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Humidification operations
Humidification operations involve transfer of the material betweena pure liquid phase and fixed gas that is nearly insoluble- Unlike absorption processes only one component is in the liquid phase(thus no concentration gradient in the liquid phase)- Both heat and mass transfer are important
Air contains water vapour. We changethe amount of water vapour in the air (humidify:increase water concentration; or dehumidify)by contacting air with water and evaporating/condensing it.
Design of cooling towers
Design of cooling towers
- forced draft- natural draft(nuclear powers)
Air/water systems^ Psychrometry: science of measuring water vapour content in the air^ Notation
: H^ -^
humidity E^ -^
enthalpy h^ -^
heat transfer coefficient
Air/water systemsHumidity: example^ Partial pressure of water vapour^ If the total pressure of moist air is 100 kPa (approximatelyatmospheric) and the humidity is measured as 0.03 kg kg-1,calculate the partial pressure of the water vapour.- If the air is at 60°C, calculate the relative humidity
Air/water systems^ Psychrometry: science of measuring water vapour content in the air^ Some useful definitions
:
- Humidity is related to the mole fraction of water in the gas as:
OH
air
OH M
H
M
M
H
y
2
/^
- Humid heat is the energy necessary to increase the temperatureof a mixture of 1g of dry air + whatever amount of vapour by 1C:
H c c c^
OH P airP s^
,^2 ,^
M is molecular weight
(gas)
Air/water systemsProperties of air water systems are summarized inpsychrometric charts:
Air/water systemsRelative humidity from wet-bulb and dry-bulb temperatures
Dry-bulb and wet-bulb temperatureDry bulb temperature:^ the ordinary temperature youmeasure with e thermometer
wet cloth/wick
Air flow
Evaporation requires energy. The wick and thereforethe thermometer bulb decreases in temperature belowthe dry-bulb temperature until the rate of heat transferfrom the warmer air to the wick is just equal to the rateof heat transfer needed to provide for the evaporationof water from the wick into the air stream.The temperature reached is called the
wet-bulb
temperature
Air/water systems
Wet-bulb temperature
wet cloth/wick
Air flow
Evaporation requires energy. The wick and thereforethe thermometer bulb decreases in temperature belowthe dry-bulb temperature until the rate of heat transferfrom the warmer air to the wick is just equal to the rateof heat transfer needed to provide for the evaporationof water from the wick into the air stream.The temperature reached is called the
wet-bulb
temperature
Air/water systems^ Wet bulb temperature isa function of:a) Temperature of airb) HumidityIs used in analysis ofair humidity
Wet bulb temperature theory
TWick
N^ H
- The rate of heat requiredto evaporate
N^ A^
(rate of vaporization):
(^
2 2 2 2
,^
Wick OH P OH OH OH
T
T
c
N
M
q^
- At the same time the mass transferrate can be expressed through concentrationgradients
yWick
y
^
^
^
A
M
H
M
H
M
H
M
H
k y
Ay y k y N
OH
air OH Wick air
Wick
y
Wick y OH
2
2
2
T^ OH air
OH MH M
M H y
2 (^2) / / 1
/^
Remember!
Wet bulb temperature theory
TWick
- Finally, rate of heat suppliedby the air to the surface
A
T
Th
q^
Wick y^
(^
4) Combine 1,2,3
^
^
2 2 2
2
,^
Wick OHp OH OH
air OH Wick air
Wick
Wick y y
T T c M H M
H
M
H
M
H
TT k y h
T docsity.com
Wet bulb temperature theory
OH
Wick y Air
Wick
y^
H H k M T T
h^
2 )
(
)
(
OH y Air
y
Wick Wick
k M
h
T T
H
H
2
)
(
)
(
Gives and equation of a straight line H vs TLewis relation
S y y^ air
c
k
h M
^
) for air-water systems
(^
Wick
Wick
T T F
H H^
Air/water systemsRelative humidity from wet-bulb and dry-bulb temperatures
H^ wick
