Results 1 to 14 of 14

Thread: finding out CFM of a unit

  1. #1
    Join Date
    Oct 2018
    Location
    saudi arabia
    Posts
    38
    Post Likes

    finding out CFM of a unit

    hi.
    i have seen a YouTube video about how to find out the actual CFM of a unit installed and running. at the beginning it was showing nice a great calculation formulas. but then i get confused when he used the electrical power input and assumed it as the cooling power output..

    he used the formula , sensible heat capacity = WATTS X 3.413 BTU/h, which its right (conversion between KW and BTU/h).

    But also he considered the WATTS in this Thermal units conversion formula is the same as the electrical power input KW.

    I get confused here, what i know is the conversion formula can be used for the output power , from kw output power to BTU/h, not from the input electrical power to BTU/h.

    what you think guy's?

    where is my mistake? or am i correct.!

    https://youtu.be/aRJH-wJZ1Gs

    https://youtu.be/aRJH-wJZ1Gs

  2. #2
    Join Date
    May 2009
    Posts
    3,375
    Post Likes
    Quote Originally Posted by mohad_i View Post
    hi.
    i have seen a YouTube video about how to find out the actual CFM of a unit installed and running. at the beginning it was showing nice a great calculation formulas. but then i get confused when he used the electrical power input and assumed it as the cooling power output..

    he used the formula , sensible heat capacity = WATTS X 3.413 BTU/h, which its right (conversion between KW and BTU/h).

    But also he considered the WATTS in this Thermal units conversion formula is the same as the electrical power input KW.

    I get confused here, what i know is the conversion formula can be used for the output power , from kw output power to BTU/h, not from the input electrical power to BTU/h.

    what you think guy's?

    where is my mistake? or am i correct.!

    https://youtu.be/aRJH-wJZ1Gs

    https://youtu.be/aRJH-wJZ1Gs

    I'm not sure I totally understand the question, but here's something that might help?

    That's the method we occasionally use to determine airflow across, (for instance) electric duct heaters in variable volume systems. It might be used if there's a particular reason why we can't do a duct traverse, or use a flow hood, or aren't particularly interested in being as accurate as possible for our airflow readings. This method has good potential for error in the field.

    It's simple. With the system stabilized, measure the volts and amps of the electric duct heater. Ohms law states that volts times amps equals watts.
    One watt is equivalent to 3.41 btu's. Multiply your watts times 3.41 which gives you your sensible capacity.

    Take the temperature of the air before it passes through the electric heating element, and take the temperature of the air downstream of the heater.
    Subtract the air temp entering from the air temp leaving for your delta T.

    Plug your readings into the formula and the result is cubic feet of air per minute. We normally don't worry about the density of the air, or the power factor of the power supply.

    This method normally works well enough for resistive loads like electric duct heaters, but there are much better methods for determining airflows. Basically, in my world, it's an extra trick in the trick bag.

  3. #3
    Join Date
    Oct 2018
    Location
    saudi arabia
    Posts
    38
    Post Likes
    Thread Starter
    Quote Originally Posted by Artrose View Post
    I'm not sure I totally understand the question, but here's something that might help?

    That's the method we occasionally use to determine airflow across, (for instance) electric duct heaters in variable volume systems. It might be used if there's a particular reason why we can't do a duct traverse, or use a flow hood, or aren't particularly interested in being as accurate as possible for our airflow readings. This method has good potential for error in the field.

    It's simple. With the system stabilized, measure the volts and amps of the electric duct heater. Ohms law states that volts times amps equals watts.
    One watt is equivalent to 3.41 btu's. Multiply your watts times 3.41 which gives you your sensible capacity.

    Take the temperature of the air before it passes through the electric heating element, and take the temperature of the air downstream of the heater.
    Subtract the air temp entering from the air temp leaving for your delta T.

    Plug your readings into the formula and the result is cubic feet of air per minute. We normally don't worry about the density of the air, or the power factor of the power supply.

    This method normally works well enough for resistive loads like electric duct heaters, but there are much better methods for determining airflows. Basically, in my world, it's an extra trick in the trick bag.

    yah, thanks... there was my mistake.
    i didn't notice he is using this method for resistive load only (electric resistive heater).

    thanks

  4. #4
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    Good video, but I disagree with the free area.
    Last edited by mgenius33; 02-28-2021 at 10:32 AM.
    "The only true wisdom is in knowing you know nothing" Socrates

  5. #5
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    Using the opening he measured, you’re only discounting 1/2” on each dimension. That’s roughly 90-95% free area of the actual listed grille size.

    You will have very high readings using the grille area explained in this video.
    On a standard RA grille I use 65-75% free air depending upon whether the louvers are tight gap or wide gap respectively.
    "The only true wisdom is in knowing you know nothing" Socrates

  6. #6
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    I will say, you can use the basic grille dimensions if you maintain a 1” distance from the grille. However, this video shows him holding the vane flat to the grille surface. The distance from the grille does make a huge difference. The velocity does change as it approaches the grille louvers.
    "The only true wisdom is in knowing you know nothing" Socrates

  7. #7
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    What distance you hold from the grille is predicted by the expected velocity. What I mean by that is, 450fpm at 1” may result in proper reading, while 150fpm may need 1/2” distance.
    "The only true wisdom is in knowing you know nothing" Socrates

  8. #8
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    Quote Originally Posted by mohad_i View Post
    I get confused here, what i know is the conversion formula can be used for the output power , from kw output power to BTU/h, not from the input electrical power to BTU/h.
    Remember energy is always conserved. If energy is not performing work, it will be conserved in the form of heat. So, if your input energy is not performing work it must be converted to heat.
    Look up “conservation of energy” law.
    "The only true wisdom is in knowing you know nothing" Socrates

  9. #9
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    12,043
    Post Likes
    I might expect holding the instrument away from the grill might entrain air next to it and effect the reading. This might be how the meter is supposed to be used by the maker. I used mine like that.
    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

  10. #10
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    Random assumptions are made with regard to methods of anemometer data collection.

    I don’t believe there is a clear set of instructions provided by any vane anemometer mfg in order to provide confident results on a return grille traverse.

    I do believe that a decent quality vane as simple as a 410 could provide excellent results a technician could feel confident with. Holding a vane free handed at various depths and speeds, using random free areas, is not going to provide those results.
    "The only true wisdom is in knowing you know nothing" Socrates

  11. #11
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    12,043
    Post Likes
    I never liked to NEED to use an anemometer because of other methods weren't possible. I didn't find them all that reliable. Following the inst. didn't always give the expected result.
    I could expect surprises. System effect etc. and the instrument itself.
    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

  12. #12
    Join Date
    Nov 2004
    Location
    NJ
    Posts
    1,332
    Post Likes
    Use a TrueFlow grid from the Energy Conservatory to get the most accurate / easiest to use results for measuring airflow in a residential application.
    Ed J

  13. #13
    Join Date
    Sep 2008
    Location
    VA
    Posts
    5,579
    Post Likes
    Quote Originally Posted by Ed Janowiak View Post
    Use a TrueFlow grid from the Energy Conservatory to get the most accurate / easiest to use results for measuring airflow in a residential application.
    Yes, but very pricey. How well do they work with insertion losses? Ideally filter back grilles are 300fpm, but we all know that’s a rarity especially in residential applications.

    Never mind, I see that you use NSOP and TFSOP, in order to calculate a correction factor.
    "The only true wisdom is in knowing you know nothing" Socrates

  14. #14
    Join Date
    Feb 2004
    Location
    New Mexico
    Posts
    12,043
    Post Likes
    I removed the grid from a Shortridge flow hood I wasn't using to use as a collector grid hooked up to my Alnor velometer.
    It worked but I had no way to certify the readings so mainly used to troubleshoot.
    But I can see the advantages of a grid on returns.
    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

Posting Permissions

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