation, temperature-dependent viscosity and appropriate correlations for the Nusselt number and the friction coefficient. When the viscosity of the two fluids is low (e.g. water) the results are identical to the classic ε–NTU relations which are obtained by neglecting dissipation and by assuming that fluid properties and heat transfer coefficients are constant. But, when one of the fluids is very viscous (e.g. glycerol) the temperatures of both fluids are significantly higher while the effectiveness can be higher or lower than the value predicted by the classic relations. In particular, for cases with a very viscous hot fluid, the effectiveness may be even higher than unity.