In rotary blasthole drilling, delivery of air in sufficient volume and at proper pressure is very essential to assure optimum bit performance.
For the most effective hole cleaning a minimum bailing velocity of 5,000-7,000 feet per minute is considered desirable for dry cuttings; and 7,000-10,000 ft./min. for wet or more dense cuttings. Right amount of air ensures efficient cuttings removal from the hole bottom to the surface and it also reduces the cutting structure wear and bearing erosive wear by means of efficient bottom hole cleaning.
This volume is calculated for the most common bit and pipe sizes. For dense materials, this volume should be increased in order to yield optimum cleaning efficiency.
The simple formula Q=AV can be applied to determine the volume flow of air (Q) necessary to give the desired velocity (V). (A) is the area of the hole through which the cuttings-laden air flows. This area is the area of the hole less the area filled by the drill pipe.
BV = 183.3 x ACFM / D2 _ d2
BV = Bailing velocity (feet per minute)
ACFM = Actual cubic feet of free air per minute delivered
183.3 = Factor for conversion
D = Diameter of the hole being drilled (inches)
d = Outside diameter of the drill pipe being utilized (inches)