Heat exchanger calculations

Logarithmic mean temperature and theta value
The logarithmic mean temperature difference, ΔTM, is used as a mean value of ΔT for calculation of the overall heat transfer.

ΔTM is defined as:

Logarithmic mean temperature

Sometimes the so-called theta value is used to define the thermal length of a duty and hence the necessary heating surface:

Theta value

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Principles of a heat exchanger
Hydraulic diameter

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