Air drilling is basically the same as mud drilling except that the mud pump is replaced by a compressor assembly including cooling system, air receiver and unloading system. The air line is connected to the swivel hose at the top of the kelly or top head drive. The air is forced down through the drilling pipe and out through the holes at the bottom of the rotary drill bit. A small stream of water is often introduced into the air system to help cool the drill bit and control dust. The air serves to cool the drill bit and force cutting up out of the hole and are collected at the top. Air is used principally in hard clay or rock formations, in as much as once the air pressure is turned off, loose cavy formations would cave-in against the drill pipe. This is not the case when mud is used.
