On the furnace bit you can actually prevent enemies from spawning by flipping the switch and getting to the ladder quick enough. Found out on my blind playthrough.

Thought it useful as you can get to the top of the catwalk without being attacked but once you get on the crane enemies will spawn from the upper catwalk doors.

Makes it easier if you flip the switch and then back away from it, turning round on the corner to go up the ladder.