Lesson 8 - Heat Exchangers

In the lesson on Radiation and Combined, you learned how to define the heat transfer rate across a multi-layered system. In this lesson, we examine how this definition can be used to determine the heat transfer rate across a heat exchanger.

Let’s consider a brewing example of a heat exchanger used to cool beer prior to filtration to show how heat is transferred through a heat exchanger.

• A warmer fluid, e.g. beer at 4°C, is pumped into the heat exchanger and is distributed across every alternative plate with a plate heat exchanger, or through the tubes with a shell and tube exchanger.

• A colder utility fluid, e.g. glycol at -6°C, is forced (pumped) into the other side of the exchanger and distributed across every alternative plate with a plate heat exchanger or through the shell with a shell and tube exchanger.

• Heat is transferred from the warmer beer to the colder glycol through the solid metal plate or the tube wall resulting in the beer stream typically cooling down to -1°C and the glycol stream warming up to around -4°C.

The mechanism for heat transfer in a heat exchanger is a combination of forced convection (through the fluids) and conduction (through the exchanger plates or tube walls).

In inline heat exchangers the overall heat transfer rate is defined by a modified equation for multi-layered heat transfer shown below:

Where

q is the overall heat transfer rate measured in W (Js^-1)

U is the overall heat transfer coefficient measured in (Wm-2 K-1)

A is the total heat exchanger surface area measured in m²

∆T_LM is the log mean temperature difference measured in °C

You will be familiar with the terms of q, A and U from the previous lessons. The equation introduces the term, log mean temperature difference. This is an approximation of the average temperature driving force across the heat exchanger and will be expanded on later in this lesson.

In inline heat exchangers, because the inlet or initial temperature of the product fluid remains constant, heat transfer rate is steady-state. This concept is defined in the lesson Principles of Heat Transfer and Conduction.

You are now more familiar with how heat transfer is defined for heat exchangers. Let’s have a look at some typical values for overall heat transfer coefficients in heat exchangers and what this means in heat exchanger design.