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Boiler Auxiliaries PerformanceMany steam generating units are operating day after day with efficiencies at or near test condition values. Any unit can operate at these efficiencies with the proper instrumentation , reasonable auxiliaries, proper maintenance program of instrumentation and auxiliary equipment and proper operating instruments. A standard procedure should be established by logging the consumption of individual auxiliaries for different loads.The auxiliaries those need to be monitored for their performance are :Fans and pumps (CC pumps if provided)Fuel burning equipment (coal mills, burners)Wind box dampers (air,fuel,overfire)Air PreheatersDust collecting equipment (ESP)Soot blowers
Forced Draft Fan, Induced Draft Fan, Primary air fan, Gas recirculation fan, Seal Air fans, Scanner Air fan, Igniter air fan.Types of fanCentrifugal Fan ( forward Curved, backward curved, radial, aerofoil shaped)Axial Fan –Axial reaction and axial impulse. Fans provided in 500 MW Trombay unit 5 (Typical example)· FD Fans :2 axial reactive single stage Variable pitch with 6.6 KV, 990 RPM. 2700 KW motors· PA Fans :2 axial reactive double stage Variable pitch with 6.6 KV, 1480 RPM. 2750 KW motors· ID Fans: 4 radial with 6.6 KV, 740 RPM, 2050 KW motors
Thermal Power Plant Fans
A fan is typically a mechanical device that causes a movement of air, vapor & other gases in a given system. Fans are widely used for circulating air in rooms & buildings for cooling/heating people, for cooling motors, materials & products, for exhausting dust & noxious fumes, for conveying light materials, for forced draft in steam boilers and in heating, ventilating & air-conditioning systems. For effecting the air flow, the fan develops a total pressure difference over the inlet & outlet air streams; the total pressure rise comprises two components viz., i) static pressure which depends on the blade profile, number of blades, pitch, hub shape etc. i.e. aerodynamic characteristics of the fan impeller and ii) dynamic pressure which develops due to velocity or kinetic energy imparted to the air stream.
What are Fans For?
A fan is an apparatus that converts electric energy into aerodynamic energy. Some of this energy is useful, other output energy is wasted energy like the air swirl at the fan's exit. We can get only two types of useful aerodynamic energies out of a fan, and these are:Static Pressure (Considered a form of energy.)Kinetic Energy (Energy due to the velocity of the air particles.)
Static Pressure:This is the energy that will make the air flow through the ducts. Static pressure is a very powerful tool for "long distance cooling“ or ‘’supplying air at a distance” (for example boiler furnace). This energy does not dissipate easily (unless the fan stops working ).We measure static pressure in inches of column of water. To give us an idea, the static pressure that the atmosphere exerts upon us due to its weight is equal to approximately 407 inches of water.
What are Fans For?
Kinetic Energy: The fan not only increases the air pressure behind it but also blows air. The kinetic energy is measured usually in cubic feet per minute (CFM) and is called a volumetric flow rate (not mass flow rate) which, by the way, does not change with altitude. To measure the energy output of a fan, we want to find out how much static pressure alone can the fan exert upon the air (shut-off condition) and how much kinetic energy alone can the air through the fan develop (free-delivery condition). These two pieces of information can be found in the extreme regions of the so called characteristic curve. See figure next slide.
Fans
Fans may be classified as centrifugal fans or axial-flow fans. In centrifugal fans, air is led through an inlet pipe to the centre or eye of the impeller, which forces it radially outward into the volute or spiral casing from which it flows to a discharge pipe. In an axial-flow fan, with the runner & guide vanes in a cylindrical housing, air passes through the runner without changing its distance from the axis of rotation.There would be no centrifugal effect on the airflow generated. Guides or stator vanes serve to smoothen/straighten the airflow and improve efficiency. In general, an axial-flow fan is suitable for a larger flow rate with a relatively small pressure gain and a centrifugal fan for comparatively smaller flow rate and a large pressure rise.
Types of Fans
Aerofoil :Airfoil also spelled as aerofoil, is a shaped surface, such as an airplane wing, tail, or propeller blade that produces lift and drag when moved through the air. An aerofoil produces a lifting force that acts at right angles to the air stream and a dragging force that acts in the same direction as the air stream.If an aerofoil is to fulfill the desired function, it must experience a lift force, as well as a drag force when in motion. The lift force arises because the speed at which the displaced air moves over the top of the airfoil (and over the top of the attached boundary layer) is greater than the speed at which it moves over the bottom.The pressure acting on the airfoil from below is therefore greater than the pressure from above. The design of the airfoil, nevertheless, has a critical effect on the magnitude of the lift force. Drag A fluid stream exerts a drag force (FD) on any obstacle placed in its path, and the same force arises if the object (aerofoil) moves and the fluid is stationary.
The fan efficiency is defined as the ratio of air power (output) to shaft power requirement (input). An optimally designed fan impeller with improved aerodynamics requires less shaft input power for desired delivery of air volume with total pressure rise as per the system requirement. With reduced shaft power requirement, there is a commensurate reduction in energy consumption.
Thus the fan efficiency is directly contingent upon selection of fan and the operating point relevant to the system resistance
The fan efficiency
------ BldAngle60deg ------BldAngle65deg ------BldAngle70deg
------ BldAngle45deg ------BldAngle50deg% ------BldAngle55deg
Axial Fan Characteristic
Centrifugal Fan Characteristic
Centrifugal Fans
Components of a centrifugal fan
Centrifugal Fans
Fig 2a , b, c
Forward-curved bladesForward-curved blades, as in Figure 2(a), use blades that curve in the direction of the fan wheel's rotation. These are especially sensitive to particulates. Forward-curved blades are for high flow, low pressure applications.Backward-curved bladesBackward-curved blades, as in Figure 2(b), use blades that curve against the direction of the fan wheel's rotation. The backward curvature mimics that of an airfoil cross section and provides good operating efficiency with relatively economical construction techniques. These types of fan wheels are used in fans designed to handle gas streams with low to moderate particulate loadings. They can be easily fitted with wear protection but certain blade curvatures can be prone to solids build-up.
Centrifugal Fans
Backward curved fans can have a high range of specific speeds but are most often used for medium specific speed applications-- high pressure, medium flow applications.Backward-curved fans are much more energy efficient than radial blade fans and so, for high power applications may be a suitable alternative to the lower cost radial bladed fan. Straight radial bladesRadial fan blades, as in Figure 2(c), extend straight out from the hub. A radial blade fan wheel is often used on particulate-laden gas streams because it is the least sensitive to solids build-up on the blades, but it is often characterized by greater noise output. High speeds, low volumes, and high pressures are common with radial fans, and are often used in vacuum cleaner, pneumatic material conveying systems, and similar processes.
Centrifugal Fans
Fans and blowers 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
Fans and Blowers
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 Fan and blower 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
Fans, Blowers & Compressor
Fans
Centrifugal Fan: The major types of centrifugal fan are : radial, forward curved and backward curved Radial fansThese fans are industrial workhorses because of their high static pressures (up to 1400mm WC) and ability to handle heavily contaminated airstreams. Because of their simple design, radial fans are well suited for high temperatures and medium blade tip speeds.Forward-curved fansThese fans are used in clean environments and operate at lower temperatures. They are well suited for low tip speed and high-airflow work - they are best suited for moving large volumes of air against relatively low pressures. Backward-inclined fansThese fans are more efficient than forward-curved fans. Backward-inclined fans reach their peak power consumption and then power demand drops off well within their useable airflow range. Backward-inclined fans are known as "non-overloading" because changes in static pressure do not overload the motor.
Centrifugal Fans
Axial Fans
Axial Fans
Axia
l Fan
s
Axial Fans
Impeller Assembly
• Blades – Wear & damage
• Pressure testing of blade bearing assemblies for oil leakage
• Slide pads- Wear & bearing damage
• Connecting lever for Cracks or damage
Fans
The fans are called Primary Air(PA), Secondary Air(SA), Induced Draft(ID) fans , Gas Recirculation (GR) fans, Seal Air fans, Scanner Air fans and Igniter fans depending on the application of their use. FGD units are provided with booster fans to boost up the pressure.• Fan control : To meet varying requirements of the system, common methods of fan output control are damper control, variable-speed control and blade pitch control in case of axial flow fans. Variable speed control is the most efficient method of controlling fan output since it also reduces power consumption. From the relationships of speed to capacity, pressure and power it follows that by reducing speed by one half, fan output will drop one half, pressure one fourth, and horsepower one eighth.
Different options of Vane control
Fan PerformanceFan Performance Curves
Fan performance curves and data show how much pressure and airflow a given fan can produce at a given speed. This data is based on the system resistance (pressure) occurring away from the fan. Fan performance curves are developed by fan manufacturers based on the testing of a prototype fan fabricated based on the theoretical design.Fan performance is best expressed in graphical form. These functional relations are the fan characteristic curves. In these curves capacity in cfm is shown horizontally as the independent variable and head (static pressure), shaft horse power and static efficiency are dependent variables plotted vertically. Fan speed is constant. Since fan operation for a given capacity must match single values of head and horsepower on the characteristic curves, a balance between fan static pressure and system resistance is required. If the system resistance for a given capacity is less than the head indicated on the fan characteristic curve, additional variable flow resistance, such control damper, must be added to the system. Varying the operating speed to yield a family of curves, will change the numerical performance values of the characteristics. However, the nature of the curves remains substantially unaltered. Performance at different speeds for the same efficiency can be related by the following :1. Capacity is directly proportional to speed.2. Head is directly proportional to speed squared.3. Power output is directly proportional to speed cubed.
Fan Performance CurvesCentrifugal Fans
Fan Performance Curves Axial Fans
System characteristic curve
The system resistance is the sum of static pressure losses in the system
It is a function of the configuration of ducts, pickups, elbows and the pressure drops across equipment
The system resistance varies with the square of the volume of air flowing through the system
system resistance increases substantially as the volume of air flowing through the system increases; square of air flow, resistance decreases as flow decreases
System Resistance ChangeThe system resistance has a major role in determining the performance and efficiency of a fan. The system resistance also changes depending on the process. For example, the formation of the coatings / erosion of the lining in the ducts, changes the system resistance marginally. In some cases, the change of equipment (e.g. Replacement of Multi-cyclones with ESP / Installation of low pressure drop cyclones in cement industry) duct modifications, drastically shift the operating point, resulting in lower efficiency. In such cases, to maintain the efficiency as before, the fan has to be changed. Hence, the system resistance has to be periodically checked, more so when modifications are introduced and action taken accordingly, for efficient operation of the fan.
System characteristic curve
The resistance to air flow (pressure) increases when the flow of air is increased and varies as the square of the flow.The pressure required by a system over a range of flows can be determined and a "system performance curve" can be developed.This system curve can then be plotted on the fan curve to show the fan's actual operating point at "A" where the two curves (N1 and SC1) intersect. At fan speed N1, the fan will operate along the N1 performance curve, flow (Q1) against pressure (P1) at operating point AFrom the two methods, first method is to restrict the air flow by partially closing a damper, causes a new system performance curve (SC2) where the required pressure is greater for any given air flow.The fan will now operate at "B" to provide the reduced air flow Q2 against higher pressure P2Second method to reduce air flow is by reducing the speed from N1 to N2, keeping the damper fully open. The fan would operate at "C" to provide the same Q2 air flow, but at a lower pressure P3
System characteristic curve
Reducing the fan speed is a much more efficient method to decrease airflow since less power is required and less energy is consumed
Fan LawsFlow ? Speed Pressure ? (Speed)2 Power ? (Speed)3
1 1
2 2
Q N
Q N
21 1
2 2
SP N
SP N
3
1 1
2 2
kW N
kW N
Varying the RPM by 10% decreases or increases air delivery by 10%.
Varying the RPM by 10% decreases or increases the static pressure by 19%.
Varying the RPM by 10% decreases or increases the
power requirement by 27%.
Where Q – flow, SP – Static Pressure, kW – Power and N – speed (RPM)
Impact of speed reduction
% Speed
% Power
100 100
90 73
80 51
70 34
60 22
50 13
Fan PerformanceSystem resistance along with the fan static pressure characteristics at various speeds, both as function of volumetric flow rate are plotted. If the fan operates at constant speed, any output less than that shown at the intersection of the system resistance and specified rpm curves must be obtained by throttling the excess fan head. This results in wastage in power that can be avoided by using a variable speed drive.Backwardly curved blade wheels are generally selected for forced draft service because the high speed is suitable for standard motor drive. The power demand is self limiting, and the static efficiency is high. These fans may be satisfactorily operated in parallel. Induced draft fans operate in gas of much higher temperature and may handle gases laden with dust. Forwardly curved blade wheels run at the lowest speed to develop a given pressure, hence are frequently chosen for induced draft service so that the centrifugal stresses in the wheels will be least. The forward curvature reduces the blade depth, but gives a large inlet opening for the gas. conditions.
Fan Performance
Fan Safety Factors : To make sure that the fans will not limit a boiler’s performance, margins of safety are added to the calculated or net fan requirements to arrive at satisfactory test block specification. These margins are intended to cover conditions encountered in operation that can be specifically evaluated. For example, variation in fuel ash characteristics or unusual operating conditions may foul heating surfaces. The unit then requires additional draft. A need for rapid load increase or a short emergency overload often calls for overcapacity of the fans. The customary margins to allow for such conditions are 15 to 20 % increase in the net weight flow of air or gas, 15to 20% increase in net head, and 25F increase in the air or gas temperature at the fan inlet.
Fan PerformanceGeneral performance requirements for force draft fans.
Reliability : Modern boilers must operate continuously for long periods (up to 18 months) without shut down for repairs or maintenance. The fan must be well balanced, and the blades so shaped that they will not collect dirt and disturb this balance.
Efficiency : High efficiency over a wide range of output is necessary because boilers operate under varying load conditions.Pressure : Fan pressure should vary uniformly with output over the capacity range. This facilitates damper control and assures minimum disturbance of air flow when minor adjustments to the fuel burning equipment change the system resistance.
Fan Performance
Capacity : When two or more fans operate in parallel, the pressure out-put curves should have characteristics similar to the straight blade or backward curve blade fans in order to share the load equally near the shut off point.
Horsepower : Motor driven fans require self limiting horse power characteristics, so that driving motor cannot overload. This means that the horsepower should reach a peak and drop off near the full load fan output.
ID fans : Induced draft fans has the same basic requirements as forced draft fan except that it handles high temperature gas which may contain erosive ash. Flat, forward curved and occasionally backward curved blades with less curvature are used. Excessive maintenance from erosion is sometimes avoided by protecting casing and blades with replaceable wear strips. Bearings, usually water cooled have radiation shields on the shaft between rotor and bearings to avoid overheating.
Fan static pressure
Fan Static Pressure r SP = SP (Fan outlet) – SP (Fan inlet)
SP (Fan outlet) – Static pressure at fan outlet, mm WC
SP (Fan inlet) – Static pressure at fan inlet, mm WC
r SP = 0.05 – (-10) = 10.05 mm W.C.
Operating Efficiency of Fan
Operating efficiency and performance evaluation of fans : All fans need to be studied for their operating efficiency(as per performance test) with the help of energy instruments in addition to online valid calibrated instruments to identify energy saving measures. The parameters to be studied in detail are :• Air/gas rates of fan / ducts•Static pressure, dynamic pressure and total pressure•Power consumption of the fan ( for estimating the operating efficiency of the fan)•Monitor present flow control system and the frequency of flow variations (for exploring application of variable frequency drive)
Compare the actual values with the design / performance test values, if any deviation is found, list the factors with the details and suggestions to overcome. •Enlist scope of improvement with extensive physical checks / observations•Based on actual operating parameters, enlist recommendations for action to be taken for improvement, if applicable such as•Replacement of fan •Impeller replacement•Variable speed drive application •Cost analysis with saving potential for taking improvement measure
Fan Application and Matching
Application and matching of Fans : The installed fan has to be thoroughly verified for its application, whether the fan is best suited for its application, duty, load variation etc. The variation options to be considered for improving energy efficiency are :• Replacement of fan with the best suited energy efficient fan• Replace/trim the impeller, if the fan is throttled to reduce flow by 10-20%.(if smaller impellers are not available).•Retrofit with variable speed drive fans if the fans are serving variable loads
Static pressureStatic pressure is the potential energy put into the system by the fan. Velocity pressureVelocity pressure is the pressure along the line of the flow that results from the air flowing through the duct, used to calculate air velocity
Total pressureTotal pressure is the sum of the static and velocity pressure. The fan flow is measured using pitot tube manometer combination, or a flow sensor (differential pressure instrument) or an accurate anemometer
Fan pressures
Total pressure is measured using the inner tube of pitot tube and static pressure is measured using the outer tube of pitot tube.
When the inner and outer tube ends are connected to a manometer, we get the velocity pressure.
Measurement by Pitot tube
Measurement of Pressures
TRAVERSE POINTS FOR VELOCITY MEASUREMENT
The velocity pressure varies across the duct. Friction slows the air near the duct walls, so the velocity is greater in the center of the duct. The velocity is affected by changes in the ducting configuration such as bends and curves. The best place to take measurements is in a section of duct that is straight for at least 3–5 diameters after any elbows, branch entries or duct size changes
The velocity should be calculated for each velocity pressure reading, and the average of the velocities should be used.
Typical Fan Characteristic Curves
The performance of two fans in parallel will result in doubling the volume flow, but only at free delivery. As Figure shows, when a system curve is overlaid on the parallel performance curves, the higher the system resistance, the less increase in flow results with parallel fan operation. Thus, this type of application should only be used when the fans can operate in a low resistance almost in a free delivery condition. By staging two fans in series, the static pressure capability at given airflow can be increased, but again, not to double at every flow point, as the above Figure displays. In series operation, the best results are achieved in systems with high resistances.In both series and parallel operation, particularly with multiple fans certain areas of the combined performance curve will be unstable and should be avoided. This instability is unpredictable and is a function of the fan and motor construction and the operating point
Operation of Fans in parallel and series
An additional fan in parallel to the first increases airflow in a low static pressure situation. An additional fan in series increases the airflow in a high static-pressure enclosure."
Operation of Fans in parallel and series
Operation of Fans in parallel and series
Fan Operating PointThe figure below show the relationship between airflow and static pressure for a typical fan. The performance of a fan in a specific application is determined by the intersection of the System Resistance/Impedance Curve and the Fan Characteristic Curve. The System Impedance Curve is a property inherent to an individual enclosure/system.
Typical Fan Characteristic Curves Effect of Speed
Flow Control StrategiesTypically, once a fan system is designed and installed, the fan operates at a constant speed. There may be occasions when a speed change is desirable, i.e., when adding a new run of duct that requires an increase in air flow (volume) through the fan. There are also instances when the fan is oversized and flow reductions are required. Various ways to achieve change in flow are: pulley change, damper control, inlet guide vane control, variable speed drive and series and parallel operation of fan
Damper ControlsSome fans are designed with damper controls. Dampers can be located at inlet or outlet. Dampers provide a means of changing air volume by adding or removing system resistance. This resistance forces the fan to move up or down along its characteristic curve, generating more or less air without changing fan speed. However, dampers provide a limited amount of adjustment, and they are not particularly energy efficient.
Fan Flow Control
Inlet Guide VanesInlet guide vanes are another mechanism that can be used to meet variable air demand. Guide vanes are curved sections that lay against the inlet of the fan when they are open. When they are closed, they extend out into the air stream. As they are closed, guide vanes pre-swirl the air entering the fan housing. This changes the angle at which the air is presented to the fan blades, which, in turn, changes the characteristics of the fan curve. Guide vanes are energy efficient for modest flow reductions – from 100 percent flow to about 80 percent. Below 80 percent flow, energy efficiency drops sharply. Variable Blade PitchAxial-flow fans can be equipped with variable pitch blades, which can be hydraulically or pneumatically controlled to change blade pitch, while the fan is at stationary. Variable-pitch blades modify the fan characteristics substantially and thereby provide dramatically higher energy efficiency than the other options discussed thus far.
Fan Flow Control
Variable Speed DriveAlthough, variable speed drives are expensive, they provide almost infinite variability in speed control. Variable speed operation involves reducing the speed of the fan to meet reduced flow requirements. Fan performance can be predicted at different speeds using the fan laws. Since power input to the fan changes as the cube of the flow, this will usually be the most efficient form of capacity control. However, variable speed control may not be economical for systems, which have infrequent flow variations. When considering variable speed drive, the efficiency of the control system (fluid coupling, eddy-current, VFD, etc.) should be accounted for, in the analysis of power consumption
Fan Flow Control
Factors to be considered in the selection of flow control methodsComparison on of various volume control methods with respect to power consumption(%) required power is shown in Figure .All methods of capacity control mentioned above have turn-down ratios (ratio of maximum–to–minimum flow rate) determined by the amount of leakage (slip) through the control elements. For example, even with dampers fully closed, the flow may not be zero due to leakage through the damper. In the case of variable-speed drives the turn-down ratio is limited by the control system. In many cases, the minimum possible flow will be determined by the characteristics of the fan itself. Stable operation of a fan requires that it operate in a region where the system curve has a positive slope and the fan curve has a negative slope. The range of operation and the time duration at each operating point also serves as a guide to selection of the most suitable capacity control system.
Fan Flow Control
Outlet damper control due to its simplicity, ease of operation, and low investment cost, is the most prevalent form of capacity control. However, it is the most inefficient of all methods and is best suited for situations where only small, infrequent changes are required (for example, minor process variations due to seasonal changes. The economic advantage of one method over the other is determined by the time duration over which the fan operates at different operating points. The frequency of flow change is another important determinant. For systems requiring frequent flow control, damper adjustment may not be convenient. Indeed, in many plants, dampers are not easily accessible and are left at some intermediate position to avoid frequent control
Fan Flow Control
Control options for centrifugal fansDamper control•Lowest capital cost •Ease of operation-automatic control•List expensive fan drive•Continuous, not step operation
Wastage of power
Axial fan-Fixed pitch, adjustable pitch, variable pitch
Performance control characteristics
•Areas of constant efficiencies run parallel to the boiler resistance line-high efficiency over a wide boiler load range
•Control range is very large both above and below the maximum efficiency
•The lines of constant blade angle are actually individual fan curves-as the curves are very steep, change in resistance produces very little volume change.
•As the blade angle can be adjusted from minimum to maximum flow change is nearly linear
Axial fans
Stall Condition.
The angular relationship between the air flow impinging on the blade of a fan and the blade itself is known as “the angle of attack”. In axial flow fan, when this angel exceeds a certain limit, the air flow over the blade separates from the surface and centrifugal force then throws the air outwards, towards the rim of blades. This action causes a build up of pressure at the blade tip, and this pressure increases until it can be relived at the clearance between the tip and the casing. Under this condition the operation of the fan becomes unstable, vibration sets in and the flow starts to oscillate. The risk of stall increases if a fan is oversized or if the system resistance increase excessively.
Axial Fans
Stall is aerodynamic phenomenon which occurs when a fan operates beyond its performance limits and flow separation occurs around the blade.
Stalling of Axial Fans
Axial Fan Stalling
Stall prevention
When axial fans are sized properly and the resistance curve is parabolic chances of stall is less
Possibility of stall increases when the fan is over sized compare to volume capacity, System resistances increases significantly or fans are operated improperly
Noise in a FanSingle tone noise is generated when the concentrated flow encounters a stationary object after leaving the rotating blade passage. The distance between the blades and the stationary objects affects the soundThe blade passing frequency and its first harmonic is most dominant.Broad band noise is produced by the fluid passing through the fan housing, contains a range of frequencies
Noise in a FanSingle tone noise is generated when the concentrated flow encounters a stationary object after leaving the rotating blade passage. The distance between the blades and the stationary objects affects the soundThe blade passing frequency and its first harmonic is most dominant.Broad band noise is produced by the fluid passing through the fan housing, contains a range of frequencies•Sound radiates from the inlet opening, the discharge duct and the fan housing. All three areas should be analyzed separately and treated in appropriate manner. Inlet sound level from the primary air fans and forced draft fans can be reduced by absorption silencer •Fan casing noise can be minimized by mineral wool insulation and acoustic lagging•Fan discharge noise needs more detail analysis to have cost effective solutions•For forced draft fans and primary air fans, absorption discharge silencer is sufficient•For induced draft fans additionally thermal insulation and lagging are needed. •Stack outlet noise can be reduced by discharge silencer.•However, it is not suitable for coal fired units as the panels gets plugged by fly ash
Noise Control
Fan Selection
PA fan constructionPrimary Air fan : The fan consist of the following components:a) Suction bend, with an inlet and an outlet side pipe for volume measurementsb)Fan housing with guide vanes (stage 1)c) Main bearings (anti-friction bearings)d) Rotor consisting of shaft, two impellers with adjustable blades and pitch control mechanism.e) Guide vane housing with guide vanes (stage 2)f) Diffuser with an outlet-side pipe for pressure measurements.Suction bend, fan housing and diffuser are welded structural steel fabrications, reinforced by flanges and gusets, resting on foundation on supporting feet. On its impeller side, the suction bend is designed as an inlet nozzle. Guide vanes of axial flow type are installed in the fan and guide vane housings, in order to guide the flow.Suction bend and diffuser are flexibly connected to the fan housing via expansion joints.
Axial Fans FD / PA Application
PA Fan Fan and guide vane housing are horizontally split, so that the rotor can be removed without having to dismount the servomotor. The fan is driven from the inlet side. The main bearings are accommodated in the core of the fan housing. The impellers are fitted to the shaft in overhung position. The centrifugal and axial forces of the impeller blades are absorbed by the blade bearings. For this purpose the blade shaft is held in a combination, of radial and axial antifriction bearings. Each blade bearing is sealed off by means of seals.Blade pitch control unit : An oil hydraulic servomotor flanged to the impeller and rotating with it adjusts the blades during operation. The servomotor consists of piston, cylinder and control parts.At pitch control, the translational movement of the servomotor piston is converted into rotational movement of the blade shafts via adjusting levers, so that the blade angles are variable.
FansOil System : The main bearings and hydraulic servomotor are supplied with oil from a common oil tank. Two oil pumps are mounted on the tank. One is operated as a main pump, whereas the other one is used as standby pump. The latter is started via the pressure switch, in the event of control oil pressure declines.FD fans construction is similar to that of PA fan. But FD fan is a single stage fan.Induced draft fans : There are four ID fans provided per boiler, 3 operating and one standby. (some 500 MW boiler are provided with 3 ID fans, two operating and one standby). These fans are single stage double inlet centrifugal fans. The principal elements of the fan are :Housing, inlet dampers, rotor with bearings and shaft seal. Regulation : The capacity of ID is changed by varying the speed(either by VFD or hydraulic coupling) and also by adjustable inlet dampers arranged in front of impeller.
FansScanner Air Fan : The function of scanner air fan is to provide a continuous supply of clean air to purge and cool the flame scanners. Air for the system is drawn from the FD fan discharge ducts through a filter by one the two scanner fans then discharged through distribution pipe work to each flame scanner. One of the fans is provided with DC supply. In case of AC failure DC scanner fan gets started. An emergency damper is provided in the suction duct to facilitate suction from atmosphere. The discharge from each fan includes a pneumatically operated isolating damper which will open and close in response to signals from FSSS. A pressure switch is provided to initiate the automatic start up of the standby fan if scanner duct to furnace differential becomes less than 6 inches.Seal Air Fans : These fans take suction from cold PA header and boost up pressure for providing sealing air to coal mills/feeders.Igniter Air Fans : These fans take suction from FD fan discharge duct and provide air for igniters.
ID FANA B C D
A FD FAN B PA 5BPA 5A
GR 5A GR 5B
ESP
RAPH-5A RAPH-5B
AIR FROM ATMOS
FD A
AIR FROM ATMOS
RAPHB
FD B
RAPHA
FROM GRF - B
FROM GRF - A
IGNITOR AIR FAN
• FURNACE
SECONDARY AIR CIRCUIT
35 DEG C
35 deg
350 DEG
-10 mm
SCAPHB
SCAPHA
SD- 2
SD
7.1
7.2
SD9.1 9.2
SD-1
SD-5
SD-6
SD
10.1
10.2
350 DEG
SD
8.1
8.2
Flue gas system
R
A
P
H A
R
A
P
H B
5 A
5 B
5 C
5 D
ESPGD- GD-
3.1
3.2
GD-7 GD
11A
11B
GD-15 GD-23
GD-
GD-19
GD-4.1
4.2 GD-10
GD
14A
14B GD-18 GD-26GD-22
12A
12B
13A
13B
GD 29.1 29.2
GD 30.1 30.2
GD 31.1 31.2
16
17
2024
21
25
GD 5.1 5.2
8
9
ID A
ID B
ID C
ID D
Stack
GD
27.1
27.2
GD
28.1
28.2
PA 5BPA 5A
RAPH-5A RAPH-5B
Primary Air System
FD Fans
Operating Efficiency of Fan
Operating efficiency and performance evaluation of fans : All fans need to be studied for their operating efficiency(as per performance test) with the help of energy instruments in addition to online valid calibrated instruments to identify energy saving measures. The parameters to be studied in detail are :• Air/gas rates of fan / ducts•Static pressure, dynamic pressure and total pressure•Power consumption of the fan ( for estimating the operating efficiency of the fan)•Monitor present flow control system and the frequency of flow variations (for exploring application of variable frequency drive)
SPECIFICATIONS OF ID FAN
SPECIFICATIONS OF F.D FANS
SPECIFICATIONS OF F.D FANS
PA FANS
Primary air fans are second high power consuming fans
in a thermal power plant.
Specifications of PA fans
Specifications of PA fans
FANS PERFORMANCE ASSESSMENT
Static Pressure
Potential energy put into the system by the fan
Velocity pressure
Pressure arising from air flowing through the duct. This is used to
calculate velocity
Total pressure
Static pressure + velocity pressure
Total pressure remains constant unlike static and velocity
pressure
The fan efficiency is defined as the ratio of air power (output) to shaft power requirement (input). An optimally designed fan impeller with improved aerodynamics requires less shaft input power for desired delivery of air volume with total pressure rise as per the system requirement. With reduced shaft power requirement, there is a commensurate reduction in energy consumption.
Thus the fan efficiency is directly contingent upon selection of fan and the operating point relevant to the system resistance
The fan efficiency
Fan total kW = Q in m3/ s x total pr. developed by fan in mmwc 102
Fan static kW = Q in m3/ s x static pr. developed by fan in mmwc 102
Fan static efficiency % Fan static kW x 100 Input kW to motor m
=
Fan mechanical Efficiency = Fan total kW x 100 Input kW to motor x m
Parameter Details Unit
Q Air flow rate m3/ s
Static pressure Difference between discharge & suction pressure mmwc
Fan static/ total kW Static / total power consumption of the fan kW
Input kW to motor Measured power consumption of the motor kW
m Efficiency of the motor at operating load
Total pressure Difference between discharge & suction pressure mmwc
FAN OPERATING EFFICIENCY EVALUATION
FAN OPERATING EFFICIENCY EVALUATION
Corrected air density, = 273 X 1.293
273 + Air temperature in 0 C
Parameter Details Unit
Cp Pitot tube constant 0.85 or as given by manufacturer
Density of air or gas at test condition Kg / m3
Velocity in m / s =Cp x 2 x 9.81 x Diff. velocity pr. in mmwc x
Volumetric flow (Q), m3/s = Velocity, m/s x Area, m2
FAN OPERATING EFFICIENCY EVALUATION
In case of gas flow measurement of ID fans,
Where it is not possible to measure the gas flow, Then the mass flow
method can be adopted, Provided the oxygen content and actual coal
flow measurements are available.
FAN OPERATING EFFICIENCY EVALUATION
For flow estimation through this method, the following are required:
Stoichiometric air requirement (work out based on the coal composition)
Oxygen content at ID fan inlet (measured)
Excess air (estimate)
% Excess air = (%O2 in flue gas x 100) / (21 – O2 in flue gas)
Coal flow (based on actual measurement or on average basis)
Fly ash content (assumed based on past data)
While in case air flow measurement for FD and PA fans the following instruments (which ever are suitable) can be used
Thermal anemometer
Vane type anemometer
Pitot tube along with micro manometer can be used
Online measuring instrument
If the fans are operating in parallel, it is advised to measure all above parameter for every fan separately to evaluate the individual performance. However combined parameters of flow and head need to be verified with Performance curve for parallel operation
FAN PERFORMANCE ANALYSIS
• If the fans are operating in parallel, it is advised to measure all
above parameter for every fan separately to evaluate the individual
performance.
• However combined parameters of flow and head need to be
verified with Performance curve for parallel operation
•Compare the actual values with the design / performance test values
if any deviation is found, list the factors with the details and
suggestions to over come.
FAN PERFORMANCE ANALYSIS
FAN PERFORMANCE ANALYSIS
The investigations for abnormality are to be carried out for problems.
Enlist scope of improvement with extensive physical checks / observations.
Based on the actual operating parameters, enlist recommendations for action to be taken for improvement, if applicable such as- Replacement of fans, Impeller replacement, VFD application.
Cost analysis with savings potential for taking improvement measures.
FAN PERFORMANCE ANALYSIS
Visual survey of insulation & the ducting system: Insulation status (measure the surface temperature with the aid of surface thermocouple / infrared pyrometer or by using thermal imaging cameras) Bends and ducting status Physical condition of insulation Identification of locations where action is required to improve the insulation (provide with detailed techno-economics) Improvement options for ducting systems if any Sources of air ingress
FAN PERFORMANCE ANALYSIS
Study of air ingress in to the system:
Before and after air pre-heater
Before and after ESP
Before and after ID fan
The difference in the oxygen gives the extent of air ingress in
to the system.
Measurements of oxygen content across all units in flue gas
path, indicates the locations where ingress is occurring.
FAN PERFORMANCE ANALYSIS
Reduction in air ingress will result in •Reduction in power consumption if ID fans•Reduced boiler losses•Improvement in boiler loading•Increased unit load•Increased margin on ID fansMinimizing air in leaks in hot flue gas path will reduce ID fan load as cold air in leaks increase ID fan loading tremendously due to density increase of flue gases and choke up capacity of fan resulting in bottleneck.
FAN PERFORMANCE ANALYSIS
Minimizing demand on the fan 1. Minimizing excess air level in combustion systems to reduce
FD fan and ID fan load.2. Minimizing air in-leaks in hot flue gas path to reduce ID fan
load, especially in case of kilns, boiler plants, furnaces, etc. Cold air in-leaks increase ID fan load tremendously, due to density increase of flue gases and in-fact choke up the capacity of fan, resulting as a bottleneck for boiler / furnace itself.
3. In-leaks / out-leaks in air conditioning systems also have a major impact on energy efficiency and fan power consumption and need to be minimized.
Energy Savings Opportunities
The findings of performance assessment trials will automatically indicate potential areas for improvement, which could be one or a more of the following .1. Change of impeller by a high efficiency impeller along with cone.2. Change of fan assembly as a whole, by a higher efficiency fan3. Impeller derating (by a smaller dia impeller)4. Change of metallic / Glass reinforced Plastic (GRP) impeller by the
more energy efficient hollow FRP impeller with aerofoil design, in case of axial flow fans, where significant savings have been reported
5. Fan speed reduction by pulley dia modifications for derating6. Option of two speed motors or variable speed drives for variable
duty conditions 7. Option of energy efficient flat belts, or, cogged raw edged V belts, in
place of conventional V belt systems, for reducing transmission losses.
8. Adopting inlet guide vanes in place of discharge damper control9. Minimizing system resistance and pressure drops by improvements
in duct system
Coal Pulverizer (Bowl Mill) Description• Pulverizer consists of grinding chamber with a classifier mounted
above it. The classifier is a multiple inlet cyclone with adjustable inlet vanes to permit varying inlet velocity as required to obtain the desired fineness of the pulverized coal. The pulverizing takes place in a rotating bowl in which centrifugal force is utilized to move the coal, delivered by the feeder, outward against the grinding ring. Heavy springs, acting through the journal saddles, provide the necessary pressure between the grinding surfaces and the coal. The rolls do not touch the grinding ring, even when the pulverizer is empty. The tramp iron and other foreign material is discharged through a suitable spout. The air and coal mixture passes upward through the classifier where the direction of the flow is changed abruptly, causing the course particles to be returned to the bowl for further grinding. The fine particles, remaining in suspension, leave the classifier and pass on through the coal piping to the wind box nozzles.
Coal Mill and Coal Feeder
The raw crushed coal is delivered from the bunkers to the individual feeders, which, in turn feed the coal at a controlled rate dictated by boiler combustion control to the pulverizes. The coal feeders normally used are of the belt type. Gravimetric coal feeders are provided to supply coal to coal mills. Feed rate is controlled by combustion control. STOCK model gravimetric feeder designed to supply 43.66 to 76.4tonnes of coal per hour is provided with each coal mill. One revolution of the feeder belt drive pulley delivers a predetermined weight of coal, regardless of its density, to the mill. A signal from the combustion control system, operating through the speed control, regulates the belt drive motor speed, and thereby regulates the coal feedrate. Load cells measure the coal flow rate accurately
Raymond Bowl Mill
A. Air flow venturiB. Throat & deflectorC. Classifier vans for improved circulation & air/fuel distributionD. Outlet cylinder for homogenization & 50 mesh particle rejectionE. Inverted cone for coarse particle rejectionF. Orifices for (coal + air) resistance as per pipe length
Coarse partials are dropped down in mill again
Fine partials goes to burners
Hot airCo
al fr
om fe
ederCOAL MILL
Coal Mills Energy Audit Objectives
The major objectives of coal mill energy audit are :• To evaluate specific energy consumption of the mills (kwh/ton of coal)• To establish air to coal ratio of the mills (ton of air per ton of coal)• To evaluate specific coal consumption of the generating unit (kg / kwh)• To compare actual coal consumption with design / PG test values)• To suggest ways to optimize energy consumption• To find out general condition of the equipment and review maintenance practices which affect energy consumption.Data Collection for energy audit Samples of raw coal, pulverized coal, mill rejects, mill gear box oil, fly ash and bottom ash should be obtained during audit and analyzed for the various parameters. Refer page 51 of guidelines for Energy Auditing Dec 2009
Milling Plant Performance
Basically any milling plant is required to fulfill the following conditions;1. It must be able to handle the design quantity of coal and produce
an acceptable product, even with worn components2. To be acceptable the product must be within the desired fineness
range at all stable loads. Grinding which is unnecessarily fine wastes milling power and could result in load limitations. If it is too coarse the combustible in ash will be too high.
3. Wet coal up to the design wetness, must be adequately dried while full out is maintained, the coal / air mixture being at an acceptable temperature (usually in the range of 70degC)
4. The turn-down ratio must be adequate without instability.
Mill PerformanceThe power consumption of coal mill should be low to minimize operating cost. The main factor of performance depends on (a) the grindability of coal. (b) Surface moisture of coal. (c) the fineness of the grind needed. Grindability : It is a term used to measure the ease of pulverizing a coal in comparison with a standard coal chosen as 100(unity) grindability. Hardgrove index is most often used to measure the grindability of a coal. The method is based on the premise that “the work done in pulverizing is proportional to the new surface produced. A definite amount of grinding energy is applied to a prepared sample in a miniature pulverizer (Hardgrove Grindability Machine) and the new surface is determined by sieving. A coal is harder or easier to grind if its grindability index is less or greater. If the coal has grindability index of 50, it would require twice the grinding power of the standard coal to produce a specified particle size. In general bituminous coals have a Hardgrove index of within the range of 45-60 but those with a low volatile content have a higher index.
Mill Performance
The capacity of a pulverizer is related to the grindability index of the coal. High HGI means Less milling power. Mill designed for higher HGI reduces the fineness as well as output while using low HGI coal. In a well designed equipment it is unnecessary for limitations in mill output to occur through a change in HGI since the plant can be designed to handle harder coal and will automatically give an increased output with softer coal. Other things being equal.Moisture : In order to pulverize and circulate coal pneumatically within a pulverizer, enough of the moisture must be removed to leave the fuel dry and dusty. Preheated air to the pulverizer is required. Drying is accomplished quickly as the coal is being circulated and ground. The use of preheated air permits control of the temperature of the fuel-air mixture to the burners for the most stable ignition. High moisture causes less drying for the same set up and plugging of coal carrying pipes apart from reducing the capacity of the mill. A relatively small increase beyond design value of moisture in coal can give a considerably reduced mill out put once the drying capacity of the mill is exceeded
Percent moisture in raw coal
Mill Capacity factor
Effect of moisture on coal mill capacity
Grinding capacity
limit
Mill PerformanceFineness : When handling bituminous coals, a mill and classifier will be designed to grind and classify the coal so that the product passing to the burners will be of a fineness of which 70-75 % will pass through a 200 mesh sieve. The fineness of the pulverized coal needed for successful burning depends largely on the ratio of volatile matter(VM) to the fixed carbon. Since the carbon burns slower the VM, low VM coals must be ground finer to expose more surface to permit more coal in a given residence time in the furnace. The finer the grind more the power needed. An increase the mill output will be obtained if over grinding is checked by an adjustment to the classifier setting. Moisture makes the coal particle adhere to each other, defeating the purpose of grinding. The coal samples for checking fineness are collected by using bag collector or a cyclone and the results of sieve analysis are plotted on a Rosin and Rammler chart. The mill fineness checks are done periodically to ensure good performance of the mills.Size of raw coal: The larger the size of the raw coal fed to the mill, the greater will the power required to break it down to pulverized fuel size.
Mill Performance
Mill wear : Mill output will be reduced as wear increases on rollers, grinding rings, and on exhauster fan blades, due principally to loss of contact resulting from excessive local wear or an irregular wear pattern of the grinding surfaces, but it is frequently found that considerable wear can take place before a reduction in output becomes pronounced and then a rapid falling off in output occurs. It is also frequently found that a falling off in fineness does not necessarily occur until wear has taken place to such an extent that output is falling off rapidly. It is generally found that increase in ash content in a coal or a change in quality of ash though not always affecting the out put of the mill, causes sufficient wear on the grinding elements to reduce interval between mill overhauls.The main factors affecting the mill out put, therefore are grindability, moisture content of the raw coal and the fineness requirement of the ground product
Mill Performance Effect of Volatile MatterEffect of Volatile matter : Low volatile coal burns slowly and very low volatile matter coal requires long flame burners. Low volatile coals are much more difficult to burn than high volatile coals from both the aspects of keeping down the carbon in dust loss and in maintaining the stable ignition. More turbulence, more surface area of coal particles and more time in the combustion zone are required for low volatile coals. This means increase in fineness and reduction in the quantity of primary air. Reduction in primary air leaves more of the total air required to be supplied as secondary or tertiary to promote turbulence and complete burnout. Theoretically hardly sufficient primary air to burn the volatile matter should be supplied but the quantity should not be reduced so much that there is too low air velocity in the fuel pipes and separation of coarse and fine particles occur.
Mill PerformancePulverized fuel fall out: Should the transport velocity be too low in the PF pipes and fittings some pulverized fuel fall out in suspension. This could result in fires inside the pipes so it is a condition to be avoided. Therefore, the minimum safe throughput of carrying air should be ascertained and the air flow always maintained above that value. Normally the minimum velocity of coal / air mixture is about 18-20 m/s.Flammability : Should the air fuel ratio become too high an explosive mixture may result. A ratio of 5:1 is regarded as the safe limit. Fan Power : The primary air fans or exhausters provide the power to transport the pulverized coal. It follows that when the fans are operated at maximum, it is a milling constraint. Investigations show that the fan performance can be represented by a simple formula with sufficient accuracy for our purpose, i.e.Wf + Wa = Constant Where Wf and Wa are fuel and air flows.This provides a means for determining the constraints for various fan speeds and various air / fuel ratios.
Mills –Nos Optimum EfficiencyThe most economical way of loading the mills is to have good mills loaded in their normal capacity and worn mills working at smaller output. The electrical energy required by a mill group per ton of coal ground is lowest when the mill is working at its normal output so that only the minimum numbers of mills necessary to provide required output should be in service. If conditions are favorable, say because of soft coal, then it may be possible to get the load with less than normal number of mills. If this can be done it will reduce the total milling power. Quite often consideration other than minimizing milling power predominate and determines which mill should be in service. It may be that steam temperature is affected by mill operation. In other cases certain combination of mills and associated burners will give an unbalanced furnace.
Coal mill operational problems
1) Mill Fires and explosions2) Mill rejects3) Coal fineness4) Mill power5) Moisture/Chocking 6) Boiler pressure variations7) Slagging and fouling
Mill Fires/ExplosionExplosion risks and fires : Raw coal on its own is safe to handle. However once it is ground to a fine powder and ultimately mixed with the transportation air, it becomes as highly flammable as gas. As little as 30 gms of coal dust, uniformly distributed in 1cum of air can cause an explosion if ignited. About 500gms per cubic meter is considered to be the concentration above which an explosion is highly unlikely. In normal operation, the coal/air ratio would be many times this value and safe operation is assured. It is only when temporary losses of coal feed occur or when mills are being started and stopped, that the mixture strength falls in the explosive range. If, under these conditions, the forward flow of air is significantly reduced or stopped and a source of ignition is present, then there is real risk of explosion. Sources of ignition in mill system vary from mechanically produced sparks, flashback from furnace or as result of mill fires developing. Fire are detected at a very early stage by CO monitoring equipment.
ESP PerformanceEfficiency of ESP is defined as the ratio of mass of all particles retained by collector (ESP) and the mass of all particles entering collector.The Collection efficiency of ESP is related to the time of particle exposure to the electrostatic field, the strength of the field, and the resistivity of the dust particle. Larger collection-surface areas and lower gas-flow rates increase efficiency because of the increased time available for electrical activity to treat the dust particles. •An increase in the dust-particle migration velocity to the collecting electrodes increases efficiency•It is easier to collect large particles than smaller particles.• Care must be taken to maintain the gas temperature entering the precipitator at or above the design temperature, otherwise local cold spots result in condensation, fouling and rapid corrosion.
CONSTRUCTION OF ESP
ESP PERFORMANCE - PROBLEMS OF ESPBROKEN WIRESELECTRODE FAILUREGAS DISTRIBUTIONALIGNMENT PROBLEMSCORROSIONBROKEN INSULATORSELECTRICAL SUPPLY PROBLEMSRAPPING SYSTEM PROBLEMSHOPPERS CHOKING
Boiler auxiliaries performanceFuel air damper will Maintain the length of the flame. Modulate in proportion to fuel burner pressure or speeder speed.
Auxiliary air damper will maintain the furnace to wind box DP.
Over fire dampers are used to control Nox.
Bottom auxiliary damper is used to control unburnt carbon falling in bottom ash hopper.
These dampers need to be checked physically at regular intervals for the proper functioning and opening as required.
RAPH should give the designed secondary and primary air temperatures at all loads and should ensure that the outlet flue gas temperatures are maintained without exceeding the air leakages across seals.
The excessive O2 is the indication of airheater leakages and differential pressure measurement across APH on air and flue gas sides indicate the fouling of airpreheaters.
Air Preheater PerformanceAir Preheater is a critical component of the boiler system and its performance has direct effect on boiler efficiency. APH leakage and APH gas side efficiency has to be checked to assess the performance of APH. Air heater leakage in APH is the weight of air passing from the air side to gas side of APH. This is the indication of the condition of the APH seals. As the APH seals wear the air leakage increases. The increase in air leakages increases the power requirement of the draft fans increasing the unit heat rate and possibly increasing limiting the unit loading.Air heater gas side efficiency is defined, as the ratio of temperature drop, corrected for leakage, to the temperature head expressed as percentage. This the indication of the internal condition of the heater.Gas side efficiency = (Tgnl – Tge) / ( Tae – Tge)
Ljungström Air Heater (RAPH)
The corrected gas outlet temperature Tgnl is defined as the outlet gas temperature for no air leakage and is given by the following formula :Tgnl = AL*Cpa (Tgl – Tae) / 100*Cpg + Tgl Where ,AL = Air leakage into the APH system %Tgnl = Gas outlet temperature corrected for no leakage deg CCpa = Mean specific heat Kcal/kg/deg CTae = Temperature of air entering the air heater deg CTal = Temperature of air leaving the air heater deg CTgl = Temperature of gas leaving the air heater deg CCpg = The mean specific heat between Tgl and Tgnl kcal/kg/deg CTge = Temperature of gas entering air heater deg CAs condition inside air heater worsens (basket wear, ash plugging etc),The air heater gas side efficiency decreases. This is generally accomplished by gas exit temperature increase and decrease in air heater outlet temperature .
Air Preheater Performance
The following relationship gives the air leakage into the APH system if the oxygen percentage is measured at the entry and exit of the APHAPH leakage , % = (O2% in the gas leaving APH – O2 % in the gas entering APH) * 100 (21 – O2 % in the gas leaving APH)Alternately, if the CO2 is measured in the exhaust gases then the air leakage is estimated byAPH leakage %, AL = (CO2 ppm in the gas entering the APH – CO2 ppm in the gas leaving the APH) * 100The use of preheated air improves combustion conditions and efficiency At all loads. The air heater reduces the physical size of the boiler. With the common fuel (coal, oil and gas) and identical furnace conditions , the efficiency of a steam generating unit as a whole will increase about 2.5 % for every 100 F decrease in exit gas temperature. This corresponds to an increase of about 2 % in efficiency for each 100 F increase in air temperature. An air heater providing air at temperatures ranging upwards from 300 F will often effect savings in the fuel ranging from 5 to 10 %.
CERC Norms of Auxiliary Power Consumption (APC)
For 200 MW - 9% with Cooling Tower - 8.5% without CT
For 500 MW - 7.5% with CT and TDBFP - 7% without CT and TDBFP - 9% with CT and MDBFP - 8.5% without CT and MDBFP
Energy Conservation in Power Utility
Factors Affecting Auxiliary Power Consumption
- Unit Generation and load Pattern
- Operation of Plant Auxiliaries
- Service auxiliary such as Illumination , air
conditioning
- Unit Startup / shutdown
Requirement of works power with load
2.78
0.22
1.381.41
0.26
0.74
0.35
0.29
0.26
2.37
BFPs
FD FANS
IDFANS
PAFANS
CEP
MILLS
CHP
AHP
Comp.air
Others
Auxiliary Consumption Pattern Typical(500MW Unit)
ID FANA B C D
A FD FAN B PA 5BPA 5A
GR 5A GR 5B
ESP
RAPH-5A RAPH-5B
Schematic representation of a balanced draft system
Balanced Draft System
