The simple reason is that they have a basic design flaw of using a cross flow exchanger in place of true counter current exchanger. These cross flow exchangers have low pressure drop compared to counter current & therefore save on compressor sizing if its a turnkey package unit.
Till date there is no guideline or formula in any of the heat transfer books for what should be the flow configuration in a given exchanger. This can come either from FE to some extent or by CFD analysis only.
- Classification & Definition of Various Compressors
- Air Compressor Troubleshooting
- Download - Energy Efficiency in Compressors
- Download - Air Compressor Audit
- Compressed Air Challenge
As said above, there is no guideline or formula which can tell you that whether water flow should be from top or from bottom but this depends on common sense & logical thinking of design engineers.
One good example is as below.
It was a interstage cooler of air compressor in which air is in cross flow pattern and coolign water flow was in two pass in the tubes. So basically one side was counter current & one was co-current but not in real sense. The Sketch is given below.
We were facing the problem of high temperature in this exchanger since commissioning. Based on my previous experience I decided to inspect it for the same flaw ( Actually I got it from experience from a lube oil cooler of big size compressor of 14 MW load - Yes you got it right it was form Fertilizer unit)
On inspection we found above configuration. In this case air exit temperature can not go below CW outlet temperature. So if exchanger is designed for 33° to 43°C CW temperatures then air temperature will always be expected to go beyond 43 + 5° (Expected approach OR ATE) = 48°C. While in true counter current it can be 33 + 5° = 38°C. So there is a huge possible gap in air cooling from 38 to 48°C.
In our case the air temperature were further high at 52°C.
Therefore, based on my previous experience I suggested to swap the CW connections to make it like
In this case, after changing the CW connection, we were able to bring down the air temperatures by 6 - 9°C in different exchangers raising the capacity of air compressor by 10% or so due to cummulative effect in 3 stages.
So, try to identify the configuration of CW inlet & outlet viz-a-viz process inlet & outlet & then you can improve the cooling just by making them right.
Hope you will find some of them in your plant as well.