The V-Cone. To determine the actual air volume (CFM) being produced, a flow meter such as a V-Cone from McCrometer can be very useful. The V-Cone conditions the air flowing through the string, and creates laminar flow (straight flowing air). It measures the pressure above the cone, then again below the cone, and converts the difference in pressure to a velocity measurement, which is later used to calculate the volume.The inside of the “V-Cone” flow meter by McCrometer.
Determining Your Specific Air Requirements, do you want to concentrate on clearing the cuttings from the cones or do you want to concentrate on removing the cuttings from the hole?
Velocity Vs. Volume
To clear chips from the cones you need: Smaller nozzles, which give you higher air velocity at face of the bit. This will blow the cuttings out and away from the cone. To evacuate the chips from the hole you need: Larger nozzles, which give you higher air volume out the bit. This will create more pressure in the drilled hole, lifting the chips out of the hole.
Let’s look at a garden hose nozzle as an example... Small nozzle opening means faster moving water, but less water exiting the nozzle. High pressure with low volume. Imagine the hose nozzle at its smallest possible opening. There is a lot of restriction.
Using the garden hose nozzle again as an example... Imagine the hose nozzle at its largest possible opening. There is almost zero restriction. A large nozzle opening provides more volume with less pressure which means a large amount of slower moving water is able to escape the hose. There is higher volume of slower moving water.
Small Nozzle Diameter - High Velocity.
Small nozzles will give you higher velocity coming out the nozzle. This means faster moving air at the cone, which will clear the bit face of cuttings very quickly. However, the lack of air volume will mean less efficient bailing, thereby not get the cuttings out of the hole as efficiently.
Large Nozzle - High Volume
Large nozzles give you higher volume coming out the nozzle. This means slower moving air at the bit face, meaning less efficient clearing of the cones. However, the high volume of air will create more bailing velocity, thereby creating more pressure around the drill steel and carrying cuttings up and out of the hole.
Breakdown of Deciding Factors
The first step is determining the actual volume of air at the end of the drill string (after all losses, ex: pipe loss, altitude, wear, etc.) A flow meter such as the V-Cone is used for this step. Next, determine the largest size of nozzle possible without exceeding the compressor’s limits (the minimum rated pressure (psi) permitted).The next step is deciding where to place your emphasis, clearing the bit face of cuttings, or clearing the hole of cuttings. After that decision has been made, choose a nozzle accordingly. Experiment with different sizes of nozzles until you have reached your ideal balance between nozzle velocity and bailing velocity.