German to English Tech/Engineering Mechanics / Mech Engineering

These terms occur in a table with the technical specifications of a regenerative heat exchanger. As far as I currently know, the Abluft is the part used to extract heat from, and the Fortluft is the part that is finally let out into the outside world. The problem is that the dictionaries give "exhaust air" as translation for both Fort- and Abluft. Additionally they mention "escaping air" for Fortluft, and I have also found "extract air" mentioned on the Internet, although I am not sure if they mean Ab- oder Fortluft bei that. I would appreciate any help on deciding which term to use for which...

TIA!