a beltgaurd after cooler is just a air cooled heat exchanger installed on a belt guard of an air compressor. here it receives a 'nominal' air flow produced from the belt guard.
I have seen references of them being designed (over all compressor hp ratings) from 10 feet per minute across the coil to up to 500 feet per minute. My question is if anyone has knowledge in designing these, what 'rules of thumb' they use in the air flow used.