HVAC Steam Condensate Return

[Mr Leaf]

Hello,

Long time lurking, first time posting.

I have an AHU with steam heating as a current system. This is being replaced with a like system. However, the unit will sit lower to the ground requiring some piping reconfiguration and the steam condensate outlets from the coils at 3" standard size from the manufacturer.

The IOM recommends the outlet be matched all the way to the trap and a 12" min pocket depth be maintained with a about 3" or so dirt trap after. Due to a very congested area, I have limited space. I cannot run the a 3" line all the way to the trap and cannot maintain a 12" pocket. Other searches I can find in text and online seem to align with the recommendation. We've had no success from the manufactures or vendors. Its me and another engineer for a small company and our thoughts as follows:

(1) Reduce at first isolation valve after outlet and then to trap at 1-1/2" (meeting current system configuration)
(2) Reduce pocket depth from 12" to allow fit, no greater than 6" min

While these seem reasonable, I wonder if there is something I should be wary about or look out for that we are not seeing? Any thoughts would be appreciated.

Thank you,
Leaf

 

[TBP]

I'm assuming that the new AHU has s the same steam requirement as the one it's replacing. I'd install a 3" 90* elbow at the outlet, then reduce the pipe size on the drop to the trap, ahead of the isolation valve. I suspect that you can easily run with a shorter drip leg as well.

Always a challenge shoehorning retrofit equipment into existing buildings.

 

[BronYrAur]

What is the load (pounds per hour) of the coil?

 

[Mr Leaf]

TBP

Correct, the steam requirements are the same.

BronYrAur

393 lbm/hr on the side. Using the x3 times rule this is 1179 lbm/hr and aligns with the traps I have been looking at.

 

[BronYrAur]

Looks to me like 1-1/2" pipe is more than adequate to serve as the drain line from the coil outlet to the trap inlet. I didn't ask you about your pressures, but 1-1/2" may even be adequate for your trap discharge, where you are carrying flash steam along with the condensate. Since the line between the coil and the trap is pressurized, you will not have flash steam in it. You will just have liquid that needs to run by gravity to the trap.