Preliminary Heat Exchanger Design

Introduction
This article will help you understand how you can estimate the initial design characteristics of a heat exchanger.  An Excel spreadsheet using the equations developed in this article is also provided. The equations are derived from a simple heat balance, and a few other elementary relationships.

These equations act as initial estimates, and the results will need to be refined by more sophisticated calculations.  If you just want the spreadsheet then click here, but read the rest of the article if you want to understand the theory.

Theory
Consider a heat exchanger operating in countercurrent flow.

Assume that we know the
  • desired input and output temperatures of all streams, 
  • specific heat capacities, 
  • the overall heat transfer coefficient, 
  • and the mass flowrate of the cold stream.  
We will now calculate
  • the flowrate of the hot stream, 
  • the heat transfer rate 
  • and the heat transfer area.
A heat balance on the cold and hot streams gives


Qc gives the overall heat transfer rate.

But Qh = Qc.  Equating both equations and rearranging for the mass flowrate of the hot liquid stream gives


The log mean temperature difference is


The overall heat transfer rate Qc can be defined in terms of the log mean temperature difference


The final two equations can be easily rearranged to give the overall heat transfer area A in terms of the heat transfer rate Qc, the heat transfer area A and the log mean temperature difference.

Excel Implementation
Implementing these basic heat transfer equations in Excel is easy, and no special explanation is required.


Download Excel Spreadsheet for Preliminary Heat Exchanger Design

Related article: Modeling the Temperature Dynamics of a Cross-Flow Heat Exchanger

3 comments:

Rajan Narasimhachari said...

The article is short and good.

It would have been great if you had included the meanings of Qh,mc and others for laymen like me.

Roy Stevens said...

Your heat transfer area is unitless??

Samir Khan said...

Roy - the heat transfer area is in metres

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