[Wreckleford]

Hi,

This is my first post. I found this forum while searching for a solution to a problem.

We have completed a chilled water installation with fan coils mounted in bulkeads blowing through short ducts to sidewall grilles. The problem we are having is that we are experiencing condnesation on some of the supply grilles. I have never experienced this problem before. Anybody had this problem before? I assume warm air somehow is passing over the cold grille and causing condnesation, but I wouldn't expect the warm air to get near to the grille as the cold air should displace it. Is there any other phenomenon at work here that I am not seeing?

Any help will be greatly appreciated. Thanks.

 

[NickTech]

well air conditioning is both a sensible heat and latent heat process. what that means is, sensible heat is the heat energy require to change the air temp. latent heat is the energy required to change the state of, in this case water. not only are you looking to remove heat from the air to make it cooler, but you are looking to condense water vapor(humidity) on the cooling coils and have it drained away leaving the air dryer. your problem is either the air is cooled too fast, shutting down the t-stat, shutting down the system too rapidly not allowing the unit to run long enough to properly dehumidify the air, leaving the moisture in the air, and allowing it to condense on the colder grille area.....OR....there is not enough temp difference between the cooling coils and the air temp where the latent process can't occur...not enough of the thermal energy is leaving the air to condense it into a liquid.....or the blower speed is too hi where the air is moving too fast to allow the moisture to collect. whew! check the blower speed first, check the water temp incoming, and outgoing from the coil. see what the temp diff. there shouldnt be too great of a diff.
it isn't that fact that the warm air is getting to the grille to have it sweat, its the still humid air coming out of the register that is making it sweat. :Thumbs:

 

[Wreckleford]

There is a big difference between the air temp and the water temp in the coil, so that is not the problem. Some of the fan coils are oversized so it is possible that they are cooling the air too rapidly. Maybe we could increase the outgoing water temp slightly at the chiller to lenghten the time it takes to cool down the air?

The fan coils have three speeds and they are running on the highest speed now, so we can slow them down also.

I didn't follow what you were saying here:

"check the water temp incoming, and outgoing from the coil. see what the temp diff. there shouldnt be too great of a diff."

Thanks, I appreciate the help.

 

[NickTech]

for example, if the water temp coming into the coil was 38 degrees, but it was coming out 65 then there is too much sensible heat, and not enough latent heat being removed from the air. sensible heat is measured by degrees but latent heat cant be measured with a thermometer it is the energy required to change the STATE of water (ie. vapor to liquid). if i was to drop the temp of air from 85 degrees with 80% RH to 60 deg w/ 80% RH then the air would be cold but clammy, due to the fact that its dew point has now changed and the humidity will condense on any surface that is only a few degrees cooler that it. i dropped the temp by 25 deg. (sensible) but the %RH stays the same, and no moisture would have been removed by condensing on the coils (latent heat). to change the temp. of one lb of water by 1 deg. F, 1 BTU is required(the water coil water). but to change water vapor to liquid 970 btus are required(humidity). see the diff. if you had 38 deg water thru your coils, and it was coming out at 44 deg. and the air was cooled and dehumidified, then the sensible heat removed was 6 deg. but the latent heat removed (which cant be measured on a thermometer only on how many lbs of water that was removed)would be greater a few times over and it won't affect your net outlet water temp.