Types of air compressors: how each works and how to choose

Portable dieselSite machines typically 100–600 CFM; hire fleet 50–2,000 CFM at 2–30 bar. Trailer, skid and towable packages for sites, drilling and blasting.Explore →Rotary screwFixed-speed and VSD, oil-injected and oil-free, 5–500 HP, with dryers and receivers.Explore →Reciprocating (piston)Single- and two-stage, tank-mounted and base-mounted, for workshops, garages and plants.Explore →Borewell and DTH600–1,500 CFM at 200–400 psig (about 14–28 bar), special units up to 35 bar, for water-well and DTH drilling rigs. All types, brands and models.Explore →Tractor-mountedPTO-driven piston compressors, 100–140 CFM at 7–10 bar, mounted or on a trolley behind a 35–60 HP tractor, for breakers, rock drills and farm work. Sales and hire.Explore →Oil-freeOil-free screw and piston compressors for food, pharma, electronics and instrument air, with ISO 8573-1 Class 0 machines where certified.Explore →Skid and stationarySkid-mounted diesel and electric packages for plants, quarries and long-term site installations.Explore →Used and refurbishedInspected and overhauled machines of all makes, with a 6-month warranty (from the invoice date; consumables excluded) and parts support.Explore →

How are air compressors classified?

Every air compressor belongs to one of two families, and the family tells you a great deal about how the machine behaves before you look at a single brochure.

Positive-displacement compressors trap a fixed volume of air and reduce that volume to raise its pressure. Piston, rotary screw, scroll, rotary vane and tooth compressors all work this way. Their flow stays fairly steady as discharge pressure changes, which is why they cover almost every workshop, factory, construction, mining and drilling duty in India.

Dynamic compressors, mainly centrifugal, accelerate air with a high-speed impeller and then slow it in a diffuser so that velocity becomes pressure. Their flow changes noticeably with pressure, and they are most efficient at large, steady flows in big plants.

On top of the compression principle, buyers usually describe compressors in three further ways:

• By lubrication: oil-lubricated (oil-injected) or oil-free compression chamber.
• By drive: electric motor, diesel engine or tractor power take-off (PTO).
• By mounting: stationary base- or tank-mounted, skid-mounted, towable portable, or tractor-mounted.

A "450 cfm diesel portable" and a "75 kW oil-injected VSD screw" are both rotary screw compressors; they simply differ in drive, mounting and control. Keeping these labels separate avoids most of the confusion in compressor enquiries.

Air compressor types compared

The table below gives typical ranges across makes. They are not limits: individual models sit above and below them, and the exact figures must always come from the manufacturer's data for the model being offered.

For side-by-side detail, the technology comparison centre goes further on each pairing.

Reciprocating (piston) compressors

A crankshaft drives a piston up and down a cylinder. On the down-stroke the inlet valve opens and air is drawn in; on the up-stroke the air is compressed until the delivery valve opens into the receiver. In a two-stage machine, air compressed in a large low-pressure cylinder passes through an intercooler before a smaller high-pressure cylinder takes it to final pressure. Cooling between stages lowers the work needed and keeps valve and ring temperatures under control.

Piston compressors are simple, robust and repairable with ordinary workshop skills. They tolerate dusty rooms better than many people expect, and spares such as valves, rings and gaskets are widely available. Their limitation is duty cycle: most are designed to run part of the time and rest in between, and a piston machine forced to run continuously will run hot, carry oil over and wear its valves early.

Variants include air-cooled and water-cooled, oil-lubricated and oil-free, single-, two- and multi-stage, and base-mounted or tank-mounted units. See the reciprocating air compressors hub, how a piston compressor works and two-stage reciprocating compressors.

Rotary screw compressors

Two meshing helical rotors, one male and one female, form pockets of air that shrink as the rotors turn, carrying air from the inlet end to the discharge end. Compression is continuous rather than pulsed, so the machine runs smoothly and is designed for long hours.

Oil-injected screws spray oil into the rotors to seal the clearances, absorb the heat of compression and lubricate the bearings. A separator removes the oil from the air afterwards. Normal discharge temperature is about 75–95 °C. These machines are the default choice for general factory air.

Oil-free screws keep the compression chamber dry, using timing gears so the rotors never touch, usually in two stages with intercooling. Water-injected designs are a further variant. They are chosen when the process cannot tolerate any risk of oil in the air.

Fixed-speed or VSD: a fixed-speed screw runs at constant speed and controls output by loading and unloading. A variable-speed drive changes motor speed to follow demand. Fixed speed suits stable demand; VSD suits demand that varies materially; larger plants often combine both as base load plus trim.

See the rotary screw compressors hub, oil-injected screws, oil-free screws, fixed-speed screws and VSD screws.

Oil-free, scroll and tooth compressors

"Oil-free" describes the compression chamber, not the whole machine: bearings and gears are still lubricated, but sealed away from the air. Oil-free air is specified where air touches the product, the patient or a sensitive process: pharmaceutical, food and beverage, electronics, laboratory, dental and medical air. Even then, intake air carries hydrocarbons and moisture, so drying and filtration are still needed to reach the ISO 8573-1 class the process requires.

A scroll compressor uses one fixed and one orbiting spiral to squeeze air towards the centre. It is quiet, has few moving parts and is usually oil-free, which suits clinics, laboratories and small users. Tooth compressors use two claw-shaped rotors and are a compact oil-free option for moderate flows.

See the oil-free air compressors hub, what oil-free means, scroll compressors and tooth compressors.

Centrifugal compressors

An impeller spinning at very high speed throws air outward; a diffuser converts that velocity into pressure. Plant-air machines normally use two or three stages on an integrally geared design with intercoolers between stages. The compression path is oil-free.

Centrifugals are efficient at large, steady flows but have a limited turndown range before surge, so they work best as base-load machines with screws handling the variation. They need clean, cool intake air and good cooling water, conditions that deserve particular attention in Indian summers.

See the centrifugal air compressors hub and how centrifugal compressors work.

Electric-driven or diesel-driven?

The drive is a separate decision from the compression type. Electric-driven compressors are the normal choice wherever reliable grid power exists: lower running cost per unit of air, no fuel logistics, no exhaust, and simpler maintenance. Stationary plant compressors are almost always electric, and electric portables and skids serve long-term sites with a power supply.

Diesel-driven compressors carry their own engine and are independent of the grid. That makes them essential for construction, road work, pipelines, blasting, borewell drilling and emergency standby. They need fuel management, engine servicing and attention to high-ambient operation; in an Indian summer of 40–45 °C, and above 45 °C on open sites, cooling-package design, clean coolers and correct safety shutdowns (high discharge temperature, low engine oil pressure, high coolant temperature) decide whether a machine runs through the afternoon.

See diesel portable compressors, electric portable compressors and diesel or electric rental air.

Portable, tractor-mounted and borewell compressors

These types are defined by how they are mounted and powered rather than how they compress.

• Portable diesel compressors are screw compressors with their own engine on a towable chassis. The 185–250 cfm class typically runs one or two breakers or a small rock drill at about 7 bar (100 psig); 250–400 cfm at 7–10 bar runs two to four breakers, rock drills or a blasting nozzle; 400–600 cfm at 7–14 bar (100–200 psig) serves crawler drills, quarrying, a No. 6 or two No. 5 blasting nozzles and shutdown standby.
• Tractor-mounted compressors are PTO-driven piston compressors of about 100–140 cfm at 7–10 bar (about 100–145 psig), carried on a 35–60 HP tractor or trolley, for breakers, rock drills and rural construction.
• Borewell and DTH compressors are high-flow, high-pressure screw packages, typically 600–1,500 cfm at 200–400 psig (about 14–28 bar), with special units up to 35 bar (about 500 psig).
• Skid-mounted compressors sit on a fixed frame for long-term site and plant use without a towing chassis.

See tractor and PTO-driven compressors, the borewell and DTH compressor library and stationary vs portable screw compressors.

Booster compressors

A booster takes air that is already compressed, typically at 7–10 bar or from a portable, and raises it further, to about 20–40 bar and beyond. Because the inlet air is already dense, a booster is much smaller than a single machine doing the whole job. Typical uses are PET bottle blowing, pressure testing of pipelines and vessels, and deep DTH drilling where a primary compressor feeds a booster on the rig.

The booster must be matched to the primary compressor's flow and pressure, and both need proper aftercooling and moisture separation. See booster compressors, PET blowing compressors and booster-assisted drilling air.

Compressor or blower?

Blowers move large volumes of air at low pressure, typically below 1 bar, for aeration, pneumatic conveying, drying and combustion air. Using a 7 bar compressor and throttling it down for a low-pressure job wastes a great deal of energy. Equally, a low-pressure requirement does not always mean a small machine: some low-pressure duties need very large flows. See blowers vs compressors.

How to choose the right type of air compressor

Choose the air first and the machine second. Horsepower and the label on the canopy follow from the duty. Work through these steps in order:

1. Define the flow as FAD. Add up what the tools or process actually consume, allow for realistic simultaneity and leakage, and compare offers only on free air delivery at the stated pressure. Comparing one maker's displacement figure with another's FAD is not like-for-like.
2. Define the pressure at the point of use. Work backwards: required pressure at the tool, plus pipe and hose loss, plus treatment loss, plus control band. Every extra bar costs roughly 7% more power, so do not add pressure as a safety margin.
3. Describe the duty pattern. Hours per day, shifts, and how much demand varies. Intermittent duty points towards piston; sustained duty towards screw; variable sustained duty towards VSD; very large steady duty towards centrifugal base load.
4. State the air quality. General tool air, painting, instrument air and product-contact air need different treatment. Decide whether oil-free compression is genuinely required or whether an oil-injected screw with the right dryer and filters meets the ISO 8573-1 class.
5. Decide the drive and mounting. Grid power available and the machine stays put: stationary electric. No power or the machine moves between sites: diesel portable or tractor-mounted. Long project with power: electric skid or portable.
6. Check the site. Ventilation and heat rejection, dust, ambient temperature, electrical supply, space, access for service and, for portables, towing and road access.
7. Decide buy, rent or used. Short or uncertain projects often suit hire; permanent plant air usually suits purchase; a refurbished machine can fit a tight budget or a standby role when its history is known.
8. Plan the lifecycle. Service intervals, spares availability, energy cost over the years and who will support the machine on site.

Quick guide by application

For the screw-versus-piston decision in particular, read rotary screw vs reciprocating compressors.

Common mistakes when choosing a compressor type

• Buying by horsepower or headline cfm. Two machines with the same motor can deliver different FAD at different pressures.
• Running a piston compressor on continuous duty. It will run hot, pass oil and need valves far sooner than it should.
• Oversizing "for safety". An oversized fixed-speed screw spends long periods unloaded, consuming power for no air.
• Raising pressure to hide pipework problems. Fix restrictive pipes, hoses and filters rather than paying for extra bar every hour.
• Specifying oil-free without defining air quality. The compressor is one link; the dryer, filters and distribution decide what reaches the process.
• Ignoring the compressor room or site. Poor ventilation, recirculating hot air and dust cause many high-temperature trips in Indian conditions.

How Reddy Compressors helps

Reddy Compressors has served industry since 2002 from our office at Tarnaka, Secunderabad. We are an independent multi-brand specialist, not an authorised dealer, and we sell, rent and service machines from ELGi, Atlas Copco, Ingersoll Rand, KAESER, Chicago Pneumatic, Doosan and others. Because we are not tied to one maker, we can recommend the type that fits your duty rather than the one we happen to stock.

• Selection: we work from your flow, pressure, duty and air quality to the right type, size and control, and compare offers on a consistent FAD basis.
• Sales: new compressors, and used or refurbished machines where their history supports it. For long-term projects we can supply brand-new compressors to site.
• Rental: our own fleet of diesel and electric machines, including high-pressure units, from 50 to 2,000 cfm and 2 to 30 bar, through our temporary air engineering centre.
• Service: more than 20 service engineers and technicians covering the whole of Telangana and Andhra Pradesh, for preventive maintenance, AMC, breakdown repair, overhaul and air audits.
• Spares: identified from the make, model, serial number, nameplate photo or old part, with fit confirmed before we quote, and supplied across India.

Our team across Reddy Generators and Reddy Compressors has also delivered temporary power-and-air packages for large projects, including multiple high-pressure compressors for an oil and gas development at Kakinada, so we understand how compressors fit into a complete site utility. Call +91 99590 60718 or +91 92468 88812, or email reddycompressors@gmail.com.

What to send us

• What the air must run: tools, machines or process, and how many at once
• Required flow (cfm or m³/min) and pressure at the point of use, if known
• Hours of running per day and whether demand is steady or variable
• Air-quality need: general, painting, instrument or product-contact
• Power available on site, or whether diesel is needed
• Existing compressor details with a nameplate photo, if replacing one
• Site location, duration (for hire) and any timing constraints

Frequently asked questions

What are the main types of air compressors?

The main types are reciprocating (piston), rotary screw, scroll, rotary vane, tooth and centrifugal compressors. The first five are positive-displacement machines that squeeze a trapped volume of air; centrifugals are dynamic machines that use impeller speed. Buyers also group compressors by drive (electric, diesel or tractor PTO), by lubrication (oil-injected or oil-free) and by mounting (stationary, skid, portable or tractor-mounted). Boosters raise already-compressed air to higher pressure.

Which type of air compressor is best for a factory?

For most factories running one or more full shifts, an oil-injected rotary screw compressor is the usual choice because it is designed for continuous duty and gives stable pressure. Choose fixed speed for steady demand and VSD where demand varies materially. Larger plants often use several screws with sequencing, or a centrifugal for base load. Product-contact air in pharma or food may justify oil-free compression with suitable drying and filtration.

Is a screw compressor better than a piston compressor?

Neither is better in every case. A screw compressor suits sustained demand, long running hours and stable pressure, and is quieter and smoother. A piston compressor suits intermittent use, small workshops and some high-pressure duties, and is simple and cheap to repair. Running a piston machine continuously shortens its life, while a screw that runs only a few minutes at a time may never warm up properly. Decide on the actual duty pattern.

What is the difference between oil-free and oil-injected compressors?

In an oil-injected compressor, oil enters the compression chamber to seal, cool and lubricate, and is separated from the air afterwards, leaving a small residual carry-over that filters remove. In an oil-free compressor, no oil enters the compression chamber at all. Oil-free is chosen where any oil risk is unacceptable to the product or process. Both still need drying and filtration to meet a defined ISO 8573-1 air-quality class.

Should I choose a diesel or electric compressor?

Choose electric where reliable grid power is available and the machine stays in one place or on a long project: running cost is lower and there is no fuel to manage. Choose diesel where there is no power, where the compressor moves between sites, or for emergency standby. On hire, the choice often depends on site power, duration and the noise and exhaust limits of the location.

What is a tractor-mounted air compressor used for?

A tractor-mounted compressor is a piston compressor driven by the tractor's power take-off, typically about 100–140 cfm at 7–10 bar (around 100–145 psig) on a 35–60 HP tractor or trolley. It runs breakers, rock drills and small pneumatic tools for rural construction, farm work, trenching and village infrastructure, where a towed diesel portable would be larger or costlier than the job needs.

How do I know what size of compressor I need?

Start with the free air delivery the tools or process consume and the pressure needed at the point of use, then allow for realistic simultaneous use and leakage rather than a fixed percentage. Add pressure losses through pipes, hoses, dryers and filters. If you are replacing a machine, its nameplate and running hours help. If you are unsure, tell us what the air must run and we will work it out with you.

Related pages

• Reciprocating air compressors
• Rotary screw compressors
• Rotary screw compressor selection guide
• Oil-free air compressors
• Centrifugal air compressors
• Scroll air compressors
• Booster compressors
• Diesel portable compressors
• Electric portable compressors
• Tractor and PTO-driven compressors
• Borewell and DTH compressors
• Rotary screw vs reciprocating compressors
• Blowers vs compressors
• Technology comparison centre