Start at the tool, not the compressor
On many sites the portable compressor is treated as a simple utility: tow it in, start it, connect a hose. Problems follow when the tool at the far end of 60 metres of hose is starved, the hammer drills slowly or the blast nozzle loses pressure in the afternoon heat. Good site air is engineered backwards from the point of use.
- Tool or process demand. Air consumption of each tool or nozzle at its working pressure, and how many run at once.
- Pressure needed at the tool, while it is working, not at rest.
- Losses in between: aftercooler, separator, manifold, hose length and diameter, couplings and bends.
- Required compressor output: free air delivery (FAD) and discharge pressure, plus a margin for wear, leaks and altitude or heat.
- Site constraints: power availability, access, noise, fuel logistics and duty hours.
Pressure is the force to overcome resistance; flow is the quantity of air that does the work. A good gauge reading at the compressor proves neither at the tool. Compare machines on FAD at stated conditions; see FAD and reference conditions and the cfm, psi and bar guide.
Types of portable compressor
Most modern portables are oil-injected rotary screw machines on a towable or skid chassis, with diesel or electric drive. Smaller units and some specialist duties still use reciprocating designs.
- Compact towable units for breakers, small tools and maintenance work.
- Medium site compressors, typically 100–600 cfm, the general workhorses of construction, plant maintenance and blasting.
- High-flow units for multiple blast pots, large pneumatic tools or temporary plant air.
- High-pressure drilling packages for borewell, DTH and quarry work, often two-stage.
- Variable-pressure machines that trade flow for pressure across a range, useful for fleets that serve varied jobs.
- Oil-free portables for process and instrument air where oil carry-over is unacceptable.
- Tractor-mounted compressors driven from a tractor power take-off, common in rural borewell and agricultural work.
Pressure classes and what they are used for
High pressure is not simply normal air with a bigger number. Higher-pressure machines usually use two-stage compression to share the pressure ratio and control discharge temperature, and they need hoses, fittings and receivers rated for that pressure. Figures below are typical (1 bar = 14.5 psi).
| Pressure class | Typical pressure | Typical applications |
|---|---|---|
| Standard | About 7 bar (101.5 psig) | Breakers, rock drills, sandblasting, general construction tools, utility air |
| Medium | About 10–12 bar (145–174 psig) | Shotcrete, pipeline work, longer hose runs |
| Intermediate high | About 14–17 bar (203.1–246.6 psig) | Production drilling, quarrying, shallower borewells, pipeline testing support |
| High pressure | About 17–28 bar (246.6–406.1 psig) | Borewell and DTH drilling; borewell duty overall is typically 600–1,500 cfm at 200–400 psig |
| Very high pressure | Special units up to about 35 bar (507.6 psig) | Deep borewells, high-performance DTH hammers, special duty |
Do not buy or hire more pressure than the job needs. Compressing to a higher pressure costs more energy, roughly 7% more power per extra bar, which on a diesel unit means more fuel every hour.
Diesel or electric portable: how to decide
| Factor | Diesel portable | Electric portable |
|---|---|---|
| Power source | Self-contained; no grid needed | Needs a reliable supply of the right capacity, or a generator |
| Running cost | Fuel and engine maintenance | Usually lower per unit of air where grid power is available |
| Mobility | Moves freely around the site | Limited by cable route and supply point |
| Emissions and noise | Exhaust and engine noise; care in enclosed areas | No exhaust at site; quieter |
| Maintenance | Engine and compressor both need service | Simpler; motor and starter |
| Best fit | Remote sites, drilling, roadworks, early construction | Long-duration jobs at plants and established sites |
See diesel or electric for site air for a fuller comparison.
Site setup: placement, cooling and fuel
- Level, firm ground with the chassis braked and chocked. Tilted machines can starve the engine of oil and the separator can carry over oil.
- Clear airflow. Keep cooling air inlets and the hot-air outlet clear of walls, other machines and stockpiles. Recirculated hot air is the commonest cause of high-temperature trips in a Telangana or Andhra Pradesh summer.
- Dust control. Position upwind of drilling and blasting dust where possible and check air filter restriction indicators daily.
- Exhaust away from people, tunnels, trenches and building intakes.
- Fuel logistics. Plan refuelling for the duty hours, keep fuel clean and dry, and use drip trays or bunding.
- Safety shutdowns tested at start-up: high discharge temperature, low engine oil pressure and high coolant temperature.
Normal discharge temperature on an oil-injected screw is typically about 75–95 °C. A machine running consistently higher needs its coolers, oil level and placement checked before it trips.
Hoses, couplings and pressure drop
The hose is part of the air system, not an accessory. An undersized or over-long hose can waste more pressure than the gap between two compressor models.
- Size hose diameter for the flow and length; longer runs need a larger bore.
- Use hoses and couplings rated above the compressor’s maximum pressure, especially on drilling and high-pressure duty.
- Fit whip checks (safety cables) across every coupling and secure hose ends, because a free hose under pressure whips violently.
- Avoid tight bends, kinks and vehicle crossings without protection.
- Use a manifold with individual valves rather than daisy-chained tees.
- Depressurise before disconnecting, every time.
Our air hoses and rig connections page covers selection and inspection in detail.
Air treatment on site
Portable compressor air is hot, wet and carries some oil. For breakers and drilling that is usually acceptable. For blasting, painting, coating, shotcrete or plant air, treatment matters:
- Aftercoolers bring air close to ambient so water condenses where it can be removed.
- Moisture separators and drains remove the liquid water.
- Filters reduce oil aerosol and particles for coating and process work.
- Dryers where the air feeds instruments or plant systems; a refrigerated dryer gives a typical pressure dew point of about +3 °C.
Coating contractors in particular should brief us on the air quality their specification requires, so the treatment train is planned with the compressor.
Operating and servicing portables on heavy duty
Site duty is harder than factory duty: dust, heat, long hours and frequent moves. On our hire jobs we change oil and service every 300 running hours, and on round-the-clock work we plan services so the machine is never pushed past its interval. Daily checks should include engine oil, coolant, compressor oil, fuel, air filter indicators, belts, leaks and the condition of hoses and whip checks. For fault finding, see diesel portable compressor troubleshooting.
Temporary air for projects
Large projects often need more than one compressor. Our team, across Reddy Generators and Reddy Compressors, has supplied and commissioned a temporary power-and-air package for an oilfield-services contractor on a natural-gas development at Kakinada, Andhra Pradesh: three compressors rated 1,100 cfm at 300 psig, two rated 1,000 cfm at 275 psig and one rated 750 cfm at 150 psig, with generators, distribution boards, operators and a technician on site, and erection, commissioning and transport. Multi-machine jobs like this need a planned manifold, a fuel plan and a service rota from day one.
How Reddy Compressors helps
From Tarnaka, Secunderabad, serving industry since 2002, we sell, rent and service portable compressors of all major makes as an independent multi-brand specialist. Our own rental fleet of diesel portables and high-pressure units covers 50–2,000 cfm and 2–30 bar. Service teams cover all of Telangana and Andhra Pradesh, and we supply equipment, rentals and spares across India, with project site support where feasible.
- Duty assessment from tool demand back to the machine.
- Selection of pressure class, flow and diesel or electric drive.
- Hoses, manifolds, aftercoolers, separators, filters and dryers to suit the job.
- Delivery, commissioning, operator briefing and scheduled servicing on hire.
- Service, overhaul and spares for portables you own.
What to send us
- Application: drilling, blasting, breaking, coating, plant air or testing.
- Tools or hammers in use and how many run at once.
- Pressure needed at the tool and hose length.
- Site location, power availability and access.
- Duty hours per day and expected duration.
- Air quality requirement, if any.
Frequently asked questions
What size portable air compressor do I need for my site?
Add up the air consumption of every tool or nozzle that runs at the same time, at its working pressure, then allow a margin for hose losses, wear and leaks. Choose a compressor whose free air delivery covers that total at the pressure needed at the tool. Most general construction duties fall in the 100–600 cfm range at about 7 bar (101.5 psig).
Watch
A short explainer on this topic
Slide-by-slide engineering explainers from Reddy Compressors. More in the video library.
Is a diesel or electric portable compressor better?
Diesel suits remote sites, drilling and work that moves around, because it needs no grid supply. Electric suits long jobs where reliable power is available, usually costing less to run, with no exhaust and less noise. Many projects start with diesel and switch to electric once site power is established and the work settles in one place.
What pressure compressor is needed for borewell drilling?
Borewell and DTH drilling typically uses 600–1,500 cfm at about 200–400 psig, which is roughly 14–28 bar. Deep holes and high-performance hammers may need high-pressure units up to about 35 bar (507.6 psig). The right figure depends on hammer size, hole diameter, depth and water inflow, so match the compressor to the hammer maker’s air requirement.
Why does my portable compressor keep shutting down on high temperature?
The usual causes are hot-air recirculation from poor placement, blocked or dusty coolers, low or degraded compressor oil, a faulty thermostatic valve or a clogged oil filter. Summer heat in Telangana and Andhra Pradesh makes marginal cooling fail quickly. Move the machine into clear air, clean the coolers, check oil level and filters, and have the thermostatic valve tested.
How long can an air hose be from a portable compressor?
There is no fixed limit, but every metre causes pressure drop, and the effect rises sharply with flow. Long runs need a larger hose diameter, fewer couplings and gentle bends. If the tool slows at the end of the line, measure pressure at the tool while it works; the fix is often a bigger hose or a shorter run rather than a bigger compressor.
Can I rent a portable compressor with an operator?
Yes. We can supply diesel and electric portables with skilled operators, delivery, commissioning and scheduled servicing for the hire period, depending on the job. Tell us the duty, hours per day, location and duration, and we will propose the machine, hoses and support. Rental equipment can be supplied across India, with service teams based in Telangana and Andhra Pradesh.
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.
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.
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.
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.