Condensation at supply air diffuser

[Cofreth]

I have an air-conditioned room which is designed to maintain a negative pressure of (-2 to -4 Pa). The room temperature need to be maintain at 22 to 23 deg C. There is no return air being duct back into the chilled water fan coil unit. Only fresh humid air is being supply to the fan coil unit through a fresh air duct. On coil temp. about 31 deg. C with an average RH of about 90%. Off diffuser air temp about 16 to 18 deg. C with RH of 85%. Supply chilled water temp about 9 deg C. I keep having the problem of condensation formed on the supply air diffuser and thus, causing water dripping and the growth of mold and fungus on the surrounding of diffuser. Any experience member please advice me how to further lower the RH in the room to prevent condensation, thank you.

[Abby Normal]

If the room is being maintained under negative pressure then you must have some exhaust that significantly over powers the supply to the room.

That is a pretty humid fresh air and you need to look at a couple of things first.

Condensation is all about dewpoint temperatures. The higher the humidity, the higher the dewpoint.

So you need to find out what is the temperature and the humidity in the room to work out the dewpoint temperature of the room and then compare this to the supply air temperature coming out of that diffuser.

If the diffuser is colder than the dewpoint of the room, which is happening right now, you will get condensation.

To lower the humidity in the room, you have to determine the source of the humidity.

Either that constant negative pressure being maintained is causing humid outdoor air to infiltrate into that space, or, perhaps the fan coil treating that outside air is not removing enough moisture from the air in the first place.

I suspect it is infiltration caused by running the room negative by 2 to 4 Pa.

Or you could have standing water, people exercising or some process occuring in that room that releases mositure to the room air.

[Abby Normal]

I should of read your whole post

Supplying air at 18C & 85% has a dewpoint of about 15.4 C, you are not getting enough mositure out of the air. If you had another AC unit in the room it could probably control the humidity.

A bit of infiltration of outside air and not difficult to have dewpoints higher than 18C in that room.

[nike123]

If air conditioner only supply fresh (conditioned) air then how are you maintain negative pressure in room.

You should recirculate thru air conditioner some amount of air to maintain lower humidity of discharged air. Or, if that amount of fresh air is necessary, then you should consider installing dehumidifier in supply air path.

[nevgee]

If you have condesation forming on the supply air diffuser then you must have air approaching the diffuser at a higher wet bulb temeprature than the diffuser temprature.
I'm not convinced that you would need to have a total loss system here but if that is the case then you need to consider what is the actual physical arrangement of the system.
Look at the actual flow arrangement of the air movement and the way it passes through the space.
If you have cool air coming from a difuser and condesation is forming there, then it must be realised that warmer moist air is gaining access to the diffuser alowing it to cause condensation.
Where is the moist air coming from?
Obviously from within the room.
This brings us back to the point of duty and capacity. Is ther more load from the room than the air con can satisfy?

This would suggest the room load is greater than the supply air duty.

[Ke-Fibertec UK]

If you were to fit a textile diffuser instead of a conventional one, it would alleviate any condensation problems.
Because the air passes through the textile material, the temperature of the air on either side of the material is the same.
Textile ducts are often used in food production areas due to the low temperatures and high RH.

This way you would save energy by being able to keep the RH higher.

[Gary]

The air as it leaves the coil needs to be about 11C in order to control the humidity in the room.

This can be accomplished by either slowing the airflow or dropping the water temperature.