Calling all humidity guru's
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  1. #1
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    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
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    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
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    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
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    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.

  5. #5
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    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.

  6. #6
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    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.

  7. #7
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    Quote Originally Posted by dlove View Post
    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.

    And to top it all off I'm 90% positive the run-outs to the air handlers are too small (3/4'' good for 4 gpm at normal design, and I'll need at least 12gpm)
    i hate repeating myself... but...now if your saying some of the lines are small and GPM isn't enough, Colder water temps, you surely need, but the GPM is your biggest issue. COIL size, GPM, airflow, return air, mixed air, all play major rolls. With your glycol being messed up how is the rest of the piping? If you plan to add more glycol after all this. I would do a major flush of the chilled water loop, check flows and balance. How has the chemical treatment program been going? sounds like you have your hands full.

  8. #8
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    Quote Originally Posted by heatingman View Post
    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.

    Here's how I would address the situation, your customer might not like the cost but if the site requires it so be it.

    1. Remove the glycol. Glycol reduces the heat transfer between the coil and the water.
    2. Run the chiller at around 40F set point
    3. Run the chilled water valves wide open.
    4. Fit VFD's to all 12 AHU's

    Run the fans at minimum speed (around 20Hz) and with 40 water and valves fully open you will be getting the best de-humidification you can get with the installed equipment. Increase the fan speed if you really need to increase the sensible cooling but you will be reducing the de-humidification. If running the fans at this speed you are still overcooling the space without reaching the RH set point then re-heat is the only option.
    The fan speed will make a bigger difference to the latent vs sensible heat removal than anything else you can do with the present equipment, if you could replace any of the coils then go with a much deeper coil 4,6 or even 8 row coils will de-humidify much better than a 1 or 2 row coil.

    IMO the de-humidification improvement with the RAWAL/APR devices is much more to do with the increased run times than limiting evaporation temperature.


    Kevin

    EDIT: Make sure the condensate drains are clear, because you will see condensate flow like you never seen before.

    ANOTHER EDIT: Make sure the coils are sparkling clean, heat transfer is the key here.
    "Profit is not the legitimate purpose of business. The legitimate purpose of business is to provide a product or service that people need and do it so well that it's profitable."

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  9. #9
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    Quote Originally Posted by klrogers View Post

    IMO the de-humidification improvement with the RAWAL/APR devices is much more to do with the increased run times than limiting evaporation temperature.


    Kevin

    EDIT: Make sure the condensate drains are clear, because you will see condensate flow like you never seen before.

    ANOTHER EDIT: Make sure the coils are sparkling clean, heat transfer is the key here.
    From speaking with Al B. the regional sales engineer for rawal, they accomplish increased run times by limiting the evap temperature.

  10. #10
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    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

  11. #11
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    Feb 2012
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    Upstate SC
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    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.

  12. #12
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    Upstate SC
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    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.

  13. #13
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    Bring your water temp down to 40, and slow the blowers.
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