# Heat Transfer through Evaporators

by recreated
Tags: evaporators, heat, transfer
 P: 51 The heat convection equation will provide a good calculation of the heat transfer but it requires a knowledge of LMTD meaning you need to know the hot fluid inlet temp, hot fluid outlet temp, cold fluid inlet temp and cold fluid outlet temp. You also need to be able to calculate U, which requires the use of empirical correlations (for condensers anyway, not sure about evaporators). It can be difficult to accurately calculate all these terms. If you are simply looking for a good approximation of the heat transfer, an energy rate equation (enthalpy eqn) should suffice. In the case of an evaporator, the working fluid (refrigerant) acts as if it has an infinite specific heat capacity and can be assumed as isothermal. When adding energy to this fluid, the enthalpy of vaporization is the major contributor and sensible heat can probably be neglected, provided the liquid is relatively saturated at inlet and outlet (isnt too subcooled or superheated) If this is the case, the energy eqn simplifies to the heat/energy added to the fluid; $\dot{Q}$=$\dot{m}$$\Delta$hfg All you need to know is the flow rate of the refrigerant and the change in enthalpy from inlet to outlet (get this in thermo tables for whatever fluid you are looking at).