offset from inlet 15 so as to avoid direct transmission of pressurized air from inlet 15 through any particular one of openings 19 plenum 13 preferably has a vertical length that is of sufficient size to allow the inlet 15 and the plurality of openings 19 not to be aligned so as to prevent disproportionate pressure through any of the openings 19 inlet 15 may be located above or below openings 19 to provide the desired offset alternatively inlet 15 may be located on the same plane as openings 19 so long as none of openings 19 is directly across from inlet 15 openings 19 may be provided in a regular or irregular pattern around the circumference of wall 10 the openings 19 may be elongated slotted curved etc and may be narrow or wide vertical horizontal or angled and may be provided in different sizes shapes and or patterns it is preferred that openings 19 be provided in pairs having opposing angles such that the air introduced through the two openings of each pair intersecting in area 12 in the flow of mud and debris that is away from the central drill pipe 25 so as to avoid causing frictional erosion against pipe 25 referring to the cross sectional view of fig1 it is seen that openings 19 are provided in pairs each pair defining a two paths 26 that intersect at locations 27 away from the outside edge of drill pipe 25 the angle u03b8 at the intersection 27 of the paths should be between about 80 and about 140 degrees preferably in the range of about 110 to about 140 degrees to be closer to wall 10 than to drill pipe 25 and to assure.