Where is compressed air used in tunnelling?
On a drill-and-blast or NATM-type tunnel, air is used at almost every stage of the cycle. On TBM drives the demand is smaller but still important for the machine’s auxiliary systems, segment handling and maintenance. Typical users are shown below; confirm exact figures from the equipment data.
| User | Typical pressure | Flow pattern | Air quality (ISO 8573-1, typical) |
|---|---|---|---|
| Pneumatic rock drills and jack-leg drills | About 7–10 bar (101.5–145 psig) at the tool | High, during the drilling phase; typically 85–145 cfm per drill | General air, water separated; lubricated at the tool |
| Hydraulic drill jumbos (flushing, shank lubrication, cleaning) | About 6–8 bar | Moderate, during drilling | Filtered, water separated |
| Shotcrete sprayers (wet-mix accelerator dosing and nozzle air, or dry-mix conveying) | About 6–7 bar | High and steady while spraying; dry-mix needs much more air | Separated and preferably dried; oil carry-over affects the lining |
| Grouting pumps, rock-bolt installation and resin mixers | About 6–10 bar | Intermittent | General air |
| Pneumatic pumps for dewatering at the face | About 6–7 bar | Continuous where used | General air |
| Breakers, scaling tools, spades, impact wrenches | About 6–7 bar | Intermittent, high simultaneity | General air |
| Blow-out of drill holes, cleaning of formwork and segments | About 6–7 bar | Bursts | General air |
| TBM auxiliaries, seals and maintenance tools | As per TBM data | Low and steady | Filtered and dried |
Size the plant from the face cycle. The heaviest demand is usually when drilling and shotcrete or several tools overlap; make sure you plan for that overlap, not only for the single largest user.
Compressed air and ventilation: keep the interface clear
Tunnel ventilation uses fans and ducting to supply fresh air and remove blast fumes, diesel exhaust and dust. It is a life-safety system designed by the ventilation engineer. Compressed air is a production utility. The two interact, and the interface must be defined:
- Compressors should take intake air from clean, ventilated surface air, never from the return air or near the portal where fumes collect.
- Exhaust air from pneumatic tools adds a small volume underground and some oil mist; it does not replace ventilation and must not be counted as fresh air.
- Diesel compressors should normally stay at the portal or shaft top. Running them underground adds exhaust and heat that the ventilation design must then handle.
- Compressed air must never be used for breathing unless it comes from a dedicated breathing-air system designed for that purpose.
Typical system architecture
A tunnel air system starts at the portal or shaft and grows with the drive.
- Compressor station at the portal or shaft top. Two or more large portable diesel compressors, or electric screw compressors once grid or site power is reliable. High-flow portable stations in the 600 cfm class and above, or several 400–600 cfm machines in parallel, are common for active drill-and-blast faces.
- Aftercooler, separator and receiver. Cooling and separating water at the surface keeps most condensate out of the tunnel main.
- Main air line into the tunnel. Steel or HDPE pipe sized for the full future length, with isolation valves at intervals and at each cross-passage or branch.
- Drain points at low points. In a rising drive water runs back to the portal; in a falling drive it collects at the face. Plan drains for each gradient.
- Face manifold. A multi-outlet manifold with valves and hose connections close to the face, moved forward as the tunnel advances.
- Standby connection. A tie-in point at the station so an extra machine can be connected without stopping the system.
Pressure drop grows with distance and flow. A main that was fine at 200 metres can leave tools starved at 2 kilometres. Recalculate the pressure budget as the drive extends, and upsize the main early rather than adding compressor pressure later; every extra bar of generation pressure costs about 7% more power or fuel.
Shotcrete and grouting air
Shotcrete is often the most demanding and most sensitive air user. Wet-mix spraying uses air at the nozzle to accelerate and place the concrete, and to dose accelerator. Dry-mix conveying needs much larger air volumes because the air carries the material through the hose. Points to plan:
- Steady pressure at the nozzle; pulsing air gives poor compaction and more rebound.
- Water separation and, where practical, drying; water and oil in the air affect lining quality.
- Dedicated supply or priority on the manifold, so a drill starting up does not starve the sprayer.
- Correct hose size and the shortest practical hose run.
Grouting and rock-bolt installation usually take less air but at times when other users are also running, so they still count in the simultaneity check. See shotcrete and guniting air supply for more detail.
Air treatment underground
Tunnel air does not need laboratory quality, but water in the main causes real damage: washed-out tool lubrication, icing in the exhausts of fast-running tools, corrosion in valves, and water slugs at the face. A practical treatment train is:
- Aftercooler at the compressor (most diesel portables have one built in or available).
- Moisture separator and drain at the station.
- General-purpose filter at the station; refrigerated dryer (typical PDP about +3 °C) where shotcrete or TBM systems need dry air.
- Drains at every low point in the main.
- Filter-regulator-lubricator sets on tool drops where tools need them.
Reliability, criticality and standby
When air stops, the face stops: drilling, bolting and shotcrete all wait, and an unsupported face is a safety concern. Plan for continuity from the start.
- N+1 compressors at the station, so one machine can be serviced or fail.
- Standby rental cover agreed in advance, with a tie-in point ready.
- Fuel planning for diesel machines running long shifts; see fuel supply and refuelling.
- Service by running hours. On hire jobs we change oil and service every 300 running hours, and tunnel duty should be planned the same way.
- Safety shutdowns on diesel portables (high discharge temperature, low engine oil pressure, high coolant temperature) checked during each service, not bypassed.
- Critical spares at site: air and oil filters, separator elements, fan belts, hoses, couplings, safety valves.
Energy and fuel
Tunnel air is expensive air: much of it is generated by diesel engines and pushed through long pipe. The main savings come from:
- Fixing leaks at hose couplings and manifolds, which multiply as the drive extends.
- Closing branch valves on idle sections and at shift change.
- Matching the number of running machines to the face cycle instead of running all compressors unloaded.
- Switching base load to electric screw compressors once reliable power is available.
- Sizing the main generously, so the station runs at the lowest pressure that still gives about 6–7 bar at the tool.
Indian context
Tunnelling in India covers metro underground sections in large cities, railway and road tunnels in hilly states, irrigation and lift-irrigation tunnels, hydropower headrace tunnels and water and sewer tunnels. Projects in Telangana and Andhra Pradesh include irrigation and water-conveyance tunnels as well as urban works. Common conditions are:
- High surface temperature at the portal in summer, typically 40–45 °C, and above 45 °C on open sites, affecting compressor cooling and dryer capacity. Shade and ventilate the station.
- Dust from muck handling and haul roads, clogging intake filters and coolers.
- Monsoon water and groundwater ingress, raising condensate load and making drainage critical.
- Remote sites with weak or no grid power in early stages, which is why diesel portables carry most of the load at first.
Common problems and fixes
| Symptom | Likely cause | What to check or fix |
|---|---|---|
| Drills slow at the face, fine near the portal | Pressure drop in a main that is too small for the length | Measure pressure at face and station; upsize main or add a booster line |
| Water slugs at tools and manifold | No separator at station, missing low-point drains | Fit separator and drains; check gradient of drive |
| Shotcrete rebound high, lining quality poor | Unsteady nozzle air, competing users | Dedicated supply or priority valve for sprayer |
| Diesel compressor trips in afternoon heat | Blocked cooler, poor ventilation at station | Clean cooler, improve shade and airflow, check oil |
| Fuel use rising without more work | Leaks, too many machines running unloaded | Leak walk, sequencing, isolate idle sections |
How Reddy Compressors helps
Reddy Compressors is an independent multi-brand compressed-air specialist at Tarnaka, Secunderabad, serving industry since 2002. We sell, rent and service compressors from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others, without being an authorised dealer for any of them.
- Rental fleet: our own diesel portables and high-pressure units, with rentals from 50–2,000 cfm at 2–30 bar, including high-flow stations and standby machines. See high-volume temporary air.
- New and used sales: portable diesel and electric screw compressors, with brand-new machines supplied for long-term projects.
- Service on site: over 20 service engineers and technicians across Telangana and Andhra Pradesh, with project-based support elsewhere in India where feasible.
- Spare parts: identified from nameplate, serial or old part, with fit confirmed before quoting.
- System advice: station layout, main sizing, drainage and standby planning as the drive advances.
Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.
Our track record. We have supplied compressors to leading companies in tunnelling across Telangana, Andhra Pradesh and beyond – new and used machines, long-term rental, and breakdown cover when a contractor’s own compressor fails – backed by our own service team.
What to send us
- Tunnel type, method (drill-and-blast, NATM, TBM) and planned length
- Number of faces and the equipment at each (drills, jumbo, sprayer, grout pumps)
- Pressure needed at the tools and expected simultaneity
- Site location, power availability and preferred diesel or electric
- Planned hire period and shift pattern
- Details of any existing compressors: make, model, nameplate photo
Frequently asked questions
How much compressed air does a tunnel project need?
It depends on the number of faces, the tools at each face and how many run at once. Add the air demand of drills, shotcrete, grouting and tools that overlap in the face cycle, then allow for pressure losses in the main and for leaks. Active drill-and-blast faces often need high-flow stations, built from several portable compressors in parallel.
Is compressed air used for tunnel ventilation?
No. Tunnel ventilation is a separate system of fans and ducts designed to supply fresh air and remove fumes and dust. Compressed air is a production utility for drills, shotcrete, grouting and tools. Exhaust from air tools does not count as ventilation, and compressed air must never be breathed unless it comes from a dedicated breathing-air system.
What compressor is used for shotcrete?
Shotcrete usually uses a portable screw compressor supplying about 6–7 bar at steady flow. Wet-mix needs moderate nozzle air; dry-mix conveying needs much more because the air carries the material. Separate water at the compressor, keep hoses short, and give the sprayer priority on the manifold so other users do not cause pressure dips.
Should tunnel compressors be diesel or electric?
Diesel portables suit the early stages and remote sites without reliable power, and are easy to move and add as standby. Once a stable power supply is available, electric screw compressors at the portal are usually cheaper to run for base load. Many projects run electric base load with diesel machines for peaks and backup.
Why do our drills lose power as the tunnel gets longer?
As the drive extends, pressure drop along the main increases, especially if the pipe was sized for the first few hundred metres. Measure pressure at the station and at the face under load. The usual fixes are upsizing the main, adding a parallel line, removing restrictions at manifolds and fixing leaks, rather than raising compressor pressure.
Can we hire standby compressors for a tunnelling project?
Yes. A standby portable compressor connected through a tie-in point lets the face keep working when a main machine fails or is serviced. Agree the machine size, connection, fuel arrangement and service cover in advance. We rent diesel and electric compressors from our own fleet and service them on site.
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.
Pipe pressure drop and sizing
Check the pressure drop and air velocity in a compressed-air line, or find the smallest bore that keeps the drop within a target.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Flow (m³/min free air): 12
- Inlet pressure (bar g): 7
- Pipe length (m): 150
- Inside diameter (mm): 80
- Roughness (mm): 0.046
- Fittings allowance (% extra length): 30
- Air temperature (°C): 30
- Target pressure drop (bar): 0.1
Result
- Pressure drop: 0.059 bar (0.839% of the inlet gauge pressure)
- Air velocity: 5.22 m/s
- Reynolds number: 2,06,066
- Smallest bore for the target drop: 73 mm inside diameter
- Velocity check: Within the common rule-of-thumb range for mains (roughly up to 15–20 m/s)
Method. Darcy–Weisbach: ΔP = f × (L/D) × ρv²/2, with the friction factor f from the Swamee–Jain relation and air density taken at mean line pressure. Fittings are allowed for as extra equivalent length. Typical commercial steel roughness is about 0.046 mm.
What to take from it. Pressure drop varies roughly with the fifth power of the bore at constant flow, so a larger main is a cheap way to hold pressure.
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.
Compressed air demand estimate
Add up the air your tools and processes use, apply duty and simultaneity, then add leakage and growth allowances to reach a capacity to enquire about.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Unit for the air each tool uses: L/s
- Group A: number of tools: 6
- Group A: free air each (in the unit above): 4
- Group A: duty (%): 40
- Group B: number of tools: 2
- Group B: free air each (in the unit above): 15
- Group B: duty (%): 60
- Group C: number of tools: 1
- Group C: free air each (in the unit above): 40
- Group C: duty (%): 80
- Simultaneity of all tools (%): 65
- Leakage allowance (%): 15
- Growth allowance (%): 10
Result
- Connected demand: 94 L/s = 199 cfm
- Average demand (duty-weighted): 59.6 L/s = 126 cfm
- Simultaneous peak: 61.1 L/s = 129 cfm
- Design basis (higher of average and peak): 61.1 L/s = 129 cfm
- Design capacity with allowances: 77.29 L/s = 4.64 m³/min = 163.8 cfm
Method. Average demand = Σ(number × air per tool × duty). Peak = Σ(number × air per tool) × simultaneity. The peak can never be lower than the average, so the design basis is the higher of the two. Design capacity = design basis × (1 + leakage) × (1 + growth).
What to take from it. Leakage and growth allowances move the answer as much as any single tool. Where you can, replace catalogue figures with measured ones.