DTH compressor sizing and duty confirmation

We supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, rentals, service and spare parts. This page sets out the step-by-step method we use when a rig owner or drilling contractor asks us to confirm whether a compressor really suits a hammer, a bit size and a target depth. Use it before you buy, hire or rebuild a machine.

Why both pressure and flow must be confirmed together

A DTH (down-the-hole) hammer is driven by compressed air. The air first works the piston that strikes the bit, then exhausts through the bit face and carries cuttings up the annulus between the drill pipe and the hole wall. The same air therefore does two jobs: it delivers impact energy and it cleans the hole.

Pressure decides how hard the piston hits. Flow decides how often it hits and whether the cuttings leave the hole. A compressor that makes high pressure but too little air will drill slowly and leave the bottom dirty, so the bit regrinds chips. A machine with plenty of air but too little pressure will stall once water and depth add backpressure. Every compressor is rated at one operating point: a quoted flow is valid only at its stated pressure. Never combine the maximum flow of one rating with the maximum pressure of another.

Our working bands for borewell and DTH machines are:

These bands are a starting point. The steps below show how to move from the band to a confirmed duty.

Step-by-step sizing method

1. Fix the hole. Record bit diameter, casing size and depth, expected total depth, and the formations you expect (overburden, weathered rock, granite or basalt, fractured zones).
2. Fix the drill string. Record drill-pipe outside diameter, pipe length and the hammer make and model. The pipe size controls the annulus area and therefore the air needed to clean the hole.
3. Take the hammer’s air demand. Use the hammer manufacturer’s consumption chart at your intended operating pressure. This is the first flow requirement.
4. Check hole-cleaning flow. Calculate the flow needed for adequate uphole velocity in the annulus (method below). This is the second flow requirement.
5. Take the larger figure, then review the air path. Hammer exhaust and cleaning air are normally the same air, so do not simply add the two. If cleaning needs more than the hammer passes, confirm with the hammer supplier whether a flushing or bypass arrangement is approved.
6. Build the pressure budget. Add hammer working pressure, water head above the hammer, line and string losses and a margin for depth (method below).
7. Correct for site conditions. Apply altitude and ambient-temperature effects to the rated free-air delivery (FAD).
8. Add a named margin. Allow for wear, deeper-than-planned holes and water. State what the margin covers rather than adding unexplained percentages at every step.
9. Match to a real machine. Check the compressor’s documented FAD at the required pressure, plus whether a booster is needed for the deepest part of the hole.
10. Write the duty statement. One paragraph: hammer, bit, pipe, depth, pressure and flow, site conditions, chosen machine and what changes would require a new review.

How much air is needed to clean the hole?

Cuttings leave the hole only if the air moving up the annulus is fast enough to carry them. The common field formula in imperial units is:

Q (cfm) = V × (D² − d²) ÷ 183.35, where V is uphole velocity in feet per minute, D is the bit (hole) diameter in inches and d is the drill-pipe outside diameter in inches.

For dry, competent rock, drillers typically aim for roughly 3,000–5,000 ft/min (about 15–25 m/s). Heavier, wetter or sticky cuttings, and holes making water, need the upper end or more. Too little velocity causes regrinding and a slow, stuck bit; much too much velocity erodes the hole wall and wastes fuel.

These are worked arithmetic examples, not settings for any particular site. They show why a larger bit on the same pipe climbs steeply in air demand, and why a pipe that is too thin for the bit can push the requirement beyond a single compressor. Our annular velocity guide explains the calculation in more detail.

Building the pressure budget

The pressure the compressor must hold at its outlet is the sum of what the hammer needs and everything that resists the air on the way. A practical budget has four parts:

• Hammer working pressure: from the hammer chart; this sets impact energy.
• Water head: each 10 m of standing water above the hammer adds about 1 bar (14.5 psi) of backpressure. In a hole with 60 m of water above the hammer, that is about 6 bar the compressor must overcome before the hammer does any work.
• Line and string losses: hose, swivel, pipe joints and check valve. Long or undersized hoses waste pressure every hour of drilling.
• Return-flow resistance: the column of air, water and cuttings in the annulus. Static head alone underestimates it when the hole is making water or the cuttings are heavy.

Worked example: a hammer rated for good performance around 17 bar (246.6 psig), 50 m of water above the hammer (about 5 bar), and 1 bar of line and return losses give an outlet requirement of roughly 23 bar (333.6 psig). That sits in the 300–400 psig band, not the 250–300 psig band many buyers would choose from the hammer chart alone. If the hole will go deeper in the same water conditions, consider a booster for drilling air or a higher-pressure unit.

Correcting for altitude, heat and wear

Compressor flow is rated as free-air delivery at standard inlet conditions. On a real site three things reduce what reaches the hammer:

• Altitude: thinner air means less mass per cubic metre. Most Deccan Plateau sites in Telangana and Andhra Pradesh are a few hundred metres above sea level, so the effect is modest, but it should be checked for hill sites.
• Ambient heat and dust: summer ambient of 40–45 °C is common, and above 45 °C on open sites. Hot inlet air, a blocked cooler or a choked air filter cut delivery and push discharge temperature towards the shutdown point. A machine that works in January can trip in May.
• Wear: an older airend, a weak engine or a sticking inlet valve can deliver well below its plate rating. For a used machine, confirm actual delivery on test before relying on the nameplate.

Size against the condition the machine will actually face in the hottest month, not the brochure condition.

Comparing options in litres, metres and hours

The right compressor is the one that drills the hole at the lowest fuel and running hours per metre, not the one with the smallest engine. Compare options in physical units only:

Illustration only: if machine A burns 40 litres per hour and drills 10 m per hour, it uses 4 litres per metre. If machine B burns 50 litres per hour but drills 15 m per hour, it uses about 3.3 litres per metre and finishes a 200 m hole in about 13 hours instead of 20. Record your own figures from the rig; they vary with formation, hammer and operator.

Common sizing mistakes we see

• Choosing by cfm alone, without the pressure at which that cfm is delivered.
• Using the hammer chart but ignoring water head, so the hammer stalls once the hole makes water.
• Putting a larger bit on the same machine and pipe, then blaming the hammer for slow drilling.
• Adding hammer air and cleaning air as if they were separate consumers, then oversizing.
• Relying on a used machine’s nameplate without a delivery test.
• Running hot, dusty summer jobs with blocked coolers and filters, then sizing up instead of servicing.

Symptoms such as low pressure on depth, slow penetration or high discharge temperature are covered in our borewell compressor performance diagnostics page.

Writing the duty confirmation

Before any machine is bought, hired or rebuilt, write down one short duty statement. It should name the hammer, bit and pipe sizes, the target depth and expected water level, the required outlet pressure in psig and bar, the required flow in cfm, site altitude and expected maximum ambient temperature, and the chosen compressor with its documented rating at that pressure. Add a line stating what would invalidate the selection: a bigger bit, a different hammer, a deeper hole or much more water. That statement lets any supplier quote against the same duty and lets your crew know the machine’s limits.

How Reddy Compressors helps

We supply all types, all brands and all models of borewell compressors: new sales, used and refurbished machines, 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, not as an authorised dealer of any one brand. That lets us recommend the machine that fits your duty rather than one product line.

• Check your hammer, bit, pipe and depth against the bands above and propose a pressure and flow.
• Supply new borewell compressors, used and refurbished machines and high-pressure units from our own rental fleet.
• Service, overhaul and repair with a team of more than 20 service engineers and technicians covering all of Telangana and Andhra Pradesh, with machines, rentals and spares supplied across India.
• Identify spare parts from the nameplate, serial number and old part, and confirm fit before supply.

Our office is at Srinilayam, 12-13-200, First Floor, Street No. 1, Tarnaka, Secunderabad 500017. Call +91 99590 60718 or +91 92468 88812.

What to send us

• Bit diameter, drill-pipe size and hammer make and model
• Target depth, casing depth and expected water level
• Formations expected and any past holes in the area
• Current compressor make, model and nameplate photo, if you have one
• Site location, and whether you want to buy new, buy used, hire or repair

Frequently asked questions

How do I calculate the cfm needed for a DTH hammer?

Take the hammer manufacturer’s air consumption at your working pressure, then check hole-cleaning flow using Q = V × (D² − d²) ÷ 183.35, with bit and pipe sizes in inches and uphole velocity typically 3,000–5,000 ft/min. Use the larger of the two after confirming they are the same air, then correct for site heat and altitude and add a stated margin for wear and depth.

What pressure compressor do I need for a 300 m borewell?

It depends mostly on how much water stands above the hammer and on the hammer itself. Each 10 m of water adds about 1 bar of backpressure. Many deep hard-rock holes need the 300–400 psig (20.7–27.6 bar) band, and some need special units up to about 35 bar. Build a pressure budget from the hammer chart, water head and losses rather than guessing from depth alone.

Why does my borewell compressor lose pressure when the hole hits water?

Water above the hammer adds backpressure, roughly 1 bar for every 10 m, and heavier wet cuttings make the return column harder to lift. If the compressor has no pressure reserve, the hammer slows or stalls. The fix may be higher pressure, a booster, better hole-cleaning flow or simply servicing a worn machine that is no longer making its rated delivery.

Is a bigger compressor always better for DTH drilling?

No. An oversized machine burns more fuel while unloaded, can erode the hole wall with excessive uphole velocity and costs more to move and maintain. An undersized machine drills slowly and wears bits. The best choice delivers the confirmed pressure and flow with a reasonable margin, giving the lowest litres of diesel and running hours per metre drilled.

Can I use a 200 psi compressor with a 350 psi hammer?

It will usually run, but the hammer will strike with much less energy, penetration will be slow and the machine will struggle once water or depth adds backpressure. Check the hammer chart at the pressure you can actually supply. For regular deep work, match the compressor to the hammer’s working band or add a booster approved for the duty.

Do you supply borewell compressors outside Telangana and Andhra Pradesh?

Yes. Our service teams cover the whole of Telangana and Andhra Pradesh, and we supply new and used borewell compressors, rental machines and spare parts across India. For project work elsewhere we can arrange site support where feasible. Send the drilling duty and location and we will confirm what we can provide.

Related pages

• Borewell and DTH compressor library
• DTH compressor selection guide
• New borewell compressor sales
• Borewell compressor rentals and hire
• Used and reconditioned borewell compressors
• Borewell compressor spare parts