Results 1 to 5 of 5
  1. #1
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,441
    Post Likes

    When Will They Learn?

    When will it become common knowledge that trying to control building static pressure and outside air quantity related to building pressure by airflow measurement and calculation doesn't work?
    I have seen that tried several times and have never seen it work.
    I am working on a job where the airflow of 6 residential AHU'S in an enclosure have a return fan that delivers air to them because the units do not have the static pressure to deliver design airflow on their own. The return fan speed is being controlled by a VFD with input from the AHU airflows. The result is the AHU fans are dragging the return fan along thus increasing the static pressure they need to deliver airflow. They of course are short.
    Control of the return fan speed is going to be changed to maintain the static pressure at the AHU inlets. The way it should have been done in the beginning.
    No man can be both ignorant and free.
    Thomas Jefferson

  2. #2
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    67
    Post Likes
    How are they controlling the outside air ?

  3. #3
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,441
    Post Likes
    Thread Starter
    At present they are not controlling outside air. The return fan speed thus negative static pressure available to draw in outside air varies with the speed of the ECM fan motors. They used the fan curve for the return fan and AHU ECM motor CFM readout.
    No man can be both ignorant and free.
    Thomas Jefferson

  4. #4
    Join Date
    Aug 2019
    Location
    Central Florida
    Posts
    67
    Post Likes
    You can figure out what negative static you need to draw in the required amount of outside air and make that your setpoint . Are the six ahu's ducted in parallel with each other ? Do the ahu's ramp up and down or do any of them shut off while the other ones stay running ? Interesting setup....

  5. #5
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,441
    Post Likes
    Thread Starter
    I talked them into changing the return fan control to maintain a negative static pressure (adjustable) at the outside air dampers. The AHU'S run continuously so no matter what they do the outside air will remain constant except when CO2 levels rise.
    This is a very interesting setup because the unit is located in the basement but serves the floor above and there is a lot of crazy ductwork. Maintaining design outside air is not possible because the AHU ESP is 0,8 inches which does not include the filter design pressure drop of 0.5 inches. CO2 control will have to do most of the fresh air work.
    I'm walking a tight rope on this one.
    No man can be both ignorant and free.
    Thomas Jefferson

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •