Thermal Performance Evaluation of Seawater Cooling
Towers
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Citation
Sharqawy, Mostafa H., Iqbal S. Husain, Syed M. Zubair, and
John H. Lienhard. “Thermal Performance Evaluation of Seawater
Cooling Towers.” Volume 1: Advances in Aerospace Technology;
Energy Water Nexus; Globalization of Engineering; Posters
(2011). © 2011 ASME
As Published
http://dx.doi.org/10.1115/IMECE2011-62977
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ASME International
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http://hdl.handle.net/1721.1/102075
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Proceedings of the ASME 2011 International Mechanical Engineering Congress & Exposition
IMECE2011
November 11-17, 2011, Denver, Colorado, USA
IMECE2011-62977
THERMAL PERFORMANCE EVALUATION OF SEAWATER COOLING TOWERS
Mostafa H. Sharqawy
Department of Mechanical Engineering
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia
mhamed@kfupm.edu.sa
Iqbal S. Husain
Department of Mechanical Engineering
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia
g200902510@kfupm.edu.sa
Syed M. Zubair
Department of Mechanical Engineering
King Fahd University of Petroleum and Minerals
Dhahran, Saudi Arabia
smzubair@kfupm.edu.sa
John H. Lienhard V
Department of Mechanical Engineering
Massachusetts Institute of Technology
Cambridge, MA, USA
lienhard@mit.edu
ABSTRACT
NOMENCLATURE
a
cp
h
hc
hd
hfg
hg
Le
Lef
packing specific area
specific heat at constant pressure
specific enthalpy
convective heat transfer coefficient
mass transfer coefficient
Seawater latent heat of vaporization
specific enthalpy of water vapor
Lewis number defined by Eq. (6)
Lewis factor defined by Eq. (5)
mass flow rate
m&
MR
inlet water to air mass flow ratio
S
seawater salinity
T
Temperature
V
volume of cooling tower
z
dimensionless height of packing in the cooling tower
Greek Symbols
Effectiveness
ε
Density
ρ
humidity ratio
ω
Subscripts
a
Air
i
Inlet
o
Outlet
s
Saturated
sw
Seawater
wb
wet bulb
Seawater has been used for long time as a cooling fluid in heat
exchangers to reduce fresh water usage in industry and power
plants. The thermophysical properties of seawater are different
from those of fresh water due to the salt content or salinity.
This difference is sufficient to affect the heat and mass
transfer processes which in turn change the thermal
performance. Thermal design of fresh water cooling towers is
described in detail in many textbooks and handbooks.
However, only a rule of thumb is frequently used for
designing of seawater cooling towers. This rule recommends
degrading the tower performance by approximately 1% for
every 10,000 ppm of salts in the feed water. In this paper, the
thermal performance of seawater cooling towers is presented
using a detailed model of counterflow wet cooling towers
which takes into consideration the coupled simultaneous heat
and mass transfer processes and uses state-of-the-art seawater
properties from the literature. The model governing equations
are solved numerically and the validity of this model is
checked using new experimental data that has been measured
using a bench top counterflow seawater cooling tower. The
effect of the variation of seawater salinity as well as other
operating conditions on the effectiveness and Merkel number
is investigated.
1
m2 m-3
J kg-1 K-1
J kg-1
W m-2 K-1
kg m-2 s-1
J kg-1
J kg-1
kg s-1
g kg-1
o
C
m3
kg m-3
kg kg-1
Copyright © 2011 by ASME
INTRODUCTION
Sharqawy et al. [9] investigated the thermal performance
of seawater cooling towers using a detailed model of a counter
flow wet cooling tower. They considered the coupled heat and
mass transfer processes in the study of model. Based on the
results of the model, they obtained a correction factor
correlation, which relates the air effectiveness of the seawater
cooling tower with that of fresh water cooling tower for the
same tower size and operating conditions. This correction
factor equation is valid up to salinity of 120 g/kg. It
characterizes the degradation of the cooling tower
effectiveness when seawater is used. They showed that an
increase in salinity decreases the air effectiveness by 5 to 20%
relative to fresh water cooling tower.
Cooling towers are used in many applications to reject
heat to the atmosphere. Heat rejection is accomplished within
the tower by heat and mass transfer between the hot water
droplets and ambient air. Seawater cooling towers have been
used since the 1970’s in facilities on the coast, as there is a
potential to reduce fresh water consumption in power plants
and other industries. In addition, the use of once-through
cooling systems where hot water is rejected back into the sea
caused many environmental problems. Therefore, seawater
cooling towers have been found to be a competitive
alternative in which seawater is recycled in a closed-loop
cooling system [1]. The salts in the water create a number of
engineering challenges including salt deposition, packing
blockage, corrosion, potentially rising salt concentration, and
salt emissions (drift). These problems can be avoided by
appropriate selection of construction material and equipment.
The use of plastic or asbestos composites for packing, pipes
and water distribution system provided a practical and
predictable solution for most of the corrosion problems.
The objective of this paper is to investigate the thermal
performance of seawater cooling towers by using the detailed
model developed by Sharqawy et al. [9] and to conduct
experimental runs to validate that model.
SEAWATER PROPERTIES
The thermophysical properties of seawater are different
from those of fresh water. This difference is sufficient to
affect the heat and mass transfer processes in cooling towers.
The literature contains many data for the properties of
seawater, but only a few sources provide full coverage for all
relevant thermophysical properties. A recent review and
assessment of seawater properties is given by Sharqawy et al.
[10]. The properties that most strongly affect the thermal
performance of cooling tower are vapor pressure, density, and
specific heat capacity. In addition, thermal conductivity,
viscosity and surface tension affect the heat and mass transfer
coefficients within the packing.
The salts in seawater change the thermophysical
properties with respect to freshwater, which in turn change the
thermal performance of the cooling tower. The thermal design
and performance of fresh water cooling towers have been
abundantly discussed in the literature. The first cooling tower
theory was developed by Merkel [2], and it included many
approximations. The major assumptions in Merkel’s model
are: the water loss by evaporation is neglected; Lewis factor is
assumed to be unity; and the exit air is assumed to be
saturated. A more accurate model was developed by Poppe
and Rogener [3] without using any of Merkel’s
approximations. The cooling tower characteristics or Merkel
number determined by Poppe’s approach is approximately
10% higher than the Merkel number determined by the Merkel
model [4]. Knowing that the effect of seawater properties on
the cooling tower thermal performance may be small at lower
salinities, it is intended in this paper to use an accurate cooling
tower model that does not make any of the Merkel
approximations.
The vapor pressure of seawater is less than that of fresh
water which reduces the potential for water evaporation. The
specific heat of seawater is less than that of freshwater which
reduces the amount of sensible heat that can be transferred at
the same temperature difference. The density of seawater is
higher than that of fresh water due to the salt content. This
increases the mass flow rate of seawater for the same
volumetric flow rate. The viscosity of seawater is higher than
that of fresh water by about 10% at a salinity of 40 g/kg. The
surface tension of seawater is higher than that of fresh water
by about 1.5% at salinity of 40 g/kg. The thermal conductivity
of seawater is less than that of fresh water by about 1.4% at 40
g/kg. For more details about the thermophysical properties of
seawater and its correlations, refer to Sharqawy et al. [10].
The thermal performance of seawater cooling towers has
not been studied carefully in the literature. The available data
are mostly in technical reports, feasibility studies, or design
guidance [5,6]. General discussion about the effect of
seawater properties on the thermal performance was given by
Nelson [7] and Warner [8]. However, no detailed performance
calculation was made. As a rule of thumb, cooling tower
vendors recommend degrading the tower performance by
approximately 1% for every 10,000 ppm of salts in the
cooling water. In practice, most engineering contractors
specify a 0.55-1.1 oC margin on the wet bulb temperature to
account for salts in the cooling water.
COOLING TOWER MODEL
A schematic diagram of a counterflow cooling tower is
shown in Fig. 1, including the important states and boundary
conditions. The assumptions that are used to derive the
modeling equations are as follows [9]:
2
Copyright © 2011 by ASME
hc
(5)
c p ,a h D
However, Lewis number (Le) is defined as the ratio of thermal
diffusivity (α) to mass diffusivity (D):
ka
α
(6)
=
Le =
D
ρ a c p ,a D
Le
• Negligible heat and mass transfer between the tower walls
and the external environment.
• Constant mass transfer coefficient throughout the tower.
• The Lewis factor that relates the heat and mass transfer
coefficients is not unity.
• Water mass flow lost by evaporation is not neglected.
• Uniform temperature throughout the water stream at any
horizontal cross section.
• Uniform cross-sectional area of the tower.
• The atmospheric pressure is constant along the tower and
equal to 101.325 kPa.
Le f = Le 2 / 3
⎛ ωs ,w + d r ⎞
⎜
⎟ -1
⎝ ω + dr ⎠
⎛ ω + dr ⎞
ln ⎜ s ,w
⎟
⎝ ω + dr ⎠
(7)
where dr is the molecular weight ratio of water to air equal to
0.622. The Merkel number is defined as
h aV
(8)
Me = d
m& sw ,i
and the mass ratio is defined as
m&
(9)
MR = sw ,i
m& a
Substituting equations (5), (8), and (9) into equations (3) and
(4) yields
Tsw,i, m˚sw,i
Packing
=
The relationship between Lewis factor and Lewis number is
given by Bosnjakovic [11] as
Tdb,o, ωo, m˚a
dz
f
z
[
[
]
dhsw = Me × Le f c p ,a (Tsw − Tdb ) + h fg (ω s ,w − ω ) dz
Tsw,o, m˚sw,o
(10)
(11)
Equations (1), (2), (10), and (11) are solved numerically
for given inlet conditions of both air and seawater streams and
known seawater outlet temperature. The values of Merkel
number (Me) and mass ratio (MR) are determined by an
iterative method such that they satisfy the inlet and outlet
temperature of seawater and the overall energy and mass
balances given by Eq. (12) and (13) respectively.
(12)
m& a (h a , o − h a ,i ) = m& sw ,i h sw ,i − m& sw , o h sw , o
Fig. 1 Schematic diagram of a counterflow cooling tower
A steady-state heat and mass balances on an incremental
volume leads to the following differential equations:
Mass balance on water vapor
d m& sw = m& a d ω
]
dha = MR × Me × Le f c p ,a (Tsw − Tdb ) + hg (ω s ,w − ω ) dz
Tdb,i , ωi
m˚a
m& a (ω o − ω i ) = m& sw ,i − m& sw ,o
(1)
(13)
EXPERIMENTAL WORK
Mass balance on salts
m&
dS = − S a d ω
m& sw
(2)
Energy balance on moist air:
m& a dh = h c a (T sw − T db ) dV + h d a h g (ω s , w − ω ) dV
(3)
Energy balance on seawater:
m& sw dhsw + hsw dm& sw = hc a (Tsw − Tdb ) dV + hd a hg (ω s ,w − ω )dV
(4)
A bench-top cooling tower from Hilton (model number
H892) is used to conduct experimental runs on forced draft
counterflow wet cooling tower (see Fig. 2). The dimension of
the tower is 150x150x600 mm with PVC packing of specific
area, a = 110 m-1. The bench-top cooling tower is modified to
reach higher mass flow ratio by increasing the mass flow rate
of water using a higher capacity pump. The water is sucked
from the water tank by the pump and delivered to the top of
cooling tower. Three heaters each of 1.5 kW are placed in the
water tank and a rotameter is used to measure the water flow
rate. The air flow rate is measured using an orifice flow meter
connected to U-tube manometer. The dry and wet bulb
temperatures are measured using dry and wet thermocouples
Equations (3) and (4) can be rewritten after introducing Lewis
factor (Lef) and the Merkel number (Me). Lewis factor relates
the heat and mass transfer coefficients as follows
3
Copyright © 2011 by ASME
respectively. The water heaters are connected to variable
transformers to control the water temperature in the tank. A
schematic diagram of the bench-top cooling tower is shown in
Fig. 2 and a photograph is given in Fig. 3.
RESULTS AND DISCUSSION
Experimental runs are conducted for both fresh water and
seawater from an initial state to a steady state condition at
which the variation of any temperature is within 0.1oC. An
example for the temperature variation with time is presented
in Fig. 4. The temperatures of water inlet and outlet, air drybulb inlet and outlet, and air wet-bulb inlet and outlet are
found from the experiments by taking the average values for
the last five minutes under steady state conditions. The steady
state condition is reached after 30-50 minutes from the starting
of the experiment. It is noticed that inlet dry-bulb and wetbulb temperatures of air remain almost constant during the
experiment, but the water inlet temperature, as expected, takes
about 30 minutes to reach the target value due to the heat
input to achieve the desired water inlet temperature.
35
Seawater (S = 44 g/kg) , MR = 1.0
30
25
Temperature [oC]
Fig. 2 Schematic diagram of bench-top cooling tower
20
15
10
5
0
0
10
T w,in
T a,in
T w,out
T a,out
20
T wb,in
T wb,out
30
40
50
60
70
80
Time [min]
Fig. 4 Temperature variation versus time for seawater
cooling tower
To illustrate the results of the present work, the air
effectiveness and water effectiveness of the cooling tower are
calculated at different mass ratios for both fresh water and
seawater. The air effectiveness defined as the ratio of the
actual to maximum possible air-side heat transfer that would
occur if the outlet air stream was saturated at the incoming
water temperature is given by [12]
Fig. 3 Photograph of bench-top cooling tower
The experimental data is obtained for fresh water and
seawater having salinity of 44 g/kg (44,000 ppm) and 85 g/kg
(85,000 ppm). The seawater was collected from the Arabian
Gulf in Al-Khobar city, Saudi Arabia. The salinity of the
collected seawater was measured to be 44 g/kg. Higher
salinity seawater used in these experiments was prepared by
evaporation of water. The salinity was measured by a salinity
refractometer from ATAGO.
εair =
hair ,out − hair ,in
(14)
hsat ,air at inlet water temp − hair ,in
The water effectiveness is the ratio of the heat transfer
from the water to the maximum heat transfer when the water
outlet temperature is equal to the inlet wet bulb temperature of
air.
The inlet air dry bulb temperature is 22.4 ± 1oC, the air
wet-bulb temperature is 16.8 ± 1.5oC, and water inlet
temperature is 31.5 ± 0.2oC for all test runs. The mass flow
ratio (MR) is varied from 0.5 to 4.8.
εwater =
4
mw ,in hw ,in − mw ,out hw ,out
mw ,in hw ,in − mw ,out hw ,at inlet wet bulb temp
(15)
Copyright © 2011 by ASME
Figure 5 shows air effectiveness for fresh water, seawater
of salinity = 44 g/kg and seawater of salinity = 85 g/kg. Air
effectiveness increases with an increase in the mass ratio
because the enthalpy of air at the outlet increases; however,
the air effectiveness value at each mass ratio decreases with
increasing seawater salinity because as the salinity increases
the vapor pressure of seawater decreases, reducing the rate of
evaporation and thus the air effectiveness.
agreement as shown in Fig. 7. The maximum deviation
between the experimental and numerical values is 0.55%.
0.65
Air Effectiveness
0.6
0.65
Fresh water (S = 0.5 g/kg)
Seawater (S = 44 g/kg)
0.6
Seawater (S = 85 g/kg)
Air Effectiveness
0.55
0.55
0.5
0.45
o
Tain = 22.4 ± 1.1 C
0.4
Twbin = 16.8 ± 1.5 C
0.35
Twin = 31.5 ± 0.2 C
o
o
0.5
0.3
o
Tain = 22.4 ± 1.1 C
o
Twbin = 16.8 ± 1.5 C
0.45
0.4
0.361
ε a = 0.363 MR
0.25
0
o
Twin = 31.5 ± 0.2 C
0.5
1
1.5
0.361
ε a = 0.363 MR
0.3
ε a = 0.345 MR
0.25
0.346
ε a = 0.333 MR
0.2
0
0.5
1
1.5
2
2.5
3
3.5
2
, R = 95.26 %
2
, R = 97.37 %
3
3.5
4
4.5
5
4
4.5
Water effectiveness values at each mass ratio for the fresh
water of the experimental readings are compared with that of
the numerical results as shown in Fig. 8. Both experimental
and numerical values are in very good agreement. The
maximum deviation between the experimental and numerical
values is 1.26%.
5
Fig. 5 Air effectiveness versus mass ratio for fresh water
and seawater
Figure 6 shows the water effectiveness for fresh water,
seawater (S = 44 g/kg) and seawater (S = 85 g/kg). As shown,
the water effectiveness decreases with increasing mass ratio;
because the enthalpy of water at the outlet increases due to
slight increase in water outlet temperature; however, the water
effectiveness value at each mass ratio increases with
increasing the salinity of the seawater because of the decrease
in enthalpy of water at the outlet due to slight decrease in
water outlet temperature.
0.7
Water Effectiveness
ε w = 0.811 exp(-0.63 MR) , R2 = 99.37 %
0.7
2
ε w = 0.902 exp(-0.605 MR) , R = 99.56 %
ε w = 0.975 exp(-0.537 MR)
0.6
2
, R = 99.16 %
R2 = 99.37 %
0.4
0.3
0.2
o
Tain = 22.4 ± 1.1 C
Twbin = 16.8 ± 1.5 o C
Twin = 31.5 ± 0.2 o C
Fresh water (S = 0.5 g/kg)
Seawater (S = 44 g/kg)
Seawater (S = 85 g/kg)
0.4
ε w = 0.811 exp(-0.63 MR) (Experimental)
0.5
0.1
0.5
Fresh water (S = 0.5 g/kg) Experimental
Fresh water (S = 0.5 g/kg) Numerical
0.6
0.8
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Mass Ratio
0.3
Fig. 8 Water effectiveness of fresh water versus mass ratio
of the experimental results compared with numerical
results
0.2
0
0
2.5
Fig. 7 Air effectiveness of fresh water versus mass ratio for
both experimental and numerical results
2
, R = 98.14 %
Mass Ratio
0.1
2
(Experimental) , R2 = 98.14 %
Mass Ratio
0.35
0.354
Water Effectiveness
Fresh water (S = 0.5 g/kg) Experimental
Fresh water (S = 0.5 g/kg) Numerical
Tain = 22.4 ± 1.1 o C
Twbin = 16.8 ± 1.5 o C
Twin = 31.5 ± 0.2 o C
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Air effectiveness for seawater (S = 44 g/kg) of the
experimental readings of cooling tower are compared with
that of the numerical analysis results and it is plotted in Fig. 9.
Both experimental and numerical values are in good
agreement. The maximum deviation between the experimental
and numerical values is 1.23%.
5
Mass Ratio
Fig. 6 Water effectiveness versus mass ratio for fresh water
and seawater
Air effectiveness values at each mass ratio for the fresh
water of the experimental readings are compared with that of
the numerical analysis results and it is plotted in Fig. 7. Both
experimental and numerical values are in very good
5
Copyright © 2011 by ASME
Water effectiveness values at each mass ratio for seawater
(S = 44 g/kg) of the experimental readings are compared with
that of the numerical results as shown in Fig. 10. Both
experimental and numerical values are in very good
agreement. The maximum deviation between the experimental
and numerical values is 1.32%.
Air Effectiveness
0.55
0.6
0.55
Air Effectiveness
0.6
compared with that of the numerical analysis results and it is
plotted in Fig. 12.
Seawater (S = 44 g/kg) Experimental
Seawater (S = 44 g/kg) Numerical
0.5
0.45
0.4
Seawater (S = 85 g/kg) Experimental
Seawater (S = 85 g/kg) Numerical
0.5
0.45
0.4
Tain = 22.4 ± 1.1 o C
Twbin = 16.8 ± 1.5 o C
Twin = 31.5 ± 0.2 o C
0.35
0.3
o
Tain = 22.4 ± 1.1 C
Twbin = 16.8 ± 1.5 o C
Twin = 31.5 ± 0.2 o C
0.35
0.25
0.346
ε a = 0.333 MR
0.2
0
0.3
0.5
1
1.5
2
2.5
(Experimental) , R2 = 97.37 %
3
3.5
4
4.5
5
Mass Ratio
0.25
0.354
ε a = 0.345 MR
0.2
0
0.5
1
1.5
2
Fig. 11 Air effectiveness of seawater (salinity = 85 g/kg)
versus mass ratio of the experimental results compared
with numerical results
(Experimental) , R2 = 95.26 %
2.5
3
3.5
4
4.5
5
Mass Ratio
0.9
Fig. 9 Air effectiveness of seawater (salinity = 44 g/kg)
versus mass ratio of the experimental results compared
with numerical results
Seawater (S = 44 g/kg) Experimental
Seawater (S = 44 g/kg) Numerical
ε w = 0.902 exp(-0.605 MR) (Experimental)
R2 = 99.56 %
0.7
Water Effectiveness
Water Effectiveness
0.8
0.6
0.5
0.4
Tain = 22.4 ± 1.1 o C
0.2
R2 = 99.16 %
0.6
0.5
Tain = 22.4 ± 1.1 o C
0.4
Twbin = 16.8 ± 1.5 C
Twin = 31.5 ± 0.2 o C
o
0.3
0.2
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Mass Ratio
Fig. 12 Water effectiveness of seawater (salinity = 85 g/kg)
versus mass ratio of the experimental results compared
with numerical results
0.1
0
0
0.7
0.1
Twbin = 16.8 ± 1.5 o C
Twin = 31.5 ± 0.2 o C
0.3
Seawater (S = 85 g/kg) Experimental
Seawater (S = 85 g/kg) Numerical
ε w = 0.975 exp(-0.537 MR) (Experimental)
0.8
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Mass Ratio
A plot of Merkel number versus mass ratio for fresh
water, seawater (S = 44 g/kg), and seawater (S = 85 g/kg) for
a cooling tower is shown in Fig. 13. It is seen that Merkel
number decreases with an increase of the mass flow ratio. At
lower mass ratios, the difference of Merkel number for fresh
water and seawater is minor and can be ignored. However, as
the mass flow ratio increases, the Merkel number for seawater
is higher than that for fresh water. This requires a cooling
tower of larger size to satisfy the same heat load.
Fig. 10 Water effectiveness of seawater (salinity = 44 g/kg)
versus mass ratio of the experimental results compared
with numerical results
Air effectiveness for seawater (S = 85 g/kg) from the
experimental readings of cooling tower are compared with the
results of the numerical analysis in Fig. 11. Both experimental
and numerical values are in good agreement. The average
difference between experimental and numerical values is
2.1%.
The water effectiveness for the seawater (salinity = 85
g/kg) of the experimental readings of cooling tower are
6
Copyright © 2011 by ASME
1.6
1.4
1.2
Merkel Number
ACKNOWLEDGMENTS
The authors would like to thank King Fahd University of
Petroleum and Minerals in Dhahran, Saudi Arabia, for funding
the research reported in this paper through the Center for
Clean Water and Clean Energy at MIT and KFUPM.
Fresh water
Sea water (S = 44 g/kg)
Sea water (S = 85 g/kg)
Tain = 22.4 ± 1.1 oC
Twbin = 16.8 ± 1.5 oC
Tw in = 31.5 ± 0.2 oC
1.0
0.8
REFERENCES
0.6
[1]
Nester D. M., 1971, “Salt water cooling tower,”
Chemical Engineering Progress, 67(7), 49-51.
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[3] Poppe, M., and Rögener, H., 1991, “Berechnung von
Rückkühlwerken,” VDI-Wärmeatlas, 1-15.
[4] Kloppers, J.C., and Kröger, D.G., 2005, “Cooling
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[5] Ying, B.Y., and David, S., 1991, “The use of cooling
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[6] Eftekharzadeh, S., Baasiri, M.M., and Lindahl, P. A.,
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[7] Nelson J.A., 1986, “Cooling tower and salt water,”
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[8] Warner, M.E., 1974, “Salt water natural-draft cooling
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[9] Sharqawy M.H., Lienhard V, J.H., Zubair, S.M., 2011,
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Towers,” ASME J. of Engineering for Gas Turbines
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[10] Sharqawy M.H., Lienhard V, J.H. and Zubair, S.M.,
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[11] Bosnjacovic, F., 1965, ‘‘Technische Thermodinmik,’’
Theodor Steinkopf, Dresden.
[12] Narayan, G.P., Mistry, K.H., Sharqawy, M.H., Zubair,
S.M., Lienhard V, J.H., 2010, “Energy Effectiveness
of Simultaneous Heat and Mass Exchange Devices,”
Frontiers in Heat and Mass Transfer 1(2), 1-13.
0.4
0.2
0.0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Mass Ratio
Fig. 13 Variation of Merkel number with mass ratio
CONCLUSION
The thermal performance of a seawater cooling tower is
investigated both numerically and experimentally. A detailed
numerical model for a counterflow cooling tower is developed
and numerical solutions are obtained for both air and water
effectiveness. It is found that the air effectiveness increases
with the increase of mass ratio; however, it decreases with
increasing salinity of the seawater. This demonstrates that a
seawater cooling tower has a lower air effectiveness than a
fresh water cooling tower at the same operating conditions.
The maximum decrease in air effectiveness was found to be
15% for seawater having salinity of 85 g/kg at a mass ratio
3.6. On the other hand, water effectiveness decreases with an
increase in the mass ratio; however, it increases with
increasing seawater salinity. This demonstrates that a seawater
cooling tower has a higher water effectiveness value than a
fresh water cooling tower at the same operating conditions.
The maximum increase in water effectiveness was found to be
87.9% for seawater having salinity of 85 g/kg at mass ratio
4.7. In addition, it is found that at lower mass ratios, the
difference of Merkel number for fresh water and seawater is
minor. However, as the mass flow ratio increases, the Merkel
number for seawater is higher than that for fresh water. which
requires a cooling tower of larger size to satisfy the same heat
load.
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