Air drilling is defined as the process of making boreholes by utilizing air or gas as the circulating fluid Studies of the hydrodynamics associated with the air drilling process have been ongoing at Penn State since 1985. In this study, dry air was utilized as the circulating medium. Field and laboratory tests have indicated that coincidental to expansion of air across bit-nozzles is the formation of water droplets which accumulate in the wellbore and along the walls of the drillpipe and hole. The presence of this free-water can become a source of error in studies of the pneumatic transport of solids. In this study, humidity and air density were measured and incorporated into the results The use of Ottawa sand for laboratory investigations of pneumatic transport can yield erroneous results because of the size attrition realized in circulating the sand To eliminate the problem with particle attrition, the experiments were conducted with sintered bauxite spheres. The experiments indicated that "Choking" occurred at low annular velocities where gravitational effects on the particles predominated and large pressure drops were observed As the annular air velocities were increased, a minimum pressure drop was observed This minimum pressure drop occurred at the optimum air velocity where air drilling is optimized As the air velocities were further increased, the pressure drops increased as the factional effects predominated Further, it was observed that in these experiments, optimum air velocity depended primarily on particle size. Minimum pressure drops also depends on particle size and on solids mass flow rate. Optimum air velocities and minimum annulus pressure drops increased when larger particle sizes were utilized.