Why a depth figure alone cannot size a compressor
An enquiry that says “300 feet borewell” or “200 metres DTH” tells us the objective. It does not tell us the hole diameter, the drill-pipe size, the hammer model, how much water the formation will make or how the casing programme changes on the way down. Those are the things that decide whether a compressor will drill the whole hole or stall halfway.
Depth matters through three physical effects. First, a longer air path between the compressor and the hammer adds friction loss in hoses, the swivel and the drill pipe. Second, any water standing above the hammer must be pushed out before the hammer can exhaust freely; each 10 metres of water column adds roughly 1 bar (14.5 psi) of back pressure. Third, cuttings must travel further up the annulus, so the return air must keep its lifting velocity for longer and through any enlarged or washed-out sections.
None of these effects grows in a neat straight line with depth. A dry hole in hard granite can be drilled deep on a modest pressure, while a shallower hole with heavy water inflow can need far more. That is why we plan by stages rather than by a single number.
Target depth, rig capability and compressor capability are different things
Before any selection discussion, label the depth figure you are using. The same number can mean very different things, and mixing them up is a common reason for an undersized or oversized machine.
| Depth description | What it tells you | What it does not tell you |
|---|---|---|
| Planned finished depth | The current project objective | The air duty needed to reach it |
| Possible extension | Extra metres the client may ask for if yield is poor | Whether the original compressor, rig and pipe can do it |
| Rig’s rated depth | The maker’s figure for a stated pipe and configuration | Performance in your formation and water conditions |
| Depth achieved on an earlier job | A result under that job’s geology and air package | The same result at a different site |
| Depth reached so far | Where the hole is today | Whether the remaining stages are covered |
Extra air does not lift the rig’s own limits. Rotation torque, pull-back capacity, mast load and pipe strength still govern how deep the rig can safely go. The compressor must match the rig and the tooling, not replace their limits.
How depth affects pressure at the hammer
A DTH hammer is rated to work at a pressure measured at the hammer inlet, not at the compressor gauge. The compressor must therefore deliver the hammer’s working pressure plus every loss and back pressure on the way:
- Pressure drop through the delivery hose, the rig manifold, valves and the rotary head swivel.
- Friction loss along the drill pipe, which rises with string length, airflow and smaller pipe bore.
- Back pressure from water standing above the hammer, about 1 bar for every 10 metres of water column.
- Back pressure from cuttings loading and any foam or injection water in the annulus.
In a dry hole the first two dominate and change slowly with depth. Once the hole goes below the static water level and the formation is making water, the water column can become the largest single term. A hammer that runs well at 17 bar (246.6 psig) near the surface may need the compressor working towards 21–24 bar (304.6–348.1 psig) to keep the same impact energy deep in a wet hole.
This is the main reason borewell compressors in India are offered in pressure bands. Typical packages for borewell and DTH work run from about 600 to 1,500 cfm at 200–400 psig (13.8–27.6 bar), with special high-pressure units up to about 35 bar (507.6 psig) for deep, wet holes.
How depth and diameter affect airflow and hole cleaning
Pressure drives the hammer; airflow cleans the hole. The return air must carry cuttings up the annulus between the drill pipe and the borehole wall. The useful quantity is annular velocity, the airflow divided by the annular area:
Annular area A = π/4 × (hole diameter² − pipe outside diameter²), and average velocity V = Q ÷ A.
Two consequences follow. A larger bore diameter increases the annular area quickly, so the same compressor gives a lower return velocity and weaker cleaning. And because air is compressed at depth, the actual volume moving up a deep, wet annulus is smaller than the free-air figure on the compressor plate. Large-diameter or deep wet holes often become airflow problems before they become pressure problems.
Signs that airflow, not pressure, is the limit include slow penetration with good hammer sound, regrinding of cuttings, fine powder instead of chips, pipe sticking on connections, and cuttings falling back when air is shut off for a rod change.
Build a stage schedule before asking for a compressor size
Divide the programme wherever the physical arrangement or expected conditions change. Typical boundaries are a casing change, a change of bit diameter, a change of hammer, the expected static water level and any known change of formation such as weathered overburden into hard rock.
- List each stage with its depth range, bit diameter, drill-pipe outside diameter and hammer.
- Note the expected formation and whether water is expected, using local drilling records where they exist.
- Estimate the back pressure from water in each stage, using about 1 bar per 10 metres of expected water column above the hammer.
- Add the hammer working pressure from the hammer maker’s data and an allowance for hose and pipe losses.
- Calculate the annular area and the airflow needed to keep cleaning velocity in the largest-diameter stage.
- Identify the stage that needs the highest pressure and the stage that needs the highest flow. They are often not the same stage.
- Choose a compressor whose rated free air delivery and maximum working pressure cover both, with a margin for wear and high ambient temperature.
- Record what would change the answer: an extension, a bigger bit, a different hammer or heavier water than expected.
Where a transition depth is uncertain, write a range. A neat schedule should not hide uncertain geology.
An illustrative stage review
The table below shows how a hypothetical programme planned to 180 metres, with a possible extension to 240 metres, might be organised. The figures are for illustration of the method only. They are not recommended casing depths, formation predictions or a compressor duty for any real site.
| Stage | Assumed arrangement | What usually governs | Selection question |
|---|---|---|---|
| 0–30 m | Larger bit through overburden, then casing | Airflow for the large annulus | Does the free air delivery keep the large hole clean? |
| 30–100 m | Smaller bit in hard rock, mostly dry | Hammer pressure plus line losses | Does the pressure at the hammer meet its working range? |
| 100–180 m | Same bit, water strikes expected | Water back pressure and wet cuttings | Is there enough pressure margin to clear the water column? |
| 180–240 m (extension) | Same bit, longer string, more water | Highest pressure case | Does the package, rig and pipe still cover this, or is a booster or larger unit needed? |
In this pattern the first stage sets the airflow and the extension sets the pressure. A compressor sized only for “180 metres” might clean the top section but run out of pressure if the client asks for the extra 60 metres in a wet formation.
Practical pressure and flow bands for borewell depth
As a starting point for discussion, the bands below reflect the classes of compressor used on borewell and DTH work in Telangana, Andhra Pradesh and across India. The final choice still depends on the stage review above.
| Pressure class | Typical flow | Where it usually fits |
|---|---|---|
| 200–250 psig (13.8–17.2 bar) | 600–900 cfm | Shallower holes, smaller bits, mostly dry formations |
| 250–300 psig (17.2–20.7 bar) | 800–1,200 cfm | Medium depths, moderate water, common agricultural and domestic borewells |
| 300–400 psig (20.7–27.6 bar) | 1,000–1,500 cfm | Deep holes, heavy water inflow, larger diameters |
| Up to about 500 psig (34.5 bar) | Special units | Very deep or very wet holes where water head dominates |
Where an existing compressor has enough flow but not enough pressure for the deep stage, a booster can sometimes raise the delivery pressure. That is a separate engineering decision with its own limits; see our page on booster-assisted drilling air.
Site conditions that change the answer
- High ambient temperature. Indian summer temperatures of 40–45 °C in parts of Telangana and Rayalaseema, and above 45 °C on open sites, reduce cooling margin. A compressor working at its pressure limit for long hours in heat is more likely to trip on high discharge temperature.
- Dust. Drilling sites are dusty. Clogged air filters and coolers reduce delivery and raise temperatures, which shows up first in the deepest, hardest stage.
- Wear. An older airend may no longer deliver its plate flow. Allow a margin, or have the machine checked before a deep programme.
- Altitude. Most of the Deccan plateau is at moderate altitude, but at higher sites thinner intake air reduces free air delivery.
How Reddy Compressors helps
We supply all types, all brands and all models of borewell compressors: new machines, used and refurbished machines, rentals, service and spare parts. As an independent multi-brand specialist we work with ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others, and we are not an authorised dealer of any of them, so our advice is neutral. We have tie-ups with all major and reputed brands.
For a depth-driven selection we review your stage schedule, hammer data and expected water conditions, and tell you which pressure and flow class covers the hardest stage and the possible extension. If your existing compressor is the question, our service engineers can check its actual delivery and condition before you commit to a deep programme. Our service teams cover the whole of Telangana and Andhra Pradesh from our office at Tarnaka, Secunderabad, and we supply compressors, rentals and spares across India. Over 20 service engineers and technicians support our own rental fleet and customers’ machines.
What to send us
- Planned depth and any possible extension
- Bit diameters and casing programme by stage
- Drill-pipe outside diameter and length per rod
- Hammer make and model, and the rig make and model
- Expected formation and water conditions, with any nearby drilling records
- Your existing compressor details (make, model, nameplate photo), if you have one
- Site location, and whether you want to buy, buy used or hire
Frequently asked questions
What size compressor do I need for a 300-foot borewell?
Depth alone does not decide it. A dry 300-foot (about 90-metre) hole with a small bit may drill well on a 600–900 cfm unit at 200–250 psig, while the same depth with heavy water and a larger bit may need more pressure and flow. Send us the bit size, pipe size, hammer model and expected water, and we will suggest the right class.
Why does my compressor drill well at the top but slow down deeper?
Usually because the pressure needed at the hammer has risen. A longer string adds friction loss, and water standing in the hole adds roughly 1 bar for every 10 metres of water column. If the compressor is already near its maximum pressure, the hammer receives less energy deeper down. Poor hole cleaning from a large diameter can also slow penetration.
How much pressure does water in the borehole add?
As a rule of thumb, every 10 metres of water standing above the hammer adds about 1 bar (14.5 psi) of back pressure that the compressor must overcome. In a wet hole with 60 metres of water above the hammer, that is roughly 6 bar extra on top of the hammer’s working pressure and line losses.
Is more cfm or more pressure better for deep drilling?
Both matter, for different reasons. Pressure gives the hammer its impact energy and pushes out the water column. Airflow keeps the return velocity high enough to lift cuttings, especially in large diameters. Deep wet holes usually need more pressure; large-diameter holes usually need more flow. A stage review shows which one limits your job.
Can I use a booster instead of buying a bigger compressor?
Sometimes. If your compressor has enough flow but not enough pressure for the deepest stage, a booster can raise delivery pressure. It adds equipment, fuel and maintenance, and it cannot create flow the primary compressor does not supply. We can review whether a booster, a higher-pressure unit or a hired machine suits your programme better.
Should I size the compressor for a possible extension?
If an extension is realistic, yes, or at least have a plan for it. Many borewells are deepened when yield is poor, and the extra metres are often the wettest and hardest. Sizing only for the planned depth can leave the rig stuck at the point the client most wants to continue.
Related pages
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 17 October 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.