The decisions behind a DTH compressor choice
Selecting a DTH compressor is really a set of linked decisions. The hammer and hole set the flow and pressure. The rig and the way you move between sites set the mounting. Your workload sets whether you should buy new, buy used or hire. Getting the first decision wrong makes all the others expensive.
This page focuses on the choices a rig owner or drilling contractor faces once the hammer is known: which pressure class, which compressor type, what mounting, single or paired machines, booster or not, and new, used or hired. Our separate guide to DTH compressor sizing and duty confirmation covers the arithmetic of hammer consumption and pressure losses in detail.
Which pressure class do you need?
Pressure class is the most important choice because it limits how deep and how wet you can drill. Hammer working pressure, line losses and the water column above the hammer all add up at the compressor.
| Pressure class | Typical flow | Typical work | Watch for |
|---|---|---|---|
| 200–250 psig (13.8–17.2 bar) | 600–900 cfm | Shallow to medium borewells, small bits, dry hard rock | Runs short of pressure in wet holes |
| 250–300 psig (17.2–20.7 bar) | 800–1,200 cfm | The common agricultural and domestic borewell range | Large bits may need more flow |
| 300–400 psig (20.7–27.6 bar) | 1,000–1,500 cfm | Deep holes, heavy water, larger diameters | Higher fuel use and cooling load |
| Up to about 500 psig (34.5 bar) | Special units | Very deep or very wet holes | Hammer and hose ratings must match |
Choose the class for the hardest work you regularly take, not your average hole. If a deeper class is needed only occasionally, hiring for those jobs can be better than buying the larger machine.
Screw or piston for DTH work?
Almost all modern DTH and borewell compressors are oil-injected rotary screw machines, usually two-stage or high-pressure single-stage designs. They deliver steady flow without pulsation, run long hours at full load and are compact enough to mount on a truck with the rig.
Older reciprocating (piston) high-pressure compressors are still found on some rigs. They can reach high pressures and some owners know them well, but they need more frequent valve and ring attention, run hotter and pulsate. When a piston machine needs a major overhaul, it is worth comparing the cost against a used or new screw unit.
Variable pressure or fixed pressure?
Many high-pressure screw compressors let the operator set the working pressure across a range. At lower pressure they deliver more flow, which suits large-diameter shallow holes. At higher pressure they deliver less flow but push through deep water. For a contractor whose holes vary from shallow overburden work to deep wet borewells, this flexibility can mean one machine instead of two.
Check the machine’s published flow at both ends of its range. A compressor that is excellent at 21 bar may not give enough flow at 14 bar for a big bit, and the reverse.
Truck-mounted, trailer or skid?
| Mounting | Best suited to | Points to consider |
|---|---|---|
| Truck-mounted with the rig | Typical Indian borewell rig moving every day or two | Vehicle payload, weight distribution, compressor driven by its own engine or by the truck engine |
| Separate truck or trailer | Larger rigs, or where the compressor serves more than one rig over time | Second vehicle, longer hose run and extra pressure loss |
| Skid | Fixed or long-term sites, quarry or mining benches | Craning to site, shelter from sun and dust |
For truck-mounted packages, the compressor’s dimensions, weight and engine arrangement matter as much as its air performance. We check these against your truck before recommending a machine.
One large compressor, two smaller ones or a booster?
Where one compressor cannot meet both flow and pressure, there are three common answers:
- A single larger machine. Simplest to run and maintain, but heavier and more fuel at part load.
- Two compressors in parallel. Adds flow for large diameters. Both machines must be set to similar pressures and properly manifolded, with non-return valves.
- A primary compressor plus booster. Raises pressure for deep, wet stages when flow is already enough. It cannot add flow and needs its own maintenance.
Which answer is right depends on how often you need the extra capacity, your vehicle space and your service capability in the field.
New, used or hired?
| Option | When it usually makes sense | What to check |
|---|---|---|
| New | Steady year-round work in one pressure class | Correct class for your hardest regular stage, parts support |
| Used or refurbished | Good value where the duty is proven and budget is tight | Airend condition, engine hours, actual delivery test, controls and safety shutdowns |
| Hire | Seasonal peaks, trial in a new depth band, short contracts, breakdown cover | Pressure class, service interval, who maintains on site |
On our hire jobs we service the compressor every 300 running hours, including the oil change, so a long programme stays on schedule.
Features that matter on Indian drilling sites
- Cooling sized for 40–45 °C ambient, and above 45 °C on open sites, with coolers that can be cleaned easily of dust.
- Heavy-duty or two-stage intake filtration, because drilling sites are dusty.
- Reliable safety shutdowns for high discharge temperature, low engine oil pressure and high coolant temperature.
- Good access to filters, separator and oil drain for quick field service.
- Parts availability for the engine and airend through multi-brand channels, not just one source.
Engine, fuel and running hours
Most DTH compressors in India are diesel-driven, either by their own engine or, on some truck-mounted rigs, through a drive from the truck engine. A dedicated compressor engine keeps the truck engine free for travel and lets each be serviced separately; a shared drive saves weight and one engine but ties the two together. Fuel use depends mainly on load: a compressor working near its maximum pressure for long hours burns far more than one idling or unloaded between rods. Good unloading control and a compressor that is not grossly oversized for your usual holes both help keep litres per metre drilled down.
Think also about annual running hours. A rig drilling most days of the year accumulates hours quickly, which favours a machine with easy service access and parts you can obtain without delay in your area.
A simple decision sequence
- Confirm the hammer and bit sizes you use most, and the deepest, wettest holes you regularly take.
- Choose the pressure class that covers those holes, then the flow that cleans your largest bit.
- Decide between fixed and variable pressure according to how varied your work is.
- Check the mounting, weight and dimensions against your rig truck.
- Decide whether occasional extra duty is better met by hire, a booster or a second machine.
- Choose new, used or hire on workload and budget, with an inspection for any used unit.
Questions to ask any supplier
- What is the free air delivery at my working pressure, and to which test standard?
- What is the maximum working pressure, and is it adjustable?
- What ambient temperature is the cooling system rated for?
- Which safety shutdowns are fitted, and how are they tested?
- Where will filters, separators and engine parts come from when I need them?
- Who services the machine in my district, and at what intervals?
Clear answers to these questions tell you more about a machine than its brochure headline.
Single screw or twin screw on a DTH drilling rig
Both rotor geometries are used in high-pressure DTH compressors, and both can do the job well. A single-screw airend has one main rotor meshing with two star (gate) rotors set on opposite sides. A twin-screw airend has a male and a female rotor meshing in an oil film. The geometry matters less than the package built around it. Judge the free air delivery at your working pressure, the fuel used per m³, the cooling rating and how easily you can get parts.
| Point to compare | Single-screw airend | Twin-screw airend |
|---|---|---|
| Rotor forces | Gate rotors on opposite sides largely balance the radial and axial loads on the main rotor, so main bearing loads are low | Gas forces load the bearings radially and axially, so the design relies on sized thrust and radial bearings |
| Planned wear parts | Star (gate) rotors are replaceable wear parts, and changing them restores sealing without replacing the main rotor | The rotors themselves are not routine wear parts. Worn clearances usually mean a bearing rebuild or an airend exchange |
| High-pressure arrangement | Typically single-stage | Single-stage or two-stage, depending on pressure class and maker |
| Efficiency at 20–35 bar | Depends on the design. Compare published FAD and fuel data at the same pressure | Depends on the design. Compare published FAD and fuel data at the same pressure |
| Parts and service support | Check local availability of star (gate) rotors, bearings and technicians who know the type before you buy | Check local availability of airend parts, bearings and technicians who know the type before you buy |
| What kills it early | Dust ingress, wrong or degraded oil, overheating | Dust ingress, wrong or degraded oil, overheating |
The bottom row usually decides how long a machine lasts on a borewell site. For a longer discussion, see single screw vs twin screw DTH compressor.
Uphole velocity in metric, and what water does to it
The annulus table in the sizing section further down this page uses inches and ft/min. The table below gives the same Q = A × V calculation in millimetres for common bit and rod combinations. It uses two targets: about 15 m/s (roughly 3,000 ft/min), the usual minimum for dry dust, and about 25 m/s for heavier or damp cuttings. Flow is free air at the surface. Above roughly 35 m/s, erosion of the hammer body, bit skirt and hole wall increases.
| Hole (bit) diameter | Rod OD | Annular area | Air for 15 m/s | Air for 25 m/s |
|---|---|---|---|---|
| 140 mm | 89 mm | 91.7 cm² | 8.26 m³/min (292 cfm) | 13.76 m³/min (486 cfm) |
| 165 mm (6.5 in) | 114.3 mm | 111.2 cm² | 10.01 m³/min (353 cfm) | 16.68 m³/min (589 cfm) |
| 165 mm (6.5 in) | 89 mm | 151.6 cm² | 13.65 m³/min (482 cfm) | 22.74 m³/min (803 cfm) |
| 203 mm (8 in) | 114.3 mm | 221.0 cm² | 19.89 m³/min (703 cfm) | 33.16 m³/min (1,171 cfm) |
| 254 mm (10 in) | 140 mm | 352.8 cm² | 31.75 m³/min (1,121 cfm) | 52.92 m³/min (1,869 cfm) |
First, a larger rod diameter narrows the annulus and raises velocity for the same air. On a 165 mm hole, moving from 89 mm to 114.3 mm rods cuts the air needed for 25 m/s from 803 to 589 cfm. Second, the velocity in the table is a surface figure. At the bit, the air is compressed by whatever is above it.
Worked example (illustrative): why wet holes need more air than the dry calculation shows
- Hole 165 mm, rods 114.3 mm, annular area 111.2 cm² = 0.01112 m².
- Compressor delivering 900 cfm = 900 ÷ 35.315 = 25.49 m³/min = 0.425 m³/s of free air.
- Velocity at the surface = 0.425 ÷ 0.01112 = about 38 m/s, which is well above the dry minimum and already in the range where wear rises if the hole stays dry.
- Now suppose 50 m of water still stands above the bit. Pressure at the bit = 1.01325 + (50 × 0.0981) = 5.92 bar(a).
- The same mass of air occupies 1.01325 ÷ 5.92 of its free volume, so the velocity at the bit falls to about 38 × 0.171 = 6.5 m/s.
That is far too slow to lift chips. Cuttings start to move only once the column is aerated and partly blown out, which lowers the pressure at the bit. This is why borewell rigs carry much more air than a dry-velocity calculation suggests, and why reserve pressure matters as much as cfm in water-bearing rock.
Pressure needed versus depth and water head
Fresh water exerts 0.0981 bar per metre of height. Water loaded with cuttings is denser. A slurry factor of about 1.15 is a reasonable working allowance. The figure that matters is the height of water above the hammer, not the depth of the hole.
| Water above the hammer | Head, clear water | Head, with cuttings (×1.15) |
|---|---|---|
| 30 m | 2.94 bar | 3.38 bar |
| 60 m | 5.89 bar | 6.77 bar |
| 90 m | 8.83 bar | 10.15 bar |
| 120 m | 11.77 bar | 13.54 bar |
| 150 m | 14.72 bar | 16.92 bar |
Worked example: restarting at depth after a rod change (illustrative)
Take a 180 m hole with the static water level 30 m below ground. While the air is off, the hole refills, so 150 m of water can stand above the hammer. With cuttings, that column needs about 16.9 bar just to break it. A 250 psig machine is 250 ÷ 14.5038 = 17.24 bar(g). Once hose and string losses of 1.5–2 bar are subtracted, it cannot clear the whole column in one go.
The usual remedy is staged unloading. When tripping back in, stop every 60–90 m and blow the hole clear before adding more rods. Each stage then only has to lift the water above that depth. A hammer fitted with a working check (foot) valve also keeps much of the water out of the string during rod changes. If staged unloading becomes routine on your sites, that is the signal to move up a pressure class or to add a booster.
Feed force, rotation and air: reading the rig as one system
A DTH hammer breaks rock by impact. Feed force only keeps the bit in firm contact with the rock, and rotation only indexes the buttons onto fresh rock between blows. Neither adds penetration beyond that point. Air pressure is what sets the blow energy. Most hammers also change to flushing mode when the bit drops in the chuck. This is a useful cue for the operator: when the bit lifts off the bottom, the gauge pressure falls and the hammer stops cycling.
Use the hammer maker’s chart for feed force and rotation speed. Typical DTH rotation is 15–40 rpm, slower for larger bits and abrasive rock. As depth increases, the weight of the string soon exceeds the feed the hammer needs. Rods weighing an illustrative 22 kg/m give 150 m of string a mass of 3,300 kg, about 32 kN on the bit. Beyond a certain depth, the rig therefore has to hold back rather than push down.
| Symptom | Likely cause | What to check or change |
|---|---|---|
| Pressure drops, hammer stops cycling, little progress | Feed too low or holdback too high, so the bit hangs in the chuck in flushing mode | Reduce holdback until the pressure rises and the hammer cycles again |
| High rotation torque, stalling, heavy button wear | Too much feed, often string weight at depth | Increase holdback. Check the rotation pressure trend against depth |
| Fine flour cuttings, slow progress | Rotation too slow or poor cleaning, so chips are being re-crushed | Raise rpm within the chart. Check uphole velocity and water inflow |
| Fast gauge-button wear, undersize hole | Rotation too fast for the rock | Lower rpm. Regrind or replace the bit before the gauge is lost |
| Penetration falls as depth increases | Pressure at the hammer falling because of water head and string losses | Compare the compressor gauge with the pressure budget. Consider staged unloading or more pressure |
Dust control, site derating and fuel per metre
Dry DTH drilling throws out fine silica-bearing dust, so dust control is a health issue as well as a site requirement. The three common approaches affect the compressor differently.
| Method | How it works | Effect on air demand | Watch for |
|---|---|---|---|
| Dry dust collector (cyclone plus pulse-jet bag filter) | A hood at the collar draws dust through a pre-separator and filter bags. Short pulses of compressed air knock the bags clean | Small. Pulse cleaning uses little air, and drilling air is unchanged | Bags blind as soon as the hole makes water. Use a drain on the pulse-air receiver. Keep the hood seal good |
| Wet or mist injection | A metered water flow is injected into the drilling air, so dust leaves as damp chips | Moderate. The air must also lift the water, which raises the pressure needed and favours higher uphole velocity | Too much water forms mud rings and collars. Hammer oil gets washed out, so raise the lubricator setting |
| Combined, often sold as “pulse wet” dust removal | A light mist damps the dust, and a pulse-jet collector catches what escapes | Slightly higher than wet alone | Keep the mist light, or the bags clog with paste |
Foam injection is a different tool. Stiff foam lifts cuttings at far lower annular velocities than air alone, so it helps in large-diameter or unstable holes where the available cfm cannot reach the velocities in the table above.
Derating for 45 °C and site altitude (worked)
- Atmospheric pressure at 500 m, roughly the height of the Hyderabad plateau, is about 0.955 bar(a), against 1.01325 bar(a) at sea level.
- Density relative to standard air at 20 °C = (0.955 ÷ 1.01325) × (293.15 ÷ 318.15) = 0.868 at 45 °C. The same cfm therefore carries about 13% less mass of air.
- The lifting force on chips scales with density × velocity². Restoring it needs 1 ÷ √0.868 = 1.073 times the velocity, or about 7% more cfm.
- For a 24 bar(g) discharge, the pressure ratio rises from (24 + 1.013) ÷ 1.013 = 24.7 at sea level to (24 + 0.955) ÷ 0.955 = 26.1 at 500 m. That is about 6% higher, which means more heat to reject on the hottest afternoons.
These steps show the size of the effect on hole cleaning. For the compressor’s actual delivery and the engine’s power at your altitude and ambient, use the maker’s derating curves.
Fuel per metre drilled (illustrative)
Assume an average shaft load of 200 kW at 225 g/kWh and diesel at 0.835 kg/litre. That gives 200 × 225 ÷ 835 = 53.9 litres/hour. At 20 m/h overall progress, including rod changes, that is 2.7 litres per metre. If short air or poor cleaning drops progress to 12 m/h, the same machine burns 4.5 litres per metre. A correctly sized compressor usually saves more fuel through faster drilling than a smaller one saves by burning less fuel per hour.
How Reddy Compressors helps
We supply all types, all brands and all models of borewell and DTH compressors: new, used and refurbished, rentals, service and spare parts. Brands we sell, rent and service include ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Sullair, Doosan, Bobcat and many more. We are an independent multi-brand specialist, not an authorised dealer, so we will recommend the class and type that suits your work rather than one maker’s range. We have tie-ups with all major and reputed brands.
We can compare candidate machines for you, inspect and test a used compressor before purchase, or hire you the right class from our own fleet while you decide. Our office is at Tarnaka, Secunderabad; service teams cover all of Telangana and Andhra Pradesh, and we supply across India.
What to send us
- Rig and truck details, including payload
- Hammer make and model and bit sizes
- Range of depths and water conditions you usually drill
- Annual drilling hours or number of holes, if known
- Whether you prefer new, used or hire
- Photos and nameplate of any existing compressor
Frequently asked questions
Which compressor is best for borewell drilling?
There is no single best brand or model. The best compressor is the one whose pressure class and free air delivery cover your deepest, wettest regular holes, fits your rig truck, and can be serviced where you work. For most Indian borewell rigs that is a high-pressure screw compressor in the 600–1,500 cfm, 200–400 psig range.
Is a 1,000 cfm compressor enough for a borewell rig?
It depends on the pressure it delivers that flow at and on your hammer and bit. A 1,000 cfm machine at 300 psig suits many medium to deep borewells. The same flow at a lower pressure may struggle in deep wet holes. Check the flow at your working pressure, not just the headline figure.
Are piston compressors still used for DTH drilling?
Some older rigs still run reciprocating high-pressure compressors, and they can work well when maintained. Most new DTH packages use rotary screw compressors because they give steady flow, run long hours at full load and need less frequent valve and ring work. We service both types.
Can I run two compressors together on one rig?
Yes, two compressors can be connected in parallel to add flow, provided they run at similar pressures and are manifolded with non-return valves so one does not back-feed the other. It suits large-diameter work. For extra pressure rather than flow, a booster is the usual answer.
Is a used borewell compressor a good idea?
A used or refurbished compressor can be good value if it is inspected properly. Check the airend for noise and wear, engine hours and condition, actual delivery under load, controls and safety shutdowns. We can inspect and test a used machine before you buy, or offer one we have checked ourselves.
Can I hire a DTH compressor instead of buying?
Yes. Hire suits seasonal peaks, short contracts, trial work in a new depth band and breakdown cover. We hire from our own fleet with service every 300 running hours. Tell us the pressure class, flow and location and we will confirm what can be offered.
Single screw or twin screw air compressor for a DTH drilling rig: which is better?
Neither is better in itself. Single-screw airends balance their rotor loads well and have replaceable star rotors. Twin-screw airends handle bearing loads with sized thrust and radial bearings, and are built in single-stage and two-stage forms. Compare the actual packages instead. Look at free air delivery at your working pressure, maximum pressure, fuel use at that duty, cooling rated for 45 °C, and check the local availability of airend parts and skilled service for the type you choose. A well-filtered, correctly oiled machine of either type will outlast a neglected one of the other.
What pressure does a high pressure air compressor for a DTH drilling rig need?
Add up the hammer’s working pressure, the losses through the hose, string and fittings, and the water head above the hammer. Water head is 0.0981 bar per metre of clear water, or about 0.113 bar per metre with cuttings. For example, a 20 bar hammer with 2.5 bar of losses and 60 m of water above it needs about 29 bar. That is more than a 25 bar machine can supply without staged unloading. Shallow dry holes need far less pressure.
What is pulse wet dust removal on a DTH drilling rig?
It combines two dust controls. A light water mist is injected into the drilling air to damp the dust at the bit. A dry collector at the collar then catches the remaining fines in filter bags, which short pulses of compressed air shake clean. Air demand rises slightly, because the air must also lift the water. Keep the mist light. Too much water turns the dust into paste that blinds the bags and builds mud rings in the hole.
How much feed force does a DTH drill need?
Only enough to keep the bit firmly in contact with the rock. Take the exact range from the hammer maker’s chart for your bit size. Extra feed does not make the hammer strike harder. Air pressure sets the blow energy. Too little feed lets the bit drop into flushing mode, so the pressure falls and the hammer stops. Too much raises torque and button wear. On deep holes, the string weight alone often exceeds the needed feed, so the rig must hold back.
Related pages
Field guide
Sizing a drilling compressor: Three questions, one pressure budget
A drilling compressor does two jobs at once. It powers the DTH hammer, and its exhaust air then lifts the cuttings up the hole. Pressure helps drive the hammer; airflow cleans the hole. More pressure cannot make up for too little flow, and a large flow at too low a pressure will not run the hammer at depth. “This job needs a 750 cfm compressor” is incomplete: 750 cfm at what pressure, for which hammer, in which bore, at what depth and altitude?
Work from the hole to the machine
The three questions
- Is there enough air for the hammer? Take it from the hammer maker’s chart, at the working pressure you will really have.
- Is there enough air to clean the hole? This depends on the space between pipe and hole wall (below).
- Does enough pressure reach the hammer? Hose, drill string, fittings, water head and back-pressure all take some.
The compressor must satisfy all three together. Required flow is the larger of the hammer demand and the cleaning demand, plus margin. They are not simply added, because the air that drives the hammer then exits at the bit and does the flushing.
Hole cleaning: The annulus
Air leaving the bit rises through the annulus between the drill pipe and the hole wall. Flow equals area times velocity:
Q = A × V, with A = π/4 × (Dh² − Dp²) (hole diameter Dh, pipe outside diameter Dp).
In inches and feet per minute this becomes cfm ≈ 0.005454 × (Dh² − Dp²) × V. Area grows with the square of the diameter, so a modestly bigger hole needs a lot more air. For 3.5 in pipe at an illustrative 4,000 ft/min:
| Hole diameter | Annular area | Air for 4,000 ft/min |
|---|---|---|
| 5 in | 0.070 ft² | about 278 cfm |
| 6 in | 0.130 ft² | about 518 cfm |
| 6.5 in | 0.164 ft² | about 654 cfm |
| 8 in | 0.282 ft² | about 1,129 cfm |
| 10 in | 0.479 ft² | about 1,914 cfm |
| 12 in | 0.719 ft² | about 2,874 cfm |
Going from a 6 in to an 8 in hole, only two inches, more than doubles the air for the same velocity. Large-diameter drilling is often an airflow problem before it is a pressure problem. The velocity itself is not fixed: it depends on the cuttings’ size, density and wetness, the formation and the maker’s guidance. The table is geometry, not a recommendation. Try your own numbers in the annular velocity tool.
Which one governs?
- Hammer needs 450 cfm, cleaning needs about 400: the hammer governs.
- 8 in hole, 3.5 in pipe, hammer 800 cfm, cleaning 1,100 or more: cleaning governs, and the hammer’s chart alone would undersize the machine.
The pressure budget
The compressor pressure must cover the hammer pressure plus every loss on the way down:
Pcompressor ≥ Phammer + hose + drill string + fittings + water head + back-pressure + margin
| Item (illustrative deep hole) | bar |
|---|---|
| Hammer needs at the tool | 20 |
| Hose and surface system | 0.7 |
| Drill string | 1.3 |
| Fittings and rotary head | 0.5 |
| Borehole back-pressure allowance | 0.5 |
| Effective water column | 3 |
| Compressor must reach at least | 26, plus margin |
A 20 bar hammer does not mean a 20 bar compressor is enough. A deep wet hole with a 22 bar hammer, 2 bar of losses, 4 bar of water and 1.5 bar of margin needs about 29.5 bar. A machine that makes the right cfm only at 25 bar is the wrong machine, however well the cfm matches. Always read the maker’s flow-versus-pressure data, not the headline pair of numbers. Where the pressure budget exceeds what a portable compressor can supply, a booster is an engineering option (see groundwater effects), and it is a specialist high-pressure system, not an improvised pressure multiplier.
Site corrections and margin
Thin air at altitude and heat both cut engine power, intake density and cooling. Atmospheric pressure is about 101 kPa at sea level, 90 kPa at 1,000 m, 79.5 kPa at 2,000 m and 70 kPa at 3,000 m. Do not apply a rule of thumb of so many percent per 1,000 m; use the engine and compressor makers’ altitude curves, and compare cfm ratings only on the same reference conditions. Add enough margin for filter loading, hose loss, hammer wear, surprise water and compressor age, but not so much that you pay for an oversized machine and burn fuel on it. More pressure than the hole needs costs fuel without making metres.
Before you quote or hire
- Bore and bit diameter, pipe outside diameter, depth, formation and expected water.
- Hammer model and its air and pressure chart.
- Hose size and length, rig details, altitude and ambient temperature.
- Future bores and depths, so the machine is not outgrown on the next job.
Send these through the selection enquiry.
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Annular area and return velocity
In drilling, the air must lift the cuttings up the annulus. Check the return velocity from the hole and rod size.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Hole diameter (mm): 115
- Rod outside diameter (mm): 89
- Free air supplied (m³/min): 12
- Bottom-hole back pressure (bar g): 1
- Target return velocity (m/s): 15
Result
- Annular area: 41.7 cm²
- Return velocity: 24.16 m/s = 4,757 ft/min
- Free air needed for the target velocity: 7.45 m³/min = 263 cfm at 15 m/s
- Assessment: Within the commonly quoted range
Method. Annular area = π/4 × (D_hole² − D_rod²). Actual flow at the back pressure = free air × (P_atm / (P_back + P_atm)). Velocity = actual flow ÷ area. A return velocity often quoted for efficient cuttings lift is around 15–25 m/s; confirm for your material.
What to take from it. Low return velocity leaves cuttings in the hole; very high velocity wears the rods and hammer. Both are warnings, not verdicts.
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.
DTH hammer duty against two compressors
Check whether each of two compressors can supply a DTH hammer’s documented air consumption at the pressure the hammer needs, after hose, string and water-column losses, and with what margin.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Hammer working pressure at the hammer (bar g): 21
- Hammer air consumption at that pressure (cfm): 950
- Pressure loss from compressor to hammer (bar): 1
- Water column above the hammer to be lifted (m): 0
- Compressor A: maximum working pressure (bar g): 17
- Compressor A: data-sheet flow at the required pressure (cfm): 1150
- Compressor B: maximum working pressure (bar g): 24
- Compressor B: data-sheet flow at the required pressure (cfm): 1000
- Site derating of delivered flow: heat, altitude, condition (%): 0
Result
- Hammer working pressure: 21 bar g = 305 psig
- Water column allowance: 0 bar
- Required compressor discharge pressure: 22 bar g = 319 psig
- Hammer consumption: 950 cfm = 26.9 m³/min
- Compressor A: Fails: cannot reach 22 bar g (maximum 17 bar g); its flow does not count
- Compressor B: flow available at 22 bar g: 1,000 cfm = 28.3 m³/min
- Compressor B: margin: +5.3 % (covers the duty)
- Result: Only compressor B covers the duty
Method. Required discharge pressure = hammer working pressure + pressure loss from compressor to hammer (hose, swivel, drill string, check valves) + water column to be lifted (0.0981 bar per metre of water). A compressor whose maximum working pressure is below the required pressure fails, whatever its flow. Available flow = data-sheet flow at the required pressure × (1 − site derating). Margin (%) = (available flow − hammer consumption) ÷ hammer consumption × 100. Use the same flow basis (FAD or scfm) for hammer and compressor. 1 bar = 14.504 psi; 1 m³/min = 35.31 cfm. Hole cleaning (annular velocity) is a separate check.
What to take from it. Compare flows at the same pressure. More cfm at a lower pressure does not run a hammer that needs higher pressure.