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
