Question
Why nozzle reinforcement calculations are performed as per UG-37 instead of UG-39?
UG-37 is for openings in shells and formed head and UG-39 is for openings in flat heads and covers which seems relevant for Air Cooler Headers.
Answer
UG-39 is no more relevant than UG-37 for air coolers headers since it applies to circular covers, and it is also based on UG-37.
APV has chosen to adapt UG-37 in consultation with air cooler manufacturers.