Workshop focus: What you need to know when buying a compressor

After the sledge, vice and vise-grip, an air compressor is of the most regularly used tools in the agricultural workshop – yet it is often overlooked.

Typically day-to-day jobs include inflating tyres, blowing down machinery or operating pneumatic tools – like (air) impact wrenches or paint-spraying equipment.

Some modern milking parlours also use compressors – to operate pneumatic gates or meal feeders.

A typical on-farm compressor will be driven by a tractor’s PTO, an electric motor or an engine. A crankshaft is connected to one or more pistons; these ‘pump’ air from the atmosphere through valves into a tank, where it is stored under pressure. Drive from the power source can be either direct or via a belt(s).

Other designs exist, such as screw or scroll compressors, but these are more usually found in specialised industrial applications – due to their considerably higher cost.

Single-cylinder designs are the most cost-effective; twin-cylinder versions tend to be quieter and smoother. Twin-cylinder compound models (where one cylinder feeds the next) tend to be more efficient, producing more air and/or higher pressure for the same power consumption.

Single-phase (electrical) power supplies can limit potential compressor size but, if power supply is an issue, some manufacturers offer twin-motor, twin-pump units. In this configuration one motor starts – then, after a few seconds, the second motor kicks in. This helps to reduce start-up current demand (while maximising air output).

The purpose of the tank is to allow pressurised air to be drawn off as required and in greater volume than the compressor’s piston(s) could supply on its own. The presence of the tank also enables the compressor to stop pumping when demand is low.

The larger the tank, the more air can be drawn before the compressor must re-start (or ‘re-prime’). Opting for a small tank may be false economy.

Key pointers

When buying a compressor, the main things to look out for are the size of the tank and the air output. While tank capacity is straight-forward (typically measured in litres), air output can be measured in different ways – which is where confusion can arise.

Traditionally, compressor output was measured in cubic feet per minute (CFM) but most modern compressors will also list output in litres per minute (L/min). A useful rule of thumb is that 3.5 CFM equals 100 L/min.

Motor horsepower or tank size is not an accurate measure of output! A low-spec compressor may have a big motor but a low air-flow.

On the other hand, portable machines with a high output motor and pump and a small tank may appear under-powered but can pack a surprising punch.

Free Air Delivery (FAD) is a measure of output of compressed air from the machine per minute at a certain pressure (typically 90 psi / 6.2 Bar) – re-expanded and collected.

Some manufacturers ‘cheat’ slightly, quoting FAD at lower pressures to make their product look better – so watch out for this.

The duty cycle of a compressor is measured as a percentage, usually per 10 minutes of operation.

A compressor with a 50% duty cycle can only run for five minutes in 10-minute period; it will need the remaining five minutes to cool down.

Cheaper models may have a duty cycle as low as 25%, while premium, industrial-spec machines should be in the 75-100% range.

Running a compressor beyond its duty cycle may cause it to overheat and burn out; so, if you are planning on air-hungry work, ensure your compressor’s duty cycle (as well as its air output) is up to the job.

Moisture problems

As compressors operate, the action of compressing, heating and cooling air causes water to build up in the tank. This must be drained periodically to prevent corrosion or frost damage.

If you’re planning on spray painting, budget for a moisture trap / oil separator – to remove droplets of water or oil from the air supply (to avoid contaminating the paint finish). Premium, industrial-spec machines can be specified with a dryer unit; it chills and filters the air to remove almost all moisture.

On the other hand, air tools such as impact wrenches operate best with some oil in the air supply; in-line oilers can be fitted to pipework to ensure adequate lubrication.

If you’re considering a portable engine-driven compressor, as used by mobile mechanics and service technicians, look for automatic throttle control and vibration damping. If you’re going to be working beside one of these machines (for any length of time) you’ll quickly appreciate the noise reduction!

Air-hungry tools

Some air tools are very demanding of air flow. When purchasing such a tool, consumption will be listed by the manufacturer. Some consumption figures will be quoted as averages; however, continuous use may call for much more air.

A typical 1/2in (air) impact gun will need 4 CFM (113 L/min) for average use. If running non-stop, this figure can be as high as 15 CFM (425 L/min).

Typical ‘average’ workshop tool air consumption: