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  1. #1
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    Default Dry Bulb vs. Wet Bulb

    Can someone explain the difference between the two temperature readings? I am looking at specing a mini-split heat pump and, after reading the definition of the terms, I'm still a bit confused.
    "American political opportunities are heavily loaded against those who are simultaneously intelligent and honest" --Richard Dawkins

  2. #2
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    Default Re: Dry Bulb vs. Wet Bulb

    Here's a nice explanation.

    It is basically the difference between the ambient air temperature and the temperature from an evaporating body. The difference between the two tells you about the humidity of the air.

    Generally wet bulb will be cooler, and the difference tells you how humid the air is- at 100% humidity a dry bulb & wet bulb should read the same.

    For a real world example, it's the difference in how warm it feels when your head is dry v. if you put on a wet hat- the evaporation will cool you down. That works a lot better in the Mojave Desert, where humidity is virtually zero, than it will in St. Louis in the summer when the humidity is hovering in the 80% range.
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  3. #3
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    Default Re: Dry Bulb vs. Wet Bulb

    Dry bulb is air temperature as you would normally take it with a thermometer. Wet bulb is the lowest temperature that can be reached by evaporation from a surface when there's airflow over it. Check out an old-fashioned sling psychrometer to see how wet bulb was measured before all these nifty digital gauges became available.

    What mini-split are you looking at, and what are the specs of interest?
    Bailer Hill Construction, Inc. - Friday Harbor, WA
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  4. #4
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    Default Re: Dry Bulb vs. Wet Bulb

    Quote Originally Posted by David Meiland View Post
    Dry bulb is air temperature as you would normally take it with a thermometer. Wet bulb is the lowest temperature that can be reached by evaporation from a surface when there's airflow over it. Check out an old-fashioned sling psychrometer to see how wet bulb was measured before all these nifty digital gauges became available.

    What mini-split are you looking at, and what are the specs of interest?
    Well- isn't it the lowest temperature possible with the air that is flowing over it; increase the airflow and you increase the evaporation, right? So wet bulb would give you the maximum at the airflow where measured... I think.

    How does that work with the heat pumps, anyway?
    And then... there's the question of wine coolers, where they also have web bulb & dry bulb used in a slightly different manner... The WhisperKool I last installed has you fill a wine bottle with water and the temperature sensor fits in there, giving you the temperature of the liquids in the cellar. In theory that gives you a more consistent temperature for wine because it cycles dependent on the average temp. of liquids in the space; that should mean that the system uses longer run times (liquids take longer to change temp. than air) and produces more consistent in-bottle temperatures.
    http://www.lavrans.com

    "He uses statistics as a drunken man uses lamp posts; for support rather than illumination." -Andrew Lang

  5. #5
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    Default Re: Dry Bulb vs. Wet Bulb

    Looking at the Sanyo mini-splits for converting a detached garage into additional living space. Probably looking at a one ton unit, though I have to check with the client and make sure he doesn't want to start hosting bicycle trainer nights in the winter (in which case, I'd consider a larger unit). I was originally looking for a unit that would work with the minimum New Jersey outside temperature and then started reading all about the wet bulb and dry bulb temperature range for the inside air.

    Good explanations from both of you. Once I saw the psychrometer tables it all made sense. I had to think back to sophomore year thermodynamics to make it clear...I think I understood the concept enough to get an A, but never realized what db or wb stood for.
    "American political opportunities are heavily loaded against those who are simultaneously intelligent and honest" --Richard Dawkins

  6. #6
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    Default Re: Dry Bulb vs. Wet Bulb

    Quote Originally Posted by Lavrans View Post

    How does that work with the heat pumps, anyway?
    It's a measure of the unit's ability to remove humidity, or at least that's my understanding of it. If you are sizing equipment for cooling, it becomes important.
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  7. #7
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    Default Re: Dry Bulb vs. Wet Bulb

    Hi
    An HVAC Unit has to remove two forms of heat, Sensible and Latent. I am not going into a lot of detail, so if others want to add in, please feel free.

    Sensible is the temp of the air that you read with a normal thermometer.

    Latent is the amount of heat in the air that is required to be removed from the air due to moisture as moist air is more dense than dry air. An HVAC unit has to be sized to both lower the sensible air temp and to remove the latent heat from the moisture in the air. Condensing the moisture in the air requires more BTUs of cooling than dropping dry air the same temp difference in degrees. I do not recall the exact number, but I think it is about 970 BTUs to change 1 pound of moisture to 1 pound of liquid at the same temperature. I will have to Google that. :-)


    In a nutshell, both factors need to be taken into consideration, in particular in NJ where we get those hot humid summer days. Or as you indicated, if they were to have Spin classes with a bunch of people sweating and putting moisture into the air by breathing.

    Hope this helps.
    Rich
    Last edited by clydewater; 04-17-2012 at 07:38 AM.

  8. #8
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    Default Re: Dry Bulb vs. Wet Bulb

    Rich, you have it right, except for the density of moist air - it's lighter than dry air. Think "low pressure area" (unsettled weather). In the vapor state, molecules of air (nitrogen & oxygen) and water each create the same volumes of gas. However, air has an average molecular weight of 29 (79% nitrogen at MW= 28, 21% oxygen at MW=32), while water has a molecular weight of only 18. Saturated air at 70 F and sea level is about 2.5 volume % water vapor, so a given volume of saturated air weighs less.

    That latent heat number 970 BTU/lb is the value at 212 F, the normal boiling point. It increases as temperature drops. At only 70 F, the value is 1054 BTU/lb.

    Just to round out things, the volume of dry 70 F air in a box 6'x6' and 10'-9" long weighs 29 lb. Saturate that air and you've displaced almost 10 cu.ft. of air molecules with water vapor, or about 0.44 lb of water. Now, to dehumidify that air to, say, 50% RH means cooling it to about 50 F. Cooling the air requires removal of about 139 BTU of sensible heat, and condensing out 0.22 lb of water will remove another 234 BTU.

  9. #9
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    Default Re: Dry Bulb vs. Wet Bulb

    I should have taken science in college.
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  10. #10
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    Default Re: Dry Bulb vs. Wet Bulb

    Hi Dick
    Thanks for correcting me on the fact that moist air is less dense than dry air.

    I do have a question though, that maybe you can help answer. I Googled this question and came back with the following which agrees with what you are saying however;


    The two most abundant elements in the troposphere are Oxygen and Nitrogen. Oxygen has an 16 atomic unit mass while Nitrogen has a 14 atomic units mass. Since both these elements are diatomic in the troposphere (O2 and N2), the atomic mass of diatomic Oxygen is 32 and the diatomic mass of Nitrogen is 28.

    Water vapor (H2O) is composed of one Oxygen atom and two Hydrogen atoms. Hydrogen is the lightest element at 1 atomic unit while Oxygen is 16 atomic units. Thus the water vapor atom has an atomic mass of 1 + 1 + 16 = 18 atomic units. At 18 atomic units, water vapor is lighter than diatomic Oxygen (32 units) and diatomic Nitrogen (28 units). Thus at a constant temperature, the more water vapor that displaces the other gases, the less dense that air will become.


    The However is I vaguely remember back to HS Chem where water cannot be H20 but must be 2H20 as oxygen is diatimic. Is this correct? (again chem was 42 years ago). So if that is the case would the atomic wt change to 2 x 18 or 36?


    EDIT -
    I answered my own question, for those who may care;
    The molar mass of water is 18.02 g/mol, as calculated from the sum of the atomic masses of its constituent atoms.

    The average molecular mass of air (approx. 79% nitrogen, N2; 21% oxygen, O2) is 28.57 g/mol at standard temperature and pressure (STP).

    Using Avogadro's Law and the ideal gas law, water vapor and air will have a molar volume of 22.414 litre/mol at STP. A molar mass of air and water vapor occupy the same volume of 22.414 litres. The density (mass/volume) of water vapor is 0.804 g/litre, which is significantly less than that of dry air at 1.27 g/liter at STP.

    STP conditions imply a temperature of 0 °C, at which the ability of water to become vapor is very restricted. Its concentration in air is very low at 0 °C. The red line on the chart to the right is the maximum concentration of water vapor expected for a given temperature. The water vapor concentration increases significantly as the temperature rises, approaching 100% (steam, pure water vapor) at 100 °C. However the difference in densities between air and water vapor would still exist


    I like these kinds of discussions. Keeps it interesting





    As for the Latent Heat, the numbers do change as you indicted between 970 and 1054 BTUs so for easy calculations we were taught to use 1000, unless it was a big system.

    I do enjoy learning something new
    Rich
    Last edited by clydewater; 04-18-2012 at 01:16 PM.

  11. #11
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    Default Re: Dry Bulb vs. Wet Bulb

    It's clear to me now why all the HVAC guys oversize.
    "First we finish the game, then we’ll deal with the Armada!"

    Sir Frances Drake

  12. #12
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    Default Re: Dry Bulb vs. Wet Bulb

    Yes, it is much easier to just guess and have "too much than not enough", but for humidity control that is not a good idea as the unit does not run long enough to dry out the air. Need the coil to be at or below the dew point to remove the moisture.

    Thanks
    Rich

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