Page 1 of 4 1234 LastLast
Results 1 to 13 of 49
  1. #1
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,508
    Post Likes

    TECHNICAL EXPLANATION OF MEASURED STATIC AIR PRESSURES

    Can anyone give a technical explanation of why measured static pressures in air systems are unreliable?
    No man can be both ignorant and free.
    Thomas Jefferson

  2. #2
    Join Date
    Jan 2010
    Location
    Sea to Sky
    Posts
    3,387
    Post Likes
    Quote Originally Posted by WAYNE3298 View Post
    Can anyone give a technical explanation of why measured static pressures in air systems are unreliable?
    I bet you can.

    Sent from my SM-G965W using Tapatalk

  3. #3
    Join Date
    Dec 2014
    Posts
    2,886
    Post Likes
    I cannot give any kind of technical explanation I'm really poor at that. But I can say that I'm very suspicious always of my static pressure readings. Now, the ones that I may trust, are when I'm looking for reasons for low air flow and I find a large pressure drop through a section of duct, particularly with some type of cooling or Heating device in there. But as far as setting up Performance, based on static pressures , I don't really trust those numbers.
    I guess that's mostly due to the variance that I see in those numbers (bouncing) ,so which one do you use?. Also you can find different static pressures in different sides of the duct, even as far as a positive on one side and a negative on the other depending upon the construction of the duct. So, to-date, I have not relied at all on static pressures to base air volume through an appliance.
    I'm probably way out in left field here as far as the topic goes but.....

    Sent from my SM-G950U using Tapatalk
    Actually that's a mistake what I wrote there. The negative pressures would be from velocity readings.
    So to expand on that, going by a strictly static pressure reading and the blower chart I could come up with an erroneous CFM because I'm reading static pressure, but, some of that air may be moving backwards. So my assumed cfm from the blower chart would be incorrect in that case.

  4. #4
    Join Date
    Mar 2008
    Location
    kansas
    Posts
    1,652
    Post Likes
    Pretty sure it’s not the technical term, but essentially it’s stratification of air through fittings, elbows, not pushing equally in all directions.
    Honeywell you can buy better but you cant pay more

    I told my wife when i die to sell my fishing stuff for what its worth not what i told her i paid for it

  5. #5
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,680
    Post Likes
    Quote Originally Posted by WAYNE3298 View Post
    Can anyone give a technical explanation of why measured static pressures in air systems are unreliable?
    Turbulence?
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  6. #6
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,680
    Post Likes
    Quote Originally Posted by WAYNE3298 View Post
    Can anyone give a technical explanation of why measured static pressures in air systems are unreliable?
    How about that term they use for water flow in a pipe, where the water is traveling faster in the center of the pipe and slower at the walls of the pipe, maybe it is the same, turbulent flow?
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  7. #7
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,508
    Post Likes
    Thread Starter
    As I expected you all have some good points. Water measured statics are a lot more reliable than air even though most water flows are not laminar. The key to this is looking at how the Pascal and Bernoulli experiments relate to static pressures and the basic difference between air and water?
    No man can be both ignorant and free.
    Thomas Jefferson

  8. #8
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,680
    Post Likes
    Well, let's see . . . Water tends to be a lot more dense than air. Did I get that one right?

    Regarding Bernoulli, sent this clip to a newbie we hired. Was explaining a particular problem where a wind tunnel was being formed by the wind blowing over a building causing problems with the A/C. But no one would believe my explanation of why they were having the problems they were having. By the way, we ended up losing that account, here's the clip:

    https://www.youtube.com/watch?v=HZClP-m9g24

    When I measure static pressures, I like to move the sensing tube around. Both into and away from the airflow. My favorite is the U-Tube manometer with 1/4" tubing; no calibration necessary. If I see much variation, then I know I need to take more precautions. Most of the stuff I work on doesn't have enough velocity to change the static much.


    Quote Originally Posted by WAYNE3298 View Post
    As I expected you all have some good points. Water measured statics are a lot more reliable than air even though most water flows are not laminar. The key to this is looking at how the Pascal and Bernoulli experiments relate to static pressures and the basic difference between air and water?
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  9. #9
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,508
    Post Likes
    Thread Starter
    BBeerme I'm surprised nobody wanted to hear your theory on the problem job. Low pressure systems are extremely vulnerable to air currents and depending on the wind direction, temperature, velocity, proximity of obstructions that cause eddy currents and stack configuration their operation can be tremendously compromised. I have read a lot of your posts and some others on here and if I was your boss no matter what you told me I would take it seriously unless I knew for a fact differently. The link you posted should have got the bosses attention especially the stack demonstration. ASHRAE has a lot of information on wind conditions versus impact on HVAC low pressure air systems. The high pressure systems can also be affected but are not as vulnerable as low pressure systems.

    As for the question here you are starting to bang on the door. Velocity of the air and density or more important compressibility of the air and water play a key role in t0he answer.
    No man can be both ignorant and free.
    Thomas Jefferson

  10. #10
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,680
    Post Likes
    The job regarding the Bernoulli effect was outdoors, and there was a history of problems with that unit before I hired on. Since my 'answer' was too simple and obvious, not to mention the outdoor coil was never being cleaned proper to begin with, no one wanted to accept the reality of the problem. Or stated another way, the powers to be would have to accept that all of the monies charged prior were in vain.

    Regarding air velocity, there have only been a very few jobs, relatively speaking, where velocity was the problem. Or, said a bit differently, the install was the problem due to the velocity of the air.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

  11. #11
    Join Date
    Feb 2016
    Location
    Louisburg Kansas
    Posts
    3,508
    Post Likes
    Thread Starter
    I was doing design on a steam power plant on unit 2. Unit 1 had been designed and built and was up and running. There was a water pipe that went outdoors under ground and when it got outside it turned up through a 3 foot thick concrete slab. The original design encased the pipe in the concrete with electric heat trace wrapped around it. My design was to install a pipe sleeve cast in the concrete bring the pipe up through it , wrap it with heat trace and install a removable cap at the top. When the heat trace failed remove the cap suck the vermiculite out with a shop vac. R&R the heat trace and button it back up. The powers that be ask why I didn't do it the way unit one was done and I told them on unit one you have to bust out concrete three feet thick to replace the heat trace. Their answer was your design is better but how do we explain to the client we did it wrong on unit one? It went out like unit 1.
    No man can be both ignorant and free.
    Thomas Jefferson

  12. #12
    Join Date
    Nov 2006
    Location
    Southeastern Pa
    Posts
    31,665
    Post Likes
    Quote Originally Posted by BBeerme View Post
    How about that term they use for water flow in a pipe, where the water is traveling faster in the center of the pipe and slower at the walls of the pipe, maybe it is the same, turbulent flow?
    You mean "laminar flow?"
    [Avatar photo from a Florida training accident. Everyone walked away.]
    2 Tim 3:16-17

    RSES CMS, HVAC Electrical Specialist
    Member, IAEI

    AOP Forum Rules:







  13. #13
    Join Date
    May 2014
    Location
    Bay Area California
    Posts
    24,680
    Post Likes
    Heh heh, yeah, I'm not saying I am THE brightest bulb in the chandelier, but I am certainly far from the dimmest !!

    I have customers tell me on a regular basis that I am the only one they trust. They put with others, but are so happy when they see me walk through the door. Just yesterday I had to just come right out and tell the customer, yeah, he's younger, didn't grow up with pneumatics, I'll take a look at it and tell you what's going on . . .

    Customer was all happy. Older guy from India who is probably some sort of engineer, and very aware and particular. No way to BS him. Just tell him the truth and he is happy.

    Quote Originally Posted by WAYNE3298 View Post
    I was doing design on a steam power plant on unit 2. Unit 1 had been designed and built and was up and running. There was a water pipe that went outdoors under ground and when it got outside it turned up through a 3 foot thick concrete slab. The original design encased the pipe in the concrete with electric heat trace wrapped around it. My design was to install a pipe sleeve cast in the concrete bring the pipe up through it , wrap it with heat trace and install a removable cap at the top. When the heat trace failed remove the cap suck the vermiculite out with a shop vac. R&R the heat trace and button it back up. The powers that be ask why I didn't do it the way unit one was done and I told them on unit one you have to bust out concrete three feet thick to replace the heat trace. Their answer was your design is better but how do we explain to the client we did it wrong on unit one? It went out like unit 1.
    If you were a real tech, you'd solder a relay on that board and call it good to go.

    I do a triple evac with nitro to remove non condensables.

    I use 56% silver on everything except steel.

    Did you really need the " If you were a real tech " ??

Page 1 of 4 1234 LastLast

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •