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Thread: Cfm question

  1. #1
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    Cfm question

    I have a quick question.
    My boss likes to say he added about a 'ton' of air conditioning (400 cfm of 55 degree air) to a space, via opening a damper on a vav box.
    Is that correct terminology, just curious?

  2. #2
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    400 cfm of air per ton so yes

  3. #3
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    Quote Originally Posted by techreptdh View Post
    I have a quick question.
    My boss likes to say he added about a
    'ton' of air conditioning (400 cfm of 55 degree air) to a space, via opening a damper on a vav box.
    Is that correct terminology, just curious?
    Your'e not going to get 400 CFM out of the typical flex that shoots off the low pressure side of a VAV.

    But it would take a air-hood to confirm anyway.

  4. #4
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    400 CFM wouldn't be unusual for a commercial app. A 10" or 12" rd. Sensible heat BTUH=1.08 x CFM x Delta T.Like said, you'd need to measure it.
    Tracers work both ways.

  5. #5
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    If the VAV is capable of it and the duct is capable of it why not? Could use the flow pickup, the flow chart along with a magnehelic or manometer to see if it is at least close to what is expected.

  6. #6
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    I guess I should have mentioned that this is an office bldg so it would naturally be a commercial application.
    But answer me this then, what if you start delivering the same cfm (400) of say 65 or 70 degree air , could you still say
    you're giving the space a " ton " ( in effect ) of air conditioning? I'm just debating the terminology , which to me is a little incorrect.

  7. #7
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    Not incorrect when you break down the formula... A "ton" is 12k btu.. so your temp rise in there is going to satisfy the ton part of the equation

  8. #8
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    400 CFM wouldn't be unusual for a commercial app. A 10" or 12" rd. Sensible heat BTUH=1.08 x CFM x Delta T.Like said, you'd neNot incorrect when you break down the formula... A "ton" is 12k btu.. so your temp rise in there is going to satisfy the ton part of the equationed to measure it.

  9. #9
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    Sorry about the above post error I was trying to combine 2 quotes and I haven't learned that nifty little trick.
    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......or maybe I'm correct and my bosses termology is a bit off.

  10. #10
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    Quote Originally Posted by techreptdh View Post
    Sorry about the above post error I was trying to combine 2 quotes and I haven't learned that nifty little trick.
    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......or maybe I'm correct and my bosses termology is a bit off.
    in general most vav boxes are not that big to increase flow by 400 cfm, thus I call his naritive hocus pocus.

    most vav for small spaces/ single rooms don't flow much above 200 cfm at 1 in duct static, then addd VFD to fan speed controls and I seriously doubt his math, if he was adding 400 cfm above what was the orginal set point, the supply grill would be singing/whistling from velocity of air flow
    If you cant cool it
    HEAT THE Hello out of it

  11. #11
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    Air conditioners are rated in three ways, by B.T.U.'s, Tons of Refrigeration, or by Horsepower. One Ton of Refrigeration removes the amount of heat needed to melt one ton of ice in 24 hours. One Ton of Refrigeration can remove 12,000 B.T.U.'s of heat in one hour. The B.T.U. is the amount of heat needed to raise 1 lb. of pure water 1 deg F.

    CFM is subjective ....400 cfm per ton is a standard used by the manufacturer. but the removal of heat is the real deal....yes adding 400 cfm to a room is like adding ton of air ..but not really if your not removing the heat, all your doing is blowing air around

  12. #12
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    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?

  13. #13
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    Quote Originally Posted by dlove View Post
    Air conditioners are rated in three ways, by B.T.U.'s, Tons of Refrigeration, or by Horsepower. One Ton of Refrigeration removes the amount of heat needed to melt one ton of ice in 24 hours. One Ton of Refrigeration can remove 12,000 B.T.U.'s of heat in one hour. The B.T.U. is the amount of heat needed to raise 1 lb. of pure water 1 deg F.

    CFM is subjective ....400 cfm per ton is a standard used by the manufacturer. but the removal of heat is the real deal....yes adding 400 cfm to a room is like adding ton of air ..but not really if your not removing the heat, all your doing is blowing air around
    You make an excellent point.

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