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  1. #1
    Join Date
    Dec 2010
    Posts
    58

    AHU with close control

    i need find some document/articles, which have deeply information about close control for AHU. My project need all-air AHU with recirculation, dehumidify cooling, humidify, electrical heating for indoor requirement like that +/- 0.5 oC; +/- 5% humidity,50000m3/h; 4000 person go in and out per hour. Please give me some advices?
    Regards

  2. #2
    Join Date
    Jan 2011
    Location
    Bartlesville, Ok
    Posts
    113
    The first thing you need to do is find the highest resolution sensors available. Use 4-20 ma custom calibrated to the lowest range, such as 30 degrees on a temp sensor. Kele will custom range them for you. You might need a second, cheaper, wider range sensor to run alongside to read values when they get out of your limited range of the high resolution sensor. Once you get solidly within the limited range sensor, use that value.

    The high resolution will allow you to see tiny direction changes, and make tiny corrections before you even move out of your target range. When you use very high resolution sensors correctly, a + or - 5% Humidity target is like the Grand Canyon.

    For instance, if your humidity starts to creep too low (such as two or three down ticks in a row without an up tiick), make a tiny increase in you humidifier output. You can even run that increase through a short duty cycle. A few seconds on, and turn it off again for a bit, in order to observe a change. If there is no change or the movement continues in the wrong direction, increase the output or the duty cycle time. When you can read the smallest change, you have time to make small corrections.

    Will this system use chilled water for cooling?

  3. #3
    Join Date
    Aug 2009
    Posts
    2,459
    Make absolutely sure that the unit is sized properly for the load. To me, this is the most important factor in this project.

    I have seen people trying to get tight temperature/humidity control using an oversized AHU and quite frankly it is a nightmare.

  4. #4
    Join Date
    Dec 2010
    Posts
    58
    Quote Originally Posted by lowdrive View Post
    The first thing you need to do is find the highest resolution sensors available. Use 4-20 ma custom calibrated to the lowest range, such as 30 degrees on a temp sensor. Kele will custom range them for you. You might need a second, cheaper, wider range sensor to run alongside to read values when they get out of your limited range of the high resolution sensor. Once you get solidly within the limited range sensor, use that value.

    The high resolution will allow you to see tiny direction changes, and make tiny corrections before you even move out of your target range. When you use very high resolution sensors correctly, a + or - 5% Humidity target is like the Grand Canyon.

    For instance, if your humidity starts to creep too low (such as two or three down ticks in a row without an up tiick), make a tiny increase in you humidifier output. You can even run that increase through a short duty cycle. A few seconds on, and turn it off again for a bit, in order to observe a change. If there is no change or the movement continues in the wrong direction, increase the output or the duty cycle time. When you can read the smallest change, you have time to make small corrections.

    Will this system use chilled water for cooling?
    Thank for advice, ,we use chilled water coil and electrical heater
    But i cant find any specified document for AHU with close control, please have some source for me
    Regards

  5. #5
    Join Date
    Jun 2009
    Location
    StL, MO
    Posts
    572
    Here's a very good book on the subject.
    http://customer.honeywell.com/Honeyw...D=77-E1100.pdf
    Pay particular attention to the "CONTROL FROM SPACE WITH SUPPLY TEMPERATURE RESET" section.
    You will need to tune the various loops.
    http://www.controleng.com/index.php?...;tt_news]=5719
    The electric heat will need some form of SCR to allow modulation of the heat.
    UA LU 562

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