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Thread: Static Pressure Frustration

  1. #61
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    I agree with heatingman to hell with tact. To get things fixed right I had no choice most of the time but to slam dunk the so called design engineer. There were times even that didn't work and the job had to bounce before anyone would listen to a lowly TAB guy.
    On one government job I repeatedly tried to get the install right before starting the TAB. I was finally directed to balance it as installed and when they had to make all the changes I told them needed in writing they thought they were going to negotiate my extra. My extra was legit and the negotiator who was from out of town realized it pretty quick and gave me every penny.
    He ask me if anyone listened to my suggestions and then added they should have.
    No man can be both ignorant and free.
    Thomas Jefferson

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  3. #62
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    I have done some work in a assisted living facility, the have smoke dampers in the duct. They have so many places where the damper motors are practically inaccessible. I know of 2 that are within a foot of a fire rated wall. 1 of those 2 I had to get the local fire inspector involved as the only way to get to it was cut the wall open. Then how do you get it back to fire rating and still leave it serviceable.

  4. #63
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    Quote Originally Posted by BNME8EZ View Post
    I have done some work in a assisted living facility, the have smoke dampers in the duct. They have so many places where the damper motors are practically inaccessible. I know of 2 that are within a foot of a fire rated wall. 1 of those 2 I had to get the local fire inspector involved as the only way to get to it was cut the wall open. Then how do you get it back to fire rating and still leave it serviceable.
    If it’s in your scope, you remove and reset the duct damper to be accessible and a contractor rebuilds the wall with 5/8” rock! On some they are doubled to keep a 2 hour rating with the fire putty

    On my end I don’t have to worry About it, it’s the smoke detector/fire alarm techs problem because their system is connected to it...I opened the inspection hatch confirm open or close and write it up .

    However I do have to replace the spring activated SA vent fuseable 160 degree links if found closed and that’s a pita! I use 2 pcs of pvc on each end to hold it open when replacing the links

    The fire marshal in my areas don’t allow inaccessible dampers and Most times a few are caught during their inspection...the duct guys correct it

  5. #64
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    On a centrifugal fan, the inlet and outlet of the fan needs to be ducted exactly correct per the installation instructions. That usually means 3 diameters of round straight duct into the inlet, and then 3 straight “diameters” of rectangular duct on the discharge. If they are installed like this...they should follow the fan curve fairly close. If it’s an exhaust fan on a roof, it needs a piece of duct on the discharge even though it’s just blowing up. If you don’t have the inlet/outlet ducted correctly they will not operate on the curve.

    I have always considered the inlet/outlet conditions the system effect. Normal duct losses away from the fan due to bad fittings or small duct, I’m not sure this is “system effect” or just duct loss. Not sure if I am correct on this.

    Over time I have discovered the fan does not always operate on the curve even if the inlet/outlet are correct. I’ve been to a fan factory and talked to the guy testing the fans and making the curves and when I questioned him he said I was doing something wrong. This has always kind of bugged me that the fan curves are not accurate. After talking with the factory guy I figured he was right, I was prolly not getting good static readings.

    This is a good topic Wayne. Thanks for bringing it up.

  6. #65
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    Fans are tested in labs w/o inlet restrictions and have little to do with field conditions. But they are useful if the tech knows their limitations.
    They are the fastest test and require the cheapest tools as long as the tech isn't swearing on the accuracy.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

  7. #66
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    You are more on target Scooby than you might think. System effect can take place anywhere in the duct system but most problems of consequences are at the fan inlet and outlet. Some duct configurations that cause system effect can be calculated but many cannot. I had a system that I think I posted on here some time back that had more than one area of system effect that could not be calculated. The owner's engineer wanted me to change sheaves on a fan to lower the duct static from above 3" to 2 inches at a certain point in the system. I took him to the ducts and showed him the problems and advised him since most of the system effect couldn't be calculated there was no way to know what speed the fan had to run to get what he wanted. He agreed to check my calculations and if he agreed I would change the sheave and whatever we ended up with would be accepted.
    I sent him the calcs and he was really impressed and over joyed and agreed to the change. Instead of 2" the static was 2.8" but was a lot better than before. The problem areas were directly at the fan inlet and for about the first 30 or 40 feet of the discharge.
    SMACNA has a manual for the known system effect factors and where they apply but to my knowledge nobody has done anything with system effect for years.
    No man can be both ignorant and free.
    Thomas Jefferson

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  9. #67
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    System effect occurs in all duct systems, how much is the difference and what's acceptable and what to do if SE is excessive enough where it won't perform to what's needed.
    The most interesting solution for myself was a small system, 15 or 20 K CFM couldn't deliver required air.
    The information was sent to the engineer in another State. His solution was the create a loop from the supply back to the supply. Pressurize the loop from both directions like some homes have done mostly in the past. Worked well and reminded me that solutions can often be just looking at a problem differently.

    Most common SE issues come from leaks, dead head ducts, not using turning vanes, sizing. All duct seams leak if not sealed. Ol'Timers often won't believe their Pittsburgh's leak but they do.
    Give me a relay with big enough contacts, and I'll run the world!

    You can be anything you want......As long as you don't suck at it.

    If a person wants to create a machine that will be more likely to fail...Make it complicated.

    USAF 98 Bomb Wing 1960-66 SMW Lu49

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  11. #68
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    There are founders and there are applicators. One day, in a galaxy far far away, a man named Willis Carrier took the theory of cooling an area by mechanical means and made that theory a fact. A few years later the application was improved on. So on and so forth. Fast forward and some Democrat, Republican or other government official saw that this was a necessary and booming industry that needed a code because of the health risks of air quality. Anyway, no system is perfect and we will be a victim of our own creation. We will try to make it better by ultimately destroying whatever it is we tried to make better. You get the idea. Science came I to play as it does with everything. We needed a standard to have a beginning reference point so that we would all be created equal. Along cam static pressure. Them minds have already been through what you guys are talking about. There was no correct or right or wrong answer. Only an idea an somewhere to start. Manufacturers got on board and starting designing systems based on efficiency. What determines efficiency is cost over life if the unit and how much a system has to fight itself to maintain a certain potential. Static pressure was born. You cannot send 1200 CFM through a 12 inch pipe without high static pressure. I used 12 inch because it was a very common size to start after a plenum. Here comes variable speed blower motors designed to run at a given pressure and rpm. When airflow is low they speed up and when it is high they ramp down. A dirty filter is the death of an x13 motor.

    I am like Dean. I would love to see the subject made better and more accurate. hat is why we have guys like you. Nothing is perfect and never will be. Great reading though. I hope this was some assistance and not pointless babble.

  12. #69
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    The determination of system effect factors has not been done for years yet it is well known that there are a lot of factors that need to be determined. It isn't an easy task but still needs to be done.
    No man can be both ignorant and free.
    Thomas Jefferson

  13. #70
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    System Effect? What's system effect? Saw this last week, been meaning to post them just for you Wayne . . .

    Yeah, I see crap like this, and just shake my head.

    Name:  20210211_132338[1].jpg
Views: 32
Size:  3.43 MB


    Quote Originally Posted by WAYNE3298 View Post
    The determination of system effect factors has not been done for years yet it is well known that there are a lot of factors that need to be determined. It isn't an easy task but still needs to be done.
    I do a triple evac with nitro to remove non condensables.

  14. #71
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    Thanks for the photo BBeerme. System effect is abnormal pressure drops due to duct configurations that standard pressure drop tables don't have. NEBB defines system effect as "the derating or loss of capacity of a fan caused by poorly designed duct fittings at, or close to, the fan discharge and inlet".
    Approximate losses can be calculated using dimensional measurements of the ductwork and data from the system effect factor tables in AMCA, ASHRAE or SMACNA.
    No man can be both ignorant and free.
    Thomas Jefferson

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  16. #72
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    Photo above may not be AT the fan, but very close to it.

    Not the worst I've seen by any means. Just not something I would want to sign my name to !!


    Quote Originally Posted by WAYNE3298 View Post
    Thanks for the photo BBeerme. System effect is abnormal pressure drops due to duct configurations that standard pressure drop tables don't have. NEBB defines system effect as "the derating or loss of capacity of a fan caused by poorly designed duct fittings at, or close to, the fan discharge and inlet".
    Approximate losses can be calculated using dimensional measurements of the ductwork and data from the system effect factor tables in AMCA, ASHRAE or SMACNA.
    I do a triple evac with nitro to remove non condensables.

  17. #73
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    Quote Originally Posted by BBeerme View Post
    Photo above may not be AT the fan, but very close to it.

    Not the worst I've seen by any means. Just not something I would want to sign my name to !!
    When I did TAB I wouldn't balance a system with that kind of problem. That didn't set well with some but I had to certify the report.
    No man can be both ignorant and free.
    Thomas Jefferson

  18. #74
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    I was on the roof doing a PM. Probably the first PM I've ever done on that roof. Couldn't really write it up, the unit was 2-3 years old. I just logged my readings.


    Quote Originally Posted by WAYNE3298 View Post
    When I did TAB I wouldn't balance a system with that kind of problem. That didn't set well with some but I had to certify the report.
    I do a triple evac with nitro to remove non condensables.

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