cfm to btu's
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Thread: cfm to btu's

  1. #1
    42yrs exp is offline Professional Member BM -bad email address
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    Does anyone remember how to calculate Btu's per hour to a room that is supplied with 160 CFM of conditioned air at 600 FPM. what else do I need to calculate Btu's per hour to the room?

    A 7" Round flex duct delivers about 160 CFM at about 600 FPM to a room. The return air room temperature is 80 degrees F. The supply air to the room is 60 degrees F.

    The thermostat is set for 76 degrees F.


  2. #2
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  3. #3
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    Btu's=CFM x 1.08 x temperature difference


  4. #4
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    You can plot your return and supply air conditions on a psychrometric chart to get the BTU change per pound of air and the density of the supply air in cubic feet per pound. You need the wet bulb and dry bulb temperatures of both the supply air and return air and the CFM.

    On the chart, plot the points where the dry bulb and wet bulb temperatures meet for the return air and the supply air, you will end up with 2 points on the chart.

    Next plot out to the enthalpy scale and subtract the enthalpy of the supply air from the enthalpy of the return air. This gives you the change in enthalpy per pound of air in BTU.

    Next get the specific volume of the supply air from the chart. This is how many cubic feet of the air equals 1 pound of air.

    Next divide the supply air CFM by the specific volume. This tells you how many pounds per minute of supply air is traveling through the duct.

    Next multiply the figure you got for the change in enthalpy by the number of pounds of air per minute through the duct, then multiply the answer by 60.

    The final answer is how many BTU are being removed from the room per hour.


    Just curious, why did you want to know?

    [Edited by mark beiser on 04-14-2005 at 12:43 AM]
    If more government is the answer, then it's a really stupid question.

  5. #5
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    0, that hurt my brain on that one, I got to hit the hay, but if you get a minute could you tell me ho many BTU's will one ounce of freon remove?
    __________________________________________________ _______________________
    “Experience is a hard teacher because she gives the test first, the lesson afterwards". - Vernon Law

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  6. #6
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    Originally posted by vw_nutt
    Btu's=CFM x 1.08 x temperature difference

    That works for sensable heating or cooling, but won't give you the total BTU removed if you are doing sensable and latent cooling. The supply air temp he listed is cooler than the return air, so he is cooling. Most of the time when you are cooling there is sensable and latent heat removal, so the formula doesn't work because latent heat removal does not affect the temperature difference.
    If more government is the answer, then it's a really stupid question.

  7. #7
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    http://www.freecalc.com/fansfram.htm

    one of these may help also

    Mark. is there a simpler way of explaining that? It sounds very confusing

    Will the HVAC Calc of Dons give you those results?

  8. #8
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    Originally posted by mark beiser
    You can plot your return and supply air conditions on a psychrometric chart to get the BTU change per pound of air and the density of the supply air in cubic feet per pound. You need the wet bulb and dry bulb temperatures of both the supply air and return air and the CFM.

    On the chart, plot the points where the dry bulb and wet bulb temperatures meet for the return air and the supply air, you will end up with 2 points on the chart.

    Next plot out to the enthalpy scale and subtract the enthalpy of the supply air from the enthalpy of the return air. This gives you the change in enthalpy per pound of air in BTU.

    Next get the specific volume of the supply air from the chart. This is how many cubic feet of the air equals 1 pound of air.

    Next divide the supply air CFM by the specific volume. This tells you how many pounds per minute of supply air is traveling through the duct.

    Next multiply the figure you got for the change in enthalpy by the number of pounds of air per minute through the duct, then multiply the answer by 60.

    The final answer is how many BTU are being removed from the room per hour.


    Okay Mark, you got my curiosity up once you mentioned using a psych chart, as those things have been part of my studies, lately. I decided to crunch a few numbers during lunch break to see where you're going with this. Feel free to tweak my figures if you see I'm off the mark. I'm learning more about these charts as I go along, and your post presented itself as a perfect opportunity to see what value there is in knowing how to use a psychometric chart.

    Let's take the OP's numbers of 80 degrees dry bulb, and add 67 degrees wet bulb for the return air temperature (52% RH). Supply will be OP's 60 degrees, let's plug in 50 for wet bulb (49 % RH). 160 CFM supply air to room @ 600 fpm.

    Enthalpy difference = 11.2 Btu/lb of dry air

    Specific volume of supply air = 13.0 cubic feet/lb of dry air

    160 CFM divided by 13.0 cubic feet/lb = 12.3 lbs/min

    12.3 lbs/min x 11.2 Btu/lb = 137.8 137.8 x 60 = 8268.00 Btuh

    So, is this near the mark? The main question I had (I need to actually hold my sling psychrometer up to a supply air duct sometime) was what to enter for wet bulb for supply air.

    Thanks for the brain stretcher.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  9. #9
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    I've never done this, so I may be wrong.

    Lets say this is on a 4 ton system set up to deliver 400cfm per ton and has a total capacity of 48,000 btu

    400*4=1600cfm
    48000/1600=30btu/cfm
    30*160=4800 btu

    Of course, This would be an approximation since the capacity is nominal and it doesn't account for heat gain in the ducts

  10. #10
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    I've found that using a psychrometric chart can provide some really useful info in the field.
    One little trick you can do is calculate the total,sensible & latent capacity of the equipment you are servicing.
    This may not be accurate to the BTU but it will be close.
    I check the total external static pressure of the blower & plot it on the fan performance data to determine the amount of CFM the fan is moving.
    Next get your delta T at the equipment,you now have the info to see what the actual sensible capacity is of the equipment 1.08 X TD X CFM = SBTU.
    Now get a wet bulb reading in the return & the supply close to the equipment,not too close on the evaporater.
    You can take these wet bulb readings & convert them over to enthalpy on the psychrometric chart. Or you can use a enthalpy chart that already has them converted for you.
    The formula for total BTU's is:
    4.5 X enthalpy change X CFM = Total BTU's.
    With these two readings total BTU's & sensible BTU's you can subtract them to obtain a number that will give you the latent BTU removal of the equipment.
    Keep in mind these are field measurements & are not taking into account the effect the duct system could be having on proper BTU delivery into the building envelope.
    ACCA Manual P is a great book to help explain the psych chart in good detail.

    Hope this helps some.
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  11. #11
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    Looks good to me Shophound..What course are you taking?

    Years ago rses used to have some great classes on doing capacity measurement..actually they had great classes on
    anything to do with hvac.

    Hey Dave are you still sticking your head in the coil cabinet and measuring free surface area?


  12. #12
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    Originally posted by simpleman
    Looks good to me Shophound..What course are you taking?

    Years ago rses used to have some great classes on doing capacity measurement..actually they had great classes on
    anything to do with hvac.

    I'm actually going through the RSES Technical Institute course, on my way to obtain CM status. I would have to say, based upon my experience so far, that RSES continues to have great classes and reference material on anything to do with HVAC/R. My knowledge of and confidence in this field has grown considerably since both joining RSES and finding this web site.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  13. #13
    Join Date
    Sep 2002
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    3,708
    I should have know..you were a rses man,just by your posts.

    They breed some great techs..no doubt! I've always enjoy reading your replys.






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