Compressed air requirements in down-the-hole drilling

Compressed air requirements in down-the-hole drilling are determined by the pressure and air flow needed to operate a down-the-hole (DTH) hammer while continuously removing rock cuttings from the borehole.

In DTH drilling, compressed air performs two different but closely related functions. First, it supplies the pneumatic energy required to drive the hammer piston. Second, the exhaust air leaving the hammer becomes the flushing medium that carries broken rock from the bottom of the hole to the surface.

Correctly matching the compressor to the hammer, hole diameter and drilling conditions is important for penetration rate, hole cleaning, fuel efficiency and equipment reliability. Selecting a compressor only by maximum pressure or engine power can result in poor drilling performance or unnecessary energy consumption.

A DTH hammer is located directly behind the drill bit. Compressed air travels from the compressor through the supply hose, drill rig and drill string before entering the hammer.

The air performs several functions:

• drives the hammer piston and produces repeated impacts at the drill bit;
• exits through passages in the drill bit and cleans the bottom of the hole;
• transports rock cuttings upward through the annular space between the drill pipe and borehole wall;
• assists in cooling and cleaning the hammer and drill bit;
• reduces accumulation and regrinding of cuttings at the bottom of the borehole.

Pressure and air flow should not be treated as the same parameter. Pressure is primarily associated with hammer operation and impact performance, while flow is especially important for flushing and cuttings transport.

Atlas Copco describes drilling-compressor selection as a balance between pressure and flow, with the required combination changing according to drilling depth and hammer size.^[1]

Several parameters should be evaluated before selecting the compressed-air supply.

The DTH hammer manufacturer normally specifies an operating pressure range and air consumption. Larger hammers generally require greater air flow because of their larger internal passages and higher pneumatic demand.

The manufacturer's hammer data should therefore be the starting point for compressor sizing.

Hole diameter strongly affects the flushing requirement.

After air leaves the hammer, it must travel upward through the annular area between the drill pipe and the borehole wall. Increasing the hole diameter increases this area. If compressor flow remains unchanged, the return-air velocity decreases.

This is one reason why a compressor that performs adequately with a smaller drill bit may provide insufficient flushing when a larger bit is installed.

The compressor must provide sufficient pressure at the hammer, not simply at the compressor outlet.

Pressure is lost as compressed air passes through:

• supply hoses;
• valves and fittings;
• filters and separators;
• the rotary head;
• drill pipes;
• connections between drill rods.

Long air lines, small internal diameters and high flow rates increase these losses.

For this reason, the pressure displayed at the compressor outlet can be higher than the pressure actually available at the hammer.

Compressor flow is commonly expressed as free air delivery (FAD).

FAD represents the useful quantity of air delivered by the compressor when expressed at defined reference or inlet conditions. It is more useful for equipment comparison than theoretical compressor displacement.

Atlas Copco describes airflow or FAD as one of the primary parameters used when sizing an air compressor.^[2]

Common units include:

• cubic metres per minute (m³/min);
• cubic feet per minute (CFM);
• litres per second (L/s).

A useful approximate conversion is:

${\displaystyle 1m^3/min\approx 35.3CFM}$

The movement of cuttings toward the surface depends partly on the velocity of the exhaust air in the annular space.

A simplified average annular velocity can be expressed as:

${\displaystyle v=\frac{Q}{A}}$

where:

• ${\displaystyle v}$ = average annular air velocity;
• ${\displaystyle Q}$ = volumetric air-flow rate;
• ${\displaystyle A}$ = annular cross-sectional area.

For a circular borehole and drill pipe:

${\displaystyle A=\frac{\pi }{4}(D_h^2-D_p^2)}$

where:

• ${\displaystyle D_h}$ = borehole diameter;
• ${\displaystyle D_p}$ = outside diameter of the drill pipe.

This relationship shows why hole diameter and drill-pipe diameter must be considered together.

For example, increasing hole diameter while keeping the same drill pipe and compressor flow increases the annular area. The same quantity of air is then distributed through a larger cross-section, reducing average return velocity.

In actual drilling, the air flow required for effective cleaning also depends on:

• rock density;
• size and shape of cuttings;
• moisture or groundwater;
• hole depth;
• hole inclination;
• leakage around drill rods;
• hammer exhaust characteristics.

The hammer and drilling equipment manufacturer's recommendations should therefore be used in addition to theoretical calculations.

A frequent sizing mistake is to focus on only one compressor specification.

A compressor with sufficient pressure but inadequate flow may operate the hammer while failing to remove cuttings efficiently.

This can result in:

• reduced penetration rate;
• excessive regrinding of cuttings;
• unstable drilling;
• increased risk of material accumulating around the drill string;
• increased drilling time and fuel use.

Conversely, a compressor with high flow but insufficient working pressure may provide effective flushing but fail to operate a high-pressure DTH hammer at its intended performance level.

Modern drilling systems therefore increasingly allow different combinations of pressure and flow.

For example, the Sandvik Leopard DI560 is designed for 4, 5 and 6 inch DTH hammers and specifies flushing-air performance at different pressure conditions.^[3]

The compressed-air delivery path should be treated as part of the drilling system.

Pressure loss becomes more significant when large volumes of air are forced through small internal diameters.

Important factors include:

• hose internal diameter;
• hose length;
• drill-pipe internal diameter;
• fittings and bends;
• valves;
• air-flow rate;
• internal surface condition.

When two compressors have similar rated pressure and FAD, the installation using a larger and shorter delivery hose may provide better pressure at the hammer.

Oversizing every hose is not always practical because larger hoses are heavier, more expensive and harder to handle. The objective is to choose a diameter that limits pressure loss without unnecessarily increasing system cost and weight.

Ambient conditions influence compressor performance.

At higher altitude, atmospheric pressure and air density decrease. A compressor therefore draws in less air mass for a given inlet volume than it would at sea level.

High ambient temperature also reduces inlet-air density and increases the thermal load on the engine and compressor cooling system.

A compressor that performs satisfactorily at sea level may therefore have less usable capacity at a high-altitude mine, quarry or water-well site.

When operating under these conditions, the equipment manufacturer's altitude and temperature derating information should be included in the sizing calculation.

A practical DTH compressor-selection process can follow these steps:

1. Identify the DTH hammer. Record hammer size, manufacturer and model.
2. Check the required operating pressure. Use the hammer manufacturer's specified range.
3. Check hammer air consumption. Determine the flow requirement at the intended operating pressure.
4. Determine bit and hole diameter. Do not assume that hammer air consumption alone guarantees adequate flushing.
5. Determine drill-pipe dimensions. Use the borehole and pipe diameters to evaluate the available annular area.
6. Consider drilling depth. Longer holes increase the air path and can make pressure losses and flushing more difficult.
7. Evaluate hoses and restrictions. Include hose length, internal diameter, valves and fittings.
8. Correct for site conditions. Account for altitude, temperature and expected groundwater.
9. Select compressor FAD and pressure. Provide a reasonable operating margin rather than selecting a machine that only meets the theoretical minimum.
10. Evaluate fuel and maintenance requirements. Compare machines at the pressure and flow actually required by the drilling operation.

Atlas Copco also provides compressor-selection tools that use factors such as DTH application and drilling depth when matching portable compressors to drilling requirements.^[4]

Compressed-air generation requires substantial engine or electrical power. Oversizing a drilling compressor may therefore increase fuel or electricity consumption when the additional capacity is not required.

Undersizing can also waste energy because poor hole cleaning and low penetration rates increase the time required to complete each hole.

An efficient system aims to provide enough pressure and flow for reliable drilling without producing significantly more compressed air than the operation needs.

Potential efficiency improvements include:

• selecting pressure according to the hammer requirement instead of always using maximum compressor pressure;
• reducing unnecessary pressure losses;
• using correctly sized hoses and drill pipes;
• repairing air leaks;
• matching compressor capacity to hammer and hole diameter;
• maintaining air filters, coolers and separator elements;
• using adjustable pressure and flow controls where available.

Some modern drilling compressors allow the operator to vary pressure and flow instead of operating continuously at one fixed maximum setting. Manufacturer documentation indicates that this flexibility can help adapt the compressor to changing hammer sizes and drilling depths while reducing unnecessary fuel consumption.^[1]

Electric portable compressors can also reduce direct combustion emissions at the drilling site where a suitable electrical supply is available. Atlas Copco lists electric-driven portable compressors as an option for drilling operations that prioritize reduced emissions.^[5]

Common problems include:

• selecting the compressor only according to engine horsepower;
• comparing theoretical displacement instead of FAD;
• considering pressure while ignoring flow;
• selecting flow based only on hammer consumption and ignoring hole cleaning;
• failing to account for pressure loss in hoses and drill pipes;
• using undersized air hoses;
• ignoring altitude and high ambient temperatures;
• applying one compressor setting to very different hammer and hole sizes;
• operating continuously at maximum pressure when it is not required.

A more reliable approach is to regard the compressor, hoses, drill string, hammer and borehole as one connected air system.

Improved compressor matching can reduce more than fuel consumption.

Faster and more stable drilling may reduce:

• compressor operating hours per borehole;
• engine fuel use;
• unnecessary idling;
• wear on drilling components;
• frequency of maintenance;
• logistical demand for fuel at remote sites.

The lowest-rated compressor is not necessarily the most efficient choice, and the largest compressor is not necessarily the most productive. The appropriate system is the one that provides sufficient usable pressure and air flow under actual site conditions while avoiding excessive energy consumption.

• Air compressor
• Energy efficiency
• Drilling
• Water well
• Mining
• Pneumatics