It is expounded theoretically that C and m of the dimensionless equation Nu=CRe Pr m,which is in common use,should not be constant.This simple dimensionless equation is unable to reflect accurately the heat transfer when Prandle number equals 0 001～7500.The authors made use of integral equations to solve the laminar flow heat transfer problem of various field flow fields around a plate of Prandtl number 0 001 to 7500.Another form of dimensionless equation is recommended which can describe the functional relationship among Nusselt number,Raynolds number and Prandtl number more accurately.
In order to save energy and metallic material,it is very important to calculate the logarithmic mean temperature difference between the two kinds of fluids and the superficial area of the heat exchanger accurately.The traditional calculational method is too complicated,but the error of simplified method is too significant,so we deduce the formulas for logarithmic mean temperature difference between the cold and hot fluids in a heat exchanger in consideration of their thermophysical properties and the formula for sueperficial area of the exchanger.The formulas are easy to use and give accurate results.
