Dehumidification by ventilation or DX?

[m2e]

I am puzzled in this subject for a while now. Typically the small pump stations and water treatment plants we design have only ventilation fans as means of dehumidification; put a humidity-stat on the wall to turn on the fan when the humidity is high. However, I don't know how effective this is and I always wonder when we should put in dehumidification unit and when we don't need to.

What would be the criteria when designing a simple HVAC system with ability to control humidity? When would you go with dehumidification unit, and when do you think ventilation is good enough?

Most of the buildings has the following characteristics:
- Wall propeller fan for supply or exhaust
- Heating by unit heaters only
- Building is typically < 1000 sq.ft.
- No open water, but piping surface would sweat.
- Ventilation rate: 6ACH when occupied, off when unoccupied
- Indoor design temperature: 50F (winter), 80F (summer)
- Climate: Washington, British Columbia, Alaska, Yukon

Thank you.

 

[MintJulep]

What is the goal?

Ventilation isn't a dehumidification process. It's impossible to remove any moisture from the air using ventilation alone.

If the outdoor air is lower humidity than the inside then you will lower the humidity in the space by replacing the wet air with dryer air, but that's not really dehumidification.

Of course, bringing in wet air does nothing, or could even make the interior humidity higher.

So if the goal is to prevent sweating of cold pipes, you need to know how cold the pipes are. Then you need to remove moisture from the air until the air's dew point is below the pipe surface temperature.

Or you could insulate the pipes sufficiently so that the jacket temperature is always warmer than the air dew point. Probably a much cheaper option.

 

[m2e]

Thanks. The goal is to lower the humidity and to reduce sweating. I understand ventilation is not dehumidification, but sometimes dehumidifier because of cost or space, we can't put one in. The water is around 40f.

 

[willard3]

You can also insulate cold surfaces to control condensation/prevent sweating.

Do you have an idea about the amount of condensation you want to control?

 

[ChrisConley]

For a dry climate, ventilation is dehumidification most of the time. The driver is what the expected maximum humidity in the space is at summer evap conditions in the zone, and whether that is acceptable in the space.

 

[Loumolito]

I struggle with Plant Operators all the time on this very subject. My projects are located in Northeast US.

Bringing moist, humid air into the plant during the summer is not a good idea. One area which is a problem is the pipe gallery underneath the filter banks. If the ventilation fans run on a hot, rainy day then not only will the pipes have condensation but the cold basement walls might grow mold with high ACH.

Winter operation is another challenge. Keeping good IAQ in the plant requires high ACH to remove chlorine odors. This is a huge make-up air heating expense.

In my experience, we design automatic controls for the ventilation systems then the Plant operator overrides manually. He will make a judgment when to turn the fans off/on.

Heat recovery with a properly coated sensible plate device is a good design option. The challenge is to convince the Plant Operators that what they are used to for 50 years is not good IAQ. Or was it?