Centrifugal Fans Handle High Static Pressure

Centrifugal Fans Handle High Static Pressure

Byline: LES GUTZWILLER AND THOMAS J. KULI

Fan applications in the mining industry have changed dramatically over the last 70 years. In 1922, when J.R. Robinson authored a technical volume entitled Practical Mine Ventilation, atmospheric control only recently had shifted from natural to mechanical methods. The large, centrifugal, mechanical draft fans that were used then ran at low speeds, about 300 rpm, and most were steam-engine driven.

In the late 1920s and early 1930s, mines became larger, increasing the pressure and volume requirements of fan equipment. Developed to meet these requirements, axial fans ran at much higher speeds (710 to 880 rpm) and were driven by electric motors. They were also smaller and less expensive to install than the cumbersome centrifugal units.

But, for higher static pressure requirements, centrifugal fans prove to be a good choice, especially for certain longwall mining applications. As the U.S. coal industry gained longwall experience, panel sizes grew to incredibly large dimensions. Even though the mined area was allowed to cave (gob), airflow had to continue through the area. Gob areas, naturally, would have a high coefficient of friction. As a result, the main mine fan, which are predominantly axial fans in U.S. coal mines, would have to provide an extremely high static pressure to meet the ventilation requirements. Some mines tried very high-speed axial fans (even a 2-stage, 5-foot (ft) diameter axial fan operating at 1,780 rpm). Experience showed that such fans were noisy and unreliable, with fan blade and bearing failures being the most common problems.

Despite these problems, methane gas being liberated at the working face as well as methane from the gob area must be removed. Clearly this is a difficult task with the main mine fan, due to its great distance from the face and its limited static pressure capability of 8 inches water (in-H [subscript]2 O).

As a result, centrifugal fans have found wide acceptance in bleeder applications (See Figure 1). Bleeder shafts are bored from the surface to the coal seam before the longwall begins mining. They are typically located near the back of the panel. As the panel retreats, the fan draws the methane, which rises, from the top of the void created above the gob. The bleeder shaft is typically lined and may be 4 to 18 ft in diameter, depending on the volume of gas to be removed. The volume requirement for the bleeder shaft fan is determined from the expected volume of methane to be liberated.

The pressure requirement for the bleeder shaft fan is dependent on the depth and diameter of the shaft, the overall volume requirement, and the friction coefficient through the partially caved airway and gob areas. Bleeder shaft fans have such high static pressure requirements that centrifugal fans are normally a much better choice than axial-style fans. Centrifugal fans offer a wide range of operating requirements from 150,000 cubic ft per minute (cfm) at 16 in-H [subscript]2 O to 250,000 cfm at 42 in-H [subscript]2 O.

Centrifugal fans are also useful for a variety of coal-related applications including:

Degas Holes – Degas holes above longwall mines also help encourage the flow of methane gas from the void above the panel. Smaller diameter holes are bored from the surface to the center of the panel and spaced equidistant throughout its length. The methane enters a perforated pipe that has been dropped from the surface. A small induced draft fan rated at approximately 1,000 cfm and 55 in-H [subscript]2 O static pressure at the surface encourages the flow. Such centrifugal fan units are often driven by natural gas engines and actually use the escaping methane gas as fuel.

Auxiliary Face Ventilation Fans – Auxiliary face ventilation fans are used to help increase gas velocities at the working face as well as to remove particulate matter suspended in the air. Both axial and in-line centrifugal fans have been used in this service. Axial fans normally operate at 3,600 rpm while the centrifugal units operate at 1,800 rpm. The centrifugal units have several advantages, including lower noise levels and better wear resistance.

Preparation Plant Dryer Fans – Preparation plant dryer fans have very high-pressure requirements. This is necessary to fluidize the bed of coal in hot, dry air. The fluidized bed provides intimate contact between the solid coal particles and the high velocity dry gas stream, resulting in very effective coal drying.

Coal dust and particulates often pass through the fan when separation is incomplete. Therefore, the fans must be of rugged construction and be lined with erosion-resistant materials. Tip speeds of these fans can reach 30,000 fpm and higher. Heavy-duty centrifugal fans are definitely the best choice for this demanding application.

Coal Mill Exhausters – Paddle wheels – or whizzer wheels – are historically the most common design employed for the movement of abrasive coal particles in coal-burning facilities. While whizzer wheels are, technically speaking, centrifugal fans, they operate at low efficiency and can incur heavy wear within short periods of time, requiring frequent and costly maintenance, replacement, and shutdowns. In most cases, erosion-resistant liners have been used to improve wear resistance, but the result is a heavy fan rotor, which undermines mechanical design considerations.

Departing from the tradition of the whizzer wheel is a backward-curved, centrifugal fan, providing improved wear resistance, energy efficiency, and fan/system flow. The fan’s design is such that it minimizes contact between the abrasive air stream and the backward-curved blade. Inlet dust deflectors – miniature fan blades placed between the main blades – direct heavy particulate along a path between the blades. The deflectors are not themselves structural members of the fan, so, if they should wear down, they can be replaced. The interior of the fan, including the blades, shroud, web, dust deflectors, and housing, is lined with a lightweight, extremely durable ceramic tile or other durable wear-resistant material. While a typical whizzer wheel might produce a volume of 13,000 cfm at 21 in-H [subscript]2 O static pressure, this larger backward-curved design produces a volume of 16,000 cfm at 28 in-H [subscript]2 O static pressure.

Though their uses vary widely, coal operators still find the centrifugal fan useful for high-static pressure applications.

Gutzwiller is president and Kuli is chief engineer at Robinson Industries Inc., based in Zelienople, Pa. Contact them at 724/452-6121.

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