guidelines for selecting sealing strip locations in shell & tube HXs

[kimdelfield]

hi

does anyone know any guidelines for selecting sealing strip locations in shell and tube HXs, any books or site or advice who really help

kim

 

[Christine74]

API Std. 660 has some guidelines for bypass sealing devices that may be helpful to you.

-Christine

 

[srfish]

If you don't let the shell side pressure drop calculation determine the number of seal strips, then a rule of thumb is:
one seal strip for every 5 tube rows that are between baffle cuts.

 

[zerok]

Hi Kim,

To make it short, the use of sealing strips depends primarily on the diametrical clearance between the shell and the tube bundle. As a general engineering practice, use of sealing strips (or another sealing device) is recommended when the shell-bundle diametrical clearance exceeds approx 30 mm. Fixed tubesheets or U-tube rear head types do not require sealing strips since the shell-bundle clearance ranges usually from 11 to 15 mm. Other rear head designs need greater clearance to accomodate the floating head so sealing strips should be used. The number of sealing strips pairs is usually 1 pair for each 5 tube rows crossed.

Refer to :

- Process heat transfer by Hewitt et al.

- Perry's chemical engineers hand book.

- Heat Exchanger design hand book (more expensive).

Hope this helps

zerok

 

[Montemayor]

No one has addressed the basic question that asks for selecting the sealing strip locations in a Heat Exchanger. Besides that, none specify the difference between sealing strips on horizontal (“long”) baffles and those on segmental baffles – a big, big difference.

The only location for a sealing strip that I could, perhaps, justify is that on a horizontal baffle – and even then I would have to scrutinize the justification, the design, the methodology, and the technique employed before I would approve such a venture. This is an area where there have been many applications with bad results – and some of them have been mine! At least I paid for my mistakes: I had to wake up in the middle of the night on several occasions to try to help a maintenance crew well behind schedule in trying to routinely extract a tube bundle without any luck. We wound up finally literally destroying a few bundles in order to salvage the shell of several “frozen” heat exchangers. No engineer has really enjoyed life until he has had to go through such a harrowing, frustrating experience and cursing the day the exchanger’s designer was born. This is not only a wasteful and expensive experience, it also defies common sense when one experiences real life, shell side fouling and mechanical fabrication shortcomings.

Allow me to illustrate what happens when one applies information one reads in a text book or else where:

If we allow a diametrical clearance of approx 30 mm to a segmental baffle in a nominal 24” OD heat exchanger shell fabricated out of seamless schedule 20 pipe (which the norm for such a relatively small exchanger), then we have allowed an available flow by-pass area of

By-Pass Flow area = (0.7854) (23.25^2 – 22.07^2) = 42 inches^2

This is the equivalent of the flow area for a pipe almost 8” in nominal size! It is fairly obvious that this can’t be acceptable “general engineering practice”. When one considers that a clearance of 1/16” is a lot to give up in exchangers over 30” ID, a person who has gone through the labor of extracting such a tube bundle will be aware of the terribly heavy weight being forced to move through such small clearances for lengths that can be up to 20 feet. This is a formidable chore for any maintenance crew and imposes a lot of safety questions. You really have to get out there with these guys to appreciate what you are forcing them to go through. A segmental baffle sealing strip that reduces the baffle clearance down to zero is simply not practical. With a miniscule-thick corrosion film it becomes an almost impossibility to pull a bundle out of a shell.

And all this becomes a vicious circle: You design an internal floating head exchanger because you have fouling service that necessitates cleaning out the bundle. Therefore, you have to extract the bundle from time-to-time. But since you seek heat transfer efficiency and low capital cost, you employ sealing strips. Then you find out that you can’t extract the bundle because it has “frozen” into fixed position. Now, you have to replace the exchanger or destroy the bundle in order to replace it with another and guess what? You have, for all practical purposes, a pseudo-fixed tube sheet exchanger performance despite the internal floating head design.

Some other solutions may work for some, such as: use Teflon or other soft material as a sealing strip; bore out the ID of the shell to as precise a machined, true dimension as you possible can (however, if corrosion works in, you’re in trouble). But I would not apply sealing strips to the segmental baffles; I would only consider them for the horizontal baffle (2-shell pass).

If all this doesn’t work, then there is always Spiral exchangers.