behind the face, whilst the upper return gate is developed as the face retreats.

Effectively this may be described as an advance/retreat face, since at the return

end it is identical in behaviour to an advancing face. Again, it requires a return air

raise behind the face. The system is known as homotropal ventilation [15] and

used in coal seams that have a very high gas content because it has a short and

stable ventilation lines.

D. ��Y�� System

This system involves two gates��the upper gate and the lower gate. Both gates are as air intakes, and the upper return gate is maintained behind the face in the gob area. Again, it requires a return air raise behind the face. The system has enough air quantity to dilute the harmful gases in return gate, which can guarantee that the machines in return gate run safely for the fully mechanized longwall mining.

E. ��W�� System

When the double-unit faces are used, three gates are pre-formed, drawn off

behind the face for retreating mining. The upper and lower gates are as air intakes and middle gate is as an air return, or the middle gate is as an air intake and the upper and lower gates are as the air returns. ��W�� system can satisfy the ventilation requirements of double-unit faces. Therefore, it is advantageous to increase the coal production.

F. Back Return System

This is the most used system for retreating face, with no airways permanently [16] maintained behind the face. A small coal rib [17] some three meters to five meters wide left between the end of the return gate and the gob area is made, supported with a roadside pack [18] (or wood chock). Periodically, a cut-through [19] is made in the coal rib, all returns are stopped off [20], except one open cut-through. In this way, the face airflow is diverted back [21], as with the Z-system, reproducing the conditions of an advancing face at the return end. The system can be used in coal seams with much higher emissions and it is probably the only system to be used successfully retreating to the rise [22].

NOTES TO THE TEXT

[1] ventilation��ͨ��

[2] tolerable heat and humidity levels�������̵��Ⱥ�ʪ�ȱ�׼

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