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Thread: Filter loading

  1. #1
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    I'm currently working on a a few large paint booths with a 30hp make up air unit and a 40hp exhaust fan on each. In the sequence it calls for changing the exhaust fan speed based on filter loading and tracking the supply fan to keep the booth -0.05 in wg compared to the hanger its in. The previous control system\sequence would blow the doors open at start up and shut down. I've got all the bugs worked out as far as space pressure, temperature requirements, and can start and stop the system with out major pressure fluctuations blowing the doors open. I just don't know how to take in account filter loading on the exhaust fans as I've never written a program for this. Any ideas? I would like to actually track exhaust fan CFM and keep it constant but that is not really and option as any velocity probe in the duct would get clogged with over spray making it way through the filters. I'm kinda think of tracking the static on the outlet of the exhaust fan compared to the booth and trying to keep this about the same as running with clean filters and the min cfm requirement. Do you think this will work? I hope this small static tip would be more likely to give an accurate reading than a long velocity probe.

  2. #2
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    Feb 2005
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    You might want to consider looking at something ultrasonic, the challenge is going to be keeping it cost effective if you go that route.
    look in here, may be helpful:
    http://www.dynasonics.com/

  3. #3
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    We too ran into a similar conundrum regarding paint booth ventilation - we tried hot-wire anemometer velocity readings after the filters, only to find the sensor always coated with paint (until the tech broke the ends off trying to clean it...) Then we went to static pressure (again, after the filters), but now the PM includes removing the static tip and cleaning it. Of course, in the long-term, I imagine the DPT sensor will internally suffer from paint clogging it up, but for now, if we can keep the static tip maintained, it works.

  4. #4
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    Fan tracking has some problems that are hard to measure and control.

    If you are trying to control space pressure - why not measure space pressure? Since there is no paint moving through a space dP sensor, the sensor shouldn't clog as often as a sensor or probe in the duct.

  5. #5
    Like cxagent said it sounds like your best bet is to stay with the booth pressure. If want to keep an eye on the filter you can either go with a switch set to trip at a given dp or use a dp transmitter to track how their doing. By using a transmitter you can alarm and have some good feedback data to trend, monitor,etc.

    Good Luck

  6. #6
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    I am tracking booth pressure and it works fine. My point was I needed a way to track exhaust filter loading to try and keep my exhaust at a constant regaurdless of filter load.

  7. #7
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    willf650

    You can't control two effective setpoints from one response unit. In otherwords, you're going to have to choose to control space static or filter differential pressure through throttling the exhaust fan. You can't do both.

    I'd set up to control space (booth) pressure. As your filter loads up and draws less flow your drive is going to ramp up in order to maintain the pressure relationship. When it reaches 100% under run conditions and as the filter continues to load the ability to control to your desired booth pressure setpoint is going to degrade. If you are monitoring this via DDC, set a limit on the actual DP meaurment and alarm from it in order to drive a filter change. Better yet, set several analog thresholds to correspond to a heightened level of warning with the first level triggering a filter-change service call.

    You can also do a similar routine but base it on the DP across the filter instead of space (booth) static. However, this could lead to an adverse pressure relationship with the atmosphere outsdide the booth...something you probably don't want.

    I'd also set up a DP switch (across the filters) tied directly to the booth controls such that is the filters are not changed and the pressure relationship is not able to be maintained, the DP switch will shut the booth down preventing an out-of-compliance situation. Alarms will propegate accordingly.

    lma

  8. #8
    I did the same set-up for TRW, what controls are you using ?

    I used air flow monitors and drove the whole system with that.

  9. #9
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    I don't see why I can't control filter load and space DP as long as I get my exhaust fan loop to settle out and not jump around I can then track the supply fan to control booth pressure. I can control booth pressure via the supply fan to setpoint regaurdless of exhaust fan speed and have already tested that. I have VFDs on both fans so I would use 2 different loops to provide control of each variable. If I can tune them to settle out all would work out in the end.
    I'm using Trane controls if that matters, Any brand could accomplish the same thing.

  10. #10
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    My apologies willf650, I misunderstod the original system description and missed the fact that you have drives on both the supply and exhaust fans.

    You are correct in your sequence assumption. Run the exhaust fans off a filter DP loop and run the supply fans off a space static DP loop per your thinking.

    I'd still be sure to test and set your upper and lower limits for both DP values and for run speeds on the fans to make sure your booth is running properly.

    You are also correct in that you should be able to do this with any DDC product....even Trane

    good luck
    lma

  11. #11
    Hey don't take this the wrong way, but that thing is going to hunt all over the placeif you try to control them seperately.

    CFM is what you are trying to control regardless if it is driven by pressure or filter loading,stick with controlling the CFM,

    In the booth you need a minimum amount of CFM,a negative pressure,and good control as the filter load-up.

    Air flow monitors on the supply and return will do this for you, punch in the CFM and let it control.

    This is after it has been balanced for pressure,have the balancer give you a report on when the booth is spraying and when it is not.The pressure will be different when the booth is in operation.

    What I am telling you to do works, controlling to different drives on two different signal sources for the same space will not work.

    [Edited by fat eddy on 03-16-2005 at 06:49 AM]

  12. #12
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    Quote Originally Posted by ;715642
    I'm currently working on a a few large paint booths with a 30hp make up air unit and a 40hp exhaust fan on each. In the sequence it calls for changing the exhaust fan speed based on filter loading and tracking the supply fan to keep the booth -0.05 in wg compared to the hanger its in. The previous control system\sequence would blow the doors open at start up and shut down. I've got all the bugs worked out as far as space pressure, temperature requirements, and can start and stop the system with out major pressure fluctuations blowing the doors open. I just don't know how to take in account filter loading on the exhaust fans as I've never written a program for this. Any ideas? I would like to actually track exhaust fan CFM and keep it constant but that is not really and option as any velocity probe in the duct would get clogged with over spray making it way through the filters. I'm kinda think of tracking the static on the outlet of the exhaust fan compared to the booth and trying to keep this about the same as running with clean filters and the min cfm requirement. Do you think this will work? I hope this small static tip would be more likely to give an accurate reading than a long velocity probe.
    The company I work for manufactures a continuous purge system that will keep pitot tubes clean, or we have a system that will do what we call an auto purge, at a predetermined time we will lock the last measured airflow and transmit the flow signal to the BMS and then blow the pitot tubes down with 80-100 psi of air. This system is used in the power industry with good success, on primary air secondary air measurements.

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