To rate an air cooled heat exchanger in ProMax, you must connect the heat exchanger to another heat exchanger side that contains air as the process fluid. (Single-sided exchangers cannot be rated in ProMax.) Usually the inlet and outlet temperatures are specified for this air side and ProMax computes the flow rate from the required duty. If nitrogen and oxygen are not present in the environment for the flowsheet, you may wish to add another flowsheet that contains nitrogen and oxygen and connect a heat exchanger in this flowsheet to the air cooler through a cross flowsheet connector.