How to manage Temp and RH on a conventional AHU System?

[najamiqbal]

We have one AHU (capacity is apprx. 70,000 cfm / hr). Temperature & Relative humidity to be maintained at 21 +/- 1 Deg C & 43 + / - 1.5 % respectively. What is the best strategy to manage Temp/RH with conventional system i.e. with 2/3 way motorized valve actuator, Temp/RH sensor, and or DDC

Your input will be greatly appreciated

[crab master]

Good luck with this - this is tight control. I am going to say you'll have to use DDC. Do you have digital scroll compressors or just a whole bunch of smaller compressors? What part of the world are you in?

[digo]

We have one AHU (capacity is apprx. 70,000 cfm / hr). Temperature & Relative humidity to be maintained at 21 +/- 1 Deg C & 43 + / - 1.5 % respectively. What is the best strategy to manage Temp/RH with conventional system i.e. with 2/3 way motorized valve actuator, Temp/RH sensor, and or DDC

Your input will be greatly appreciated

cfm / hr? Perhaps you meant cubic meters per hour?

from wikipedia (http://en.wikipedia.org/wiki/Cubic_feet_per_minute):
Cubic feet per minute (CFPM or CFM) is a non-SI unit of measurement of the flow of a gas or liquid (or air) that indicates how much volume in cubic feet pass by a stationary point in one minute.

To convert cubic feet per minute to cubic meters per hour, multiply by 1.699.

[najamiqbal]

Thank you Fellows.

I am in Dubai where they still us metric system (non-IS measurement).

[kostasgreece]

Your description is a litle vague...You have to maintain T/RH conditions on supply or return, is it a single coil or dual coil application, constnt fan or variable?
The concept in all ways has a common base use a ddc controller with ability to set P,I,D during tests (so you can achieve your tight control), use a good quality valves and actuators, same applies to sensors dont be cheap to that point.
As a base control strategy supply air temperature should modulate heat or coll respectively to maintain design values.
In addition a separate control loop should maintain supply air RH by modulating cool coil lower than deisgn RH value (if you need to also take into consideration space latent RH).

Maximum of each loop finally drives heat , cool output.

Thats as brief as it gets, for details you can review strategies from control theory books.

Good luck.

[Control Doctor]

Single Zone? VAV? RTU? what arrangement do the coils have? Outside Air?

To get the most constant temperature and humidity I would use a supply air reset utilizing room temperature as the reset function. When you keep a constant supply temp it will be a lot easier to control your supply humidity, or at least know what you have to do to control it. Do you have humidifiers? So many questions......

[mujtaba12]

cooling coil will be 100% as long as humidity has to be maintained called wild condition but due to continous cooling if ur req. temperature drops then ur hotwater valve operates accordingly to increase the temperature..once the humidity has been achieved then move ur hotwater valve to 0% and cooling coil will operate with the return temperature

[leaflying]

What kind of environment the unit is serving? Does it has indoor humidity load or humidity load is 100% from outside air? Depends on this, dew point temperature control may or may not be applicable.

And to satisfy said setpoints, you almost always need reheat coils to recover dry bulb temperature after water content is removed precisely.

[Under_Control]

As stated, more info is needed. Are you designing this from scratch, including the unit? Just controls? Chilled water or DX? I see +/- on humidity, so you need to be able to add moisture if needed to maintain? Amount of OA fixed, or variable? If variable, based on what? CO2? Flow measurement? What is indoor RH avg? Outdoor RH avg? What kind of reheat? And what about all the questions I forgot about? There will be many more I'm sure. Try to get as much info as possible, do not assume we know anything about your site, location, requirements, specs, unit location, RTU vs indoor AHU, preconditioned OA unit (the best if you have very humid enviroment). Good luck! If this is a very critical location, it would probably be best to have someone that can check the job/info and consult, and not something you want to chance on a forum, since if you leave out 1 vital bit of info, it might be disasterous.

[exwtk]

cooling coil will be 100% as long as humidity has to be maintained called wild condition but due to continous cooling if ur req. temperature drops then ur hotwater valve operates accordingly to increase the temperature..once the humidity has been achieved then move ur hotwater valve to 0% and cooling coil will operate with the return temperature
100% cooling would only be required for de-humidification. Since you are in Dubai then high humidity won't be your problem. Your humidification would be more important. Does your system have humidifiers.
Sorry to tell you this... but maintaining 43 %RH with +/- 1.5% seems a little unrealistic

[najamiqbal]

Thank you Fellows for all your help and useful tips. I will update you about the outcome.

Regards