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  1. #1
    Join Date
    Jun 2011
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    Currently working in KSA
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    Space Pressure Control.

    Hi Guyz,

    I experience a problem controlling space pressure in the building.

    As per the sequence of operation space pressurization shall be controlled by maintaining a differential airflow offset between the system exhaust and the system outdoor air.

    in the event the system differential airflow exceed the offset setpoint the DDC controller shall modulate the outdoor air damper from open to close and the return air damper from closed to open.

    My problem is the pressure is so erratic just why my control is going on and off or modulating from 0 to 100 or vice versa.

    any tips and tricks controlling like i have?

    I hope you guys could help me.

    Thank you in advance.

  2. #2
    Join Date
    Jul 2008
    Posts
    1,430
    what control system?
    what PID parameters?
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  3. #3
    We currently use our mixed air for modulation.

    Plus keep the variables set for min and max to not overshoot your building. Are you using a program to control the ddc, we currently have Seimens Apogee and if so make sure that noone before you placed dampers in Oper priority. This would force the dampers closed, open amd wouldnt be able to be overrode by the system letting it do its job.

  4. #4
    Join Date
    Jun 2011
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    Currently working in KSA
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    Quote Originally Posted by MatrixTransform View Post
    what control system?
    what PID parameters?


    Siemen PXC controller.

    gain= 20
    tn= 10 minute

    im trying to play with rise time of the control as well. i dont know if does make sense.


    jojit

  5. #5
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    Jun 2011
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    Quote Originally Posted by Notasupertech View Post
    We currently use our mixed air for modulation.

    Plus keep the variables set for min and max to not overshoot your building. Are you using a program to control the ddc, we currently have Seimens Apogee and if so make sure that noone before you placed dampers in Oper priority. This would force the dampers closed, open amd wouldnt be able to be overrode by the system letting it do its job.

    we are currently in commissioning stage. so still functionality test is on going, i am pretty sure no one override the damper command.

  6. #6
    Join Date
    Jul 2008
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    Quote Originally Posted by lightbearer1925 View Post
    Siemen PXC controller.
    gain= 20
    tn= 10 minute
    Hmmm... sorry I dunno what 'gain' actually means in a PXC controller.
    Maybe somebody can help enlighten us all

    Maybe try ziegler-nichols method and see how you go.
    1 + 1 = 3 ( *** for very large values of 1)

    ...everybody wants a box of chocolates and long stemmed rose

  7. #7
    Join Date
    Sep 2007
    Location
    Sacramento, Ca.
    Posts
    35
    First, make sure you have all your exhaust fans accounted for, and that they are monitored for status. If the EF is ON, then add that EF's flow value into the equation for total exhaust. If you have EF's in the building that you cannot account for status-wise, you'll never get the correct amount of offset between suppl and exhaust.

    Is the OA Damper modulating? This will effect the amount of Supply air.

    Is the exhaust damper modulating? This will effect the amount of Exhaust Air (naturally), and if your AFMS for return/exhaust is located before the RF, you won't know exactly how much is exhausted, and how much is being fed back through the MIxed Air section.

    Presuming this is all OK, when in the PPCL editor, look up the definition of LOOP - presuming you are using LOOP control for the BSP algorithm - and do the math to determine your P and I values.

    If you are using an ADAPT statement (ADAPTS actually) again look at the Help section in the PPCL editor, and do the math that is specified there. Then run a BUMP test to get a more refined set of values for your variables. If you are unfamiliar with the BUMP test, ask the Siemens tech about it. The explanation can be found in the Adaptive Control Application Guide.

  8. #8
    Join Date
    Jul 2011
    Location
    East coast USA
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    925
    PID is important, dampers speed is important. The type of dampers are important. you should look at the stoke of the dampers and see how much they open. Sometimes a damper could be 50% open and pull as much air as if they were 100%. I would also, trend my damper positions, static pressure cfm and VFD Hz (if you have them) this will help pinpoint where the bouncing is..

    Also, check the calibrations and accuracy of the controls. Just because they are new don't mean they good.

  9. #9
    Join Date
    Nov 2009
    Location
    Northern, Kentucky
    Posts
    175
    A couple of questions: has the air balance been done? did the air balancer check and record the actual building pressure when all the equipment was runnig at design and the building closed up? if the equipment is not maintianing the correct pressure per the design, balance report, no tuning will fix an incorrect balance or AFMS calibration. Your best bet would be to control based on buliding pressure differential. Good luck.

  10. #10
    Join Date
    Sep 2007
    Location
    Sacramento, Ca.
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    Dennis, I disagree, though your point is valid. It appears the spec calls for air flow differential control. Switching to building static pressure control could be an affront to the designer. Once the spec is satisfied, but the building does not perform to expectations, then change to BSP control. If the Cx Agent does this on his own, he is treading on very thin ice, indeed.

  11. #11
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    Quote Originally Posted by BenControllin View Post
    Dennis, I disagree, though your point is valid. It appears the spec calls for air flow differential control. Switching to building static pressure control could be an affront to the designer. Once the spec is satisfied, but the building does not perform to expectations, then change to BSP control. If the Cx Agent does this on his own, he is treading on very thin ice, indeed.
    i guess it will come down to the type of system they have, the OP doesn't mention, other than DDC controls, what type of system he has. but if its constant volume and exhaust than air flow differential seems ok. But I do prefer Static pressure.

    I just think using dampers to control pressure is difficult and a one time balance will not do. you need to spend time and experience the system in action. Not even sure what the OP means by space. is it for the whole building or a space, like a clean room?

    assuming the balance was done correctly, fixing the dampers until a good balance in pressure is established. than begin to look what causes the pressures to become erratic, are doors opening, ? I still say trending will help a lot here.

  12. #12
    Join Date
    Sep 2007
    Location
    Sacramento, Ca.
    Posts
    35
    We agree. Not enough info to drill down to a good answer. All your concerns are valid, and the questions you have are unanswered by the OP's description. Also, the spec must be followed until found to be unworkable. If the Cx agent departs from that, he will be assuming a role of the design engineer. A role he wasn't compensated for, and perhaps is unprepared to take on.

  13. #13
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    Jun 2011
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    Currently working in KSA
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    Quote Originally Posted by MatrixTransform View Post
    Hmmm... sorry I dunno what 'gain' actually means in a PXC controller.
    Maybe somebody can help enlighten us all

    Maybe try ziegler-nichols method and see how you go.

    in PXC controller gain is use instead of proportional. it is only reverse with proportional.

    for example;

    proportional or p = 1 meaning if you have 1 error from setpoint than your control will be 100%.

    gain = 100 then if you get 1 error from the setpoint than your control will be 100%

    thanks for reply

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