Energy and Power Engineering, 2013, 5, 651-656
doi:10.4236/epe.2013.54B126 Published Online July 2013 (http://www.scirp.org/journal/epe)
Study on Operating Characteristics of Power Plant with
Dry and Wet Co o l ing System s
Tao Tang1, Jian-qun Xu1*, Sheng-xiang Jin2, Hong-qi Wei1
1School of Energy and Environment, Southeast University, Nanjing, China
2Beijing Energy Company Limited, Beijing, China
Email: tangtao_1026@163.com, *qlj1062@163.com
Received February, 2013
ABSTRACT
The represent paper will study the performance of the power plant with the combination of dry and wet cooling systems
in different operating conditions. A thermodynamic performance analysis of the steam cycle system was performed by
means of a program code dedicated to power plant modeling in design operating condition. Then the off-design behav-
ior was studied by varying not only the ambient temperature and relative humidity but also several parameters con-
nected to the cooling performance, like the exhaust steam flow rate, the air cooling fan load and the number of operat-
ing cooling water pumps and cooling towers. The result is an optimum set of variables allowing the dry and wet cooling
system be regulated in such a way that the maximum power is achieved and low water consumption.
Keywords: Dry and Wet Condenser; Cooling Tower; Off-design; Characteristic Curve; Operational Optimization
1. Introduction
There are three ways of thermal power plants’ cooling
systems: dry cooling system, wet cooling system, and dry
and wet cooling system. In China, the wet cooling system
use in the power plans commonly, but in the northwest
and northeast China where the water is shortage use air
cooling system. The wet cooling systems have high
thermal economy, but with high water consumption. The
air cooling systems can save a lot of water, but the ex-
haust steam pressure is high and varying all the time for
the impact of ambient temperature [1]. The wet and dry
cooling systems combines the both advantages, it not
only make full power when the ambient temperature is
high but also with low water consumption [2,3].
The present study was inspired by the operation of a
power plant with the combined wet and dry cooling sys-
tem (Figure 1), placed in Northwest China. The wet and
dry cooling system is composed of an air cooled con-
denser in parallel with a water cooled condenser. In the
wet cooling system, the cooling water which shared by
two 300 MW Units, taken from the condenser passes
through four wet mechanical draft cooling towers and
returns to the condenser by two cooling water pumps.
The off-design performance of an air cooling con-
denser or water cooling condenser separately is well
deeply investigated [4-6], but the study on the perform-
ance of complies wet and dry cooling system is rarely
find. So the critical element of this study is the wet me-
chanical draft tower. The heat transfer in cooling tower is
a very complex phenomenon. But it could be described
by several equations [7-9] with some simplifying as-
sumptions.
The purpose of the present paper is to explore the im-
pact of a dry and wet cooling system on the thermo-dy-
namic performance of a power plant. This paper offers an
original contribution for cooling system performance
analysis by considering the dry and wet system together.
2. Mathematical Model of Direct Air Cooling
System
2.1. The Pressure of Air Condenser
Using η-NTU method to calculate the condensate tem-
perature of air condenser [1]:
'
11
()
1
1
cc c
s
a
NTU
yfyfa p
Dh h
tt
Sv ce
(1)
Figure 1. Schematic of the wet and dry cooling systems.
Copyright © 2013 SciRes. EPE
T. TANG ET AL.
652
1000yfyfa p
KS
NTU Sv c
(2)
where: Dc is exhaust steam flow rate. hc
is condensate
enthalpy. hc is exhaust steam enthalpy. ta1 is ambient
temperature. cp is air specific heat. Sy is frontal area. vyf is
face velocity. ρa is air density. NTU is heat transfer units.
K is heat transfer coefficient. S is total area.
Using Equation (3) to calculate the condenser pressure
of air condenser:
7.46
1
1
100
9.81 ()
57.66
s
s
t
p
 (3)
Exhaust steam pressure is:
1cs
pp p
1
(4)
where: Δp1 is air condenser pressure drop.
2.2. Air Cooling Condenser Heat transfer
Coefficient
The total heat transfer resistance including internal ther-
mal resistance, external thermal resistance, and wall
thermal resistance:
0
00
11 1111
()( )
b
i
iibm wai
K
SSS

 S
)
i
(5)
00
()/ln(/
mi
SSS SS (6)
00
()/
f
chi wai
SSS

 (7)
where: K is total heat transfer coefficient. S is total area.
αi and α0 are internal and external tube convective heat
transfer coefficient. εi and ε0 are internal and external
tube fouling resistance. δb is base pipe wall thickness. λb
is base pipe wall thermal conductivity. Si and S0 are in-
ternal and external surface area of tubes. Sm is the num-
ber heat transfer area of the base pipe. Schi is fin surface
area. Swai is outer heat transfer area. ηf is fin efficiency. η0
is the total tube fin efficiency.
3. Mathematical Model of Water Cooling
System
3.1. The Pressure of Water Condenser
Water condenser temperature could be got by Equation
(8):
21
4187
1
[1 ]
4.187 1
cc
sw AK
Dw
hh
tt me
 
(8)
where m = Dw/Dc is circulation ratio. D
c is steam flow
rate. Dw is cooling water flow rate. hc-hc
is 1kg steam’s
latent heat. tw1 is cooling water temperature to condenser.
K is heat transfer coefficient. A is cooling area.
Then the water condenser pressure ps2 can be calcu-
lated by Equation (3), the exhaust steam pressure is:
2cs
pp p
2
 (9)
where: Δp2 is water condenser pressure drop.
3.2. Cooling Water Temperature to Water
Condenser
In a closed-loop cooling water system, cooling water
temperature to condenser equals cooling water tempera-
ture from cooling tower, it is not only affected by envi-
ronmental conditions, but also by the design parameters
and operating conditions of the cooling tower.
At present, the cooling tower thermodynamic calcula-
tion use enthalpy method commonly [7,8]. The equations
are not shown in this paper.
4. Results and Discussion
The power plant with dry and wet cooling systems can
operate as three cases: direct air cooling (Dry), dry and
wet cooling system with one cooling water pump and
two wet mechanical draft towers (D&W1), and dry and
wet cooling system with two cooling water pumps and
four wet mechanical draft towers (D&W2). Assuming
two Units at the same operating conditions, each Unit
can get half of the circulating cooling water flow rate.
In order to optimize the operation, it must study the
respective operating conditions off-design characteristics
fistly.
4.1. Design Parameters
Table 1 shows the main design parameters of the Unit
with wet and dry cooling systems.
Each considered variable is subjected to the constraints
listed in Table 2, it contains any possible the power plant
operating condition during the year.
Table 1. Design parameters.
Name Unit Content
Ambient temperature 23.6
Atmospheric pressure kPa 90.06
Relative humidity % 86.84
Gross power output MW 300
Exhaust steam flow rate t/h 614.23
Exhaust steam enthalpy KJ/kg 2437.9
Exhaust steam pressure kPa 15
AC cooling area m2 492 810
AC frontal area m2 5128
AC face velocity m/s 2.91
Wet condenser cooling area m2 3700
Cooling water flow rate t/h 12100
Gas-water ratio / 0.506
CT cooling number / 1.12
Copyright © 2013 SciRes. EPE
T. TANG ET AL. 653
4.2. Dry configuration
The exhaust steam flow rate and ambient temperature
influence on exhaust pressure can be got by using ma-
thematical model of direct air cooling systems mentioned
before. Figures 2-5 reports some of the parametric anal-
ysis results for the Dry configuration. As can be seen
from these Figs, the exhaust pressure rise with exhaust
steam flow rate increases and ambient temperature rises.
And the higher the ambient temperature, this trend is
more obvious. The exhaust pressure will drop when AC
fan load increases. The AC fan load should be operated
according to the maximum load to get the highest power
production, because auxiliary power consumption will
rise as AC fan load rising.
Table 2. Parametric analysis ranges.
Input variables Unit Ranges
Ambient temperature -20 - 35
Relative humidity % 20 - 100
Air condenser fan load % 10 - 100
Exhaust steam flow rate t/h 250 - 650
Figure 2. Exhaust steam flow rate and ambient temperature
influence on exhaust pressure (AC fan load 100%)-Dry.
Figure 3. Exhaust steam flow rate and ambient temperature
influence on exhaust pressure (AC fan load 75%)-Dry.
Figure 4. Exhaust steam flow rate and ambient temperature
influence on exhaust pressure (AC fan load 50%)-Dry.
Figure 5. Relative humidity and ambient temperature in-
fluence on exhaust pressure-W&D.
4.3. W & D Configuration
The wet and dry cooling system off-design process was
carried out by excel VBA code by using the mathemati-
cal model mentioned before.
Table 3 compares the model results against experi-
mental date in three cases. The three cases show three
different environmental conditions and power output. For
all the cases, a good agreement was found between mod-
el results and experimental date: the maximum difference
is lower than 5%.
Figures 5-10 show the influence of the parameters
mentioned before on exhaust pressure and dry proportion
from W&D model.
Figure 5 shows the exhaust pressure variation versus
ambient temperature and relative humidity. As expected,
exhaust pressure increases with rising relative humidity.
The effect becomes more and more appreciable with in-
creasing temperature. In the same range of ambient tem-
perature changes, the greater the relative humidity the
more obvious exhaust pressure changes. This is consis-
tent with the dry proportion behabious shown in Figure 6:
Copyright © 2013 SciRes. EPE
T. TANG ET AL.
Copyright © 2013 SciRes. EPE
654
it is obvious that the steam flow rate entering the AC
increases with raise in relative humidity. Relative humid-
ity increases, the heat transfer capacity of the cooling
tower decline, so wet proportion decreases.
The influence of the AC fan load on the cycle per-
formance is shown in Figures 7-8. Obviously, the ex-
haust pressure decreases with rising AC fan load. The
effect becomes more and more appreciable with increas-
ing temperature. The steam flow rate entering the AC
increases with raise in AC fan load.
Table 3. Comparison between model result (M) and experimental date (EXP).
Case 1 Case 2 Case 3
M EXP Error M EXP Error M EXP Error
Ambient temperature() 22 22 - 30 30 - 26 26 -
Relative humidity (%) 68 68 - 48.3 48.3 - 74.5 74.5 -
Power output (MW) 300 300.008- 265 265.102- 197 196.8 -
Exhaust steam flow rate (t/h) 614.23 - - 540 - - 397.5 - -
steam flow rate to wet (t/h) 273.216 274.9690.64% 255.11 258.5941.37% 190.75 185.2582.88%
Air condenser fan load (%) 93 93 - 93 93 - 94 94 -
Cooling water temperature from CT() 29.287 29.768 1.64% 34.3 34.271 0.08% 30.85 31.536 2.22%
Cooling water temperature to CT () 38.394 38.547 0.40% 42.804 43.584 1.82% 36.875 37.747 2.36%
Exhaust steam Pressure KPa 14.707 14.717 0.07% 17.364 17.371 0.04% 10.73 11.058 3.06%
Figure 7. AC fan load and ambient temperature influence
on exhaust pressure -W&D.
Figure 6. Relative humidity and ambient temperature in-
fluence on dry proportion-W&D.
Figure 9. Exhaust steam flow rate and ambient temperature
influence on exhaust pressure -W&D.
Figure 8. AC fan load and ambient temperature influence
on dry proportion -W&D.
T. TANG ET AL. 655
The influence of the exhaust steam flow rate on the
cycle performance is shown in Figures 9-10. The ex-
haust pressure progressively increases with exhaust
steam flow rate increases. The effect becomes more and
more appreciable with increasing temperature. The way
in which the exhaust steam is shared into the two con-
densers is shown in Figure 10, that a decreasing steam
flow rate goes through the air cooling system as the am-
bient temperature increases from 10 to 25, but an
increasing team flow rate goes through the air cooling
system as the ambient temperature increases from 30
to 35. This behavious is consistent with the water
cooling condenser has better performances in high tem-
perature.
4.4. Optimization of the Operation
At the low temperature, the unit operate with direct air
cooling system. Figure 11 report the results of the con-
densing system optimization procedure for the Dry con-
figuration. The AC fan load which meet the power load
at diffient ambient temperature is given in Figure 11.
The AC fan load increses with rising ambint temperature.
When the power load is 300MW, it must open D&W1 at
16, and D&W1 open at 24 when power load is
225 MW, and D&W1 open at 31 when power load is
150 MW. When power load is lower than 120 MW, it’s
no need to open D&W1.
Figure 10. Exhaust steam flow rate and ambient tempera-
ture influence on dry proportion -W&D.
Figure 11. Power load and ambient temperature influence
Figure 12. Power load and ambient temperature influence
on AC fan load – D&W1.
Figure 13. Power load and ambient temperature influenc
Figures 12-13 report the results of the condensing
sy
-15 report the results of the condensing
sy
5. Conclusions
on of a wet and dry cooling system
e
on dry propotion – D&W1.
stem optimization procedure for the D&W1 configura-
tion. The AC fan load which meet the power load at dif-
fient ambient temperature is given in Figure 12. The AC
fan load increses with rising ambint temperature. When
the power load is 300 MW, it must open D&W2 at 24,
and D&W2 open at 31 when power load is 225 MW.
When power load is lower than 150MW, it’s no need to
open D&w2.
Figures 14
stem optimization procedure for the D&W2 configura-
tion. The AC fan load which meets the power load at
different ambient temperature is given in Figure 14. The
AC fan load increses with rising ambint temperature.
When the power load is 225 MW, the exhaust pressure
can keep 15 KPa at any ambient temperature. But when
the power load is 300 MW, the exhaust pressure can not
keep 15 KPa at ambient temperature is above 28.
A detailed simulati
installed in a steam power plant was developed and some
conclusions were made as follows:
on AC fan load – Dry.
Copyright © 2013 SciRes. EPE
T. TANG ET AL.
Copyright © 2013 SciRes. EPE
656
Figure 14. Power load and ambient temperature influence
on AC fan load – D&W2.
Figure 15. Power load and ambient temperature influenc
1) A parametric analysis was carried out in order to
ch
, the exhaust press
de
ution of dry and wet cooling
sy
decrease decided by
th
hile discarding the remaining heat in the air
condenser.
M,
hanical En-
dry cooling towers,” CTI
and Cooling
ergy Engineer-
5, pp. 430-433.
e
on dry propotion – D&W2.
eck the influence of ambient temperature, relative hu-
midity, exhaust steam flow rate and air condenser fan
load on the thermodynamic performances of a power
plant with dry and cooling system.
2) In dry and wet cooling systemure
creases with rising AC fan load, and increases with
rising relative humidity and exhaust steam flow rate. The
effect becomes more and more appreciable with increas-
ing ambient temperature.
3) The heat load distrib
stem in different operation situations was well devel-
oped. Steam flow rate to AC decreases with increasing
ambient temperature, and increases with increasing air
cooling condenser fan load and relative humidity, but
when the exhaust steam flow rate increases, steam flow
rate to AC may increase or may be
e ambient temperature conditions.
4) The air cooled condenser resulted the best way to
reject heat if the temperature is lower than 16, 24℃,
31, when the power load is 300 MW, 225 MW, 150
MW. At higher ambient temperature, the condensation
should exploit the cooling capacity of the tower as much
as possible w
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