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Thread: cfm to btu's

  1. #14
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    Aug 2004
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    Originally posted by shophound
    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.
    It looks like you are mostly on the right track. Try doing it again with a more realistic supply air WB temp. With no duct loss, the supply air will be saturated, so the DB and WB temperatures will be the same. There is always some duct loss, but the WB temperature will still be very close to the DB temperature, unless duct losses are very bad.

    Try refiguring it with supply air conditions of 60 DB and WB to represent more realistic supply air conditions.

    Also look at the specific volume again. With the supply air conditions of 60 DB and 50 WB you figured it with, the specific volume should have been close to 13.2 cubic feet per pound. At 60 DB and 60 WB, the specific volume will be about 13.26 cubic feet per pound.

    With return air conditions of 80 DB and 67 WB, supply air conditions of 60 DB and 60 WB, and 160 CFM, you would have about 3675 btuh of cooling. That is a much more realistic number.


    Actually, now that I look at it, it looks like you figured it with supply air conditions of 50 DB and 50 WB instead of the 60 DB and 50 WB you stated, wich is why I was getting different numbers than you with the conditions you stated.


    I don't know what Psychrometric chart you are using, but if you don't have one already, you should get the big laminated one Trane sells. It is 11"x17", so it is easy to read everything. You can draw on it with a soft pencil and erase the pencil marks when you are done.
    It is "Form Number 1-43.191 Jan. 1983"
    If more government is the answer, then it's a really stupid question.

  2. #15
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    Aug 2003
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    t looks like you are mostly on the right track. Try doing it again with a more realistic supply air WB temp. With no duct loss, the supply air will be saturated, so the DB and WB temperatures will be the same. There is always some duct loss, but the WB temperature will still be very close to the DB temperature, unless duct losses are very bad.
    Okay, THIS is where I was fuzzy, as I alluded to in my last post. It only makes sense that the supply air would be saturated. When I ran the numbers again (using your specific volume figure since my chart seems harder to nail down an accurate number), I came up with 3261 Btuh, which isn't far off your figure of 3675 Btuh.

    I copied my chart out of my RSES technical manual and blew it up on the copier so I could read it without squinting. I'd like a copy of the Trane chart but since I'm not hooked up with Trane, I'm not sure how to get one (I'd also like a Trane ductulator). Just walk into a Trane parts house and ask to buy one?

    Thanks for your help, Mark. I knew I was close but I also knew something was amiss....turned out my suspicion was right.
    • 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.

  3. #16
    Join Date
    Apr 2004
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    7,680
    I dont know if I posted this or not but I ran accross it while looking for something else. For those who might want a brief tutorial on Psycrometric chart use, go here:

    http://www.uwsp.edu/it/tlrn/LOs2003/paperlo/

    Have your sound on.

  4. #17
    Join Date
    Apr 2004
    Posts
    7,680
    BTW....

    BTUH = CFM*HD*4.5

    HD means Enthalpy BTU per pound Difference.

    Assuming a 67 WB return and a 55 WB supply....

    BTUH = 160 * (31.62-23.22) * 4.5
    BTUH = 160 * 8.4 * 4.5
    BTUH = 6048

    Assuming 65 WB return and 58 WB supply....

    BTUH = 160 * (30.06-25.12) * 4.5
    BTUH = 160 * 4.94 * 4.5
    BTUH = 3557

    [Edited by docholiday on 04-15-2005 at 07:48 AM]

  5. #18
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
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    Originally posted by simpleman
    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.

    Simpleman, thank you.

    Nice to hear I'm contributing to this site in a good way. I believe I've learned as much as I've shared. That's how it should be.
    • 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.

  6. #19
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,914
    Originally posted by docholiday
    BTW....

    BTUH = CFM*HD*4.5
    Whats the fun in using a constant to replace part of the math?

    The equation can also be expressed as BTUH=CFM*HD*(60/specific volume)

    Thanks for sharing that, I had forgotten about that equation. Using the 4.5 constant gives an answer within a few btuh of what you would get if you did it with the actual specific volume the way I had described.
    If more government is the answer, then it's a really stupid question.

  7. #20
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
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    Please refresh me...what does the "HD" stand for in this formula?

    Thanks.
    • 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.

  8. #21
    Join Date
    Apr 2004
    Posts
    7,680
    See my post where I defined it.

    Earlier I posted a link to a psyc chart tutorial. A specific temperature at a specific relative humidity gives you a specific wet bulb and enthalpy.

    You need to get a total heat chart or psyc chart to make these caluclations. Ask your Rheem Distributor for P/T Chart - Total Heat Chart (P/N OLC005)

    ----------------------------------------------------------

    Mark, you're right, there are altitude adjustments for the costant I used. I should have posted them.

    Altitude / Sensible Heat Constant/ Total heat constant
    0000 / 1.08 / 4.50
    2000 / 1.01 / 4.14
    4000 / 0.94 / 3.84
    6000 / 0.87 / 3.57
    8000 / 0.81 / 3.31

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