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Humidity

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  • Illustrating HumidityAIHA Conference & Exposition

    Philadelphia, PAJune 7, 2007

    PO 128 IEQ General Interest

    Presented By : Paul Haas CSP, CIH

  • Teaching Psychrometrics

    We teach classes describing dehumidification

    Students range from PhDs to remediation workers

    Many students had difficulty understanding the concepts

    We developed animations to illustrate the behavior of water vapor

  • Water

    As everybody knows water is simply two hydrogen atoms bonded to one oxygen atom (H2O)

    Water Molecule H2O as a single molecule is a gas (Water

    Vapor).

  • Hydrogen Bond

    The molecules of water are charged. Positive on one side, negative on the other. This makes them want to stick together positive to negative in what is called a Hydrogen Bond.

    Hydrogen Bond

  • Kinetic Energy

    Heat makes the molecules of water vibrate and bounce off one another

    Kinetic Energy - Latent Heat This vibration is kinetic energy as it is

    energy in the form of movement of the molecules. The more heat the more vibration.

    If it were not for this kinetic energy we would have ice in a crystal structure (Ic)

  • Liquid Water

    If there is kinetic energy between 32 and 212F the molecules bounce off one another sufficiently to allow movement as a liquid

    Liquid water behaves as large agglomerates of water molecules held together by hydrogen bonds

    It behaves more like H252O126

  • Liquid Water (Continued)

    This is why membranes like Tyvek work they have holes big enough to allow H2O to pass, but too small to allow H252O126 to pass.

    The first part of this animation shows how water molecules are held together to form liquid water Coalescence - Evaporation

  • Evaporation

    At all temperatures above absolute zero there will be enough kinetic energy that the occasional water molecule will escape as water vapor Below 32F this is called sublimation Between 32 and 212 F it is called evaporation Above 212F it is boiling releasing steam

    The second part of this animation illustrates evaporation

    Coalescence - Evaporation

  • Condensation

    Water vapor stays as a gas because there is enough kinetic energy to keep the individual water molecules bounding off each other

    If the temperature is cooled the molecules slow enough that the hydrogen bond sticks them together and they condense out as water

    Condensation

  • Condensation

    Typically this condensation occurs when the water vapor encounters a chilled surface

    Where there are mold problems with drywall the chilled surface is typically drywall panels that have been cooled by air conditioning

  • Psychrometrics

    Not surprisingly the behavior of water vapor is of great importance in air conditioning and other building issues

    The relationships were studied at length and equations developed to describe the phenomenon

    These relationships are expressed graphically in the psychrometric chart

  • Psychrometric Chart

    30 40 50 60 70 80 90 100 110 120 130

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    0.1

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    2030

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    86

    90 %

    70 %

    50 %

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    2 %

    Entha

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    Btu

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    Relat

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    10

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    85 F Wet bulb temperature

    90

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  • Dry Bulb Temperature

    Temperature of the air as measured by a dry thermometer

    When you refer to temperature you are generally referring to the dry bulb temperature70

    Dry bulb temperature

  • Relative Humidity

    Relative Humidity The amount of moisture in the air as a percent of the amount of moisture the air can hold

    Warm air can hold more moisture than cool air

    50%

    Relative

    Humid

    ity

  • Specific Humidity

    The amount of moisture in the air in grains of water vapor per pound of air

    55 grS

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  • Vapor Pressure

    Pressure due to the water molecules in gaseous water

    Expressed in inches of mercury

    Convert to inches of water by multiplying by 13.6

    0.37"V

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  • Dew Point

    The temperature at which the air can no longer hold the moisture that is in it.

    At the dew point moisture condenses out as dew on a surface or as a fog.

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  • Dew Point

    Temperature of surface above dew point Surface stays dry

    Temperature of a surface below dew point Moisture condenses on surface

    Air below dew point fog

  • Enthalpy

    The amount of heat in the air.

    This is the sum of the heat that is in the air temperature plus the heat of conversion to keep water in a vaporous form.

    Entha

    lpy

    25.4 Btu

  • Wet Bulb Temperature

    Air temperature as measured by a wet bulb thermometer

    Wet bulb temperature when compared to dry bulb temperature can be used to determine relative humidity

    58.5

    Wet bulb temperature

  • Psychrometrics

    Relative humidity Changes with temperature Not a measure of the total amount of moisture in the

    air. Specific Humidity and Dew Point

    both are a measure of the total moisture in the air. Dew Point

    Depends on total moisture in air Can be directly related to temperature measurements

    of surfaces to determine condensation risk.

  • Example

    Consider a hot humid day 90F and 90% RH

    We want to condition the air to 70F at about 50% RH

    We do this by chilling the air far enough to condense out enough moisture to dehumidify it

    Our goal is air with 50 to 55 grains of moisture per pound of air

  • Using the Psychrometric Chart

    Plot 90 and 90% RH on the chart There is 195 grains of moisture per pound

    of air Cool the air to 50 Now there is 53 grains

    of moisture and 100% RH. 142 grains of moisture have condensed

    out the air is now dehumidified. Cool from 90 degrees, 90% RH to 50

    degrees

  • Using the Psychrometric Chart (2)

    The air is dehumidified, but cold (50F) and is at 100% RH

    However it only has 53 grains of moisture When warmed to 70F the RH drops to

    49% Warm Back Up to 70 degrees Summary

  • Tying It All Together

    We had hot humid air chilled it to condense

    the moisture out warmed it back up to

    reach our target humidity

    Chart at left gives the temp, RH and specific humidity at each step

    53 gr49%70

    53 gr100%50

    195 gr.90%90

    Specific Humidity

    Relative HumidityTemp

  • Tying It Together (2)

    Water molecules as a gas more kinetic energy

    Water molecules as liquid less kinetic energy

    Temperature and RH at each step

    Psychrometric Chart for each step

    This animation ties the whole thing together On the left we see what the water

    molecules are doing On the right we see the temp, RH and

    Psychrometric chart Dehumidification

  • Dehumidification

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    DRY BULB TEMPERATURE - F

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    DHUMIDIFICATION- 90 & 90% RH TO 50

    DEHUMIDIFICATION

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