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CHAPTER 19
Humidification Operations
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Humidification Operations
• Humidification
and
dehumidification
involve transfer
of
the
material
between
a
pure
liquid
phase and fixed gas that is nearly insoluble
‐
Simpler process than
absorption
and
stripping
as liquid
contain
only
1
component
(thus
no
concentration
gradient
and
resistance
for
mass tranfer)
‐
Both
heat
transfer
and
gas
phase
mass
transfer influences each other
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Definitions
1.
Vapor
–
Component
present
as
gaseous
and liquid form; referred as component
A
2.
Gas –
component present only
in
gaseous
form; referred as component
B
• Gas‐vapor mixture follow the Ideal Gas Laws
3.
Humidity
– mass
of vapor carried
by
a
unit
mass of vapor‐free gas
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Prepared by, Dr. Nora JULLOK/ UniMAP4
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Prepared by, Dr. Nora JULLOK/ UniMAP5
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Prepared by, Dr. Nora JULLOK/ UniMAP6
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Phase equilibria
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Figure 19.1: Equilibria for the system air‐water at 1 atm.
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Adiabatic saturator
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Humidity Chart
Figure 19.2: Air‐water at 1 atm.Prepared by, Dr. Nora JULLOK/ UniMAP
10
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Humidity Chart• Ts
is obtained by trial‐and‐error calculation; for the air water system,
by using humidity chart
• The curved line marked 100% gives humidity of saturated air as a function of air temperature (coordinate of point in this line are found
from Eq. 13)
• Any point above
& to the left of saturation line represent a mixture of saturated air & liquid water.
• Any point below
saturation line represent undersaturated air
• Point on
the temperature axis represent dry air
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Humidity Chart (cont..)
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• A portion of Humidity Chart.
Figure 19.3: Use of humidity chart.
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Use of Humidity Chart1.
Assumption:
A
given
stream
of
undersaturated
air
have
a
temperature T1
and percentage humidity, HA1
2.
Point
a
represent
air
(this
point
is
the
intersection
of
constant temperature line for T1
and constant percentage humidity line for
HA1 )
3.
The
humidity
H1
of
the
air
is
given
by
point
b,
the
humidity coordinate by point a
4.
Dew point – found by following the constant‐humidity line through point
a
to
the
left
to
point
c
on
the
100%
line,
and
then
read
at
point d
on the temperature axis.
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Use of Humidity Chart (cont..)
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Wet‐bulb Temperature• Evaporation requires energy. The wick and therefore the thermometer bulb
decreases
in
temperature
below
the
dry‐bulb
temperature
(ordinary temperature
measure
with
thermometer)
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.
Figure 19.4: (a) Wet‐bulb thermometer. (b) Gradients in the gas boundary layer.
• The temperature reached is called the wet‐bulb temperaturePrepared by, Dr. Nora JULLOK/ UniMAP
16
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Wet‐bulb Temperature (cont..)• Wet‐bulb temperature is a function of:
1. Temperature of air2. Humidity
• Precautions to measure the wet‐bulb temperature:1. The wick must be completely wet, so no dry areas of the wick are
in contact with the gas2.
The
velocity
of
the
gas
should
be
large
enough
(at
least
5m/s)
to
ensure
that
the
rate
of
heat
flow
by
radiation
from
warmer surroundings to the bulb is negligible in comparison with the
rate of
sensible heat flow by conduction and convection from the gas to the bulb.
3.
If
makeup
liquid
is
supplied
to
the
bulb,
it
should
be
at
the
wet‐ bulb temperature.
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Wet bulb temperature theory
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Prepared by, Dr. Nora JULLOK/ UniMAP19
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Prepared by, Dr. Nora JULLOK/ UniMAP20
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COOLING TOWERS
Figure 19.5: Natural‐draft cooling tower.
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Figure 19.6: Typical cooling towers: (a) crossflow tower; (b) counterflow tower
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• A cooling tower is a special type of heat exchanger in which the warm water and the air are brought in direct contact for
‘evaporative cooling’.
• It provides a very good contact of air and water in terms of the contact area and mass transfer coefficient of water vapor while
keeping air pressure drop low.
• Enthalpy of air is lower than enthalpy of water. Sensible heat and latent heat transfer take place from water drop to surrounding air.
Temperature profiles in cooling tower is presented in Figure 19.7
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Figure 19.7: Conditions in cooling tower: (a) , (b) at bottom of
tower, (c) at top of tower
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Figure 19.8: Flow diagram of countercurrent gas‐liquid contactor
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• Thus, cooling is accomplished by sensible heat transfer from water to air and evaporation of a small portion of water.
• The hot water which is coming from heat exchanger is sprayed at the top of the cooling tower.
• Air enters through the louvers at the two opposite walls of the cooling tower.
• During cooling process of water, around 2% water is evaporated.
• Make water is used to compensate the water loss due to evaporation.
• Blowdown is there to drain a part of water containing solid deposit.
• The exit cold water from the cooling tower is used in the heat exchanger
or
other
unit
operation
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Prepared by, Dr. Nora JULLOK/ UniMAP27
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Figure 19.9: Operating diagram for cooling tower; plot the enthalpy of the air versus water temperature.
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Prepared by, Dr. Nora JULLOK/ UniMAP29
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Prepared by, Dr. Nora JULLOK/ UniMAP30
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Prepared by, Dr. Nora JULLOK/ UniMAP31
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Prepared by, Dr. Nora JULLOK/ UniMAP32
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Prepared by, Dr. Nora JULLOK/ UniMAP33
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Tutorial 4
• Problems:19.1
19.3
19.6
19.10
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