7 replies to this topic

[fatimah]

dear friends

as for shell & tube for example, why sometimes clients want to add margin on the surface area? planning to use higher flow rate next time? i will say it may not. adding margin into the surface area will make the HE larger but lesser or near-to-zero margin still make it works. can anybody explain?

Edited by fatimah, 06 January 2010 - 12:57 AM.

[Root]

Fatimah:

Designer keeps over-deisgn due to fouling factor and during his design calculation nornally, he will add more Q required and then he will further calculate the surface area UA.

For gas processing facilities, you can ask the designer why he is giving you a fouling factor while the gas processing facilities don't have tendency of fouling and, in an other words, while they over-design they are making more money.

Hope you will get good idea.
Cheers

[srfish]

After the heat exchanger is put into operation and later the flows are increased, the resulting change in performance depends on the nature of the fluids involved. It is possible that there will be no degradation in performance. This is possible if the fluid inside plain tubing is controlling the overall heat transfer coefficient and the Reynolds number is in the transition range between laminar and turbulent flow.

[fatimah]

Dear friends

actually the over design i mean here is at the surface area.
if the flow rate is going to be increased, the margin will be added at the flow rate.
then for fouling, the margin is added when we set the fouling factor.
i still dont get the use of surface area margin as there is no specific effect i can see at the end.
we know the output; that's the bigger surface area but we unsure the input, the max flow rate or maybe max temp they able to use.

[Zauberberg]

Actually is as simple as that: adding 10-20% surface area accomodates for exchanger fouling. Remember the basic heat transfer equation: Q = UA x LMTD.
As U declines over time, Q will decrease as well. Adding surface area during the initial design is an attempt to maintain design duty, even when the exchanger starts experiencing fouling.

As being said many times in these forums, try to design an exchanger in such way that accounting for fouling will not actualy promote fouling during exchanger operation, by maintaining high velocities and turbulence within the allowable pressure drop. Speaking in real terms and examples, it's always better to add an extra surface area by choosing longer tubes rather than increasing the number of tubes. The former will add some margin over design and avoid fouling (at the expense of slightly higher dP), while the latter will add some initial margin but at the expense of reduced velocities and lower dP, and therefore higher probability of fouling in future.

[Andres Valencia Michaud]

Actually, this is the first time I see someone with a margin in HE's area.
Thanks to Zauberberg my simulation runs smoothly now and there wasn't any margin in the area design. While "U" is not constant, it would mean that a larger area, would get a larger "U" and a smaller "LMTD" for the same fluid flow, meaning the output temps won't be the ones designed for, making the HE under perform until it gets dirty where for the same fluid flow, "U" will be smaller and also "A".
I might be wrong with all of this, but this is very interesting so, if someone can prove me wrong, please explain.

[Aleq Barry]

Dear friends

actually the over design i mean here is at the surface area.
if the flow rate is going to be increased, the margin will be added at the flow rate.
then for fouling, the margin is added when we set the fouling factor.
i still dont get the use of surface area margin as there is no specific effect i can see at the end.
we know the output; that's the bigger surface area but we unsure the input, the max flow rate or maybe max temp they able to use.

There are two ways to account for fouling. You include a fouling factor and then get the HT area. Until the fouling to same extent is achieved, this acts as margin in the HT area (margin between clean and dirty HT coefficients). Problem is that for certain fluids, one may not have a guideline for the fouling factor. In this case, you arrive at the HT area without assigning any fouling factor and allow certain design margin in HT area per your judgement.

If one wants to add design margin on HT area for allowing higher flow rate to the extent of this margin, it needs to be seen more carefully. The reaosn for this is that coefficients change at higher flows, so this also should be taken into account, instead of blindly allowing more HT area in terms of number of tubes or leangth. Pressure drop at such higher flow is also another factor that should be accounted for in such intended overdesign.

[breizh]

Hi Fatimah ,
attached a link to Norsok standard (page 17) about extra surface
http://www.standard..../1132/P-100.pdf

Hope it helps
Breizh