Humidification operations

1
Humidification operations
Humidification operations involve transfer of the
material between a 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 change the amount
of water vapour in the air (humidify increase
water concentration or dehumidify) by contacting
air with water and evaporating/ condensing it.
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Design of cooling towers
3
Design of cooling towers
- if water comes in contact with unsaturated air
part of water evaporates and the remaining
water cools down - this is exploited in cooling
towers for cooling re-circulated water from
condensers and heat exchangers
4
Design of cooling towers
- forced draft - natural draft (nuclear
powers)
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Air/water systems
Psychrometry science of measuring water vapour
content in the air
Notation H - humidity E - enthalpy h
- heat transfer coefficient
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Air/water systems
Psychrometry science of measuring water vapour
content in the air
Some useful definitions 1) Humidity is the
measure of water content in the air
- Absolute humidity -
Saturated humidity - Relative
humidity - Percentage humidity
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Air/water systems
Humidity example
Partial pressure of water vapour If the total
pressure of moist air is 100 kPa (approximately
atmospheric) and the humidity is measured as 0.03
kg kg-1, calculate the partial pressure of the
water vapour. - If the air is at 60C,
calculate the relative humidity
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Air/water systems
Psychrometry science of measuring water vapour
content in the air
Some useful definitions 2) Humidity is related
to the mole fraction of water in the gas as
M is molecular weight
3) Humid heat is the energy necessary to increase
the temperature of a mixture of 1g of dry air
whatever amount of vapour by 1C
(gas)
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Air/water systems
Psychrometry science of measuring water vapour
content in the air
Some useful definitions 4) Humid volume is the
total volume of 1g of air-vapour mixture. From
the ideal gas relations
5) Total enthalpy Eair is the enthalpy of 1g of
air-vapour mixture with the reference
temperature T00C
with the definition of Cs at ref T0C
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Air/water systems
Properties of air water systems are summarized
in psychrometric charts
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Air/water systems
Relative humidity from wet-bulb and dry-bulb
temperatures
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Air/water systems
Relative humidity from wet-bulb and dry-bulb
temperatures
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Air/water systems
Dry-bulb and wet-bulb temperature
Evaporation requires energy. The wick and
therefore the thermometer bulb decreases in
temperature below the dry-bulb temperature until
the rate of heat transfer from the warmer air to
the wick is just equal to the rate of heat
transfer needed to provide for the evaporation of
water from the wick into the air stream. The
temperature reached is called the wet-bulb
temperature
Dry bulb temperature the ordinary temperature
you measure with e thermometer
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Air/water systems
Wet-bulb temperature
wet cloth/wick
Wet bulb temperature is a function of a)
Temperature of air b) Humidity Is used in
analysis of air humidity
Air flow
Evaporation requires energy. The wick and
therefore the thermometer bulb decreases in
temperature below the dry-bulb temperature until
the rate of heat transfer from the warmer air to
the wick is just equal to the rate of heat
transfer needed to provide for the evaporation of
water from the wick into the air stream. The
temperature reached is called the wet-bulb
temperature
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Wet bulb temperature theory
T
TWick
Twick Hwick ywick
Equilibrium properties at the interface
NH20
yWick
y
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Wet bulb temperature theory
T
TWick
1) The rate of heat required to evaporate NA
(rate of vaporization)
NH20
yWick
y
2) At the same time the mass transfer rate can be
expressed through concentration gradients
Remember!
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Wet bulb temperature theory
3) Finally, rate of heat supplied by the air to
the surface
TWick
T
4) Combine 1,2,3
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Wet bulb temperature theory
Simplifications
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Wet bulb temperature theory
Gives and equation of a straight line H vs T
for air-water systems
Lewis relation
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Air/water systems
Relative humidity from wet-bulb and dry-bulb
temperatures
Hwick
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Design of cooling towers
- forced draft - natural draft (nuclear
powers)
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Design of cooling towers
Objectives - find the minimum required air
rate to obtain particular water
temperature - find the height of packing
Temperature profiles in cooling towers
TH2O
bottom (cooler water)
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Design of cooling towers
1) Overall energy balance
(water)
(air)
Energy balance at intermediate point
subscript x properties of water subscript y
properties of air
(air)
(water)
vapour free mass flow rate of air
mass flow rate of water
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Design of cooling towers
1) Overall energy balance
Eair
equilibrium line
Energy balance at intermediate point
Eair,a
This is operating line equation
Eair,b
TH2O
TH2O,b
TH2O,a
Slope of the operating line
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Design of cooling towers
Eair
equilibrium line
Eair,a
Minimum air flow condition
Eair,b
TH2O,b
TH2O,a
TH2O
Sits on equilibrium line
Slope of the operating line
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Design of cooling towers
Differential volume dZ 1) Enthalpy balance
(water)
(air)
dZ
Z
(air)
(water)
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Design of cooling towers
The rate of sensible heat transfer from water to
the interface is
air
water
Here A is the contact surface area in dZ, a is
the contact surface area per unit of height of
packing. (In packed towers we used a that was the
contact area per unit volume of packing and
differential volume was dVSdZ, with S being the
cross section of the column.)
The rate of heat transfer from the interface to
bulk air
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Design of cooling towers
The rate mass transfer of water through the gas
film into the bulk
air
water
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Design of cooling towers
Lewis relation
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Design of cooling towers
Analogy to packed towers!
the height of packing
slope of equilibrium line
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Design of cooling towers