Many factors drive modern plants to make process equipment changes during outage season. Two of the most common are 1) demands for increased production and 2) process variable changes. Centrifugal fans used throughout the power production process are frequently the subjects of these changes. What follows is an account of the key modifications to centrifugal fans that plants should consider implementing during outage season.
Two Simple Modifications to 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 two ways to achieve the needed increase in capacity without installing new fans to meet the increased demands:
- Increase the diameter of the existing fans
- Increase the speed of the existing fans
Option 1: Increase the Diameter
Because of the simplicity and relatively low cost of increasing a fan’s diameter, plants frequently choose to make this sort of modification during scheduled outages. Increasing the fan’s diameter involves adding extensions to the blades of the fan impeller and adjacent components (often referred to as tipping out). This type of change always increases the structural loading on the impeller, lowers the natural frequency of the shaft/bearing/rotor support system, increases the inertia of the impeller, and increases the horsepower requirements.
Option 2: Increase the Speed
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 (a 4/3 ratio) 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 that allows them to perform analyses, such as finite element analysis (FEA) 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. Outage season is the ideal time to do so.
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 such circumstances.
If you’re considering a fan upgrade during outage season, get in touch with a professional service provider to have your system evaluated. Our mechanical draft fan services has lots of experience analyzing air handling systems and recommending economical, cost-savings fan upgrades that can be accomplished during scheduled downtime. Give them a call at 1-888-663-2028 and they’ll be happy to help.