# SATURATION AND HUMIDITY

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```					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)

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 views: 356 posted: 5/25/2011 language: English pages: 20