AC Humidification

[dale1627]

Is it a wise idea to use a humidification system feeding directly into an AC duct. The system does not have heat, just cool air. I need to maintain a humidity level of about 40% in an office environment and this is the best method that I can come up with. My fear is that the moisture will accumulate in the duct and promote mold growth. Any opinions would be appreciated.

Dale

 

[KenRad]

Proper design and installation methods are crucial to a successful duct humidification system. Follow manufacturer's installation instructions, use a competent and experienced contractor, and perform required maintenance throughout the life of the system and it will serve you well. Neglect any of these items and you will cause major grief for the building owner.

Duct humidification has received more than its share of bad publicity, for the reasons stated above. Some things to remember:

1. Isothermal (steam injection) types are best when humidifying cold air, as you are. Stay away from water atomizers (adiabatic humidification).

2. Never install a duct humidifier in a section of internally lined ductwork.

3. Get the blessing of the humidifier manufacturer on location for the humidifier in the system.

4. Humidifier should be selected for a short absorption distance, and there should be no duct obstructions until well beyond this distance.

5. Control design and setup is another key.

6. If possible, go with an all-welded humidifier with no replaceable parts (o-rings, plastic inserts). This will avoid problems if the unit goes for a few years without being looked at.

Some reputable manufacturers are Pure Humidifier, Dristeem, Armstrong, and Carel.

---KenRad

 

[dale1627]

Thanks KenRad.

Dale

 

[cme]

Specify a welded stainless steel duct section where the dispersion grid is located ..... length as required per absorption distance ... slope the bottom to a common drain with a trap piped to indirect waste .... provide a stainless steel access door on the duct side for internal access to the grid and for cleaning

 

[tombmech]

Great comments by KenRad and cme. I don't disagree with them.

However, I'm a little curious about the problem in the first place. Cooling only - in an office environment - should not have low humidity problems. Normal, off-the-shelf, properly sized A/C units are sufficient to keep humidity above the value you state. Typically, it's heating season that may bring humidity down to 30%-40%. Yet, you state "cooling only."

Given - that's only a general rule-of-thumb, but I have found it dependable. The biggest problem is the temptation to oversize the A/C unit or the coil. That will prevent the coil from operating down into its dehumidifying regime. The cooling will stay at the sensible level only, and humidity will rise out of control.

It is very seldom that cooling warm air - especially with a decent latent load {office environment} - results in humidities of less than 50%. The act of cooling - just by itself - increases the value of Relative Humidity.

 

[UtilityLouie]

I was originally thinking along your lines tombmech, but then I remembered a couple of paper testing labs that I did where 50% RH was the design condition. The only way I could reliably control it was to dehumidify, reheat and rehumidify. Expensive energy costs, but compared to increased product costs - not so bad. These labs were inside a mill with no heat loss, so essentially a cooling only application - I just had to add heating and humidification to make things work.

 

[tombmech]

UtilityLouie,
I don't disagree, controlling to 50%RH with any sort of reasonable tolerance will require the systems you describe. Computer rooms have similar requirements for instance, and demand 50% +-5% year round. So, A/C units are specifically made for those applications and contain humidify, dehumidify, cooling and heating all in the same unit (along with super-high sensible ratio coils).

That is still a lot different than maintaining a "minimum" level of around 40% in a cooling only, office scenario.

 

[dale1627]

The ac unit is indeed cool only. The office environment is in a printing facility, the room has computer workstations and some high end equiment. The room is pretty much in the center of our facility and in the 9 years that I have been here have never seen a need for heat in the room. The unit in place now did not come equiped with humidification systems or heat. The original system did but was changed out several years ago by our prior maintenance manager, so now I get to deal with the issues at hand.
To make matters worse the department in question decided to remove a double set of doors right next to the air return for the system that they want humidified and refuses to replace them. Makes sense to me that it will be hard to maintain an accurate level of humidity in a room when your return is pulling from two seperate systems. I think that my best bet is to just wash my hands of it and call in a contractor.

Dale

 

[tombmech]

Dale,
Nortec offers some fairly reliable, bucket-in-a-box humidifiers that are pretty good in small applications - like an office environment. I have even used them in laboratories. You can also find some units that are wall-mounted and will shoot up an atomized mist on a semi-regular basis - something akin to the perfume boxes in commercial restrooms these days.

40% is not too bad - it's just above the level that causes problems (<30%), with a reasonable cushion. You can install an add-on system that's completely independent from the rest of the AHU - in the duct that supplies the office area, or even on the wall.

When I was growing up, my mother used to put frozen orange juice cans full of water on top of our electric baseboard heaters. If the application is small and non-critical - it's not rocket science.

You need RH levels consistently above 60% to promote mold growth. That is very unlikely if all you're trying to achieve is 40% max at the outset.