Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant

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ARCHIVE OF MECHANICAL ENGINEERING
DOI: 10.24425/ame.2022.144076 2023, Vol. 70, No. 1, pp. 129–149
Mohamad Hasan MALEKMOHAMADI1
, Hossein AHMADIKIA 2
,
Siavash GOLMOHAMADI3
, Hamed KHODADADI4
Evaluation of wet cooling tower replacement by Heller
cooling tower in a power plant
Received 23 July 2022, Revised 27 September 2022, Accepted 28 November 2022, Published online 8 December 2022
Keywords: Heller cooling tower, wet cooling tower, condenser, power plant
Water resources are the main component of natural systems affected by climate
change in the Middle East. Due to a lack of water, steam power plants that use wet
cooling towers have inevitably reduced their output power. This article investigates the
replacement of wet cooling towers in Isfahan Thermal Power Plant (ITPP) with Heller
natural dry draft cooling towers. The thermodynamic cycle of ITPP is simulated and the
effect of condenser temperature on efficiency and output power of ITPP is evaluated.
For various reasons, the possibility of installing the Heller tower without increasing
in condenser temperature and without changing the existing components of the power
plant was rejected. The results show an increase in the condenser temperature by
removing the last row blades of the low-pressure turbine. However, by replacing the
cooling tower without removing the blades of the last row of the turbine, the output
power and efficiency of the power plant have decreased about 12.4 MW and 1.68
percent, respectively.
Nomenclature
Symbols
𝐴face front surface of heat exchanger 𝐶𝑟ratio of heat capacity
𝐶heat capacity 𝑑diameter
𝑐𝑝specific heat capacity 𝑒roughness
BHossein Ahmadikia, e-mail: ahmadikia@eng.ui.ac.ir
1University of Isfahan and Isfahan Thermal Power Plant, Isfahan, Iran
2University of Isfahan, Isfahan, Iran; ORCID: 0000-0003-2167-5237
3Isfahan Thermal Power Plant, Isfahan, Iran
4Department of Electrical Engineering, Khomeinishahr Branch, Islamic Azad University, Isfahan,
Iran
0
©2023. The Author(s). This is an open-access article distributed under the terms of the Creative
Commons Attribution (CC-BY 4.0, https://creativecommons.org/licenses/by/4.0/), which permits
use, distribution, and reproduction in any medium, provided that the author and source are cited.

130 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
𝑓friction NTU number of heat transfer unit
𝐺velocity of mass flow rate 𝑃pressure
𝑔coefficient of gravity pass number of passes in heat exchanger
𝐻height 𝑄heat
ℎcoefficient of convection heat transfer 𝑞heat transfer
ˆ
ℎenthalpy Re Reynolds number
ℎ𝑙head loss RH relative humidity
𝑘coefficient of conduction heat transfer 𝑅𝑗other thermal resistance
𝐿length 𝑇temperature
𝑚mass flow 𝑈overall heat transfer in the area
𝑁number 𝑊work
Greek symbols
Δdifference 𝜃angle
𝛿thickness 𝜅coefficient without dimension
𝜀efficiency 𝜌density
𝜂efficiency
Subscripts
𝑎air 𝑜outer or outlet
𝑖inner or inlet out outlet
in inlet 𝑇total
max maximum 𝑡tube
min minimum 𝑤water
1. Introduction
In recent years, climate change and global warming have caused irreparable
damage to some countries. Studies have shown that in the Middle East and North
Africa average temperature has increased and rainfall has decreased. The water
crisis and the drought in Isfahan City, in the central area of Iran, have made the
water supply scarce throughout the country. Dziegielewski and Baumann [1] state
that valid long-term forecasts of water demand are essential to all types of water-
related planning. Without such forecasts, water planners cannot efficiently allocate
water resources among competing uses or ensure long-term sustainability. For
power plant cooling systems, optimizing and retrofitting the performance of the
cooling tower reduces the power plant’s internal electricity consumption and water
consumption [2]. Isfahan Thermal Power Plant (ITPP) is a natural gas-fired steam
power plant with five units and total output power of 835 MW, which uses wet
cooling towers. The water consumption of the cooling tower is 2000 m3/h. Due to
the lack of water, it is necessary to adopt methods to reduce the water consumption
of the power plant in large volume. For steam power plants, a well-known solution is
to change the wet cooling tower to a dry one, which reduces water consumption by
more than 95%. In addition, the fog of wet cooling towers and its combination with
ITPP combustion products cause acidic rain, which must be avoided and prevented

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 131
(see Fig. 1). Also, the blowdown of wet towers causes environmental pollution.
These problems can be resolved by changing the wet to dry system and installing
the Heller tower.
Fig. 1. Wet tower of ITPP and their output plume
One of the indirect dry cooling systems is the Heller cooling tower. This system
was first presented in 1965 by Professor Heller at the University of Budapest,
Hungary. The heat of the water leaving of the condenser of the power plant is
absorbed by the ambient air passing through a number of compact heat exchangers
called the Heller-Forgo in the Heller cooling tower. These heat exchangers are in
the form of a delta with a vertical position in the circular environment around
the cooling tower. The deltas are made of a special type of heat exchanger called
Forgo, which consist of 15-meter columns stacked on top of each other. Fig. 2
shows the Heller tower and a view of a delta. The air flow rate is controlled by
louvers. The Heller cooling tower does not need compensation water, so it does not
(a) Heller cooling tower (b) Forgo heat exchanger
Fig. 2. Heller cooling tower and delta heat exchanger

132 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
pollute the environment and also does not need a fan. Of course, one of the major
disadvantages of the high Heller tower is its inefficiency in high wind speeds and
high ambient air temperature.
Due to the retrofitting of the cooling tower and the installation of a new tower,
many parts of the existing cooling system must be replaced. New circulating water
pumps are required to provide additional heat. Therefore, the effect of changing the
pumps and circulating water piping system on other parts must be evaluated. Many
studies and researches have been done to change the design of the cooling system
of various power plants. Burns et al. [3] discussed the retrofit cost estimation of an
alternative cooling system. They described the characteristics of the cooling tower,
circulating water pumps and pipes, and the changes required in the condenser. Loew
et al. [4] investigated the cost of retrofitting combined cycle power plants in Texas
with alternative cooling systems, either hybrid towers or air-cooled condensers.
They determined the projected annual reductions in water withdrawals and the
cost per gallon of water saved by the retrofit. They concluded that replacing the
once-through cooling with wet recirculating towers has a lower cost per reduced
water withdrawal.
Conradie and Kroger [5] presented the modeling equations of the Heller cool-
ing tower, including mass, momentum, energy, and experimental relations. In an-
other study, they evaluated the optimal performance of the Heller cooling tower
in its economic concept and studied the dimensional changes and size of heat
exchangers used in the Heller towers [6]. Buyz and Kroger [7] studied the improve-
ment of the shape, dimensions and size of Heller towers. Zou et al. [8] presented
a new concept called solar enhanced natural draft dry cooling towers, in which
solar collectors are added to traditional natural draft dry cooling towers to increase
their performance. Bagheri and Nikkhoo [9] have optimized the location of two
new Heller towers that will be added to the Shazand Power Plant (SPP). In their
study, the air flow inside and around the old and new towers has been studied
numerically, and the best optimal location for the constructing two new towers has
been suggested according to wind direction and speed.
In past years, studies that have been done about Heller tower are usually
about improving its performance when the weather is hot, or windy [10–13]. Since
different weather conditions, especially the ambient temperature, have a great effect
on the performance of a dry cooling tower, then the condenser vacuum is also highly
dependent on weather changes. Under the influence of this change in condenser
vacuum, the amount of output power and thermal efficiency of the power plant will
change. Therefore, it is necessary to evaluate the effect of condenser vacuum on
the performance of the steam power plant. By performing such an evaluation, the
effect of changing the wet tower to the dry tower on the thermodynamic cycle of the
power plant will be determined. Alizadeh et al. [14] have evaluated the performance
of Heller tower of the Shahid Montazeri Power Plant (SMPP) in different weather
conditions for condenser vacuum, output power and thermal efficiency of that
power plant. Jahangiri and Rahmani [15] have numerically modeled the Heller

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 133
tower of SMPP. They studied the effects of wind and ambient temperature on the
performance of the Heller tower and the thermodynamic cycle of the power plant.
According to them, the output power is reduced by 25% in high wind speeds and
ambient temperatures. Samani [16] presented a model for a combined cycle power
plant with a Heller dry cooling tower. He checked the output of the power plant in
different operating conditions.
The above studies show that replacing a wet cooling tower with Heller requires
a lot of knowledge and is more difficult than designing a Heller tower for a new
power plant. Changes in turbine, condenser and Heller system installation are some
of the challenging aspects of cooling tower replacement. Here, the design of the dry
Heller tower and the changes of some components of the power plant are studied,
and their qualitative and quantitative effects on the thermal efficiency and output
power of the power plant are investigated. In this study, a unit of ITPP with a nominal
capacity of 320 MW is modeled according to its real data and at the rated load. The
condenser of this unit is of the surface type and a wet cooling tower is designed and
installed to dissipate heat from it. Changing each part of the power plant makes it
possible to achieve changes in efficiency, fuel consumption and output power. In
this study, due to the replacement of the cooling tower, it is possible to determine
any changes in the characteristics of the power plant. Also, the replacement of the
wet cooling tower with natural draft cooling tower is discussed.
2. Heller cooling tower analysis
In the Heller cooling tower design, the temperature of the air and water entering
the heat exchanger are known. Here, using 𝜀–the NTU method for analyzing the
heat exchanger is preferable to the logarithmic thermal difference method.
2.1. Heat transfer analysis of Heller cooling tower
The maximum allowable heat is obtained through a counter-flow heat ex-
changer with the maximum allowable thermal change ((𝑇𝑤,in–𝑇𝑎,in). The maxi-
mum heat exchange 𝑞𝑚𝑎𝑥 and the actual heat exchange 𝑞are obtained through the
following equations:
𝑞max =𝐶min 𝑇𝑤, in −𝑇𝑎,in ,(1)
𝑞=¤𝑚𝑤𝑐𝑝𝑤 𝑇𝑤 ,in −𝑇𝑤 , out=¤𝑚𝑎𝑐𝑝𝑤 𝑇𝑎,out −𝑇𝑎,in ,(2)
𝜀=𝑞𝑞max ,(3)
where 𝜀, the coefficient of performance defined in the actual heat exchange, is
the maximum allowed heat exchange ratio. From Eqs. (1) and (3), the following
equation is the result:
𝑞=𝜀𝐶min 𝑇𝑤,in −𝑇𝑎,in .(4)

134 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
The number of transfer units, NTU, is a dimensionless parameter given as:
NTU =UA𝐶min ,(5)
where UA is the total heat transfer coefficient of the heat exchanger. The following
equation, specific to heat exchangers with finned pipes and cross-sectional flow,
can be used in the design of heat exchangers in Heller cooling towers [17].
𝜀=1−exp "NTU0.22 exp −𝐶𝑟NTU0.78−1
𝐶𝑟#,(6)
𝜀=𝐶min 𝐶max .(7)
The Forgo heat exchanger is used in Heller cooling towers. Louver dampers
are installed to regulate the flow of air passing through the heat exchanger. The
dependence of the total heat transfer coefficient on the heat transfer coefficient
inside and outside the pipe is presented as [6]:
1
UA
=1
𝜂𝑜ℎ𝑜𝐴𝑜
+1
ℎ𝑖𝐴𝑖
+𝛿𝑡
𝑘𝑡𝐴𝑡
+𝑅𝑗.(8)
To have a simple equation, Eq. (8) is rewritten as:
1
𝑈
=1
𝑈𝑖
+1
𝑈𝑜
,(9)
where, 𝑈𝑖and 𝑈𝑜are determined by the following experimental relations [18]:
𝑈𝑖=319 +5.67𝑇𝑤, in +𝑇𝑤 ,out
2¤𝑚0.8
𝑤,(10)
𝑈𝑜=1180 "¤𝑚𝑎
𝐴face 2𝜌0,𝑎
𝜌𝑎,in +𝜌𝑎 ,out 0.64 #0.515
.(11)
According to Fig. 3, the specifications of the Forgo type single column heat
exchanger are listed in Table 1.
Table 1. Specifications of Forgo heat exchanger
Parameter Value Parameter Value
Width (m) 2.4 Free surface of passing air (m2)17.5
Height (m) 15 Fin thickness (mm) 0.33
Depth of airflow (mm) 150 Fin distance (mm) 2.8
Front surface of heat exchanger (m2) 34.5 Number of tubes 240
Inner tubes diameter (mm) 17.1 Number of passes 2
Outer tubes diameter (mm) 18.25 Number of rows 6

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 135
Fig. 3. Forgo heat exchanger
2.2. The Heller tower
The air enters the tower and its upward movement is due to buoyancy forces
caused by its heat gain in the heat exchanger. The air suction through the tower is
calculated by [9,18,20]:
Δ𝑃=𝑔𝐻 Δ𝜌𝑎,(12)
where, Δ𝜌𝑎is the air mass density difference between the inside and outside of
the Heller tower. The tower must be tall enough to compensate for any possible
resistance to air flow. Therefore, the height of the tower is calculated by:
Δ𝑃= Δ𝑃delta +Δ𝑃louver +Δ𝑃exit =𝑔𝐻Δ𝜌𝑎,(13)
in which, Δ𝑃delta is the air pressure drop in passing through the deltas around
the tower, Δ𝑃louver the air pressure drop in the louvers, and Δ𝑃exit is the air
pressure drop exiting the tower shell, which are determined based on experimental
relations [18–20]:
Δ𝑃louver =0.00548 ¤𝑚𝑎
𝐴face
𝐶0.5
𝑘2
,(14)
Δ𝑃delta =h0.147 +0.007 cot2𝛼
2i¤𝑚𝑎
𝐴face
𝐶0.5
𝑘1.76
,(15)
Δ𝑃exit =𝜌air,mean
2𝑔𝐺𝑑
3.6𝜌air,mean
4
𝜋(𝐷2−2)22
,(16)

136 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
where, 𝜃is the delta angle and 𝐶𝑘=𝜌𝑎,0/𝜌𝑎,mean where 𝜌𝑎, 0and 𝜌𝑎,mean are the
standard conditions and average air density, respectively. 𝐺𝑑is the total discharge
of air passing through the deltas in tons per hour and 𝐷out is the diameter of the
outlet of the tower.
The Forgo heat exchangers are placed next to each other with a 40◦to 60◦
delta angle. The diameter of the tower base is determined according to the degree
between the delta columns and the number of heat exchangers installed in a delta
shape around the tower. In practice, some distance is allowed among the heat
exchangers for expansion and contraction, and this automatically increases the
diameter of the tower base. The base diameter is calculated as follows (see Fig. 4):
𝐷base =2𝜅2.4 sin(𝜃/2) + 0.15 cos (𝜃/2)
tan 𝛽+0.15 sin(𝜃/2),(17)
𝛽=360/𝑁delta .(18)
Fig. 4. Delta-type heat exchanger
The diameter of the concrete foundation of the tower is about 1.5 times the
diameter of the base of the tower, which must be considered according to the plant
site. From the balance of forces, it is concluded that the diameter of the base should
be at least 65% of the height and at most equal to the height of the tower [21].
A computer program is developed based on the Heller tower simulation design
and the data available at SMPP and SPP according to the data presented in Table 2.
Table 3compares the output data with the results obtained from the two mentioned
Table 2. SMPP and SPP Heller towers data
Parameter SMPP SPP
Nominal production (MW) 200 325
Circulating water (m3/h) 25200 34000
Entrance water temperature (◦C) 58 60
Exit water temperature (◦C) 48 50
Air humidity (%) 40 35
Entrance air temperature (◦C) 30 30
Entrance air pressure (Pa) 86813 81810
Between delta angle (degree) 60 49.03

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 137
power plants. Comparing the obtained values with real data validates the developed
computer program.
Table 3. Comparison of calculated and real data of SMPP and SPP Heller towers
Parameter SMPP SPP
calculated real calculated real
Number of heat exchangers 238 238 268 264
Tower base diameter (m) 105.91 109 109.73 110
Upper opening diameter (m) 62.07 62 61.83 62
Tower height (m) 121.32 120 150.12 150
3. Thermodynamic simulation of power plant
The ITPP configuration is shown in Fig. 5. The outlet pressure and temperature
of the ITPP boiler are 170 kg/cm2and 540◦C, respectively. The steam flow rate of
the boiler is 1056 Ton/hr. The power plant has six closed heaters, one open heater,
and one boiler with a reheating system. The turbines here include one in high
pressure, one in medium and one in low pressure. The condenser is also surface
type. There is also a flow meter at points 1, 13, and 21 of Fig. 5.
Fig. 5. The schematic of ITPP [22]
Using the law of conservation of mass and thermodynamic laws, the thermo-
dynamic simulation of the power plant has been done. With every change in power
plant parts, output power, efficiency, fuel consumption and other characteristics
are calculated. Therefore, according to the replacement of the cooling tower and
the change of the condenser temperature, the change of the thermal efficiency and
power output of the power plant will be determined. The values of the necessary
parameters at different points of the power plant cycle and nominal load are listed

138 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
in Table 4, which are in accordance with the technical documents of the power
plant.
Table 4. Thermodynamic characteristics and mass flow rate of different points of the power plant
according to Fig. 4at nominal load
Number Pressure (MPa) Temperature (◦C) Enthalpy (kJ/kg) Flow (kg/s)
1 17.462 540 3395 293.33
2 7.823 428.5 3196 37.56
3 3.708 327.4 3034.65 255.78
3’ 3.708 327 3034.64 26
3” 3.708 327 3034.64 229.8
4 3.414 540 3543 229.8
5 1.609 434 3111 19.79
6 0.632 319.8 3334 8.21
7 0.632 319.8 3111 197.4
7’ 0.632 319.8 3111 9.23
8 0.164 212.4 2852 8.21
9 0.0745 74.56 2728.6 8.13
10 0.0294 68.65 2602 8.6
11 0.0095 44.75 2447 172.5
12 0.0096 44.85 188.4 205.9
13 1.464 46.52 190.7 205.9
14 1.65 66.3 287.3 205.9
15 1.544 74 383.7 205.9
16 1.29 125.9 478.4 205.9
17 0.0294 68.6 287.3 8.6
18 0.0745 71.6 383.7 16.34
19 0.156 94.4 478.4 8.21
20 0.6102 165.4 679.4 293.33
21 20.01 167.7 705.1 293.33
22 19.74 203 855.4 293.33
23 16.66 245.7 1061 293.33
24 19.37 292 1303 293.33
25 1.558 172.5 859.9 76.09
26 3.56 207.9 1066 63.56
27 7.58 251 1308 37.56
28 0.632 319.8 3111 10.56
29 0.618 166 2772 2.14
30 0.098 34 142.5 8.42
The temperature and pressure of different points of the cycle according to
Fig. 4and Table 4, as well as the flow rate of points 1, 13 and 21 (where there
is a flow meter in these three points), are the input parameters of the simulation

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 139
program. The control system automatically adjusts the air flow and fuel flow rate.
The thermodynamic properties of all cycle points are shown in Table 4. Continuity
and energy laws in the various components of the power plant cycle and empirical
relationships are used by Engineering Equation Solver (EES) software. In order
to validate the simulation, parameters such as pressure, temperature and flow at
different points of the cycle and turbines output power, pumps consumption, etc., at
different loads have been matched with real values like ITPP. According to Fig. 5,
the energy flow diagram at nominal load is shown in Fig. 6. This diagram shows
the flow rate at the nominal load. In Fig. 6, the amount of heat given to the boiler
is the heat absorbed by the water and steam inside the boiler, and was calculated
using the laws of conservation of energy and mass.
Fig. 6. ITPP power cycle diagram at rated load (numbers are rounded and in MW)
3.1. Effect of condenser temperature on efficiency and output power of ITPP
According to the data related to ITPP, the output power and thermal efficiency
of the ITPP steam cycle are computed based on different condenser temperatures.
The obtained results are presented in Fig. 7, where it can be seen that the increase
Fig. 7. Effect of condenser temperature on output power
and thermal efficiency in ITPP steam cycle

140 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
in condenser temperature can reduce the output power and thermal efficiency of
the power plant units.
3.2. Effect of condenser temperature on Heller tower size
The ambient temperature severely affects the efficiency of the Heller tower, re-
sulting in changes in the temperature of the water leaving the tower, leading to wide
variations in condenser pressure. Due to climatic change, the water temperature of
the cooling tower changes seasonally and as a result the pressure and temperature
of the condenser changes. Fig. 8shows the effect of the ambient temperature on the
outlet water temperature of the Heller tower of the SMPP, where this power plant
is located close to the ITPP.
Fig. 8. The change in the outlet water temperature of the Heller tower
based on the change in the ambient temperature and comparison
with the experimental data of SMPP [19]
Heat is dissipated in natural draft dry cooling towers due to the temperature
difference between the water and the ambient temperature, with great dependence
on climatic and wind conditions. In order to compensate for the increase in ambient
temperature and wind blow, a high condenser temperature should be considered in
the design of the condenser. This fact, in addition to installation costs, significantly
reduces the dimensions of the cooling tower.
The specifications of a Heller cooling tower for a 7 m3/s flow, 30◦C of am-
bient temperature and 25% RH to have a temperature difference of 7.8◦C in in-
let and outlet water temperatures (according to data from ITPP), are shown in
Fig. 9. As the temperature of the condenser increases, the dimensions, the num-
ber of heat exchangers and the size of the Heller cooling tower are drastically
reduced.

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 141
Fig. 9. Effect of condenser temperature on the Heller cooling tower dimensions
4. Heller cooling tower design without changes in turbine
and pumping system
First, the pressure drop coefficients should be determined according to the
second power of the mass flow rate in the condenser. Then, the head drop of the
riser water pipes of the wet cooling tower should be reduced. The pressure drop
levels of the condenser, pipes and Heller tower are calculated based on the water
flow rate. The pressure drop curve in terms of system flow rate (system resistance
curve) is compared with the characteristic curve of the existing pump, and the new
operating point of the system and pump is determined.
4.1. Tower design without increasing condenser temperature
Based on ITPP data, a Heller tower is designed at 30◦C, 25% relative humidity,
and is related to the atmospheric pressure of Isfahan. The circulating water flow
is considered to be 13.167 m3/s. The new inlet and outlet water temperatures of
the cooling system are 45◦C and 37.2◦C, respectively, according to the current
conditions of ITPP. The Heller tower computer program has been developed for
one or more towers with different conditions with variety of water flow rates in
a single or parallel arrangement of towers. Some similar data of SMPP and SPP
without increasing condenser temperature are presented in Tables 5and 6.
Tables 5and 6show that the obtained dimensions do not correspond to what
might be manufacturable, and this is because the design conditions have deviated
from their true path. Due to the small temperature difference between the water and
the environment, the determined number of heat exchangers is very high. Therefore,
the lower water speed in heat exchangers reduces the heat transfer coefficient on
the water side. Considering the mentioned drawbacks, the installation of the Heller
tower assuming no increase in the temperature of the condenser temperature is

142 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
Table 5. Data for a tower similar to SMPP towers without increasing condenser temperature
at a delta angle of 60◦
Flow
(m3/s)
Number of heat
exchangers
Base diameter
(m)
Upper opening
diameter (m)
Height
(m)
12 720 304.9 179.9 165.7
14 840 355.7 209.9 209.5
16 960 406.5 239.8 239.8
Table 6. Designed data for two parallel towers similar to SPP towers without increasing in condenser
temperature at a delta angle 49◦
Flow
(m3/s)
Number of heat
exchangers
Base diameter
(m)
Upper opening
diameter (𝑚)
Height
(m)
6 380 155.2 87.64 171. 1
7 442 180.7 101.92 166.65
8 406 207.4 116.67 163.25
neither logical nor justifiable. As a result, the condenser must operate at a higher
temperature despite reduced power and thermal efficiency.
4.2. Tower design by increasing condenser temperature
For Heller cooling tower design, the water flow rate, inlet and outlet water
temperature, and conditions of the environment must be known. With an estimated
flow rate at 58◦C intake water temperature, the design ambient air temperature
entering the Heller tower for SMPP and SPP is assumed to be 30◦C. By determining
the number of heat exchangers, the pressure drop curve of the new system is
compared with the characteristic curve of the existing pumps and the crossing
point shows the flow rate. This process is repeated several times with different
numbers of towers and arrangements until all the designed and simulated towers
are acceptable and the pump efficiency is kept as high as possible. To calculate
the friction coefficient on the water side of the Forgo heat exchanger, the following
equation is applied [23]:
𝑓=0.25 log 𝑒
3.7𝑑𝑖
+5.74
Re0.9
𝑤−2
.(19)
Using the friction coefficient obtained through Eq. (19), the pressure drop of
the Forgo heat exchanger is calculated from Eq. (20), which is then added to the
pressure drops associated with the condenser and other parts of the water pipes.
ℎ𝑙,Forgo =8𝑓 𝐿 pass ¤𝑚𝑇 , 𝑤 /120𝑁delta 2
𝜋2𝑔𝑑5
𝑖.(20)

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 143
Here 𝑚𝑇 ,𝑤 is the water flow rate entering to Heller tower. The new head loss system
is as follows:
ℎ𝑙,new =ℎ𝑙,other +ℎ𝑙 ,Forgo .(21)
Since the riser pipes in the existing wet tower are eliminated in the new system,
the slope of the pressure drop curve will be lower than the current one. Another
thing is that the temperature difference between the environment and the system
increases. This means that the contribution of water flow in heat dissipation is
reduced. Reducing the flow in this way also causes a further reduction in the
slope of the pressure drop curve. As a result, the flow is higher than before. This
contradiction means that the existing pumps are larger than necessary and must be
replaced with smaller pumps. This issue is clear in Fig. 10. By using the current
pumps and increasing the flow rate, the number of heat exchangers increases and
finally the designed tower is large and structurally unacceptable. As a result, it
cannot be designed using the existing pumps.
Fig. 10. System resistance curves and characteristic curve of pumps
5. Effect of temperature increase on turbine blades
As the power of the turbines increases, the size of the rotors, discs and blades
increases proportionally, which causes problems. Because the last rows of low-
pressure turbines are long, pressure, temperature, and steam quality affect efficiency
and blade life. Increasing the dimensions increases the tension and vibration of the
blades. Reducing the temperature of the turbine exhaust increases the speed of the
steam, which reaches the speed of sound, and the generated sonic waves create
cyclic stresses. Since the longer blades change cross-section in a revolving man-
ner (due to a change from impulsing to reactive conditions), they are exposed to
twisting and rotating forces. Due to the large dimensions and different hardness at
disc connection points, they have different natural vibrating frequencies. In order

144 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
to reduce vibration (self-imposed vibration), a group of a few blades is connected
through a lashing wire. Such groups consist of eight blades in ITPP. The tem-
perature increase at the last row causes the breakage of the lashing wire, which
is the first factor that intensifies the cycle fatigue caused by the separation of the
grouped blades. In a sense, high temperature causes low cycle fatigue through high
centrifuge force in long blades, and in a case where this cycle is rated as overspeed,
the condition is worsened, especially when the chemical environment of the steam
is unsuitable. It causes cracks in the blades that eventually break and cause drastic
vibration in the adjacent blades [24–26].
The increase in exhaust pressure disturbs the steam flow pattern on the blades.
This pattern depends on the steam speed in the turbine stages. Hence, a different
kinetic energy is is converted to work. The increased pressure causes thermal
friction, also known as thermal windage. It should be noted that this thermal
condition is due to the high speed of the blade (about 500 m/s at the top of the
blades). In this condition, the steam flow pattern due to the pressure increase in the
exhaust is such that the steam speed is behind the rotor movement and the blades
fan the steam using the energy released from the other primary stage blades. This
phenomenon not only reduces drastically the efficiency, but also intensifies friction
at the top of the blades.
An increase in the condenser temperature of a low-pressure turbine has a
destructive effect on the blades of the last stage. As the steam is ejected through
the nozzles and passes through the moving blades, it causes the moving blades
to vibrate. The high turbine speed assists this phenomenon, and as a result, the
vibration frequency of the blade increases. A few natural frequencies are inevitable
for these blades. The condition where the steam flow pattern causes the blades to
vibrate is called “stall flutter” and is caused by low vacuum.
6. Design based on increasing condenser temperature
and changing the pump system
The characteristics of ITPP are similar to those of SPP, and we used Heller-
type SPP cooling tower for the new ITPP system. The last row of blades removal
slightly affects the output power of the power plant. Here, two cases with and without
last-row blades of a low-pressure turbine are evaluated for design. In both cases,
the output power and thermal efficiency reduction conditions are compared and
discussed with the existing situation (condenser temperature of 45◦C, see Fig. 7).
6.1. Cooling tower design using existing last row turbine blades
The inlet and outlet water temperature difference in the current condenser is
7.8◦C, which is estimated to be 10°C through the new system, which is related
to SMPP and SPP. It can reach 14◦C for ITPP [22]. An increase in condenser
temperature results in a decrease in output power and thermal efficiency (see Fig. 7)

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 145
compared to the existing 45◦C and output power and thermal efficiency decrease
by 12.68 MW and 1.68%, respectively at the modified 58◦C temperature. Based
on the SPP data, the tower design is based on an ambient temperature of 30◦C,
a circulating water flow rate of 34000 m3/h and a delta angle of 49◦. Considering
the low vacuum of the power plant condenser in the summer and the lack of water
spraying in the ITPP condenser, the speed of the air passing through ITPP heat
exchangers was considered lower than SPP, which increased the number of heat
exchangers. The geometric specifications of the new design are determined and
compared with SPP specifications in Table 7.
Table 7. Specifications of the proposed cooling tower without blades
Parameter Flow rate
(m3/h)
Number of heat
exchangers
Base diameter
(m)
Upper opening
diameter (m)
Height
(m)
calculated 34000 286 116.05 65.4 158.8
suggested 34000 286 116 65 160
SPP 34000 264 110 62 150
6.2. Cooling tower design by removing the last row of blades
The low-pressure turbine in the ITPP is symmetrical, which technically allows
the elimination of the last row blades. The calculations related to the design of
a condenser with a temperature of 58°C are repeated assuming the removal of
the last row blades. In this case, the low-pressure water heater (heater No. 1
in Fig. 5) and the steam extraction located near the last row of blades are also
removed. Calculations of the new conditions of the ITPP thermodynamic cycle
show that the output power, thermal efficiency and condenser temperature will be
301.5MW, 40.79% and 58◦C, respectively. In other words, according to Fig. 7,
the output power is reduced by 17.5 MW and the thermal efficiency is reduced
by 2.36%.
By removing the last row of blades, the increase in heat loss in the turbine
without removing the last row of blades is observed at the condenser temperature
of 58◦C. The design conditions are calculated according to the specifications for
SPP. There are two possibilities to consider: Increasing the temperature difference
in the cooling tower up to 13◦C or increasing the circulating water flow rate up
to 44.36 m3/h in the existing temperature difference of 10◦C. A slight increase in
temperature or water flow rate confirms that at high condenser temperatures, the
efficiency of the last row blades is low compared to low condenser temperature.
The geometric specifications of the proposed Heller cooling tower are pre-
sented in Table 8. In both of the above assumed cases, if the blades are not
removed, there is no significant change in the number of heat exchangers and the
base diameter of the Heller tower. A significant reduction in output power and
thermal efficiency of the power plant is evident compared to the current situation.

146 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
Table 8. Suggested cooling tower specifications with removed blades
Parameter Flow rate
(m3/h)
Number
of heat
exchangers
Based on
diameter
(m)
Upper opening
diameter (m)
Height
(m)
Increasing temperature 34000 290 117.68 66.31 158.34
Increasing flow 34044.36 286 116.05 65.4 158.8
7. Heller cooling tower plant layout study
There is insufficient space to install Heller cooling towers in ITPP, even if
the wet towers are removed. The power plant is located on the eastern side of the
Zayandehrood River and these areas are used for agriculture, so the Heller towers
can be installed there. In Fig. 11, the dashed lines represent the water intake of
the tower while crossing the river, and the circles of the tower and the solid lines
represent the water exiting from the tower. The rectangular section represents the
proposed location of the new pump house and the new pipelines will be on the
internal streets of the plant. The distance between the units and the installation site
of the proposed tower is 250 m. The advantage of this location is that the plant
does not shut down while the new system is being installed.
Fig. 11. Heller tower installation location in ITPP
8. Conclusions
The replacement of Heller tower with ITPP wet towers was evaluated. In all
the different methods of investigation, it was assumed that there is no change in the
structure of the condenser because it was not economical at all. First, the thermal
performance of Heller Tower was analyzed and simulated by a computer program.
The results of the computer program were compared with the Heller Tower of SMPP
and the Heller Tower of SPP and the correctness of the program was confirmed.

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 147
The ideal thermodynamic cycle of the power plant was simulated based on
the characteristics of the 320 MW ITPP units, and the output power and thermal
efficiency of the cycle were obtained based on the condenser temperature in the ideal
state. Then, the effect of the condenser temperature on the size of the Heller tower
was investigated. It was observed that the amount of output power and efficiency has
an inverse relationship with the size of the Heller tower and an agreement should be
made between these two issues. The design was investigated assuming no change in
the temperature of the condenser, turbine and current pumping system and several
modes and arrangements. These tower were large in terms of its dimensions and
size and most of them were structurally inappropriate.
The design was done with the current system and assuming an increase in
condenser temperature and two factors can reduce the head losses in the system
and the reduction of the system head losses leads to an increase in the flow rate.
One is to remove the rising pipes of the wet tower, and the other is to reduce the
necessary flow of circulating water due to the increase in temperature difference
between the tower and the environment. This contradiction is due to the large size
of the existing pumps for the new system, so it was concluded that the new pumps
should be smaller than the existing pumps. After that, the effect of temperature
increase on the turbine blades was investigated and it was stated that the increase
in temperature leads to the creation of various stresses in the blades of the last
row of the low-pressure turbine. Then, the design was carried out assuming an
increase in the condenser temperature and with the specifications of the SPP due
to the closeness of its rated output power to units of ITPP. This design was initially
carried out with the assumption of not removing the blades of the last row of
the low-pressure turbine, as the designed tower was very close to the SPP Heller
tower. In this case, the output power decreased by 12.4 MW and the thermal
efficiency decreased by 1.68% compared to the current state. In the next step,
the design was carried out assuming the removal of the blades of the last row in
two cases. One mode is to increase the temperature difference in the tower, and
the other mode is to increase the flow of circulating water. It was shown that the
case of increasing the temperature of the proposed tower was not significantly
different from the case without removing the last row of blades. The case with
increased flow rate was the same as the case without removing the last row of
blades. The decrease in output power and the decrease in thermal efficiency were
calculated to be 17.5 MW and 2.36%, respectively, in this mode compared to
the current mode. At the end, the placement of the designed Heller towers and
the new pumping system was discussed according to their required and available
space.
Acknowledgements
The authors thank and support the management staff of Isfahan, Shahid Mon-
tazeri, and Shazand power plants.

148 M.H. MALEKMOHAMADI, H. AHMADIKIA, S. GOLMOHAMADI, H. KHODADADI
References
[1] B. Dziegielewski and D. Baumann. Tapping alternatives: The benefits of managing urban water
demands. Environment: Science and Policy for Sustainable Development, 34(9):6–41, 2010.
doi: 10.1080/00139157.1992.9930929.
[2] D. Marmer.Water conservation equals energy conservation. Energy Engineering, 115(5):48–63,
2018. doi: 10.1080/01998595.2018.12027708.
[3] J.M. Burns, D.C. Burns, and J.S. Burns. Retrofitting cooling towers: estimates required to
achieve the next level of CWA 316(b) compliance. In Proceedings of the ASME Power Confer-
ence, pages 25–33, 2004. doi: 10.1115/POWER2004-52051.
[4] A. Loew, P. Jaramillo, and H. Zhai. Marginal costs of water savings from cooling system
retrofits: a case study for Texas power plants. Environmental Research Letters, 11(10):104004,
2016. doi: 10.1088/1748-9326/11/10/104004.
[5] A.E. Conradie and D.G. Kröger. Performance evaluation of dry-cooling systems for power
plant applications. Applied Thermal Engineering, 16(3):219–232, 1996. doi: 10.1016/1359-
4311(95)00068-2.
[6] A.E. Conradie, J.D. Buys, and D.G. Kröger. Performance optimization of dry-cooling systems
for power plants through SQP methods. Applied Thermal Engineering, 18(1-2):25–45, 1998.
doi: 10.1016/S1359-4311(97)00020-3.
[7] J.D. Buys and D.G. Kröger. Dimensioning heat exchangers for existing dry cooling towers.
Energy Conversion and Management, 29(1):63–71, 1989. doi: 10.1016/0196-8904(89)90014-9.
[8] Z. Zou, Z. Guan, H. Gurgenci, and Y. Lu. Solar enhanced natural draft dry cooling tower for
geothermal power applications. Solar Energy, 86(9):2686–2694, 2012. doi: 10.1016/j.solener.
2012.06.003.
[9] S. Bagheri and M. Nikkhoo. Investigation of the optimum location for adding two extra Heller-
type cooling towers in Shazand power plant. Proceedings of the 17th IAHR International
Conference on Cooling Tower and Heat, pages. 74–83, Australia, 2015.
[10] W. Peng and O.K. Sadaghiani. Presentation of an integrated cooling system for enhancement
of cooling capability in Heller cooling tower with thermodynamic analyses and optimization.
International Journal of Refrigeration, 131:786–802, 2021. doi: 10.1016/j.ijrefrig.2021.07.016.
[11] M.A. Ardekani, F. Farhani, and M. Mazidi. Effects of cross wind conditions on efficiency of
Heller dry cooling tower. Experimental Heat Transfer, 28(4):344–353, 2015. doi: 10.1080/
08916152.2014.883449.
[12] A. Jahangiri, A. Borzooee, and E. Armoudli. Thermal performance improvement of the three
aligned natural draft dry cooling towers by wind breaking walls and flue gas injection under
different crosswind conditions. International Journal of Thermal Sciences, 137:288–298, 2019.
doi: 10.1016/j.ijthermalsci.2018.11.028.
[13] A.R. Seifi, O.A. Akbari, A.A. Alrashed, F. Afshari, G.A.S. Shabani, R. Seifi, M. Goodarzi,
and F. Pourfattah. Effects of external wind breakers of Heller dry cooling system in power
plants. Applied Thermal Engineering, 129: 1124–1134, 2018. doi: 10.1016/j.applthermaleng.
2017.10.118.
[14] R.A. Kheneslu, A. Jahangiri, and M. Ameri. Interaction effects of natural draft dry cooling
tower (NDDCT) performance and 4E (energy, exergy, economic and environmental) analysis
of steam power plant under different climatic conditions. Sustainable Energy Technologies and
Assessments, 37:100599, 2020. doi: 10.1016/j.seta.2019.100599.
[15] A. Jahangiri and F. Rahmani. Power production limitations due to the environmental effects on
the thermal effectiveness of NDDCT in an operating powerplant. Applied Thermal Engineering,
141:444–455, 2018. doi: 10.1016/j.applthermaleng.2018.05.108.
[16] A.D. Samani. Combined cycle power plant with indirect dry cooling tower forecasting using arti-
ficial neural network. Decision Science Letters, 7:131–142, 2018. doi: 10.5267/j.dsl.2017.6.004.

Evaluation of wet cooling tower replacement by Heller cooling tower in a power plant 149
[17] T.L. Bergman, F.P. Incropera, D.P. DeWitt, and A.S. Lavine. Fundamentals of Heat and Mass
Transfer. John Wiley & Sons, 2011.
[18] Archive of Isfahan Mohammad Montazeri Power Station. Isfahan, Iran, 1984.
[19] H. Ahmadikia and G. Iravani. Numerical and analytical study of natural dry cooling tower in a
steam power plant. Journal of Advanced Materials in Engineering (Esteghlal), 26(1):183–195,
2007. (in Persian).
[20] H.G. Zavaragh, M.A. Ceviz, and M.T.S. Tabar. Analysis of windbreaker combinations on steam
power plant natural draft dry cooling towers. Applied Thermal Engineering, 99:550–559, 2016.
doi: 10.1016/j.applthermaleng.2016.01.103.
[21] K.F. Reinschmidt and R. Narayanan. The optimum shape of cooling towers. Computers &
Structures, 5(5-6):321–325, 1975. doi: 10.1016/0045-7949(75)90039-5.
[22] Isfahan Thermal Power Plant documents, No. C.583 and C.749, Islam Abad Power Plant,
Isfahan, Iran, 1988.
[23] I.H. Shames. Mechanics of Fluids. 4th ed. McGraw-Hill, New York, 2003.
[24] C.R.F. Azevedo and A. Sinátora. Erosion-fatigue of steam turbine blades. Engineering Failure
Analysis, 16(2):2290–2303, 2009. doi: 10.1016/j.engfailanal.2009.03.007.
[25] H. Kim. Crack evaluation of the fourth stage blade in a low-pressure steam turbine. Engineering
Failure Analysis, 18(3):907–913, 2011. doi: 10.1016/j.engfailanal.2010.11.004.
[26] L.K. Bhagi, P. Gupta, and V. Rastogi. Fractographic investigations of the failure of L-1 pressure
steam turbine blade. Case Studies in Engineering Failure Analysis, 1(2):72–78, 2013. doi:
10.1016/j.csefa.2013.04.007.