6,000-12,000 feet per minute cuttings speed (uphole velocity) is ideal for evacuation. Over 20,000 FPM backpressure makes air find an easier way (through pebbles or sand). The penetration rate drops and the drill can get stuck. Less than 6,000 FPM and it will only be able to evacuate sand; diesel and time get spent pulverizing rocks. If your setup (Bit-Pipe-Compressors) can’t get you into the sweet spot, you’ll need some tricks to get your cuttings out efficiently. Foam can help or sometimes more air is needed. Switching to a Reverse Circulation will provide for the most effective evacuation, penetration rate, control over the uphole velocity and minimal air/diesel usage.