Study and analysis of the role of Condenser and NDCT on the performance of 220 MWe indian PHWRs discussed.

1. i
A PROJECT REPORT
ON
A STUDY ON ROLE OF CONDENSER AND NATURAL
DRAFTCOOLING TOWER ON THE PERFORMANCE
OF RAPS 3&4
BY
SAIKAT HALDER (8910292)
&
RAVI KUMAR KAPADIYA (8910292)
EXECUTIVE TRAINEE
AT
NPCIL
RAWATBHATA RAJASTHAN SITE

2. ii
ACKNOWLEDGEMENT
We are thankful to Shri. S. Kumar Training Superintendent, RR Site, for
extending valuable training reserve unit facilities.
We bring out profound and overwhelming gratitude to Shri C.S. Tiwari, SCE
(RAPS 3&4) and Shri B.S Dhakad, ACSE (RAPS 3&4), Shri. J.S Shaktawat,
SO/D (TSS Conventional), MMU Unit, Shri R. Pradhan (SO/D), Shri. Arshan
Abid (SO/D) and Shri A. Sharan (SO/C) for their relentless guidance, valuable
suggestions, constant encouragement throughout this dissertation work, which
were of immense help in successful completion of this project.
We take this opportunity to thank NPCIL, RAPS 3&4 and authorities for
providing necessary guidance to carry out this project work.
Finally, we also thank all those who have helped directly and indirectly during
this project and also for the successful completion of the same.

3. iii
ABSTRACT
In a nuclear power plant the efficiency and performance of its individual
equipment and component plays a very vital role in deciding the overall power
plant efficiency. As the overall efficiency of thermal power plant is product of
reactor, turbine, generator and cycle efficiency, where, turbine efficiency is
around 80-90% and generator efficiency is about 98%. But when cycle efficiency
is included the turbo-generator efficiency is reduced to below 40%, due to which
the overall plant efficiency drops to 32-42 %.
As nuclear power plant is based modified Rankine cycle, the most important
parameters of this cycle are pressure and temperature of superheated steam at
inlet of high pressure turbine and pressure and temperature of exhaust steam at
outlet of low pressure turbine. So to increase the cycle efficiency either inlet
parameter is to be increased or outlet parameters to be decreased.
The most of the heat loss in any power plant occurs at the condenser side. This
heat which is lost cannot used for further work done in turbine to rotate the turbo-
generator and produce electricity. So the performance of the condenser plays a
very vital role on deciding the overall performance of power plant. Therefore our
aim in this project is to study, analyse and various factors and parameters which
are effecting the condenser performance and efficiency, calculate its performance
and how its optimum performance can be achieved.
Since the cooling tower is also an important component of power plant, as the
circulating cooling water which coming out of condenser taking the latent heat
from exhaust steam changing its state to water during this heat exchanging
process and losses its heat to the atmosphere .We will also study the how
performance of cooling towers effects the performance of condenser and its
impact on plant efficiency.

4. iv
TABLE OF CONTENTS
Sl. No. TITLE OF CHAPTER PAGE No.
1. INTRODUCTION 1-7
1.1 RAPS 3&4 220MWe System Rankine Cycle 1-2
1.2 Role of condenser in thermal cycle 2
1.3 Principle of Condenser 2-3
1.4 Advantage of Condenser 3-4
1.5 Pressure and its measurement 4-5
1.6 Vacuum and its measurement 5
1.7 Terminal Temperature Difference 5-6
1.8 Initial Temperature Difference 6
1.9 Logarithmic Mean Temperature Difference 6
1.10 Saturation Temperature 6
1.11 Cleanliness Factor 6-7
2 Constructional Details of Condenser 8-14
2.1 Types of Condenser 8
2.2 Materials for Condenser Tubes 8-9
2.3 Tube nest arrangement of Condenser Tubes 9
2.4 Condenser Support 9
2.5 Air extraction System 10-11
2.6 Air leaks in Condenser 11-12
2.7 Condenser Tube leak 12-13
2.8 Online Condenser Tube Cleaning System 13
2.9 Electronic Low Vacuum Unloading 13
2.4 Major Design Parameters of RAPS 3&4 Condenser 13-14
3 Evaluation of Condenser Performance 15-20
3.1 Cooling Water inlet temperature or performance of NDCT 15-18
3.2 Cooling water flow 18
3.3 Change of heat transfer 18-20
3.4 Remedial suggestions to improve Condenser Performance 20
4 Natural Draft Cooling Tower 21-24
4.1 Cooling Towers in Power plant 21
4.2 Principles of Operation 21-22
4.3 Types of Cooling Tower 22-23
4.4 Difference between NDCT and IDCT 23
4.5 Terminologies 23-24
5 Description of NDCT System of RAPS 3&4 25-28
5.1 Natural Draft Cooling Tower 25
5.2 Make up Source 25-26
5.3 Blow Down Requirement 26
5.4 CCW Circuit of RAPS 3&4 26-27
5.5 Design Parameters of NDCT 27
5.6 Components of RAPS 3&4 NDCT 28
6 NDCT Performance Assessment 29-32
6.1 NDCT Performance Parameters 29
6.2 Factors Affecting NDCT Performance 29-31
6.3 Performance Assessment of NDCT 31-32
7 Reference 32

5. v
LIST OF FIGURES
Sl.
No.
Figure Name Page No.
1.1 Schematic of 220MWe RAPS 3&4 standard PHWR 1
1.2 Secondary side T-S Diagram of RAPS 3&4 220 MWe standard PHWR 1
1.3 Pressure Diagram 5
1.4 Terminal Temperature Difference 6
2.1 Single and double pass surface condenser 8
2.2 Condenser Dog-bone joint 9
2.3 Air Extraction System 11
2.4 Bubbler Method 13
3.1 Effect of CW Inlet Temperature on Condenser Vacuum (Unit-3) 16
3.2 Effect of CW Inlet Temperature on Condenser Vacuum (Unit-4) 16
3.3 NDCT performance chart 17
4.1 Typical Cooling Tower Circuit 21
4.2 Schematic of a Cooling Tower 22
4.3 Cross Flow and Counter Flow Natural Draft Cooling Tower 22
5.1 CCW System of RAPS 3&4 27
6.1 Performance of Cooling Tower 30
LIST OF TABLES
Sl.
No.
Table Name Page No.
3.1 Condenser Performance related Data Sheet 15
3.2 CCW inlet temperature or NDCT related performance Data 16-17
3.3 CCW pump data 18
3.4 Condenser Heat Load Data 18
3.5 Condenser Fouling Data 19
3.6 Condenser Vacuum Related Data 20
5.1 CCW Load Data 25
5.2 Blow Down Requirement Data 26
6.1 NDCT Performance Data 32

6. 1
CHAPTER - 1
INTRODUCTION
Condenser serves as the closing link in the thermal cycle. The entire heat energy in the steam entering
the turbine cannot be converted into mechanical work. The unutilized heat energy has to be rejected to
a ‘sink’. Condenser acts as a heat sink in the thermal cycle in which rejection of heat energy takes place
on condensation of exhaust steam of turbine.
1.1)RAPS 3&4 220MWe System Rankine Cycle
RAPS 3&4 are the Pressurized heavy water type reactors design to generate 235 MW electricity (MCR)
by each unit. Design of SSC (system, structure & components) are identical in both units with very few
exceptions. The Schematic of the standard 220MWe are shown below.
Fig 1.1: Schematic of 220MWe RAPS 3&4 standard PHWR
The T-S Diagram of the secondary side is given below.
Fig 1.2: Secondary side T-S Diagram of RAPS 3&4 220 MWe standard PHWR

7. 2
The cycle description of the system is given below.
The nearly dry saturated (maximum allowable wetness 0.276%) steam is taken from steam generator
(SG) goes to HP turbine at a pressure of 40 kg/cm2
and a temperature of 250°C. It is then expanded in
the HP turbine up to 160°C at a pressure of 6 kg/cm2
. Then it goes to Moisture Separator cum Reheater
(MSR) and reheated up to 232.9°C at the same pressure and then it goes to double flow LP turbine. It
is then expanded in the LP turbine up to 50°C (may be changed) and condenser vacuum pressure (design
value 63.5 mmHg). After that it is condensed in the condenser at the same temperature and pressure.
The condensed steam is then pumped by Condensate Extraction Pump (CEP) up to 19.80 kg /cm2
and
reheated through a series of LP heaters (total 5, 4 closed type and one open type i.e. Deaerator) and a
HP heater up to 171°C and goes to the steam generator through Boiler Feed Pump (BFP) at SG pressure
to SG. Steam extraction pressures and temperatures for different heaters (LP &HP) are shown in the
diagram. The different thermodynamic processes occurring in the cycle at different equipment are given
below.
Process 1-2: Pumping of saturated liquid by CEP from condenser pressure.
Process 2-3: Reheating of sub cooled liquid in LP heaters up to deaerator.
Process 3-4: Pumping of sub cooled liquid by BFP up to SG pressure.
Process 4-4a: Reheating of sub cooled liquid in HP heater.
Process 4a-5-6: Heat addition and vaporization of liquid at constant SG pressure in SG.
Process 6-7: Expansion of saturated steam in HP turbine.
Process 7-8: Reheating of expanded wet steam in the MSR up to superheated condition.
Process 8-9: Expansion of superheated steam in LP turbine.
Process 9-1: condensation of expanded wet steam in the condenser up to saturated liquid condition.
1.2)ROLE OF CONDENSER IN THERMAL CYCLE:
Condenser is simply not a closing link, but a vital one in the thermal cycle. Constant temperature heat
rejection is taking place here. The latent heat of exhaust steam of turbine is absorbed as sensible heat
by the circulating water.
To extract maximum work from the steam expanding in the Turbine, expansion of steam should be
high. Since the condenser helps in maintaining high vacuum that is practically possible, maximum work
can be expected. If the steam had exhausted at atmospheric pressure, this would not have become
possible.
1.3)PRINCIPLE OF CONDENSER:
1.3.1. VOLUME OF STEAM:
If water is put into a closed vessel and heated, a quantity of heat known as sensible heat is required to
bring the water to boiling point and if further heat is added to convert the water into steam this is known
as latent heat. The volume of the steam formed is far greater than that of the water and consequently
the pressure in the vessel rises. Thus the application of the latent heat has caused an increase in pressure.
1.3.2. REMOVAL OF HEAT:

8. 3
Now reverse the process and remove some heat by cooling the vessel. During this cooling the latent
heat is removed from the steam which is reduced to water (or condensed) with a consequent fall in
pressure. This removal of latent heat happens on a very large scale in a turbine condenser.
1.3.3. CONDENSER PRESSURE:
The condenser is an airtight vessel where the steam exhausted from the turbine is cooled and condensed.
The condensation is so complete that the pressure inside the condenser is reduced below that of the
atmosphere and this condition is referred to as the vacuum in the condenser.
To maintain this low pressure condition it is essential that any air or other incondensable gases, passing
in to the condenser with the steam must be continuously removed and, in addition to condensing the
steam, the condenser must separate, these gases from the steam for discharge by an ejector or air pump.
1.4)ADVANTAGES OF CONDENSER:
1.4.1. Improvement of thermal cycle efficiency:
The minimum absolute pressure, that is practically possible (or the maximum possible vacuum), which
is maintained in the condenser helps to extract maximum work from the steam expanding in the turbine.
This achieves considerable efficiency increase of the turbine. Overall efficiency of the cycle is raised
due to this.
1.4.2. Easy handling of the working fluid, i.e. by condensing and reuse:
Since the condensed steam is reused, make up water requirement is drastically reduced. The make-up
water rate is very less in normal working conditions, practically it will be high. After exhaust steam
from turbine, which occupies the first place, the drain condensate from LP heaters is the major source
of recovery of the working fluid.
1.4.3. Hot-well is a water reserve and hence provides flexibility in operation:
Even though the cycle is a closed one, constant water levels are maintained in condenser, Deaerator and
Steam generator. System losses are made up continuously.
During start up, shut down, load raising, load reduction or during any emergency situation, the water
storage in the Hot-well serves as a reservoir. This gives flexibility in operation of the plant. During the
situations mentioned above, the incoming fluid rate and outgoing fluid rate may not exactly match but
still the plant can be operated smoothly because the Hot-well serves as a reservoir.
1.4.4. Helping to conserve DM water since the drains are diverted to condenser through flash
tanks:
Many drains are provided in steam and water lines in Turbine area. If these drains are not properly
diverted, wastage of DM water will be there mainly during start up and shut down. Since condenser is
the reserve operating at the lowest pressure in thermal power plant, its serves as the receiving point.
Maximum possible recovery of working fluid is accomplished by diverting all the drain to the flash
tanks, which in turn divert them to the condenser in the form of steam and water.
1.4.5. Thermal cycle water losses are advantageously made up at the Hot-well:
Thermal cycle water losses are made up at the Hot-well. This arrangement has many advantages over
other options.
Advantages of adding make up water at the Hot-well:

9. 4
i) Pumping power required is bare minimum since the pressure required is very small. Higher pressure
will be required to inject the make-up water, if it is done at any other point of the cycle. In fact, without
any pumping, the make-up water can be made to be sucked by the condenser from the CST.
ii) Temperature difference between the make-up water and main condensate water in the condenser hot-
well is the lowest encountered anywhere in the cycle. Hence the problem of thermal shock (violent heat
transfer) is avoided.
iii) As the make-up water undergoes deaeration in the condenser, a portion of the dissolved oxygen
coming along with the make-up water is removed through ejectors.
iv) Since the make-up water is added/injected at the neck of the condenser, i.e., right into the flow of
exhaust steam, condensation of exhaust steam is further improved as the make-up water directly
quenches the exhaust steam.
1.4.6. Provision for steam dumping:
Normally whenever the demand load at the generator terminal decreases the turbine governing system
acts on HP governing valves to reduce the steam from SG to turbine to match the turbine load with the
steam requirement and during this period some excess steam is generated.
In case, when the load rejection force the reactor to operate at the load less than 70% of the previous
power level for more than half an hour, the reactor is poisoned out. Hence, for the survival of the reactor
from getting poised out is such circumstances, the reactor load cannot be brought down below steam
70% of its previous power level, thus producing excess steam in SG than required.
In both the above cases the system has to get rid of excess steam produced in SG. Instead of discharging
this excess steam into atmosphere, thus loosing costly DM water, the excess steam is dumped into
condenser by means of steam dumping system. This facility is provided by two sets CSDVs (Condenser
Steam Dump Valve, 2*30% large CSDVs & 2*5% small CSDVs) which connects the condenser and
main steam line.
1.5)PRESSURE AND ITS MEASUREMENT:
Pressure may be defined as the force per unit area applied in a direction perpendicular to the surface of
an object.
Mathematically, pressure may be expressed as:
p = F/A
Where: p is the pressure, F is the force and A is the area.
Everyday pressure measurements are usually made relative to ambient air pressure. In other cases
measurements are made relative to a vacuum or to some other specific reference. When distinguishing
between these zero references, the following terms are used:
Absolute pressure: It is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus
atmospheric pressure.
Gauge pressure: It is zero-referenced against ambient air pressure, so it is equal to absolute pressure
minus atmospheric pressure. Negative signs are usually omitted. To distinguish a negative pressure, the
value may be appended with the word "vacuum" or the gauge may be labelled a "vacuum gauge."
Differential pressure: It is the difference in pressure between two points.
From above figure we can establish the following relations:-

10. 5
1. Absolute pressure = Gauge pressure + Atmospheric pressure
i.e., Pabs = Pg + Patm
2. Vacuum pressure = Atmospheric pressure – Absolute pressure
i.e., Pvac = Patm - Pabs
1.3: Pressure Diagram
1.6) VACUUM AND ITS MEASUREMENT:
Vacuum is sub-atmospheric pressure. It is measured as the pressure depression below atmospheric. The
term vacuum in the case of a condenser means pressure below atmospheric pressure. It is generally
expressed in mm of Hg (mercury).The vacuum is measured by means of a vacuum gauge (Kentometer).
Usually for calculation purpose the vacuum gauge reading is taken as 730 mm of Hg (as of elevation of
RAPS 3&4 from sea level given in design manual) as follows:
Corrected vacuum in mm of Hg = (730-absolute pressure in mm of Hg)
= 730- (actual barometric reading – actual vacuum)
Vacuum Efficiency: - It is defined as the ratio of the actual vacuum to the maximum obtainable vacuum
Vacuum Efficiency =
𝐴𝑐𝑡𝑢𝑎𝑙 𝑣𝑎𝑐𝑢𝑢𝑚
𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑜𝑏𝑡𝑎𝑖𝑛𝑎𝑏𝑙𝑒 𝑣𝑎𝑐𝑢𝑢𝑚
Condenser Efficiency: -It is defined as the ratio of the difference between the outlet and inlet
temperatures of cooling water to its difference between the temperature corresponding to the vacuum
in the condenser and inlet temperature of cooling water.
Condenser Efficiency =
𝑅𝑖𝑠𝑒 𝑖𝑛 𝑡𝑒𝑚𝑝.𝑜𝑓 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟
𝑇𝑒𝑚𝑝 𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 𝑡𝑜 𝑣𝑎𝑐𝑢𝑢𝑚 𝑖𝑛 𝑐𝑜𝑛𝑑𝑒𝑛𝑠𝑒𝑟 − 𝑖𝑛𝑙𝑒𝑡 𝑡𝑒𝑚𝑝.𝑜𝑓 𝑐𝑜𝑜𝑙𝑖𝑛𝑔 𝑤𝑎𝑡𝑒𝑟
1.7) TERMINAL TEMPERATURE DIFFERENCE (T.T.D.):
The temperature difference between the exhaust steam and the cooling water is least at the top of the
condenser where the cooling water leaves. Here the cooling water has its highest temperature. This
particular temperature difference is very important and is given a special name. It is called the terminal
temperature difference. The important point is that any increase in this terminal difference leads directly
to increase in the saturation temperature of the exhaust steam and a higher back pressure.
T.T.D. = Condensing Steam saturation temperature – Cooling Water outlet temperature
i.e. 𝜃2 = 𝑇3 – T2

11. 6
Fig 1.4: Terminal Temperature Difference
During a period of high air leakage, when air blankets tube surfaces, the absolute pressure, air leakage,
steam temperature and terminal difference will rise and again upon correcting the leakage, will return
to normal. Also, during a period of dirty condenser tubes, the absolute pressure, steam temperature and
terminal difference increases and after cleaning will return to normal.
1.8) INITIAL TEMPERATURE DIFFERENCE (I.T.D.):
It is defined as the difference between saturation temperature of the condensate water in the condenser
and temperature of cooling water coming into the condenser.
I.T.D. = Condensing Steam saturation temperature – Cooling Water inlet temperature
i.e., 𝜽1= T3 – T1
1.9) LOGARITHIMIC MEAN TEMPERATURE DIFFERENCE:
The logarithmic mean temperature difference (LMTD) is used to determine the temperature driving
force for heat transfer in flow systems, most notably in heat exchangers. The LMTD is a logarithmic
average of the temperature difference between the hot and cold streams at each end of the exchanger.
The larger the LMTD, the more heat is transferred. The use of the LMTD arises straightforwardly from
the analysis of a heat exchanger with constant flow rate and fluid thermal properties.
Mathematically it can be given as,
LMTD =
𝜃1− 𝜃2
ln(
𝜃1
𝜃2
⁄ )
1.10) SATURATION TEMPERATURE:
Saturation temperature means boiling point. The saturation temperature is the temperature for a
corresponding saturation pressure at which a liquid boils into its vapour phase. The liquid can be said
to be saturated with thermal energy. Any addition of thermal energy results in a phase transition.
If the pressure in a system remains constant (isobaric), a vapour at saturation temperature will begin to
condense into its liquid phase as thermal energy (heat) is removed. Similarly, a liquid at saturation
temperature and pressure will boil into its vapour phase as additional thermal energy is applied.
1.11) CONDENSER CLEANLINESS FACTOR
The condenser cleanliness factor was one of the parameters to characterize the tube dirt degree, which
indicated that the ratio of actual Heat transfer to ideal heat transfer about the old and the new tubes at
the same flow rates. The condenser cleanliness factor was the average of all the cooling pipe cleanliness
factors.
CF =
𝜟𝑻 𝒂𝒄𝒕𝒖𝒂𝒍
𝜟𝑻 𝒅𝒆𝒔𝒊𝒈𝒏

12. 7
A brand new condenser, on day one, before the CW fouled it would theoretically, then, would show a
cleanliness factor of 100%. The designer then hopes, that once it reaches its initial fouled condition
that it does not exceed his design cleanliness factor of 95% (as our condenser is designed based on 95%
CF) and less than that means loss of performance. As day by day due to scaling in the tube CF starts
decreasing and due to this condenser back pressure falls.

13. 8
CHAPTER -2
CONSTRUCTIONAL DETAILS OF CONDENSER
Since the condenser is one of the critical components of a power plant and it’s also don’t have another
backup condenser, so knowledge of its constructional details of every elements becomes vital before
knowing other details on it. Study of its elements will help us to understand its importance and function
in condenser. In this chapter we will deal with each elements of a condenser in brief to get an idea about
it importance.
2.1) TYPES OF CONDENSER
Condenser is basically a heat exchanger and hence can be of following two types:
2.1.1. Direct contact type (Jet Condenser)
In jet condenser the exhaust steam and water come in direct contact with each other and temperature of
the condensate is the same as that of cooling water leaving the condenser. The cooling water is usually
sprayed into the exhaust steam in a closed vessel to cause rapid condensation.
2.1.2 Surface Condenser
Condensation of exhaust steam takes place on the outer surface of the tube, which are cooled by water
flowing inside them. The condenser essentially consists of shell, which encloses the steam space. Tubes
caring cooling water pass through the steam space. The tubes are supplied cooling water from inlet
water box on one side and discharge, after taking away heat from the steam to outlet water box generally
on the other side. The arrangement is called single pass arrangement. In two pass arrangement cooling
water passes twice through condenser so that the inlet water box and the outlet water box are on the
same side.
Instead of one inlet and one outlet water boxes, there may be two or more pairs of several inlet and
outlet water boxes, each suppling cooling water to separate bundle of tubes. This enables cleaning and
maintenance of part of tubes while turbine can be kept running on a reduced load.
Fig 2.1: Single and double pass surface condenser

14. 9
2.2 MATERIALS FOR CONDENSER TUBES
The selection of tube material depends mainly on quality of cooling water and cost. Copper has
very high heat transfer coefficient but low mechanical strength. Copper alloys such as Brass, Bronze
and Cupro-Nickel are widely used for condenser tubes. Nickel alloy Monel being high corrosion
resistance material is also preferred.
Stainless Steel (SS) tubes have a high strength and good corrosion resistance though heat transfer
coefficient is lower than copper alloys. High strength of SS permits use of tubes of lower wall,
which compensates the effect of lower heat transfer coefficient. SS tubes have an added advantage
that ends can be welded to tube sheet thereby eliminating the probability of leak through the rolled
joint.
2.3) TUBE NEST ARRANGEMENT OF CONDENSER TUBES
To achieve maximum low back pressure to turbine ii is necessary to prevent undercooling of
condensate and to minimise the pressure drop during steam flow through the tune nest.
After the steam condenses the saturated liquid continues to transfer heat to the cooling water as it
falls to the bottom of the condenser or hot well. This is called sub cooling and a certain amount is
desirable. A few degrees of sub cooling prevents condensate pump cavitation. The deference
between saturation temperature for the exiting condenser vacuum and temperature of the
condensate is termed as condensate depression. Excessive condensate depression or under cooling
decreases the operating efficiency of the plant because the sub cooled condensate must be reheated
in the steam generator, which in turn requires more heat from the reactor.
If tube bundles are tightly packed, only a little quantity of steam will reach to the bottom of the
condenser and most of heat will be condensed in the upper part.as the droplets of the condensate
fall through the condenser tube nest and strikes tubes at lower portion of tube nest, they may get
sub cooled.
ample space is provided around tube bundles and the number of path are provided in the tube
bundles to allow steam to promptly penetrate deep in to the tube nest with minimum resistance and
even heat distribution in the tube nest and hence chances of under cooling are reduced. Further
condensate deflector are introduced which collects condensate droplets and direct them away from
the lower tubes so that they fall directly in to the hot well avoiding chances of under cooling.
2.4) CONDENSER SUPPORT
LP cylinder is supported on pedestals and sole plates. Skirt portion of exhaust chamber of LP
cylinder is connected to condenser by flexible dog-bone joint if condenser have rigid foundation.
The flexible joint prevents transmission of forces to LP cylinder caused by expansion of condenser
walls. The impact of movement of condenser (heavy mass) during seismic accident will not be
transmitted to LP cylinder due to failure of flexible joint.
To avoid the weight of condenser being transmitted to the turbine exhaust casing, in spring
supported condenser, the water box, tubes and hot well are filled with water before welding the
main exhaust joint.
Fig 2.2 Condenser Dog-bone joint

15. 10
2.5) AIR EXTRACTION SYSTEM
2.5.1 Need of the system:
Air leaks into the condenser shell through flanges, glands. Some air also comes along with steam,
which has leaked into the exhaust end of the turbine along the shaft. This air affects the performance
of the condenser because of following reasons.
(i) Air forms film over condenser tubes. Air has low thermal conductivity, so it reduces heat
transfer considerably.
(ii) It reduces condenser vacuum and increase the turbine exhaust pressure thus reducing the turbine
output.
As air water vapour mixture approaches the cold tube surface, water vapour condenses. Whereas
Air being non-condensable, forms an air film around condensate film. Since air has low thermal
conductivity, the heat transfer is greatly reduced.
Hence, the air has to be removed continuously from the condenser shell.
If Pst = Saturation pressure of steam at measured shell temp.
Pair = Partial pressure of air inside the shell.
Psh = Measured total pressure of the shell.
Psh in the condenser shell is approximately constant, and the steam and air mixture enter the
condenser in a fixed proportions when steady conditions prevail. As some of the steam is
condensed, the partial pressure of the remaining steam (Pst) decreases and hence partial pressure of
air increases to maintain the same total pressure (Psh). At reduced partial pressure (Pst), the steam
corresponding saturation temperature is below that of incoming steam. So, condensation proceeds
at progressively lower temperature. If air is not removed, Pst further decreases and consequently Tst
decreases which may not be achieved by condenser cooling water. So effective condensation of
steam will not take place which will increase shell pressure i.e. exhaust steam pressure and reduces
Turbine output. For convenience of air removal an air cooler section is provided in the condenser
shell. Most of the condensation is carried out on upper portion of tube banks. At lower portion of
tube banks air removal tube is provided. To guard against excessive water vapour extraction along
with air, the space beneath the air extraction baffles has been provided with its own, cooling tubes
in order to condense as much water vapour as possible and thus preventing its removal from
condenser.
2.5.2 System description:
RAPS-3&4, KGS-1&2, RAPP-5&6, mechanical vacuum pump type unit has been adopted for air
removal due to following advantages:
(i) Since system is independent of steam supply, it can be operated at any time desired during the
start-up of unit with electric power supply only. However steam supply is required for L.P.
Gland sealing during vacuum pulling. So, this arrangement can be utilized for leak checking in
partial vacuum conditions.
(ii) The system is capable of completely automatic operation.
(iii) It operates efficiently over the entire range of vacuum.
The air extraction system consists of the following equipment:
(a) 2 x 100% mechanical vacuum pumps 4212-PM-l00l & 4212-PM- 1002.
(b) 2x100% heat exchangers 4212-HX-1001 & 1002.
(c) Moisture separators 4212-SP-1001 & 1002.

16. 11
(d) Associated valves and piping.
The mechanical vacuum pump is a positive displacement type motor driven pump. The mixture of air,
non-condensable gases and water vapour is removed from the condenser by the mechanical vacuum
pump. The discharge from vacuum pump is separated in a separator and the water is recirculated to
vacuum pump through heat exchanger, where the heat is removed from the CCW. Here CCW absorbs
the heat of compression and condensation. In heat exchanger, CCW rejects heat to NDCT. Water cooled
by CCW serves the purpose of seal cooling and jacket cooling of air extraction pumps.
2 mechanical vacuum pumps one will be operating and other will be standby. Vacuum pump is driven
by 75 KW, 415 V, CL-IV motor with a full load motor speed of 590 rpm. Design capacity of each pump
is 130.8 kg/hr with a suction pressure of 25 mmHg and design suction temperature of 22° C.
There are two heat exchanges, with a design cooling water flow rate of 49.9 m3
/hr. HX is a shell and
tube type. Condenser cooling water (CCW) flows in the tube side of condenser. During starting of the
system, air/non-condensable gases should be evacuated from the condenser at a higher rate to
establish condenser vacuum. For this hogging duty, both pumps will work. When both the pumps are
in operation during hogging duty, the pressure can be reduced to
178 mm of Hg in about 30 min. The approximate total volume of air to be removed during start up is
1800 m3
. After reaching the designed hogging pressure, pumps will be switched off manually. When
vacuum fails standby pump start when condenser vacuum falls to ≤615 mm of Hg.
Fig: 2.3 Air Extraction System
2.6) AIR LEAKS IN CONDENSER
Air leakage into the condenser cusses drop in vacuum. The dog-bone joint is kept submerged with
water to prevent ingress of air. A drop in water level indicates leak in the dog-bone joint.

17. 12
The traditional method of locating air leaks, when turbine is on- load is to pass a lighted candle round
the joints, which are suspected of having a leak. The flame of the candle is drawn towards the place
where air is being drawn into the condenser.
Off-load leak searches are carried out by filling steam space of condenser with water to a level below
the turbine blades. Care must be taken to ensure that the condenser supports have first been set in
the correct position to cater for extra load in the condenser. Fluorescence is often added to water,
and if any leakage take place the fluorescence can be detected by the use of ultra-violet lamp.
2.7) CONDENSER TUBE LEAK
The cooling water (raw water) carries impurity with it. Leakage of cooling water into the condenser
spoils the chemistry of condensate. It increases the conductivity of DM water, used as feed water.
Increase in conductivity can increase the rate of corrosion of all the equipment through which the
feed water flows. Salts present in the raw water when mixed with the feed water accelerates the
formation of scale at tube sheets of SG and other places in the feed water system. Internal leakage
of the cooling water should therefore be detected at earliest. Major probable source of leakage of
cooling water into the condenser are:
1) Tube to tube sheet rolled joint failure.
2) Tube leak due to corrosion and erosion of tubes.
3) Fatigue and stress corrosion cracking of tubes.
The principle methods of detection of leak location are:
1) Bubbler method: This is used for on-load detection.Fig.2.4 shows conventional bubbler
method.one end of the condenser tube is plugged and the other end of the tube is connected to a
flexible tube. The other end of flexible tube is connected to air side of bubbler bottle. Bubbles
are formed when atmospheric air enters the bubble bottle to compensate the air sucked by the
vacuum in leaking tube. A simple manometer can also be used in place of bubbler tube. The
leaking tube will suck the liquid out of glass tube of manometer because of vacuum in the
condenser. Rise in level in manometer will indicate tube leak.
2) The blanket effect:
In this method the tube plate is covered by thin plastic sheeting or by foam. The leaking tube
will tend to pull the foam or sheet in to it.
3) Sonic detection:
As air is drawn in to the leaking tube it creates a supersonic whistle. Microphone placed in the
entrance to the tube detect the whistle, and the resulting signal is amplified.
If on-load detection is not successful it may be required to shut down the turbine. Condenser’s
steam space is filled with DM water to submerge the condenser tube. Water will come in water box
through leaky tube.as mentioned previously, the condenser support spring must be jacked up before
the steam space is filled with DM water to conduct water fill test.

18. 13
Fig 2.4: Bubbler Method
2.8) ONLINE CONDENSER TUBE CLEANING SYSTEM
The condenser is having SS tubes (SS 316L) designed with a cleanliness factor of 95% to maintain
backpressure at its design value. For maintenance of above degree of cleanliness of the tubes and
to remove any incipient scaling on the tube, on line ball recirculation type tube cleaning system for
the condenser is employed. The online condenser tube cleaning system will maintain a circulation
of resilient ball in close loop through the condenser tubes. While passing through the tubes ball gets
deformed there by cleaning the inner surface of the tube. The ball will be injected at the CW inlet
pipe by ball recirculation pumps which subsequently will be distributed evenly throughout the cross
section of condenser tube sheet to effect uniform cleaning of tubes. The balls after passing through
the tube will be taken out of the CW outlet pipes. The balls are then let to the CW inlet pipe once
again for recirculation.
Besides this condenser tubes are cleaned by water jet method in off-load condition.
2.9) ELECTRONIC LOW VACCUM UNLOADING
With the turbine running at normal speed and carrying a given load the governor valve are passing
a certain flow in the turbine and then steam goes to condenser. Any interruption in the supplying of
the cooling water to the condenser while the load remains constant tends to decrease the vacuum in
the condenser.
The governor system is require to detect an increase in the condenser pressure and to reduce the
governor valve opening independently of the designed electrical loading so that the inflow to the
turbine substantially matches the temporarily reduced condenser capacity.
Electronic low vacuum unloader acts on speeder gear motor in case in drop in condenser vacuum
to reduce the load by means of controlling High Pressure Governor Valve (HPGV). It overrides the
governor control in doing so and thereby reduces HPGV opening.
The unloading of the machine starts on 590 mmHg and it continues till 570 mmHg at which point
the governor valve will admit only 10% of the rated steam flow. If condenser vacuum falls further
this unit trips the turbine at the condenser vacuum of 559 mmHg by energizing the trip solenoid of
the trip gear.
2.10) MAJOR DESIGN PARAMETERS OF RAPS 3&4 CONDENSER
1. Type of condenser: Horizontal, surface type, double pass, shell & tube construction
2. Heat load: 4.452 x 108
kcal/hr (518 MW)
3. Total cooling water flow through the condenser tubes: 33230 m³/hr
4. Cooling water velocity: 2.2 m/s
5. Cleanliness factor: 95%
6. Design heat transfer surface area: 20,000 m2
7. Condenser tube: 25.4 mm outer diameter, 0.711mm wall thickness, 13.5 m long, 18938 no. of
tubes & 25.4 mm outer diameter, 1.245mm wall thickness, 13.5 m long: 286 no. of tubes

19. 14
8. Nos. of water boxes: 2 at inlet and 2 at outlet
9. Tube material: Stainless Steel 316 L
10. Condenser shell material: Carbon Steel
11. Design cooling water inlet temperature: 25.9°C
12. Design cooling water outlet temperature: 39.298 °C
13. Design temperature difference across condenser: 13.398°C
14. Temperature of condenser at hot well outlet: 42.60°C
15. Condenser Back pressure: 63.5 mm Hg
16. Steam flow to the condenser: 857164 Kg/hr
17. Design LMTD: 8.52°C

20. 15
CHAPTER-3
EVALUATION OF CONDENSER PERFORMANCE
Power plant steam cycle efficiency largely depends on the condenser vacuum. Higher condenser pressure
results in lower generation of electrical output leading to economical losses to station.
At our station exhaust steam from LP turbine is condensed in a surface type condenser which is cooled
by Condenser Cooling Water. This condenser cooling water is in turn cooled in Natural Draught Cooling
Tower (NDCT). Condenser vacuum is maintained by condensing the steam and continuously removing
the air and other non-condensable gases accumulated in condenser by means of condenser air extraction
pump. The air and non-condensable gases collecting inside the condenser are removed by 2X100%
(capacity) water ring type vacuum pumps, thus maintaining the high heat transfer and low back pressure.
The principle factors affecting condenser performance and efficiency are enlisted below.
3.1)Cooling Water Inlet Temperature or Performance of NDCT
High cooling water inlet temperature (t1) leads to higher saturation temperature and corresponding rise in
condenser saturation pressure (i.e. lower condenser vacuum) for a design specified cooling water
temperature rise and terminal temperature difference. As the t1 is an uncontrollable parameter as it depends
upon the external environmental condition and hence required to be incorporated in the other controllable
operating parameters such as rise in cooling water flow subjected to maximum critical erosion limits.
However, the gain which results from this is almost cancelled out by the additional pumping power
required. In addition to the above (t1) is attempted to be minimized through the cooling towers in which
heat of cooling water is rejected to ambient air but it also depends upon the performance capacity of
NDCT and environmental constraints like dry bulb temperature and relative humidity.
These are two effects caused by the cooling water inlet temperature. The primary one is to alter the steam
saturation temperature by the same amount as the change, assuming all the other factors remain constant.
This, in its turn, will change the corresponding back pressure. The secondary effect is caused by the fact
that the heat transfer of the cooling water film in contact with condenser tubes change with temperature
of the water.
A representative for both the units are taken at different time interval on 25/05/2017. The effect of
variation cooling water of inlet temperature on condenser vacuum is represented in the Fig 3.1 & 3.2 for
both the units is given below.
Unit Time
(hrs)
CW Inlet
Temperature
to condenser
(T1) (°C)
CW Outlet
Temperature
from
condenser
(T2) (°C)
Condenser
Back
Pressure(Pb)
in bar
Saturation
Temperature
(T3) (°C)
t2-t1
(°C)
TTD
(°C)
Unit 3 00:00 32.89 44.8 94.92 49.55 12.91 3.75
05:00 32.42 44.54 94.14 49.26 13.12 3.42
07:00 32.04 45.28 91.67 48.86 13.24 3.58
12:00 34.45 46.97 104.66 51.53 12.52 4.56
15:00 34.61 46.9 106.05 51.8 12.39 4.9
19:00 34.05 46.67 102.81 51.17 12.62 4.5
Unit 4 00:00 33.57 46.38 100.02 50.61 12.81 4.23
05:00 32.32 45.34 94.46 49.96 13.02 4.62
07:00 32.82 45.78 97.7 50.13 12.96 4.35
12:00 35.06 47.57 110.23 52.59 12.51 5.02
15:00 35.15 47.58 111.16 52.76 12.43 5.18
19:00 34.46 47.12 105.56 51.71 12.66 4.59
Table-3.1 Condenser Performance related Data Sheet

21. 16
Fig: 3.1 Effect of CW Inlet Temperature on Condenser Vacuum (Unit-3)
Fig: 3.2 Effect of CW Inlet Temperature on Condenser Vacuum (Unit-4)
Both the above figure clearly depicts that as the cooling water inlet temperature increases, the
corresponding saturation temperature as well as condenser back pressure increases. A comparative value
of change in back pressure due to CCW inlet temperature or NDCT performance is given below.
Sl.
No
Description Unit-3 Unit-4
1. Reactor power (MW) 189 216
2. CCW inlet temperature WB-1 (°C) 34.1 35.8
3. CCW inlet temperature WB-2 (°C) 34.2 35.8
4. CCW outlet temperature WB-1 (°C) 47.1 48.2
5. CCW outlet temperature WB-2 (°C) 47.1 47.2
6. Condenser Back Pressure (Kentometer) (mm Hg) 102.6 106.2
7. Saturation Temperature (w.r.t Condenser Back Pressure) (°C) 52.7 52.9
8. Hot Well Temperature (°C) 52.6 52.6
9. NDCT Range (°C) 12.65 11.9
10. Terminal Temperature Difference (TTD) (°C) 6.4 5.2
90
92
94
96
98
100
102
104
106
108
31.5 32 32.5 33 33.5 34 34.5 35
BackPressure(mmHg)
CCW Inlet Temperature (ºC)
Unit-3
92
94
96
98
100
102
104
106
108
110
112
32 32.5 33 33.5 34 34.5 35 35.5
BackPressure(mmHg)
CCW Inlet Temperature (ºC)
Unit-4

22. 17
11. Dry Bulb Temperature (°C) 40 40
12. Wet Bulb Temperature (°C) 28 28
13. Relative Humidity (%) 35.0 35.0
14. NDCT approach 6.45 7.8
16. CCW inlet temperature (from NDCT Performance Curve °C) 34 34
17. CCW actual inlet temperature (°C) 34.15 35.8
18. Back Pressure at CCW inlet temperature(w.r.t curve of 0.95 CF
in mmHg)
92 92
19. Back pressure at actual CCW inlet temperature (w.r.t curve 0.95
CF in mmHg)
94 96
20. Loss in Back pressure due to NDCT of CCW inlet temperature
(mmHg)
2 4
Table 3.2 CCW inlet temperature or NDCT related performance Data
Thus as per above analysis due to NDCT performance and external weather condition as inlet
temperature increases causes a loss of 2 mmHg in condenser vacuum in unit-3 and 2 mm Hg for Unit-
4, but as the inlet temperature is more than the design value (25.9°C) due to external weather condition,
a loss of condenser back pressure of 30.5(94-63.5 = 30.5) mm Hg for Unit-3 and 37.5 mm Hg for Unit-
4 is there. The NDCT performance curve is shown, which is given by the manufacturer.
Fig: 3.3 NDCT performance chart

23. 18
By knowing Relative Humidity, Wet Bulb Temperature and NDCT cooling Range, the design CCW
inlet temperature or NDCT outlet temperature can be found out. Here flow to NDCT is considered to
be 100%. Then by taking that value and following that temperature, at 95% Cleanliness Factor, the
design Condenser Back pressure at that CCW inlet temperature can be found out. Similarly by following
the same curve and taking actual CCW inlet temperature, actual back pressure can be found out. The
difference in these two indicates the loss in back pressure due to NDCT performance or CCW inlet
temperature.
3.2) Cooling Water Flow
Reduced cooling water flow shall increase the cooling water temperature rise, which leads to higher
saturation temperature at design terminal temperature difference and corresponding saturation pressure.
Cooling water pressure drop across the condenser tubes reduces the cooling water flow. Higher pressure
drop indicates internal tube surface deposits, which adversely affects the cooling water flow and heat
transfer. Adequate flow can be maintained by higher discharge pressure but heat transfer shall remain
condenser vacuum control parameters. A frequent on line tube cleaning and proper condenser
maintenance can help.
The primary effect of a change of cooling water flow is to alter its temperature rise. The secondary
effect, which operates in the same direction as the primary, results from the change of heat transfer rate,
due to the changed thickness of the cooling water boundary film. As the CCW flow measuring Anubar
is not working properly Power measurements of CCW pumps was carried out, following are the data
of power measurements.
Sl.
No
Pump Unit-3
current
drawn
(Amp)
Unit-4
current
drawn
(Amp)
Voltage
(kV)
Power
Factor
Unit-3
power
drawn
(Kw)
Unit-4
power
drawn
(Kw)
1. P-1 103.10 104.17 6.6 0.9 612.41 618.76
2. P-2 102.91 103.86 6.6 0.9 611.28 617.16
3. P-3 103.63 102.78 6.6 0.9 615.60 614.57
Table 3.3: CCW pump data
From the above data it is clear that total power drawn by each unit’s pump is approximately same for
both the units. Hence we can say that flow of the water in the both the units is about same and there is
no significant effect in condenser performance.
3.3) Change of Heat Transfer
Change in heat transfer due to any reason, proportionately changes the condenser vacuum, which is
briefly described here under.
The following parameters which affects heat transfer are discussed below.
3.3.1) Level on Condenser Hot Well
Level more than that of the design value, covers some of tubes and make them unavailable for
condensation. Condenser vacuum falls in similar way as in case of inadequate cooling water flow
followed by significant sub cooling. Condensate pumps and associated condenser hot well control
system must be maintained healthy enough to keep the condensate level within design specified ranges.
As the hot well level remains approximately same (230 mm WC) well below the condenser tube level,
there is no possibility of condenser tube covering effect due to it.
3.3.2) Steam Flow
Increasing steam flow shall increase the saturation temperature and corresponding saturation pressure.
Steam flow is a very important input parameter which is regulated as per the requirement of electricity

24. 19
demand. Load raising should preferably be restricted within the condenser capability limit to prevent
wastage of heat at higher potential, but as the steam flow is not altered under loaded condition, there is
no effect of it on condenser vacuum.
3.3.3) Internal/External Tube Deposits
Dirty condenser tubes reduce the heat transfer, which adversely reflects upon condenser vacuum. The
effect of the dirty tubes on the heat transfer is to increase the TTD above optimum. The effect of this
loss on vacuum can be minimized by increasing the flow of cooling water through the condenser on
account of increase pumping power. So that we have to observe that this should be less than the gain in
output from the machine. On-line tube cleaning and chlorination of CCW water are effective method to
reduce this problem.
Sl.
No.
Description Unit-3 Unit-4
1. Actual Cleanliness Factor (C.F) 0.79 0.87
2. Back Pressure (at actual CCW inlet temperature and 0.95 CF) (mm Hg) 94 100
3. Back Pressure (at actual CCW inlet temperature and actual CF) (mm Hg) 102 104
4. Total loss in Back Pressure due to fouling (mm Hg) 8 4
Table 3.5: Condenser Fouling Data
Hence, the above data shows an increase in condenser back pressure of 10 mm Hg for unit-3 and 6 mm
Hg for Unit-4 due to fouling or deposits in the condenser tube.
3.3.4) Air Ingress in Condenser
Practically all the air entering the condenser does so through leakages into the turbine spaces which are
under vacuum and can have one or more of the following ill effects on operation:-
a) Air entering to the outside to the condenser tubes adds considerable resistance to the heat flow. To
overcome this, in order to maintain the flow of heat the exhaust temperature must rise. This is known
as air blanketing.
b) The corresponding backpressure will rise as a result of increased exhaust temperature.
c) The condensate temperature in relation to that of the exhaust temperature in a similar manner to the
pressure drop.
Possible areas of air ingress:
LP Turbine Glands, LP Turbine Diaphragms, LP Turbine Parting Plane, CEP Glands, Hot well, Valve
Glands, Flange Joints etc.
The air ingress data has been given below.
Sl.
No.
Parameter Description Unit Unit-3 Unit-4
1. Condenser Vacuum Pump Flow Kg/hr 28 35
2. Loss in Condenser Vacuum mmHg 0 6
Fig 3.6: Condenser Vacuum Related Data
Removal of non-condensable gases from condenser play crucial role in condenser performance. As per
above observation there is no contribution of air ingress in the deterioration in condenser vacuum of
Unit 3 but, in Unit 4 there is a change of 4 mmHg in back pressure due to air ingress.
3.4) Remedial Suggestion to improve Condenser Performance
1. Higher gaseous impurities in the steam can be managed by proper deaeration and chemical
treatment.

25. 20
2. Air ingress can be avoided by frequent leak detection test and effective steam sealing of low
pressure turbine.
3. External tube deposits can gradually increase terminal temperature difference which needs
better de mineralized water quality management.
4. Internal tube deposits causing decreased heat transfer in the condenser can be effectively
minimized by on-line condenser tube cleaning and chlorination of CCW.
5. CCW pump performance has to be continuously monitored for maintaining optimum CW flow
to the condenser.
6. NDCT fills can be checked routinely in every BSD and replaced in case of chocking of water
due to deposition.
7. Condenser air extraction system performance should be monitored continuously and proper
measures should have to be taken in case of any leak on the system or degraded performance
of vacuum pump and heat exchanger.
8. Air leak check to the condenser should be done routinely for any air ingress and proper
maintenance should be taken to resolve it instantly.
9. Load raising should preferably be restricted within the condenser capability limit to prevent
wastage of heat at higher potential.
10. Condenser tube leak detection should be done in every BSD and should be replaced in case of
major tube failure.
11. CCW flow measuring Anubar has to be made in service.

26. 21
CHAPTER-4
NATURAL DRAFT COOLING TOWER
4.1) COOLING TOWERS IN POWER PLANTS:
A cooling tower is equipment used to reduce the temperature of a water stream by extracting heat from
water and emitting it to the atmosphere. Cooling towers make use of evaporation whereby some of the
water is evaporated into a moving air stream and subsequently discharged into the atmosphere. As a
result, the remainder of the water is cooled down significantly. The make-up water source is used to
replenish water lost to evaporation. Cooling towers are able to lower the water temperatures more than
devices that use only air to reject heat, like the radiator in a car, and are therefore more cost-effective
and energy efficient.
Fig: 4.1 Typical Cooling Tower Circuit
4.2) PRINCIPLE OF OPERATION:
Cooling water is pumped from the turbine condenser by the tower pump to the cooling tower. Inside
the tower the water passes through sprinklers, and sprays out in find drops. The water then falls as
droplets, passing over pickings where it is made to present a greater surface area to the cooling air. The
water then falls into the cooling tower pond.
Air is drawn in near the bottom of the tower, either by natural draught or by a fan. The air passes up the
tower and cools the water. Any water droplets which have been carried up by the air are removed by
the water droplet.
The theory of cooling: - As a water droplet falls through the tower, air flows past it and cooling takes
place in three ways:
(a) A small proportion of heat is lost from the droplet by radiation of heat from its surface.
(b) Approximately a quarter to one-third of the heat transfer is by conduction and convection between
the water and the air the amount of heat transferred depends on the temperature of water and air.
(c) The remainder of the heat transfer is by evaporation. As the air evaporates some of the water into
water vapour, the vapour takes with it the latent heat of evaporation. The remaining water therefore
has a lower heat content than it had originally, and is also at a lower temperature.
The amount of evaporation which takes place depends on a number of factors; these include the total
surface area the water presents to the air (the reason the packing design is so important), and the amount
of air flowing. The greater the air flow, the greater the cooling achieved. Besides this it depends upon
ambient atmospheric condition.

27. 22
Fig: 4.2 Schematic of a Cooling Tower
4.3) TYPES OF COOLING TOWERS
This section describes the two main types of cooling towers: the natural draft and mechanical draft
cooling towers.
4.3.1) Natural draft cooling tower:
The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the
ambient air and the hotter air inside the tower. As hot air moves upwards through the tower (because
hot air rises), fresh cool air is drawn into the tower through an air inlet at the bottom. Due to the layout
of the tower, no fan is required and there is almost no circulation of hot air that could affect the
performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers
are mostly only for large heat duties because large concrete structures are expensive.
There are two main types of natural draft towers:
a) Cross flow tower:
Air is drawn across the falling water and the fill is located outside the tower.
b) Counter flow tower:
Air is drawn up through the falling water and the fill is therefore located inside the tower, although
design depends on specific site conditions.
Fig: 4.3 Cross Flow and Counter Flow Natural Draft Cooling Tower
4.3.2) Mechanical draft cooling tower:
Mechanical draft towers have large fans to force or draw air through circulated water. The water falls
downwards over fill surfaces, which help increase the contact time between the water and the air -

28. 23
this helps maximize heat transfer between the two. Cooling rates of mechanical draft towers depend
upon various parameters such as fan diameter and speed of operation, fills for system resistance etc.
Mechanical draft towers are available in the following airflow arrangements:
a. Counter flows induced draft.
b. Cross flow induced draft.
In the counter flow induced draft design, hot water enters at the top, while the air is introduced at the
bottom and exits at the top.
In cross flow induced draft towers, the water enters at the top and passes over the fill. The air, however,
is introduced at the side either on one side (single-flow tower) or opposite sides (Double-flow tower).
4.4) Difference between NDCT and IDCT
Sl.
No
Natural Draft Cooling Tower Induced Draft Cooling Tower
1. Air flow is natural and based on exit and inlet
conditions of air due to density difference.
Cooling is accomplished by draught of air created
by a fan.
2. No power required apart from pumping of
water to the tower.
Power for fan operation is additional
requirement.
3. Maintenance free as far as rotary equipment
are concerned.
Regular maintenance of rotary equipment
required.
4. Generally operates on very low water loading
about 7.5 m3
/hr/m2
max for film fill and
generally up to 6 m3
/hr/m2
for splash fills, as
generally applicable to tropical conditions.
Can operate at very high water loading up to 25
m3
/hr/m2
for film and 19m3
/hr/m2
for splash film.
5. Low operating fill air velocities. Low operating air fill velocities.
6. Tower requires greater area due to lower water
loading.
Tower requires smaller area due to higher water
loading.
7. Larger pumping head is required to cater to
large diameters at the bottom to enable proper
air flow through the system.
Requires smaller pumping head for the same
application.
8. Generally suitable for larger ranges and bigger
approaches than IDCT, generally greater than
4.5° C.
Suitable for lower approaches also.
9. Capacity cannot be enhanced once the tower
is built.
Additional cells can be added or fan can be
operated at higher pitch to increase capacity.
10. Natural draught towers performance are
affected greatly by wind both at discharge and
inlet.
Effect of wind is far less than that of NDCT.
11. Tower sizing is directly affected by relative
humidity of the inlet air.
RH of inlet air has very negligible effect on the
tower performance once the inlet WBT is taken
into consideration.
12. Difficult to repair concrete at great heights due
access related issues and safety aspects.
Easier to repair concrete due to lower heights.
13. Not very suitable for wet climates with high
dry bulb and relative humidity since effective
evaporative cooling cannot be achieved by
natural draft of air.
Suitable for all weather conditions.
4.5) TERMINOLOGIES:
The following basic terms are used in cooling towers:-
4.5.1) Dry Air:
The international joint committee on Psychometrics Data has adopted the following exact composition of
air N2= 78.084%, O2=20.947%, Argon=0.934%, CO2=0.033% (% by volume). Dry air is never found in

29. 24
practice. Air always contains some moisture. Hence the common designation “air” means moist air. The
term “dry air” is used to indicate the water free contents of air having any degree of moisture.
4.5.2) Saturated air:
Air that contains the maximum amount of water vapour it can hold at its saturation temperature and
pressure. For a given temperature, a given quantity of air can be saturated with a fixed quantity of
moisture. At higher temperature, it requires a larger quantity of moisture to saturate it.
4.5.3) Dry-bulb temperature:
The temperature of air measured by an ordinary thermometer is called dry bulb temperature (Tdb),
commonly referred as DBT. When ordinary thermometer is exposed to atmosphere, it indicates the dry
bulb temperature, which is nothing but atmospheric temperature.
4.5.4) Wet-bulb temperature:
It is the temperature measured by the thermometer when the bulb of the thermometer is covered by a
wetted cloth and is exposed to a current of rapidly moving air (Twb), commonly referred as WBT. When
the air comes in contact with the wet cloth it absorbs some moisture and gives up some heat, due to
which the temperature of the air reduces. This reduced temperature measured by the thermometer is
called the wet bulb temperature.
If the moisture content of the air is very low, it will give up more heat to the cloth and wet bulb
temperature of air will also be comparatively low. On the other hand, if the moisture content of air is
high it will lose lesser heat to the air and the wet bulb temperature will be higher. Thus the wet bulb
temperature indirectly indicates the moisture content present in the air. The wet bulb temperature of the
air is always less than the dry bulb temperature of the air,
i.e., Twb< Tdb.
4.5.5) Wet bulb depression:
It is the difference between dry-bulb and wet bulb temperatures (Tdb-Twb).
4.5.6) Relative humidity (RH):
The amount of water vapour in the air at any given time is usually less than that required to saturate the
air. The relative humidity is the percentage of saturation humidity, generally calculated in relation to
saturated vapour density. Relative Humidity can be expressed as follows:
RH = ( 𝑽𝒂𝒑𝒐𝒖𝒓 𝑷𝒓𝒆𝒔𝒔𝒖𝒓𝒆 𝒐𝒇 𝒂 𝒈𝒊𝒗𝒆𝒏 𝒗𝒐𝒍𝒖𝒎𝒆 𝒐𝒇 𝒂𝒊𝒓 𝒂𝒕 𝒂 𝒑𝒂𝒓𝒕𝒊𝒄𝒖𝒍𝒂𝒓 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆
𝑷𝒂𝒓𝒕𝒊𝒂𝒍 𝒑𝒓𝒆𝒔𝒔𝒖𝒓𝒆 𝒐𝒇 𝑺𝒂𝒕𝒖𝒓𝒂𝒕𝒆𝒅 𝒗𝒂𝒑𝒐𝒖𝒓 𝒂𝒕 𝒕𝒉𝒂𝒕 𝒕𝒆𝒎𝒑𝒆𝒓𝒂𝒕𝒖𝒓𝒆
)* 100 %
4.5.7) Carrier’s Equation
When DBT and WBT are known, for calculating the partial pressure of water vapour in air many co-
relations have been proposed of which Dr. Carrier’s equation is most widely used. It is given below:
𝑷 𝒗 = (pg) wb -
(𝑷−𝑷 𝒈)−(𝑻 𝒅𝒃− 𝑻 𝒘𝒃)∗𝟏.𝟖
𝟐𝟖𝟎𝟎−𝟏.𝟑∗(𝟏.𝟖∗𝑻 𝒅𝒃+𝟑𝟐)
Where, (pg) wb = Saturation pressure at wet bulb temperature.
Pv = Partial pressure of water vapour
Pg = Partial pressure of saturated vapour
Tdb = Dry Bulb Temperature in °C
Twb = Dry Bulb Temperature in °C
4.6) DALTON’S LAW OF PARTIAL PRESSURE:
Dalton’s law of partial pressure state that:
In a container in which gas and a vapour are enclosed, the total pressure exerted is the sum of partial
pressure of the gas and partial pressure of the vapour at the common temperature.
Let T = Temperature of mixture of air and water vapour in the container in °C
Pa= Partial pressure of air at temperature T,
Ps= Saturation pressure of water vapour at temperature T
P = Total pressure in the container.
Therefore, P = Pa + Ps

30. 25
Chapter 5
Description of Natural Draft Cooling Tower System of RAPS 3&4
5.1) Natural Draft Cooling Tower
The heat load for the CCW system is from the main condenser and other equipment put in parallel to the
main condenser. The heat load is 4.452 x 108
Kcal/hr. and CCW flow is 33230 m 3
/hr. for main
condensers. Apart from the condenser heat load the cooling tower will also dissipate the heat from
Non Active Process Water (NAPW) system hot water of which will be discharge into circulating water
discharge pipe down stream of condenser. The heat load and flow for this system is 8.77x 106
kcal/hr and
2106 m 3
/hr. respectively. The different heat loads are given below.
Equipment Flow required (m3
/hr) Heat load (Kcal/hr)
Main Condenser 33230 4.452 x 108
Turbine oil cooler 550 1.06 x 106
Boiler blow down cooler 90 0.9 x 106
Air extraction unit 108 0.4 x 106
Table 5.1: CCW Load Data
Counter flow type wet cooling tower is a direct contact type heat exchanger that facilitates removal of
heat from hot water by ambient air. In this mechanism, hot water is sprayed downwards while the ambient
air moves upward (hence counter flow). In this process a major portion of heat exchange takes place by
evaporation of water into air. However this phenomenon is satisfactorily possible only when the heat
transfer media in the tower provides sufficient surface area and time of contact between air and water. In
a NDCT, which is a tall chimney type structure, natural draught is created due to the density difference
between the air column inside the tower and the ambient air outside the tower. For structural stability,
shell of NDCT is hyperbolic shell of revolution. Due to its hyperbolic shape it helps in reduction of
drift losses.
An open recirculation type cooling water system incorporating a cooling tower is selected since there
is practical difficulty in adopting once through cooling water system due to very high plant site level
compared to reservoir (Rana pratap sagar) level. The static head involved would be very high and
uneconomical. In view of the large amount of heat load and flow involved natural draft cooling tower is
selected which involves no operating cost and no maintenance cost.
The main design parameters defining the duty of NDCT are the total heat load, recirculating water flow,
design wet bulb temperature, design relative humidity and approach to the design wet bulb temperature.
The design dry bulb temperature is 35.6 °C and wet bulb temperature is 26.6 °C. The cooling water flow
governs the size of the CCW pumps cooling water tunnel and hot water piping which is about 36000 m 3
/hr including other small coolers. NDCT will dissipate the heat load from the secondary cycle through
condenser along with small heat load of non-active process water system.
NDCT is wet type hyperbolic reinforced concrete structure consisting of an external shell for creating
a natural draft of air with PVC film type fill. NDCT is divided into two sections; each can be isolated for
maintenance keeping other section in operation with the help of isolation gates. Each section of the basin
is provided with one outlet having two screens and two steel gates. For desludging and draining the basin,
pipe connection with sluice valve and drain sump is provided for each section of the basin. Trash racks
and traveling water screens have also been provided for individual sump for preventing entry of floating
debris etc.
The basin level for NDCT-3 is selected as 97.5m and that for NDCT-4 is selected as 103.5m to suit the
site contour and keeping the relative elevations between the units same.
The main source of firewater is the storage available in the natural draft tower basin (11300 m3) and
C.W intake tunnel (200m3) connecting the basin with C.W pump sumps. The fire water pump house
sumps connected with the CW pump sumps of both the units by 500NB diameter pipes with isolation
valves so that fire water can be made available from any one unit or both unit.
5.2) Make Up Source
The makeup water as well as initial fill up to this system is supplied from the plant water system. The
plant water pumps for units 3&4 are located in CW pump house of RAPS-1&2 for catering make-up
water to NDCT and IDCT of both units. There are two plant water pumps 0-7112-P-1001, 1002, common

31. 26
for both units, each of capacity 3000 m3
/hr. The source of plant water is Rana pratap Sagar Lake into
which the long closed inlet tunnel is extended.
5.3) Blow Down Requirement
In order to maintain Cycle of Concentration (COC) & hence turbidity in CCW water below permissible
limits around 400 m3
/hr blown down from cooling water tunnel i.e. from condenser inlet is carried out.
Part of this blow down water is used for supplying cooling water to the D2O upgrading plant. In addition
CCW system from each unit has an in built blow down of 165m3
/hr in the form of direct discharge to
drainage system from GT air coolers, which is served by NAPW system as the return from NAPW system
is also cooled by the same NDCT. Blow down and make up water data for NDCT (Under Normal
Operation)
Sl.
No
Component Flow (m³/hr)
1. Normal evaporation loss 800
Drift loss 40
Main condenser 33230
Turbine oil coolers 550
Boiler blow down cooler 90
NAPW system 2050
Total flow = 35920, (~36000 m³/hr)
Blow down required for cycle of concentration
equal to 3.0
400
Blow down required for cycle of concentration
equal to 4.0
267
Blow down from transformer oil cooler 165
Total water quantity available for D2O
upgrading plant
235 (400-165)
Table 5.2: Blow Down Requirement Data
Blow down required =
(𝐄𝐯𝐚𝐩𝐨𝐫𝐚𝐭𝐢𝐨𝐧 𝐥𝐨𝐬𝐬𝐞𝐬)
(𝐂𝐎𝐂−𝟏)
5.4) CCW Circuit of RAPS 3&4
Condenser Cooling Water System uses water from natural draft cooling tower basin. The water from the
basin is made available in the sump, located in Cooling Water Pump House (CWPH) through a deep RCC
tunnel by gravity. Three numbers of CCW Pumps 7121-P-1001, P-1002 and P-1003 (capacity of each
pump is 14350 m3
/hr at a Head of 9.2 mWC) takes suction from their respective sumps and individual
pump discharge pipes are connected to make 2 numbers 1800 NB headers for each unit. The two headers
for each unit are mainly buried and supply cooling water to two inlet water boxes of the condenser.
Discharge pipe of the same size from two condenser outlet water boxes return hot water from condenser
to NDCT. (Refer Fig: 5.1)
From the two-inlet headers to the condenser, a part of water is diverted for Turbine oil coolers, Boiler
blow down coolers, Condenser and Air extraction unit heat exchangers. Water after dissipating heat to
the atmosphere in NDCT gets collected in the basin for further recirculation through the system.
The makeup water as well as the initial filling up to this system is supplied from the Plant water system.
The plant water pumps for RAPS-3&4 are located in C.W. pump House of RAPS-1&2 for catering make
up water to NDCT and IDCT for both the units. The Condenser Cooling Water System is open
recirculation type with natural draft Cooling tower dissipating the heat to the atmosphere, which acts as
final heat sink. All The three pumps will be kept running continuously for each unit. The pumps are
vertical, turbine type, single stage self-water lubricated.

32. 27
Blow down lines of 250 NB are taken from 1800 NB supply header of each unit and are interconnected
with each other. A common 250 NB line carrying the water is going to the heat exchangers located in
D2O upgrading plant.
Chlorine dosing has been incorporated in order to prevent biological growth in the system. Intermittent
dosing of chlorine solution is done in the cooling water outlet tunnel in staggering manner.
A simplified schematic of CCW system is given below.
Fig 5.1: CCW System of RAPS 3&4
5.5) Design Parameters OF NDCT
Circulating water flow-36000m3
/hr
Wet bulb.temp-26.60
C
Relative humidity-50%
Total height-119m
Storage capacity-12042m3
Type and material of fill-film type PVC fill
Dry bulb temp-400
C
Evaporation loss-800m3
/hr
Range-12.70
C
Approach-5.80
C
Total number of spray-6000(Tentative)
Spray material-polypropylene
L/G Ratio: 1.76
CCW
SUMP
NAPW
SUMP
P
-
1
0
0
1
P P
RAPS 1&2
PUMP
HOUSE
BBD
COOLER
TURBINE
OIL
COOLER
AIR
EXTRACTION
UNIT
MAIN
CONDENSER
Make up water
N
D
C
T
RCC tunnel
1800 NB Header

33. 28
5.6) COMPONENTS OF NATURAL DRAFT COOLING TOWERS OF RAPS 3&4
5.6.1) Tower features:
The cooling tower shell forms largest structural component. The shape of the tower is defined by two
hyperboloids. The one above the throat is called upper hyperbola and the one below the throat up to air
entry opening is called lower hyperbola. The centre line shell profile is fixed based on the internal
diameters at the air entry level, at the throat level and the thickness of shell at these levels. These
diameters are obtained from thermal design carried out by our thermal consultants, GEA, Germany.
The curvature of the shell at throat for the two hyperbolas is the same.
The shell is supported on 36 pairs of diagonal columns which follow the meridional tangent of the shell
at the air entry level to facilitate smooth flow of axial forces. Each column pair is joined to pedestal
which rests on ring footing. The ring footing forms the foundation of the shell structure and transfers
all the loads to bearing rock.
No major structural connection is provided between any parts of the shell to other ancillary structures.
One peripheral walkway platform above air entry on the top of the fill level is provided to facilitate
inspection and maintenance of the distribution system of the hot water.
The hot water pipe enters the shell at the specified level at two locations. No structural connection
between HW pipes and shell is provided. The pipe will enter the shell through the opening which will
larger than the pipe and will be connected to RCC hot water duct which provide vertical reaction only.
The gap between the shell wall and the pipe will be closed by providing a simple metallic device made
of mild steel and the same will be painted suitably. The internal space will be packed with mineral wool
so as to provide a seal made up of inorganic material.
One peripheral walkway platform at the top of the shell is provided at inside. Handrails will be provided
on the platform. The platform will be formed by approximately 50segment separated from each other
by about 10mm.this is to avoid cracks in the shell due to circumferential rigidity along the walkway
platform.
One ladder with cage will be provided throughout the height. The ladder will be staggered and will
follow the shell profile. The ladder will be located outside the tower from the bottom of the shell up to
throat level and will be inside the tower from the throat level to top of the shell. An opening of 450mm
width and the 2000mm height will be provided at the throat level to facilitate crossing over from the
outside the ladder to inside ladder.an aluminium door of the suitable size will be provided at the opening.
Two RC platform of necessary size will be provide with necessary handrails, on either side of opening.
Four wind screens have been provided in order to reduce losses of cooling water by drift out of the air
inlet and to ensure a uniform air flow through the heat exchanger fill during strong winds. The screens
are located in the tower radially at ninety degree intervals. They extend vertically from the lower edge
of the fill to the pond water level. Laterally they project from the pond edge to the tower centre for a
distance of one quarter of the pond diameter. The screen consist of precast concrete elements fastened
to columns or legs of the support frames.
For balancing the pressure within the cooling tower, openings are provided between the individual wind
screen elements allowing the air to pass between neighbouring tower sectors and to avoid a dead space
on the leeward side of the vertical wind screen.
5.6.2) Access to top of the top of fill:
One RCC spiral staircase outside the cooling tower leads to access door permitting entrance to the
tower. The top of the two hot water distribution ducts serves as regularly walkway. A circumferential
walkway is provided. One central walk way is also provided to facilitate access to the middle of tower.
Permanent hand railing made up of hot dip galvanized steel will be provided for all walkways. A RC
spiral staircase is provided outside the tower for access in to cooling tower to inspect the fill. Aluminium
doors is provided in the shell at this entry point.
Aviation warning lights will be provided. Lighting arrester will be provided. Suitable earthling system
will be designed.

34. 29
CHAPTER-6
NDCT PERFORMANCE ASSESSMENT
6.1) NDCT PERFORMANCE PARAMETERS
The important parameters, from the point of determining the performance of cooling towers, are:
6.1.1) Cooling range
The extent by which the hot water is cooled in the tower is known as cooling range.
6.1.2) Approach
The difference between the cold water leaving the tower and the wet bulb temperature of air
entering is called ‘Approach’. Presumably the wet bulb temperature is the ambient minimum
temperature.
6.1.3) Heat Load
It is the amount of heat exchanged in a cooling tower between the hot water and the cold air in
calories/minute.
6.1.4) Drift
The windage loss or carry over is otherwise known as drift and this indicates the amount of water
that is carried from cooling towers in the form of fine droplets entrained in the circulation air.
6.1.5) Evaporation Rate
The rate at which the water is being evaporated to cool the hot water is called evaporation rate
and circulating air carries this evaporated water vapour away.
6.1.6) Capacity
The average volume of circulating water that is cooled in the tower at any time is the capacity of
the tower.
6.1.7) Sprinkling density
The rate at which the water is falling through unit fill area of the tower is known as sprinkling
density
6.1.8) Cycles of concentration (C.O.C)
It is the ratio of dissolved solids in circulating water to the dissolved solids in make-up water.
6.1.9) Blow down losses
Blow down depend upon cycles of concentration and the evaporation losses and is given by
relation: Blow Down = Evaporation Loss / (C.O.C. – 1)
6.1.10) Liquid/Gas (L/G) ratio
It is the mass ratio of water (Liquid) flowing through the tower to the air (Gas) flow. Each tower
will have a design water/air ratio. An increase in this ratio will result in an increase of the approach,
that is, warmer water will be leaving the tower.
Thermodynamics also indicate that the heat removed from the water must be equal to the heat
absorbed by the surrounding air:
Let, L/G = liquid to gas mass flow ratio (kg/kg)
T1 = hot water temperature (°C)
T2 = cold water temperature (°C)
h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature (kJ/kg)
h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature (kJ/kg)
L (T1 – T2) = G (h2 – h1)
𝑳
𝑮
=
(𝒉 𝟐 − 𝒉 𝟏)
(𝑻 𝟏 –𝑻 𝟐)
6.2) FACTOR AFFECTING NDCT PERFORMANCE
6.2.1) Capacity
Heat dissipation (in kJ/hr) and circulated water flow rate (m3
/hr) are not sufficient to understand
cooling tower performance. Other factors, which we will see, must be stated along with flow rate m3
/hr.
For example, a cooling tower sized to cool 33000 m3
/hr through a 6°C range might be larger than a
cooling tower to cool 33000 m3
/hr through 4°C range.

35. 30
6.2.2) Range
Range is determined not by the cooling tower, but by the process it is serving. The range at the
exchanger is determined entirely by the heat load and the water circulation rate through the exchanger
and on to the cooling water.
Range (°C) =
𝐇𝐞𝐚𝐭 𝐋𝐨𝐚𝐝 𝐢𝐧 𝐤𝐉/𝐡𝐨𝐮𝐫
𝑾𝒂𝒕𝒆𝒓 𝑪𝒊𝒓𝒄𝒖𝒍𝒂𝒕𝒊𝒐𝒏 𝑹𝒂𝒕𝒆 𝒊𝒏 𝒎 𝟑/𝒉𝒓
L (T1 – T2) = G (h2 – h1)
L =
(𝒉 𝟐 − 𝒉 𝟏)
(𝑻 𝟏 –𝑻 𝟐)
∗ 𝑮
Thus, Range is a function of the heat load and the flow circulated through the system.
Fig 6.1: Performance of Cooling Tower
6.2.3) Cooling Tower
Cooling towers are usually specified to cool a certain flow rate from one temperature to another
temperature at a certain wet bulb temperature. For example, the cooling tower for RAPS 3&4 be
specified to cool 36000m3
/hr with a range of 12.7°C at 26.6°C wet bulb temperature and at 50% RH.
Cold Water Temperature 32.4°C – Wet Bulb Temperature (26.6 °C) = Approach (5.8°C)
As a generalization, the closer the approach to the wet bulb, the more expensive the cooling tower due
to increased size. Usually a 2.8°C approach to the design wet bulb is the coldest water temperature that
cooling tower manufacturers will guarantee. If flow rate, range, approach and wet bulb had to be ranked
in the order of their importance in sizing a tower, approach would be first with flow rate closely
following the range and wet bulb would be of lesser importance.
A) Heat Load:
The heat load imposed on a cooling tower is determined by the process being served. The degree of
cooling required is controlled by the desired operating temperature level of the process. In most cases,
a low operating temperature is desirable to increase process efficiency or to improve the quality or
quantity of the product. In some applications (e.g. internal combustion engines), however, high
operating temperatures are desirable. The size and cost of the cooling tower is proportional to the heat
load. If heat load calculations are low undersized equipment will be purchased. If the calculated load is
high, oversize and more costly, equipment will result. Process heat loads may vary considerably
depending upon the process involved. Determination of accurate process heat loads can become very
complex but proper consideration can produce satisfactory results. On the other hand, air conditioning
and refrigeration heat loads can be determined with greater accuracy.
B) Wet Bulb Temperature:
Wet bulb temperature is an important factor in performance of evaporative water cooling equipment. It
is a controlling factor from the aspect of minimum cold water temperature to which water can be cooled
by the evaporative method. Thus, the wet bulb temperature of the air entering the cooling tower
determines operating temperature levels throughout the plant, process, or system. Theoretically, a

36. 31
cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load.
However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering
air wet bulb temperature, when a heat load is applied. The approach obtained is a function of thermal
conditions and tower capability. Initial selection of towers with respect to design wet bulb temperature
must be made on the basis of conditions existing at the tower site. The temperature selected is generally
close to the average maximum wet bulb for the summer months. An important aspect of wet bulb
selection is, whether it is specified as ambient or inlet. The ambient wet bulb is the temperature, which
exists generally in the cooling tower area, whereas inlet wet bulb is the wet bulb temperature of the air
entering the tower. The later can be, and often is, affected by discharge vapours being recirculated into
the tower. Recirculation raises the effective wet bulb temperature of the air entering the tower with
corresponding increase in the cold water temperature. Since there is no initial knowledge or control
over the recirculation factor, the ambient wet bulb should be specified. The cooling tower supplier is
required to furnish a tower of sufficient capability to absorb the effects of the increased wet bulb
temperature peculiar to his own equipment. It is very important to have the cold water temperature low
enough to exchange heat or to condense vapours at the optimum temperature level. By evaluating the
cost and size of heat exchangers versus the cost and size of the cooling tower, the quantity and
temperature of the cooling tower water can be selected to get the maximum economy for the particular
process.
6.2.4) Range, Flow and Heat Load
Range is a direct function of the quantity of water circulated and the heat load. Increasing the
range as a result of added heat load does require an increase in the tower size. If the cold water
temperature is not changed and the range is increased with higher hot water temperature, the driving
force between the wet bulb temperature of the air entering the tower and the hot water temperature is
increased, the higher level heat is economical to dissipate.
If the hot water temperature is left constant and the range is increased by specifying a lower cold water
temperature, the tower size would have to be increased considerably. Not only would the range be
increased, but the lower cold water temperature would lower the approach. The resulting change in both
range and approach would require a much larger cooling tower.
6.2.5) Approach & Wet Bulb Temperature
The design wet bulb temperature is determined by the geographical location. Usually the design
wet bulb temperature selected is not exceeded over 5 percent of the time in that area. Wet bulb
temperature is a factor in cooling tower selection; the higher the wet bulb temperature, the smaller the
tower required to give a specified approach to the wet bulb at a constant range and flow rate.
6.2.6) Fill Media Effects
In a cooling tower, hot water is distributed above fill media which flows down and is cooled due
to evaporation with the intermixing air. Air draft is achieved with use of fans. Thus some power is
consumed in pumping the water to a height above the fill and also by fans creating the draft. An energy
efficient or low power consuming cooling tower is to have efficient designs of fill media with
appropriate water distribution, drift eliminator, fan, gearbox and motor. Power savings in a cooling
tower, with use of efficient fill design, is directly reflected as savings in fan power consumption and
pumping head requirement.
6.3) PERFORMANCE ASSESSMENT OF NDCT
In operational performance assessment, the typical measurements and observations involved are:
a) Cooling tower design data and curves to be referred to as the basis.
b) Intake air WBT and DBT at each cell at ground level.
c) CW inlet temperature to NDCT.
d) CW outlet temperature from NDCT.
6.4) COOLING TOWER SAMPLE PERFORMANCE CALCULATION
Wet bulb temperature (WBT) Twb = 26.6ºC
i) So, Range = Circulating water temperature at cooling tower inlet - Circulating
Cooling water at the outlet of cooling tower.

37. 32
Design Range= 12.7 °C
Now,
Approach = Circulating cooling water at the inlet of cooling tower - Wet bulb temperature
Design Approach= 5.8 ºC
ii) Effectiveness of cooling tower =
𝑹𝒂𝒏𝒈𝒆
(𝑹𝒂𝒏𝒈𝒆+𝑨𝒑𝒑𝒓𝒐𝒂𝒄𝒉)
∗ 𝟏𝟎𝟎 %
12.7/ (12.7+5.8)
= 68.64 %
.
Unit DBT
(°C)
WBT
(°C)
RH
(%)
NDCT inlet
temperature
(°C)
NDCT outlet
temperature
(°C)
Range
(°C)
Approach
(°C)
Effectiveness
(%)
3 40 28 35 47.1 34.15 12.25 6.45 62.23
4 40 28 35 47.7 35.8 11.9 7.8 60.04
Table 6.1: NDCT Performance Data
7) REFERENCE
1. Condenser Design Manual RAPS 3&4
2. CCW System Design Manual RAPS 3&4
3. Condenser Tube Cleaning System Design Manual RAPS 3&4
4. NDCT Design Basis Report RAPS 3%4 (by L&T)
5. Training Manual on Common Service System Part-2 RAPS 3&4
6. Training Manual on Turbine Auxiliaries and Feed Water System RAPS 3&4
7. COIS and Field Data RAPS 3&4
8. Thermal Engineering By R.K Rajput
9. Study of Cooling Tower from Bureau of Energy Efficiency (Govt. of India)