The general equation for heat transfer across a surface is: Q = UA Δtm                                                                                           (eq – 1.7) Where Q = heat transferred per unit time (W) U = the overall heat transfer coefficient (W/m2  oC) A –heat-transfer area, m2, Δtm = the mean temperature difference, the temperature driving force, oC. […]

The principal types of heat exchangers used in chemical and allied industries which will be discussed in this chapter, are listed below: Double-pipe exchanger: the simplest type, used for cooling and heating. Shell and tube exchangers: used for all applications Plate and frame exchangers (plate heat exchangers): used for heating […]

As the physical layout of the exchanger cannot be determined until the area is known the design of an exchanger is of necessity a trial and error procedure. The steps in a typical design procedure are given below: Define the duty: heat transfer rate, fluid flow-rates, temperatures. Collect together the […]

Before equation (eq – 1.7) can be used to determine the heat transfer area required for a given duty, an estimate of the mean temperature difference Δtm must be made. This will normally be calculated from the terminal temperature differences: the difference in the fluid temperatures at the inlet and […]

Estimation of the “true temperature difference” from the logarithmic mean temperature difference by applying a correction factor to allow for the departure from true counter-current flow: ΔT m = Ft ΔT tm                          (eq – 1.10) Where ΔT m = true temperature difference, the mean temperature difference for use in the […]

A general expression for U can be easily obtained as follows. Consider a double pipe heat exchanger in which one fluid through the inner pipe and the other fluid through the annular space between space between the two pipes. Let       L          = length of heat exchanger, m ri          = inside […]

An important parameter in the design and monitoring of heat exchangers is the overall heat transfer coefficient, U, between the two fluids. A value for U can be easily obtained by knowing the followings: Mass flow of the fluid, Specific heat of the fluid, Difference in temperature of the fluid […]

In most heat exchangers some scale formation will take place on both sides of the heat transfer surface after the heat exchanger has been in use for some time (unless scale inhibition mechanism is in place). This introduces two additional resistances in the heat flow path. Thus the total thermal […]

Steam is frequently used as heating medium. The mean temperature difference is given here:  A pure, saturated, vapour will condense at a fixed temperature, at constant pressure. For an isothermal process such as this, the simple LMTD can be used in equation 1.3; no correction factor for multiple passes is […]

In some type of heat exchanger, the temperature of both the fluids is varying and there fore logarithmic mean temperature will have to be calculated. The temperature changes of water and hot fluid may be represented graphically by the following figure. During the process the hot fluid will reject heat […]