Due to increased production in recent years, Holnam Texas Limited Partnership's cement plant in Midlothian, Texas, experienced buildup and was forced to shutdown periodically while its workers cleaned off the extremely hard, brick-like substance on the preheater I.D. fan. The process took about 24 hours-16 hours for the fan to cool down and another eight for sandblasting. In the year prior to the new fan's installation, the plant was down for a week because of fan-related difficulties.

In 1997, Robinson Industries was brought in to manufacture and install a backward-curve fan that significantly reduced buildup on both the fan's front and back surfaces. Since the fan's installation, Holnam has yet to stop production because of buildup or any other fan-related problem.

In many cement plants, the buildup breaks off in chunks, which can throw the fan off balance, resulting in high bearing vibration and, in some cases, mechanical damage. To combat the problem, Holnam resorted to periodic cleanings that necessitated costly downtime in its around-the-clock operation.

The original fan at the plant was obtained from Robinson in the 1980s. Running at its original design capacity, the fan did well, but problems developed when Holnam increased production. By 1994, the fan was running 30% over its intended capacity, and Holnam was having difficulty tolerating shutdowns, which were increasing in frequency and severity.

Robinson was conducting research specifically addressed to buildup on preheater fans in cement plants. In one study, the company determined that buildup can be reduced by matching the shape of fan blades as closely as possible to particle streamlines so the impact energy of dust particles is minimized. Blade angle must be inclined enough to prevent the "hard" buildup on the blade's front surface yet radial enough to prevent the "soft" buildup on the blade's back surface. Engineers determined that the best blade for the Holnam cement plant would be the backward curve.

It was also decided that the new fan at Holnam would have to be larger than the old one, not only to accommodate the plant's production but also to accommodate the blade's new design. To receive the same performance for a given application, a backward-curve fan needs to be larger than a radial-bladed fan. Holnam's parent company, Holderbank, also conducted research on the buildup problem, which corroborated this fact.

Three design challenges typical in the design of kiln I.D. fans were addressed. The fan would have to (1) handle a steady stream of dirty air, (2) tolerate high temperatures, and (3) tolerate fast changes in temperature. Robinson faced an additional challenge specific to the Holnam plant: the fan would have to handle extreme stress because of its size. The new fan would need to be 179 in. diam (4.55 meters, the largest Robinson had built in its 106-year history), rated at 4,500 hp (3,356 kW), and deliver 430,000 cfm.

As design work began, a number of finite element analyses were conducted, including steady-state centrifugal stress analysis, transient thermal/stress analysis, and mode frequency analysis. Engineers knew that rapid changes in temperature-from 450 degrees to 840 degrees F (218 degrees to 434 degrees C-would present difficulties because the fan and shaft would expand and contract at different rates. While the plate steel in the fan would respond rapidly to temperature change, the forged steel in the shaft would not. Under certain conditions, the fan could become loose on the shaft. The engineers, therefore, designed an integral flange on the shaft to which the fan wheel could be bolted. The more typical solution to this problem-a radial arrangement of pins between the shaft and the fan hub-would not have worked on such a large fan.

Supply engineers also determined the number of bolts and type of bolt material necessary to ensure steady-state operation at high temperatures. The bolts needed to withstand stresses created during rapid changes of temperature. Thus, a computerized model was created to conduct steady-state stress analysis and transient thermal analysis. The analyses took into consideration the preload on the bolts since preload was critical to preventing movement between the web plate and shaft flange. Proper clamp load and tension was imperative.

Engineers analyzed the bolts for short-term temperature properties and for long-term temperature-or temperature creep-properties. Using a computer, engineers created a finite element model, enabling them to conduct tests on the web plate thickness, the bolts and the shaft flange. The tests examined steady-state operation and the effect of temperature change on the bolts and the bolted connection.

The original I.D. fan at the Holnam facility was rated at 3,500 hp (2,610 kW) and designed for a speed of 1,180 rpm. The new motor, while rated for 4,500 hp (3,356 kW), would be designed for just 880 rpm. Robinson engineers designed the fan for lower speeds because their studies and tests found that lower-speed fans are less susceptible to buildup. Lower-speed fans also minimize imbalances.

The size of the fan and its weight-67 tons including the wheel, shaft, housing, inlet dampers, bearings, and bearing pedestals-presented fabrication challenges. The rotor was welded on a positioner so the tilt would be perfect for welding at every point. In addition, the fan's size necessitated splice welds in the shrouds and web plates.

The fan's plate steel was ultrasonically tested for defects. Before the welds, destructive tests were conducted on sample plates. After construction, a dry magnetic particle inspection was made of all final wheel welds. All shrouds and web plates were subjected to X-ray testing. All bolts were ultrasonically tested for defects. The bolts were tested again during tightening to ensure that they were being stretched to the proper length.

The fan wheel and shaft assembly were shipped from Robinson's headquarters in Zelienople, Pa., to Texas via a process that took nearly two weeks. The fan casing was fabricated at Robinson Fans Florida, Inc., in Lakeland, Fla. Robinson Fans Service and Equipment Co. oversaw the installation, which took 19 days and involved the removal of a concrete foundation and the pouring of 415 cu yd (317 m3) of new concrete.

This article was adapted from information provided by Robinson Industries.