Air flows from right to left in this multi-stage heat exchanger. In the first stage, air passes through the green pre-filter and is then cooled to 50 degrees F. Cooling the air condenses humidity in the process. The second stage re-heats the air to 75 degrees F with steam. In the last stage, air passes through a pink HEPA filter. The process provides air at a consistent 75 degrees F that''s highly filtered with 40 percent relative humidity. C-125 Heat Exchanger.