Ted Older and Dan Banyay, The New York Blower Company, explain how to select the correct fan rotor design and configuration for each cement application.
The cement industry utilises centrifugal fans for various crucial processes, including material handling, pollution control, combustion air supply, and exhaust ventilation. These include raw mill, kiln ID, cooler exhaust, coal mill, filter, recirculation, finish mill, separator, electrostatic precipitator, and cooler vent applications. While these processes differ, there are typically three primary factors that are challenging in this industry: dust load, temperature, and process variations. Dust load is the most difficult as it can cause severe wear to fan components as well as excessive vibration due to particulate build-up. High temperature and rapid temperature changes require close attention to fan material selection and mechanical design. Process variations mean that the fan flow rate needs to be modulated to meet each process condition. Careful fan selection is critical to cost-effectively meet these challenges with maximum energy efficiency.
Several parameters, including rotor type, speed, arrangement, materials of construction, control method, coupling selection, bearing type, and motor size need to be considered with the fan selection.
Fan rotor type
There are several basic centrifugal rotor types utilised in high dust load industrial applications, designated by blade profile, including straight radial, radial tip, backward curved, and airfoil.
The most basic is straight radial, where the blades are flat and extend radially from the centre hub. These provide the highest pressure with the lowest blade stress and are best suited for high pressure/low flow processes. However, the efficiency is relatively low. The paddle-wheel design is included in this category.
The radial tip design has a radial profile at the outer edge and curves into the direction of rotation toward the inlet edge. It is used for higher flows than straight radial blades with slightly higher efficiency.
Backward curved fans lean back relative to the direction of rotation with a curved profile. This allows smoother flow through the blade channel compared to radial blades resulting in higher efficiency. However, the rotor size is inherently larger with higher blade stress levels.
Airfoil blades are similar to the backward curve design. However, instead of a single thickness blade, it has an airfoil profile, similar to an airplane wing. It is generally the most energy efficient design. Its main drawback is potentially severe imbalance should any particulate get inside the hollow blades.
In high particulate load applications, the proper blade profile is critical in minimising wear and build-up, which are affected by velocity and impingement angle. Wear can be controlled by liner material selection but minimising build-up is a function of blade profile. For example, the kiln ID fan is particularly challenging, as build-up can occur in two ways. If the profile is too radial, the front side of the blade will accumulate build-up due to the direct impingement of the dust on the surface. If the blade is inclined too much, the dust will accumulate on the back side and be held in place by centrifugal force. The ideal selection will be a trade-off which minimises both effects.
Speed
The rotating speed directly affects the size of the fan. For a given aerodynamic performance requirement (flow and pressure), higher speed results in a smaller fan. This means lower initial cost than a slower, larger fan and requires less space. The negative factors are higher velocities and greater concentration of particulate load. Higher velocities cause greater pressure drops through the fan components and connected ductwork which increases the fan power consumption. Fan wear due to particulate load increases exponentially with velocity. These are significant and must not be overlooked.
Inlet and casing configuration will also impact fan size, so it is important to understand the differences between single-width/single-inlet (SWSI) and double-width/double-inlet (DWDI) fans. A single-width design has the air entering the fan on one side. The inlet duct can be oriented directly along the axis of the fan shaft or perpendicular by including an inlet box. The arrangement is typically used for small to moderate size fans.
The DWDI configuration has the air entering both sides of the casing utilising both sides of a double width impeller. It can handle twice the flow of a similar single width design and is used for the largest fans. With air entering both sides, there is better mechanical stability with balanced axial loading in each direction.
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