Designing a test heat exchanger
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 fluid physical properties required: density, viscosity, thermal conductivity.
- Decide on the type of exchanger to be used.
- Select a value for the overall coefficient, U.
- Calculate the mean temperature difference, Δtm .
- Calculate the area required from equation Q = UA Δtm
- Decide the exchanger layout.
- Calculate the individual coefficients.
- Calculate the U and compare it with selected U value. If the calculated value differs significantly then return to step 6.
- Optimize the design: repeat steps 4 to 10, as necessary, to determine the cheapest exchanger that will satisfy the duty. Usually that will be one with the smallest area.
Typical values of the overall heat-transfer coefficient for various types of heat exchanger are given below:
Shell and tube exchangers
|Hot fluid||Cold fluid||U (W/m2 oC)|
|Organics (some non-condensables)||Water||500-700|
|Water||Dilute aqueous sol.||200-500|