The following is a list of each part used for processing air and pressurizing rooms.
Processes air to reclaim ice and any bit of Oxygen and convert it into breathable air.
A relatively cheap item to create and is the main part of the Air System.
A Condenser needs to be close to a source of open "air", best way to do this is to place it in your outer walls, and then connecting Ducts to areas where needed.
The second part needed in creating a working Air System.
Simply connect a vent either directly to the Condenser or by Ducts to have an approximate area of 15x15 from the vent.
Not necessarily required for a working System.
Ducts allow you to connect multiple Vents to a single Condenser to expand the oxygen filled area, but are not needed if you have a small base.
You will know when you have a valid System when you get a Blue fan icon in your retical while looking at any part of it. You will also see a fan icon under your Oxygen bar. A green icon means more oxygen while a red one means low oxygen and you'll need to expand.