2.
SURFACE EVAPORATION
01. AIM AND OBJECTIVE
i) To study the basic principles involved and industrial applications of surface
evaporation.
ii) To study the basic equations involved in surface evaporation operation and to
evaluate overall mass transfer co-efficient for the given system.
iii) To carry out the laboratory experiment for verification of Himus equation and
determination of equation constants.
iv) To study the various factors affecting surface evaporation operation
02. THEORY AND BACKGROUND INFORMATION
When a liquid surface is kept open to atmosphere a concentration gradient is set in
between the liquid surface and the atmospheric air. At the liquid surface the partial
pressure of the liquid surface will be the vapor pressure of the liquid at the surface
temperature .The partial pressure of the liquid vapor far away from the liquid surface
is determined by the amount of liquid vapor present in the atmosphere. Because of the
concentration difference there will be steady transfer of liquid vapor into the
atmosphere or the liquid will evaporate.
The steady rae of evaporation of water is given by the Himus equation
dW/A.dt = k (pI-pA)n
dW- mass of water evaporated ,kg
A-Area of evaporating surface, m2
dt-time taken for evaporating dW quantity of water, hr
pi-vapor pressure of water at surface temperature, mm Hg
pA-partial pressure of water in air, mm Hg
k, n – are Himus constants
The values of the constant k and n are calculated by conducting evaporation
experiment at different temperatures
03. APPARATUS, EQUIPMENT, INSTRUMENTS, MATERIALS USED
1.Single pan balance (3.11 kg capacity)
2.Beaker (2000 ml)
3.Thermometer (0-110C)
4.Immertion Heater (1000W)
5.Dimmerstat
6.Water
Department of Chemical Engineering
Bapuji Institute of Engineering and Technology
04. EXPERIMENTAL SETUP
05:EXPERIMENTAL PROCEDURE:
i) Beaker is filled with water upto ¾ th capacity
ii) The beaker is placed on the pan of single pan balance
iii) A 1000W heater is immersed in the water freely without touching beaker. The
heater is connected to dimmerstat and then to power supply
iv) A thermometer is freely suspended from the top so that the thermometer bulb
just touches the surface of water
v) Start the power supply. Maintain constant temperature of water in the beaker
by using dimmerstat
vi) Adjust weights on the scale in such a way that the pointer shows exactly zero
on the scale
vii) Shift weights 2 g to the left so that the pointer moves upward. Immediately
start the stop watch.
viii) As the water evaporates the pointer slowly moves in downward direction.
When the quantity of water evaporated equals 2 g the pointer indicates zero
on scale
ix) Note down the time taken for evaporation of 2g of water.
x) The experiment is repeated for different water temperatures say 50,60,70,75
C
xi) The dry bulb and wet bulb temperatures of the atmospheric air are noted
down.
xii) A graph of ln (dW/A.dt) vs ln (pi-pA) is plotted and the Himus constants k
and n are reported.
Department of Chemical Engineering
Bapuji Institute of Engineering and Technology
06. DATA :
Diameter of the vessel = …………. m
Cross-sectional area of the vessel, A= ……………. m2
Dry bulb temperature = …………….. C
Wet bulb temperature = …………….. C
07. FORMULAE
Saturated molal absolute humidity
YS =pA /(pt - pA)
pA – vapor pressure of water at prevailing temperature
Saturated Absolute Humidity, YS’ = YS (MA/MB)
MA- Molecular weight of water=18.02 kg/kmol H2O
MB- Molecular weight of air=28.97 kg/kmol, air
YS’ = pA /(pt - pA) (MA/MB)
08.NOTATIONS
YS - Saturated molal absolute humidity
YS’ - Saturated Absolute Humidity
pA - vapor pressure of water at prevailing temperature
pt - Total pressure, mm Hg
MA - Molecular weight of water=18.02 kg/kmol H2O
MB - Molecular weight of air=28.97 kg/kmol, air
dW -mass of water evaporated, kg
A - Cross-sectional area of the vessel, m2
dt -Time taken for evaporating dW quantity of water, hr
pi -vapor pressure of water at surface temperature , mm Hg
Department of Chemical Engineering
Bapuji Institute of Engineering and Technology
pA -partial pressure of water in air (at equilibrium, the partial pressure of a in the
vapor gas mixture equals the vapor pressure pA at the prevailing temperature)
k, n- Himus constants
d -diameter of the beaker
09.OBSERVATION TABLE
Diameter of the vessel = m
Cross-sectional area of the vessel, A= m2
Dry bulb temperature = C
Wet bulb temperature = C
Sl No Surface Weight of water Time
temperature,C evaporated, kg taken, s
1 50
2 55
3 60
4 65
5 70
6 75
10.SAMPLE CALCULATIONS
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Bapuji Institute of Engineering and Technology
11.CALCULATION TABLE
Sl Temperature Wt. of Time dW/Adt pi pA (pI-pA) ln(dW/Adt) ln(pI-pA)
No C water taken
evaporat kg/ m2 hr mm Hg mm Hg mm
ed, kg hr Hg
12. GRAPHS
ln (dW / A dt) vs ln(pi-pA)
ln (dW/A.dt)
ln (pi-pA)
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Bapuji Institute of Engineering and Technology
13. RESYULTS AND DISCUSSIONS
Himus equation constants
K=
n=
14.PRACTICAL APPLICATIONS OF EXPERIMENTAL STUDY
15.TYPICAL VIVA QUESTIONS
16.REFERENCES
i) R.H. Perry, “ Chemical Engineers’ Handbook”, Seventh Edition, Mcgraw Hill,
New York
ii) Robert E. Treybal, “Mass Transfer Operations”, Third Edition, Mcgraw Hill,
New York
iii) W.L.McCabe et.al. “Unit Operations of Chemical Engineering”, Fifth Edition,
Mcgraw Hill, New York
Department of Chemical Engineering
Bapuji Institute of Engineering and Technology
