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  1. #1

    Latent heat calculations

    Hey all,

    This is my first post. I work for a company called Durable Machinery which works out of the Brooklyn Navy Yard in New York. We are a relatively recent company, and we are trying to figure out how to gauge the power of our units. My understanding is that total heat = latent heat + sensible heat. Sensible heat is simple enough, however latent heat seems to be another story.

    So far, I have that latent heat = 0.68 x cfm x change in W. W is presumably moisture content.

    Using a psychrometric chart, I have been able to calculate W in grains or moisture/lb of dry air.

    The problem is this. Our unit got these test results:
    Dry bulb in: 77
    Web bulb in: 55
    (W in: 29.5)

    Dry bulb out: 55
    Wet bulb out: 46
    (W in: 31.8)

    W in - W out = -2.3, which means that the latent heat is going to be negative.

    Please let me know if there is something I'm doing wrong or the numbers are just impossible.

    If there are any better ways people have out there to calculate latent heat, I'm all ears.

    Thanks in advance,
    Durable

  2. #2
    Join Date
    Jul 2007
    Location
    Virginia
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    5,591
    Quote Originally Posted by durable machinery View Post
    Hey all,

    This is my first post. I work for a company called Durable Machinery which works out of the Brooklyn Navy Yard in New York. We are a relatively recent company, and we are trying to figure out how to gauge the power of our units. My understanding is that total heat = latent heat + sensible heat.
    Just use the Total Heat formula:

    CFM x 4.5 x enthalpy change (Δ H) = BTUH

    The enthalpy for your IN wet bulb of 55f = 23.22

    The enthalpy for your OUT wet bulb of 46f = 18.16

    Your Δ H = 5.06

    CFM x 4.5 x 5.06 = Total BTUH
    Instead of learning the tricks of the trade, learn the trade.

  3. #3
    Join Date
    May 2000
    Location
    Stafford, Va. USA
    Posts
    213
    Hi Durable Machinery,
    Welcome.

    Your leaving wet bulb temperature is too low, does not make sense.
    I would expect the wet bulb temperature to be around the 55 degrees.
    When the air leaves the cooling coil, it is typically saturated, meaning the Dry Bulb (DB) and Wet Bulb (WB) are the same. It also means that the coil is removing the moisture (latent load) as it is supposed to do.

    Another formula that should be used is Q (total load) = 4.5 X CFM X enthalpy (h) difference.
    The two enthalpy numbers can be found on the psychrometric chart. They are difficult to derive.
    I would recommend that you hire a local air balance contractor and have them test a couple of your systems. You should work with them to see how they do the WB-DB measurements and use the the psychrometric chart and do the calculations.
    The balance contractor can tell you if the system is moving the proper amount of air and how it is performing.

    Let us know how it works out or if you have more questions.
    Good Luck.
    Allen
    UA Local 602
    A man who says that he can't learn anything in a class is exactly right.

  4. #4
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    I also like the total heat formula. Of course the key, as usual, is the determination of the airflow. I have also found an app for my iphone which has the psychrometric chart in computer format. All you have to do is input two of the air measurements and enthalpy, as well as the remainder of the air qualities are outputted. No more chart plotting in the field.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,095
    You might want to check with the AHRI about how you are rating your units.

    One of your WB readings is probably very incorrect. Check the calibration of your meters/gauges.
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  6. #6
    Join Date
    May 2000
    Location
    Stafford, Va. USA
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    Hi Waregl82,
    Where did you get your psychrometric app for your Iphone?
    Thank you,
    Allen
    UA Local 602
    A man who says that he can't learn anything in a class is exactly right.

  7. #7
    Join Date
    Nov 2009
    Location
    Tennessee
    Posts
    75
    Quote Originally Posted by Allen View Post
    Hi Waregl82,
    Where did you get your psychrometric app for your Iphone?
    Thank you,
    Allen
    One I found seems pretty good is by Munters Psychroapp. Seems pretty good. I don't get to use it much but when I do it comes in handy

  8. #8
    Join Date
    Jan 2013
    Location
    Cincinnati
    Posts
    28
    Durable,

    Latent Heat is normally only an issue if a straight line drawn between the two points intersects the Saturation Curve of the Psychrometric Chart. Since in this case it does not intersect, you are simply changing the relative humidity and density of the air with the temperature change. The numbers should not go negative (should stay zero), but the volume of air changes since cooler air is denser - which could explain the querky reading.

    NEBB uses the following formulas for calculating Q (heat transfer rate or heat flow):

    Q (total) = BTU (latent) + BTU (sensible)

    Q (total) = 4.5 x CFM X delta h

    Q (sensible) = 1.08 x CFM x delta T

    Q (sesnible) = 60 x Cp X d X CFM x delta T
    for Standard air : Cp = 0.24 BTU/lb x degree F

    Q (latent) = 0.69 x CFM X delta W (gr)

    Q (latent) = 4840 x CFM x delta W

    Delta T = Sesnible BTUs / (1.08 x CFM)

    Latent Heat of Vaporization = 970 BTU/lb

    Latent Heat of Fusion = 144 BTU/lb

    Hope this helps.

    As far as other ways to c

  9. #9
    Join Date
    Feb 2007
    Location
    Georgia
    Posts
    162
    Gary makes an important point you are using cfm you have to look at mass flow in #/min

  10. #10
    Join Date
    Dec 2012
    Location
    Daphne, AL
    Posts
    60
    Hey Allen, The app I got and like is called "Psychrometric Plus". I may have paid a couple of bucks for it, but it was the one I like best. You can input any two quantities and the rest are calculated and displayed. You can even view two steams and see the deltas. I see you are in UA local 602. Where is that? I'm in UA local 91, Birmingham, AL.
    Good Luck, waregl82

  11. #11
    Join Date
    May 2000
    Location
    Stafford, Va. USA
    Posts
    213
    Hi Waregl82,
    Thank you for the information on the app.
    Local 602 serves Washington, DC, Maryland and Northern Virginia.

    Good luck to you my friend.
    Allen
    UA Local 602
    A man who says that he can't learn anything in a class is exactly right.

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