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Air Properties and Psychrometrics

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					            Air Properties and
             Psychrometrics
                  MET 150




3/27/2006        Air Properties and Psychrometrics   1
Atmospheric and Gauge Pressure

    Atmospheric pressure
    Measure with a barometer
    Standard pressure (dry air at sea level)
          14.7 psia
          29.92 in. Hg. absolute

    Gauge pressure scale
    Conversion between atmospheric and
     gauge pressure

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Pressure Conversions


     What is the absolute pressure if the
      gauge pressure reads 42 psig?

     What is the gauge pressure if the
      absolute pressure reads 27 psia?



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Pressure in Inches of Water

    Common measure in air distribution
     systems
    14.7 psi = 406.9” water = 33.9’ water
    1 psi = 27.68 inches water
    Convert inches water to psi: divide by
     27.68
    Convert psi to inches water: multiply by
     27.68
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Converting Between Inches Water
and PSI


      What is the pressure in inches water if
       the gauge reading is 2.2 psi?

      What is the pressure in psi if the
       manometer reads 3.8 inches water?



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Gas Laws

     Dry air is a gas

     Follows specific laws pertaining to
      relationships between pressure,
      temperature and volume

     Known as the gas laws

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Dalton’s Law

      Total pressure of a mixture of confined
       gases is the sum of the pressures each
       gas would exert if it alone occupied the
       volume of the mixture at the same
       temperature.

      Each gas acts independently

      TP = P1 + P2 + … Pn

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Boyle’s Law

      At constant temperature, the pressure on
       a given quantity of confined gas varies
       inversely with the volume of the gas.

      At constant temperature, the volume of a
       given quantity of gas varies inversely
       with the applied pressure.

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Boyle’s Law

    Po x Vo = Pn x Vn
    Vn = Vo x Po / Pn
    Pn = Po x Vo / Vn
    Where:
          Po = original absolute pressure (psia)
          Pn = new absolute pressure (psia)

          Vo = original volume (cubic feet)

          Vn = new volume (cubic feet)


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Boyle’s Law Examples:

      What is the new volume of 3 cubic feet of
       gas at 25 psig if it is compressed to 40
       psig?

      4 cubic feet of gas is expanded from 45
       psig to 10 psig. What is the new volume
       in cubic feet?


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Charles’ Law

      With a constant pressure, the volume of
       a given quantity of confined gas varies
       directly with its absolute temperature

      With a constant volume of gas, the
       pressure varies directly with its absolute
       temperature

      Absolute temperature = oF + 460
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Charles’ Law

      At constant pressure:
          Vo x Tn = Vn x To
          Vn = Vo x Tn / To

          Tn = Vn x To / Vo

          Where:
              To = original absolute temperature
              Tn = new absolute temperature

              Vo = original volume (cubic feet)

              Vn = new volume (cubic feet)


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Charles’ Law

      At constant volume:
          Po x Tn = Pn x To
          Tn = Pn x To / Po

          Pn = Po x Tn / To

          Where:
              To = original absolute temperature
              Tn = new absolute temperature

              Po = original absolute pressure (psia)

              Pn = new absolute pressure (psia)


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Charles’ Law Examples

   What is the new volume of 10 cubic feet
    of gas at 50 degrees if the temperature is
    raised to 130 degrees at constant
    pressure?
   What is the new pressure (in psig) of a
    quantity of gas in a cylinder whose
    pressure is 30 psig at 40 degrees if the
    temperature is raised to 95 degrees?

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Psychrometrics

      Study of dry air and water vapor mixtures

      Condition of the air (temperature and
       humidity) affect human comfort

      Dry air a mixture of gases (nitrogen,
       oxygen and others)

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Definitions

      Specific volume: how much space 1
       pound of dry air occupies.
            At 70oF and sea level: 13.33 ft3/lb
      Specific density: Weight of dry air per 1 ft3
            At 70oF and sea level: 0.075 lb/ft3
      Specific heat: ability to get hot
            At 70oF and sea level: 0.24 Btu/lb/oF


 3/27/2006                Air Properties and Psychrometrics   16
Sensible Heat

    The amount of heat which, when added
     to air, causes a change in temperature
     with no change in the amount of
     moisture present
    Measured with a thermometer
    Sensible heat = specific heat x specific
     density x 60 min/hr x CFM x DT (Btuh)
          Btuh = 0.24 x.075 x 60 x CFM x DT
          Btuh = 1.08 x CFM x DT


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Humidity

    Amount of humidity affects rate at which
     perspiration evaporates, making skin
     cooler
    Comes from evaporation of earth’s
     ocean and other bodies into the
     atmosphere
    Inside, comes from cooking, showers,
     human respiration and perspiration

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Humidity
   Moisture contained in the air
       Pounds moisture per pound of dry air
       Grains moisture per pound of dry air
       At 70oF and sea level, 7000 grains per pound
        of water
 Saturated air: holds all the moisture it can
  at that temperature and pressure
 Saturated air at 70oF and sea level
  contains 110.5 grains of water or 0.01579
  pounds of water per pound of dry air
 3/27/2006           Air Properties and Psychrometrics   19
Relative Humidity

    Amount of water held by the air as a
     percent of what it can hold at that
     temperature and pressure
    Expressed as a percentage
    Warmer air will hold more water
    Relative humidity affects comfort
          Winter: 67-76oF, 30% RH
          Summer: 72-81oF, 40% RH


 3/27/2006           Air Properties and Psychrometrics   20
Comfort Zones
                               Dressed in typical
                                summer or winter
                                clothing
                               Engaged in
                                sedentary activity
                               Air motion in
                                occupied zone does
                                not exceed 30 fpm
                                (winter) or 50 fpm
                                (summer)
                  ASHRAE Comfort Chart
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Measuring Humidity

   Sling psychrometers
   Electronic psychrometers
   Hygrometers with thermometers
   Measure both air temperature and
    moisture content




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Sling Psychrometer

    Two thermometers – dry bulb and wet
     bulb
    Dry bulb measures sensible heat
    Wet bulb thermometer has wick
     saturated with distilled water,
     evaporation lowers temperature, taking
     into account moisture content of air


 3/27/2006       Air Properties and Psychrometrics   23
Sling Psychrometer

    Insure that wick is wet
    Whirl psychrometer with a steady motion
     through surrounding air (30 sec.
     minimum)
    Periodically check wet bulb readings
    Accept wet bulb reading when two
     consecutive readings the same
    Always read wet bulb before dry bulb
    Do not touch wick when taking reading
 3/27/2006       Air Properties and Psychrometrics   24
Sling Psychrometer

    Determine approximate relative humidity
     from sliding scale
    More precise readings can be made by
     plotting the dry bulb and wet bulb
     temperatures on the psychrometric chart
     (covered next week)




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Digital Psychrometers and Hygrometers

    Measure dry bulb and wet bulb
     temperature directly or measure relative
     humidity directly
    Read out relative humidity




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Dewpoint

   Temperature at which water vapor in the
    air becomes saturated with moisture and
    the moisture starts to condense into
    water droplets
   If relative humidity is 100%, dewpoint,
    wet bulb and dry bulb temperatures are
    all the same (no water can be
    evaporated from the wet bulb
    thermometer)

3/27/2006       Air Properties and Psychrometrics   27
Enthalpy

    Total heat content of air and water vapor
     mixture
    Measured from pre-determined base
     point
    Expressed in Btu/lb
    Find enthalpy of air stream by measuring
     dry and wet bulb temperature and
     plotting on a psychrometric chart

 3/27/2006        Air Properties and Psychrometrics   28
Equipment Capacity

   Btu/hr
   Enthalpy difference of air entering and
    leaving equipment x specific density of
    air x cfm x 60 min/hr
   Btu/hr (Btuh) = 4.5 x cfm x DH




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Example

     Enthalpy of 10,000 cfm of air entering
      and leaving a cooling coil are as follows:
         41.5 btu/lb
         22.3 Btu/lb

     What is capacity of unit in Btu/hr? Tons?
         Btu/hr = 4.5 x 10000 x (41.5-22.3) =
          864,000 Btu/hr
         864,000 Btu/hr x 1ton/12,000 Btu/hr = 72 T



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Homework/Labs

   Read pages 11.1 – 11.7
   Answer “Think about it” in complete and
    thoughtful sentences and paragraphs.
    Watch spelling and grammar. Neatness
    counts!
   Worksheet on gas pressure relationships
   Lab on RH measurements



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