Aarge number of the world’s power generation lants are operating with 50-year-old-plus fan quipment. These fans originally were selected rom low- to moderate-efficiency designs that were the most reliable and economical when the lants were built. Higher efficiency fans were available, but 50 years ago operating costs were low and energy costs expended on these fixed loads were more easily tolerated.
More recently, power generators have been forced to examine every aspect of their internal energy consumption. Fan designs are being upgraded to the most efficient available. Environmental systems have imposed additional fan loads that require replacing existing fans with larger, more efficient machines.
What’s more, technology advances have introduced new fuel schemes to power generation, which require larger fans and higher pressures. Changes in fuel chemistry along with the introduction of biomass and low energy solids are creating complex combustion requirements. The largest new-design coal-fired boilers require multiple forced draft and induced draft fans, which can exceed 10,000 horsepower each. In many cases, the induced draft fans are axial design units with in-flight adjustable blades. These fans are efficient and possibly are capable of larger airflows than can be produced by single centrifugal fans.
structural members are possible in some energy upgrade situations. Fan designers can apply the newest high-efficiency fan designs to existing fan breaching and supports. Old fan designs, which once were required to handle abrasive and erosive gases with low efficiency impellers, can now be replaced with higher efficiency backward-curved and airfoil impeller fans thanks to improved metallurgy and armor protection for airstream surfaces.
Forced draft fans that supply combustion air to the boiler are usually designed to push clean air through an air preheat system before entering the boiler. New design fans are airfoil centrifugal or axial fans. In colder climates, fans have been placed in acoustic enclosures with air preheat steam coils, air filtration and intake silencers. All of these features add pressure drop to the fan systems. These increased pressure drops have the potential to increase the fans’ sound level, meaning that installations now require acoustical study and compliance documentation. New design forced-draft fans usually include variable speed fan controls along with “man safe” isolation dampers. Depending on the type of fans that are selected, removing some of the old design volume control dampers is possible. This results in reduced leakage and superior volume control when coupled with the variable speed fan drives.
In considering a fan upgrade, it is always advisable for a utility company to consult with the original plant designer. Many of these original fan installations were given a design life of 30 years. This means many fans are currently well beyond their intended 30-year design lifespans. Regulatory decisions concerning fuel allocations and grid changes have turned old baseload stations into peaking units that are cycled twice a day. Changes in combustion systems and flue gas volumes have tripled the amount of pressure drop in some boilers. Newly added environmental controls and complex control systems create a requirement for completely different fan designs or even for multiple fans in some retrofit systems.
Improvements in fan technology have identified weaknesses in old fan designs that can lead to problems with the oldest fan systems. Turning these fan systems on and off frequently causes additional stresses on foundations, bearings, couplings and fan rotating elements as older fans systems were not designed for these extra cycles. All of these factors mean that fan component replacement is increasingly necessary.
Induced draft fans convey products of combustion and fly ash from the boiler and air preheat system to the power station’s environmental systems. Because of the increased pressure drops required by collection and suppression systems, these fans have grown into the largest and highest-horsepower fans at the power station. Most larger power stations use multiple induced draft fans with more than one particulate and gas cleaning device dedicated to each fan. The noise levels of these fans are often the greatest issue in relation with local compliance regulations.
When retrofitting these induced draft fans, it is possible to increase service life and operating efficiency. In particular, airfoil blade fans are commonly used in induced draft service with special attention paid to abrasive and erosive wear. Variable speed fans are also applicable in induced draft service to reduce power, abrasion and erosion. Removing some of the old volume control dampers can increase system efficiency and reduce fan input power. Large variable pitch axial fans can provide more airflow in a given space when more airflow is required by process conditions. Occasionally, it is possible to reduce the number of induced draft fans by retrofitting with more compact designs. Reducing the number of fans can also result in lower total power consumption.
The digital design power available to fan engineers creates solutions to upgrades and retrofits that save a great deal of time and money. By combining physical dimensions at an existing facility with theoretical fan design data, timelines can be significantly shortened. Retrofitting new fans to existing foundations and
Forced draft fans that convey powdered coal to the combustion system have increased size and pressure as moisture content and impurities in coal have increased. Originally, these fans were provided with low-efficiency fan wheels along with highly abrasion-prone “whizzers” on the mill discharge to convey material to the
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