which cooling fluid is available for this processc?
As long as pressures are low, flat plate heat exchangers are cheapest to construct. If pressures are above 1 atm by much plate and tube is the next cheapest. Cooling fluids are either R-12 or its replacement (R-134a?) and ammonia (if food is involved) . Ammonia probably is cheapest and best refrigerant for industrial use. Other refrigerants at this temp are CO2 and propane
If you are cooling a gas along with pressure letdown you must use the thermodynamic properties of that gas to cool down by the same gas.This will reduce the refrigeration duty for cooling down the gas straight from -15 to -30 oC. If you do not have that choice you can use a plate type heat exchanger with Ammonia as refrigerant.
