You'll enter the main room but you'll be under it in the vents. Go through the vents and come out the other side. Once there, you'll be able to stand up. Take a right and then crouch and go left. follow the path until you see more pipes with steam and three green buttons on the other side. You have to hit all three of these quickly to shut off the steam. To do so, tap the aim button to quick throw instead of aiming. Head back the way you came and crawl under the metal steps back towards the center room. You'll enter a squarish area from which you can enter the center room. From this spot, you can actually enter above with a takedown, eliminating an enemy right from the start.
The group above consists of about 8 enemies. Most of them are nothing special however you should watch out for flying objects. Occasionally, an enemy will grab a milk crate or brick from the side to throw at you. Keep an eye out for this and dodge as appropriate. Aside from this, the rest of the enemies are pretty simple, not even carrying any kind of weapon.
