Build the selection around a documented drilling duty and a defined delivery boundary.
Selecting a compressor for down-the-hole drilling starts with the installed equipment and intended work. The hammer requirement is an important input, but the decision also needs the rig's other air demands, relevant flushing requirements, delivery arrangement and site conditions.
A defensible selection explains how those inputs became a requirement and why the offered compressor can meet it. It also identifies the conditions under which that conclusion remains valid. A familiar model, broad CFM band or successful previous hire cannot supply that explanation on its own.
Reddy Compressors considers DTH air-supply enquiries against documented requirements and confirmed equipment arrangements. High-pressure supply requires supplier confirmation. This guide provides a selection workflow; the final equipment match needs review for the actual assignment.
1. Establish the configuration being selected for
Start a duty sheet with the rig, hammer, bit and relevant drill-string identities. Confirm the installed drilling mode and any modifications. Attach the applicable technical documents and record their revisions where available.
Define the compressor's role: normal external supply, replacement for an existing source or part of a separately engineered combined arrangement. Do not carry a requirement from one role into another without checking its boundary.
The duty sheet should describe the intended hole and operating activities, along with available formation and water information. State which inputs are confirmed and which remain estimates. Have the drilling specialist identify the implications of uncertain conditions.
Output from this stage: an identifiable equipment configuration, intended assignment and list of unresolved application questions.
2. Establish the hammer requirement at the proposed duty
Obtain the air-consumption information for the exact hammer and configuration. Record the pressure associated with the required flow, its measurement or reference basis, and the conditions stated by the manufacturer.
A hammer family name or nominal size should not be used to invent a consumption value. If the document does not support the proposed operating point, obtain clarification from the tooling authority. Do not extrapolate a missing value simply because two nearby figures are available.
Keep this evidence attached to the duty sheet. It will be needed if the hammer, configuration or intended operating pressure changes. Detailed interpretation of a particular consumption curve belongs with that tool's documentation.
Output from this stage: a supported hammer input or a specific information request that must be closed before selection.
3. Reconcile the complete air demand
Determine which rig functions and accessories rely on the proposed source. Manufacturer rig literature can distinguish hammer demand, feed, rotation, dust collection and totals with different inclusions. Use the actual configuration rather than assuming that every DTH rig has the same additional loads.
Create a demand ledger before calculating a total.
Ledger entry | Evidence to record | Check before inclusion |
|---|---|---|
Hammer | Identified manufacturer duty data | Does it apply to the installed configuration and selected pressure? |
Other rig functions | Rig requirement and documented inclusions | Are these already included in a stated rig total? |
Air-powered accessories | Applicable equipment requirement | Are they supplied by this compressor and active in the relevant operating case? |
Additional flushing requirement | Drilling-specialist assessment | Is this genuinely additional to the air already accounted for? |
Agreed design provision | Stated reason, basis and approving person | Does it address a defined uncertainty or operating need? |
Do not automatically add a separate “flushing CFM” to hammer consumption. Establish how the documented requirement treats the air used for flushing and whether additional flow is needed. A published rig brochure explicitly notes that further flushing demand can vary with drilling depth and formation; it does not supply one allowance for every project.
Output from this stage: reconciled demand for each relevant operating case, with no unexplained omissions or duplicate entries.
4. Separate operating cases before combining requirements
Identify which functions operate together. Production drilling, a separate clearing activity and other planned air uses may require different assessments. Their relationship must come from the equipment and work programme, rather than an assumed average utilisation.
For each case, record the required flow, pressure, delivery point and duration or operating pattern. Obtain confirmation of any proposed non-simultaneous use. A scheduling assumption should not silently remove a demand that can occur during drilling.
Keep each case as a paired requirement. The largest flow and the highest pressure may belong to different activities. Check whether a candidate supports every required case rather than constructing an undocumented combination of the separate maxima.
An average air figure can also conceal a required sustained duty. Describe the actual operating requirement before deciding whether any averaging is appropriate.
Output from this stage: a set of operating cases that suppliers can answer consistently.
5. Establish the pressure required at the supply boundary
Identify where the application pressure requirement applies. A rig-inlet requirement and a hammer operating requirement are not interchangeable descriptions. If the requirement begins at the hammer, the responsible specialist must establish the implications for the upstream system.
For the surface delivery route, assess hoses, connections, valves and any intermediate equipment at the relevant flow. Pressure losses depend on the actual arrangement. Do not assign a fixed allowance merely because a delivery line is described as “long.”
The following simplified illustration explains boundary arithmetic only. All values are invented, assume a steady operating case and use the same gauge-pressure reference:
Illustrative item | Assumed value |
|---|---|
Required pressure at the rig inlet | 20.0 bar(g) |
Assessed pressure loss between compressor outlet and rig inlet | 1.2 bar |
Resulting outlet pressure for that case, before any separately justified provision | 21.2 bar(g) |
The arithmetic is 20.0 + 1.2 = 21.2. It is not a recommended DTH pressure, a control setting or a calculated loss for an actual hose. The rig-inlet requirement is assumed to have been established separately; this illustration does not determine hammer pressure or account for downhole conditions.
Review excessive delivery losses as an arrangement issue. Increasing source pressure is not an automatic remedy, particularly when equipment limits and other operating cases must also be respected.
Output from this stage: a pressure requirement at the chosen supply boundary, supported by an assessment of the actual route and applicable limits.
6. Review water, formation and environmental conditions
Water influx and back pressure are recognised challenges in deep-hole DTH work. The drilling specialist should assess their relevance to the selected hammer and operating method. Keep this review distinct from the surface-hose pressure-loss assessment.
Bore depth alone does not establish a water column, a flushing requirement or the pressure needed from the compressor. Do not add a generic pressure increment per metre and describe the result as an approved selection.
Also record site elevation and expected ambient conditions. Portable-compressor manufacturer guidance identifies altitude as relevant to performance and cooling. Request the offered machine's applicable operating limits and performance confirmation for the site.
A broad environmental statement should not be converted into a universal percentage correction. Where the manufacturer supplies an applicable correction or operating restriction, retain it with the comparison and apply it on the stated basis.
Output from this stage: explicit site assumptions, supported adjustments where applicable, and any conditions that still prevent a conclusion.
7. Compare candidates against the complete requirement
Issue the same duty sheet to each supplier. Ask for the exact offered machine and configuration, supported flow at each required pressure, applicable reference conditions and relevant site restrictions.
Compare flow figures on a reconciled basis. A unit conversion changes the unit; it does not by itself reconcile different definitions of airflow. If the reference conditions or reporting basis are unclear, ask for clarification before calculating a surplus.
Use a candidate record that keeps technical and delivery questions visible.
Assessment area | Evidence needed for the offered unit |
|---|---|
Duty coverage | Supported flow-pressure combinations for all required cases |
Site applicability | Relevant operating limits and manufacturer-backed site assessment |
Supply arrangement | Delivery boundary, connection scope and approved integration where applicable |
Operating pattern | Confirmation that the intended work pattern is supported |
Equipment identity | Model, configuration and traceability to the performance evidence |
Availability and support | Actual offer, deployment arrangements and agreed support scope |
A candidate may meet the stated duty but remain unconfirmed for delivery. Another may be available but lack evidence for a required operating point. Keep those findings separate.
Do not treat a numerical difference between supply and demand as an automatically adequate reserve. Explain what provision the design requires and why. Neither a universal spare-capacity percentage nor an arbitrary increase in compressor size resolves missing technical evidence.
Output from this stage: a documented comparison showing supported duties, shortfalls and unanswered questions for each candidate.
8. Resolve an incomplete selection through the right question
Consider an illustrative enquiry in which the rig owner supplies a complete-rig air figure, the tooling supplier supplies a hammer sheet, and the compressor offer quotes free-air delivery at a stated pressure. The rig total's inclusions and the demand's flow basis are unknown.
Adding the two demand figures would not repair that uncertainty. The next actions are specific: ask the rig authority whether the hammer is included, establish the demand reference basis and reconcile the requirement to the offered supply boundary.
If the answers show that an accessory is missing from the rig total, assess that accessory's contribution in the relevant operating case. If the answers show it is already included, preserve the documented total. The decision follows the evidence rather than a preferred compressor size.
This example supplies no invented customer, model or performance result. It shows how the selection progresses when the paperwork is incomplete.
9. Record the selected package and its conditions
The selection record should identify the accepted configuration, duty cases, supporting documents and responsible reviewers. Include any limitations, unresolved items and conditions that require reassessment.
Examples of material changes include a different hammer configuration, revised delivery route, additional air-powered equipment or an altered operating requirement. Assess the change against the original basis before carrying the selection forward.
Agree how the delivered package will be checked against the accepted scope. The method, measurement points and applicable operating conditions should be established by the responsible technical parties. A compressor running unloaded is not evidence that the full drilling duty has been demonstrated.
Keep the technical selection separate from the drilling outcome. It does not guarantee penetration rate, completed depth, water yield or project duration.
Frequently asked questions
Is hammer size enough to choose a compressor?
No. Obtain the exact hammer duty and reconcile it with the actual rig, operating cases, delivery arrangement and site conditions.
Should I add a standard margin to every DTH air requirement?
Use a provision with a stated technical basis. A fixed percentage cannot compensate for unknown inclusions, incompatible flow references or missing site information.
Can a compressor with a higher maximum pressure be accepted automatically?
No. Confirm available flow at the required pressure and the supported operating range. Maximum pressure alone does not establish duty coverage.
Should flushing air always be added to hammer consumption?
First determine what the supplied figures already include and what additional flushing the application requires. Blind addition can misstate the requirement.
Can the previous compressor be replaced with the same CFM label?
The label is only a starting point. Review the offered configuration, paired pressure-flow evidence, reference basis and delivery scope against the current requirement.
What if no candidate has sufficient evidence?
Keep the selection open and request the missing information. A proposed package should not be described as technically accepted while its required duty remains unestablished.
Prepare a DTH selection enquiry
Provide the rig and hammer documents, intended operating cases, available site information and proposed connection arrangement. Include existing demand calculations with their assumptions, rather than only the final CFM figure.
Reddy Compressors serves enquiries from Secunderabad/Hyderabad, with Telangana and Andhra Pradesh as priority areas. Wider project support and specialist supply depend on confirmed arrangements for the assignment.
For the broader equipment and service offering, visit Portable Compressors.
The selection should explain the requirement, the evidence and the conditions of acceptance.
*Content prepared: 29 September 2026.*
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.