Heat Transfer HEAT TRANSFER TERMINOLOGYHeatFluxThe rate at which heat is transferred is represented by the symbol . Common units for heatQtransfer rate is Btu/hr. Sometimes it is important to determine the heat transfer rate per unit area,or heat flux, which has the symbol . Units for heat flux are Btu/hr-ft^{2}. The heat flux can beQdetermined by dividing the heat transfer rate by the area through which the heat is beingtransferred.(2-1)QQAwhere:= heat flux (Btu/hr-ft^{2})Q= heat transfer rate (Btu/hr)QA = area (ft^{2})ThermalConductivityThe heat transfer characteristics of a solid material are measured by a property called the thermalconductivity(k) measured in Btu/hr-ft-^{o}F. It is a measure of a substance’s ability to transfer heatthrough a solid by conduction. The thermal conductivity of most liquids and solids varies withtemperature. For vapors, it depends upon pressure.LogMeanTemperatureDifferenceIn heat exchanger applications, the inlet and outlet temperatures are commonly specified basedon the fluid in the tubes. The temperature change that takes place across the heat exchanger fromthe entrance to the exit is not linear. A precise temperature change between two fluids acrossthe heat exchanger is best represented by the log mean temperature difference (LMTD or DT_{lm}),defined in Equation 2-2.(2-2)DT1m(DT_{2}DT1)ln(DT_{2}/DT_{1})where:DT2 = the larger temperature difference between the two fluid streams at eitherthe entrance or the exit to the heat exchangerDT1 = the smaller temperature difference between the two fluid streams at eitherthe entrance or the exit to the heat exchangerRev. 0 Page 3 HT-02