To understand the problem created with excess surface area in an air-cooled heat exchanger, it is necessary to go back to the basic heat transfer equation.


Q is the overall heat transferred.
U is the overall heat transfer coefficient.
A is the heat transfer surface area.

LMTD is the logarithmic mean temperature difference, which is defined as


TPi is the the process temperature in
TPo is the process temperatures out
TAi is the air-side temperatures in
TAo is the air-side temperatures out

If the overall heat transfer coefficient U is purposely derated, real-world performance will be a unit that actually rejects more heat (Q) with a lower outlet temperature on the process (TPo), given the rest of the conditions being held constant. Basically, that means that the heat exchanger will overcool the process.

For more on air-cooled heat exchangers, read the related article, "Keeping It Cool, But Not Too Cool."