Cmo:
Thanks for the quick reply. I apologize for requesting info that I overlooked by focusing on the 600 kJ/gmole figure and missed your 8MM Btu/hr figure. This is a healthy and significant heat load. I did not expect this size of heat load from a liquid mixing operation. I can visualize several banks of Marley or Hudson aerial coolers being dedicated to this service, if the 8 Million Btuh figure is a design, and not a peak value. If this is a consistant and steady load, this calls for a serious and detailed engineered design - not just a utility cooling coil.
The -40 oC (-40 oF) doesn't bother me as much as the total heat rejection quantity and the fact that you find yourself in a situation where you don't have normal plant utilities. I would suspect you're in field gathering or in a remote location. The low temperature levels can be easily handled with glycol solutions. However, I'm coming up with an estimated 600 to 800 gpm of cooling fluid recirculated - depending on the design conditions. This is a formidable cooling service. Aerial coolers this size are expensive to temperature-control. Additionally, as I stated, they are very inefficient compared to liquid-cooled units.
All heat transfer operations dependent on a transfering heat through a gas film are hampered by the notoriously low values of the gas film heat transfer coefficient. This is the trade off for using a quick, simple heat sink like the free air atmosphere. Gas are great thermal insulators - especially when they are static. This is why the good insulators, like expanded polyurethane, use air or Freon as gas-trapped bubbles to do the actual insulation (or heat transfer prevention). You have to move a lot of air and use a lot of fins (extended tube surface) to try to make up for the relatively low heat transfer coefficient on the air side - which is the controlling factor in an aerial cooler. This doesn't make the aerial cooler a bad application, it just paints it in the true light of its shortcomings. While a liquid-cooled exchanger is more efficient, it may also have its respective trade offs also. However, in this application I suspect the answer may be a refrigeration unit (which I always prefer not to place out in the field) servicing this need. A cooling water tower or evaporative cooler is probably not practical due to the low environment design temperature.
There isn't much you can do about the aerial coolers. They will work best in the colder climes (due to the larger delta T), but they still require enclosures to control the outlet process temperature. I don't know how important or economically driven this application is in your operations, but I kind of suspect that if you're accumulating heat in the magnitude of 8 MM Btuh and you're trying to make money by processing natural gas, this is costing you profits.
Do you have a potential for refrigeration in your NG processing unit? Perhaps some of your process fluids offer this potential - much like an NGL unit works. Just a thought since I don't know enough.
Good Luck.
Art Montemayor