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  1. #1
    Join Date
    Mar 2008
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    4

    How to measure BTU/hr output of heat exchanger?

    I have built a simple air to air heat exchanger that flows 3 cubic feet per minute of air at a delta-T of 10 degrees F from input to output.

    As best I understand the formulas, my device is outputting 3 X 10 X 4.5 BTUs per hour, i.e. 135 BTU/hr.

    Is this correct?
    Thanks

  2. #2
    Join Date
    Aug 2007
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    WI
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    Quote Originally Posted by Gregte View Post
    I have built a simple air to air heat exchanger that flows 3 cubic feet per minute of air at a delta-T of 10 degrees F from input to output.

    As best I understand the formulas, my device is outputting 3 X 10 X 4.5 BTUs per hour, i.e. 135 BTU/hr.

    Is this correct?
    Thanks
    No, the formula you are looking for is: CFM X Delta T X 1.08

    3 X 10 X 1.08 = 32.4 btu per hour
    How do you know you have 3 cfm? Even with some pretty sophisticated equipment, this low of a number is very hard to measure.

  3. #3
    Join Date
    Mar 2008
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    4
    Quote Originally Posted by DesMech View Post
    No, the formula you are looking for is: CFM X Delta T X 1.08

    3 X 10 X 1.08 = 32.4 btu per hour
    How do you know you have 3 cfm? Even with some pretty sophisticated equipment, this low of a number is very hard to measure.
    What I did is measure the fact that the air flowing in my 4 inch diameter duct (100 ft. long) will have to remain in that duct for 3 minutes in order to drop 10 degrees F. Since 100 ft. of duct is just under 9 cu.ft. I adjust the flow rate until the output temp is 10F lower than the input temp. This calculates to be 3 CFM.

    The way I know that the air has to be in the duct for 3 minutes in order to drop 10F is that I put a length of duct (closed ends) with 71 degree air in it into an environment that was 19F cooler. I measured the internal temperature drop of the air in this length of duct and recorded it every minute for 8 minutes. I found that it had lowered 10F in 3 minutes and so used this as an acceptable temperature drop.

    So, every unit volume of air that is in the duct for 3 minutes should get lowered 10F. The 100 ft. of ducting will always be in an environment that is about 19F cooler than the air being input into the duct.

    I may not have made this clear but its about the best I can do.

    Anyway, thanks for the information.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,928
    I think you confusing CFM, and FPM.
    Your heat loss test was inacturate.
    The test piece didn't have constant conditioned air being supplied to it.

    3 CFM being moved in a 6" pipe would be roughly 15 FPM. Or 6 minutes for the air to go from one end to the other.

    Volume and velocity are often confused as being the same thing..
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  5. #5
    Join Date
    Mar 2008
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    4
    Quote Originally Posted by beenthere View Post
    I think you confusing CFM, and FPM.
    ...
    As stated I am using 100 feet of 4 inch diameter pipe. This pipe would hold 8.73 cubic feet of air. At a flow rate of 2.91 CFM (roughly 3 CFM) the FPM would be about 33 FPM.

  6. #6
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,928
    Not sure why I was thinking you said 6".

    Again, the temp drop of your test piece is not an acturate test for using on your run.

    I come up with 2.88 CFM at 33 FPM, for 31 BTU's.
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