Performance optimization of Forced Draft Fan of BBGS Unit # 1
1. On
PERFORMANCE OPTIMIZATION OF FORCED DRAFT
FAN
Of
BUDGE BUDGE GENERATING STATION UNIT # 1
By
Saikat Halder
Of
MECHANICAL ENGINEERING DEPERTMENT
INDIAN INSTITUTE OF ENGINEERING SCIENCE AND
TECHNOLOGY, SHIBPUR
Under the guidance of
MR. SAMIR BANDYOPADHYAY
SENIOR MANAGER, MECHANICAL MAINTAINANCE
BUDGE BUDGE GENERATING STATION
CESC LIMITED
In
2. Page I
BUDGE BUDGE GENERATING STATION
CESC LIMITED
PUJALI, BUDGE BUDGE
SOUTH 24 PARGANAS, WEST BENGAL
CERTIFICATE OF DECLARATION
This is to certify that Mr. Saikat Halder, 3rd year undergraduate student of Mechanical
Engineering, Indian Institute of Engineering Science and Technology, Shibpur has
successfully completed the project titled “ Performance Optimization of Forced Draft
Fan “ under my supervision and guidance during the summer internship programme,
UNMESH 2015.
Dated: ……………………………...
(Mr. Samir Bandyopadhyay)
Senior Manager, Mechanical
Maintenance Department
Budge Budge Generating
Station
CESC Limited.
3. Page II
ACKNOWLEDGEMENT
At the onset I must thank all the people at BBGS without whose active support this project
would not have materialized. In view of this I would like to extend my sincere thanks and
gratitude to everyone who has supported me during the ‘UNMESH 2015’ internship
programme.
I take this opportunity to express my sincere thanks to my project guide, Mr. Samir
Bandyopadhyay, Senior Manager, Mechanical Maintenance Department for his
invaluable guidance, advice, constant encouragement and enlightening discussions during the
course of the Summer Internship Programme, Unmesh-2015 at Budge Budge Generating
Station without which it would not have been possible for me to give the progress report in this
shape.
I would also thank Mr. Debashish Chatterjee, Mr. Subir Roy, Mr. Jagannath
Chakrovorty, Mr. Debashish Mandal and Mr. Sujoy Sahu for their constant support and
guidance.
I would also thank my Institution and the HRD Department of CESC Limited without whom
this project would have been a distant reality. I also extend my heartfelt thanks to my family
and well-wishers.
Dated: ………………………………
(Saikat Halder)
B.E 3rd year Undergraduate
Mechanical Engineering Department
IIEST, Shibpur.
4. Page III
CONTENTS
1. An overview of fans
1.1. Fans
1.2. Fan Types
1.3. Basic Fan Theory
2. A Brief Overview of Budge Budge Generating Station
3. Air and Flue Gas Path of BBGS Unit # 1
4. Result and Discussion
4.1. Measurement of various fan parameters
4.2. Fan design efficiency
4.3. Example: Performance Test Calculation on FD Fan
4.4. Discussion and analysis
4.5. Causes of low operating efficiency and high power consumption
4.6. Remedial suggestions for improving the operational efficiency
5. Conclusion
5. Page IV
ILLUSTRATIONS
LIST OF DIAGRAMS
Sl. Description Page Number
1 Centrifugal Fan 2
2 Axial Fan 2
3 Forward-Curved Centrifugal Fan Blades and its Performance Curve 3
4 Radial Blade Centrifugal Fan 3
5 Radial Blade Fan curve 4
6 Radial-Tip Centrifugal Fan 4
7 Backward-Inclined Centrifugal Fans 5
8 Backward-Inclined Centrifugal Air foil Fan and Backward-Inclined Fan Curve 5
9 Propeller Fan and Propeller Fan Curve 6
10 Tube axial Fan and Tube axial Fan Curve 7
11 Vane axial Fan and Vane axial Fan Curve 8
12 Air and Flue gas path of BBGS Unit # 1 16
LIST OF GRAPHS
Sl. Description Page Number
1 Fan Characteristics Curve 10
2 H-Q Curve of FD Fan-A 31
3 H-Q Curve of FD Fan-A 31
LIST OF TABLES
Sl. Description Page Number
1 Differences between Fans, Blower and Compressor 1
2 Fan Efficiencies 2
3 FD Fan parameter and Motor parameter 17-18
4 Parameters of FD Fan-A 21-22
5 Parameters of FD Fan-B 23-24
6 Calculated Data of FD Fan-A 26-28
7 Calculated Data of FD Fan-B 28-30
6. Page V
LIST OF ABBREVIATIONS USED
ASME American Society of Mechanical Engineers
BBGS Budge Budge Generating Station
BMCR Boiler Maximum Continuous Rating
BHEL Bharat Heavy Electricals Limited
BHP Break Horse Power
FD Forced Draught
FDF Forced Draught Fan
HVAC Heating, Ventilation and Air Conditioning
ID Induced Draught
kW Kilowatt
kV Kilovolt
PA Primary Air
RPM Revolutions per Minute
SC System Curve
VFD Variable Frequency Drive
WC Water Column
7. Page VI
Executive Summary
Objective
The objective of this project is to monitor, analyse and optimise the performance of Forced
Draft Fans of Budge Budge Generating Station Unit #1.
Introduction
Energy projects are among the most capital intensive infrastructure investments. Decisions
made today will form our lives for decades, and it is important that these decisions are based
on facts and a proper economic assessment of available options. The global power sector is
facing a number of issues, but the most fundamental challenge is meeting the rapidly growing
demand for energy services in a sustainable way, at an affordable cost and in the
environmentally acceptable manner. This challenge is further compounded by the fact that the
major part of the increase in demand for power and hence in the emissions in the future, will
come from developing countries, who strive to achieve a rapid economic development.
A power plant produces electrical energy and also consumes substantial amount of this energy
in the form of auxiliary consumption. Auxiliary power comprises the power consumption by
all the unit auxiliaries as well as the common station requirement such as station lighting, air
conditioning etc. Plant ‘auxiliaries’ include all motor-driven loads, all electrical power
conversion and distribution equipment, and all instruments and controls.
This auxiliary equipment has a critical role in the safe operation of the plant and equipment
used for auxiliary power are varying for different types of power plant. Reduction of auxiliary
power consumption could thus help increase the efficiency of a power plant.
Forced Draft (FD) fans provide control for draft and forced air zoning of fuel burned furnaces
of steam generation plant of a thermal power plant. Forced Draft (FD) fans are used for
supplying the combustion air into the furnace of a boiler. A good design of fan and its control
system increases plant reliability by improving furnace pressure control and airflow control,
which is most critical control part of combustion control system. In this report, the performance
evolution, monitoring and optimization of Forced Draft fan of BBGS unit #1 has represented.
8. 1
Chapter-1
An overview of fans
1.1 Fans:
Fans provide air for ventilation and industrial process requirements. Fans generate a
pressure to move air (or gases) against a resistance caused by ducts, dampers, or other
components in a fan system. The fan rotor receives energy from a rotating shaft and transmits
it to the air.
Difference between Fans, Blowers and Compressors
Fans, blowers and compressors are
differentiated by the method used to
move the air, and by the system pressure
they must operate against. As per
American Society of Mechanical
Engineers (ASME) the specific ratio –
the ratio of the discharge pressure over
the suction pressure – is used for
defining the fans, blowers and
compressors (see Table 1.1).
1.2 Fan Types
Fan selection depends on the volume flow
rate, pressure, type of material handled, space
limitations, and efficiency. Fan efficiencies
differ from design to design and also by types.
Typical ranges of fan efficiencies are given in
Table 1.2.
Fans fall into two general categories: centrifugal
flow and axial flow.
In centrifugal flow, airflow changes direction
twice - once when entering and second when
leaving (forward curved, backward curved or
inclined, radial) (see Figure 1.1).
In axial flow, air enters and leaves the fan with
no change in direction (propeller, tube axial,
vane axial) (see Figure 1.2).
TABLE 1.1 DIFFERENCES BETWEEN
FANS, BLOWER AND COMPRESSOR
Equipment Specific Ratio Pressure rise
(mm Wg)
Fans Up to 1.11 1136
Blowers 1.11 to 1.20 1136 – 2066
Compressors more than 1.20 –
TABLE 1.2 FAN EFFICIENCIES
9. 2
Figure 1.1: Centrifugal Fan Figure 1.2: Axial Fan
1.2.1 Centrifugal Fans
Centrifugal fans are the most commonly used type of industrial fan. Centrifugal fans
are capable of generating high pressures with high efficiencies, and they can be constructed to
accommodate harsh operating conditions. Centrifugal fans have several types of blade shapes,
including forward curved, radial-blade, radial-tip, backward-inclined, backward-curved, and
air foil. Some centrifugal fan types are capable of serving widely varying operating conditions,
which can be a significant advantage.
I. Forward-Curved Centrifugal Fans
This fan type, shown in Figure 1.3, has blades that curve in the direction of rotation. This
fan type is typically used in applications that require low to medium air volumes at low
pressure. It is characterized by relatively low efficiency (between 60 and 65 percent). This fan
type can operate at relatively low speeds, which translates to low levels of noise. Forward
curved fans are commonly selected because of their small size relative to other fan types. Stress
levels in fans are closely related to operating speed; consequently, forward-curved fans do not
require high-strength design attributes. Their low operating speed also makes them quiet and
well suited for residential heating, ventilation, and air conditioning (HVAC) applications. A
typical performance curve is shown in Figure 1.3. The dip in the performance curve represents
a stall region that can create operating problems at low airflow rates.
Forward-curved fans are usually limited to clean service applications. These fans are typically
not constructed for high pressures or harsh service. Also, fan output is difficult to adjust
accurately (note how the fan curve is somewhat horizontal), and these fans are not used where
airflow must be closely controlled. Forward-curved fans have a power curve that increases
steadily with airflow toward free delivery; consequently, careful driver selection is required to
avoid overloading the fan motor.
10. 3
II. Radial Blade Centrifugal Fan
Shown in Figure 1.4, this type is commonly used in applications with low to medium
airflow rates at high pressures. The flat blade shape limits material build-up; consequently,
these fans are capable of handling high-particulate airstreams, including dust, wood chips, and
metal scrap.
This fan type is characteristically rugged. The
simple design of these fans allows many small
metalworking shops to custom build units for
special applications. In many cases, the blades
can be inexpensively coated with protective
compounds to improve erosion and corrosion
resistance. The large clearances between the
blades also allow this fan to operate at low
airflows without the vibration problems that
usually accompany operating in stall. The
characteristic durability of this fan type is a
key reason why it is considered an industry
workhorse. Figure 1.4: Radial Blade Centrifugal Fan
A typical fan curve for radial fans is shown in
Figure 1.5.
Figure 1.3: Forward-Curved Centrifugal Fan Blades and its Performance Curve
11. 4
Figure 1.5: Radial Blade Fan curve
III. Radial-Tip Centrifugal Fan
This fan type fills the gap between clean-air fans and the more rugged radial-blade fans.
Radial-tip fans are characterized by a low angle of attack between the blades and the incoming
air, which promotes low turbulence. A radial
tip fan is shown in Figure 1.6.
Radial-tip fans have many of the characteristics
of radial-blade fans and are well-suited for use
with airstreams that have small particulates at
moderate concentrations and airstreams with
high moisture contents. Radial-tip fans can
have efficiencies up to 75 percent. These fans
are commonly used in airborne-solids handling
services because they have large running
clearances.
Figure 1.6: Radial-Tip Centrifugal Fan
IV. Backward-Inclined Centrifugal Fans
This fan type is characterized by blades that tilt away from the direction of rotation.
Within backward-inclined fans are three different blade shapes: flat, curved, and air foil. Flat
blade types, shown in Figure 1.7, are more robust. Curved-blade fans tend to be more efficient.
Air foil blades, shown in Figure 1.8, are the most efficient of all, capable of achieving
efficiencies exceeding 85 percent. Because air foil blades rely on the lift created by each blade,
this fan type is highly susceptible to unstable operation because of stall.
12. 5
A consequence of backward-incline blade orientation is a low angle of impingement with the
airstream. This promotes the accumulation of particulates on the fan blades, which can create
performance problems. Thin air foil blades are more efficient than the other blade types because
of their lower rotating mass. However, this thin walled characteristic makes this fan type highly
Susceptible to erosion problems. Loss of blade wall thickness can lead to cavity formation in
the blades, which can severely interfere with fan performance.
A common application for backward-inclined fans is forced-draft service. In these applications,
the fan is exposed to the relatively clean
airstream on the upstream side of the process.
The high operating efficiencies available
from this fan type can provide low system
life-cycle costs. A typical performance curve
is shown in Figure 1.8. The motor brake
horsepower increases with airflow for most
of the performance curve but drops off at
high airflow rates. Because of this non-
overloading motor characteristic, this fan
type is often selected when system behaviour
at high airflow rates is uncertain.
Figure 1.7: Backward-Inclined Centrifugal Fans
’
Figure 1.8: Backward-Inclined Centrifugal Air foil Fan and Backward-Inclined Fan Curve
1.2.2 Axial Fans
The key advantages of axial airflow fans are compactness, low cost, and light weight. Axial
13. 6
fans are frequently used in exhaust applications where airborne particulate size is small, such
as dust streams, smoke, and steam. Axial fans are also useful in ventilation applications that
require the ability to generate reverse airflow. Although the fans are typically designed to
generate flow in one direction, they can operate in the reverse direction. This characteristic is
useful when a space may require contaminated air to be exhausted or fresh air to be supplied.
I. Propeller Fans
The simplest version of an axial fan is the propeller type, shown in Figure 1.9. Propeller
fans generate high airflow rates at low pressures. Because propeller fans do not generate much
pressure, they are usually not combined with extensive ductwork. Propeller fans tend to have
relatively low efficiencies, but they are inexpensive because of their simple construction.
Propeller fans tend to be comparatively noisy, reflecting their inefficient operation.
As shown in Figure 1.9, the power requirements of propeller fans decrease with increases in
airflow. They achieve maximum efficiency, near-free delivery, and are often used in rooftop
ventilation applications.
Figure 1.9: Propeller Fan and Propeller Fan Curve
II. Tube axial Fans
A more complex version of a propeller fan is the tube axial fan. This type, shown in Figure
1.10, is essentially a propeller fan placed inside a cylinder. By improving the airflow
characteristics, tube axial fans achieve higher pressures and better operating efficiencies than
propeller fans. Tube axial fans are used in medium-pressure, high airflow rate applications and
are well-suited for ducted HVAC installations. The airflow profile downstream of the fan is
uneven, with a large rotational component. This airflow characteristic is accompanied by
moderate airflow noise. Tube axial fans are frequently used in exhaust applications because
they create sufficient pressure to overcome duct losses and are relatively space efficient. Also,
14. 7
because of their low rotating mass, they can quickly accelerate to rated speed, which is useful
in many ventilation applications. The performance curve for tube axial fans is shown in Figure
1.10. Much like propeller fans, tube axial fans have a pronounced instability region that should
be avoided. Tube axial fans can be either connected directly to a motor or driven through a belt
configuration. Because of the high operating speeds of 2-, 4-, and 6-pole motors, most tube
axial fans use belt drives to achieve fan speeds below 1,100 revolutions per minute.
Figure 1.10: Tube axial Fan and Tube axial Fan Curve
III. Vane axial Fans
A further refinement of the axial fan is the vane axial fan. As shown in Figure 1.11, a vane
axial fan is essentially a tube axial fan with outlet vanes that improve the airflow pattern,
converting the airstream’s kinetic energy to pressure. These vanes create an airflow profile that
is comparatively uniform.
Vane axial fans are typically used in medium- to high-pressure applications, such as induced
draft service for a boiler exhaust. Like tube axial fans, vane axial fans tend to have a low
rotating mass, which allows them to achieve operating speed relatively quickly. This
characteristic is useful in emergency ventilation applications where quick air removal or supply
is required. Also, like other axial fans, vane axial fans can generate flow in reverse direction,
which is also helpful in ventilation applications. Depending on the circumstances, these
applications may require the supply of fresh air or the removal of contaminated air. Vane axial
fans are often equipped with variable pitch blades, which can be adjusted to change the angle
of attack to the incoming airstream. Variable pitch blades can change the load on the fan,
providing an effective and efficient method of airflow control.
As shown in Figure 1.11, vane axial fans have performance curves that have unstable regions
to the left of the peak pressure. These fans are highly efficient. When equipped with air foil
15. 8
blades and built with small clearances, they can achieve efficiencies up to 85 percent. Vane
axial fans are frequently connected directly to a motor shaft.
Figure 1.11: Vane axial Fan and Vane axial Fan Curve
1.3. Basic Fan Theory
1.3.1. Principle of Operation
System that require air flow are normally supplied by one or more fans of
various types, driven by a motor.
The motor rotates the fan which delivers air to the system as it develops a
pressure in the ductwork (or air pathways) that causes the air to move through
the system.
Moving air in a streamline has energy due to the fact that it is moving and it is
under pressure.
In terms of air movement, Bernoulli’s theorem states that static pressure plus velocity pressure
at a point upstream in the direction of air flow is equals to the static pressure plus velocity
pressure as measured at a point downstream in the direction of air flow plus the friction and
dynamic losses between the two measuring point.
The motor imparts energy to the fan, which in turn transfers energy to the moving air. The duct
system contains and transports the air. This process causes some losses in static pressure due
to friction with walls and changes in the direction of flow (due to elbow and other fittings), as
well as air losses through unintentional leaks.
16. 9
1.3.2. System Characteristics
The term "system resistance" is used when referring to the static pressure. The system
resistance is the sum of static pressure losses in the system. The system resistance is a function
of the configuration of ducts, pickups, elbows and the pressure drops across equipment-for
example back filter or cyclone. The system resistance varies with the square of the volume of
air flowing through the system. For a given volume of air, the fan in a system with narrow ducts
and multiple short radius elbows is going to have to work harder to overcome a greater system
resistance than it would in a system with larger ducts and a minimum number of long radius
turns. Long narrow ducts with many bends and twists will require more energy to pull the air
through them. Consequently, for a given fan speed, the fan will be able to pull less air through
this system than through a short system with no elbows. Thus, the system resistance increases
substantially as the volume of air flowing through the system increases; square of air flow.
Conversely, resistance decreases as flow decreases. To determine what volume the fan will
produce, it is therefore necessary to know the system resistance characteristics. In existing
systems, the system resistance can be measured. In systems that have been designed, but not
built, the system resistance must be calculated.
1.3.3. Fan Characteristics
Fan characteristics can be represented in form of fan curve(s). The fan curve is a
performance curve for the particular fan under a specific set of conditions. The fan curve is a
graphical representation of a number of inter-related parameters. Typically a curve will be
developed for a given set of conditions usually including: fan volume, system static pressure,
fan speed, and brake horsepower required to drive the fan under the stated conditions. Some
fan curves will also include an efficiency curve so that a system designer will know where on
that curve the fan will be operating under the chosen conditions (see Figure 1.12). In the many
curves shown in the Figure, the curve static pressure (SP) vs. flow is especially important.
The intersection of the system curve and the static pressure curve defines the operating point.
When the system resistance changes, the operating point also changes. Once the operating point
is fixed, the power required could be found by following a vertical line that passes through the
operating point to an intersection with the power (BHP) curve. A horizontal line drawn through
the intersection with the power curve will lead to the required power on the right vertical axis.
In the depicted curves, the fan efficiency curve is also presented.
17. 10
Figure 1.12: Fan Characteristics Curve
1.3.4. Fan Laws
Rotational speed: Fan rotational speed is measured in terms of Revolution per Minute
(RPM). Fan rotational speed affects fan performance, as shown by the following fan laws. Air
flow rates vary in direct proportion to the rotational speed of the fan:
𝐴𝑖𝑟𝑓𝑙𝑜𝑤 𝑓𝑖𝑛𝑎𝑙 = 𝐴𝑖𝑟𝑓𝑙𝑜𝑤𝑖𝑛𝑖𝑡𝑖𝑎𝑙 *
𝑅𝑃𝑀 𝑓𝑖𝑛𝑎𝑙
𝑅𝑃𝑀𝑖𝑛𝑖𝑡𝑖𝑎𝑙
…………………………….. (1)
Pressure built up by the fan varies as the square of the rotational speed of the fan:
𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒𝑓𝑖𝑛𝑎𝑙 = 𝑃𝑟𝑒𝑠𝑠𝑢𝑟𝑒𝑖𝑛𝑖𝑡𝑖𝑎𝑙 * (
𝑅𝑃𝑀 𝑓𝑖𝑛𝑎𝑙
𝑅𝑃𝑀 𝑖𝑛𝑖𝑡𝑖𝑎𝑙
)
2
…………………… (2)
Power required by the fan varies with the cube power of the rotational speed of the fan:
𝑃𝑜𝑤𝑒𝑟𝑓 𝑖𝑛𝑎𝑙 = 𝑃𝑜𝑤𝑒𝑟𝑖𝑛𝑖𝑡𝑖𝑎𝑙 * (
𝑅𝑃𝑀 𝑓𝑖𝑛𝑎𝑙
𝑅𝑃𝑀 𝑖𝑛𝑖𝑡𝑖𝑎𝑙
)
3
……………………… (3)
Care needs to be taken when using the fan laws to calculate the effect of changes in fan speed,
since these laws apply to a specific density of gaseous medium. When fan speeds changes are
accompanied by significant changes in other parameters such as gas composition, moisture
18. 11
content and temperature, the fan laws will need to be adjusted accordingly to compensate for
the resulting change in medium density.
To avoid overloading the motor, some types of fans must be sized appropriately for the air flow
rate and pressure requirement.in particular, forward-curved blade centrifugal fans, which are
capable of generating high airflow at relatively low speeds, can readily provide excessive
airflow and pressure and overload the motor if operated at too high a speed for the application.
Moreover, operating the fan bellow the required speed can cause insufficient air flow through
the system.
Air stream temperature has an important impact on fan-speed limits because of the effect of
heat on the mechanical strength of most materials.
1.3.5. Fan Efficiency
Performance Terms and Definitions
Static Pressure: The absolute pressure at a point minus the reference atmospheric pressure.
Dynamic Pressure: The rise in static pressure which occurs when air moving with specified
velocity at a point is bought to rest without loss of mechanical energy. It is also known as
velocity pressure.
Total Pressure: The sum of static pressures and dynamic pressures at a point.
Motor Input Power: The electrical power supplied to the terminals of an electric motor drive.
Fan Shaft Power: The mechanical power supplied to the fan shaft.
Fan manufacturers generally use two ways to mention fan efficiency: mechanical
efficiency (sometimes called the total efficiency) and static efficiency. Both measure how well
the fan converts horsepower into flow and pressure.
The equation for determining total efficiency is:
𝜂 𝑡𝑜𝑡𝑎𝑙 % =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑚ᵌ/ sec ∗ 𝛥𝑃 ( 𝑡𝑜𝑡𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒)𝑖𝑛 𝑚𝑚𝑊𝐶
102 ∗ 𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒 𝑓𝑎𝑛 𝑠ℎ𝑎𝑓𝑡
* 100…….………... (4)
The static efficiency equation is the same except that the outlet velocity pressure is not added
to the fan static pressure:
𝜂 𝑠𝑡𝑎𝑡𝑖𝑐 % =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑚ᵌ/ sec ∗ 𝛥𝑃 ( 𝑠𝑡𝑎𝑡𝑖𝑐 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒)𝑖𝑛 𝑚𝑚𝑊𝐶
102 ∗ 𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒 𝑓𝑎𝑛 𝑠ℎ𝑎𝑓𝑡
* 100………………… (5)
Determination of Power Input to the fan shaft
Power Measurement: The power measurements can be done using a suitable clamp- on power
meter. Alternatively by measuring the amps, voltage and assuming a power factor of the power
can be calculated as below:
19. 12
𝑃 = √3 * V*I*cos 𝛷……………………………………………... (6)
Transmission Systems: The interposition of a transmission system may be unavoidable
introducing additional uncertainties.
𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒 𝑓𝑎𝑛 𝑠ℎ𝑎𝑓𝑡 = 𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒 𝑚𝑜𝑡𝑜𝑟 ∗ 𝜂 𝑚𝑜𝑡𝑜𝑟 ∗ 𝜂 𝑡𝑟𝑎𝑛𝑠𝑚𝑖𝑠𝑠𝑖𝑜𝑛 ……... (7)
Best Efficiency Point
The best efficiency point (BEP) is a point on the operating characteristics of the fan where a
fan operates most efficiently and cost-effectively in terms of both energy use and cost of
maintenance/replacement.
Operation of a fan near its BEP results in high efficiency and reduced wear and tear on the
equipment. Operation far away from the BEP results in lower fan efficiency, increased bearing
loads and higher noise levels.
1.3.6. Fan Design and Selection Criteria
Precise determination of air-flow and required outlet pressure are most important in
proper selection of fan type and size. The air-flow required depends on the process
requirements; normally determined from heat transfer rates, or combustion air or flue gas to be
handled.
System pressure requirement is usually more difficult to compute or predict. Detailed analysis
should be carried out to determine pressure drop across the length, bends, contractions and
expansions in the ducting system, pressure drop across filters, drop in branch lines, etc. These
pressure drops should be added to any fixed pressure required by the process (in the case of
ventilation fans there is no fixed pressure requirement). Frequently, a very conservative
approach is adopted allocating large safety margins, resulting in over-sized fans which operate
at flow rates much below their design values and, consequently, at very poor efficiency.
Once the system flow and pressure requirements are determined, the fan and impeller type are
then selected. For best results, values should be obtained from the manufacturer for specific
fans and impellers.
The choice of fan type for a given application depends on the magnitudes of required flow and
static pressure. For a given fan type, the selection of the appropriate impeller depends
additionally on rotational speed. Speed of operation varies with the application. High speed
small units are generally more economical because of their higher hydraulic efficiency and
relatively low cost. However, at low pressure ratios, large, low-speed units are preferable.
20. 13
Chapter-2
A BRIEF OVERVIEW OF BUDGE BUDGE GENERATING STATION
Capacity 750 MW (3 x 250 MW)
Location PUJALI, BUDGE BUDGE, 24 PGS(S), WEST BENGAL
COMMERCIAL GENERATION
Unit # 1 07.10.97
Unit # 2 01.07.99
Unit # 3 28.01.10
Fuel source ECL, BCCL, ICML & Imported Coals
Fuel requirement 2.45 million tons of coal per annum
Mode of transportation Rail
Water source River Hooghly
Land area 225 acres
Ash dumping area 91 acres
Type and Make of Boiler:
Type- Horizontal Single Drum, Natural Circulation, Water Wall Tube, Two Pass, Balanced Draft,
Single Reheat, Pulverized Fuel Boiler with Common Cold PA Fan.
Make- M/S ABB ABL Limited, Durgapur, W.B. (Unit#1&2) M/S BHEL (Unit#3)
Type and Make of Turbine:
Type- Tandem Compounded, Three Cylinder, Single Reheat, Double Flow LP cylinder, Condensing
Type with uncontrolled Extraction.
Make- NEI Parsons, UK.
Type and Make of Generator:
Type- 250 MW, 3Ø Alternator with Hydrogen cooled Rotor and Stator Core and DM water cooled
Stator Windings (Unit 1&2) at a speed of 3,000 rpm.
Make- NEI Parsons.
Others
ISO 9001:2008, ISO 14001:2004 & OHSAS 18001:2007 certified
21. 14
Chapter-3
AIR AND FLUE GAS PATH OF BBGS UNIT # 1
The detailed description of air and flue gas path of BBGS Unit # 1 is described below. There
are three basic type of fans used, namely ID, FD and PA fan. Besides these three types of fans
there are two important fans are there one is seal air fan and other is scanner fan. A brief
description is given below.
2.1. Purpose of Fans
Supply air for combustion in the furnace and for evacuation of the flue gases formed
from the combustion.
Maintain Balanced Draft inside the furnace.
Supply air for cooling of equipment working in hot zones.
Supply air for sealing of gates, feeders & mills bearings etc.
Air used for combustion is divided into 2 parts:
1. Primary Air
Portion of total air sent through mills to the furnace. This air dries the pulverized coal and
transport it to the furnace for combustion.
2. Secondary Air
Large portion of total air sent to furnace to supply necessary oxygen for the combustion.
FD FAN
Supplies secondary air to the furnace through APH to assist in combustion. Supply total air
flow to the furnace except where an independent atmospheric P.A fan is used. Provides air for
sealing requirement and excess air requirement in the furnace.
PA FAN
Supply high pressure primary air through APH needed to dry & transport coal directly from
the coal mills to the furnace.
Primary air for mills is divided into cold & hot primary air.
ID FAN
Suck the gases out of the furnace and throw them into the stack by creating sufficient negative
pressure in the furnace (5-10 mm WC) in the balanced draft units.
Located in between the ESP and Chimney in the flue gas path. Radial Fans -double suction-
backward curved vane with inlet guide vane (IGV) control and VFD control is use in all boilers
22. 15
Handles large volume hot dust/ash laden flue gas (temp up to 150° C) from furnace and all
leakages occurring in the system till the inlet of the fan.
Overcome the pressure drop inside the furnace, Super heater, Re -heater, Economiser, Gas
ducting & ESP.
Consumes max. power in all boiler auxiliaries as it handles the large volume and heavy pressure
drop of the flue gas.
SCANNER AIR FANS
Scanner Air Fan is belong to Centrifugal Fan category.
Supply cooling air to all the flame scanners at different elevations housed in the furnace for
sensing flame.
A.C scanner fan boost the pressure of cold secondary air from F.D fan discharge duct in normal
operation.
D.C scanner operates only in case of a.c power failure and sucks air directly from the
atmosphere.
SEAL AIR FANS
Seal Air Fan is also belong to Centrifugal Fan category.
Supply seal air at a pressure higher than system or equipment pressure.
Supply seal air to raw coal feeders, mills bearings, gates etc.
Seal fan either boost the pressure of cold secondary air from F.D fan discharge duct or takes
air directly from the atmosphere in normal operation.
ID is "Induced Draft" and FD is "Forced Draft." In an induced draft system, the fan is at the
exit end of the path of flow, and the system is under negative pressure - that is, the pressure in
the flow area is below atmospheric, because the air is being drawn through the fan. In a forced
draft system the fan is at the entry end of the path of flow. It operates at positive pressure
because outside air is drawn into the fan and forced into the system.
The detailed diagram of air and gas path of BBGS Unit #1 is given in the Figure 3.1.
2.2. Air and Flue gas path of BBGS Unit # 1
The detailed diagram of air and flue gas path of BBGS unit #1 is given in the figure 3.1.
The atmospheric air enters through inlet air damper, filter and silencer to FD Fan A and B at
atmospheric pressure and then it is released at some higher pressure by the FD Fans to a
common discharge bus. From this discharge bus the air is divided into four segments. Two of
which goes to the secondary air heater A and B, where the cold air is pre-heated by the flue
gases coming out from the boiler and then this air goes to the boiler for combustion. Cold air
by-pass arrangement is there.
24. 17
During starting of the boiler after shut down, there is not enough temperature of the flue gasses,
so on that time the cold air is directly by-passed to the boiler through cold air by-pass without
sending it to the secondary air heater. In normal operating condition the cold air by-pass duct
is closed and the cold air is directed to the boiler through secondary air heater.
The other two segments from the common air bus is directed to PA Fan A and B and some
pressure rise took place there. After discharge from PA Fan A and B the air from both fan
comes to a duct and from there by a cold air tapping arrangement some cold air goes to the
mill. Rest of the air goes to the primary air heater and the air is heated by the flue gasses in the
primary air heater. After coming out from the primary air heater the hot gasses also directed to
the mill. In the mill the hot air from primary air heater and cold air from cold air tapping
arrangement before primary air heater get mixed. The temperature of the mixed air is controlled
by controlling the flow of cold and hot air after the PA Fan. The mixed air is then goes to the
mill dries down and flows away the pulverised coal to the boiler.
The hot flue gasses from boiler first goes to the economiser. After coming out eco top and
bottom bank the flue gasses goes to the primary and secondary air heater. From there it comes
to a common duct and then goes to ESP A and ESP B where ash particles present in the flue
gases get separated and then it goes to the atmosphere through ID Fan A, B and C and chimney.
The primary parameters of the forced draft fan and motor are as follows:
FD Fan parameter Motor parameter
Particulars Unit Design parameter Particulars Unit Design parameter
Type Double inlet box,
constant speed
Type SCIM/ 1LA7 716-
6H.70-Z1
Specification Z9 size 207000 Rated voltage kV 6.6
Control
mode
Inlet Louvre
Dampers
Rated power kW 1182
Max Air
Quantity
mᵌ/sec 196.65 Rated speed RPM 995
Air pressure mm
WG
397 No load
current
Amps 41.0
Temperature °C 45 Full load
current
Amps 126.0
Absorbed
power
Kw 1035 Max.
permitted
medium
temperature
°C 70
Rotation
speed
RPM 980 No of pole 6
Size and
design of
bearing
6’’ dia Sturtevant
type H.D. Bearing
Direction of
rotation
Clockwise
Method of
lubrication
Oil Disc Bath with
Circulating System
Method of
cooling
TETV (IC 0151)
25. 18
Manufacturer
Howden Sirocco
limited (Rotating
parts)
ACC Babcock,
Sturtevant
Bearing(Static
parts)
Manufacturer
Bharat Heavy
Electricals Limited
2.3. Principal Parts of FD Fan in BBGS Unit #1
IMPELLER
The impeller is of double inlet design, comprising a centreplate, parallel side plates,
backwardly inclined flat plate blades, hub and hub plates to provide lateral stiffness.
It is an all welded fabrication, constructed from suitable high quality materials, having all welds
inspected by an appropriate non-destructive (N.D.T.) method, before and after stress relieving.
The finished impeller is pressed on and keyed to its shaft.
The complete assembly (rotor) is dynamically balanced.
FAN CASING
The fan casing, complete with inlet boxes, forms a composite fabrication. It is of all welded,
steel plate construction, stiffened adequately where necessary.
Bolted and/or site welded joints are incorporated to suit transportation and installation.
A section is made removable to simplify replacement of the impeller/shaft assembly.
The casing is supported on landings at its base, and these are drilled for the necessary holding
down bolts.
INLET CONE
This is a steel plate, all welded fabrication fitted at the inlet to the impeller. It is shaped so as
to create a smooth passage for the gas flow into the impeller.
The cone is flanged for attachment to a convenient member and its fixing holes are large enough
to allow any necessary adjustment for alignment. After final positioning, the inlet cone is
dowelled to prevent movement.
SHAFT
The impeller shaft is designed to give a critical speed well above its running speed and is
accurately machined from a suitable grade of forged or rolled steel.
The machining surface finish on the panels which take the bearings, seals, impeller and
coupling is to the grade required for the part.
26. 19
SHAFT SEAL
Where the fan shaft penetrates the fan casing, a seal is fitted to reduce the leakage to a
minimum.
It is of split construction, to simplify fitting, and consists of a disc of suitable flexible material,
e.g. Klingerit, supported between a steel carrier and cover plate.
When correctly fitted, the flexible disc would just touch the rotating shaft.
BEARING PEDESTALS
The pedestal is a robustly constructed, all welded, steel fabrication designed to support the
shaft bearing.
It is adequately stiffened and, where necessary, provided with hand holes to give access to
bolts.
COUPLING AND SHAFT GUARDS
Guards are fitted over all exposed running parts and are arranged for easy removal for access
purposes.
SHAFT BEARING
These are self-aligning sleeve bearings installed in robustly constructed cast iron housings.
Lubrication is provided by means of a rotating disc which lifts oil from the sump and deposits
it into the journal area at the top of the sleeve.
LOUVRE DAMPER CONTROL
The inlet Louvre Damper Assembly, which is fitted at the fan inlets, is a multi-bladed louver
damper of robust construction. The blades are of streamlined cross section and, when fully
open, create negligible resistance to gas flow.
The direction of closing of the blades is arranged so that, when partially closed, the gas is
induced to whirl in the direction of impeller rotation. This has the effect of reducing the
pressure volume characteristic of the fan with corresponding reduced power consumption
The blades are arranged in a frame. Each blade is carried by to flanged bearings. One end of
each spindle is extended and carries a lever, these levers are linked together and connected to
an operating shaft, which is connected with the power source, and is common to both fan inlets.
The blades are not individually adjustable.
27. 20
Chapter-4
Result and Discussion
4.1. Measurement of various fan parameters
To assess the performance efficiency of FD fan A and B of unit #1, the readings of the
following parameters were taken:
1. Pressure of air at the discharge to each FD fan at different load.
2. Flow of each FD fan at different load.
3. Current requirement (Amps) of each FD fan at different load.
4. Inlet damper position of each FD fan at different load.
5. The average temperature of the air passing through the FD Fan.
The rated voltage of each fan is taken as 6.6 kV, the motor efficiency is taken as 95%, as there
is no transmission system attached with the motor to fan, the transmission efficiency is taken
as 99% with considering some slippage loss. The power factor of the motor is taken as 0.85 as
supplied by E&I department of BBGS. The suction pressure to the FD fan is considered as
atmospheric suction. The collected data of each fan are tabulated on the next page.
32. 25
Hence the design fan efficiency,
𝜂 𝑑𝑒𝑠𝑖𝑔𝑛,𝑡𝑜𝑡𝑎𝑙% =
𝑉𝑜𝑙𝑢𝑚𝑒 𝑖𝑛 𝑚ᵌ/ sec ∗ 𝛥𝑃 ( 𝑡𝑜𝑡𝑎𝑙 𝑝𝑟𝑒𝑠𝑠𝑢𝑟𝑒)𝑖𝑛 𝑚𝑚𝑊𝐶
102 ∗ 𝑃𝑜𝑤𝑒𝑟 𝑖𝑛𝑝𝑢𝑡 𝑡𝑜 𝑡ℎ𝑒 𝑓𝑎𝑛 𝑠ℎ𝑎𝑓𝑡
* 100 (eqn
. 4)
=
196.65 ∗ 397
102 ∗ 1027
* 100 = 74.53 %
4.3. Example: Performance Test Calculation on FD Fan
The following is a typical report on calculations made for FD Fan with respect to the
measured parameters provided and the fan efficiency is calculated.
a) Measured Parameters (FD Fan)
Mass flow rate = 607.5 tons/hr.
Total pressure (Δp) = 300 mm WC
Air temperature = 34°C (at site condition)
Speed = 980 RPM
b) Measured Parameters (Fan motor)
Rated current = 100 A
Rated voltage = 6.6 kV
Power factor = 0.85
Motor efficiency = 0.95
Transmission efficiency = 0.99
c) Air density & volume flow rate in mᵌ/sec
Air density =
273 ∗ 1.293
273+34
= 1.1498 kg/mᵌ
Calculated volume flow rate =
607.5 ∗ 1000
3600 ∗ 1.1498
= 146.76 mᵌ/sec
d) Power input to the motor
𝑃 = √3 * V*I*cos 𝛷 (eqn
. 6)
= √3 * 6.6 *100 *0.85 kW
= 971.68 kW
37. 30
145.7 297 913.8 46.4
145.8 298 913.8 46.6
146.0 297 913.8 46.5
146.8 300 913.8 47.2
146.5 297 923.0 46.2
147.3 300 923.0 46.9
148.9 304 932.1 47.6
4.4. Discussion and analysis
It is clearly observed from the data that the capacity of the FD Fan was considerably higher
than required. When the unit was operated at full load, the FD Fan-A ran at 60% to 70% of its
rated capacity and FD Fan-B ran at 70% to 80% of its rated capacity. In some cases like at low
load as the air requirement in boiler is less, on that case only FD Fan-A was operated and FD
Fan-B was closed. On such situation also the FD Fan-A ran at a maximum of 85% to its rated
capacity. The actual current to the fan motor was 80% to 85% of the full load rated current
when both the fan was operating at maximum power output.
Data showed that the type of fan was not suitable and the forced draft fan was operated within
a low power output area over a long period of time, thereby increasing power consumption and
increasing costs of operating the unit. Due to this the fan efficiency is also considerably low
with respect to its rated design efficiency in the operating zone. With FD Fan-A efficiency
ranging from 28.3% at low load to 41.9% at full load which is 32.63% lower than its design
efficiency. In case of FD Fan-B also the efficiency range is from 34.8% to 47.6% where an
amount of 26.93% efficiency declination is noted. Also during low load operation, when only
FD Fan-A was operated, the fan efficiency was in between 30% to 35%.
As per the test data, it was observed that the high capacity of the boiler FD Fan is resulting in
the poor operating efficiency of the fan and substantially higher energy consumption. A
considerable negative impact on energy conservation was noted.
Furthermore, the operational performance of the two fans (FD Fan-A & FD Fan-B) was
mismatched; the output of FD Fan-B is far higher than the output of FD Fan-A, e.g. the
maximum operating efficiency of FD Fan-B is 47.6% which is 5.9% higher than FD Fan-A and
minimum operating efficiency of FD Fan-B is 32.83% which is 4.03% more than FD Fan-A.
This poses a threat to the stable operation of the unit.
On the next page, a plot of the design H-Q curve as reproduced in the same scale as supplied
by the manufacturer are given for both the fans. From the calculated data some representative
datas at different loads (from 50% to 100%) are selected, as well as the design points are also
plotted for both the fans as supplied by the manufacturer. In case of FD Fan-A the operating
points are plotted from 50% load to 100% load with 10% load interval and in case of FD Fan-
B they are from 60% load condition to 100% load condition because at 50% or bellow that load
condition, only FD Fan-A was being operated and FD Fan-B was closed.
39. 32
From the two curve it is clear that the design point is very far from the actual operating point.
The operating point for both the fans at full load condition are (128.95 mᵌ/sec, 303 mm WC) f
and (143.59 mᵌ/sec, 299 mm WC) for FD Fan-A and FD Fan-B respectively. That means at
this operating condition both the fans are meeting the system demand of 100% Boiler
Maximum Continuous Rating (BMCR) which is 60% to 80% of its design capacity. Besides
this, in case of loading condition of 50% or bellow this, when only FD Fan-A was operated, in
this case also the maximum capacity not meets the demand of full capacity of the fan
(maximum of 85% of its full capacity). Hence this clearly implies an overdesign of the fan due
to which the fans were operated within an area which are inefficient with respect to its design
flow and power consumption thereby deteriorating the operational efficiency. Moreover this
the different lines shown in the H-Q curve implies H-Q curve of the fan at different inlet damper
opening position (% of damper opening are given at the bottom of each curve), provided by
the manufacturer. But in actual situation the operating points are not matching to its design
curve, for an example, in case of FD Fan-A at a flow of 128.95 mᵌ/sec and a head of 303 mm
WC, the inlet damper position as noted was 72.7%, but in the design curve it fits in between
50% and 60%. This might be due to the unintentional leaks in the system and due to the
corrosion of blades as it is working for a long period. This deviation shows a loss incorporating
in the system.
Besides this, both the fan rotates at the same speed (980 RPM) irrespective of the flow
requirements at different loading condition. Hence, there are not much variation in power
consumption. The power consumption range for FD Fan-A was in between 831.6 kW and 923
kW. In case of FD Fan-B also much variation in power consumption was not seen as it also ran
at the same speed irrespective of the load requirement. As there is no measures to control the
speed of the fan, airflow to the boilers of the generating units is regulated via mechanical inlet
louvre dampers which restricts the amount of air pushed in by the fans. The intake fan motors
remain at maximum speed but the inlet damper controls the air flow to ensure an appropriate
volume of air is supplied to the generating unit. Due to this the power consumption range at
low load to full load are near to each other which results in a poor operating efficiency and
excess auxiliary power consumption that finally affects the overall efficiency of the plant and
lowers it.
4.5. Causes of low operating efficiency and high power consumption
After analysing the various data as calculated in the previous discussion, the following
causes are incorporated for low operating efficiency and high power consumption of both the
FD Fans. The causes are enlisted bellow.
1) Oversizing of the fan:
It is one of the major cause of low operating efficiency and high power consumption of both
the fans. A conservative design tendency is to source a fan/motor assembly that will be large
enough to accommodate uncertainties in system design, fouling effects, or future capacity
increases. Designers also tend to oversize fans to protect against being responsible for
inadequate system performance. However, purchasing an oversized fan/motor assembly
creates operating problems such as excess airflow noise and inefficient fan operation. The
incremental energy costs of operating oversized fans can be significant. In this case, the FD
40. 33
Fan-A ran at 60% to 70% of its rated capacity at full load and FD Fan-B ran at 70% to 80% of
its rated capacity at full load. In some cases like at low load as the air requirement in boiler is
less, on that case only FD Fan-A was operated and FD Fan-B was closed. On such situation
also the FD Fan-A ran at about 85% to its rated capacity. The actual current of the fan was 80%
to 85% of the full load rated current when both the fan was operating at maximum power
output. At full load the maximum actual power of FDF-A was 923 kW, hence the redundant
capability of the motor was (1182-923) = 259 kW which was 21.9% of its rated power and for
FDF-B at full load the maximum actual power was 932 kW with a redundant capability of
(1182-932) = 250 kW, which was 21.2% of its rated power. This oversizing creates long term
problem with respective to both operation and cost and decreases the operating efficiency.
2) Constant speed of the fan:
This is the most important cause of low operating efficiency and high power consumption
of both the fans. Both the fan rotates at the same speed (980 RPM) irrespective of the flow
requirements at different loading conditions. As a result of this much variation in power
consumption was not seen. The power consumption range for FD Fan-A was in between 831.6
kW and 923 kW and for FD Fan-B also much variation in power consumption was not seen.
As there is no measures to control the speed of the fan, airflow to the boilers of the generating
units is regulated via mechanical inlet louvre dampers which restricts the amount of air pushed
in by the fans. The intake fan motors remain at maximum speed but the inlet damper controls
the air flow to ensure an appropriate volume of air is supplied to the generating unit. Due to
this the power consumption range at low load to full load are near to each other which results
in a poor operating efficiency and excess auxiliary power consumption that finally affects the
overall efficiency of the plant.
3) Role of dampers:
The major frictional loss takes place in the dampers where by throttling the air and restricting
the amount of air entering the fan, the flow is controlled. Throttling processes are highly
irreversible process where a huge amount of irreversibility loss takes place. Besides this
dampers control airflow by changing the amount of restriction in an airstream. Increasing the
restriction creates a larger pressure drop across the damper and dissipates some flow energy,
while decreasing the restriction reduces the pressure differential and allows more airflow. This
losses are one of the major issue in the decrement of operating efficiency of the fan.
4) Mismatching in performance of both FD Fans:
This is also a severe cause of performance declination of both the fans. As per the data, the
output of FD Fan-B is far higher than the output of FD Fan-A. As for example, the maximum
volume flow rate as noted for FD Fan-B was 148.9 mᵌ/sec and for FD Fan-A was 165.5 mᵌ/sec.
The noted head for FDF-A and B at different loading condition was also different. This can
hamper the stable operation of the unit.
5) Characteristics of the system: The system characteristics has a great role in the
performance of the fan. For optimizing the performance of the fan it is important to know the
exact characteristics of the system.
41. 34
4.6. Remedial suggestions for improving the operational efficiency
The possible remedies for improving the operating efficiency are given bellow. The
adaptation of any of the solution requires cost-benefit and other analysis.
The oversizing of the fan problem can be removed by properly designing the fan but as it
is an installed unit so some design modifications can be made by consulting with the
manufacturer or some consulting engineering firms.
Another method of increasing the fan efficiency is to control fan rotational speed. Use of a
motor that has multiple speeds and to select a lower rotational speed during low airflow
requirements can be adopted.
Fans that operate over a wide range of their performance curves are often attractive
candidates for Variable Frequency Drives (VFD). VFDs use electronic controls to regulate
motor speed which, in turn, adjusts the fan output more effectively. The principal advantage
offered by VFDs is a closer match between the fluid energy required by the system and the
energy delivered to the system by the fan. As the system demand changes, the VFD adjusts
fan speed to meet this demand, reducing the energy lost across dampers or in excess airflow.
Also, VFDs tend to operate at unity power factors, which can reduce problems and costs
associated with reactive power loads. Because VFDs do not expose mechanical linkages to
potential fouling from contaminants in the airflow, they can also lead to reduced
maintenance costs. The energy and maintenance cost savings provide a return that often
justifies the VFD investment.
For obtaining various rotational speed of the fan a fluid coupling with scoop control method
can be adopted between the motor driving shaft and fan driven shaft.
Proper monitoring and maintenance of the fan and its paths also required.
42. 35
Conclusion
Based on the overall report, the following conclusions can be drawn.
1. The capacity of the FD Fan was considerably higher than required. When the unit was
operated at full load, the FD Fan-A ran at 60% to 70% of its rated capacity and FD Fan-
B ran at 70% to 80% of its rated capacity. In some cases like at low load as the air
requirement in boiler is less, on that case only FD Fan-A was operated and FD Fan-B
was closed. On such situation also the FD Fan-A ran at about 80% to its rated capacity.
The actual current of the fan was 80% to 85% of the full load rated current when both
the fan was operating at maximum power output.
2. The FD fans operated within a low power output area over a long period of time, thereby
increasing power consumption and increasing costs of operating the unit. Due to this
the fan efficiency is also considerably low with respect to its rated design efficiency in
the operating zone.
3. Both the fan rotates at the same speed (980 RPM) irrespective of the flow requirements
at different loading conditions. As a result of this much variation in power consumption
was not seen. As there is no measures to control the speed of the fan, airflow to the
boilers of the generating units is regulated via mechanical inlet louvre dampers which
restricts the amount of air pushed in by the fans. The intake fan motors remain at
maximum speed but the inlet damper controls the air flow to ensure an appropriate
volume of air is supplied to the generating unit. Due to this the power consumption
range at low load to full load are near to each other which results in a poor operating
efficiency and excess auxiliary power consumption that finally affects the overall
efficiency of the plant.
4. The operating points are not matching to its design curve, this deviation shows a loss
incorporating in the system.
5. The operational performance of the two fans (FD Fan-A & FD Fan-B) was mismatched.
This poses a threat to the stable operation of the unit.
6. Modifying the design of the fan, use of multiple speed motors, use of Variable Speed
Drives (VFDs), use of fluid coupling and proper operation and maintenance of the fans
are some corrective measures for improving the operational efficiency of the FD Fan.
