As part of powering the engine, you need some cold gas and hot gas. This room, found east of the engine proper - itself found in the northern part of the station, inner side - concerns itself with the former, by running gases through a set of three freezers and then circulating it through pipes outside.
Wrench gas canisters down and open the relevant valves - be careful not to open the wrong one, or it may just pump your gases out a vent into space! You'll need to turn the freezers on, too.
There is no vent. Unless you count the one outside when you need to free gases from their steel confinement.
The APC for this room is on the northwest corner.