A compressor runs workshop tools, spray guns, glass and window machinery and many other machines, so it is one of the most common items on the used market. The air receiver, the tank that stores the compressed air, is a pressure vessel, so the condition of the receiver matters more than anything else on the machine.
Work out what you need
- Air output. The important figure is the free air delivery (FAD), shown in litres per minute or cubic feet per minute. It must meet the total demand of the tools you run at once. A big tank on a low-output machine will run out of air.
- Pressure. Check the maximum working pressure on the data plate suits your machines.
- Power supply. Smaller compressors run on a 13 amp plug or a single-phase supply. Larger ones need three-phase.
- Oil-lubricated or oil-free. Oil-free compressors suit spraying and clean air uses. Oil-lubricated machines usually last longer under heavy use.
The air receiver
- Read the data plate on the receiver, which shows the maker, the year, the serial number and the maximum pressure.
- Look for rust, pitting and dents, especially around the base, welds and the drain. A tank that has been left without draining can corrode from the inside.
- Check that the drain valve opens and that the water from it is not heavily rusty.
- Check the safety valve is present and not painted over, and that the pressure gauge works.
The pump and motor
- Run the compressor from empty and time how long it takes to reach full pressure. Compare the time with the stated output.
- Listen for knocking, rattling or a squealing belt. Check the belt tension and condition on belt-driven machines.
- Check the oil level and colour on oil-lubricated machines. Milky oil suggests water getting into it.
- Check the hours counter if there is one, and ask when it was last serviced and when the oil, filters and valves were last changed.
- When it reaches pressure, shut it off and listen for leaks. The pressure should hold, not drop quickly.
The law: pressure systems
The Pressure Systems Safety Regulations 2000 apply to compressed air systems that include a receiver above a certain size and pressure. In general, a written scheme of examination is needed when the receiver’s maximum pressure in bar multiplied by its volume in litres comes to more than 250. That covers most workshop compressors: a 100-litre receiver at 10 bar comes to 1,000. The user, or the owner for a mobile compressor, must have the scheme drawn up by a competent person, and the receiver must be examined in line with it. Ask the seller for the scheme and the latest examination report. If you buy a compressor with no record of examination, plan to have the receiver examined before it is used. HSE publishes guidance for small businesses on safe use of compressed air.
Paperwork and payment
- Check the serial number on the machine matches the paperwork.
- Get the manual, the examination records and any service history.
- Pay by bank transfer once you have seen the machine run.
- A large compressor is heavy, so plan its transport. Read the guide to transporting used machinery.
Summary: buying a used compressor
Check the output against what you need, the condition of the air receiver and its examination record, the safety valve, the drive, and how quickly it reaches pressure. Then browse used compressors for sale, or read the general guide to what to check when buying used machinery.
Used compressors for sale
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