There is very limited information available regarding the air pressures, flow rate, and velocity profiles in blast hole air drilling applications. Currently, many people are using incompressible flow correlation to calculate the bailing velocity. In this article, air pressure, flow rate, and velocity profiles are calculated using compressible flow theory. The airflow at the bottom of the hole is simulated using the jet flow theory. The concept of choking condition is introduced, and it is concluded that the velocity at the nozzle exit cross section cannot exceed sonic velocity. The minimum pressure required for achieving choked condition at the nozzle cross section is calculated. Choked and overchoked flow conditions at the nozzles are examined, and the maximum achievable cleaning power is correlated to the nozzle size and pressure.The bailing velocity down the hole is calculated as a function of air pressure at the bit (upstream the nozzles). This article concludes that for a specific drill compressor, optimum nozzling of a bit results in highest achievable bailing velocity (considering drill operating conditions). Optimum nozzle sizes are calculated using the compressor evaluation data, and the disadvantages of improper nozzling of the bits are highlighted using field test data. The effect of very high bit pressures on bit life and bailing velocity is discussed, and optimum bit pressure is calculated based on choked condition at the nozzle exit cross section.
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