Condensation on the 3 slot linear diffuser grille

[Eddylim]

Hi all,

I am facing the condensation problem on the surface of the 3 slot linear diffuser grille. The reading taken as showed below:

Room air dry bulb temperature : 22'C
Room air wet bulb temperature : 17.5'C

Supply air dry bulb temperature : 16'C
Supply air wet bulb temperature : 14'C

Ceiling air dry bulb temperature : 24.5'C
Ceiling air wet bulb temperature : 18'C

The aircond system installed for the building is a ceiling return system.

Who can give advise?

Thank you

Eddy

[Tesla]

Hi Eddy and welcome to our forum.
I suspect you have a problem of insufficient air flow. Check via the BMS and find how many VAVs are at 90% or above. Do a physical check of the affected area for restrictions or excess heat load. If you increase the static pressure set point the supply air temp should rise - this will depend on how it is programed, there may be a minimum supply air temp set point. Compare the affected floor with other floors and let us know how you go

[Eddylim]

Hi Tesla,

Thank for your reply. My BMS does not interface with VAVs, its only interface with AHU & Chiller system. Actually, there are 3 rooms at the same floor and served by the same AHU. The air supply to 2 rooms (call Room A & B) with 1 VAV box & same air con ducting, and another room (Call Room C) with another VAV box.
The condensation only occur on room A & B. The room temperature for room A & B are constant.
The AHU data as showed below:
Static pressure set point = 150pa
Static Pressure = 469pa
Supply air temp set point = 14.8'C
supply air temp = 14.9'C
Return air temp = 23.4'C

Do you think the static pressure on AHU will affect the air flow to the room?

There are no condensation occur on others floor. Its happen only on that 2 rooms.

Is there any other solution? Please advised.

Thank you

Eddy

[Eddylim]

Hi all expert

I had increased the air flow of the VAV box, & the room temperature also from 22'C become 20'C, but the condensation still happen.

Who can help? please.

Eddy

[dougheret0]

First, the room is pretty humid, at 65%. If the building is just started up, the building may still be drying out from being open during construction. Also note that the room dew point is 15C based on your measurements, within the measurement error of the supply air dry bulb temperature of 16C. Ideally, supply air should be closer to 13C, but if the problem is moisture gain in the room, reducing the supply air temperature would just make it worse.

Plotted on a psych chart, it looks like the supply air is not cold enough and the room air is not dry enough. The room may have a lot of infiltration, or a big load of people, or some other source of humidity. If the problem is poor dehumification by the cooling coil (supply air not cold enough), you would have it in all of the rooms, so the problem must be a moisture source in the affected spaces.

You surely don't want to have the supply air any warmer than 15C, so you need to work on finding why the rooms are so humid. You might look for outdoor or unconditioned space leaks into the return.

[Tesla]

Hi Eddylim
I firstly note that your static pressure is way above the set point - this just don't look right. Is there a possibility that the other room C is hot and calling for cooler air which in-turn is too low for rooms A + B.
There is a balance for VAVs where you need a measured amount of cooling which can be provided by lower air temp or increased air flow. Considering condensation is occurring it is indicative of a lack of air flow/too low air temp.
You need to check why the static pressure is so high, there could be restrictions in the supply or return air. Also check if there is too much fresh air.
One other thing is I think your room temp is too cold @ 22°C - for a hot ambient place like Singapore I would use around 23.0°C - 23.5°C. At 22°C is simply pouring money down the drain wasting energy. So if you are able increase the room temp set point, even a little will help.
Is there condensation on all outlets in rooms A + B or just some of them and do you have a graphical representation of the VAV positions with litres/sec?

[Magoo]

The supply air is below the dew point of the room, raise the the supply air control point.

[ysfan]

Hi Eddy Lim,

I am Condensation Specialist located in Singapore. I'm glad to help you if you can give me your location or contact me at my mobile 97998801 and for more info please visit our website at seculate.com.sg .

Thanks

Fan.

[Gary]

The air flowing through the conditioned space is gaining humidity.

Assuming there is no moisture source in the room, I would be looking for supply air leaks.

If the return air pulled from the space exceeds the supply air delivered to the space, the space will have negative pressure and humid outdoor air will be sucked into the room.

[Eddylim]

Hi All,

Thank you for all your advised & info. Currently, there are no more condensation on the room due to increase the minimum air flow of the VAV box from 750CMH to 1900CMH.

[Tesla]

Hi Eddylim
Thank you for getting back to us and letting us know the result. We will all learn from this post as VAVs can be tricky to troubleshoot even from experience.
On another note the static pressure can and should be varied and lowered for low load conditions to save on power and wear and tear of equipment - provided minimum flow rates are maintained.