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  1. #14
    Join Date
    Sep 2002
    Posts
    12,698
    I wasn't sure how much I was allowed to help you before - but what I said is true: the air coming off a really properly performing cooling coil should be about 100% RH.

    Moving on: if the air leaving your chilled water is not nearing 100% RH then you need a colder coil, less moisture in the incoming air, a larger chilled water coil, or less air flow across the chilled water coil.

    The easiest two to do would be to lower the chiller LWT set-point as much as possible and reduce the air flow through the chilled water coil.

    Can you talk to the chiller guy(s) there? And ask them if they have tweaked the approach on the chiller barrel as close as it's possible to get? And then add 33 to that approach number and set the LCWT for the resulting number?

    Another way to tweak is to increase the water flow rate though the chilled water coil. This will reduce the water TD through the coil and so keep more of your coil surface area at a lower temperature - further below the dew point of the air. One of the reasons that water coils are larger than direct expansion coils is that most of the surface area gets warmer than entering water temperature water. Plus; with a chilled water system you lose twice: first in the chiller's heat exchanger and then again in the water-to-air heat exchange. Lost of surface area is the only way to compensate.

    PHM
    ------



    Quote Originally Posted by Usafwhite View Post
    Good afternoon, I'm having issues controlling humidity throughout several operating rooms. It is a VAV system. AHU has a cooling coil only and we use VAV hot water reheat. AHU has 44 supply water temp and 53 degree return water temp. The supply water pressure is 66PSI and return pressure is 55PSI. Discharge air is 52 degrees leaving the AHU (I dropped the disharge air setting to 50 hoping to remove more moisture). AHU has a common return (shares a mechanical room with another AHU). Humidity level in the mechanical room is 48%. I climbed in to the mixed air section of AHU before filter section and humidity is 49%. I climb in to the coil section, after filters but before cooling coil and it's 53%. I climbed into the blower section of AHU, after the cooling coil and it's gaining 20 to 25% humidity. The supply air has anywhere from 70 to 75% humidity. Drain pans are clear and there is a steady flow of water coming out the drain of the AHU. Inlet guide vanes on the AHU are not working and are stuck open. Is it possible the air is moving too fast over the coil to remove all the moisture? I'm thinking of working on the vanes in order to slow the air down to test my theory.
    Most of the rooms are set between 62 and 65 degrees. I noticed if I turn the thermosat up, the humidity drops. It's in direct relation to how much air is supplied from the AHU. Any thoughts or ideas would be greatly appreciated. Thanks for your time.
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

  2. #15
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,357
    If the inlet guide vanes are stuck on the AHU, and the system uses VAV units, how are you controlling static pressure in the ducts? I would lean on management to either fix the guide vanes or convert the blower to VFD (better option...saves energy).

    Others have mentioned dew point temperatures above. Using dew point is far more reliable than relative humidity for troubleshooting humidity control problems. Had you recorded dew point temps as you climbed through your air handler, you would have seen where the actual absolute moisture gains are as the air travels down the AHU. If you also know what the conditioned space target parameters are (in the OR, for instance), you can tell by the dew point of the air leaving the AHU, along with the dry bulb temp, if you'll ever get there.

    For example, if the target OR condition is 65F at 50% relative humidity, the dew point temp of air leaving the AHU would need to be 45F or less to reach target. If you have a deep coil in the AHU you might achieve this with a 5 degree approach between entering chilled water temp and leaving air temp, meaning entering chilled water would be at 40 degrees. Otherwise, with higher approaches you will need colder water or less airflow. With inlet guide vanes stuck, I would work on that first before talking to the chiller operators.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #16
    Join Date
    Jun 2010
    Location
    Philadelphia PA
    Posts
    2,190
    The problem is that chillers are unable to provide cooling near freezing like a DX coil can. Chilled water warms 1^F per 1^F btus of cooling provided. DX is able to do the majority of their cooling at the refrigerants evaporating temp. You have a coil that is 33^F providing 35^F dew point air exiting the coil.
    I thought that the idea of chilled water is to get a deeper (more rows) coil to get at that latent load. a DX coil would freeze with the deeper coil
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  4. #17
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,533
    Quote Originally Posted by genduct View Post
    I thought that the idea of chilled water is to get a deeper (more rows) coil to get at that latent load. a DX coil would freeze with the deeper coil
    Much easier to remove latent with a DX coil at a uniform coil temp. Chilled water gains temp as it absorbs heat. Less latent removal with chilled water.
    Regards TB
    Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
    Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
    Use +Merv 10 air filter. -Don't forget the "Golden Rule"

  5. #18
    Join Date
    Sep 2002
    Posts
    12,698
    That's the same thing that the guy in post # 14 said, isn't it? <g>

    PHM
    ------


    Quote Originally Posted by teddy bear View Post
    Much easier to remove latent with a DX coil at a uniform coil temp. Chilled water gains temp as it absorbs heat. Less latent removal with chilled water.
    Regards TB
    PHM
    --------
    The conventional view serves to protect us from the painful job of thinking.

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