Rob:
What you are describing is a half-pipe coil (or “limpet” coil) wrapped around a pressure vessel. I have written and discussed this heating/cooling technique many times in our Forums, and these can be found in our SEARCH engine. The youtube video you submit is not a true half-pipe. What those guys are doing is taking a relatively think steel (stainless?) plate in a roll and forming it into a semi-half circle that looks more like a half-oval and bending this into a helical coil. That’s fine, but the ultimate strength won’t be the same as that of a true half-pipe. This is usually an important point involving reactors because the external coil is welded onto the external shell surface and, as such, forms part of the pressure vessel. Since it forms part of the pressure vessel, it must conform to the pressure – and vacuum – needs of the vessel. This means that the pressure in the external limpet coil represents “external pressure” to the main vessel walls and that must be designed and allowed for in resisting the internal pressure AS WELL AS the total external pressure. By external pressure here, I mean the possibility of rating the vessel for full vacuum. Vacuum is considered external pressure and must be designed for if it is to occur.
What you see in the video is probably NOT what happens in the majority of fabricating these types of coils. The reason being that the need for these type of coils is not that great and fabrication shops can’t justify the purchase, operation, training, and maintenance of a special fabrication machine to fabricate a coil “once in a blue moon”. Consequently, fab shops take normal schedule 40 (sometimes schedule 30 pipe) and bend it into a coil while it is filled with a fluid – usually machine oil. Then they cut the coil longitudinally and weld it to the shell of the pressure vessel. I have never seen coil made like they show in the video. Making a limpet coil as I’ve described is a very laborious and tedious task. It involves a lot of manhours (in the USA manhours are very expensive) and takes a lot of welding. The welding is tedious because it tends to locally heat up the vessel walls and must be controlled for quality. The last coil I had fabricated for an existing vessel took about 2 months.
As I have often pointed out in many previous threads, after going to all the time, trouble, and expense described above, the amount of heat transfer obtained by use of a limpet coil is minimal when compared to other means and techniques. This is due to the fact that the area covered by the coil is limited to the available vessel surface – and the vessel surface is hardly ever selected or sized for the sake of the coil. The vessel (usually a reactor) is sized based on other important points and not on the optimum shell heating it can produce. The limpet coil, then, is nothing more than token heat transfer and can’t be relied upon for such important process needs such as quick, rapid heat-up and cool-down as well as careful and accurate heat transfer. At best, any jacket is meant for simply maintaining a certain vessel temperature while the contents are static – which is defined in reactor operations as dead time – i.e., non-profitable operation. For accurate, rapid, and efficient heating and cooling of reactors, the usual methods relied upon are internal coils mated to agitators or external circulation pumps and heat exchangers.
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