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  1. #1
    Join Date
    Apr 2001
    Posts
    327
    I have a dual duct system that the spec calls for the following "mix hot and cold deck to maintain room setpoint. On a call for cooling modulate hot deck closed to 50% of total air quantity, on a further call for cooling modulate hot deck closed and cold deck to 50% of total air quantity. On a further call for cooling modulate the cod deck to 100% open."

    The boxes are modulating to 50 and 100% like it asks but where does the mixing start. These are invensys controllers, is it possible to " mix the air to maintain setpoint " with out a discharge air temp sensor.
    If any of you read this spec how would you program the boxes to operate.

  2. #2
    Join Date
    Feb 2004
    Location
    Philadelphia
    Posts
    104
    a dual duct system is nothing more than a multizone ahu with the dampers located at the "mixing" or terminal boxes instead of at the ahu. the hot duct and cold duct air flow is typically regulated by a damper. the damper is mechanically setup, for example, so that with no control signal to the damper motor the hot duct air flow is at 100% (fully open) and the cold duct air flow is at 0% (fully closed). on a rise in space temperature above the space temperature setpoint the control signal to the damper motor increases and the hot duct air flow is decreased while the cold duct air flow is simultaneously increased by the damper motor driving the damper. on a fall in space temperature the reverse occurs.

    the "mixing" box is supposed to be where the "mixing" of the two airstreams takes place. typically the space temperature sensor, thru a control loop in the controller, directly controls the operation of the damper motor based on space temperature deviation from setpoint. there is no discharge air temperature sensor involved at the "mixing" box. The box is supposed to magically deliver the correct proportion of hot air to cold air in order to get the perfect temperature air into the space to maintain the setpoint.

    i have not seen a spec that calls for what your spec calls for. that spec kinda defeats the whole purpose of the system. you should be able to position the damper in any number, or an "infinite", number of positions in order to maintain the space temperature setpoint by mixing the air however it needs to be mixed to accomplish this. i don't quite see how driving the damper motor like a three position switch (hot open - cold closed, hot half open - cold half open, hot closed - cold open) really works. just my two cents worth.

  3. #3
    Join Date
    Mar 2004
    Location
    Janesville WI
    Posts
    419
    Very well put fms2k, we have a lot of these around still all pneumatic, damper seems to stick with the hot duct open a lot, looking to convert over to ddc havent found a damper act. that fits between hot-cold duct. took one of these boxes down holy crap was it heavy.

  4. #4
    Join Date
    Feb 2004
    Location
    Philadelphia
    Posts
    104
    jtricor,

    thanks for your remarks. yea, i think they sold those mixing boxes by the pound back in the day. i have seen the dampers unfortunately start to bind, or stick, as you termed it. the old pneumatic controls really did an ok job if they were maintained and periodically checked for calibration. that was back in the old days - i think those days are long gone now due to ddc. i have retrofitted/up-
    graded a bunch of these over the past few years from pneumatic control to ddc. have had good success with using Belimo LF series motors on the dampers to replace the pneumatic motors originally used, if that helps you any.

  5. #5
    Join Date
    Apr 2005
    Location
    Pittsburgh, PA
    Posts
    42
    Now, I could be wrong, been known to be on occasion, but I don't think that spec is calling for a 3 position. Key word for me was "Modulate". Now it sounds strange to me that they are calling for the hot deck to modulate closed to 50% and not modulate the cold deck open to 50% at the same time, but I guess they have their reasons. From what I can tell, if you are using a heating/cooling loop, 0-66% would be 100-0% hot deck damper and 33-100% loop would be 0-100% cold deck damper. I'm guessing your in a hospital, so you might want to question the lower air volume, but I think this might work. I graphed it out and it looks like you're going to have a minimum of about 50% total volume when the loop signal is at 50%. Let me know if this helps you at all.

  6. #6
    Join Date
    Apr 2004
    Posts
    34
    cardinal ac
    I think that we have come to a solution on this problem. Give me a call and I will discuss it with you.


    A

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