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  1. #1
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    Question Pressure correction equal friction question.

    Most the time people I have seen derive values from the equal friction loss method off of .10 per one hundred feet of supply air; however, nobody uses a correction factor let alone EL or a roughness multiplier. Most systems I have tested in the fall before hurricane sandy had ESP's near 1 which is still with in recommended residential settings, but on the higher side of the scale. How imperative is using the corrected pressure drop yet disregarding equivalent length and roughness for sizing prefabricated sheetmetal ducts when they only come in even sizes?

  2. #2
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    Ie. today, we serviced a reducing trunk of twenty eight feet with six branch runs totaling another sixty feet, approximately. For a two ton system, our supply started in a crawl under the down flow free return carrier beast. There were three sections of fourteen by ten cheated down to four sections of twelve by ten all four foot long. I think it's better to up the size to sixteen by ten and fourteen by ten to lower velocities and static but please give me your thoughts. Keep in mind the unit is centered in the apartment and the plenum dives straight into the center of the duct in the crawl, wrapped up tight to get full of hot air...and swell like a balloon, but hey that's a different post..for a different day..

  3. #3
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    12X10 is a bit small for 2 tons/800 CFM. 6 branches, hope that they're all 7".
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  4. #4
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    Where do you find 1" as a recommended pressure?
    I find that even VS motors are reaching their max at 0.85" on most brands.................
    And I also find that most everything else with a psc motor is only rated to 0.50". Consider that the average evap coil pd is 0.25" and the average filter rack is another 0.125". Throw in a bull head and a ra drop 90 coupled with some 0.10 slide rule duct and well your right...........1.0"

    All Man D EL's are factored with a 900 fpm sa and a 700 fpm ra. That should tell you something........................................
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  5. #5
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    Actually there are eight runs, all six inch and it blows pretty hard but did not get a static yet

  6. #6
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    Quote Originally Posted by ACFIXR View Post
    Where do you find 1" as a recommended pressure?
    I find that even VS motors are reaching their max at 0.85" on most brands.................
    And I also find that most everything else with a psc motor is only rated to 0.50". Consider that the average evap coil pd is 0.25" and the average filter rack is another 0.125". Throw in a bull head and a ra drop 90 coupled with some 0.10 slide rule duct and well your right...........1.0"


    All Man D EL's are factored with a 900 fpm sa and a 700 fpm ra. That should tell you something........................................
    I realize that but nobody directly answered the question. The recommended
    residential settings are detailed on my duct slide rule.

  7. #7
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    The duct slide calculator is just that, it is not meant to size ductwork until a friction rate is known to use on the slide calculator.

    To find the proper friction rate, one needs to first know the proper size unit by performing a manual J, then select proper size unit using manual S, then design proper duct system by following steps in Manual D.

    A residential gas furnace is normally rated at .5 TESP, and most air handlers are normally rated at .3 TESP. If you are testing units with close to one inch of TESP then it is not likely that the system is properly moving design airflow. Most standard furnace PSC motor can handle up to .6TESP and most variable speed units up to .7TESP before airflow is reduced or noisy variable speed blowers.. But on the contrary most airhandlers with PSC motors can handle up to .4TESP and most variable speed units can handle up to .5 before airflow is reduced or noisy variable speed blowers.. Always consult the blower table of the manufacturer to confirm because some units suffer severe loss of airflow at lower statics past the rated static more than others.

    I had a PSC gas unit today that was running at 1.11 TESP that had very bad ductwork, this unit had a 3 1/2 ton blower but at 1.11 TESP was only moving about 900 cfm on high. We are going to try to lower the static as much as possible on the return side but the supply is buried in a concrete slab, and customer doesn't want to jackhammer up the floor to fix the supply..lol.. Luckily the A/C is only a 2 1/2 ton and lucky the blower unit is was oversized or the airflow would be too low in current conditions.

    Goodluck
    J

    Learn Manual D inside and out and go to some duct design course and the misteries of duct design will be given, also you will find less than 10% of ducts are truely sized correctly, most are serverely undersized not just boarderline...

    On your question if the friction rate that is needed once found by performing a manual D, if the duct needed fall between the prefab sizes then go to the larger size; i.e. needs a 11x22 duct according to the table use a 12x22 duct.

  8. #8
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    This should help.
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  9. #9
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    Quote Originally Posted by Advanced Response View Post
    The duct slide calculator is just that, it is not meant to size ductwork until a friction rate is known to use on the slide calculator.....most airhandlers with PSC motors can handle up to .4TESP and most variable speed units can handle up to .5 before airflow is reduced or noisy variable speed blowers.. Always consult the blower table of the manufacturer to confirm because some units suffer severe loss of airflow at lower statics past the rated static more than others.
    Exactly.

    Using EL, and Friction correction is good, but you need to be sure you start from the right place, and that place is whatever the fan can give you AFTER you account for the Filter drop, (sometimes) the A coil pressure drop, and so on. 1" off of the back of the slide rule doesn't come close to most of what is out there, although you are to be commended as you at least want to get there, and are studying and asking questions to do so.

    Once you get that, correcting for the EL being over 100' and so on - like you are doing - will put you ahead of the pack.

  10. #10
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    Quote Originally Posted by Mr.HVAC View Post
    Exactly.

    Using EL, and Friction correction is good, but you need to be sure you start from the right place, and that place is whatever the fan can give you AFTER you account for the Filter drop, (sometimes) the A coil pressure drop, and so on. 1" off of the back of the slide rule doesn't come close to most of what is out there, although you are to be commended as you at least want to get there, and are studying and asking questions to do so.

    Once you get that, correcting for the EL being over 100' and so on - like you are doing - will put you ahead of the pack.
    What I was missing was the design friction rate value. I've learned that the available static pressure multiplied by one hundred and divided by the total equivalent length allows you to use the correct starting point on the slide rule.

  11. #11
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    The other perplexing quandary with airflow in my mind is registers. Most of what I see is four by ten which doesn't have the c.f.m at face value equal to a six inch duct. I'm not sure if its an issue for comfort more than a design; however, in my mind a five inch galvanized metal pipe or even that new poly material is as high as you could go with a four by ten without a minor increase in velocity coupled by a reduction in c.f.m. I want my ducts to emulate a slow steady warm consistent feeling like reliable radiant heat.

  12. #12
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    Duct velocity and register velocity have nothing to do with each other.
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  13. #13
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    Quote Originally Posted by Joseph'bidness View Post
    The other perplexing quandary with airflow in my mind is registers. Most of what I see is four by ten which doesn't have the c.f.m at face value equal to a six inch duct. I'm not sure if its an issue for comfort more than a design; however, in my mind a five inch galvanized metal pipe or even that new poly material is as high as you could go with a four by ten without a minor increase in velocity coupled by a reduction in c.f.m. I want my ducts to emulate a slow steady warm consistent feeling like reliable radiant heat.
    You should go through Acca manual T. That gives you a breakdown of not only velocity at the registers, but of how and where to place them to avoid dead spots, and to ensure proper churning of the air to avoid stratification. You need enough face velocity to ensure proper throw, the amount of throw depends on the size of the space, and the design of the register... But too much flow being forced through it means restriction which puts you back at manual D.

    A good jump start into all of this that walks you through it but is geared specifically towards the smaller residential stuff you seem to work on is National Comfort institute classes. I think you'd like them. And since they deal a lot with this type of thing, they offer a bunch of cheat sheets and support on their site:

    https://www.nationalcomfortinstitute.com/index.cfm?

    Need to be registered through them to get access to a lot of it though.

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