Borewell and DTH compressor engineering centre

Why a borewell compressor is not chosen by one number

A drilling contractor often asks for “a 1,000 cfm machine” or “a 350 psi compressor”. Either figure on its own says very little. The compressor is one link in a chain that runs from the airend, through the discharge valve, hoses, swivel and drill pipe, to the hammer, the bit face and back up the annulus with the cuttings. A weakness anywhere in that chain shows up at the rig as slow penetration, a stalled hammer or a hole that will not clean.

Two machines with the same headline rating can behave very differently at depth. One may deliver its full free air delivery (FAD) at the pressure the hammer needs; another may only reach that pressure by throttling back its flow. A good selection starts with the drilling job and works back to the compressor, not the other way round.

This engineering centre brings together the main topics a rig owner, driller or project engineer needs: hammer air demand, annular velocity, depth, groundwater, the air path, diagnostics, mobilisation and the decision to buy, hire, repair or replace. Each section links to a detailed page in our borewell library.

Start with the drilling job, not the compressor model

Before comparing machines, write the job down in a form another engineer could check. Unknowns should be marked as unknown, because a visible gap is safer than a confident guess.

• Rig make, model and drilling method (DTH, rotary, tophammer or combined).
• Hammer make and model, bit diameter and the maker’s air-consumption data at stated pressures.
• Planned hole diameters, casing depth and the target depth.
• Drill-pipe outside diameter and any changes in pipe size down the hole.
• Expected geology: soil and weathered overburden, hard rock, fractured zones.
• Expected water: depth to water, likely inflow, and whether foam or water injection is used.
• Site conditions: ambient temperature, dust, altitude, access for a towed or truck-mounted unit.
• Whether the need is a purchase, a hire, a replacement, a repair or fleet support.

Our borewell project enquiry brief sets out the same list in a form you can fill in and send to us.

Pressure and flow must be read together

Pressure and flow do different jobs. Pressure drives the hammer piston and overcomes the resistance of the air path and the water column above the hammer. Flow feeds the hammer and, after it exhausts, lifts the cuttings out of the hole. A compressor is only suitable if it delivers the required flow at the required pressure at the same time, under the site’s temperature and altitude.

Brochures sometimes quote a maximum pressure from one configuration and a maximum flow from another. These cannot be added together. Ask for the FAD at the actual working pressure, measured to a stated standard such as ISO 1217, and confirm it applies to the configuration being offered.

These are typical bands, not a substitute for the hammer maker’s data. See DTH compressor sizing and duty confirmation and high-pressure borewell air for the detailed method.

How hammer, bit and pipe set the air requirement

The hammer is usually the largest single consumer of air on the rig, and its consumption rises with supply pressure. Hammer makers publish air consumption against pressure; keep that data attached to the exact hammer model and bit size, and do not mix it with figures for a different hammer. Add any other air users on the same supply, such as rig functions or a water-injection pump, but do not count the hammer twice if a “complete rig” figure already includes it.

Bore diameter alone does not size the compressor. What matters for hole cleaning is the annular area between the hole wall and the drill pipe, A = π/4 × (D² − d²), where D is the hole diameter and d the pipe outside diameter. The same nominal bore drilled with a smaller pipe has a larger annulus, so the return air moves more slowly and the cuttings lift less well. That is why bore diameter vs air requirement and annular velocity are treated together.

Annular velocity figures only mean something when their basis is stated: free air or compressed air at bottom-hole pressure, and the assumed hole and pipe sizes. Many drilling references quote a minimum uphole velocity for dry cuttings; wet or sticky conditions usually need more. Our cuttings transport and borehole cleaning page explains how to check it.

Depth, groundwater and high-pressure duty

Depth is a programme, not a single pressure setting. As the hole deepens, the air path gets longer and, once water is struck, the column of water above the hammer adds back-pressure that the compressor must overcome. As a rule of thumb, every 10 m of standing water above the hammer adds roughly 1 bar (14.5 psi) that the supply must beat before the hammer works properly.

• Dry hard rock at moderate depth: a medium or high-pressure DTH unit is usually enough.
• Heavy water inflow: pressure margin becomes the deciding factor, and flow must still clear water and cuttings together.
• Unstable overburden or collapsing zones: casing planning matters as much as the air; see casing and unstable formation planning.
• Very deep work: a booster can raise the pressure of air from a primary compressor; see booster-assisted drilling air.

The detail is covered in drilling depth and compressor selection, wet hole drilling and water ingress and groundwater effects on drilling air.

The air path from compressor to hammer

Air that leaves the compressor at the right pressure can still arrive at the hammer short. Every hose, coupling, swivel, check valve and pipe joint takes a share of the pressure, and a leaking joint wastes flow before it reaches the bit. In Indian summer conditions, hot discharge air and long hose runs in direct sun add to the problem.

1. Check the compressor discharge pressure while drilling, not at idle.
2. Read pressure at the rig inlet or swivel to see the loss in hoses and connections.
3. Inspect hose bore, length and condition, and check couplings and whip-checks.
4. Check drill-pipe threads, seals and internal bore for wear or damage.
5. Confirm the lubricator delivers the hammer oil the maker specifies.

Read more in air hoses and rig connections, drill string and air path review and DTH lubrication and air treatment interfaces.

Diagnose before you upsize

Slow drilling is often blamed on a small compressor when the real cause lies elsewhere. Before buying or hiring a bigger machine, rule out the common faults.

Our borewell compressor performance diagnostics page walks through these checks, and diesel compressor starting and charging diagnostics covers the engine side.

Mobilisation and site acceptance

A sound compressor can still fail on site if it arrives without the right hoses, with old oil or with dust-choked coolers. Before a machine goes to a rig, check its oil and filters, cooler cleanliness, safety shutdowns (high discharge temperature, low engine oil pressure, high coolant temperature), battery and charging, tyres and towing gear, and hose fittings. At site, run it loaded and record discharge pressure and temperature. See drilling air mobilisation and site acceptance.

After drilling, the job changes: yield testing, development and pump installation have their own air and equipment needs. The pages on borewell yield testing and production duty and final bore diameter and pump passage cover that stage.

Own, hire, repair or replace

Running cost per metre drilled is often a better yardstick than purchase price; see drilling productivity and cost per metre and borewell rig reliability and drilling records.

Borewell library by topic

• Selection: DTH compressor selection, DTH drilling compressors, water-well drilling compressors, DTH hammer air requirement, DTH hammer and bit selection.
• Rig and ground: the borewell rig as an integrated system, groundwater, geology and drilling method selection, deep hole drilling air planning.
• Machines and support: new borewell compressor sales, used and reconditioned borewell compressors, borewell compressor rentals and hire, borewell compressor service and repairs, borewell compressor spare parts, parts and lifecycle support.

Rig-integrated compressors: single screw or twin screw, and sizing the diesel engine

Many crawler and truck-mounted DTH rigs carry their own compressor, driven by the same diesel engine that powers the rig’s hydraulics for rotation and feed. Two compressor designs are common on these rigs.

The design type matters less than the delivered flow at working pressure, cooling capacity at 45 °C ambient, engine margin and the machine’s condition. A full comparison is on single screw vs twin screw DTH compressor.

Worked example: diesel engine power for a 1,000 cfm, 24 bar(g) rig compressor

1. Flow: 1,000 cfm ÷ 35.315 = 28.32 m³/min free air, or 0.472 m³/s.
2. Pressure ratio: discharge 24 + 1.013 = 25.01 bar(a); inlet 1.013 bar(a); ratio 25.01 ÷ 1.013 = 24.7.
3. Ideal (isothermal) power: 101,325 Pa × 0.472 m³/s × ln 24.7 = 153 kW.
4. Compressor shaft power: assuming an isothermal efficiency of 55% for a high-pressure oil-injected package (illustrative; use the maker’s shaft power where available), 153 ÷ 0.55 = 279 kW.
5. Cooling fan and auxiliaries: allow 8%: 279 × 0.08 = 22 kW, giving 301 kW.
6. Rig hydraulics: rotation and feed drawing 60 litres/min at 180 bar(g) need 180 × 10⁵ Pa × 0.001 m³/s = 18 kW hydraulic, or 18 ÷ 0.85 = 21 kW at the pump shaft. Total demand is about 301 + 21 = 322 kW.
7. Site derating: allow 10% for 45 °C ambient and Deccan altitude (illustrative; take actual figures from the engine’s derating curve): 322 ÷ 0.90 = 358 kW, about 480 hp, as the engine’s continuous rating.

At a specific fuel consumption of about 220 g/kWh and diesel at 0.835 kg/litre, full output of 322 kW burns about 322 × 0.22 ÷ 0.835 = 85 litres/hour. Real consumption is lower because the compressor unloads during rod changes.

Dust control on DTH rigs: dry collection or wet suppression

DTH drilling in granite and gneiss produces fine dust with a high proportion of quartz. Respirable crystalline silica is a lung hazard for the crew, and the same dust chokes compressor coolers and air filters. Rigs use one of two approaches, and some offer both, switched to suit the ground.

Air and water budgets (illustrative)

A collector rated at 0.3 m³/min of pulse air uses 0.3 × 35.315 = 10.6 cfm, about 1% of a 1,000 cfm compressor; include it in the flow budget below. A wet system injecting 3 litres/min uses 3 × 60 × 8 = 1,440 litres in an 8-hour shift. Once the hole meets significant groundwater, the formation itself wets the cuttings; see wet-hole drilling and water ingress.

Feed force and rotation speed by hammer size

A DTH hammer does the cutting by impact. Rotation only indexes the bit to fresh rock between blows, and feed force only keeps the bit in contact with the bottom. Too little feed lets the hammer bounce and damages threads; too much stalls rotation and wears the buttons quickly.

Rotation is set by a target bit rim speed of about 0.25 to 0.50 m/s: rpm = 60 × v ÷ (π × D), with D in metres. For a 165 mm bit at 0.25 m/s: 60 × 0.25 ÷ (π × 0.165) = 29 rpm; at 0.50 m/s, 58 rpm. Feed force typically falls between about 0.4 and 1.2 kN per cm of bit diameter, with around 0.85 kN per cm a common setting near 16 bar(g); for 165 mm, that is 6.6 to 19.8 kN, about 14 kN at 16 bar(g).

Use the lower rotation in abrasive rock and the higher rotation in softer, fractured or non-abrasive ground. Higher air pressure pushes the hammer back off the rock harder, so it needs more feed; start low and raise feed in small steps until rotation is smooth and penetration steady. The hammer maker’s figures take precedence; see DTH hammer and bit selection.

Worked example: when the string outweighs the feed

Rods weighing about 16 kg/m (illustrative; check your own rods) at 150 m depth, plus a 90 kg hammer, weigh (16 × 150 + 90) × 9.81 ÷ 1,000 = 24.4 kN in air. Submerged in water, buoyancy removes 1 ÷ 7.85 of it (about 13%, from the density of steel), leaving about 21.3 kN. If the bit needs 14 kN, the rig must hold back about 10.4 kN in a dry hole, or 7.3 kN with the string submerged, rather than push down. Many slow, rough-drilling deep holes are simply over-fed by their own string. To lift the string with a 1.5 times margin for friction and stuck cuttings, the rig’s pullback needs at least 24.4 × 1.5 = 36.6 kN.

Matching compressor output to the rig’s hammer range

A rig rated for 4 to 6 inch hammers needs a compressor matched to the largest hammer at the highest pressure the job requires. Hammer air consumption rises roughly in proportion to absolute inlet pressure, so a figure quoted at one pressure must be corrected before comparing compressors.

Worked example (illustrative)

1. Hammer chart: a 6 inch hammer uses 750 cfm at 17.2 bar(g) (about 250 psig).
2. Planned pressure at the hammer: 20.7 bar(g) (about 300 psig) for deeper, wetter sections.
3. Corrected flow: 750 × (20.7 + 1.013) ÷ (17.2 + 1.013) = 750 × 21.71 ÷ 18.21 = 894 cfm.
4. Leakage and losses at swivel and joints: add 5%: 894 × 1.05 = 939 cfm.
5. Dust collector pulse air: add 10.6 cfm: about 950 cfm.
6. Pressure at the compressor: 20.7 bar(g) plus about 1.0 bar loss in hoses, swivel and drill pipe = 21.7 bar(g), 314.7 psig.

A compressor delivering 1,000 cfm at 24 bar(g) (about 348 psig) covers this with about 5% flow margin and 2.3 bar of pressure reserve. A 4 inch hammer on the same rig uses far less air. Reduce the set pressure to within that hammer’s maximum rating, and if the hole is not cleaning, fit a choke (bypass) plug that sends extra air past the hammer to the bit for flushing. Confirm the result against annular velocity and DTH hammer air requirement. For special high-pressure duties, the rental fleet includes DTH and borewell units up to 35 bar(g).

How Reddy Compressors helps

We supply all types, all brands and all models of borewell compressors: new machines, used and refurbished units, rentals, service and spare parts. We work as an independent multi-brand specialist with ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and other makes; we are not an authorised dealer of any brand. We have tie-ups with all major and reputed brands.

Our office is at Tarnaka, Secunderabad, and we have been serving industry since 2002. Our team of more than 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply compressors, rental machines and spares across India. We run our own rental fleet, including high-pressure diesel units, and on hire jobs we change oil and service every 300 running hours.

For a rig owner, that means one contact for choosing the right pressure band, checking a machine before you buy, hiring cover for a deep or wet contract, repairing a breakdown or identifying a part from the nameplate before it is quoted.

What to send us

• Rig, hammer and bit details, and the hammer maker’s air data if you have it.
• Hole diameter, drill-pipe size, target depth and expected water.
• Current compressor make, model, serial number and a nameplate photo.
• Pressure and temperature readings while drilling, and any fault symptoms.
• Whether you want to buy new or used, hire, repair or buy spares.
• Site location in Telangana, Andhra Pradesh or elsewhere in India.

Frequently asked questions

What size compressor do I need for a borewell?

It depends on the hammer, bit size, drill-pipe size, depth and water. Most water-well DTH rigs in India use 600–1,500 cfm at 200–400 psig (13.8–27.6 bar). Shallow agricultural bores sit at the lower end; deep, large-diameter or wet holes need the higher bands. Start from the hammer maker’s air consumption at your working pressure, check annular velocity, then add margin for hose losses and heat.

Is more pressure or more cfm better for borewell drilling?

Neither on its own. Pressure makes the hammer strike hard and overcomes water standing above it; flow feeds the hammer and lifts cuttings out of the hole. Too little pressure and the hammer slows or stalls in water. Too little flow and the hole does not clean, so the bit regrinds cuttings. You need the right flow at the right pressure together.

Why does my borewell compressor lose pressure at depth?

The usual reasons are water above the hammer adding back-pressure, longer air paths with more losses, leaking pipe joints, worn hoses or couplings, or a compressor that cannot hold its rated flow at that pressure. A tired airend, blocked air filter or faulty unloader can also limit pressure. Measure at the compressor and at the swivel while drilling to find where it is lost.

Can I hire a high-pressure compressor for one deep borewell?

Yes. Hiring suits one-off deep or wet bores, seasonal peaks and breakdown cover. We run our own fleet of high-pressure diesel compressors and can match the unit to your hammer and depth. Tell us the rig, hammer, bit, depth and site location, and we will advise the pressure band and accessories needed, including hoses where required.

Should I buy a new or used borewell compressor?

A new machine suits steady year-round work and gives full maker support. A used or reconditioned unit can be good value if the airend, engine and cooling system are sound and spares for that model are available. Ask for running hours, service records and a loaded test at working pressure before deciding. We sell both, and test-run used machines under load before purchase.

Do you supply spare parts for all borewell compressor brands?

Yes. We supply spares for all brands and models of borewell compressors as an independent multi-brand specialist. We identify the part from the make, model, serial number, nameplate photo or the old part, and confirm fit before quoting. Common items include air and oil filters, separators, oil, unloader and minimum pressure valve kits, hoses, belts and engine parts.

Is a single screw or twin screw compressor better for a DTH drilling rig?

Neither is better in every case. A single screw compressor balances gas loads across its main rotor, so its bearings are lightly loaded, but its gaterotors are wear parts that need periodic replacement. A twin screw compressor is the more widespread design, usually two-stage at high pressure, with rotor bearings as the main life-limiting item. For a DTH rig, compare delivered cfm at working pressure, cooling capacity at 45 °C, engine margin, spares availability for the exact model and the machine’s condition before the rotor type.

What size diesel engine does a DTH drill compressor need?

Start from the compressor’s shaft power at full flow and pressure, add the cooling fan and auxiliaries, add the rig’s hydraulic power for rotation and feed if the same engine drives them, then derate for heat and altitude. In the worked example on this page, a 1,000 cfm, 24 bar(g) compressor needs about 279 kW at the shaft, 322 kW in total with fan and hydraulics, and an engine of about 358 kW (about 480 hp) continuous rating after a 10% site derating.

What is pulse wet dust removal on a DTH drilling rig?

The phrase covers the two main dust-control methods on DTH rigs, which some rigs carry together. Dry collection uses a collar hood, a cyclone and filter bags or cartridges cleaned by short pulses of compressed air. Wet suppression injects water into the drilling air so dust is wetted at the bit. Dry collection uses a little pulse air and no water; wet suppression needs a water supply of a few litres per minute and higher hammer lubrication. Choose by rock type, water availability and how close the site is to people.

How much feed force does a DTH drill need?

Only enough to keep the bit firmly on the rock. Typical figures are 0.4 to 1.2 kN per cm of bit diameter, about 6.6 to 19.8 kN for a 165 mm bit, with around 14 kN common near 16 bar(g). Start low and raise feed in small steps, more at higher air pressure, until rotation is smooth. In deep holes the drill string often weighs more than the bit needs: 150 m of 16 kg/m rods plus the hammer weighs about 24 kN. The rig must then hold back rather than push down.

Related pages

• Borewell drilling season: the rig compressor pre-season checklist
• Borewell and DTH compressor library
• New borewell compressor sales
• Borewell compressor rentals and hire
• Borewell compressor service and repairs
• Borewell compressor spare parts