AFT Product Tips
Increasing Heat Exchanger Accuracy for Large Temperature Differences and Non-Linear Heat Capacity
Many times when modeling heat transfer using heat exchangers in AFT Fathom, significant temperature differences across the heat exchanger can have a major impact on the accuracy of the heat transfer results. This can especially become a problem with non-linear heat capacity relationships. The heat transfer across the heat exchanger in the model below is calculated from the temperature difference across the heat exchanger and the arithmetic average heat capacity.
One way to enhance the accuracy of the heat transfer results is to model the single heat exchanger junction, J2, as multiple heat exchangers as shown in the below model.
The “Specified Heat Rate In vs. Flow” thermal model would be reasonable to use for each heat exchanger (for a discussion on this type of thermal model see Problems With Heat Exchanger Specified Heat Rate In Constants?). A design factor could be used for reducing the junction heat transfer for each heat exchanger (ie, if using five heat exchanger junctions to model one heat exchanger, a design factor of 0.2, or 1/5 would be used for each junction).
Apply the full pressure drop of the original heat exchanger to the first heat exchanger in the series. Pipes P101 and P106 will have the same specifications as pipes P1 and P2, respectively. The remainder of the pipes can be short frictionless pipes.
The end result will be a more accurate heat transfer solution due to the smaller increments of the average heat capacity used for each heat exchanger calculation.
Great solution! Having a more accurate heat transfer solution would give a lot of benefits, especially to those who are having much more problems with this. To better analyze these, I guess having software for heat transfer analysis can be a good thing to start with.