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Sensible heat calculations

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bob_46
bob_46 Member Posts: 813
Sol, when the steam is discharged into the air and mixes it is contained in the air as superheated steam at low pressure. I don't have a psych chart handy to give you the latent but there is no sensible. Once the steam is condensed it's water laying in the condensate pan in the evaporator. You don't have to cool it further.

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  • Solomon Rosenbaum
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    Sensible heat calculation

    I am drawing a blank on this since it has been several years since I did this sort of stuff.

    Can someone please tell me how you calculate the btu saved when you have a sensible heat only change from 212 degF to 55 degF.

    thanks.
  • Brad White
    Brad White Member Posts: 2,398
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    A sensible question

    When you say "sensible heat", I think the temperature-only based portion of air conditioning, but you mention a steam-potential temperature of 212 F to 55 F., so I am just thinking you mean a water heat exchange.

    From that, I am going to assume that the real term "sensible" does not apply because the temperatures we can assume are all below the condensing point if steam. Now that I framed that (whew!):

    If you know your flow rate or water content, you are dealing with 157 degrees F. difference. Your pounds of water (gallons multiplied by 8.33 or thereabouts and depending on your starting temperature) will give you lbs. of water.

    Gallons per minute if a flow rate, would, when multiplied by 8.33 and then by 60 and then by the temperature change, would give you BTU's per Hour.

    For example, 10 GPM x 8.33 is 83.3 lbs. of water per minute. Multiplied by 60 is 5,000 lbs. of water per hour. Multiplied by the 157 degrees is 785,000 BTU's.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
  • Solomon Rosenbaum
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    more details on my situation

    I have steam humidifiers maintaining the humidity in a building. We are in a climate that gets very humid in the summer, yet we still use these things. I am trying to figure out what I would save by shutting the steam in the summer and using better controls on the AHU/Chiller to maintain the humidity. I am assuming the chiller has to work harder because it is now fighting the steam humidifiers on two fronts. First, the added heat, second the extra dehumidification it has to do.

    So, I first assume that I was saving 907 btu/lb for the condensing of the steam part of the equation.

    Now, I am trying to calculate the energy needed to get the 212 deg F water to 55 deg F which is my supply air temp.

  • Brad White
    Brad White Member Posts: 2,398
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    Mutually exclusive equipment, Sol

    The humidifiers run in the winter only or at least do not run when the AC system would be. Typically you would humidify to 30%, maybe 35% RH and cool to whatever temperature but also to an RH of 45% to 50%.

    You can see that you have a "deadband" between 30-35% and 45-50% during which time the humidifier would not run.

    There is no point in running both at the same time unless you enjoy high energy bills.

    To answer your question though, the 212 to 55 question means 157 BTU's per pound. The condensing or latent portion of the steam produced would be 970 BTU's per pound depending on pressure.

    EDIT: I see Bob's take on your question and agree with it. There is no sensible gain to speak of from having a steam humidifier in a duct, beyond the manifold jacket losses.

    These days, virtually no one uses jacketed manifolds constantly bathed in steam. (Energy Code here in MA). Rather, we use a pre-warming valve to pre-heat the manifold before steam is introduced. When there is no call for humidification, the jacket is cold.

    Due to the absorption of the steam into the airstream (evaporation really), the adiabatic cooling absorbs and cancels out any sensible gain to the ductwork. A few feet away, the duct air temperature is about the same as it was when it entered, just more humid.
    "If you do not know the answer, say, "I do not know the answer", and you will be correct!"



    -Ernie White, my Dad
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