Technical Report
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CITATIONS
This report was prepared by
EPRI Fossil Maintenance Applications Center (FMAC) 1300 W.T. Harris Blvd.
Charlotte, NC 28262
This report describes research sponsored by EPRI.
The report is a corporate document that should be cited in the literature in the following manner:
REPORT SUMMARY
A continuous flow of air and combustible gases in fossil power plants is required to supply the correct amount of combustion air and to remove the gaseous combustion products. This flow, which passes through ducts, the boiler, heat exchangers, and flues and stacks, is created and sustained by stacks and/or fans. In fossil stations, supply air fans are often referred to as forced
draft (FD) fans and are used to push air through the combustion air supply system into the
furnace. Some stations also use fans to move the gaseous combustion products through heat exchanger surfaces to the stack. These are often referred to as induced draft (ID) fans. Background
Reliability of this equipment is important to plant efficiency and availability, and maintenance of these components becomes an important task for plant personnel. This issue ranked Number 1 in the 2003 FMAC Maintenance Issues Survey and, because of this, FMAC plans to begin a project to produce a guide that will address many of the common problems that members are facing with this equipment. Maintenance issues that are most often cited include bearing and alignment problems, lubrication, vibration problems (resulting from improper balancing or buildup of deposits), and erosion and wear of blades from entrained particles. Also reported are problems with dampers, particularly on flow control dampers.
Objectives
• To provide information on axial and centrifugal fans used for boiler draft service
• To assist fossil power plant maintenance personnel in troubleshooting and maintaining fans • To provide routine and preventive maintenance guidance to assist in improving the reliability
of fans Approach
In cooperation with interested FMAC members, a task group of utility engineers, equipment suppliers, and industry experts was formed. This group identified key design and maintenance issues facing plant personnel and provided input that was used in the preparation of the guidance set forth in this report. Experience-proven practices and techniques were identified during this effort and are summarized here for use by all power plant personnel.
Results
This guide provides the user with an understanding of FD and ID fans, including elemental component descriptions, common materials of construction, and typical applications. The scope of the guide includes common applications and criteria for selection, failure modes and
troubleshooting guidance, condition monitoring and predictive maintenance techniques, preventive maintenance strategies, recommendations on fan repair and inspection techniques, and good installation practices, including “how to” information on important steps.
EPRI Perspective
The information contained in this guide represents a significant collection of technical
information, including techniques and good practices, related to the maintenance, monitoring, and troubleshooting of this important piece of plant equipment. Industry knowledge from recent experiences and improvements has been included in this report. Assembly of this information provides a single point of reference for power plant personnel, both now and in the future. Through the use of this guide, EPRI members should be able to significantly improve and optimize their existing plant predictive, preventive, and corrective maintenance programs related to this equipment. This will help members achieve increased reliability and availability at a decreased cost. Keywords Plant maintenance Plant operations FD and ID fans Fossil
ACKNOWLEDGMENTS
FMAC would like to acknowledge the following individuals for their contributions during the development of this report.
Mark Litzenger AmerenUE
Alan Parkinson AmerenUE
Arnold Van Geuns ESKOM
Scott Hall Salt River Project
Edward Weeks Salt River Project
Michael Stewart AmerenUE
Herman Kleynhans ESKOM
Ken Leung Hong Kong Electric
Imraan Dindar ESKOM
Travis Houn Great River Energy
Christopher Rauch AmerenUE
Ray Henry Sargent and Lundy
Petrus Kruger ESKOM
Robert Vihnicka Sargent and Lundy
Bala Gogineni Sargent and Lundy
Alan Grunsky EPRI
FMAC also acknowledges the following organizations for permitting the generous use of figures and various materials from their literature and in-house resources and for reviewing and
providing valuable comments on this document. Howden Buffalo – John Magill and Robin Flemming Flatwoods – Jim Greenzweigs
CONTENTS
1 INTRODUCTION ...1-1 1.1 Purpose ...1-1 1.2 Organization ...1-1 1.3 Key Points ...1-2 2 GLOSSARY OF TERMS ...2-1 2.1 Terms and Acronyms ...2-1 2.2 Conversions for Units Used in This Report ...2-3 3 TECHNICAL DESCRIPTION...3-1 3.1 Introduction ...3-1 3.2 Fan Applications...3-3 3.2.1 Induced Draft Fans ...3-3 3.2.2 Forced Draft Fans...3-3 3.2.3 Balanced Draft...3-4 3.2.4 Cold Primary Air Fans ...3-5 3.2.5 Hot Primary Air Fans ...3-5 3.2.6 Gas Recirculation Fans ...3-6 3.2.7 Number of Fans...3-6 3.3 Fan Types ...3-7 3.3.1 Centrifugal Fans ...3-7 3.3.2 Axial Fans...3-13 3.4 Fan Drives ...3-15 3.5 Fan Controls...3-16 3.5.1 Centrifugal Fan Controls...3-16 3.5.1.1 Inlet Vanes ...3-16 3.5.1.2 Inlet Dampers ...3-17 3.5.1.3 Two-Speed Motors...3-183.5.1.4 Fluid Drive...3-18 3.5.1.5 Variable-Speed Motors ...3-19 3.5.1.6 Turbine-Driven Fans ...3-19 3.5.2 Axial Fan Control ...3-20 3.5.2.1 Variable-Pitch Blades...3-20 3.5.2.2 Variable-Speed Drives ...3-21 3.5.2.3 Variable Inlet Vanes...3-22 3.5.2.4 Cooling Fans...3-22 3.6 Other Components ...3-22 3.6.1 Bearings ...3-22 3.6.2 Lubrication Systems ...3-23 3.6.3 Turning Gear ...3-24 3.7 Fan Performance...3-24 3.7.1 Axial Fan Performance ...3-28 3.7.2 Fan Pressure Definition ...3-29 3.8 Selection of Fan Type ...3-32 3.9 Fan Requirements...3-33 3.10 Fan Testing ...3-33 3.11 Operation ...3-34 3.11.1 Prestart Checks ...3-34 3.11.2 Startup Procedures...3-34 3.11.2.2 Sequence of Steps for Startup...3-35 3.11.3 Alarm Conditions to Monitor ...3-35 3.11.4 Operating Parameters to Monitor ...3-35 3.11.5 Emergency Actions...3-36 3.11.6 Fan Control...3-36 3.11.6.1 Electric Motor Restrictions ...3-37 3.11.7 Control of Vanes and Dampers ...3-38 3.11.8 Fan Outlet Dampers ...3-38 3.11.9 Stall Prevention for Axial Fans ...3-39 3.11.10 Draft Fan Shutdown...3-39 3.11.10.1 Controlled Shutdown...3-40 3.11.10.2 Uncontrolled Shutdown...3-40
3.12 National Standards...3-42 4 FAILURE MODES AND EFFECTS ANALYSIS ...4-1 4.1 Blades ...4-6 4.2 Bearings ...4-7 4.3 Foundations ...4-8 4.4 Inlet Vanes ...4-9 4.5 Couplings ...4-10 4.6 Hydraulic Actuating Mechanism ...4-10 4.7 Electric Motors...4-10 4.8 Hubs ...4-11 4.9 Housing ...4-11 4.10 Turning Gears ...4-12 4.11 Shaft ...4-12 4.12 Center Plate ...4-13 4.13 Inlet Dampers ...4-13 4.14 Isolating Dampers ...4-13 4.15 Variable-Speed Drive ...4-14 4.16 Controls ...4-14 4.17 Ductwork ...4-14 5 TROUBLESHOOTING ...5-1 6 CONDITION MONITORING ...6-1 6.1 Vibration Monitoring ...6-1 6.1.1 Parameters ...6-1 6.1.1.1 Amplitude ...6-2 6.1.1.2 Frequency ...6-2 6.1.1.3 Phase Angle ...6-2 6.1.1.4 Vibration Form ...6-2 6.1.1.5 Vibration Mode Shape ...6-3 6.1.2 Vibration Analysis ...6-3 6.1.2.1 Amplitude Versus Frequency Analysis ...6-3 6.1.2.2 Real-Time Spectrum Analysis...6-4 6.1.2.3 Time Waveform Analysis ...6-4
6.1.3 Proximity Probes ...6-5 6.1.4 Velocity Probes...6-6 6.1.5 Accelerometer Probes ...6-7 6.1.6 Data Acquisition...6-8 6.1.6.1 Machine Diagram...6-8 6.1.6.2 Tri-Axial Readings ...6-9 6.2 Oil Analysis ...6-10 6.3 Nondestructive Examination...6-11 6.4 Infrared Thermography...6-11 6.5 Motor Current Analysis...6-11 7 MAINTENANCE ...7-1 7.1 Developing a Preventive Maintenance Program ...7-2 7.2 Basic Rules for Conducting Maintenance ...7-3 7.3 Periodic Maintenance Recommendations...7-3 7.4 Component Maintenance ...7-5 7.4.1 Bearings ...7-6 7.4.1.2 Routine Maintenance Recommendations ...7-7 7.4.1.2 Bearing Overhaul ...7-8 7.4.2 Lubrication System ...7-10 7.4.2.1 Routine Maintenance ...7-10 7.4.2.2 Circulating Lube Oil System Overhaul ...7-10 7.4.3 Couplings...7-12 7.4.3.1 Routine Maintenance Recommendations ...7-12 7.4.3.2 Coupling Overhaul ...7-13 7.4.3.3 Coupling Alignment...7-14 7.4.4 Variable Inlet Vanes and Control Dampers ...7-15 7.4.4.1 Routine Maintenance ...7-15 7.4.4.2 Inlet Vane Overhaul ...7-15 7.4.5 Centrifugal Fan Wheels ...7-16 7.4.5.1 Centrifugal Fan Wheel NDE...7-18 7.4.5.2 Blades ...7-18 7.4.5.3 Center Plate/Side Plate ...7-21
7.4.8 Structural Support System...7-22 7.4.8.1 Concrete Foundation ...7-22 7.4.8.2 Repairing Concrete Foundations ...7-22 7.4.8.3 Surface Cleaning ...7-23 7.4.8.4 Crack Repair ...7-23 7.4.8.5 Anchor Bolts...7-24 7.4.8.5.1 Forces Affecting Anchor Bolts...7-24 7.4.8.5.2 Proper Installation ...7-24 7.4.9 Housing ...7-25 7.4.9.1 Housing...7-25 7.4.9.2 Inlet Cones...7-26 7.4.9.3 Fan Wheel Clearance ...7-26 7.4.9.4 Access Plates/Doors...7-28 7.4.10 Expansion Joints ...7-28 7.4.11 Electric Motors...7-29 7.4.11.1 Dirt ...7-29 7.4.11.2 Moisture ...7-30 7.4.11.3 Friction ...7-30 7.4.11.4 Vibration...7-30 7.4.11.5 Rotor Shaft End Play ...7-31 7.4.12 Fluid Drives...7-32 7.4.12.1 Routine Maintenance ...7-32 7.4.12.2 Overhaul Fluid Drive ...7-33 7.4.13 Turning Gear ...7-34 7.4.14 Hydraulic Supply System...7-35 7.4.15 Axial Fan Blade Adjustment System ...7-35 7.4.16 Axial Fan Blade Bearings ...7-35 7.4.17 Axial Flow Fan Rotor Overhaul...7-36 7.5 Fan Wheel Balancing ...7-36 7.5.1 Size of the Balance Weight ...7-38 7.5.2 Location of the Balance Weight...7-38 7.5.3 Vibration Sensitivity ...7-38 8 SPECIAL MAINTENANCE TASKS...8-1 8.1 Extended Shutdown ...8-1
9 FAN UPGRADE OPTIONS ...9-1 9.1 Reasons for Fan Upgrade ...9-1 9.2 Tipping ...9-1 9.3 Wheel Replacement ...9-2 9.4 Housing Modifications ...9-3 9.5 Coatings ...9-3 10 SAFETY...10-1 10.1 Rotating Equipment...10-1 10.2 Confined Space...10-1 10.3 Burn Hazards ...10-1 10.4 Electrical...10-1 10.5 Operation Testing...10-2 10.6 Cleaning Operations...10-2 10.7 Fan Movement ...10-2 11 TRAINING...11-1 12 REFERENCES ...12-1
A CENTRIFUGAL FAN WHEEL INSPECTION AND REPAIR ... A-1
B KEY POINT SUMMARY ... B-2
C TRANSLATED TABLE OF CONTENTS ... C-1
Français (French) ... C-2 日本語 (Japanese) ... C-12 Español (Spanish) ... C-24
LIST OF FIGURES
Figure 3-1 Boiler Air Flow Schematic...3-2 Figure 3-2 Section of a Centrifugal Fan ...3-8 Figure 3-3 Cross-Section View of a Centrifugal Fan...3-9 Figure 3-4 Centrifugal Fan Components and Accessories ...3-10 Figure 3-5 Typical Rotor with Forward Curved Blades for a Centrifugal Fan...3-11 Figure 3-6 Wheel Blade Types and Rotation (Viewed from the Drive End) ...3-12 Figure 3-7 Centrifugal Fan Rotor Components ...3-13 Figure 3-8 Two-Stage Axial Fan Assembly...3-14 Figure 3-9 Two-Stage Axial Fan Impeller ...3-14 Figure 3-10 Inlet Vane Control Assembly ...3-17 Figure 3-11 Variable-Pitch Axial Fan Components ...3-20 Figure 3-12 Sleeve Bearing Components ...3-23 Figure 3-13 Typical Centrifugal Fan with Variable Inlet Vanes ...3-26 Figure 3-14 Typical Centrifugal Fan with Variable Inlet Vanes and Showing System
Curve...3-27 Figure 3-15 Typical Centrifugal Fan with Speed Control ...3-28 Figure 3-16 Performance Field for Variable-Pitch Axial Flow Fan ...3-29 Figure 3-17 Fan Pressure Definitions ...3-30 Figure 3-18 Fan Correction for Inlet Density ...3-31 Figure 4-1 Centrifugal Fan Housing Components ...4-12 Figure 7-1 Centrifugal Fan Wheel...7-17 Figure 7-2 Wear and Erosion Protective Accessories ...7-20 Figure 7-3 Cross-Section View of a Fan Illustrating Clearance Requirements Between
the Wheel Inlet and the Inlet Bell ...7-26 Figure 7-4 Enlarged View of Figure 7-3 ...7-27 Figure 7-5 Basic Block Diagram of a Hydraulic Regulating System ...7-32 Figure 9-1 Airfoil Blade with the Blade Tipped...9-2 Figure A-1 Blade Map Example ... A-6 Figure A-2 Blade Map Example ... A-7 Figure A-3 Blade Map Example ... A-8 Figure A-4 Example of Undercut and Overlap ... A-11 Figure A-5 Hand-Held Yoke Probe with Articulated Legs ... A-15
Figure A-6 Orientation of an Articulated Yoke Probe to Produce Flux Lines at Right
Angles ... A-16 Figure A-7 Indications Are Consecutively Numbered and Circled ... A-19 Figure A-8 Marking Multiple Indications ... A-19
LIST OF TABLES
Table 4-1 Summary of Centrifugal Fan Problem Areas ...4-2 Table 4-2 Summary of Axial Fan Problem Areas...4-3 Table 4-3 FD Fan Failure Data for U.S. Fossil Plants from 1982 Through 1995
(NERC/GADS Data) ...4-4 Table 5-1 Fan Troubleshooting...5-1 Table 5-2 Bearing Troubleshooting...5-3 Table 5-3 Lubrication System Troubleshooting ...5-4 Table 5-4 Hydraulic System Troubleshooting ...5-5 Table 5-5 Troubleshooting Noise Level ...5-6 Table 5-6 Troubleshooting Fluid Drive ...5-7 Table 5-7 Fan Performance Troubleshooting ...5-9 Table 7-1 Surveillance and Preventive Maintenance Frequencies ...7-3 Table A-1 Summary of Inspection Practices for Centrifugal Fan Wheels ... A-3 Table A-2 Percent of Fans Undergoing NDE ... A-13 Table A-3 Typical Materials in Power Plant Fans ... A-25
1
INTRODUCTION
Fired steam generators (that is, boilers) require large draft fans to move air into the furnace and to remove the combustion products. Draft fans are primary auxiliaries that support boiler operation for all types of fuel and firing methods. Draft fans are typically used in four principal applications: forced draft (FD), primary air (PA), induced draft (ID), and gas recirculation (GR). Other applications include booster fans and mill exhausters. This guide addresses FD and ID fans, which are the largest and most common applications. The maintenance recommendations for these fans are easily adapted for most other draft fan applications.
1.1 Purpose
This FD and ID fan maintenance guide provides information on axial and centrifugal fans used for boiler draft service and is intended to assist fossil power plant maintenance personnel in troubleshooting and maintaining fans. A discussion of fan characteristics and components that serves as a reference for understanding the basics of fan performance and mechanical
construction is provided. Routine and preventive maintenance guidance is provided to aid in improving the reliability of fans. A troubleshooting guide assists in diagnosing problems that have been encountered in various fan failure reports. Data for this guide were obtained from direct experience in fossil plants, industry surveys on failure reports, vendor input, and reviews of plant literature and industry documentation.
1.2 Organization
The organization of this guide is as follows:
Section 1 is an introduction and discussion of the guide’s purpose and organization. Section 2 is a glossary of items, definitions, and acronyms used in this guide.
Section 3 provides technical description, basic discussion of fan performance characteristics, and the mechanical components.
Section 4 provides an analysis of failure modes and effects as well as failure data. Section 5 provides guidance for troubleshooting and corrective actions.
Section 7 provides guidance for preventive maintenance.
Section 8 includes recommendations for special maintenance tasks. Section 9 presents upgrade options.
Section 10 provides recommendations for safety. Section 11 provides training recommendations. Section 11 is a list of references.
Appendix A discusses centrifugal fan wheel inspection and repair. Appendix B provides a list of all the key points indicated in the guide.
1.3 Key Points
Throughout this guide, key information is summarized in “Key Points.” Key Points are bold-lettered boxes that succinctly restate information covered in detail in the surrounding text, making the key points easier to locate.
The primary intent of a Key Point is to emphasize information that will allow individuals to take action for the benefit of their plant. The information included in these Key Points was selected by Fossil Maintenance Applications Center (FMAC) personnel, the consultants, and utility personnel that prepared this guide.
The Key Points are organized according to three categories: Operation and Maintenance (O&M) Costs, Technical, and Human Performance. Each category has an identifying icon, as shown below, to draw attention to it when quickly reviewing the guide.
Appendix B contains a listing of all of the key points in each category. The listing restates each key point and provides reference to its location in the body of the report. By reviewing this listing, users of this guide can determine whether they have taken advantage of key information that the writers of the guide believe would benefit their plants.
Key O&M Cost Point
Emphasizes information that will result in reduced purchase, operating, or maintenance costs.
Key Technical Point
Targets information that will lead to improved equipment reliability.
Key Human Performance Point
Denotes information that requires personnel action or consideration in order to prevent injury or damage or ease completion of the task.
2
GLOSSARY OF TERMS
2.1 Terms and Acronyms
ABMA American Boiler Manufacturers Association acfm actual cubic feet per minute
AMCA Air Movement and Control Association, Inc. ASM American Society for Metals
ASTM American Society for Testing and Materials AWS American Welding Society
BTS Blade tip speed cfm cubic feet per minute
EPRI Electric Power Research Institute FD Forced draft
FMAC Fossil Maintenance Applications Center fpm feet per minute
FSP Fan static pressure FTP Fan total pressure FVP Fan velocity pressure
GADS Generating Availability Data System GR Gas recirculating
hp Horsepower
ID Induced draft
ksi kips per square inch
MT Magnetic particle testing MTBF Mean time between failures NDE Nondestructive examination
NEMA National Electrical Manufacturers Association NERC North American Electric Reliability Council NFPA National Fire Protection Association
O&M Operation and maintenance
PA Primary air
PM Preventive maintenance psi pound(s) per square inch
psig pound(s) per square inch, gauge PWHT Post-weld heat treatment
SCR Selective catalytic reduction SMAW Shielded metal arc welding SPR Static pressure rise
SSS Structural support system UT Ultrasonic test
VT Visual test
WG Water gauge
WMP Wet fluorescent magnetic particle (test) WR2 Moment of inertia
2.2 Conversions for Units Used in This Report
°C = (°F – 32) x 5/9 1 hp = 746 W 1 lb = 0.45 kg 1 lb/ft3 = 16 kg/m3 1 psi(g)= 6.9 kPa 1 in. = 25.4 mm 1 fpm = 0.30 mpm 1 ft = 0.3 m 1 in2 = 6.45 cm2 1 ft-lb = 1.35 joules 1 ksi = 6.9 MPa
3
TECHNICAL DESCRIPTION
3.1 Introduction
A fan is a device that produces a flow of gas by the movement of a surface. As used in this guide, a fan is defined as a turbo machine with a rotating impeller enclosed in a casing. Fans are similar to compressors; the difference is that fans create a flow of gas whereas
compressors increase the pressure of the gas. To increase a flow, fans must increase the pressure of the gas and compressors must create a flow. In the past, there were specific criteria defining the difference between fans and compressors. For example, the 1946 edition of ASME PTC-11, “Performance Test Code for Fans,” defines a fan as providing a compression ratio of 1.1 or a density change of 7%. ISO 5801 defines the upper limit of fans as a pressure increase of 120 inches Wg (30 kPa). ASME PTC-10, “Performance Test Code on Compressors and Exhausters,” states that compressors are usually intended to produce considerable density change. The choice of whether a device is a fan or a compressor is not regulated or standardized.
Draft fans provide one or a combination of the following functions in a boiler: • Supply air required for combustion
• Remove products of combustion • Deliver fuel to the burners
• Circulate the gases for better heat transfer
In addition, the fan selected must be adequate for the required duty with regard to air volume, static pressure, horsepower, and noise. Discussion of these topics follows.
3.2 Fan Applications
3.2.1 Induced Draft Fans
ID fans move the combustion flue gas through the boiler, air heater, and precipitator or the baghouse, scrubber, and chimney to the atmosphere. They are a major component of a fossil-fired plant and typically consume approximately 2% of the gross electrical output.
ID fans have the largest design margins of any major equipment in a fossil-fueled power plant. The margins are typically 15% on flow, 30% on head, and 25°F on temperature. The large margins are intended to allow for the following:
• Uncertainty in determining system requirements • Allowance for wear
• Operating flexibility
• Allowance for pluggage and leakage • Air infiltration
Even with these large margins, it is not uncommon for the ID fans to be the limiting factor on the output of a coal-fired unit. ID fans are included in the top 25 causes of fossil plant outages and are responsible for approximately 2% of the total outages of fossil-fired units.
The temperature of gas to be handled by the ID fan is based on the calculated unit performance at maximum boiler load. Temperature affects fan performance, and thus, a margin on temperature should be included to allow for variations in operation.
Key Technical Point
Temperature affects fan performance, and thus, a margin on temperature should be included to allow for variations in operation.
3.2.2 Forced Draft Fans
FD fans provide combustion air for boilers. The FD fan inlet is open to the atmosphere and discharges through air preheating coils, an air heater into the boiler windbox, and finally through the burners into the furnace.
In pulverized coal-fired boilers, approximately one-third of the combustion air is PA that is used to transport the pulverized coal to the burners. Some boiler designs use PA fans, which may take suction from the atmosphere and operate in parallel with the FD fans or may take suction from the FD fan discharge and operate in series with the FD fans. The PA fan application is similar to the FD fan; therefore, the description, problem area, and maintenance requirements described for
FD fans are essentially the same for PA fans. Other boiler designs use mill exhausters that take the air and pulverized coal mixture from the mill outlet and transport the mixture to the burners. Because of the erosive nature of the pulverizer coal-air mixture, mill exhausters have a very different application than the FD, ID, or PA fans and are not addressed in this guide.
For pressurized units without ID fans, the FD fan is sized for the entire system to the stack or to the pollution control system.
FD fans for coal-fired plants rank close behind ID fans as the cause of outages. The causes of FD fan failures are similar to those for ID fans. The FD fans for a coal-fired plant consume
approximately 0.7% of the gross electrical output.
The design margins on FD fans are typically smaller than the margins on ID fans but still larger than on other major equipment. Margins of 15% on flow and 30% on head at the maximum expected ambient temperature are common.
FD fans are normally equipped with sound trunks (inlet boxes) for noise attenuation. When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration. An alternative method of noise attenuation is using a fan room. This involves the use of open inlet FD fans located in a specially designed room with acoustical baffles for air entry.
Key Technical Point
When specifying FD fans, pressure loss through the silencers (if they are provided) must be taken into consideration.
3.2.3 Balanced Draft
The balanced draft system uses both an FD fan system and an ID fan system to move air through the boiler.
FD fans on a balanced draft boiler must have the necessary volume output of air required for combustion, plus air heater losses and discharge pressure high enough to equal the total
resistance of air ducts, air heater, burners, and any other resistance between fan discharge and the furnace.
ID fans in a balanced draft boiler move the gaseous products of combustion over convection heating surfaces, pollution control system(s), plus the gas passages between the furnace and stack.
The weight of gas to be handled by the ID fan is the sum of the following: • Theoretical air for combustion
• Excess air required at burner • Infiltration
• Leakage air-to-gas through the air heater
The draft to be provided by the fan is determined by losses through the following boiler components:
• Furnace
• Boiler and superheater • Economizer
• Selective catalytic reduction (SCR) • Air heater
• Precipitator or baghouse • Ductwork
• Flue gas desulfurization system (scrubber) • Stack
For fan design, safety margins are added to the net weight requirement, net draft requirement, and gas temperature.
3.2.4 Cold Primary Air Fans
Cold primary air fans take ambient air and discharge it through the air heater, where the air is heated up to 650ºF (the actual temperature depends on the moisture content of the coal), and then into the pulverizers—where it is used to dry, heat, and convey the pulverized coal to the burners. This system is used on large boilers where fans are installed in parallel in order to service a bank of pulverizers.
3.2.5 Hot Primary Air Fans
Hot primary air fans take heated air from the air heater and blow it into the pulverizers where it is used to dry, heat, and convey the pulverized coal to the burners.
3.2.6 Gas Recirculation Fans
A gas recirculation system performs either or both of the following functions:
• Controls steam temperature over a wide boiler load range. To accomplish steam temperature control, a portion of the flue gas from the economizer outlet is introduced in the lower part of the furnace by means of a suitable fan and ducts. This is known as gas recirculation.
• Controls furnace gas temperature when a portion of the flue gas from the economizer outlet is recirculated to the furnace outlet. This is called gas tempering and may be used to control NOX.
The volume requirements of the gas recirculating fan are determined by the amount of
recirculation necessary to obtain the required steam temperature. Maximum flow can occur at either full boiler load or some reduced boiler load point, depending on boiler design. The gas recirculation fan must be sized so its pressure capability will always exceed the pressure differential developed by the boiler; otherwise, backflow of high-temperature furnace gas will result through the fan, with serious consequences.
Radial tip blade fans (see Figure 3-6) can be applied for gas recirculation duty, but the straight blade fan may be needed where high concentrations of fly ash will be encountered, depending on the ash properties. Inlet dampers are the principal means of accomplishing volume control.
3.2.7 Number of Fans
One of the major design decisions for a fan system is the number of fans. The factors to be considered in selecting the number of fans are initial cost, operation and maintenance (O&M) costs, flexibility of operation, and reliability.
When evaluating initial cost, the cost of motors, ductwork, insulation, control equipment, electrical equipment, and foundations must be considered in addition to the cost of the fans. The fewest number of fans usually results in the lowest initial cost.
Key O&M Cost Point
The fewest number of fans usually results in the lowest initial cost.
Operating cost usually decreases as the number of fans increases. Fans usually have their highest efficiency near their design points. At lower loads, some of the fans can be shut off in a system with more fans. The remaining fans will then operate closer to their design points and, therefore, more efficiently. An important parameter for evaluating operating cost is the projected loading schedule for the generating unit. The variation in operating cost with the number of fans will be less for a unit that operates at or near full load most of the time than for a unit that operates at
It is difficult to assign cost values to the differences in reliability and flexibility of operation with different numbers of fans. It is also difficult to assign a cost value to plant arrangement. Plant arrangements can be improved by reducing the number of fans. For these reasons, the selection of the number of fans is not a straightforward economic evaluation.
In addition to the factors discussed above, practical aspects should also be considered. For centrifugal fans, because of fan size limitations, the maximum unit size for which two ID fans can be used is approximately 700 MW. The same number of FD and ID fans is usually selected to simplify operation of the fans.
The trend in the power industry has been to use two FD and two ID fans up to approximately 700 MW. Above this size, it is not practical to build ID fans that are large enough to use two fans. In recent years, this concept has been challenged, and units with one FD and one ID fan have been built up to a limit of 500 MW. Obviously, the cost for one fan and motor is less than for two, but the bulk of the savings comes from the reduced number of ducts as well as capital costs.
The number of ID fans can have an effect on unit availability. Many owners believe that two fans will provide better availability than one. However, with two fans, the probability of a fan failure is roughly twice that for one fan, but the impact of a failure is approximately one-half. Thus, the equivalent availability is about the same. There is not a large enough database of boilers with single FD and ID fans to verify this probability, but statistical studies (using the NERC-GADS database) of boiler feed pumps verify this theory. A boiler with one full-size boiler feed pump has more full forced outages than units with two 50% capacity pumps but fewer forced deratings. The overall equivalent availability of one full-size feed pump is slightly higher than two half-size pumps.
On large units with four ID fans, the unit will probably be capable of operating at full load with three fans under normal operating conditions. Thus, the unit will essentially have an installed spare, which should result in improved availability.
3.3 Fan Types
Fans are a type of turbo machinery that transfers energy to air in order to increase pressure to induce flow. They are usually classified by the direction of flow through the impeller. The two types used in boiler draft applications are centrifugal fans, where the flow is radially outward, and axial fans, where the flow is axial along the fan shaft. Centrifugal fans are sometimes referred to as radial fans.
3.3.1 Centrifugal Fans
The centrifugal fan produces movement by throwing air off the blades in a radial direction by means of centrifugal force. The air or gas enters the inlet boxes and travels through the inlet vanes into the spinning fan wheel and then discharges through the scroll outlet, which is perpendicular to the fan shaft. Centrifugal fans are available with and without inlet boxes and with single or double inlets. The flow through the fan is controlled with inlet dampers or variable-pitch inlet vanes. Figure 3-2 is a photograph of a cut-out section of a centrifugal fan.
Figure 3-2
Section of a Centrifugal Fan (Courtesy of Howden Buffalo, Inc.)
Figures 3-3 and 3-4 illustrate the cross-sectional view of various parts of a typical centrifugal fan.
Figure 3-3
Figure 3-4
Centrifugal Fan Components and Accessories (Courtesy of Howden Buffalo, Inc.)
Figure 3-5
Typical Rotor with Forward Curved Blades for a Centrifugal Fan (Courtesy of Howden Buffalo, Inc.)
Centrifugal fans are available with the following various blade profiles, as shown in Figure 3-6: • Airfoil backward inclined
• Flat bladed backward inclined • Radial tip
• Straight radial • Forward curved
Figure 3-6
Wheel Blade Types and Rotation (Viewed from the Drive End)
In most power plant applications where the fans will handle clean air or clean gas, the highly efficient backward inclined airfoils are the preferred design. In an application where the fan is
The airfoil centrifugal fan is the most frequently applied fan for forced draft duty because of its inherently high efficiency, low noise, stable performance, and steep rising pressure curve. These features ensure good operation, particularly when fans are operating in parallel. Forced draft service tends to be a good application for axial fans. The parts of a typical centrifugal fan rotor are shown in Figure 3-7.
Figure 3-7
Centrifugal Fan Rotor Components
3.3.2 Axial Fans
Axial fans produce movement of air along their axis or in an axial direction. Axial fans are offered in single- and two-stage models. They are available with fixed-pitch and variable-pitch blading. The variable-pitch fan is more sophisticated and efficient than the fixed-pitch fan. Figures 3-8 and 3-9 illustrate sectional views through a two-stage axial fan. Air or gas enters through a single inlet box where it makes a 90-degree turn through straightening vanes, passes through the impellers, and exits through the diffuser section.
Figure 3-8
Two-Stage Axial Fan Assembly
Figure 3-9
Where higher pressures are necessary, two-stage fans may be required. Two-stage fans are offered for large-capacity induced draft service. A brief description of each of the main components of the axial fan follows.
Rotor Hub. The rotor hub is the major component of an axial fan. The hub serves as the
retaining ring for the blades and their bearings and as the housing and protective cover for the blade-actuating mechanism.
Blade Shaft Bearings. Blade shaft bearings are used to transfer the centrifugal force from the
blades to the fan hub.
Main Shaft and Main Shaft Bearings. In the axial fan, the main shaft does not carry large loads
over long spans between bearings as in a centrifugal fan. The rotating masses are low, and the bearings are located close to the rotor. The shafts, therefore, are small in diameter and relatively inexpensive in comparison to the large-diameter, long, forged steel shafts required for centrifugal fans.
Blades. Various materials are used for axial fan blades. For forced draft service, relatively
inexpensive cast aluminum blades are generally used. For induced draft service, where high wear resistance is required, ductile iron, cast steel, cast steel with hardened surfaces, or forged
aluminum blades with stainless steel inserts are available. Blade material selection is influenced by the blade velocity and the size and hardness of the dust particles.
In induced draft service, if the fans are operated for long periods with precipitators performing at low efficiencies, excessive blade wear will occur no matter what blade material is used. This is true for either type of fan. In the axial fan, however, a worn set of blades can be replaced in a few shifts. In the centrifugal fan, days of welding and balancing may be required to repair the fan. Axial flow fans are more complicated than centrifugal fans but have the advantage, in most cases, of higher efficiency over a wider load range.
3.4 Fan Drives
Electric motors are normally used for fan drives because they are less expensive and more efficient than any other type of drive. For fans of more than a few horsepower, squirrel-cage induction motors are more widely used. This type of motor is relatively inexpensive, reliable, and highly efficient over a wide load range. It is frequently used in large sizes with a hydraulic coupling for variable-speed installations.
Two-speed ac electric motors can be used in conjunction with inlet vanes to obtain slightly higher efficiencies at lower loads. This arrangement has a higher initial cost and is less reliable than a single-speed motor and inlet vanes. Variable-speed motors are the optimum type of drive
for centrifugal fans. They allow the fan to operate near its peak efficiency over the entire load range; controllability is good, and fan erosion is substantially reduced at lower speeds. Recent developments in variable-speed motors make them an excellent choice for centrifugal fans, although the high initial cost of variable-speed motors is not justified in all applications. Steam turbines have higher initial and maintenance costs and are less reliable but do provide a variable-speed drive. Steam turbines are not competitive with electric motors for fan drives.
3.5 Fan Controls
3.5.1 Centrifugal Fan Controls
As the load on a boiler varies, the pressure and flow requirements from the fans in the system vary. The most widely used methods to control centrifugal fans are by means of inlet vanes and variable-speed drives.
3.5.1.1 Inlet Vanes
Inlet vanes introduce a swirl to the flow entering a fan. This changes the angle of attack between the flow and the fan blade and effectively changes the fan characteristics. Inlet vane control has a low initial cost, is a simple method of control, and is very common for ID fans. Figure 3-10 is an illustration of an inlet vane control assembly. The major disadvantage of inlet vanes is poor efficiency at lower loads. Inlet vanes are subject to erosion if ash concentrations are high. The vane linkage and bearings can bind or become damaged if they are located in the gas stream. Therefore, these components should be located outside the fan inlet housing, where inspection and maintenance can be performed without entering the fan. The duct connections at the fan inlet or outlet should be flexible to isolate the fan from duct expansion and vibration. The duct should be separately anchored and not supported by the fan.
Figure 3-10
Inlet Vane Control Assembly
Inlet vane control is more efficient than inlet damper control because inlet vanes use part of the pressure head loss to accelerate the incoming gas in the direction of wheel rotation.
To optimize system performance, an operator should perform the following:
• Operate with all dampers fully open in order to limit pressure head losses. (Vane position should be set based on the load requirement.)
• Watch for control or mechanical problems that will cause vane flutter.
• Manually position the inlet vane control during initial startup to avoid pressure excursions. • Ensure that shut-off dampers are closed tightly to provide air/gas seal before startup; this
limits the inertia acceleration load on the motor. 3.5.1.2 Inlet Dampers
Inlet dampers control air flow by introducing a swirl in the flow and pressure drop. Inlet dampers have a low initial cost, are simple, and are not as prone to erosion as inlet vanes. The control linkage on inlet dampers is simpler than that for inlet vanes and can be located completely outside the duct. However, system pressure pulsations are more common with inlet dampers than inlet vanes. Inlet dampers can create vortices in the inlet boxes or around the fan shaft. The biggest disadvantage of inlet dampers is their low efficiency at low loads.
3.5.1.3 Two-Speed Motors
The fan selection with two-speed motors is the same as with inlet vane control or inlet damper control. The fans for two-speed motors are often sized so that the fan can operate on low speed at full load and at normal operating temperature. The high speed provides the design margin. Operators at some plants with two-speed ID fan motors do not change the motor speed while the unit is operating. This is usually based on their past experience, when the unit has tripped
because of furnace pressure excursions that occur when speeds are changed. This problem can be overcome by a careful design of the control system and requires a review of the damper or vane control response, the allowable furnace pressure limits, and the time it takes for the motor to change speed.
3.5.1.4 Fluid Drive
Fluid drive is a method of varying the fan speed for flow control. The fan selection is essentially the same as the inlet damper alternative, except that a fluid drive is located between the motor and the fan to control the fan speed. Inlet dampers are typically used in addition to the fluid drive to increase the speed of response to avoid furnace pressure excursions during transients. The use of the dampers for control during normal operation is typical but can be eliminated in most installations. Using speed control with the dampers full open can result in a significant power savings (200 hp on a 6000-hp fan) with only minor modifications.
Speed control allows the fan to operate near peak efficiency over the entire load range. However, the fluid drive has a maximum efficiency of approximately 95%, and it decreases at lower
speeds. The combined efficiency of the fan and fluid drive is slightly lower than inlet vane control at full load, but it is higher at lower loads. The major disadvantage of a fluid drive is the high initial cost, which is approximately the same as that of the fan.
Fluid drive control provides better overall efficiency than inlet vane configuration. To optimize system performance for a fluid drive system, the operator should perform the following:
1. Start the motor under a no-load condition (that is, ensure that the fluid coupling is empty and the inlet vane or damper is closed). This will limit the inertia acceleration load on the motor by accelerating the inertia of the fan to its running speed at a controllable rate.
2. Verify that all shut-off dampers (that is, non-control dampers) are fully open before startup to prevent pressure head losses.
3. Do not use louver dampers or inlet vanes as flow control devices to throttle the fan output volume in a system that was backfitted with a fluid drive. This will cause energy losses. The control system should be tuned to allow a variable-speed drive to have control with the dampers in the fully open position for a given flow range.
4. Watch for control or mechanical problems that could cause the system to hunt from one point to another.
3.5.1.5 Variable-Speed Motors
Variable-speed motors are directly connected to the fan. The speed of the motor is continuously variable from approximately 10% up to the full speed. Synchronous or induction motors can be used with variable frequency drives, and the frequency of the power to the motor is controlled by an electronic system. The incoming ac power is converted to adjustable voltage dc power by a thyristor. The adjustable dc power is connected to an inverter, which converts it to an adjustable ac power output.
Speed control is the optimum method of controlling centrifugal fans. The system resistance for ID service, and most FD fan service, is essentially a square curve. Because fan efficiency is essentially constant over similar flow-head squared curves, a variable-speed fan can operate near its best efficiency over the entire load range. Also, the control is stable down to essentially zero flow. Fans with variable-speed motors do not require a turning gear because the main motor can operate at the turning gear speed for extended periods.
3.5.1.6 Turbine-Driven Fans
Turbine-driven ID fans have been studied by architect-engineers and turbine manufacturers, but very few have been installed. The advantage of turbine-driven fans is the improved plant heat rate. Based on previous studies, the capital cost of turbine-driven fans is considerably more than the cost for centrifugal fans with variable-speed motors.
In most cases, turbine-driven ID fans do not result in a fuel cost savings over ID fans with variable-speed motors. The efficiency of a turbine drive is typically 81%, which is less than the main turbine efficiency of 88%. For a motor-driven fan, the efficiencies of the main generator (98%), transmission (98%), and motor (95%) result in an overall efficiency approximately the same as that of turbine-driven fans.
The National Fire Protection Association (NFPA) standards for explosion prevention require that a master fuel trip shall not trip the fans. To meet this requirement, an auxiliary steam source, such as a cross connection to a second unit or an auxiliary boiler on continuous standby, would be required with turbine-driven fans. The costs for this auxiliary source would have to be added to the estimated costs above.
Turbine drives are less reliable than motor drives. Because the availability of variable-speed motors—including the associated electrical equipment—is higher than that of turbine drives, turbine drives are not recommended for ID fans.
3.5.2 Axial Fan Control
Axial flow fans can be controlled by using variable-pitch blades, inlet vanes, or variable-speed drives.
In a variable-pitch axial fan, blade adjustment levers are located within the hub and are actuated hydraulically. Variable-pitch axial fans respond quickly and smoothly to system demand
changes. Variable-speed axial fans are not normally used because variable-pitch axial fans are lower in cost and achieve similar efficiency.
3.5.2.1 Variable-Pitch Blades
Axial fans can be controlled by varying the blade pitch or speed or by using variable inlet vanes. Either varying the blade pitch or using variable inlet vanes controls the flow by operating on the same principle as do variable inlet vanes on a centrifugal fan. Varying the blade pitch is more efficient than using variable inlet vanes because the flow resistance of the vanes is absent. Variable-pitch blades can provide efficiency as high as that of variable-speed control over most of the load range for a lower initial cost. Variable-pitch blades are the most common method of control; variable inlet vanes are used occasionally, and variable-speed control is rare. Figure 3-11 shows the components of a variable-pitch axial fan.
The most common blade actuation system for variable-pitch axial fans is a hydraulic system with a rotating union between the rotating and stationary parts. A critical design area of variable-pitch axial fans is the blade thrust bearings. These bearings experience very little movement and loads in excess of 100,000 pounds. Also, the shaft rotation acts as a centrifuge that separates particles out of the bearing lubricant and separates the lubricant if it is a mixture of elements of different densities. Ball bearings are accepted as the best type of bearing for this application.
The impeller blade adjustment system is made up of a stationary servomechanism, rotating seal, and rotating piston rod. Alignment between the servomechanism and rotating piston rod is maintained by antifriction-type bearings. This unit provides a transition between the non-rotating regulating levers and the rotating piston rod; and the servomechanism’s pilot valve transforms a mechanical input signal from the regulating lever to a hydraulic signal. This hydraulic signal, in turn, is received by the hydraulic cylinder in the rotor assembly.
The impeller blade adjustment system provides interface between the lever assembly and the rotor assembly. The impeller blade adjustment system receives its input signals from the boiler control system through an electric actuator (located outside the axial fan housing) and the hydraulic supply system. A clevis arm provides a mechanical link between the lever and the servo. Lever motion is translated into the clevis arm moving axially. The clevis arm’s motion causes three oil ports inside the servo to close or open in a specific sequence in order to allow oil to flow to or from the hydraulic supply system. Oil movement determines which side of the hydraulic cylinder is under pressure, ultimately resulting in movement of the blades to an ordered position.
In some applications, a combined lube oil/hydraulic supply system is used to provide oil that both lubricates main fan bearings and provides pressurized hydraulic oil to serve as the working medium to vary the blade pitch.
Basic components of the hydraulic oil system include primary and secondary pumps, heat exchangers (either air- or water-cooled type), reservoir, instrumentation (such as pressure switches, temperature switches, gauges, and alarms), supportive piping, and isolation valves. 3.5.2.2 Variable-Speed Drives
It is possible to have fixed pitch blades with an axial fan and control the fan using a variable-speed drive. Although the arrangement would be slightly more efficient at low flow rates than a variable-pitch blade axial flow fan, the cost would be higher. Variable-speed axial flow fans are therefore rarely used in boiler draft applications.
3.5.2.3 Variable Inlet Vanes
This alternative has fixed blades and variable inlet vanes. The inlet vanes change the angle of attachment between the flow and the blades, similar to the effect of inlet vanes on a centrifugal fan. The design is less complicated than the variable-pitch blade design but is also considerably less efficient. Fixed blade axial fans have most of the disadvantages of variable-pitch blade axial fans without the advantage of high efficiency.
3.5.2.4 Cooling Fans
All the bearings (that is, main shaft bearings and blade bearings) in an axial flow fan are inside a fairing inside the fan. For an ID fan or any other hot air or gas service, an external cooling fan is used to provide cool, clean air to pressurize and cool the main shaft bearings and vibration probes.
3.6 Other Components
3.6.1 Bearings
Two general types of bearings used in draft fan applications are rolling contact and sliding contact. Both types, depending on the application, can be designed to support axial or radial loads.
Both centrifugal and axial fans can use either ball or roller bearings; however, ball and roller bearings are more common on axial fans. Sliding contact bearings are more common on centrifugal fans. Ball and roller bearings consist of four major components:
• Outer race • Inner race • Rolling elements
• Spacer for the rolling elements
A sliding contact type bearing known as a journal or sleeve bearing is used extensively on centrifugal draft fans and some axial fans. This type of bearing is made up of four parts: journal, upper and lower sleeves, and (where required) thrust collars and oil rings. A journal bearing can be further categorized as a fixed or floating type, depending on whether the axial movement of the shaft is allowed. Fixed bearings can have either one or two thrust collars. Floating-type bearings are used to allow for thermal growth in center-hung fans and are installed at the opposite end of the fan motor. A typical sleeve bearing is shown in Figure 3-12.
Figure 3-12
Sleeve Bearing Components
3.6.2 Lubrication Systems
Various techniques are used to provide oil to fan and motor bearings. Static lubrication in which each bearing has a fixed supply of oil in their sumps is very common. This method is simple and very cost-effective; however, it relies on operator vigilance to detect low oil levels or poor oil quality. Use of temperature sensors to provide remote warning to the control room operators, in case of a hot bearing, offers added protection for this method. A gear pump attached to the input shaft of the driver is a second method used to provide lube oil to fan and motor bearings. A third method, the use of fluid drives to supply oil to the bearings, is also used on some fans. A fourth method involves the use of a dedicated circulating lube oil system.
Two variations of this system involve the use of a forced lubricating oil system in which oil is supplied to the bearings under pressure. The second variation involves the use of a circulating oil system, which supplies oil to the bearing sump; oil rings then move the oil to the bearing surface. The latter variation is the one used most often.
3.6.3 Turning Gear
Turning gears are sometimes installed with ID or gas recirculating fans where exposure to high-temperature flue gas (while the fan is idle) could result in warping or thermal growth of fan internals. Once a fan is stopped and the inlet and outlet dampers are closed, a fixed volume of hot flue gases remains inside the fan housing. As these hot gases cool down, a natural temperature gradient forms. Consequently, the fan rotor cools unevenly. The result is thermal distortion, which causes the fan shaft to bow upwards in the center. It is interesting to note that many mistakenly believe that the shaft sags under the weight of the wheel.
A thermally distorted fan wheel will cause vibration when the fan is restarted because the fan is out of balance due to the distortion. Power stations may choose any one of three solutions to resolve vibration caused by thermal growth. One approach involves starting the fan and
accepting the accompanying vibrations. Over a period of time, the fan rotor geometry is restored as a consequence of the rotor becoming evenly heated once again. The second approach involves the operator’s initiating a series of start-stop cycles to allow the fan wheel temperature to balance out. As with the first approach, high vibration becomes an accepted condition. This, however, in addition to exposing the fans to low cycle stresses, makes this option even less attractive. The third approach involves the installation of a turning gear. A turning gear consists of a small motor, a reduction gear, and an overrunning clutch. The turning gear is used to keep the fan rotor turning at a slow speed (approximately 60 to 90 rpm) when the fan is hot but not operating. This provides an optimum solution to prevent thermal distortion.
The design speed of the turning gear is critical. Most centrifugal fans have sleeve bearings that have a minimum speed. Below the minimum speed, the oil film between the journal and the sleeve is not adequate to prevent metal-to-metal contact, and the bearing will be damaged. The turning gear should be designed to operate above the minimum bearing speed (usually 60 to 90 rpm). The bearing manufacturer should be consulted to determine the minimum speed.
Key Technical Point
The design speed of the turning gear is critical. Most centrifugal fans have sleeve bearings that have a minimum speed. Below the minimum speed, the oil film between the journal and the sleeve is not adequate to prevent metal-to-metal contact, and the bearing will be damaged.
3.7 Fan Performance
Because boiler draft fans are among the highest auxiliary power consumers in the plant, the performance of the fans is important for efficient plant performance. Deficiencies in fan performance can cause a load limitation on plant output. In many coal-fired plants, the ID fans are the limiting factor on plant electrical output. Although the ID fans may be the apparent cause of a load limit, in many cases the root cause is high air heater leakage, air heater pluggage, high gas temperatures, precipitator infiltration, or something similar.
Key Technical Point
In many coal-fired plants, the ID fans are the limiting factor on plant electrical output. Although the ID fans may be the apparent cause of a load limit, in many cases the root cause is high air heater leakage, air heater pluggage, high gas temperatures, precipitator infiltration, or something similar.
As with most other turbo machinery, the performance of fans is best illustrated by a curve of pressure versus flow. Fan curves are usually plotted as pressure in inches of water gauge ("Wg) versus volumetric flow in actual cubic feet per minute (acfm). Note that most curves are labeled as cfm, where it is understood that cfm is acfm. Because fan performance depends on inlet density, the fan curve should specify the density. Some European fan suppliers plot curves in head—in feet (meters) of fluid (rather than pressure, in inches of water)—versus volumetric flow. These curves are essentially independent of inlet density, similar to a centrifugal pump curve that is plotted as head versus volumetric flow.
Key Technical Point
Note that most curves are labeled as cfm, where it is understood that cfm is acfm. Because fan performance depends on inlet density, the fan curve should specify the density.
Figure 3-13 charts the performance field for a typical centrifugal fan with inlet vane control. It is a customary design practice to specify a test block condition (which contains flow) and head margin above the expected operating requirements at full unit load. Both centrifugal and axial fans must be capable of meeting conditions well beyond the expected conditions at full plant operating load.
When using these curves, care must be taken not to confuse inlet vane position in degrees and percent open. Fan manufacturers usually present their curves in terms of vane angle, with 90 degrees being the full open position. Many boiler controls identify inlet vane position in terms of percent open, with 100% being full open. Because control room and actuator position indicators may not accurately indicate actual vane positions, the actual vane position should be verified.
Figure 3-13
Typical Centrifugal Fan with Variable Inlet Vanes
Key Technical Point
Fan manufacturers usually present their curves in terms of vane angle, with 90 degrees being the full open position. Many boiler controls identify inlet vane position in terms of percent open, with 100% being full open.
Figure 3-14
Typical Centrifugal Fan with Variable Inlet Vanes and Showing System Curve
Note that at the minimum flow (25% per NFPA), the inlet vanes would need to be less than 15 degrees. The inlet vanes are designed for flow control and not to isolate the fan, and—with the vanes fully closed—the performance will be approximately the same as with the vanes 15 degrees open. Thus, controllability at this low vane opening may be a problem. Most large boilers have two 50% capacity FD and ID fans; single-fan operation at low flow rates usually provides better controllability.
Key Technical Point
The inlet vanes are designed for flow control and not to isolate the fan, and—with the vanes fully closed—the performance will be approximately the same as with the vanes 15 degrees open. Thus, controllability at this low vane opening may be a problem.
Figure 3-15 is a typical curve for a centrifugal fan with speed control (applicable to a fluid drive or a variable frequency drive). The system curve is also shown in this figure. The minimum flow (25%) is achieved at 200 rpm. A stable speed of 200 rpm could be a problem with a fluid drive that has a maximum speed of 900 rpm. With variable frequency drives, a minimum speed of 200 rpm should not be a problem.
Figure 3-15
Typical Centrifugal Fan with Speed Control
A centrifugal fan’s most efficient area of operation is near the full-load condition in the test block area. Because the lines of constant efficiency run approximately perpendicular to the system resistance line, as load drops, the efficiency of a centrifugal fan also drops rapidly. A centrifugal fan sized for a test block condition will not operate in its most efficient region under normal conditions. For a typical centrifugal fan using inlet vane control, efficiency at the test block condition may be as high as 88% but will be only 70–75% at the 100% unit load condition. At 50% unit load, fan efficiency may be as low as 25%. A centrifugal fan with a variable-speed drive operates near its peak efficiency at all loads.
3.7.1 Axial Fan Performance
A typical performance field for a variable-pitch axial fan is shown in Figure 3-16. The boiler resistance curve and test block condition are the same as those used in the previous example for a centrifugal fan. For axial flow fans, maximum operating efficiencies occur below the stall line, which represents the maximum capability of the fan. This makes it possible to select a fan that
Figure 3-16
Performance Field for Variable-Pitch Axial Flow Fan
Furthermore, for axial fans, the areas of constant efficiency run approximately parallel to the boiler resistance line. As load decreases, efficiency does not drop off as drastically as it does with a centrifugal fan. Thus, at 50% load, efficiency may remain as high as 65%—more than double the efficiency of a centrifugal fan with inlet vane control.
Higher operating efficiencies and the resulting fuel savings are the most significant factors favoring axial fans.
3.7.2 Fan Pressure Definition
The fan pressure should be defined on the fan curve. Possible definitions are fan static pressure (FSP), static pressure rise (SPR), or fan total pressure (FTP). These are defined as follows: • FSP = SP2 – TP1
• SPR = SP2 – SP1
Where the subscripts 1 and 2 refer to the fan inlet and outlet, SP is static pressure, and TP is total pressure. Note that fan static pressure is not the same as static pressure rise. These definitions are illustrated in Figure 3-17.
Key Technical Point
Note that fan static pressure is not the same as static pressure rise.
Figure 3-17
Fan curves are useful in evaluating actual fan performance, which is usually measured in terms of pressure rise across the fan and volumetric flow rate. The measured volumetric flow can be applied directly to the fan curve (assuming that the units are the same). The pressure rise across the fan must be converted to the density on the fan curve.
Key Technical Point
The pressure rise across the fan must be converted to the density on the fan curve. Figure 3-18 is an example of correcting the fan performance to the operating conditions. It shows a typical fan curve based on an inlet density of 0.075 lb/ft3. It also shows an operating point based on measured data. Due to a temperature difference, the density at the measured point is 0.0696 lb/ft3. Comparing the operating point to the fan curve appears to show that the fan is not performing as designed. However, correcting the fan curve to the actual density at the operating point shows that the fan is actually performing better than design.
Figure 3-18
The procedure for correcting a fan curve to the operating density is to correct the fan head as follows: 1 2 1 2 ρ ρ H H = Eq. 3-1 Where:
H2 = fan pressure at density 2, "Wg
H1 = fan pressure at density 1, "Wg
ρ1 = density 1, lb/ft3
ρ2 = density 2, lb/ft3
To convert the fan curve, use Equation 3-1 at several flow rates and plot the new curve. Note that the volumetric flow rate (cfm) for point 1 and point 2 is the same. A fan is a constant-volume machine, regardless of density.
3.8 Selection of Fan Type
The following are some of the considerations in selecting the type of fan to be used:
• Volume/Pressure Characteristics. As the flow through an axial fan decreases, the output pressure decreases. The characteristic curve for a centrifugal fan shows an increase in static pressure as flow decreases. This feature results in the maximum pressure capability of a centrifugal fan that is substantially higher than that required to satisfy the test block condition. This higher maximum pressure capability can affect the design pressure of ductwork, precipitators, and other components in the draft system.
• Erosion/Corrosion. In general, a centrifugal fan has much greater capability to withstand erosion than axial fans. Protective liners and nose and tip plates are easily applied. Protective nose pieces and coatings are available for axial fan blades, but generally, axial fans are best suited for air or clean gas applications.
• Mass and WR2 (moment of inertia). Axial fans weigh less than centrifugal fans and have
fewer massive rotating elements, which reduces foundation mass requirements. The lower weight results in a lower WR2, which tends to reduce the cost of the drive motor.
• Evaluation. If a decision is made to consider axial fans, bids should be taken for both centrifugal and axial fans. The final decision can then be based on the results of a comprehensive economic evaluation.
