# Heat Transfer

Energy Balance Equation for Heat Exchangers - The heat energy balance equation for a heat exchanger essentially states that, heat lost from hot fluid is taken up by the cold fluid. Hence depending on the mass flow and mass heat capacity on both sides, the hot and cold sides of a heat exchanger undergo temperature change.

Heat Transfer by Conduction - Heat transfer through solids occurs by means of heat conduction. Conduction is explained using Fourrier's equation. Equations are derived for steady state heat conduction through a flat wall and a cylindrical wall.

Heat Transfer by Convection - Heat Transfer through fluids generally occurs by means of heat convection. Convection typically occurs between a solid surface and a fluid which could be gas or liquid. The fluid motion could be caused purely by density differences, which is known as ‘natural convection heat transfer’ or it could be caused by other mechanical devices such as fans, which is known as ‘forced convection heat transfer’. Role of heat transfer coefficient to describe convective heat transfer is also emphasized.

Heat Transfer Coefficients for Heat Exchangers - For heat transfer equipments, separate ‘heat transfer coefficients’ (h) can be defined for the hot and cold media of the equipment. These coefficients can be used to describe the heat transfer on each side of the heat exchanger. In addition an ‘overall heat transfer coefficient’ (U) can be defined which describes the overall heat transfer occurring in the exchanger.

Residence Time - Concept of residence time is important in understanding the operation of heat transfer equipments. Residence time for a vessel, equipment or a piece of pipe can be defined as the average time spent by a random fluid particle passing through the vessel during a continuous process.