The compressed air system was one of the most important systems on the submarine. It was used to blow the ballast tanks, fire torpedoes, and to start the main engines. Without a functional air system the submarine became inoperable. Docking facilities were usually provided with the necessary high-pressure air supply so that the submarine would not have to produce its own. Air tanks tended to accumulate condensed moisture (water), thus they were regularly drained for maintenance. On VELELLA the primary air compressor was installed in the auxiliary room.
Additional information regarding the air system is limited, but we know that this boat was equipped with:
VELELLA was equipped with an air filtration system for the removal of CO2 (carbon monoxide). There were also oxygen tanks used to oxygenate the depleted air while submerged. The submarine was equipped with external hookups so that, in case of malfunction, a support ship could blow the ballast tank and also provide breathable air. Due to the primitive nature of the air circulation system, when submerged, the crew would experience very uncomfortable conditions and the air was reportedly foul within a very few minutes.