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  1. #1
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    Ebtron Gold AFMS and Duct Static Pressure Sensors

    Not sure if to post this in this forum or not, but I figured I would try here first. If it belongs in a different forum I apologize.

    So I have a new space that is used for sterilization of medical equipment, it has a new Constant Volume 100% OA unit with all spaces controlled by a VAV box (supply side). We specified Ebtron Gold fan inlet mounted units on both return and supply fans. Both Airflow and pressurization are critical in this area due to the activities in the space. The controls contractor for the project included duct static pressure sensors in their S.O.O. (but it is not in the VA's), and I questioned it. They are insisting that instead of setting fan speed via the airflow reading of the Ebtron Units, they want to set fan speed via a duct static pressures sensor. Their claim is that the fan may be able to spin slower in the event that some of the VAV box dampers are "almost closed".

    I was under the impression that the Fan inlet sensors like Ebtron Gold were more precise than using static pressure, which was the reason they are in the VA specifications. Further, I thought that the AFMS would compensate for things like changes in Atmospheric pressure, temperature and humidity that a differential pressure sensor would not.

    So if I could hear your opinions on the pro's and con's of DPS vs and AFMS I would appreciate it.

    Thanks in Advance!
    Tim

  2. #2
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    This isn't computing yet. You have a "constant volume" unit but the fans are varying off duct static or AFMS + VAVs?

    Guessing the OA unit is supposed to deliver a constant volume and the "VAVs" in the spaces have fixed airflow set points?
    Propagating the formula. http://www.noagendashow.com/

  3. #3
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    1) If they are able to 'set fan speed' then it's not constant volume. (I assume it has a VFD?)

    2) Yes, Ebtron is more accurate than a DP sensor to determine airflow, HOWEVER - if the VAV boxes are trying to maintain a specific airflow, and the AHU (a fan in series with the VAV's) is also trying to maintain a specific volume, they will invariably fight. If the VAV sensors are just a couple of CFM different than the fan inlet sensors, they will both try to adjust to make setpoint. Therefore, controlling the SF VFD based on Duct Pressure is a better control option.
    You will still get CFM controlled by the VAV's, and you will get an accurate airflow reading from the Ebtrons. If the ebtron is reading too low or high, adjust a VAV box airflow setpoint up or down.

  4. #4
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    Quote Originally Posted by orion242 View Post
    This isn't computing yet. You have a "constant volume" unit but the fans are varying off duct static or AFMS + VAVs?

    Guessing the OA unit is supposed to deliver a constant volume and the "VAVs" in the spaces have fixed airflow set points?
    Correct, all spaces are supposed to maintain a constant airflow 24/7. The original contract docs did not include a duct static pressure sensor, only the AFMS and the VAV's, but the controls contractor wants to do control via a DPS. I am sure that the A/E who did the original Sequence of Operation was not as clear as could be, but the intent was AFMS determines Fan Speed, which I assumed would be adjusted to be design CFM plus some fudge factor for leakage and the VAV's being off a little, which would be worked out during the commissioning and test and balance. Seems pretty simple to me, but perhaps I am missing something.

  5. #5
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    I would request they give you both with a software switch to select between them if it where me. Try both see what happens. One could easily setup a airflow out of wack alarm if the duct static is maintained but the AFMS is off target by x%.

    I would agree with John's thoughts that it may cause the system to be unstable. Maybe not. Keep in mind the OA unit will have to take in account the +/-5% (prob best case) error in the VAV pickup + sensor + PID tune. That error is not going to be a constant so it cannot be taken out of the equation.

    Regardless what Ebtron claims their TD probes can get fouled out by contaminates in the air stream, which a duct static sensor is normally not effected by. Would assume in your case the air is hepa filtered, but fan inlets over time seem to collect grease & belt dust around them. That crap will certainly cause issues if any of it gets in the TD probe.

    If space pressure was super critical, you would have venturi valves not VAV/CAV boxes. If it really matters...just measure it. Not too costly to add a few space pressure sensors in the critical spots or even just the low tech ping pong ball style of indicators.
    Propagating the formula. http://www.noagendashow.com/

  6. #6
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    Quote Originally Posted by JohnLoves2ski View Post
    1) If they are able to 'set fan speed' then it's not constant volume. (I assume it has a VFD?)

    2) Yes, Ebtron is more accurate than a DP sensor to determine airflow, HOWEVER - if the VAV boxes are trying to maintain a specific airflow, and the AHU (a fan in series with the VAV's) is also trying to maintain a specific volume, they will invariably fight. If the VAV sensors are just a couple of CFM different than the fan inlet sensors, they will both try to adjust to make setpoint. Therefore, controlling the SF VFD based on Duct Pressure is a better control option.
    You will still get CFM controlled by the VAV's, and you will get an accurate airflow reading from the Ebtrons. If the ebtron is reading too low or high, adjust a VAV box airflow setpoint up or down.
    1) Yes, VFD controlled Fans.

    2) OK, that kind of makes sense, but wouldn't you see the VAV's going full open or full closed, at which point you could adjust the setpoint (or something similar?). If not, where would you recommend the DPS sensor be located? In this case, the contractor wants it located 2/3 of the way down the supply duct, the reason given is so that all VAV's "have proper flow".....

    Now that I am thinking of it, we may not have VAV's in every single space, I think a couple of the non critical spaces do not (2 bathrooms and the mechanical room come to mind), if we added a little the Design CFM and controlled the fan speed from the AFMS, wouldn't the "extra" air just go to a space like the mechanical room?

    I am not opposed to using the DPS, just trying to rationalize the correct implantation, This area will receive a lot of attention from the people in Washington DC when it opens up and would need to justify any deviation from the already accepted plans.

  7. #7
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    I like the idea of a software switch!

    I just checked on the documents, I was incorrect in stating that all areas have a VAV box, although we have a dozen or so of them, this doesn't cover every space, there are still a bunch of areas that just have a supply damper and will be set manually at the time the T&B is done.

    As far as pressurization goes, it is critical and we have 2 AFCV's in the return duct for the 2 rooms that share a common wall where the pressurization is absolutely critical, this wall separates the clean and dirty sides of the areas, but of course material needs to transfer between the 2, which is done via an air lock pass through box where both doors cannot be open at the same time. We are also supposed to have a "ball in the wall" sensor with an electronic switch, but apparently the manufacturer does not offer the version with an electrical switch anymore, so may end up just substituting a differential pressure sensor and a "regular" ball in the wall for a visual indication. Which would make sense because checking the sequence of Operation on the contract drawings reveals that it never mentions how the space pressurization is to be monitored, I guess the A/E figured if we knwo for certain the airflow into and out of the 2 rooms and the volumes of the rooms, we will have no issues?

  8. #8
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    If you have supplies with the volumes being set by static balance damper, would suspect things would control stable regardless how you do it. Any fluctuation from the VAVs will just get sucked up there.

    Quote Originally Posted by VAEngineer View Post
    I guess the A/E figured if we knwo for certain the airflow into and out of the 2 rooms and the volumes of the rooms, we will have no issues?
    Define certain. The day after TAB & CX or 4yrs down the road? Ball in the wall, cheap, easy and nearly fail proof.
    Propagating the formula. http://www.noagendashow.com/

  9. #9
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    Quote Originally Posted by orion242 View Post
    If you have supplies with the volumes being set by static balance damper, would suspect things would control stable regardless how you do it. Any fluctuation from the VAVs will just get sucked up there.



    Define certain. The day after TAB & CX or 4yrs down the road? Ball in the wall, cheap, easy and nearly fail proof.
    I could be wrong, but I believe the area is supposed to be re certified for airflow every 6 months or 1 year. I know our chemo mixing clean room is every 6 months, can't remember if the sterilization is the same or longer.

  10. #10
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    Quote Originally Posted by VAEngineer View Post
    re certified for airflow every 6 months or 1 year.
    Put a check in the certain box then
    Propagating the formula. http://www.noagendashow.com/

  11. #11
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    Quote Originally Posted by orion242 View Post
    Ball in the wall, cheap, easy and nearly fail proof.
    And again to the ball in the wall, people in these types of spaces typicaly have a fairly rigorous protocol to follow already. Adding thou shall check ones balls are the right spot before continuing, not a big ask vs the cya factor & cost.
    Propagating the formula. http://www.noagendashow.com/

  12. #12
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    If I understand what you are trying to achieve listen to the control guy. The idea of controlling space static pressure by reading airflows and doing a calculation from that data NEVER WORKS. If that's what you are trying to do don't do it. The VAV needs enough stable static pressure to control airflow. It doesn't matter if the supply air duct DP sensor is dead on accurate but it does have to be repeatable. With the VAV accurately calibrated you will get the airflow you want as long as you have enough duct static. If you are trying to maintain the space static pressure with VFD driven exhaust fans control the VFD'S from space pressure. Use a sensor with local readout and alarm if it is critical.
    No man can be both ignorant and free.
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  13. #13
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    Quote Originally Posted by VAEngineer View Post
    1) Yes, VFD controlled Fans.

    2) OK, that kind of makes sense, but wouldn't you see the VAV's going full open or full closed, at which point you could adjust the setpoint (or something similar?). If not, where would you recommend the DPS sensor be located? In this case, the contractor wants it located 2/3 of the way down the supply duct, the reason given is so that all VAV's "have proper flow".....

    Now that I am thinking of it, we may not have VAV's in every single space, I think a couple of the non critical spaces do not (2 bathrooms and the mechanical room come to mind), if we added a little the Design CFM and controlled the fan speed from the AFMS, wouldn't the "extra" air just go to a space like the mechanical room?

    I am not opposed to using the DPS, just trying to rationalize the correct implantation, This area will receive a lot of attention from the people in Washington DC when it opens up and would need to justify any deviation from the already accepted plans.
    I vote for using the DP sensor. As long as the pressure is constant you will always move the same amount of air through the fixed orfices in the system.

    The DP sensor will also take into account the filter loading and speed the fans up to keep the pressure at setpoint downstream.

    In addition the DP does not need to be cleaned/calibrated every year. AFMS are highly prone to getting dirty and reading inaccurately.

    If you used the AFMS how will you insure the VAVs sensors have enough static to operate? A JCI vav needs a minimum of a quarter inch to operate correctly. Other manufacturers need a half inch. Without the DP sensor your just guessing.

    Your controls contractor is correct. Keep the AFMS but use the DP.


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