30 CFR § 18.22
Boring-type machines equipped for auxiliary face ventilation.
Each boring-type continuous-mining machine that is submitted for approval
shall be constructed with an unobstructed continuous space(s) of not less
than 200 square inches total cross-sectional area on or within the machine
to which flexible tubing may be attached to facilitate auxiliary face
For More Information:
See MSHA's Program Policy Manual