Follow the tunnel from the room with the huge gun turret until you hear voices. There's a squad of Solarii busy with various tasks in the next room. The first man is standing guard near the tunnel where Lara is crouching. He has his back to you.
So while the others are busy, you can sneak up behind him and, when prompted, press the Melee button to dispatch him with your axe.
Before the others notice, take cover behind the stack of pipes with the 2 bundles of arrows leaning against it.
Once you step out into the room, a steam pipe in the passageway where you entered bursts, preventing you from retreating that way. You'll need to kill everyone here in order to move on.
If you're quick, you can take out several more men without attracting attention. First, the welder up on the ledge to the left...
...then another man working on the ledge to the right.
If you still have time, you can also get one or two more ahead on the left. Be sure to use arrows or silenced pistol or rifle shots to the head so the others don't hear. For the one on the upper level on the left, it may help to shoot an arrow into the wall to his left. When he turns to look, he'll expose more of himself, so you can take him out with a quick headshot.
Don't spend too much time on the guys on the far left though. Focus on the 3 men working on the generator in the sunny area directly ahead. You can kill all of them at once using the generator itself. One will call out, "Keep it steady. We're bringing it up." Shortly afterward, the men will start hauling the generator up toward the ceiling. As soon as they get it all the way up, target the pulley and fire.
This will drop the generator right on top of them, causing an explosion that will also ignite the nearby fuel barrel, killing all 3 at once—and earning you a nice XP bonus.