7 replies to this topic

[UofUChemE]

Hello, 

 

I am trying to calculate the allowable pressure drop for a shell and tube heat exchanger. Instead of going through all of the calculations is there any fairly accurate rules of thumbs that I can use?

 

 

Thanks.

[Bobby Strain]

One calculates the actual pressure drop. One specifies the Allowable.

 

Bobby

[UofUChemE]

Thank you for sharing this I appreciate it. 

 

I am completely new to the world of heat exchangers so how can I know what psig values are appropriate when specifying the allowable pressure drops?

[Bobby Strain]

If you have a real job that requires this, talk to one of your co-workers. Or supervisor.

 

Bobby

[Pilesar]

Allowable pressure drop depends on the service. In general, higher pressure drop means higher velocity which corresponds to higher heat transfer coefficient. So the exchanger designer will try to use most of the pressure drop allowed by the process engineer since it will give the cheapest exchanger cost. The process engineer may allow a lot of pressure drop where the pressure has to be reduced anyway. An example is a stream sourced from a high pressure pipeline. Where pressure is critical, such as a vacuum distillation column, the dP of the condenser can greatly affect plant operation and only a fraction of a psi may be allowed. For liquid/liquid exchangers which are pumped, 5 psi allowed is typical. For compressed gases perhaps 2 psi is more appropriate since compression is much more expensive than pumps. Plant design should take into account economics. Low pressure drop exchangers have higher capital cost but usually with a lower operating cost. Somewhere there is an optimal choice for equipment design, but it is difficult to take short cuts without 'going through all the calculations' at least in the beginning. You can try to rely on 'rules of thumb' but lasting learning is by experience. You get the experience by repetition, reworking unsatisfactory designs, economic comparisons of alternate designs, etc. Engineering can be hard work and is not for everybody. Taking shortcuts that the engineer has not personally verified is a snake waiting to bite. As a student, you may not have time for thorough research. But take the time in your career to really learn engineering and not just parrot rules of thumb. There is much more to designing a heat exchanger than specifying a duty.and calculating a pressure drop.

  If you are trying to calculate pressure drop of an existing exchanger design, there are simplifications you can make. But you really should go through the rigorous calcs first and figure out for yourself which parameters are important and which can be neglected. After you have done that for a few exchangers, you may decide that rigorous calcs are really not that difficult after you become familiar with them.

[breizh]

Hi ,

You should be prepared for calculation , rules of Thumb are useless .

attached resources , many more using the search engine in this forum

Good luck,

 

Breizh

Edited by breizh, 14 June 2018 - 08:02 AM.

[Art Montemayor]

Once again, our good friend and long-time Forum member, Breizh, has submitted some excellent reading and studying material.

 

The subject of pressure drop calculations for the shell side of shell and tube heat exchangers has been one that always presented a problem for me during my process design and project engineering years.  I now see that Breizh has contributed an article that I long sought but was not successful in obtaining - the great discussion and design article by Dale Gulley - otherwise known to our Forum members as our fellow member "SRFISH".

 

Dale's formidable and expert experience and knowledge in heat exchanger design and fabrication is exhibited in this excellent article that every chemical engineer and student should read and study in detail.  This is expert information coming from an expert in the field and won't be found in common text books on the subject of Process Heat Transfer.  Even Donald Kern in his famous "Process Heat Transfer" text book does not deal with this subject as well as Dale has done.

 

Thank you, once again, Breizh for your timely and kind contribution.  This piece of information fills a void that existed in my Heat Transfer data library for a long time.

 

And thank you Dale Gulley - albeit belatedly.

[UofUChemE]

Thank you all for your responses. This has helped me quite a bit.

 

Thank you! 

UofUChemE