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Thread: VAV pressures

  1. #1
    Join Date
    Mar 2008
    Location
    Bulgaria, Sofia
    Posts
    2

    VAV pressures

    Hi all,

    I have some questions concerning VAV systems. Differential pressure must be measured at the entrance of each VAV box. The VAV box tupe is in series fan powered (about 2000 - 2500m3/h).

    1.I need to specify the scope for the transmitter - how many Pascal should it be able to read by the VAV box???

    2.The main fan of the AHU has frequency modulation and the pressure is measured near the AHU. I also need to know the scope of the transmitter there having in mind that the external static pressure of the AHU fan at full airflow is 600Pa.

    Any help will be greately appreciated.

  2. #2
    Join Date
    Nov 2007
    Posts
    388
    Quote Originally Posted by Joro-- View Post
    1.I need to specify the scope for the transmitter - how many Pascal should it be able to read by the VAV box???
    The transmitter will be reading velocity pressure, so determine the maximum primary air velocity at maximum flow & that is your upper range. Select a transmitter that has a low range of 45 Pa, which is equivalent to 2.5 m/s velocity or the velocity pressure of your minimum primary airflow rate.

    Most manufacturers can help you w/ this.

    Quote Originally Posted by Joro-- View Post
    2.The main fan of the AHU has frequency modulation and the pressure is measured near the AHU. I also need to know the scope of the transmitter there having in mind that the external static pressure of the AHU fan at full airflow is 600Pa.
    The static pressure port should not be at the AHU! It should be located about 2/3 down stream of the AHU, in a duct main. Otherwise you are wasting fan energy.

    You need to evaluate the minimum system pressure needed at that point in the supply duct to deliver air to the hydraulically most remote VAV terminal in the system.
    Last edited by flyrfan; 03-28-2008 at 12:37 PM. Reason: -

  3. #3
    Join Date
    Mar 2008
    Location
    Bulgaria, Sofia
    Posts
    2
    Hi flyrfan,

    Thanks for responding to my questions. I'm happy to know the opinion of an experienced professional. VAV is not very popular in Bulgaria and I'd appreciate some more clarifications:

    What do you mean "a low range of 45 Pa". Does it mean from 0 to 45Pa or over 45Pa.? The company engaged with the delivery provides a range of :

    0 to 100Pa
    0 to 200Pa
    0 to 500Pa
    0 to 1000Pa
    0 to 2000Pa

    The velocity will really be 2.5-3m/s. Does it mean the best choice is 0 to 100Pa? And will it be bad idea to use a wider rage like 0 to 500Pa?

    It seems we are not to measure the pressure at 2/3 downstream of the AHU due to other reasons. So we will need to put it near the AHU. The available pressure of the AHU fan is 600Pa and the total is about 900. Is 0 to 1000 Pa range enough. Not sure is it worth considering the situation when the fan works at an airflow less than maximum and then it can provide bigger pressure?

    My questions might seem funny to you but I've really got no experience with VAV systems. And just for information, supposedly we could measure the pressure at 2/3 downstream, does it mean we could use a transmitter with lower range?

  4. #4
    Join Date
    Nov 2004
    Location
    SW FL
    Posts
    5,997
    Quote Originally Posted by Joro-- View Post
    Hi all,

    I have some questions concerning VAV systems. Differential pressure must be measured at the entrance of each VAV box. The VAV box tupe is in series fan powered (about 2000 - 2500m3/h).

    1. how many Pascal should it be able to read by the VAV box???

    2. The main fan of the AHU has frequency modulation and
    the pressure is measured near the AHU.

    ... having in mind that the external static pressure of the AHU fan at full airflow is 600Pa.

    Any help will be greately appreciated.
    Use the blower performance curves
    Calculate your air flows based on Your layout
    and
    assure it is balanced.

    THEN
    Ask Titus
    to assist in selecting size and proper noise level

    http://www.titus-hvac.com/ecatalog/g...px?fileid=7564
    Designer Dan
    It's Not Rocket Science, But It is SCIENCE with "Some Art". ___ ___ K EEP I T S IMPLE & S INCERE

    Define the Building Envelope and Perform a Detailed Load Calc: It's ALL About Windows and Make-up Air Requirements. Know Your Equipment Capabilities

  5. #5
    Join Date
    Dec 2005
    Posts
    107
    http://www.nailor.com/IOM_Manual/vav...f/IOM_FPTU.pdf

    See page 4 of this IOM manual for the pressure ranges of the inlet sensors for Nailor series fan powered terminals.
    Basically the 0-200 Pa sensor is the one you need.

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