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Thread: Duct Sizing.

  1. #1
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    Duct Sizing.

    I've been sizing duct systems for years. I go by feet per minute. For square duct I figure the supply at 650 FPM and on returns I figure 450. That's what I was taught in school. I'm working with some guys now that like to use the static pressure method. .08 to .1. This makes a big difference. 2000 CFM at 650 FPM gives you about a 20x25. 2000 CFM at .08 static gives you about 20x15. That a huge difference. What's the preferred method?

  2. #2
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    I size by friction loss, mostly.

    Velocity becomes more of a concern the closer you are to the register.

    If you sized all the returns in a commercial building to 450 fpm, the tinknocker would by you a beer, breakfast, lunch and dinner.

  3. #3
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    I'll assume you're not really talking about the duct size but rather the face velocity of a supply or return appliance. The manufacturer's specs should be followed closely and it all depends on the noise factor as to how big the return will be. The grilled itself depends on the spacing of the louvers as they come anywhere from 3/8-in to 5/8-in. So the same size grille could give you two entirely different noise factors. If you're ducting a library or bedroom, you can't have the same noise you might have in a basketball court. All the man's specs I've seen deal with velocity, which is what you were taught to use, FPM. Static pressure needs to be factored into the total duct load but if it were me, I'd select the grilles based on FPM and noise and then take whatever static loss/drop that comes with it.
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  4. #4
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    same here "friction loss" size ducts to accomadate blower that best fits the load.

  5. #5
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    Quote Originally Posted by RefSupply View Post
    I've been sizing duct systems for years. I go by feet per minute. For square duct I figure the supply at 650 FPM and on returns I figure 450. That's what I was taught in school. I'm working with some guys now that like to use the static pressure method. .08 to .1. This makes a big difference. 2000 CFM at 650 FPM gives you about a 20x25. 2000 CFM at .08 static gives you about 20x15. That a huge difference. What's the preferred method?
    If they just use .08 or .1,without doing a Manual D to determine the actual friction Rate,then you are both wrong.Good news is your way is likely closer to being correct then their's.

    Your method will be larger ducts then needed at least some of the time,their's will be too small most of the time.

    Manual D from www.acca.org to learn more.

  6. #6
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    If their just using .08 and .1, they are just using a rule of thumb.
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  7. #7
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    Most of the time when your dealing with commercial, you start with static. Reason being is the job is designed for a specific static rating. Most of the time 1" is standard, but, I've done jobs that spec'd 4" and 7".
    Jim

  8. #8
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    Quote Originally Posted by RefSupply View Post
    I've been sizing duct systems for years. I go by feet per minute. For square duct I figure the supply at 650 FPM and on returns I figure 450. That's what I was taught in school. I'm working with some guys now that like to use the static pressure method. .08 to .1. This makes a big difference. 2000 CFM at 650 FPM gives you about a 20x25. 2000 CFM at .08 static gives you about 20x15. That a huge difference. What's the preferred method?
    I suspect the guys you are working with are using friction rate, not static pressure.

    0.08 or 0.1 are very low static pressure numbers and would require much larger ducts that 20x15 for 2000 CFM. However my program indicates that if you have a 20x15 duct at 0.08 friction rate then you get the 2000 CFMs, further suggesting to me they are using friction rates not static pressure to size the ducts.

    By the way, your 2000 CFM at 650 FPM using a 20 x 25 duct results in a friction rate of 0.02.....

    Key1
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  9. #9
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    There are several methods of duct sizing. Static regain, Velocity Reduction, Equal friction are three. Most people here seem to use the equal friction method.
    Like Beenthere said .1 or .08 are just thumbers.
    Tracers work both ways.

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