Heat transfer - jacketed or double pipe

[MJCronin]

To all,

What are the limitations of cooling a powder/air mixture within a jacketed/double pipe system ?

We have a client that is asking about the pros/cons of such a powder system as compared with other, more traditional methods of cooling ( fluidized bed etc)

Specifics:

Inner pipe: 12 dia -calcinated gypsum -
Specific heat - 1.090kJ/jg C
Temp 350 F
Flow 50 metric TPH

Outer pipe 16 dia - 50% ethylene glycol
at 150 F

Does jacketed pipe cooling make sense for this type of application ?

Are there any websites/reference texts that anyone can point me toward ???

Thanks

MJC

 

[Montemayor]

MJ:

Several features about this proposal immediately stick out as impediments to a successful, efficient, and economical application:

1) The inherent nature and configuration of a double pipe configuration make it almost impossible to achieve access to the annular space for inspection and cleaning - except in the ultra-large pipe sizes (6"+). But even in the conventional, smaller pipe sizes, the amount of heat transfer surface offered by this method is very small and it comes at a very expensive cost as well as required space. This configuration (pipe within a pipe) is what justified the introduction and continued development of the shell & tube heat exchanger.

2) The apparent simplicity of the configuration is ruined by the mechanical difficulty inherited while handling any fluid with solids in it (always in the "tube" side, never in the annular space) because of the inherent low velocities that are achieved. You can use multiple tubes inside a larger pipe, but then you have multiple welding and joint problems on the outside - and you start to lose the ability to gain access to the tube side (not to mention the inaccessibility to the annular space!). The lower velocities will stimulate powder (solids) deposits within the exchanger and make life (& the process) impossible to sustain.

3) while it is true that you can achieve true, theoretical counterflow in such a configuration, experience has proven that the cost of this pure efficiency gain is not worth it. Rarely do you see any double pipe configuration larger than 25-50 ft2 of heating surface. There are applications for double pipes, but never in the large sizes.

The proven efficient and dependable way to cool (or heat) any solids + gas stream is by using an acceptable third gas stream to mix with the principle stream, removing (or adding heat) during its pneumatic transport and separating it later downstream. Any internal hardware, changes in direction, internal obstacles, low spots, low velocities that you can avoid should be done in order to preserve the fluidization of the powder and, thereby enabling it with a continued transport capability. The moment you drop velocity, start precipitating powder (& accumulating it internally) you are in trouble. In order to process a gas + powder mixture, you must maintain fluidization. A long, smooth trajectory with very long radius bends (not elbows) is the type of transport system that is used and this can also be a vehicle for a second, cold gas inserted into the stream and recirculated after doing its cooling. If you can use air as the cooling medium without introducing an explosive atmosphere, this should be the way to go. Direct, conductive heat transfer through intimate contact should be the most efficient way. I hope these thoughts are of some help.


Art Montemayor
Spring, TX

 

[quark]

You may also face probles with sticking of powder to pipe walls if the powder is hygroscopic(you should not cool the mixture below due point of air). This is not a problem with fluidized bed systems.

Regards,

 

[MJCronin]

To all,

Thank you for your answers...

I have found other references in Heat Transfer textbooks that also describe the limitations of double pipe units.

Additionally, there is a company that offers coolers for powders and pellets (

http://www.bulkflow.com)

MJC