COOLING TOWER
HUMIDIFICATION/COOLING TOWER
Saddawi
The Goal of the Experiment
The goal of this experiment is to determine heat and mas
balance for countercurrent air-water system in a Packed
Cooling Tower.
To find the Characteristic equation, Number of Transfer
Units NtoG and Number of Heights Transfer Units HtoG
Murphree gas phase stage efficiency and the Overall cooling
tower effectiveness efficiency
Experimental Setup
Base unit components include:
1. Air distribution chamber.
2. A tank with heaters to simulate cooling loads of
0.5, 1.0 and 1.5kW.
3. A makeup tank with gauge mark and float
operated control valve.
4. A centrifugal fan with intake damper to give
0.06kg s-1 max. air flow.
5. A water collecting basin.
6. An electrical panel
Note
Use distilled water to fill the makeup tank . Monitor and record the amount
of water evaporated during all of the test operations of the cooling tower.
This can be done by measuring the time needs to spend by added amount of
water to the make-up tank.
Check wet bulb thermocouple reservoir for water. Add if necessary.
After the system reach to study sate,
Record all temperatures, dry and wet bulb temperatures of the air and water
temperature of all sections, mass flow-rate of ware and air.
Some background theory
The basic function of a cooling tower is to cool water by
intimately mixing it with air.
This cooling is accomplished by a combination of:
Sensible heat transfer between the air and the water (Conduction
and Convection) and it controlled by temperature differences
and area of the contact between air and water.
And the evaporation of a small portion of the water.
In the cooling towers, the evaporation is the most effective part
in the cooling process
Mass Balance and Enthalpy Balance on Cooling Tower
*Please see page 12 equations (1,2,&3)
Take mass balance over a differential section (see the fig.)
mw - mw1 = ma (Y -Y1' )
dmw = ma dY
Water
Inlet
(1)
T2
H2
mw
Air
outlet
t2
h2
ma
(2)
2
*Mass velocity of dry air remain constant through the
cooling tower
Take enthalpy balance over the same differential section
mw H + mah1' = mw1' H1' + mah
dz
z
1’
(3)
*Because the latent heat of water is a big value, so a
small amount of water evaporation will produce large
cooling effect.
Therefor we can assume the mass velocity of the
water falling down through the tower is constant with
out large consequences error
Please see equation (4) on page 12
mw (H - H1' ) = ma (h1' - h)
(4)
1
Water
Outlet
T1
H1
mw
t1
h1
ma
Air
Inlet
Equation (4) can be rewritten in term of heat balance as in equation (5)
mwCpw dT = ma dh
Where
m a Dh = ma (Cpair dt + l3dY )
(5)
Take the integral of eq (5) over entire Column
mwCpw (T2 -T1 ) = ma (h2 - h1 )
(6)
Eq (6) represent Air Bulk Operating Line by plotting air enthalpies
versus water temperatures.
mw
Cpw
ma
Enthalpy of Air
Slope of (NO) line =
h2
h1
O
N
T1
Cooling Tower Operating line
(Air bulk operating line)
Water Temperature
T2
Saturated Air Operating line
Saturated Air
water vapor Film
If you assume that the drops of water
falling through the tower are surrounding
by a thin air film,
* This film must be saturated with water
vapor.
* The heat and mass transfer take place
between the film and the upstream air bulk
Where there is no resistance to heat flow in the
interface between the saturated air film and
water. In other words, the interface temperature
can be assumed to be equal to the bulk water
temperature (Merkel assumption)
T(wart temperature) ≈ ti (interface temperature)
Water
bulk at
temp T
Heat movement
By plotting the enthalpies of the saturated air–water vapor mixture
(film) and water bulk temperatures will produce a curve, please see
the Figure.
This carve represent Saturated Air Operating line or can be called
Water Operating line
Air bulk at temp t
The relation between the temperature and
enthalpy of the saturated air
Enthalpy
H2
This curve applies to the air film
surrounding the water
It called Water Operating Line
And limited for hot and cold water
temp (T2 and T1)
Air Operating Line or
Tower Operating Line
Represent Air condition
through the column
H3
h2
H1
h3
h1
T1
T3
T2
Water Temperature
Driving Force Diagram
Enthalpy Driving Force
H2-h2
Cooling Range
T2-T1
Mass Balance and Enthalpy balance on Cooling Tower
In terms of mass and heat transfer coefficients.
*Please see page 15-19
mwCpw dT = ma dh
Where
m a dh = ma (Cpait dt + lw dY )
(5)
ma dh = hg a(ti - t)dz + lw K y a(Yi -Y )dz
By rearrange eq 7
pleas see eq 11&12 on page 17
Kya
dh
=
dz
(H i - h)
ma
h2
Take integral over entire Tower
(7)
ò
h1
K ya
dh
=
(H i - h) ma
z
ò dz =
o
(8)
K y az
ma
(9)
h2
ò
h1
K ya
dh
=
(H i - h) ma
K y az
Z
ò dz = m = H
o
a
toG
z
NtoG = Number of Air
Enthalpy Transfer Units
HtoG = Heights of
Transfer Units
ma
H toG =
Kya
By combing eqs (5 &9)
Merkel’s Equation
T2
KaV
dT
= Cpw ò
mw
T1 H w - ha
H w - ha = Dhm
This equation is commonly referred to as the Merkel equation. The
left-hand side of this equation is called the ”Tower Characteristic,”
which basically indicates the 'degree of difficulty to cool' the water or
the 'performance demand' of the tower.
The tower characteristic and the cooling process can be explained on
a Psychrometric Chart
KaV Cpw (T2 - T1 )
=
mw
Dhm
Please note that V=Z =Volume occupied by packing
per unit plan area
To obtain mean driving force (∆hm) Carey and Williamson method
can be used. This depends upon the application of correction factor
f to the observed value of Hm- h3 (at the arithmetic mean of inlet
and out let water temps T1 & T2)
g1 = H1 - h1
g 2 = H 2 - h2
g m = H 3 - h3
Dhm = f g m
Characteristic Cooling Tower Equation
KaV
mw
By ploting values of
versus
mw
ma
KaV
mw n
= b[ ]
mw
ma
The cooling tower effectiveness .ε. is defined as the ratio of the
actual energy transfer to the maximum possible energy transfer
h2 - h1
e=
H 2 - h1
Murphree gas phase stage efficiency
Y2 -Y1
EMG =
Yas -Y1
Yas
Y2
Y1
tas t2 t1
Air Temps