9 replies to this topic

[seth]

Dear Experts,

I want to know what is the formula to be used in calculating the no. of Baffles for shell and tube condenser considering that the shell side nozzle size and distance are 2" NB Sch 40 and 2000mm.

Thanks,

Seth

[riven]

There is no one method never mind one equation.

Suggest searching the forms for exchanger design and consulting Kern process heat transfer amoung other references.

[djack77494]

I don't know what heurisitcs are built into heat exchanger design software, and I don't know how easy it would be to find out. I've seen tables suggesting spacing commonly employed, but didn't get close enough to know the limitation. Practically, there will be mechanical limitations, max and min. As you increase the number of baffles, you are increasing the shell side turbulence. Results in better heat transfer, but also higher pressure drop. This might be something best solved by trial and error. Pick a baffle spacing and see how well the heat transfer and hydraulics meet your constraints.

[seth]

actually when i was in Dammam saudi arabia 3 years ago, my colleague in engineering imparts me such formula, is very effective particularly in preliminary design, it is our starting point so that we can adjust the required pressure drop in shell side. unfortunately i misplaced it.

[Jiten_process]

I dont think there is any hard core formula for that and even if it is there it would be used as a guideline coz in course of optimization of thermal design you have to manipulate the no. of baffles considering allowable pressure drop, vibration, stream analysis etc. which is case specific, i would say. One guideline which I use when i am optimizing thermal design is that the ratio of central baffle spacing to shell id should be in between 0.3 to 0.6. This is what experienced base guideline that u might gate good trade off of your pressure drop and heat transfer coefficient. Allowing higher pressure drop doesnt mean always you get higher heat transfer. Increasing the no. of baffle more than desired would increase your A & E stream fraction and u may loose your heat transfer coeff.

what others have to say on this...

[Rudra]

Normally the minimum baffle spacing is 20% of shell ID. Maximum baffle spacing is limited by the maximum unsupported length as per TEMA standard to avoid the vibrations.

By 2000 mm distance, u mean to say 2000 mm exchanger length?

You can try with initial guess of baffle spacing as 20% of shell id. With this u will know the baffle spacing.
number of baffles can then be calculated by = ( tube length/baffle spacing ) - 1. However this formula assumes that inlet and outlet spacing are same as central spacing.

However this may or may not be optimum baffle spacing.

If you are designing a condenser, number of baffles may not play an important role because condensing coefficeints are higher. On the other hand if the shell side fluid is single phase, then effect of number of baffles will be more because it will increase the turbulance and hence the sensible coefficient.

But as Jiten mentioned, too many baffles may increase baffle to shell leakage stream and hence the exchanger performance.

In nut shell, you can decide number of baffles as an initial guess, but further optimisation is needed.

[breizh]

Dear Seth,

I would suggest you to visit www.gulleyassociates.com where you can find plenty of good information about heat transfer and design.

There is also a very good paper from Ravij Mukherjee in Chemical engineering progress -feb 1988
ref : Effectively design shell & tube Heat exchangers.

regards
Breizh

[seth]

Rudra,

Actually, the 20% shell ID baffle distance is not practically working as it depends upon the required reynolds number of shell side fluid that controls the required pressure drop. the formula that you had given (number of baffles can then be calculated by = ( tube length/baffle spacing ) - 1) is this applicable to u- tube bundle? shall i calculate the total tube length including the bended section? the 2000mm distance is the center to center distance of shell nozzle.

second thing the formula 20% shell ID baffle distance is not applicable anymore in shell and tube condenser, am i correct?

Seth

[Rudra]

Baffle spacing equal to 20% shell ID is the minimum.
Maximum baffle spacing is limited by TEMA unsupported length span.

If Shell diameter is too small, the baffle spacing of 20% shell ID will become too small too mechanical place the baffles in the exchanger.

Thats why i said optimum baffle spacing needs to be calculated.
Find attached a paper by MUkherjee on Effectively design shell and tube exchangers

Attached Files

•  0_Effectively_design_S_T_HEs_17_pages.pdf   241.91KB   472 downloads

[seth]

Rudha,

Thank you very much, actually i got already that reference attachment since 2004 when i am in saudi arabia.

Thanks,

seth