How to Control Humidity in HVAC Application

[khurram440]

Hi
I want the control sequence to be used to control the Humidity in HVAC application.
Basically wht we do is as the Humidity goes above the setpt the Cooling is switched ON to squeze out the moisture from the air and the Heaters are used to ReHeat the air to bring it to required setpt.
But the problem is that when the Cooling is ON the temp is very low and the space is very cold.....im not able to find that perfect balance between the two.
Plz advice as to wht PID setting should be used for this application.
Khurram Shaikh
Application Engg.

 

[Macbeth]

When controlling temp and humidity I usually allow the unit discharge temp to control Cooling cycle and Heating Cycle. And have humidity override the heating cycle to warm the air witch in turns starts the cooling cycle. I have found this to be better for if the cooling cycle starts first the humidity will rise.
As for the PID setting that’s hard to say depends on sensor placement cooling and heating size PID algorithm.

 

[ChasBean1]

Khurram -

Start from the space and work your way out. What are the temperature and humidity requirements for the space served? The control you mention is common and preferred in this application, so you're at the right starting point. The rest is sensor location and what control parameters you desire.

Normally, the space thermostat will provide the right amount of coil reheat to keep the space at the desired temperature. The humidity in the space will be good (only at the space temperature setpoint) with known desired conditions... Pls repost with further detail regarding the space requirements.

 

[VEEKRISH]

Instead of Electric Heating, why don't you try re-heating with a hot water coil.The Hot water Coil can be controlled by a 3-way valve and dew point controller. This will provide good control without any swing that you are experiencing.