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370 Advanced Mine Ventilation

Figure 22.5 A typical arrangement for antiignition back spray and a water-jet cutting spray in
front of the bit.

22.4 Machine Design Parameters

Machine design parameters that have an influence on the probability of a frictional
ignition are as follows:

1. Speed of the bit,
2. Size of the bit or depth of cut,
3. Angle of attack, and
4. Material of construction for the bit tip and bit block.

22.4.1 Speed of the Bit
It is generally agreed that when the bit speed is reduced below 40 ft/s, there is a
significant reduction in the probability of ignition [10]. However, such low
speeds may lower the coal production rate and may require deeper cuts and larger
motors.

22.4.2 Size of the Bit
A large bit with a depth of cut of at least 1.5 inch is desirable. Deeper cuts at lower
speeds also tend to reduce respirable dust production. It would appear from above
that there is an optimum for the combined parameters of speed of cutting and depth
of cut to yield the maximum coal output per unit of energy consumed.

22.4.3 Angle of Attack

Fig. 22.6 shows the definition of angle of attack, q A , bit angle, q T , and angle of clear-
ance, q C [7].

Field experiments confirmed laboratory findings that when q A is greater than 57 ,
the probability of ignition was substantially reduced [9]. Frictional ignition probability