Links on how chilled water systems work

[Clevor]

I am an industrial hygienist on a Navy base in Japan and we are having numerous problems every summer with mold. Many of the buildings on base have chilled water systems. I don't quite understand how these systems work, particularly in regards to how humidity is controlled.

The problems we see are high indoor humidity levels (as high as 65-75%) and condensation around supply air ducts where mold subsequently grows. I feel there is too much outdoor air being allowed in the building and moisture isn't being removed from the air via the usual cooling coil system where air is directed over a condensor. Many of the buildings have no obvious source of external water intrusion.

I have difficulty communicating with facilities maintenance engineers on base as they are Japanese nationals. They keep telling me they cannot control humidity in the building. Can someone direct me to a link with a simplistic explanation and diagrams on how chilled water systems work?

From what I understand there are fresh air dampers and AHUs on chilled water systems? Where is moisture removed from the air?

Thanks.

 

[imok2]

Sounds to me like you are having a control or
a damper problem with the outside and return air dampers. They should go to minium OSA and full return air when OSA temperature and humidity go over a set point. On the other hand your supply chilled water temperature may be to hi or a combination of poor maintenance. Go here for simplicity

http://home.howstuffworks.com/ac.htm

 

[marcoh]

There are any number of causes that cause the problems.

The FM guys are probably correct in that the AC systems are not designed for specific RH control (most AC systems are not) but I would guess the AC systems have been 'economically' designed with cooling coils incorrectly selected for sensible cooling only and disregarding latent cooling (ie moisture removal). Coils should be selected at correct coil air on (mixture of outdoor and indoor air) and coil air off conditions. Am taking a guess here but your cooling coils should probably 6 or 8 row coils in lieu of the 4 row coils you probably have. Unfortunately there is no easy or cheap way to fix this!

Also check your outdoor air into the system, supply air quantities, minimise infiltration into the building, calibrate controls etc.

 

[Clevor]

Thanks for the link, Imok2. I do know the basics of how air conditioners work, since I do my own automotive A/C work. I didn't get the link between cooling towers and chilled water systems (CWS), but I do now.

So on CWS, there is an air handler to blow air to the rooms, and I guess there are coils there which you blow the hot air past to cool it. How much condensation is there? If there is a lot, I'd think you'd need a drain line for each room.

If there is too much ambient room air at high humidity, the chilled air coming out of the supply will cause condensation around the ducts, which is what we are seeing.

As Marcoh mentioned, not much you can do to change the coils in each room (even then, you'd have to drain the system). Conclusion: FM needs to control the outdoor air. I don't think they even monitor this. I also notice the buildings on base have vents in the wall between floors. I suspect the crawl space is being passively ventilated on the misconception this will prevent mold. Correct - if you don't use A/C in the building at all! The excess outdoor air may be infiltrating here.

My recommendation would be to seal off all vents to the crawl space and maintain better control of outdoor air being allowed in the building.

If the problem still reoccurs, I suppose the existing CWS in the buildings could possibly be retrofited with cooling coils at the AHU, to remove moisture from the air? Anybody know if this is practical or has actually been done?

 

[krb]

McQuay has some good info here:

http://www.mcquaybiz.com/McQuay/Des...?models=/McQuay/DesignSolutions/GreenWayPage3

Not knowing much about how your system is controlled, it would be hard to answer as to how to fix it. Another option is a humidistat, which can help control humidity as well.