16 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

[behrouzifar]

Hi all,

 

When preparing a process datasheet for equipment, should the allowable pressure drop be increased above the PFD value to account for the overdesign on flow rate?

 

Assuming a 10% overdesign, some propose scaling the allowable DP using this relation: (Design DP) = (1 + 0.1)^2 * (PFD DP). Is this method correct, or should the PFD value be specified in the datasheet?

 

Using this approach, if the allowable pressure drop in the PFD is 50 kPa at a normal flow rate of 1000 kg/h, the datasheet will report an allowable DP of 60 kPa for a 10% overdesign.

[Pilesar]

Specify the allowable pressure drop according to the process design flow. If the allowable pressure drop in the PFD is 50 kPa at a normal flow rate of 1000 kg/h, then the datasheet should reflect the maximum pressure drop allowed for the design. Overdesign for equipment is used as a "safety factor" to allow for operation that does not exactly conform to expected conditions. Note that the PFD pressure drop probably includes some piping that is not included within the boundaries of the equipment data sheet. If that is the case, then the equipment datasheet should probably show a different pressure drop than the PFD. Consider elevation changes between pressure nodes on the PFD! Just because there is a change in pressure, then that does not mean this is all due to friction! Note also that if you are specifying the equipment for 110% of flow, then the pressure drop will likely be less at 100% of flow. The equipment designer should use the maximum flow (in this case the 110%) and the maximum allowable equipment pressure drop in the design constraints.

[Pilesar]

For heat exchanger design, there is sometimes better heat transfer when the entire allowable pressure drop is used and sometimes it is better to use the smallest possible pressure drop depending upon the service. The data sheet notes can be quite extensive to describe the process needs. I have always added notes to the heat exchanger datasheets. These help in troubleshooting, designing replacements when the exchanger wears out, and pinning down the actual process needs for the designer. The designer is not a mind reader and there may be constraints which conflict with each other. The datasheet preparer sometimes must work with the equipment designer to resolve issues that only arise during the detailed design. There are design standards such as TEMA but there is also much flexibility in actual design. TEMA allows for this.

[behrouzifar]

Thank you for your response.

 

I wanted to clarify my previous mention of the PFD pressure drop. I mean the allocated pressure drop that we include for the equipment in our process simulation. We account for all additional losses, such as pipe losses and static head losses, separately from the equipment pressure drop itself.

My question is how we should represent the allowable pressure drop in the datasheet. Since our simulation is based on a flow rate corresponding to 100% operating conditions, should we report a higher allowable DP value in the datasheet? We have a 10% overdesign factor for the equipment sizing.

 

Thanks again.

 

Specify the allowable pressure drop according to the process design flow. If the allowable pressure drop in the PFD is 50 kPa at a normal flow rate of 1000 kg/h, then the datasheet should reflect the maximum pressure drop allowed for the design. Overdesign for equipment is used as a "safety factor" to allow for operation that does not exactly conform to expected conditions. Note that the PFD pressure drop probably includes some piping that is not included within the boundaries of the equipment data sheet. If that is the case, then the equipment datasheet should probably show a different pressure drop than the PFD. Consider elevation changes between pressure nodes on the PFD! Just because there is a change in pressure, then that does not mean this is all due to friction! Note also that if you are specifying the equipment for 110% of flow, then the pressure drop will likely be less at 100% of flow. The equipment designer should use the maximum flow (in this case the 110%) and the maximum allowable equipment pressure drop in the design constraints.

[Pilesar]

Do not increase specified maximum allowable pressure drop above the maximum allowable pressure drop. If the exchanger is designed for 110%, then the same maximum allowable equipment pressure drop should be used without scaling it up. I think you do not understand why. Perhaps you might explain your thinking.

[behrouzifar]

I agree with your point about not increasing the allowable pressure drop. However, I noticed some engineers scaling the pressure drop, which made me question my own understanding. Thank you again for your feedback.

 

Do not increase specified maximum allowable pressure drop above the maximum allowable pressure drop. If the exchanger is designed for 110%, then the same maximum allowable equipment pressure drop should be used without scaling it up. I think you do not understand why. Perhaps you might explain your thinking.

[Pilesar]

You are in the student forum. Is it some student engineers who are scaling the pressure drop? The reason for the 110% you are using in the equipment design is to allow for uncertainties in thermo, stream composition, stream properties, utilities, etc. It is good practice so that the equipment is not blamed for inability of the plant to perform to capacity. When the equipment is designed inadequately, then all eyes look to the engineer who specified and approved the equipment design! The operators may even name the equipment after the engineer as a mark of shame! Scaling the allowable pressure drop to 110% is less conservative than not scaling the pressure drop. It removes some of the safety factor the 110% is intended for. In real life, it may not be a problem since the exchanger pressure drop will probably be below the maximum allowable. But in theory, it is incorrect to scale the pressure drop.
  In a process plant, the pressure nodes are often the fixed contraints for control. The plant will likely try to push flow rates to the maximum which may be well above the design flows. For example, the heat exchanger is sized to include a fouling layer on the heat transfer surfaces. When the surfaces are clean, the exchanger has greater capacity. Winter operation capacity may be greater because the cooling utility is colder. Process pressure changes affect the condensing and boiling temperatures. Piping and equipment are designed for a maximum pressure which determines the pressure safety valve (PSV) setpoints. The process is not intentionally operated all the way to the PSV setting since the PSV begins to open prior to the PSV setpoint. Typically, the pressure at the top of distillation towers is controlled. In the scope of the overall plant equipment cost, it is not usually much more expensive to make sure the heat exchangers are not a bottleneck. As long as the pressure drop is reasonable, the equipment designer can build an exchanger to conform. Usually the actual pressure drop of the design will be less than the maximum allowable assuming a good design and a reasonable max pressure drop. If there is a constraint in maximum pressure drop that causes the designer a big problem, then that should be discussed between the designer and the process equipment specifier. If this is anticipated to be a specific concern, then it is something to add to the notes on the datasheet! Sometimes other specifications can be relaxed without a problem. To the designer, all specifications are equal, but that is usually not the case with the process specifier.

[breizh]

A few resources about HX

Heat Exchanger Thermal Design Margin Explained

 

A lot of stuff to read in this website .

 

 

Breizh

Attached Files

•  Shell&tube exchanger design NPTEL.pdf   6.21MB   1 downloads
•  Shell Pressure Drop.pdf   6.64MB   1 downloads

[behrouzifar]

Thanks, Pilesar and Breizh, for your responses and the resources.