AHU drain / trap sizing

[Kirky]

Hi all need your help again

We have been tasked with installing drains to a number of large externally mounted roof ahu. These AHU's do not have cooling coils they only offer heating via a steam coil. The problem they are having is for some reason these AHU's keep filling with water and hold this water until they are shutdown. The reason for the water build up is unclear, the heating coils are not leaking, the AHU roof is sound. We have managed to narrow it down to two possibilities.

1. During the winter the outfall from a number of extract fans located in front of the AHU may be getting drawn into the AHU. These fans extract from a very warm humid environment and so it could be condensation being drawn in


2. The other theory is that on the odd occasion the AHUs are shutdown (fire alarm tests etc) then the chimney effect draws the hot humid air back up into the AHU where it condensates inside the fan chamber

Anyway I'm going off point here. Our customer has requested we install a drain to each AHU in order to drain away any water build up. My question is how do I size the trap, size the pipework and what materials can I use.


ALL GUIDANCE MOST WELCOME


CHEERS

[Brian_UK]

You need to measure the static air pressure, positive or negative, at the point where the drain is required.

The depth of water within the drain trap needs to be greater than the measured pressure.

The drain can be run in copper or plastic to avoid corrosion problems. Don't forget to allow for the removal of the trap itself for cleaning or for topping up the water level in the trap if required.

[chemi-cool]

How many liters per hour?
I make water traps 2 diameters.

There must be a leak somewhere in the AHU, if there is air vent inside, have it checked, it might stop water but not steam.

[Kirky]

Ok cheers guys.