This section is pretty short, but it's also a bit difficult to enter. Probably the best way place is the raised grate in front of Fralin, but it's pretty conspicuous, so you'll have to wait until there is no one around. It might also be possible to enter through the mechanical room of Dietrick, but it's likely that the door into the tunnel is kept locked.
After you open the grate, there's about a two foot drop into the grate and then you'll have to slide down into the main tunnel, which is about a four foot drop. The tunnel is pretty dirty here; if it's been raining recently, you will probably end up covered like mud, as I was on my first trip to this tunnel.
After dropping in, you will find yourself in one of the tallest steam tunnels on campus, with steam pipes running along the wall opposite you. You'll want to go left, following the tunnel toward Dietrick. You probably won't be able to go right, since the tunnel is blocked by pipes in that direction. As you continue down the tunnel, you will pass a short door to a mechanical room in Latham Hall on your left. I am not sure if this door is locked or alarmed (unlikely, but possible), as I have not tried it.
The tunnel turns sharply to the right shortly after Latham and the tunnel continues, now heading north toward AJ. Eventually, the tunnel appears to come to an end, but there is a smaller section of tunnel that leads into Dietrick's mechanical room on the left.
The mechanical room is worth checking out and makes a pretty good exit, so climb up into that section of tunnel and continue on through the unlocked grate to the end of the tunnel. On your left side, you should notice a door leading into Dietrick. Open it and you will find yourself in the mechanical room in the basement of Dietrick.
When you're done checking out the mechanical room, head up the stairs in the back on Dietrick. One level up, there's a door that leads out on to the loading dock.