Many factors drive modern plants to make process equipment changes during outage season, and two of the most common are demands for increased production and process variable changes. Centrifugal fans and duct systems that are used throughout the power production process are frequently the subjects of these changes. This article will address some of the key modifications that plants should consider implementing with their centrifugal fans and duct systems during outage season.
How plant changes affect industrial fans
When increased production is needed, it almost always translates into a need for more flow and pressure capacity from the process fans. There are several ways to achieve the needed increase in capacity:
- Increase the diameter of the existing fans
- Increase the speed of the existing fans
- Install new fans sized to meet the increased demands
Because of the simplicity and relatively low cost of increasing a fan’s diameter, this is the most common choice and can be easily implemented during scheduled outages. Increasing the fan’s diameter involves adding extensions to the blades of the fan impeller and adjacent components. This type of change always increases the structural loading on the impeller, lowers the natural frequency of the shaft/bearing/rotor support system, and increases the inertia of the impeller.
If a speed increase is selected as the best method of increasing fan capacity, the entire fan rotor and all of its associated components (impeller, hubs, shaft, bearings, couplings, motors, etc.) must be evaluated to determine if they can handle the higher speed. Any one of these changes can prove detrimental to the long-term health and successful operation of the fan and its associated equipment.
Increasing fan rotor operating speed increases the stresses in the rotor by the square of the speed change ratio; in other words, increasing the speed from 900 rpm to 1,200 rpm will increase the stress levels by a factor of 1.78, if no changes are made to the impeller structure. Normally, a significant speed increase requires a finite element stress analysis to ensure that the impeller structure can withstand the higher stress levels at the new speed. Experienced fan designers should evaluate the increased stress levels and make recommendations regarding additional stiffening or higher strength materials to handle the higher speed.
Most fan designers have software, such as computational fluid dynamics (CFD), that allows them to determine the effects of additional impeller weight on the natural frequencies of the fan rotor and support system. This type of evaluation, referred to as a shaft critical speed analysis, should always be performed when impeller changes are planned and is an ideal consideration during outage season.
If the additional weight causes a large shift in the rotor and support system’s first natural frequency, it might be necessary to increase the shaft or bearings size, or modify the bearings to achieve the required support stiffness. When a substantial increase in build-up on the fan rotor is anticipated, the fan rotor and support system design can be evaluated to determine its response to the higher loading.
Increasing fan rotor operating speed also reduces the separation between the first natural frequency of the fan rotor and support system and the operating speed. A shaft critical speed analysis also should be performed in these circumstances.
Practice good duct design
System losses, other than those of the boiler and ancillary equipment of the boiler, are normally a result of the total pressure losses from friction in the ductwork, elbows, changes in duct cross-sectional area and losses associated with emissions control equipment. A certain amount of loss is unavoidable, but losses might be reduced many times by simple and relatively inexpensive modifications during outage season. These modifications can include turning vanes, implementing well-developed and longer sections for diverging and converging flows, redesigning compound elbows and aerodynamically enhancing or removing obstructions in the flow path.
