Docstoc

SATURATION AND HUMIDITY

Document Sample
SATURATION AND HUMIDITY Powered By Docstoc
					SATURATION AND HUMIDITY
             What is a Vapour?
• The term "vapor" is applied to the gas of any
  compound that would normally be found as a
  liquid at room temperature and pressure.
• For example, water, gasoline, rubbing alcohol,
  and finger nail polish remover (ethyl acetate)
  are all normally liquids, but they all evaporate
  to give a gas.
       Air does not “hold” water




All gases co-exist in the same volume
                   Evaporation




• Vaporization or evaporation
  – Endothermic
Water vapor saturation
              • Water molecules move
                between the liquid and
                gas phases

              • When the rate of water
                molecules entering the
                liquid equals the rate
                leaving the liquid, we
                have equilibrium
                 – The air is said to be
                   saturated with water
                   vapor at this point
                 – Equilibrium does not
                   mean no exchange
                   occurs
        Evaporation and Vapor Pressure
Vapor Pressure

• Vapor pressure - pressure of the vapor present at
  equilibrium with its liquid
    – Vapor pressures vary widely - relates to intermolecular forces
                  e = vapor pressure




                              es = es(T)




es = saturation
vapor pressure
          Saturation Vapor Pressure
              and Temperature


es = 12.3 mb; T = 10°C




es = 23.4 mb; T = 20°C




es = 42.4 mb; T = 30°C
                           Saturation and Temperature


•   The saturation vapor pressure
    of water increases with
    temperature
     – At higher T, faster water
        molecules in liquid escape
        more frequently causing
        equilibrium water vapor
        concentration to rise
     – We sometimes say “warmer
        air can hold more water”
•   There is also a vapor pressure
    of water over an ice surface
     – The saturation vapor
        pressure above solid ice is
        less than above liquid water
                   Relative Saturation

• Relative Saturation
   – ratio of actual vapor pressure to saturation vapor pressure
   – Range: 0-100% (+)

• Can be changed by
   – Changes in water vapor content,
   – Changes in temperature
                            Dew Point
• The dew point is the
  temperature at which the air is
  saturated with water vapor

• If a surface cools below the dew
  point, water condenses on the
  surface and dew drops are
  formed

• Dewpoint is a measure of the
  water vapor content of
    the air
• It is not a measure of
  temperature!
Which environment has higher water
          vapor content?
            IMPORTANT!!
• SATURATION & HUMIDITY

• RELATIVE SATURATION & RELATIVE HUMIDITY

• ABSOLUTE SATURATION & ABSOLUTE
  HUMIDTY
                Numericals
• Helium contains 12% by volume of ethyl
  acetate. Calculate percent relative saturation
  at a temperature of 30C and a pressure of
  98KPa. Vapour pressure of He at these
  conditions is 15.9 Kpa. (74)
• The %R.H of air at 86F and a total pressure of
  750 mmHg is 20.7%. At 86F vapour pressure
  of water is 31.8 mmHg. What is the partial
  pressure of the water vapour?(658)
• Ethyl ether at a temperature of 20C exerts a vapour
  pressure of 442 mmHg. Calculate the composition of a
  saturated mixture of nitrogen and ether vapour at a
  temperature of 20C and a pressure of 745 mmHg
  expressed in the following terms:
   –   %Volume
   –   %Weight
   –   Pounds of vapour per cubic foot of mixture
   –   Pounds of vapour per pound of vapor free gas
   –   Pound moles of vpour per pound mole of vapour free gas.
• What is the minimum number of cubic meters
  of dry air at 20C and 100 Kpa that are
  necessary to evaporate 6 kg of ethyl alcohol if
  the total pressure remains constant at 100
  kpa? Assume that the air is blown through the
  alcohol to evaporate it in such a way that the
  exit pressure of the air-alcohol mixture is at
  100kpa. Vapour pressure of alcohol at 20C is
  5.93kpa and molecular wt. 46.07. (503m3)
• A gas holder contains 10,000 ft3 of gas
  saturated with water vapour at 80F and a
  pressure 6.0 in H2O above atmospheric. The
  barometer reads 28.46 in Hg. Calculate the
  weight of water vapour in the gas. Vapour
  pressure of H2O at 80 ⁰F is 1.032 in Hg. (158)
• The weather reported on the radio this
  morning was that the temperature this
  afternoon would reach 94 ⁰F, the relative
  humidity would be 43%, the barometer 29.67
  in Hg, partly cloudy to clear, with wind speed
  of 8 mi/hr. How many pounds of water vapour
  would be in 1 mi3 of afternoon air? 5280ft =
  1mile while vapour pressure of water at 94 ⁰F
  = 1.61 in Hg. (1.52108lb)

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:356
posted:5/25/2011
language:English
pages:20