Down-the-Hole (DTH) rock drilling is one of the most demanding compressed air applications in terms of simultaneous pressure and volume requirements. The DTH hammer at the bottom of the drill string is powered entirely by compressed air. The air drives the hammer piston at high frequency, simultaneously providing the percussion energy that breaks the rock and purging cuttings from the hole up through the annulus between the drill pipe and hole wall. Get the air supply wrong and the drill rate falls, the hammer jams or the hole fills with cuttings.
| Q: What air pressure and CFM is needed for DTH drilling?
DTH drilling air requirements depend on hammer size, rock hardness and hole depth. For 4-inch to 6-inch diameter water well drilling in medium rock: 100 to 200 CFM at 100 to 150 PSI (7 to 10 bar). For 6-inch to 8-inch blast hole drilling in hard rock: 250 to 400 CFM at 150 to 200 PSI (10 to 14 bar). For large diameter DTH drilling above 8 inches: 400 to 900 CFM at 200 to 350 PSI (14 to 24 bar). More pressure improves hammer performance; more CFM clears cuttings and prevents hole collapse. |
How DTH Drilling Uses Compressed Air
In Down-the-Hole drilling, the hammer assembly sits at the bottom of the drill string. Compressed air is piped down through the hollow drill rods to the hammer. Inside the hammer, the air pressure drives a piston that reciprocates at high frequency, typically 800 to 2,400 blows per minute depending on hammer design and air pressure. Each blow impacts the drill bit, which transfers the impact energy to the rock face.
After driving the piston, the exhaust air exits through ports in the drill bit and flows upward through the annular space between the drill rods and the hole wall, carrying the rock cuttings generated by the bit to the surface. This simultaneous cuttings lifting function explains why both pressure (for hammer performance) and volume (for cuttings velocity) must both be adequate. Insufficient volume causes cuttings to accumulate at the bottom of the hole, burying the bit and stopping penetration.
Minimum Cuttings Velocity Calculation
The minimum air velocity in the annulus required to lift cuttings varies with rock density and particle size, but a general target is 15 to 20 meters per second annular velocity. Air velocity is calculated from the air flow rate and the annular area between the drill rod outside diameter and the hole diameter. For a 6-inch hole with 3.5-inch drill rods: annular area = pi/4 x (6^2 – 3.5^2) inches^2 = 18.8 square inches = 0.0121 m2. For 15 m/s minimum velocity and 6 bar absolute: required mass flow is approximately 0.0121 x 15 x 6 / 1 = 1.09 m3/s at operating conditions = 65 m3/min FAD. This is approximately 2,300 CFM, confirming why large diameter DTH drilling requires very high flow rate compressors.
DTH Hammer Selection and Air Pressure Requirements
DTH hammers are available for operating pressures from standard 7 to 10 bar up to high-pressure 25 bar and above. Higher pressure hammers deliver greater impact energy per blow, improving penetration rate in hard rock. However, the compressor must be able to maintain the required operating pressure at the hammer while simultaneously providing sufficient flow volume for cuttings removal.
Standard rotary screw compressors in the 7 to 10 bar range suit smaller hammers in softer rock. High-pressure diesel portable compressors from DEHAHA in the 14 to 24 bar range provide the pressure needed for productive drilling in hard granite, quartzite or similar formations.
Effects of Hole Depth on Air Requirements
As a DTH hole deepens, the hydrostatic pressure of the air column in the drill rods increases slightly, but more significantly, the pressure required to lift cuttings over the increasing depth increases. A rule of thumb is to add approximately 0.1 bar to the required hammer operating pressure for every 10 meters of hole depth for compressed air drilling in dry holes.
For water well drilling, groundwater encountered during drilling enters the annulus and must also be lifted to surface. The additional hydrostatic head of the water column significantly increases the back pressure against which the exhaust air must work. DTH water well drilling in high water table areas typically requires 25 to 50 percent more compressor pressure than dry rock drilling at the same depth.
DEHAHA Portable Diesel Compressors for DTH Drilling
DEHAHA diesel portable screw compressors in the high-pressure range cover DTH drilling requirements from small water well rigs to large blast hole drills. Our 185 CFM to 900 CFM range includes high-pressure variants at 14 bar, 17 bar and 24 bar for demanding hard rock drilling applications. Browse: Portable diesel air compressor range
>>> Request DEHAHA DTH Drilling Compressor Specifications and Pricing
Связанный: Screw Compressor for Mining Applications
Связанный: Три основных типа буровых установок
