How much air does a DTH hammer need?

What a DTH hammer actually does with the air

A down-the-hole hammer is a pneumatic piston that sits directly behind the bit at the bottom of the hole. Compressed air travels down the drill pipe, enters the hammer through a check valve, and is switched by the hammer’s internal porting so that it drives the piston onto the bit shank and then returns it for the next blow. The exhaust air leaves through the bit face and flushes the cuttings up the annulus between the drill pipe and the borehole wall.

That gives the air two jobs at the same time. It is the energy source for every blow, and it is the transport medium that keeps the hole clean. A compressor that satisfies one job and not the other produces a familiar set of field complaints: a hammer that “beats” weakly, slow penetration, regrinding of cuttings at the bit, stuck strings in broken ground, or a hole that simply stops making progress below the water table.

Because the hammer behaves broadly like a fixed set of ports and chambers, it does not take a fixed volume of air. At higher supply pressure the air is denser, more of it passes through the hammer in each cycle, and the hammer both hits harder and consumes more free air. That is why a hammer air requirement is meaningless unless the pressure is quoted with it.

Why the pressure and the flow must always travel together

Hammer makers publish air consumption as a table or a curve: consumption in cfm or litres per second against supply pressure. Reading across that curve shows the basic relationship every rig owner sees on site. Raise the pressure and the hammer drills faster, but it also asks for more air. Run a large hammer at a pressure the compressor can reach only with a reduced flow, and the hammer starves.

Three practical consequences follow:

• A figure such as “900 cfm” is incomplete. You need “900 cfm at 20 bar at the hammer”, from the document for that specific hammer model and configuration.
• A compressor quoted as “1,100 cfm” and “25 bar” may not deliver both at once. Many portable drilling compressors have a rated flow at a rated pressure, and some offer selectable pressure–flow settings. Ask for the delivered flow at the pressure you will actually run.
• Some hammers have interchangeable or selectable air-cycle parts that change consumption. Record which arrangement is fitted before you use any chart.

Where a published row is marked as estimated, treat it as an estimate in your own calculations. Where the paperwork does not say whether pressure is gauge or absolute, ask the tooling supplier rather than guessing. One bar of difference changes the answer.

Typical hammer size, bit diameter and air bands

The table below shows typical borewell practice in India. It is a planning guide only; the hammer maker’s chart for the exact model governs the final number.

The band rises with depth for two reasons. Deeper holes usually need higher pressure to overcome the water column above the hammer, and larger bits need more air simply to keep the wider annulus clean.

Pressure at the hammer is not pressure at the compressor

The gauge on the compressor panel reads discharge pressure. The hammer sees something lower, and the difference grows with depth and with every restriction on the way. When you translate a hammer requirement into a compressor requirement, work down the air path:

1. Start with the hammer chart. Note the supply pressure you want at the hammer and the consumption at that pressure.
2. Add water back-pressure. Below the standing water level, the hammer exhausts against the column of water and cuttings in the annulus. A full column of clean water exerts roughly 1 bar for every 10 m of submergence (about 1.45 psi per metre). Once air is lifting the water the aerated column exerts less, but restarting after a rod change can see close to the full head. The hammer needs its working pressure difference on top of that.
3. Add path losses. Discharge hose, rig swivel and rotary head, drill pipe bore, check valve and any lubricator all take pressure, especially at high flow through small bores.
4. Add a control margin. The compressor’s regulation band means discharge pressure moves; the minimum in that band must still meet the requirement.
5. Correct the flow for site conditions. Free-air delivery is quoted at stated inlet conditions. In an Indian summer, with intake air at 40–45 °C, and above 45 °C on open sites, the mass of air delivered is lower than on a cool morning, and dusty intakes reduce it further if filters are neglected.

This is why a rig that drills well at 60 m may stall at 150 m on the same compressor: the pressure the compressor can make is fixed, but the share left for the hammer shrinks as the water column grows. A booster compressor, or a higher-pressure package, is the usual answer for deep, wet holes.

Hammer flow versus hole-cleaning flow

The air that leaves the hammer is the same air that cleans the hole, so do not add a hammer figure and a flushing figure together. Instead, check that the hammer’s exhaust flow is enough to give a sound uphole velocity in the annulus.

Uphole velocity is the volume of air moving up the annulus divided by the annular area: the area of the hole minus the area of the drill pipe. Drilling practice commonly quotes a dry-air working range in the region of 15–25 m/s (about 3,000–5,000 ft/min) near the collar. Heavy, wet or sticky cuttings, foam work and enlarged holes need careful judgement beyond that rule of thumb.

These figures treat the air near the collar as close to atmospheric. Lower in the hole the air is compressed by the back-pressure, so its actual velocity is lower than at the top. A wider hole with a slim pipe is the classic case where a hammer that is well fed for energy still leaves the hole poorly cleaned. Matching pipe diameter to bit size, or adding air, is the correction.

Symptoms of a hammer that is short of air

A worn compressor delivering less than its rated flow looks, from the rig, very much like a hammer that is too large. Testing the delivered air before blaming the tooling saves a lot of argument.

A simple method to size the compressor from the hammer

1. Identify the hammer by maker, model and configuration, and the bit diameter you will drill.
2. From the hammer document, pick the hammer supply pressure you intend to run and read the consumption at that pressure.
3. Estimate the deepest submergence you expect below the water table and add the back-pressure.
4. Add hose, swivel, string and lubricator losses, and the compressor’s regulation band, to reach the required discharge pressure.
5. Check uphole velocity for your hole and pipe sizes, and take the larger of the hammer flow and the hole-cleaning flow.
6. Derate for site conditions, especially summer intake temperatures and dust.
7. Compare that requirement with the delivered flow of each candidate compressor at that discharge pressure, not with its maximum flow or its maximum pressure separately.

Our DTH hammer duty comparison tool follows the same logic, and the practical guide to reading hammer charts explains each step in more detail.

How Reddy Compressors helps

Reddy Compressors, Tarnaka, Secunderabad, has supported drilling and site air since 2002. For borewell and DTH work we supply all types, all brands and all models of borewell compressors: new machines, used and refurbished units, rentals from our own fleet, service and spare parts. We work across ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands.

When a rig owner sends us the hammer, bit and expected depth, we work through the air path with them, recommend a pressure–flow class, and say plainly when a booster or a higher-pressure machine is the better answer. Our team of more than 20 service engineers and technicians covers the whole of Telangana and Andhra Pradesh, and we supply compressors, rentals and spares across India. Where a rig’s compressor is underperforming, we can test delivered flow and pressure, inspect the airend, coolers, intake and regulation, and repair it.

What to send us

• Hammer make, model and size, and the bit diameter you drill
• Drill pipe outside diameter and the hose and swivel arrangement
• Expected hole depth and standing water level, if known
• Your current compressor make, model, and the pressure and flow it shows under load
• Whether you want to buy new, buy used, hire, or repair your existing machine
• Site location and the period of work

Frequently asked questions

How much air does a 6 inch DTH hammer need?

Typical 6 inch borewell work in India uses roughly 800–1,200 cfm at 250–300 psig (17.2–20.7 bar), rising towards 1,500 cfm and 400 psig for deep, wet or large-diameter holes. The exact figure depends on the hammer model, its configuration and the supply pressure. Always read the maker’s consumption chart at your intended pressure, then check uphole velocity for your bit and pipe.

Does a DTH hammer use more air at higher pressure?

Yes. Because the hammer’s porting is fixed, denser air at higher pressure means more free air passes through each cycle. Higher pressure gives harder blows and faster drilling, but consumption rises too. A compressor that can reach a high pressure only at reduced flow may starve the hammer at exactly the setting the driller wants, so always compare flow at the required pressure.

Why does my rig drill slower as the hole gets deeper?

Below the water table the hammer exhausts against a column of water and cuttings, up to roughly 1 bar for every 10 m of water when the annulus is full. That back-pressure reduces the pressure difference across the hammer. If the compressor is already at its limit, penetration falls. A higher-pressure compressor or a booster restores the margin.

What uphole velocity is needed for DTH drilling?

For dry air drilling, a commonly used guide is about 15–25 m/s (around 3,000–5,000 ft/min) in the annulus near the collar. Heavy or wet cuttings and enlarged holes need more care. Velocity depends on the gap between hole and drill pipe, so a large bit on slim pipe can need much more air than the hammer alone consumes.

Can I use a higher-pressure compressor with a smaller hammer?

Often yes, provided the hammer is rated for that pressure and the compressor’s regulation can be set to suit. Running a hammer beyond its rated pressure risks damage and voids the maker’s guidance. A larger compressor running lightly loaded can also waste fuel. Tell us the hammer and duty and we will suggest a sensible setting or a better-matched machine.

Do you rent compressors for borewell drilling?

Yes. We rent borewell and DTH compressors from our own fleet, along with selling new and used machines and providing service and spares for all brands. Send the hammer, bit, depth and site details and we will recommend a pressure–flow class. Our service teams cover Telangana and Andhra Pradesh, and we supply across India.

Related pages

• Borewell and DTH compressor library
• Reading DTH hammer air charts: a practical guide
• DTH hammer and bit selection
• Booster-assisted drilling air
• New borewell compressor sales
• Borewell compressor rentals and hire