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  1. #1
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    Jul 2012
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    Dehumidify and control cooilng temperature

    Hello,
    I'm developing a software to control a simple air handling unit (ahu)

    My goal is to control the temperature & humidity with a cooling and a heating coil.

    Right now, I'm controlling the temperature easily, but I can't control the humidity at the same time !

    With a web research, I think I have to produce more cooling capacity and the reheat the air with the cooling coil , but I can't find a relation between how much I have to cool and how much I wan't to reheat to maintain the relative humidity between 50-60% RH !

    I would appreciate any help since my area is electronics !

  2. #2
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    Technically your trying to control or lower dewpoint

    You would need a humidity sensor that can supply a analog input to your controller.

    That input would have priority over your cooling command. Your output would be a discreet command to a solenoid to open and close hot gas flow to the coil

    Reheat is an inefficient way to dehumidify BTW. It works but its expensive.

  3. #3
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    Quote Originally Posted by Six View Post
    Technically your trying to control or lower dewpoint

    You would need a humidity sensor that can supply a analog input to your controller.

    That input would have priority over your cooling command. Your output would be a discreet command to a solenoid to open and close hot gas flow to the coil

    Reheat is an inefficient way to dehumidify BTW. It works but its expensive.
    Thank you for the reply

    The system is part of a air handling unit with a cooling coil and a heating coil.

    My main goal is to control temperature, but maintain the humidity controlled 50-60 % RH

    I can open the cooling valve and heating valve from 0-100% independently at the same time.

    I have the temperature control of the AHU donee, with 2 PIDs (cooling + heat) but I have to limit the humidity !

    If I supply the air at 13C I have +- 90% RH !!!

  4. #4
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    So you have a PID loop controlling a 3 way chill water valvle and a PID loop controlling a 3way heating valve ?

    Is this hypothetical or a real installation ?

    In order to dehumidify your chill water coil will have to get down to below dewpoint temps and then you will have to use reheat to control space temp.

    Some dedicated dehumidifiers run coil temps 30 degrees below dewpoint.

    MANY different factors contribute to the effectivness of a system to both remove heat and humidity from the return air and not the least of which is total system capacity compared to load.

    At 93% RH it sounds like you are NOT removing any latent heat, only sensible heat.

    Your relative humidity will climb as your air temp lowers and your dewpoint remains constant.

    Many a IT tech, engineer, electronics guy ( I used to be heavy into electronic hobbie building myself ) think our vocation and proffesion is based on such simple concepts that anyone can do it.

    Oh well...

  5. #5
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    Jul 2012
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    Quote Originally Posted by Six View Post
    So you have a PID loop controlling a 3 way chill water valvle and a PID loop controlling a 3way heating valve ?

    Is this hypothetical or a real installation ?

    In order to dehumidify your chill water coil will have to get down to below dewpoint temps and then use reheat to control space temp.

    MANY different factors contribute to the effecticness of a system to both remove heat and humidity feom the return air and not the least of which is total system capacity compared to load.

    At 93% RH it sounds like you are NOT removing any latent heat, only sensible heat.

    Your relative humidity will climb as your air temp lowers and your dewpoint remains constant.
    No I have a 2way chill water valve and a 2way heat water valve.

    Each one of this is controlled by a PID. It's a real AHU isntallation.

    So in order to dehumdify I have to exceed the 100% RH barrier, right !? then reheat the air to the desired temperature?!

    My RH% and temperature sensor are in supply duct, after the AHU.

    Which is the efficient way of dehumidify and cool !?

    My cooling PID is controlling the cooling valve based on temp in a direct action !

    If the Xctl (temp) rises, the Yctl (chill valve output), rises too.

    I can add another PID to overcool based on info from te RH sensor.

    I order to dehumidify, Xctl (RH%) rises, the Yctl (chill valve output)

    Is this right ?!

  6. #6
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    The humidity sensor needs to be placed in the return not the supply.

    Yes it sounds like your chill water valve is direct acting. Typically energy adding devices ( hot water , steam valves fail to close) and valves that remove energy fail to open.

    The chill water actuator should be spring to open and hot water should be spring to close.

    More effecient ways of dehumidification are through desicant wheels with a reactivation process and dedicated dehumidifiers.

    To dehumidify coil temp should be kept below dewpoint but first get a proper measurement of humidity from your return air.

    Andl leave the sensor there. Typical office building supply air temps are 55 degrees with a coil temp in the low to mid 40s.

    Also if they're two way valves where is the by-pass ?

  7. #7
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    air leaving the cooling coil will be saturated. as 6 suggests relocate your humidity sensor to the zone or return.
    IV IV IX

    use your head for something other than a hat rack.......Gerry

  8. #8
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    Typically when high humidity removal is needed the cooling cool is deeper than a sensible cooling application, 6, 8 or more rows deep is not uncommon, but has others have suggested moving the sensor location is the first step.

    Kevin
    "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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    Psychometric Chart - It's very complex, but as your finding out "a simple air handling unit (ahu)" ain't so simple when you get all the controls in it, especially getting them working properly.

    Disclaimer above link was just found on Google, so don't know how good it is, but Wikipedia and then you have to adjust for elevation...

    Agreed on moving the RH sensor to the return, but what are you really trying to control and why? Electronics room/Operating room? Then with 2 way valves are you accounting as well for a time delay for flow and temp? How hot/cold are you controlling the water temp temps to or is that a fixed value? How about fan speed?
    "How it can be considered "Open" is beyond me. Calling it "voyeur-ed" would be more accurate." pka LeroyMac, SkyIsBlue, fka Freddy-B, Mongo, IndyBlue
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  10. #10
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    Jul 2012
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    There is something like that (see attachment pls).

    But if i'm supplying air at, imagine, 15C and 90% rH, after some time the rH of the return will rise, this ir right ?!
    Attached Images Attached Images  

  11. #11
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    Jul 2012
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    Quote Originally Posted by crab master View Post
    Psychometric Chart - It's very complex, but as your finding out "a simple air handling unit (ahu)" ain't so simple when you get all the controls in it, especially getting them working properly.

    Disclaimer above link was just found on Google, so don't know how good it is, but Wikipedia and then you have to adjust for elevation...

    Agreed on moving the RH sensor to the return, but what are you really trying to control and why? Electronics room/Operating room? Then with 2 way valves are you accounting as well for a time delay for flow and temp? How hot/cold are you controlling the water temp temps to or is that a fixed value? How about fan speed?
    hot water = +-55C
    Chilled water = +-10C

    the space to control is a warehouse that must mantain a temp of 21-22C and 50-55% rH

  12. #12
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    Jul 2012
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    Northern NY
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    A very simple answer to this without regards to energy efficiency or any of the such is below. I have done this in many pharmaceutical warehouses.

    When your space humidity is above set point you need to either raise the space temperature if you can (this will decrease space RH) or if not you need to be in a dehumidifying mode.

    When in a dehumidifying mode you open the cooling coil 100% and then run the reheat coil to maintain a discharge air temp that will satisfy the space temperature. This will drop the discharge of the cooling coil as low as possible and "dry out" the air but be very cold, then you need to "reheat" the air to actually maintain the space at the temperature you want.


    as far as just putting in 15 Deg C air at 90% RH, if there is no heat load in the space then eventually you would get the space down to 15 Deg C. In reality most spaces have a heat load from both external sources as well as the mass of the items in the space. This will cause the space to rise in temperature and as the temperature rises the RH goes down. You can see this by use the psychometric chart to look at 15 Deg C air with 90% Rh then increase it to say 21 Deg C and then the Rh would be around 65% or so. I just did a quick check on the chart, more accurate versions and details can be given by those who use them regularly.


    On many AHU units like this I have done I run the discharge after the cooling coil around 12 Deg C then run the discharge after the heating coil as necessary to keep the space temperature in compliance. It is very energy wasteful but with out some other hardware such as hot gas reheat on DX, or heat wheels there is not much you can do, but it will work.

    -Jeremy

  13. #13
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    Jul 2012
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    First of all thank you !

    So, to cool and dehumidify, do you think the procedure of opening the cooling valve to 100% and then control the temperature with heating valve is a good option ?!

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