Who this page is for: drilling contractors, quarry owners and borewell rig owners buying, hiring or running a DTH rig, especially an integrated rig where one diesel engine drives both the compressor and the rig hydraulics, who need to know whether screw type matters and whether the rig’s air suits their hammer, hole and site.
Key facts
- DTH compressors work at roughly 10–25 bar(g) (145.0–362.6 psig), with deep or wet borewell duty up to about 28 bar(g) (406.1 psig) and special units to about 35 bar(g) (507.6 psig). At these pressures both screw types are oil-injected.
- Compressing 45 °C air to 20 bar(g) (290.1 psig) in one stage without cooling would give a discharge temperature near 480 °C. Injected oil holds it at typically 80–105 °C and carries 80–90 % of the shaft power to the cooler.
- A twin-screw airend carries heavy axial and radial bearing loads; bearings are its main overhaul item. A single-screw main rotor is nearly balanced, but its two gate rotors have wearing tooth faces that set the delivered air.
- A rig’s air figure means something only as free air delivery (FAD) at a stated pressure. Theoretical displacement is typically 5–15 % above FAD on a healthy oil-injected screw.
- At 45 °C each cubic metre of intake air carries about 7.9 % less mass than at 20 °C; the m³/min figure does not change, but the hammer feels the loss.
- On an integrated rig the compressor typically takes 70–80 % of engine output at full load, while the engine may typically lose 5–8 % of its power at 45 °C (the maker’s curve decides).
- At Hyderabad, about 540 m, atmospheric pressure is about 95.0 kPa against 101.3 kPa at sea level: intake air is about 6 % less dense before any heat correction.
How each screw type compresses
Twin-screw: male and female rotors
Two meshing helical rotors turn in a figure-of-eight casing: a male rotor, commonly with four lobes, and a female rotor, commonly with five or six flutes. Trapped air is squeezed into a shrinking pocket until it reaches the discharge port, whose position sets the built-in volume ratio. In oil-injected machines the male rotor drives the female through the oil film, so no timing gears are needed. See our rotary screw explainer.
For DTH duty the consequence is load. Discharge pressure pushes both rotors towards the suction end and sideways apart, and these forces rise roughly in proportion to the pressure difference, so a 20 bar airend carries about twice the gas load of a similar 10 bar airend. Heavy thrust bearings are used, and many 25–35 bar portable units split the work into two stages to keep each stage’s pressure ratio and bearing load sensible.
Single-screw: one main rotor and two gate rotors
One cylindrical main rotor, commonly with six helical grooves, meshes with two star-shaped gate rotors (star wheels) set on opposite sides, their shafts at right angles to it. Each gate rotor tooth sweeps along a groove like a moving wall, shrinking its volume until the groove reaches the discharge port. With two gate rotors, each groove compresses twice per revolution.
That symmetry cancels most radial gas force, and in the usual design both ends of the main rotor sit at suction pressure, so axial force largely cancels too and the main rotor bearings run lightly loaded. The load does not vanish: it moves to the gate rotor shafts and their bearings. The gate rotors carry the gas load on their teeth, which are usually a polymer composite on a metal carrier so they can run at very small clearance. Those tooth faces wear; as they do, internal leakage grows and free air delivery falls gradually rather than suddenly.
Why both are oil-injected for DTH, and what follows
Dry screw compression reaches only about 3–3.5 bar(g) per stage before discharge temperature runs away. At DTH pressure ratios of 15–28, injected oil absorbs the heat, seals the clearances and lubricates the bearings and, in a single-screw machine, the gate rotor faces. The quantities are large: a typical airend delivering 21 m³/min (741.6 cfm) at 20 bar(g) (290.1 psig) needs about 185 kW at the shaft, and if 85 % of that heat goes into oil warming by 25 K, the circuit moves about 3.1 kg/s, roughly 217 litres per minute.
- Separator: the receiver-separator and its coalescing element strip oil before the hose. A failed element sends oil down the string and starves the airend. See separator and receiver vessels.
- Oil: hot, humid, dusty air at high pressure makes grade, interval and cleanliness critical; on a single-screw machine the oil must also suit the gate rotor material.
- Cooling: a cooler designed with a 60 K difference between oil at 95 °C and air at 35 °C has only 50 K at 45 °C, about 17 % less heat removed before any dust settles on the fins.
The monsoon problem: water in the oil
At high pressure, intake moisture condenses at a surprisingly high temperature, because compression multiplies the partial pressure of the water vapour by the absolute pressure ratio. Air at 35 °C and 80 % relative humidity carries water vapour at about 4.5 kPa; compressed from 101.3 kPa to 20 bar(g) (a ratio of about 20.7) that becomes about 93 kPa, which saturates at about 98 °C; at Hyderabad’s 95 kPa intake it is about 99 °C. A 30 °C, 85 % monsoon day still gives about 92 °C. Air at 45 °C and 25 % humidity in a dry summer saturates at about 81 °C. A machine whose oil runs at 90 °C is just clear in May but condenses water into its separator in a humid monsoon week. That is why high-pressure machines hold oil temperature well up with their thermostatic valve, and why fitting a cooler-running element is a mistake. Milky oil after the rains is the warning on either type.
Single-screw versus twin-screw: the comparison
Independent drilling data is thin; uncertain points are marked.
| Aspect | Twin-screw | Single-screw |
|---|---|---|
| Mechanism | Two meshing rotors; pocket shrinks along the mesh | Gate rotor teeth sweep main rotor grooves; two compressions per groove per revolution |
| Bearing loads | High thrust and radial loads, rising with pressure | Main rotor nearly balanced; gate rotor bearings carry the tooth load |
| Main wear parts | Bearings, shaft seals; rotor profiles wear slowly | Composite gate rotor teeth, gate rotor bearings, seals |
| Typical DTH pressure | 10–25 bar(g) (145.0–362.6 psig) single-stage; two-stage common to 35 bar(g) (507.6 psig) | About 13–25 bar(g) (188.5–362.6 psig) on integrated rigs, usually single-stage |
| Efficiency at DTH pressures | Mature profiles; good while bearings and clearances are sound | Comparable when new; evidence mixed; delivery falls as gate rotor teeth wear |
| Dust and heat tolerance | Dust in oil wears bearings | Dust and hot oil also attack composite tooth faces |
| Overhaul pattern | Airend rebuild or exchange at bearing life; workshop job | Planned gate rotor replacement with clearance setting; full rebuild less often |
| Parts in India | Wide: bearings, seals and rebuild skills common across makes | Narrower: gate rotors are model-specific, often only via the rig or airend maker |
The hammer cannot tell which screw made its air; it responds only to the mass of air arriving at pressure, so judge any rig compressor by its delivered free air at working pressure on a hot day, its cooling margin and the parts you can buy for it.
Integrated rig or separate compressor
An integrated rig mounts compressor, hydraulic pumps, coolers and usually a dust collector on one crawler driven by one diesel engine; a separate arrangement pairs a rig with a portable compressor that has its own engine. Our page on the borewell rig as one system describes the parts; the question here is power.
| Consumer on an integrated rig | Typical share of engine output | When it peaks |
|---|---|---|
| Compressor | 70–80 % | Loaded at full pressure |
| Cooling fan (often hydraulic) | 5–8 % | Hot afternoons, dirty coolers |
| Rotation | 8–12 % | Collaring, broken ground, jammed bit |
| Feed and pull-back | 2–4 % drilling; more on pull-back | Freeing a stuck string |
| Dust collector fan | 3–4 % | Continuously while drilling dry |
When the compressor is loaded and the hydraulics call for peak torque together, the engine either has margin or lugs down. Compressor delivery falls with engine speed, hammer pressure sags, penetration slows and rotation torque rises further. Good rigs use power-limiting pump controls; operators help by not collaring at full feed with the compressor loaded.
Diesel engines lose power above their rated ambient; use the engine maker’s curve. Altitude is not negligible on the Deccan plateau: at Hyderabad, about 540 m, intake air is about 6 % less dense than at sea level, while coastal sites lose almost nothing. Heat adds its own penalty on top. A separate compressor isolates hydraulic peaks from the air, at the cost of a second engine and hose losses.
When reading a rig’s air figure, look for FAD at a stated pressure, engine speed and intake temperature (see FAD and reference conditions); a machine rated at 17 bar(g) (246.6 psig) delivers less at 20 bar(g) (290.1 psig), and dust collector pulses take their share before the hammer.
Matching the compressor to the hammer and the hole
Sizing starts at the bottom of the hole: the hammer’s consumption at pressure (see DTH hammer air requirement) and enough air leaving through the annulus to clean the hole (see annular velocity and the annular area and velocity tool). The compressor must supply the larger of the two at hammer pressure plus every loss, as our DTH compressor selection guide sets out.
Annulus area is π/4 × (hole diameter² − rod outside diameter²). A typical rule-of-thumb target for uphole velocity with dry cuttings, worked out on free air delivery, is 15–25 m/s (about 3,000–5,000 ft/min); heavier, wetter or coarser cuttings need the upper end. In quarry blast-hole sizes the hammer’s consumption usually gives more than enough velocity, so the hammer governs; in larger water-well holes with slim rods cleaning can govern. Water standing above the hammer adds about 1 bar (14.5 psi) of back-pressure per 10 m of head; see wet-hole drilling.
Worked example (illustrative)
Situation: a quarry contractor runs an integrated rig rated 21 m³/min (741.6 cfm) FAD at 20 bar(g) (290.1 psig), assumed rated at 20 °C intake. He drills 115 mm holes with 76 mm rods and wants to move to 152 mm holes with 89 mm rods. All figures are illustrative.
- Pressure budget, 115 mm. The 4-inch-class hammer works at 18 bar(g) (261.1 psig). Losses through hose, rotary head and rods are taken as a typical 1.5 bar (21.8 psi). Required: 18 + 1.5 = 19.5 bar(g) (282.8 psig), leaving 0.5 bar (7.3 psi). With 10 m of water above the hammer, add 1.0 bar: 20.5 bar(g) (297.3 psig), beyond the rig.
- Uphole velocity, 115 mm. Annulus = π/4 × (0.115² − 0.076²) = 0.00585 m². The hammer uses 16 m³/min (565.0 cfm): 16 ÷ 60 ÷ 0.00585 = 45.6 m/s, far above an illustrative 20 m/s target. The hammer governs.
- Uphole velocity, 152 mm. Annulus = π/4 × (0.152² − 0.089²) = 0.01192 m². A 20 m/s target needs 20 × 0.01192 × 60 = 14.3 m³/min. The 5-inch-class hammer needs 20 m³/min (706.3 cfm) at 18 bar(g), giving 20 ÷ 60 ÷ 0.01192 = 28.0 m/s. The hammer governs again.
- Heat correction. At 45 °C intake mass per cubic metre is 293.15 ÷ 318.15 = 0.9214 of the 20 °C value. The rig’s 21 m³/min becomes 21 × 0.9214 = 19.35 m³/min (683.3 cfm) of 20 °C-equivalent air, 0.65 m³/min (23.0 cfm) short of the 152 mm hammer’s 20 m³/min. The hammer will run below 18 bar(g) and drill slower on hot afternoons. On a plateau site near Hyderabad (about 540 m) the air is also about 6 % less dense, giving 21 × 0.9214 × 0.938 = 18.15 m³/min (641.0 cfm) of sea-level, 20 °C-equivalent air if the rating was stated at sea level, so the shortfall grows.
- Engine budget. Taking a typical specific shaft power of 8.8 kW per m³/min at 20 bar(g) for a single-stage oil-injected airend (an illustrative figure; use the maker’s data), the compressor needs 21 × 8.8 = 184.8 kW. Illustrative hydraulic loads of cooling fan 16 kW, rotation 25 kW, feed 5 kW and dust collector fan 9 kW add 55 kW: total 239.8 kW. The 250 kW engine, with an assumed 6 % derating at 45 °C, gives 250 × 0.94 = 235.0 kW, 4.8 kW short. Expect lugging when collaring.
- Conclusion. Adequate for 115 mm dry holes, marginal for 115 mm wet holes, short for 152 mm on 45 °C days. Options: a lower-consumption hammer, a larger compressor, or a separate high-pressure compressor for the 152 mm work.
Dust control: dry collection or wet suppression
Dry collection seals the collar with a hood and uses a fan, usually hydraulic, to pull the return air into a collector. A cyclone or pre-separator throws out coarse chips; fine dust is caught on cartridge or bag filters cleaned by short pulses of compressed air from solenoid valves, the “pulse” in pulse-jet. The fan must draw more than the air leaving the hole or dust escapes round the hood; torn filters or a split hood seal let much of it out.
Wet suppression pumps water into the air line so a mist travels down the rods, through the hammer and up the annulus, damping dust at source. Flow is typically a few litres per minute (more with foam for broken ground), set to the least that stops visible dust. The concerns are the hammer and the hole. Some rigs carry both systems.
| Factor | Dry collection | Wet suppression |
|---|---|---|
| Hammer | Stays dry; normal lubrication | Water washes oil out; raise lubricant to the hammer maker’s advice; risk of corrosion when idle |
| Cuttings and hole | Dry chips, easy to log | Paste and mud rings in clayey ground; needs more uphole velocity; excess water can cave weak collars |
| Engine and air | Fan takes a few percent of engine power; small pulse air use | Small pump only; nearly all air reaches the hammer |
| Best fit in India | Hard-rock quarries in dry ground with dust limits to meet | Borewells already making water; sticky ground where foam helps |
Either way, protect the compressor intake. A collector exhausting near the intake or a split hood seal can cut filter life from weeks to days in summer. Point the exhaust downwind of the intake, check the restriction indicator every shift, and keep muddy spray away from the cooler fan. Our page on DTH lubrication and air treatment covers the hammer side.
Rock dust from granite and quartz-bearing ground contains respirable crystalline silica, which causes silicosis. Whichever method a rig uses, it should work every shift, operators near the collar should wear suitable respiratory protection, and the dust control arrangements should be agreed with the site’s safety officer under the rules that apply to the mine or quarry.
Service and reliability on site
- Intake filtration: two-stage filters with pre-cleaner and restriction indicator; never run with a punctured element. Dust that passes reaches the bearings and, on a single-screw, the gate rotor teeth.
- Oil temperature: log discharge temperature against ambient. A rise of a few degrees at the same ambient means fouled coolers, low oil or a sticking thermostatic valve; one stuck in the full-cooling position lets monsoon water condense.
- Separator: replace at the maker’s pressure-drop limit; watch for oil at the hammer.
- Coolers: blow out daily in dusty work, washing caked oily dust without bending fins. See portable compressor cooling.
- Diesel side: on an integrated rig every engine fault is an air fault; drain the fuel water separator daily and keep the engine air filter as clean as the compressor’s. See portable compressor fuel systems.
| Symptom | Likely on a twin-screw | Likely on a single-screw | First check |
|---|---|---|---|
| Delivered air falling over months | Clearance growth, worn inlet valve | Gate rotor tooth wear, the commonest cause | Delivery test at rated pressure and speed |
| New noise or vibration | Thrust bearing wear, rotor contact | Gate rotor bearing wear, chipped tooth | Stop; check oil for metal |
| High temperature trip | Fouled cooler, low oil, thermostatic valve | Same, with added risk to composite gate rotors | Cooler, oil level, valve |
| Milky oil | Condensation from cool running or humid intake | Same; water also degrades gate rotor material | Thermostatic valve, oil change |
| Oil in delivered air | Separator element, scavenge line, overfilling | Same | Separator pressure drop, scavenge orifice |
Buying or replacing: decisions by duty
| Duty | What matters most | Leaning |
|---|---|---|
| Quarry blast holes, 90–152 mm, long hours | FAD at 17–20 bar(g) on hot afternoons; cooling margin; local parts | Either type; prefer proven local parts supply |
| Borewells, 150–200 mm, deep or wet | 21–28 bar(g) capability for water head | Twin-screw, often two-stage or a dedicated portable unit |
| Integrated rig, remote owner-operator | Parts availability, simple service, fuel tolerance | Single-screw only if gate rotors can be stocked locally |
| Occasional larger holes | Extra flow for weeks | Hire a separate compressor |
Checking a used rig compressor
- Delivery test at rated pressure and full engine speed (more than 10 % short of rated FAD points to airend wear), discharge temperature after 30 minutes with ambient recorded, and an oil sample for water, metals and silica.
- Single-screw: gate rotor history, inspection through service covers where possible, and a confirmed supply source. Twin-screw: bearing noise and airend overhaul record.
- Separator vessel records, safety valve, and engine smoke and hours; see used and reconditioned borewell compressors.
DTH rig compressor duty and condition sheet
| Item | Record |
|---|---|
| Rig type (integrated or separate compressor) | |
| Compressor type (twin-screw or single-screw; stages) | |
| Rated FAD (m³/min or cfm) at rated pressure (bar g) | |
| Engine rated power (kW) and speed (rpm) | |
| Hole and rod diameters (mm); hammer air demand at pressure | |
| Annulus area (m²) and uphole velocity (m/s) | |
| Typical depth and water level (m) | |
| Dust control and water injection rate (litres per minute) | |
| Ambient temperature at test (°C) | |
| Loaded engine speed (rpm) and discharge pressure (bar g) | |
| Discharge temperature after 30 min (°C) | |
| Separator pressure drop (bar); intake filter indicator | |
| Oil appearance and hours since change | |
| Cooler condition (clean, dusty, caked) | |
| Hours since gate rotor or bearing work | |
| Checked by and date |
Exceptions and cautions
- Hammer charts use differing reference conditions; ask which the free air figure assumes before correcting for heat or altitude.
- Engine derating differs by engine and charge-air cooling; use the maker’s curve, not the 6 % in the example.
- The receiver-separator is a pressure vessel; examination and testing follow the applicable rules, its design code and the insurer. Confirm your obligations with a competent person.
- Do not mix oils in a single-screw airend without confirming compatibility with the gate rotor material.
Common mistakes
- Comparing one rig’s displacement with another rig’s FAD.
- Choosing on screw type without asking where gate rotors or bearings come from.
- Expecting a rig that drilled well in January to drill the same in May.
- Collaring at full feed and rotation with the compressor loaded, then blaming the compressor for lugging.
- Fitting a cooler-running thermostatic element to hide a dirty cooler.
- Injecting more water than needed and washing the oil out of the hammer.
- Upsizing the hole without checking the new hammer’s air against the rig’s compressor.
How Reddy Compressors helps
Reddy Compressors is an independent multi-brand compressed-air specialist, based in Secunderabad since 2002. We are suppliers, not dealers, and not an authorised dealer of any make. We have tie-ups with all major and reputed brands. On DTH work our part is the compressor and the air it delivers to the hammer.
- Sales of new, used and refurbished borewell and DTH compressors of all major makes, including Atlas Copco, Ingersoll Rand, ELGi, KAESER, Chicago Pneumatic and others, with high-pressure units up to 35 bar(g) (507.6 psig) for special DTH duty.
- Service, AMC, overhaul and spare parts for borewell and DTH compressors of all major makes, by more than 20 service engineers and technicians; see borewell compressor service and spare parts.
- Rental from our own fleet, 50–2,000 cfm (1.4–56.6 m³/min) at 2–30 bar(g) (29.0–435.1 psig), for larger holes, seasonal peaks or breakdown cover; see borewell compressor hire.
- Coverage across Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad, with supply across India.
What to send us
- Nameplate photographs of compressor and engine; hole and rod diameters, hammer size and typical depth.
- Working pressure, loaded engine speed and discharge temperature, with the ambient at the time.
- Dust control method, water injection rate, trips, running hours and last overhaul date.
Use the equipment selection enquiry for a new, used or hired compressor, or the service and maintenance enquiry for an existing machine. Call/WhatsApp 99590 60718 or 92468 88812.
Frequently asked questions
Is a single-screw or a twin-screw compressor better for a DTH rig?
Neither in every case. A twin-screw carries heavier bearing loads but is more common, with parts and rebuild skills widely available in India. A single-screw has a balanced main rotor but gate rotors that wear and must be replaced. Judge a rig by free air delivery at working pressure on a hot day, cooling margin and local parts.
Watch
Short explainers on this topic
Slide-by-slide engineering explainers from Reddy Compressors. More in the video library.
What are gate rotors and how often do they need replacing?
They are the two toothed star wheels that compress against the main rotor of a single-screw compressor. Their composite tooth faces wear at a rate set by oil condition, oil temperature and filtration. Replace them on the maker’s interval or when a delivery test shows air falling, and confirm a parts source in advance.
Why are DTH compressors oil-injected?
Compressing air to 15–25 bar in one stage without cooling would heat it to several hundred degrees. Injected oil absorbs the heat, seals clearances and lubricates bearings, holding discharge at roughly 80–105 °C.
How do I know if my rig’s compressor is big enough for a larger hole?
Take the new hammer’s air demand at its pressure, add losses and any water head, and check annular velocity for the new hole and rods. Compare with the rig’s free air delivery at that pressure, reduced for heat and altitude. If short, hire a separate compressor or choose a lower-consumption hammer.
Why does my integrated rig slow down on summer afternoons?
Hot air carries less mass, coolers lose temperature difference, the fan needs more power and the engine may derate, all on one engine. Clean coolers and filters, and avoid peak hydraulic demand while the compressor is loaded.
Is dry dust collection or water injection better for a DTH rig?
Dry collection keeps hammer and hole dry and suits hard-rock quarries. Water injection suppresses dust well but washes oil from the hammer, wets cuttings and can damage weak hole walls; it suits borewells already making water. Either way, keep dust away from the compressor intake.
Why does my compressor oil turn milky in the monsoon?
At 20 bar(g), moisture in humid monsoon intake air condenses at roughly 92–99 °C, so oil running at 90 °C collects water. Check the thermostatic valve, avoid long light-load running, and change the oil if it stays milky.
What should I check before buying a used rig compressor?
A delivery test at rated pressure and full speed, discharge temperature after 30 minutes, and an oil test for water, metals and silica. Ask for gate rotor history on a single-screw and the airend overhaul record on a twin-screw.
Related pages
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.