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Humidification OperationsHumidification 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
‐ Both heat transfer and gas phase mass transfer
influences each other
Prepared by, Dr. Nora JULLOK/ UniMAPDefinitions
1. Vapor – Component present as gaseous and
liquid form; referred as componentA
2. Gas – component present only in gaseous form;
referred as componentB
• 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/ UniMAPPhase equilibria
Prepared by, Dr. Nora JULLOK/ UniMAPFigure 19.1: Equilibria for the system air‐water at 1 atm.
Prepared by, Dr. Nora JULLOK/ UniMAPAdiabatic saturator
Prepared by, Dr. Nora JULLOK/ UniMAPHumidity Chart
Figure 19.2: Air‐water at 1 atm.
Prepared by, Dr. Nora JULLOK/ UniMAPHumidity Chart
• Tsis 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 belowsaturation line represent undersaturated air
• Point onthe temperature axis represent dry air
Prepared by, Dr. Nora JULLOK/ UniMAPHumidity Chart (cont..)
Prepared by, Dr. Nora JULLOK/ UniMAP• A portion of Humidity Chart.
Figure 19.3: Use of humidity chart.
Prepared by, Dr. Nora JULLOK/ UniMAPUse of Humidity Chart
1. Assumption: A given stream of undersaturated air have a
temperature T1and 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
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 don the temperature axis.
Prepared by, Dr. Nora JULLOK/ UniMAPUse of Humidity Chart (cont..)
Prepared by, Dr. Nora JULLOK/ UniMAPWet‐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
Figure 19.4: (a) Wet‐bulb thermometer. (b) Gradients in the gas boundary
• The temperature reached is called the wet‐bulb temperature
Prepared by, Dr. Nora JULLOK/ UniMAPWet‐bulb Temperature (cont..)
• Wet‐bulb temperature is a function of:
1. Temperature of air
• 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 gas
2. 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
3. If makeup liquid is supplied to the bulb, it should be at the wet‐
Prepared by, Dr. Nora JULLOK/ UniMAPWet bulb temperature theory
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Prepared by, Dr. Nora JULLOK/ UniMAPCOOLING TOWERS
Figure 19.5: Natural‐draft cooling tower.
Prepared by, Dr. Nora JULLOK/ UniMAPFigure 19.6: Typical cooling towers: (a) crossflow tower; (b) counterflow tower
Prepared by, Dr. Nora JULLOK/ UniMAP• A cooling tower is a special type of heat exchanger in which the
warm water and the air are brought in direct contact for
• 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
Prepared by, Dr. Nora JULLOK/ UniMAPFigure 19.7: Conditions in cooling tower: (a) , (b) at bottom oftower, (c) at top of tower
Prepared by, Dr. Nora JULLOK/ UniMAPFigure 19.8: Flow diagram of countercurrent gas‐liquid contactor
Prepared by, Dr. Nora JULLOK/ UniMAP• 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
• During cooling process of water, around 2 water is evaporated.
• Make water is used to compensate the water loss due to
• 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
Prepared by, Dr. Nora JULLOK/ UniMAP
Prepared by, Dr. Nora JULLOK/ UniMAPFigure 19.9: Operating diagram for cooling tower; plot the enthalpy of the air versus
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Prepared by, Dr. Nora JULLOK/ UniMAPTutorial 4
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