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  1. #1
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    Relationship between velocity pressure and static pressure

    I am working on a military project with an odd duct design. I found an equivalent fitting in SMACNA's "HVAC Duct System Design" book, and calculated a velocity pressure loss of 45%. How does velocity pressure relate to static pressure? There is only .35" external SP available to get the air through the ducts and diffusers. This is insufficient if 45% is lost through the first transition.

  2. #2
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    When a fan is moving air through a duct system, two types of pressures are encountered, velocity pressure and static pressure. The sum of these pressures is referred to as total pressure.

    Velocity pressure is the pressure caused by air in motion. It is velocity pressure that we feel when the wind blows; likewise it is velocity pressure that moves tree branches in the wind. Faster air speeds create greater velocity pressures. The energy of air in motion is kinetic energy.

    Static pressure is the amount of resistance measured in inches of water, produced when air is moves through an object like duct work. The higher the static pressure or resistance, the more energy it takes to move air through the object. In an air conditioning system as the static pressure or resistance increases the horsepower of the fan motor is depleted. Eventually the static pressure will be higher than the airflow and it will stop.

    As you can see from the above you are missing elements in your equation .

    Tell me more about your experience as a mechanical engineer. Are you a professional mechanical engineer ?

  3. #3
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    I am not a PE yet. Most of my mechanical engineering experience comes from oil and gas companies, but the recession forced a career change. I have been designing ventilation systems in Alaska for 6 months.

    The fan outlet is roughly 25"x18" and encounters an abrupt transition to 52"x26". From a table, I determined that the fitting loss is equal to 45% of the velocity pressure.

    The air velocity in the small duct is over 2100 FPM, and the velocity pressure is about 19 in.w.g. The fan curve gives .35" SP for the required CFM. The crux of the issue is the question: "Do we need to increase the motor size to get our required flow?"

    How do I determine if I still have pressure enough to make it thorough the remaining ducts and diffusers?

  4. #4
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    What is the design air flow required and motor RLA ? Are we working with a DX coil ?

  5. #5
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    There is indeed a DX coil prior to the fans, of which there are three on a single shaft run by a 10 HP motor (with a 15 HP motor option). The electrical information deals with the unit as a whole, and not just the motor, but it runs at 172 FLA @ 208V. Each fan is responsible for 4865 CFM at 20 degrees delta T.

  6. #6
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    Sounds like a bunch of hot air to me.. haha.... just kidding.

  7. #7
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    Is it three blowers into one common duct at 25x18 or three blowers into three dusts ?

    I work with PE’S daily who design systems 10% over the load calculations as a buffer for error and leakage. The data provided assuming the 45% loss in transition is correct puts your system at 78%. This does not include accurate data for system total loss for which I plugged in a k correction factor of .880.

    All being said 78% is not terrible. It does indicate a speed increase will be required.

    The 15 HP motor with adjustable sheaves should provide the air flow required.
    As you know the pressure drop will increase the square of the flow increase.

  8. #8
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    fitting

    Quote Originally Posted by farrago View Post
    I am working on a military project with an odd duct design. I found an equivalent fitting in SMACNA's "HVAC Duct System Design" book, and calculated a velocity pressure loss of 45%. How does velocity pressure relate to static pressure? There is only .35" external SP available to get the air through the ducts and diffusers. This is insufficient if 45% is lost through the first transition.

    If at all possible make the appropriate transition in the duct instead of blasting energy at it.
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  9. #9
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    Quote Originally Posted by kdocsr05 View Post
    Is it three blowers into one common duct at 25x18 or three blowers into three dusts ?
    Three blowers into three separate ducts.
    Quote Originally Posted by kdocsr05 View Post
    I work with PE’S daily who design systems 10% over the load calculations as a buffer for error and leakage. The data provided assuming the 45% loss in transition is correct puts your system at 78%.
    How is that calculated?

  10. #10
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    Quote Originally Posted by ACFIXR View Post
    If at all possible make the appropriate transition in the duct instead of blasting energy at it.
    It is not possible, without a complete redesign of the ducts. We are contemplating this very seriously.

    Thank you for your help everybody.

  11. #11
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    Arrow STATIC REGAIN

    Static Pressure is the pressure that causes air in the duct to flow, & velocity pressure is the pressure that results from the air movement.

    The goal is to achieve 100% Effective Duct Length for Optimal Static Regain.

    To achieve 100% Effective Duct Length, a straight run from the blower or a turn, etc., has to be long enough to achieve a uniform Velocity across the cross-section of the duct.

    If the Outlet velocity is less than 2,500-fpm: 100 percent-effective duct length = 2.5 X duct diameter.

    If velocity is more than 2,500-fpm USE: FPM/1000 X duct diameter.

    There is a chart U can use to get the duct diameter for rectangular duct.
    It is somewhat above the square of the sum of the sq.ins. -

    Many times U will only be able to get a percentage of a 100% effective length, get what U can. - Darrell

  12. #12
    Join Date
    Nov 2001
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    I am not an expert at this, so take this with a grain of salt:

    I think that you are saying that you are going from a smaller duct to a larger duct. I believe your velocity pressure will go down a lot because your velocity will go down a lot. That is, even with no losses, if your FPM was 2500, and you transition to a duct with 2X the cross section, your new velocity is 1250 FPM, and your velocity pressure is lower.

    However, your static actually increases. Total pressure is velocity pressure plus static pressure. Your table says that you lose 45% of your velocity pressure, but you do not mention total pressure change. I think that in this case, your static pressure will actually increase. I believe this is what people are referring to when they use the term "static regain."

    I don't know much about duct design - it's far from my area of expertise, but I am fairly certain that when you transition from a smaller duct to a larger one, it should not kill your air flow.

    If I am reading correctly that you are going from smaller duct to larger duct, you need to check this out much more carefully before you recommend changing the ducts that are in place.

    If I did misread this, my apologies.

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