Dehumidification with cooling coils only

[drfloyd]

Hi all,

I am currently employed as an intern at a lyophilization facility. The facility is broken into zones including an office space, mechanical space and clean rooms for lyophilization. The relative humidity requirement for the clean rooms is 40% or lower to discourage the growth of mold. However, in summer months the HVAC system has been unable to maintain below about 60%. Our chillers are set to put 42 degree chilled water through the cooling coils in our makeup air unit and 2 air handler units.

The majority of the moisture is removed by the cooling coil inside the make up air unit. Once this air has entered the air handlers, the cooling action inside of those air handlers removes relatively (compared to the make up coil) little moisture from the air because it is 90% recycled from the building. I believe our humidity problems are due (at least partially) to the cooling coil in the make up unit being completely saturated with condensed moisture, making it unable to remove as much moisture as it could potentially do at that temperature, as well as possibly dispersing water droplets into the air.

My question is, is there any way that we can achieve 40% RH in the rooms using only cooling coils? Would it be practical to simply add another cooling coil within the makeup unit to extract more moisture? A dehumidification skid would be extremely difficult to fit into our mechanical space.

If that won't work, we thought about possibly adding a refrigerant coil (with compressors and all that fun stuff) as an alternative to an entire dehumidifier unit.

I apologize if my question is unclear. I would appreciate any help I can get on this!

 

[AnotherEllis]

Where are you geographically?

It has a huge impact on how moisture is removed. Clean rooms are interesting in that the air recirc units generally flow so much air that if you actually drove the LAT off the coil low enough to remove humidity you would turn the space into a meat locker or waste tremendous amounts of energy on reheat. I've successfully handled humidity 3 ways in clean rooms:

In arid climates, we generally have to add humidity to hold 68-72dB and ~45%rh.

In climates with higher humidity I have used both munters units and simple 8 row coil chilled water units to provide very low dew point makeup air and modulated how low we brought the dewpoint based on cleanroom conditions (ie: backed off the regen if humidity is within design range.)

Also in hot humid climates or spaces that internally generate humidity, I have used recirc fans in lieu of ARUs and had a side stream style cooling coil, so I could take some percentage of the recirc air, cool it using only chilled water to as low as possible (say 42-44wB) and mixed that back with the 72°F recirc air to provide both cooling and dehumidification to the space.

Of course, in clean room design every solution must be application specific.

 

[mybuilding]

I sure think that a good reheat coil will greatly changed your RH but make sure all other conditions such as OA is adequate for the space.

 

[BKS60]

Had someone start me on understanding controlling humidity with this explanation.

Think of your airflow like a sponge that is saturated with water. If you sqeeze the sponge (pass air across a cold coil)the water is dropped out but the sponge is still just as wet per volumn. Now if you open your hand and let the sponge expand (reheat the air causing it to expand)the sponge is drier per volume and so is the air.

With this in mind your air leaving the coil while cold is compressed and still containing a high percentage of moisture. Reheat is needed.