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Thread: Cfm question

  1. #14
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    Quote Originally Posted by JohnLoves2ski View Post
    Could he be using it as a colloquial term? for instance, my boss just added a ton of charcoal to the BBQ?

    If he's actually trying to say air conditioning tons, and he actually bumped it up by 400CFM os 55F air, then yes, he added a refrigeration ton. I rarely hear people try to quantify space cooling that way...
    Is he trying to impress you because he just learned a 55/400CFM thumbrule?
    No, I don't think he's using a figure of speech. And he's not trying to impress me. He's used that expression in this instance a few times and I just thought it wasn't quite right. I'm not going dispute with him I just don't think it's quite the right way of putting it. That's the reason I asked.

  2. #15
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    Quote Originally Posted by techreptdh View Post
    Any how when I apply the formula to my situation the btuh comes to 8208 since I am adding 400 cfm of 55 degree air into a 74 degree space.
    Someone please tell me what I'm not seeing.
    Sir, you are applying a "sensible formula" but expecting a "total heat' figure equaling 12,000 btu's. The sensible formula is fine for heating calculations but the latent work is not accounted for when applied to cooling calcs. The gap between your 8k figure and the 12k figure is latent work being done and going down the drain pipe.

    Apply the total heat formula of: 4.5 x (Rth-Sth) x cfm = total heat. You can measure dry and wet bulb temps to arrive at enthalpy(total heat) by plotting on a pyschrometric chart or phone app(psycro lt).

    When you look at the capacity specs of a piece of equipment, you have a variety of air conditions that account for the tonnage. Sensible Heat Ratio, the percentage of latent vs. sensible work, is at play.

  3. #16
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    Quote Originally Posted by btuhack View Post
    Sir, you are applying a "sensible formula" but expecting a "total heat' figure equaling 12,000 btu's. The sensible formula is fine for heating calculations but the latent work is not accounted for when applied to cooling calcs. The gap between your 8k figure and the 12k figure is latent work being done and going down the drain pipe.

    Apply the total heat formula of: 4.5 x (Rth-Sth) x cfm = total heat. You can measure dry and wet bulb temps to arrive at enthalpy(total heat) by plotting on a pyschrometric chart or phone app(psycro lt).

    When you look at the capacity specs of a piece of equipment, you have a variety of air conditions that account for the tonnage. Sensible Heat Ratio, the percentage of latent vs. sensible work, is at play.
    So then you can't go around saying a "ton" of cooling was added to a space due to all the variables that need to be accounted for, is that correct?

  4. #17
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    Sir kindly reread your original post. The initial description was "ABOUT a ton". About a ton is correct for 400 cfm of vav air(constant/steady state discharge of 55 unless discharge air has a reset) and the room air temp is near normal. The "about" part is a rough guess on your bosses part on the volume, the latent and sensible percentages vary but still total to 100%(70/30, 65/35, etc.)

    I would give you the formula I posted and some instruments to go check the values for yourself and prove me wrong, then we'd both find out. If you pressed the issue further, I might make my boo boo face towards you out of frustration.

    Get ACCA's book on psycrometrics, the answers are all in there. You'll find that my sensible heat ratio explanation wasn't really correct, it was about correct. SHR expresses what % of total heat is sensible heat. The balance of total would be the latent%

  5. #18
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    Quote Originally Posted by btuhack View Post
    Sir kindly reread your original post. The initial description was "ABOUT a ton". About a ton is correct for 400 cfm of vav air(constant/steady state discharge of 55 unless discharge air has a reset) and the room air temp is near normal. The "about" part is a rough guess on your bosses part on the volume, the latent and sensible percentages vary but still total to 100%(70/30, 65/35, etc.)

    I would give you the formula I posted and some instruments to go check the values for yourself and prove me wrong, then we'd both find out. If you pressed the issue further, I might make my boo boo face towards you out of frustration.

    Get ACCA's book on psycrometrics, the answers are all in there. You'll find that my sensible heat ratio explanation wasn't really correct, it was about correct. SHR expresses what % of total heat is sensible heat. The balance of total would be the latent%
    Point taken........and please, not the boo boo face

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