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  1. #1
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439

    Calling all humidity guru's

    What would be the best evaporating temperature, or in this case chilled water set point to maintain good humidity control w/o over cooling the air?

    Also, anyone using the Rawal device APR valves?

    www.rawal.com

  2. #2
    Join Date
    Sep 2004
    Location
    Kansas City, Kansas, United States
    Posts
    13,832
    the warmer the air the lower the RH.

    look at a psych chart, typical leaving air off coil is 95% RH

    leaving air is like 55* dry bulb, 50-53* wet bulb.

    much colder like 50* it is like 97% or something like that???

    we use rawal devices all the time, guess that is how to spell it????

    they work great on small roof top units!
    I WILL SELL WORK,GENERATE BUSINESS, GO GET NEW CUSTOMERS!
    YOU SHUT THE HELL UP AND QUIT RUNNING YOUR MOUTH!

  3. #3
    Join Date
    Sep 2004
    Location
    Kansas City, Kansas, United States
    Posts
    13,832
    this is 80* dry bulb and like 68* wet bulb.

    [IMG]http://4.bp.********.com/-hwSKNdbNwDU/Ti0TfswXqCI/AAAAAAAAAIg/4cyGJTVjxUQ/s1600/Psychrometric_chart_simplified.png[/IMG]
    I WILL SELL WORK,GENERATE BUSINESS, GO GET NEW CUSTOMERS!
    YOU SHUT THE HELL UP AND QUIT RUNNING YOUR MOUTH!

  4. #4
    Join Date
    Feb 2012
    Location
    Upstate SC
    Posts
    84
    Do you have any modulating valves downstream? I'm thinking you should be good at 45 Deg. F set point. If you are having a humidity problem in spaces re-post the specifics. I have dealt with humidity issues on hydronic systems and the problems are usually with the terminal units.

  5. #5
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439

    Thanks Supertech

    But, I know that the relative humidity is high leaving the discharge.

    The humidity reset once it is re-acclimated to the surrounding air. Thus lowering overall humidity in the space.

    Part of what I understand the APR to do is set a low limit evaporating temperature to prevent over cooling the air and allow for longer run cylces.

    I want to apply the same thinking to a chilled water system to remove more humidity without over cooling the space.

    The customer in this case wants to maintain "40 to 45% RH in the low 70's (db)"

    So to get that, I'm thinking my chilled water set point needs to be around 48 degrees. giving me a theoretical discharge air of 58 db 70% rh

    Let me know if my thinking is off on this.

  6. #6
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439

    JB

    I only spent about an hour there on friday. I'll be going back on Monday.

    It's a bastadrized process chiller applied for comfort cooling/ process dehumidification. It's a printing operation, so humidity is way more critical then cooling.

    60 ton screw machine. With 36,000 cfm on the air side from 12 air handlers (3000 cfm per)

    I'm going to try to slow the air way down to try and get closer to 350 or 400 cfm per ton.

    The chiller had an open tank for water feed, but was filled with a glycol mix. The mix went septic during the off season due to air infiltration. So I've got a lot of clean up to due before I even start the chiller.

    But, I know it never performed, which is why glycol was added. They figured colder water would provide better humidity control, but I think it likely caused the sensible load to satisfy way before the humidity load was met leaving them with too much humidity.

  7. #7
    Join Date
    Feb 2012
    Location
    Upstate SC
    Posts
    84
    I think you are on the right track, lowering the air speed is one of the tricks I would try first. Keeping the ambient set point as high as you can get away with is another. Keep your chilled water set point low. Clean coils would also help.

  8. #8
    Join Date
    Jul 2011
    Location
    East coast USA
    Posts
    955
    Airflow across the cooling coil, coupled with sufficient run-time, has a significant effect on proper humidity removal. Too much air will result in poor dehumidification. Too little air can cause the ductwork to sweat in the unconditioned spaces .

    but The size of the coil will play a part to help remove latent heat as well, that's what your asking how remove latent heat with out changing temp. But if your system was not designed this way it will be hard. Running your chilled water cold to ring out moisture and than a reheat to maintain a discharge air temp is common. to maintain constant temps and humidity does require some controls. from what you said you have chilled water what type of fan system do you have? does it have mixed air, outside,return dampers. reheat coils, does it have temperature sensors like humidity, SAT, RAT, MAT. ? if you just lower your water temp you will ring out moisture, but you will lower air temp. To achieve what you want its not just one thing.

  9. #9
    Join Date
    Feb 2006
    Location
    Float'N Vally, MS
    Posts
    1,807
    Quote Originally Posted by heatingman View Post
    But, I know that the relative humidity is high leaving the discharge.

    The humidity reset once it is re-acclimated to the surrounding air. Thus lowering overall humidity in the space.

    Part of what I understand the APR to do is set a low limit evaporating temperature to prevent over cooling the air and allow for longer run cylces.

    I want to apply the same thinking to a chilled water system to remove more humidity without over cooling the space.

    The customer in this case wants to maintain "40 to 45% RH in the low 70's (db)"

    So to get that, I'm thinking my chilled water set point needs to be around 48 degrees. giving me a theoretical discharge air of 58 db 70% rh

    Let me know if my thinking is off on this.
    I do not believe you will be able to do that without reheat.
    We are in a very humid area and have several clean rooms with tougher conditions. Basically all the stars have to be aligned, cfm ducting etc.
    We drive the chw vlv wide open and temper it with reheat. The trick is to have very slow control loops (if DDC).
    Life is too short, Behappy!
    TFMM

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,721
    Bring your water temp down to 40, and slow the blowers.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  11. #11
    Join Date
    Dec 2004
    Location
    Chicago area
    Posts
    1,439

    Evap temperature.

    If I lower my chilled water loop, that will dehumidify, but it will also cool the air too much.

    The way I see it, as long as the chilled water is below the dew point I will dehumidify.

    The closer the spread between the chilled water and the incoming air, the less sensible transfer there will be.

    With the entering air at 72db and 45% rh, the dew point is about 49 degrees. So as long as the coil is below that, I will remove humidity.

    If the coil was 40 or 45 that would dehumidify, but the spread between the coil temp and the incoming air is that much more, which will cause the the sensible load to be addressed even more.

  12. #12
    Join Date
    Sep 2004
    Location
    Kansas City, Kansas, United States
    Posts
    13,832
    i agree with all posts!

    i am usually or always working on vav stuff with vfd,s so lowering fan speed and maintaining proper duct static is not usually much of an option for me?

    i have labs that run the heat all night and start cranking on the air an hour before they start their process!!!!!!!!!!

    this is not efficient but helps the labs!!!!!!!!!
    I WILL SELL WORK,GENERATE BUSINESS, GO GET NEW CUSTOMERS!
    YOU SHUT THE HELL UP AND QUIT RUNNING YOUR MOUTH!

  13. #13
    Join Date
    Apr 2003
    Location
    St. Louis
    Posts
    18
    Make shure coil is piped correctly coldest water/ coldest air, and how much building infiltration is their.

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