Literature Review
The purpose of a power plant is to create power distribution, as necessary. However, to
create a large amount of power a plant must also use a large amount of power. Reduction in
the amount of energy a power plant uses will increase the overall efficiency and profit of the
plant. One of the simplest ways to achieve this is to install variable frequency drives (VFDs), for
the motors controlling the boiler fans. Following is a discussion of the basics of power plants,
the different types of fans within the plant, the function of VFD’s, ways to integrate VFD’s into
existing systems, possible companies to purchase VFD’s from, and the requirements of the
National Electric Code standards for all variable frequency drives.
Forced Draft and Induced Draft Fans
Electrical power plants that use a boiler, add heat to feed water and then supply steam
to one or more turbines. This heat is generated by the combustion of a fuel such as coal. The
air needed for the combustion is supplied by forced draft fans. Electrical power plants run on
the balanced draft principle. A forced draft (FD) fan is used to push air into the boiler for
combustion. An induced draft (ID) fan is used to draw the unused air and the gaseous
combustion products out of the boiler and into the chimney for release. These two fans must
work together for optimum performance. Both the inlet and outlet air flows of the boiler must
be controlled in order to have optimal combustion and to maintain a balanced draft.
Additionally, they have to work under safe operating conditions. If there is too much air flow,
the energy is being wasted [2]. However, if there is not enough air flow then the area can
become polluted with fuels and there is risk of the boiler explosion. Also, if not enough air is
drawn from the boiler, the boiler will become pressurized and small particles will be blown out
of the openings and into the atmosphere polluting it. On the other hand, if too much air is
drawn from the boiler, a trip will happen so that the boiler doesn’t implode. So it’s a very
delicate process to get the induced draft fans and the forced draft fans set correctly for the
ideal operation. In most power plants, the air flow is controlled so that it holds the boiler
pressure at one-half inch water column. The fans that power plants use to control this air flow
are usually very large and have single speed fan motors. The air flow is controlled by
modulating the inlet vanes on each force draft fan while the vanes on induced draft fans are
modulated to control the pressure of the boiler. These fans are one of the largest consumers of
power in the whole power plant [2]. Forced Draft Fans are usually the most efficient fans in a
power plant. Forced and induced draft fans are made up of nine main components: the fan, the
fan ring, the plenum, the nozzle, the header, the tube bundle, the drive assembly, the column
support, and the inlet bell. The Forced Draft Fan must be able to supply the air necessary for
fuel combustion. It also must be able to handle stoichiometric air and the excess air from
burning the actual specific fuel it is designed to use [3]. Forced draft fans not only supply the
air for the total airflow, but they also provide air to make up for heater leakage and some
sealing air requirements. The forced draft fan is kept in the cleanest environment possible,
meaning it is kept in room air and is not around the high temperature, acidic environment of
exhaust gasses. Because FD fans are kept in such a clean environment, it is much less
susceptible to failures, unlike an induced draft fan. Also because of the FD fans’ well-kept
location, it makes access much easier for things like maintenance and replacements. Compared
to the ID fans, a lower initial cost, are much cheaper to maintain, have fewer vibration
problems, have a shorter fan shaft, and require less space because of its lesser plot width. [4]
Variable Frequency Drives and Controls
Optimizing control systems for the Variable Frequency Drives (VFD’s) in coal plants may
increase efficiency of the coal plant. Variable speed fans need to be adjusted to give the desired
airflow. There are many different kinds of VFD’s and they can have different features as well
depending on the fan they will be used on. Variable drive fans utilize hydraulic or magnetic
couplings. These couplings make the fan wheel speed independent of the motor speed. Fan
speed controls are usually integrated into automated systems to maintain the correct speed of
the fan. The speed of the FD and ID fans can also be controlled by electronic variable-speed
drive, which controls the speed of the motor. This method means more energy efficiency of at
reduced speeds, than the hydraulic or magnetic couplings. [5] Fan dampers can be used to
control gas flow in and out of the fan. They can be installed on the inlet or outlet side. These
dampers can slow the flow of gasses or change how the gas enters the fan wheel. [6] Fan hubs
can be arranged in three ways: Forward, Backward, and Radial. These different configurations
can results in different outcomes. Forward use blades curved toward the direction of rotation
of the fan wheel. Backward are used to design to handle gas steams with relatively low
particulate loadings. The backward are more energy efficient than radial fans. Radial fans have
blades that extend straight from the hub. [7]
A study conducted at Alabama Power to create more efficient control devices for 2 units
that have more than 25 years of service has resulted in better efficiency. This resulted in 0.15
percent heat rate improvement and $120,000 per year in savings. Two damper drives control
heated and tempering primary air to a common duct. A third damper feeds hot hair to the
pulverizer. A distributed control system (DCS) positions 21 dampers to maintain a target
temperature within the pulverizer and to develop air flow to the correct amount, so that
ground coal can be transferred to furnace burners. Secondary air from the two FD fans turns to
wind boxes. A pulverizer then feeds the coal to a row of burners. This controls ID fan inlet
dampers while fans speeds are between 17-25 percent maximum. High speed fans mean the
dampers are open and the draft is controlled only by fan speed. These drives come with fieldmounted contract electronic units and HART protocol communication capability. The unit is
processor controlled and receives 20mA set point signals from the digital control system. It
feeds back 20 mA signals representing the signals precise actuator positions. This unit controls
the need for torque or limit switches. When voltage is cutoff, the motor is cutoff from its power
source. This device utilizes digital positioning, known as a smart drive. These digital drives can
use advanced frequency converter technology to vary torque or travel speed of the intelligent
actuators. This unit eliminates the need for torque or limit switches. [8]
1. 𝑅𝑃𝑀 =
120 𝑥 𝐹𝑟𝑒𝑞𝑢𝑒𝑛𝑐𝑦
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑜𝑙𝑒𝑠
2. Where frequency is in Hertz
3. 𝐻𝑜𝑟𝑠𝑒 𝑃𝑜𝑤𝑒𝑟 = 𝑇𝑜𝑟𝑞𝑢𝑒 𝑥 𝑅𝑃𝑀
Where Torque is in Newton meters
Figure 1: Speed Equations [9]
To calculate the speed of the AC motor (at the shaft), equation 1 can be used. The rated
frequency and number of poles is both nameplate data that is known. This relationship implies
that a change in frequency will lower the speed of the motor. The second equation in Figure 1
shows the relationship between horsepower, torque, and speed (RPM). This equation shows
that if the speed is increased then the torque will decrease while the horsepower remains the
same. One more basic property is the voltage divided frequency ratio (V/Hz). For a desired
speed and assuming constant current, this ratio remains the same so it can be inferred that if
the frequency increases, then the voltage can decrease, leading to lower power consumption at
the rated torque. These basic properties of AC motors lead to the implementation of the VFD.
A VFD makes use of the properties just discussed. A VFD varies the frequency of the AC voltage
that is input into the AC motor. The VFD can increase and decrease the frequency. A fan
motor’s efficiency can be raised with VFD’s in a couple ways. One way is that a VFD can allow a
fan to operate at many different speeds. When less cooling is required, the VFD reduces the
speed. Currently, fan output is regulated by mechanical dampers, which limit airflow, but not
power input on the motor. [9] Also, a VFD can provide a slow rate of acceleration when the fan
is needed to run in a low power mode. [10] According to Harshman, VFD’s have accurate and
precise controls that are simple to work. Also, he states that certain VFD’s can achieve 95%
rated voltage (power) when the load is lessened. Overall, a VFD is a device that will make fans
more efficient by utilizing the properties of AC motors concerning frequency and speed. A fan
with a VFD can run more efficiently, and will also be easier to control and monitor.
Theoretically, VFD’s would be an excellent option to consider for ensuring fans to run more
efficiently.
Integration of VFD’s
The VFD can be very easily integrated into the existing system of a plant by putting in
line between the power source and the motor itself. Switches will be put in place to operate
the VFD and have an override in which the fan would run at full speed, in case of VFD failure.
VFD integration used on an ID fan of a 300MW coal fired power plant located in north-east of
Yunnan province is estimated to have a power saving rate of 43.8%. [11] The VFD will enable
the motor to run at lower frequencies causing the fan to run at lower speeds, which will result
in lower power consumption.
Possible Vendors
VFDs may be obtained from many different vendors including Kirby Risk, Springfield
Electric, Siemens, and Flanders Electric. Flanders Electric has a local office in Marion, IL.
Codes and Standards
As stated in the National Electrical Code; when dealing with AC motors with adjustable
voltage or variable torque drive systems the ampacity of conductors and the ampere rating of
all safety switches including, branch-circuits, short-circuits, and ground-faults shall be based on
the maximum operating current that is stated on the motor nameplate [12]. In addition, the
over current protection should be set to 125% of the phase converter nameplate single-phase
input full-load amps [13].
Summary
Coal power plants operate on a few basic principles; however, many of them are aging,
and are in need of more efficient operating methods. The forced draft and induced draft fans
are some of the highest consumers of energy in the entire power plant. One of the possible
solutions to cut down on energy use and be more efficient would be to look at placing variable
frequency drives on FD and ID fans throughout the plant. Overall, the focus of this project is to
increase efficiency of the auxiliary loads at coal power plants, and in turn help lower consumer
prices.
References
[1]
Electric Power Plant Design, Headquarters, Department of the Army, Washington, DC,
1984. pp. 1-1
[2]
Binstock, Morton. "Coordinated control for power plant forced and induced draft fans
during startup and fan speed changes." Patent Storm. N.p., 1980. Web. 7 Oct 2010.
http://www.patentstorm.us/patents/4181099/description
[3]
ProcessRegister. "Forced Draft Fans." Process, Energy and Greater Manufacturing
Industries Manufacturer Directory - Buy, Sell, Trade with Industrial Suppliers, Exporters and
Importers. 07 Oct. 2010. Web. 07 Oct. 2010.
http://www.processregister.com/Forced_Draft_Fans/Suppliers/pid2983.htm
[4]
Hudson Products Corporation. "FinFin Technical Info." Welcome to Hudson Products.
2009. Web. 07 Oct. 2010. http://www.hudsonproducts.com/products/finfan/tech.html
[5]
(2010, October 6) Centrifugal_fan. [Online]. Available:
http://en.citizendium.org/wiki/Centrifugal_fan
[6]
(2010, October 7) The Steam Generator. [Online]. Available:
http://en.citizendium.org/wiki/Conventional_coal-fired_power_plant#The_steam_generator
[7]
(2010, October 7) Digitial Positioners. [Online]. Available:
http://www.controlglobal.com/articles/2010/DigitalPositioners1003.html
[8]
(2010, October 6) Convential Coal Power Plant. [Online]. Available:
http://en.citizendium.org/wiki/Conventional_coalfired_power_plant#The_steam_generator
[9]
ABB Inc. - Drives and Power Electronics. (2010, October 4) Variable-Frequency Drives
Upgrade Reactor Circulating Pumps. [Online]. Available:
http://www.joliettech.com/what_is_a_variable_frequency_drive.htm
[10] M. Harshman. (2010, September 26) Variable-Frequency Drives Upgrade Reactor
Circulating Pumps. [Online]. Available: http://www.powermag.com/o_and_m/2710.html
[11]
mediumvoltagedrive.de/1-power-generation/6.doc
[12] National Electrical Code. 2008 Edition. National Fire Protection Association.
Quincy, MA. 2007 pp. 304
[13] National Electrical Code. 2008 Edition. National Fire Protection Association.
Quincy, MA. 2007 pp. 347