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Calculating Static Pressure Through Duct Network

  1. Apr 28, 2010 #1

    My name is CK and I have question on calculating static pressure through a ducting network.

    Some context:
    The ducting network services 360 identicle components. This is achieved by dividing a main duct into 4 branches, each servicing approximately 90 components. There is a desired volumetric flow rate of 500 m3/s through the main duct, which theoretically breaks down to 1.39m3/s per component.

    The 4 branches that the main duct breaks off to, has a tapered design. Components furthest from the branching point have smaller ID's while ones close have a larger, this is done to ensure a 'semi-consistent' air velocity (m/s), pressure drop etc. Additionally, each component is equally spaced out along the branches. The air flow from each component can be adjusted by a gate setting.

    The question:
    Currently we use a manometer to measure static pressure at a point very close to the component. This is achieved through a 3mm drilled hole on a side of the duct, close the component. It is believed that static pressure is sufficient in evaluating the balance of air flow between components.

    What I would like to do is to put together a spreadsheet, that captures the variation in static pressure from component to component. I have drawings of the entire ducting network.

    It would be great if you may provide me with some direction or key equations that would be useful in putting this spreadsheet together.
    Thanks in advance!

  2. jcsd
  3. Apr 29, 2010 #2
    If you designed your ductwork to maintain a constant supply air VELOCITY, this method is called "static regain". This maintains a CONSTANT static pressure in all segments of the duct. Hence, all 360 terminals have the full system static pressure available on the duct side. The individual terminals should have a data sheet which correlates P.D. (pressure drop) with either velocity (fpm) or volume (cfm). Naturally, this is a logarithmic graph.
    A more accurate measure would be to use a pitot tube to measure velocity pressure and directly determine the volume of air flow. Sounds like you already have a pressure transmitter, you would only need to replace your probe with a pitot tube to have true volume flow.
    I have been in building automation for 30 years, and while the simple calculations you desire are theoretically possible for a system in equilibrium, introducing a VFD drive or a VAV terminal will quickly get your calculations into a second order differential equation.
    In your system, is the supply air fan constant speed and the terminal dampers fixed?
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