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# [Need Help] Clean-Dirty-Service Heat Transfer Coefficient

## [Need Help] Clean-Dirty-Service Heat Transfer Coefficient

(OP)
Hello,

I need to reconfirm with those that have experience with the said issue. I'm in a never ending argument on the following issue in determining the overall heat transfer coefficient in different condition.

One party says that U_dirty is actually U_service and another party says that it is not. I have attached sample calculation with two methods, one method with U_dirty and U_service calculated differently and another method is U_dirty and U_service is consider the same.

When it comes to excess design, one party stands by saying adding extra area on dirty area is not going to be efficient since it's just an exchanger working under dirty condition while other party saying that extra area over the dirty area will cause the unit to not work effectively and will cause greater margin on the excess area.

All helping hands is greatly appreciated and thank you is in advance

### RE: [Need Help] Clean-Dirty-Service Heat Transfer Coefficient

You need to be careful about making HX too large.
If you increase the tube count you might reduce the flow velocity which could lead to more fouling.
If you make it longer you will increase pressure drop and require more pump hp.
Over sizing by more than 15% makes me wonder how you arrived at the values to use for fouling.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

### RE: [Need Help] Clean-Dirty-Service Heat Transfer Coefficient

(OP)
Hi all,

Thanks for the response. After back and forth argument we finally drag in the supplier to explain their exchanger philosophy. Another point I failed to informed was that this isn't shell and tube type but it's a plate type heat exchanger, a plate and shell type to be exact.

Now, the supplier data sheet specifically stated there Excess Design 59% and Excess Area 167%. This was the issue that led to the argument between clean-dirty-service heat transfer rate. In summary, depending on the size of the plates the excess design and excess area will vary. For our case, it was found out that the overall heat transfer in service is similar to dirty and this is due to (what they say) to minimize excess area if the heat transfer rate in service is consider as a separate.

This is our finding between the supplier at the moment and thanks 25362 and EdStainless for your respond. Appreciate it a lot, thank you

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