2 replies to this topic

[Rajesh India]

Vessels that have full or half pipe limpet coils are often provided with heat transfer cements for enhancing the heat transfer area. Particularly for the full pipe limpet coil, the point of contact between the limpet pipe and the vessel is minimal. Experience shows such arrangements do prove effective for heat transfer, yet specifying them in absence of this information could be error prone. How to estimate the available heat transfer area for such arrangements?, i.e. how much credit should be taken for the additional area that is made available by application of heat transfer cement?
Thanks for your help in advance.

[Art Montemayor]

Rajesh:

A Limpet coil is simply a "half-pipe" welded to the shell (usually the outside surface) of a vessel (usually a reactor). This is ilustrated in various websites as such.

A full-pipe coil in contact with the vessel shell is problematic. The conduction surface, as you state, is minimal and not effective at all. The result, in my opinion is ineffective and not practical to fabricate nor to apply. I've never - in all my years - seen one or heard of one. A half-pipe coild is always welded and never "cemented" to the shell. Even the half-pipe coild is not very effective and any cement, in my opinion, is a waste of time and doesn't do anything for the heat transfer.

You make several claims regarding effectiveness. Can you give us references or cite personal experience? A vessel jacket is merely an attempt at trying to maintain some degree of heat transfer. It is never employed as the total answer to the vessel's required heat transfer. A circulating pump and an external exchanger is the only, positive way to control or manage the total heat transfer in a reactor or a vessel - if that is the requirement.

[Rajesh India]

Thanks Art for your reply.

Full pipe limpet coil design is very common in India. The pipe is simply tack welded (at intermittent points, not even continuous weld) to the shell so that it more than holds on to the shell on its own. Since the point of contact is minimal, the gap is filled with heat transfer cement (different from the conventional cement). The heat transfer area obtained this way is at least corresponding to the diameter of the pipe. At times, the heat transfer cement is applied to even largeer surface such that it can cover the width which is as high as twice the pipe diameter.

Similar to the above, for half pipe coil also, heat transfer cement is applied to get enhanced heat transfer area.

While the above arrangement does provide additional heat transfer surface, the question is how to de-rate the half pipe or full pipe coil side heat transfer coefficient. If the utility side coefficient in itself is not to be de-rated, additional resistance term in terms of the thickness of the heat transfer cement should be introduced in the calculation of overall heat transfer equation, but unfortunately, this resistance unlike the shell metal wall thickness is not exactly in series with the heat transfer direction.

I have used this type of cement in the past, but not sure what make it was. The closest reference that I can find can be seen from http://www.csiheat.c.../controheat.htm