Drawbacks of the UA Model

Quite often when I talk with process engineers, they are quite happy with the modelling of heat exchangers done in the process simulators.

Most this modelling is done via an approximation: a constant UA value.

This approach is perfectly adequate if you do not change your process conditions. When you go from a single phase application to a 2-phases application the coefficient U is changing dramatically; but even in a single phase application, the coefficient can change.

The property of the fluid can change with the temperature and pressure so changing the conditions, will change the coefficient.

Even if you consider the same temperature and pressure, the flowrate is enough to change the coefficient. The coefficient is linked to the reynolds number:

this mean that if you change your process conditions, the local heat transfer changes then the overall U value alters, which means that as the UA is constant, the Area MUST change!

I don't think that there are a lot of exchangers that can provide this feature.