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Coal mine ventilation.

Les Gutzwiller(*) expounds on the advantages of centrifugal fans for longwall ventilation, and for use in coal preparation plants

Mining practices and the industry's fan application problems have changed dramatically over the last 70 years or so since J.R. Robinson authored a technical volume entitled Practical Mine Ventilation in 1922. At that time mine atmospheric control had only recently changed from natural ventilation to mechanical ventilation. Large mechanical draft fans used then were centrifugal units.

As changes in mining result in the need for higher and higher static pressure requirements, centrifugal fans are once again proving to be a very good choice. Applying centrifugal 0 fans to the various coal mining industry applications requires a close working relationship between the mine ventilation specialist (or preparation plant designer) and the fan designer/manufacturer to ensure a successful installation. In addition to volume and pressure requirements, concerns about noise levels, reliability, flow control, efficiency (power requirements) and wear protection must also be carefully considered in making the best fan selection.

In US coal, the regulations require specific ventilation standards that include minimum gas velocity at the working face of 72 ft/min (22 m/min), less than 2% methane in all air courses and less than 1% methane at the face. Even though the area behind a retreat face is allowed to cave, forming a 'gob area', the air flow must continue. Of course, the caved areas will have a considerably higher coefficient of friction than an open air shaft. The main mine fan would have to provide an extremely high static pressure capability to meet the ventilation requirements. In the early days of retreating longwalls in the US in the 1970s, some tried very high speed axial fans - even a two-stage, 5 ft (1.52 m) diameter axial operating at 1,780 rpm. The experience showed that such fans were very noisy and were very unreliable from a maintenance standpoint. Fan blade failures and bearing failures were the most common problems.

Bleeder fans

Figure 1 shows a plan view of a retreating longwall operation where panel one has been completed and panel two is about 30% complete. Methane gas being liberated at the working face as well as methane from the gob area must be removed. Clearly this is difficult to do with the main mine fan (lower right corner of the figure) due to its great distance from the face and its limited static pressure capability (that is 8.0 in [H.sub.2]O).

A bleeder shaft has been bored from the surface to the end of the planned system of longwall panels [ILLUSTRATION FOR FIGURE 1 OMITTED]. The bleeder shaft may be 4 to 18 ft (1.2 to 5.5 m) in diameter, depending on the volume of gas to be removed, and is typically lined. The volume requirement for the bleeder shaft fan is determined from the expected volume of methane to be liberated in the mine.

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. Fans have been provided for a wide range of operating requirements. Single-inlet fans are used for the lower volume requirements and double-inlet fans for the highest volume needs. A comparison of centrifugal fans versus axial fans for the bleeder shaft is given in the adjacent table:

Other applications

Degas holes above longwall mines help encourage the flow of methane gas from the 'dome' above the panel [ILLUSTRATION FOR FIGURE 2 OMITTED].

The methane enters a perforated pipe that has been dropped from the surface. A small induced draft fan rated at approximately 1,000 [ft.sup.3]/min and 4 in Hg (equivalent to 55 in [H.sub.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 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 [ILLUSTRATION FOR FIGURE 3 OMITTED].

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 have very high pressure requirements [ILLUSTRATION FOR FIGURE 4 OMITTED]. This is necessary to fluidise the bed of coal in hot, dry air. The fluidised 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 very rugged construction and be lined with erosion-resistant materials. Tip speeds of these fans can reach 30,000 ft/min and higher! Heavy duty centrifugal fans are definitely the best choice for this demanding application.

Example: Fan volume requirement

1,200 [ft.sup.3]/min of methane will be liberated(*)

2% methane (maximum) allowed at fan

1,200/0.02 = 60,000 [ft.sup.3]/min

1.25 safety factor x 60,000 = 80,000 [ft.sup.3]/min

* One company approximates methane liberation based on the longwall panel size as follows:

Methane ([ft.sup.3]/min) [approximately equal to] WL/2800

W is panel width (ft) and L panel length

For a 600 ft by 5,500 ft panel then:

Methane = 600 x 5500/2800 = 1,200 [ft.sup.3]/min

Axial fans

* Best suited for high volume/low pressure applications

* Potential for stalling if the volume drops too low

* Flow control via variable pitch blades (typically manually adjusted, but hydraulic 'on-the-fly' is available at high cost)

* Mechanical efficiencies are typically 85% (some vendors quote even higher)

* Large diameter, high speed units are a maintenance problem. Bearings and adjustable pitch blades are in the gas stream

* Steel construction with cast aluminium blades alloy rotor material allows higher tip speeds

* Horizontal discharge causes highly directive noise. Often requires 90 [degrees] elbow or silencer. Close running clearance rotor-to-casing and straightening vanes generates harmonics

Centrifugal fans

* Best suited for high pressure/low volume applications

* No stall point in the fan performance curve

* Flow control 'on-the-fly' by means of inlet dampers. Variable speed is also available adjustment at some cost

* Mechanical efficiencies are typically 82-85% for airfoil bladed designs

* Typically reliable with low maintenance even in high pressure applications. Fan bearings and damper bearings are typically located outside the gas stream for good accessibility

* All steel construction. High strength low- and more design safety factor for stresses due to centrifugal force

* Discharge can be horizontal, vertical or at an angle. Norse can usually be directed up or a silencer can be fitted. Large rotor-to-casing clearance and airfoil shaped blades

Example: Fan pressure requirement

Pressure required for a given mine:

[Mathematical Expression Omitted]

K = mine coefficient of friction(*)

L = length of shaft (ft)

O = perimeter of shaft (ft)

V = velocity desired (ft/min)

A = shaft cross-sectional area ([ft.sup.2])

WG = pressure requirement (in [H.sub.2]O)

Q = gas volume desired ([ft.sup.3]/min)

*K = 100 x [10.sup.-10] (typical, use for approximations)

K = 200 x [10.sup.-10] (for crooked or fallen-in air shafts)

K = 40 x [10.sup.-10] (for circular, lined air shafts)

Bleeder shaft: 6 ft diameter and 850 ff deep (K = 40 x [10.sup.-10])

Airway size: 15 ft wide by 5,500 ft long by 6 ft high

Gob and caved-in airways (K = 200 x [10.sup.-10])

Volume required = 80,000 [ft.sup.3]/min

[Mathematical Expression Omitted]

[Mathematical Expression Omitted]

* Chief engineer with Robinson Industries, P.O. Box 100 Zelienople, PA 16063-0100, US. Fax:(+1 412) 452 6121. Fax:452 0388
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Author:Gutzwiller, Les
Publication:Mining Magazine
Date:Dec 1, 1997
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