Ventilation – Design Considerations Part 2

This is the second in a series of articles about the design considerations of main and booster fan installations. If you haven’t already, go ahead and read Ventilation – Design Considerations Part 1, written by Cam Seeber and Jim Wywrot.

In the last issue we talked about how volume and static pressure requirements for primary and booster fans are determined. The next step is to select the type of fan.

General guidelines that have been around in the industry will state that for a fan operating with a Static Pressure (SP) of 1.75kPa or less (<7¨ w.g.), an axial fan offers the best choice. One advantage of an axial is simplicity of overall design. At the higher end, where conditions dictate a fan to provide static pressures at 4.5 kPa (18¨ w.g.) or greater, a centrifugal is the obvious winner because of its ability to develop higher pressures. Over time axial fans have increased in size and capacity. Correspondingly their ability to deliver higher static pressures has also increased. However, in smaller fans, such as generally found in booster fans,these guidelines generally hold.

Regardless of which is used or at what range, selecting the fan is only half of the problem. The layout–both on the drawing board and on site–is really the ‘heavy-half’ of the design.

When a fan’s performance is measured in a test lab, it is under idealized conditions. The entrance to the fan will be preceded by a straight length of duct 10 times the diameter of the fan’s impeller diameter or a smooth inlet bell. This ensures that the air streamlines are as close to laminar as practical, leading to the optimum fan performance. However, in the field it is often the case that there isn’t sufficient room to incorporate these large straight sections of inlet, and compromises must be made. Generally the rule of thumb is that at least six diameters upstream be straight without any changes to diameter. The greater the deviation from the ideal conditions, the larger the losses will be.

The rules also apply to the fan discharge. Any abrupt change such as an elbow can have detrimental effects. For a centrifugal fan, selecting the discharge and rotation of the fan to eliminate additional bends is a good practice and relatively easy. Out fitting elbows with adequate turning vanes, or splitters, will help in maintaining an even velocity profile to reduce the turbulence that leads to increased static pressure losses.

In order to develop useful static pressure, a fan rotates and therefore expels the air at a much higher velocity than will eventually travel down the mineshaft. In some fan literature, Fan Total Pressure is quoted. Fan Total Pressure as its name implies is the summation of the fan’s Velocity and Static Pressure. In this energy system the velocity pressure is not useable energy and must be converted to static pressure. This can easily be accomplished by utilizing a discharge cone or evasé.

A well-designed evasé slows down the air in a uniform manner to keep the amount of turbulence generated to a minimum. By doing this the maximum amount of available energy can be reclaimed back into useable static pressure. On the other hand, with an abrupt transition, less of the available energy from the velocity pressure is converted into static pressure and the rest simply ends up as heat.

Consider the following example, which demonstrates the importance of the evasé in providing an energy-conserving solution to a mine operator.

A fan is operating at 165 m3/s at 5.0 kPa (350,000 cfm @ 20¨ w.g.) static pressure with no evasé. In this example the fan is discharging at 51 m/s (10,000 fpm) directly into a larger duct with a sudden expansion and the fan is drawing 1,328 kW (1,780 hp). If the same fan uses an evasé to reduce the discharge speed to 20.3 m/s (4000 fpm), this will result in a 0.92 kPa (3.7¨ w.g.) static pressure regain. With the evasé in place the fan would now draw 1,204 kW (1,616 hp) or a savings of 124 kW (166 hp). If energy costs are $0.08/kW-hr, the power cost savings that result are $92,000 annually.

Even for a well-selected fan with good inlet and outlet conditions, this does not guarantee that the goals the fan was selected for are being met. The fans deliver the output, but where it goes from there, as Paul Harvey might say, “is the rest of the story”.

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