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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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