humidity control

[2muchinfo]

If i need to control humidity as well as the sensible heat load in a space with dx cooling and wet heat available, what is the most economical and efficient sequence of operation?

[jimgpe]

I assume you are trying to DEhumidify, that is, limit the maximum relative humidity?

If so, your only choice given the equipment to hand, is to cool the air then re-heat. (Note: preheat doesn't work, but I bet you knew that....)

Provide a humidistat in the space - "make" on rise in relative humidity past set point. Provide a wall thermostat with auto change over cool to heat.

Either a call for cooling or a call for dehumidification shall start the cooling cycle. If only the duhumidistat is calling, the space will over cool and the wall thermostat will bring on the reheat.

Good luck!

Jim.

[viceman]

try a honeywell t7350 thermostat. it provides several dehumidification options and is cheap

[2muchinfo]

thanks for the info. if i have a call for cool and a call for dehumidification comes in, does it make any sense to immediately use the current space temp for the new discharge air temp? why not go ahead and satisfy the cooling request by maintaining a 55deg discharge temp, and then reheat if needed to maintain spacetemp and dehumidify? in case you haven't figured it out i'm trying to understand the logic of a control sequence already in place on a piece of equipment.

[cxagent]

To dehumidify the discharge air dew point temperature needs to be LESS THAN 55F. A 55F leaving dewpoint will only reach 51%RH at 74F if a) all the air in the space is replaced with discharge air or b) there is NO moisture generation in the space. There would have to be no people, plants, envelope air leaks, toilets/plumbing, etc. to have no moisture generation in the space. Not many building like that.

Since the original post asked about the most economical and efficient dehumidification, I should add that reheat is not good at either. It is effective so it is usually the first place people look for dehumidification. More economical would be to first: 1)reduce outside air if that is where the latent load is coming from, 2) slow the airflow to get a lower discharge temperature, 3) add dedicated dehumidification equipment (higher first cost but more energy efficient). Only then should the reheat be considered if you are looking for economical and energy efficient dehumidification.

I hope that helps. Some of the options may be limited by the equipment currently available, budget, etc.

[jimgpe]

Yes, there are more economical ways, and yes I was limiting my comments to the available equipment - DX and hot water reheat.

If we are allowed move outside the box, You gave some good suggestions. Here's another one: a run-around precool/reheat system by Q-Dot or some similar product will also save reheat energy.

[2muchinfo]

i appreciate the help, but can anyone help me understand why the current sequence works the way it does? why wouldn't you continue to cool with the dx until the call for cool was satisfied and then if there is still a need to dehumidify, bring on the reheat to keep from overcooling the space.

[viceman]

the 7350 will ignore the call for dehumidification if the unit is in cooling.

[cxagent]

Originally posted by 2muchinfo
i appreciate the help, but can anyone help me understand why the current sequence works the way it does? why wouldn't you continue to cool with the dx until the call for cool was satisfied and then if there is still a need to dehumidify, bring on the reheat to keep from overcooling the space.

I guess I didn't understand your original question. What you asked in your second post (reset the leaving air temp to current space temp) will do little to remove humidity from the space. As your most recent post said, I would want it to continue cooling until the call for cooling is satisfied (while keeping the LAT below 55F), then bring on enough reheat to offset the cooling. The cooling (with LAT [actually leaving air dewpoint] below 55F) is what will dehumidify. The reheat doesn't dehumidify, it only keeps from overcooling.

Be warned - any "dehumidification" sequence that doesn't maintain the leaving air dewpoint below 55F will not dehumidify the space. Draw the process on a psychrometric chart to prove it for yourself.

[acman01]

Can anyone recommend a chemical for cleaning cooling tower coils? I am a bldg mgr...not a professional HVAC guy....Thanks for the help