Noel:
You really should be specific in your query. The term "best" should be prohibited from engineering description if you don't qualify it. Does this mean from an economical, safe, operational, optimum value?
I presume you mean "generally best" due to a latent concern that CO2 will pose a corrosive problem in an offshore platform. That being the case, my experience dictates the use of 316 Stainless Steel tubes. This is not a preference, but a precaution against a potential tube corrosion causing a leak offshore from a 2,500 psig gas source. If the unit were onshore, I would contemplate using carbon steel tubes (with a corrosion allowance); but on an offshore platform I wouldn't chance a leak for safety and logistic reasons. Today's cost difference and availability between the carbon and stainless tubes doesn't warrant economic concern in my opinion.
I'm assuming the gas pressure is in the tube side and that the cooling water is sweet water and not seawater. If you employ seawater you are out of the stainless application due to chloride stress corrosion. You are now talking a more expensive alloy solution. But I assume you have an indirect glycol-water cooling system for this application.
