Compressor Buyers' Guide

By: Steve Kealy

Presented by
  • Earthmovers & Exacavators

Kaeser Mobilair M31E electric compressor Some compressors use electric motors, like this Kaeser Mobilair M31E Kaeser Mobilair M31E electric compressor

If you’re looking for the best air compressors for sale in Australia your first stop should be this handy guide.


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Anyone who passed year six physics will know that liquids cannot be readily compressed, while gasses can. Try and compress a liquid and the pressure exerted is just applied to the vessel containing the liquid – whereas gas can be forced into a smaller space, though it gives off heat in the process and, if squeezed (or cooled) enough, becomes liquid.

Looked at in reverse, if you heat a liquid it becomes a gas: a litre of water will create 1700 litres of steam. Put it into less than a 1700-litre container and you have the makings of a steam engine. Thus, liquids can be used to transmit energy, while gasses can be used to store it.

Think of hydraulic systems like a car’s brakes, where pressure exerted at the foot pedal is transmitted to the brake pads, versus a steam-driven locomotive.

The job of compressing gas is the work of the imaginatively named compressor – predictably, there are several different types. In common industry, most are interested in compressing normal air.

During the process, it’s not uncommon for moisture to be forced out and to collect in the bottom of the holding tank; if that’s made of steel, the risk of corrosion is high and most compressors will have a moisture trap to remove accumulated water and/or oil.

It goes without saying that high-pressure gas of any type must have appropriate high-pressure hoses and fittings, and they must be routinely inspected and immediately replaced if found to be chafed, worn, kinked, damaged or abraded.

 

What is an air compressor?

An air compressor converts the power of an electric motor, diesel or petrol engine into potential energy stored in pressurised or ‘compressed’ air. By one of several methods, an air compressor forces air into a storage tank capable of withstanding high pressure.  Depending on parameters, this may be certifiable pressure vessel.

When pressure in the tank reaches an upper limit, the air compressor switches off and the compressed air is held in the tank until needed. The energy contained in the compressed air can be used for various things, using its kinetic energy for propulsion or other applications. Once the tank pressure reaches a lower limit, the air compressor turns on again and re-pressurises the tank.

A compressor’s capability is measured in kilopascal (kPa) and litres of air per minute or cubic feet per minute (CFM), while tank capacity is measured in litres. A cubic foot is 28.32 litres. Used industrial units will often also record the number of hours of use they’ve had.

 

Applications

Air compressors can supply high-pressure clean air to inflate heavy-duty vehicle tyres or drive various air-powered tools, from pneumatic rattle guns to industrial jack hammers or piledrivers.

Lower-pressure air can drive construction tools such as nail guns, be piped to a submerged surface-supplied diver or large-scale industrial processes such as blast furnaces, or to aid oxidation in petroleum, coking or cement plant purging systems. Older readers might remember how office memos were shot through buildings in little capsules through complex piping using compressed air. Nowadays, that’s done by email.

 

Kaeser Classic 210 reciprocating air compressor
Reciprocating piston air compressors like those in Kaeser’s Classic range require minimal maintenance


Reciprocating piston air compressors

The reciprocating piston designs are essentially a variation on a pump and operate much like an internal combustion engine, but without the fuel and ignition components: air is drawn in, compressed and exhausted – not to atmosphere, but via a one-way valve into a pressure-capable tank.

There are two alternative piston designs: oil-lubricated and oil-less. The typical oil-less system is more technically developed and delivers better-quality, cleaner air, but is more expensive, louder, and has a shorter operating life than oil-lubricated pumps.

Most commonly, compressors are driven by electric motors or petrol/diesel engines; some smaller medical compressors use electro-magnets. Obviously those using an engine are more portable but are often noisy and need ventilation, whereas electric machines are smoother, quieter, and are more suited to fixed locations such as workshops and factories.

 

Atlas Copco GR 200-FF rotary screw air compressor
The Atlas Copco GR 200-FF is a two-stage rotary screw air compressor. Photo: Atlas Copco


Rotary screw air compressors

A rotary-screw compressor uses two meshing helical screws – known as rotors – to compress the gas, and most commonly replace piston compressors where large volumes of high-pressure air are needed: either for large industrial applications or to operate high-power air tools. The compression process of a rotary screw is in a continuous sweeping motion, so there is very little pulsing or surging of air flow, as occurs with piston compressors. Many automotive superchargers use a similar principle.

In a dry-running rotary-screw compressor, timing gears keep the male and female rotors exactly aligned. In oil-flooded rotary-screw compressors, the oil gives a hydraulic seal and transfers mechanical energy between driving and driven rotors. Efficiency is dependent on precise clearances between the rotors and the chamber for sealing compression cavities, and very high rotational speeds are needed to cut leakage flow rate losses.

Modern screw compressor rotors have different profiles: the male side has convex lobes which mesh with the convex cavities of the female rotor, and usually the male rotor has fewer lobes than the female rotor, so it spins faster.

Rotary-screw compressors are compact and range in pumping capabilities from a few dozen litres per minute to tens of thousands of litres per minute.

 

Becker DT 4.40K rotary vane air compressor
Becker DT 4.40K rotary vane air compressors have a simple design with one shaft and direct drive


Rotary vane air compressors

A rotary vane compressor uses an elliptical slotted rotor in a cylinder. The rotor has slots along its length, with each slot containing a vane.

The vanes are forced outwards by centrifugal force when the compressor is operating, and the vanes move in and out of the slot because the rotor’s axle or crank is eccentric to the casing in which it spins. The vanes sweep the cylinder, sucking air in on one side and ejecting it on the other.

In general, vane compressors are used for smaller applications where space is an issue, but generally they are not as efficient as rotary-screw compressors. With appropriate design and components, rotary vane compressors are suitable for moving moist air in places like mines.

 

Ingersoll Rand C1000 centrifugal compressor
The Ingersoll Rand C1000 centrifugal compressor has few moving parts and is particularly suited for high-volume applications


Centrifugal air compressors

Centrifugal compressors are a type of dynamic or turbo-compressor, usually with a radial design. They’re common in very large applications and work slightly differently to other machines: unlike displacement compressors which work at a constant flow, dynamic compressors work to deliver a constant pressure, although their performance is affected by external conditions such as variations in intake air temperature.

Air is drawn to the middle of a rotating impeller’s radial blades and is pushed to the centre. This radial movement of air results in a pressure rise and the generation of kinetic energy. Before the air is fed to the centre of the impeller, kinetic energy is converted into pressure by passing through a diffuser and volute – a curved funnel that increases in area as it approaches the discharge port.

Each stage takes up a part of the overall pressure rise of the compressor unit. Depending on the pressure required for the application, a number of stages can be arranged in a series to achieve a higher pressure. This multi-stage arrangement is often used in the oil and gas processing industries. In wastewater treatment plants, low pressure, single-stage applications are used to achieve desired pressures.

In modern centrifugal air compressors, high-speed electric motors are used to drive the impellers. This results in a compact compressor without a gearbox needing a lubrication system, thus making it oil-free and appropriate for applications that require 100 per cent oil-free air.

 

Buying advice

When buying a new or used industrial grade compressor, it’s worth dealing with a reputable firm with runs on the board. They’ll have the experience to know what’s good and what’s not, and, with a hard-won reputation to protect, they’ll not risk dudding a client by knowingly selling on a defective or sub-standard unit for the sake of a few hundred dollars.

There are specialist aspects of compressors that require specialist knowledge, particularly around the certification and ongoing routine testing of pressure vessels. Thus it’s worth contacting a reputable dealer or plant broker such as Maxon Machinery or NJ Devine in NSW or Applied Machinery in Victoria – they all have new and used stock inventory.

Their resident experts will be able to make informed suggestions about what sort of compressor would suit your application, find it, and arrange to have in installed and commissioned too if necessary.

Happy blowing!

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