Heat Exchanger Design Steps, Calculations, Example Problem

Heat exchanger is a widely used equipment in process industry. Heat exchanger is used to transfer heat to reduce energy losses. You should follow several design steps to design a effective heat exchanger. There are several types of heat exchangers and design methods will vary according to that type.

In this tutorial, first we will see some basic things of heat exchangers such as types and uses before we come to the design problem of heat exchangers.

Types of heat exchangers

There are different types of heat exchangers and they are different in their structures and operations.

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 and cooling
Plate-fin exchangers
Spiral heat exchangers
Air cooled: coolers and condensers
Direct contact: cooling and quenching
Agitated vessels
Fired heaters

Applications of heat exchangers

In petrolatum refining industry, heat exchangers play a big role to reduce energy loses. You can see, lot of heat exchangers are installed simultaneously to other unit operations (ex: distillation).

Design steps of heat exchanger

Define the heat duty: heat-transfer rate, fluid flow-rates, temperatures.
Collect physical properties of fluids such as density, viscosity, thermal conductivity. These data can be found in engineering books and online sources.
Decide on the type of exchanger to be used.
Select a trial value for the overall heat transfer coefficient (U).
Calculate the mean temperature difference, δTm by using correct equation.
Calculate area required.
Decide the exchanger layout.
Calculate individual coefficients.
Calculate overall heat transfer coefficient and compare with the trial value. If the calculated value differs significantly from the estimated value, substitute the calculated value for the estimated value and return to step 6 to continue the calculation.
Calculate pressure drop of heat exchanger; if that is unsatisfactory, you must return to steps 7 or 4 or 3, in that order of preference.
Optimize the design: repeat steps 4 to 10, as necessary, to determine the cheapest exchanger that will satisfy the duty. Usually this will be the one with the smallest area.

Design Problem

You have to design a heat exchanger to cool an ammonia gas stream which has a flow rate of 5000 kg/h in 67 bar. Temperature of ammonia stream should be decreased to 40⁰C from 120⁰C. As the coolant, water is going to be used and outlet temperature of water cannot exceed 40⁰C. Pressure drops over the heat exchanger must not exceed 0.5 bars for ammonia stream and 1.5 bars of cooling water. Design a shell and tube heat exchanger for this requirement.

Find physical data for water and ammonia gas

Use books or online web services to find all required data. Density, viscosity, thermal conductivity, specific heat capacity values and more required. Try to obtain, average values for some parameters when other parameters will affect them. As an example, density vary on temperature.