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  1. #1
    Join Date
    Aug 2011
    Location
    houston, texas
    Posts
    20

    wet bulb dry bulb

    can anyone explain this to me. what does wet bulb/dry bulb mean? i havnt gotten a straight answer yet...

  2. #2
    Join Date
    May 2009
    Posts
    480
    Wet bulb is the temperature at which moisture evaporates. It is a reference point on a chart that allows us to know what the correct superheat a piston evaporator should have.

    You can measure the wet bulb with a thermometer with a tiny wet sock around it. The more humid it is inside, the higher the wb will be. Hence you will notice a higher superheat.

    I like a digital psychrometer. It is a tool you absolutely have to have.

    Dry bulb is simply the temp you are accustomed to reading on a thermometer.

  3. #3
    Join Date
    Dec 2002
    Location
    SouthEast NC ICW & Piedmont Foothills
    Posts
    7,635
    this was discussed elsewhere on here not long ago.

    wet bulb temperature is the measure of how much heat is actually in the air
    ( that's the best I remember it)
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    It`s better to be silent and thought the fool; than speak and remove all doubt.

  4. #4
    Join Date
    Aug 2011
    Location
    houston, texas
    Posts
    20
    thanks guys...

  5. #5
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,371
    A practical definition for wet bulb: for a sample of air at a given temperature and moisture content, it is the temperature to which air can be cooled via evaporation.

    Example: dry bulb temperature outdoors is 90 degrees. Wet bulb temperature is 70 degrees. Turn on a swamp cooler fan and wet the pads real well. In theory the air emerging from the swamp cooler should be 70 degrees. In reality it's higher because some air bypasses the swamp pads without getting cooled.

    While wet bulb is a way to measure the amount of moisture in a given sample of air, dew point temperature is more consistent, since it indicates absolute moisture content, unlike relative humidity, which measures the amount of moisture a sample of air can hold at a given temperature. Dew point can be directly converted to grains of moisture per pound of dry air on a psychrometric chart, as can wet bulb. However, it's important to know that if you have a sample of air with a dew point temperature that does not change, the wet bulb temperature will change as the dry bulb temperature is changed.

    Let's say you start at 70 degrees dry bulb, 70 degrees wet bulb, and 70 degrees dew point. This means the air is 100% saturated, same as saying it's at 100% relative humidity. Raise dry bulb temp without changing the dew point temp, wet bulb temp goes up. Lower dry bulb temp without changing the dew point temp, wet bulb goes down.

    IMO wet bulb, in HVAC work, is more useful when dealing with evaporative cooling concerns, such as with cooling towers and swamp coolers. Dew point is more useful for controlling indoor humidity, which is a large part of what we're after with air conditioning. Why the HVAC industry chose to rate and service comfort cooling systems by wet bulb vs. dew point is anyone's guess. Perhaps as an indirect reference to the ability of human skin to evaporate perspiration, which is directly affected by wet bulb temperature.

    However, I advocate dew point as a point of reference since if a tech were to measure dew point inside a space, he could at a glance tell how well the a/c is dehumidifying. For human comfort cooling the indoor room temperature band is rather narrow, say 72-78 degrees. If you chose to aim for a target indoor dew point temperature of 55 degrees, at 72 dry bulb your relative humidity (RH) would be 55% and at 78 dry bulb it would be 45%. At a 50 degree dew point, at 72 dry bulb the RH would be 46% and at 78 dry bulb it would be 37%. If you were to walk into a house on a service call and measure a dew point temperature of 60 degrees and a dry bulb temp of 78, RH would be 64%, which many people would find uncomfortable, and an underlying reason for your service call. It would also mean the air coming off the evaporator is warmer than 55 degrees, which means something is wrong with the a/c (again, it's why you're there...to find out why your customer is uncomfortable). While the 64% RH could also tip you off to something being amiss, seeing a high dew point right off the bat clues you into a dehumidification problem.
    • 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. #6
    Join Date
    Feb 2010
    Location
    In a boiler room
    Posts
    7,186
    When the evaporator is wet, then the temp it is "seeing" or "feeling" is wet bulb temp. This IMO is why wet bulb is used when evaluating an A/C unit or finding target SH for fixed metered systems.

  7. #7
    Join Date
    Aug 2011
    Location
    Hawii
    Posts
    2

    Wet & Dry Bulb

    In order to determine total heat (Enthalpy) you need to know both & plot it on a psychrometric chart. This will allow you to see different properties such as Dewpoint,Relative Humidity, Enthalpy, etc. This process is very important to use when determining total capacity that a evap coil is operating at.

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