When it comes to sizing a compressor for your application there are a few pitfalls to watch out for. The most common is people believing a bigger vessel means more air, which is not entirely true. The combination between the pump output (CFM) the vessel size (v) and the application is what is important. There is also the maximum pressure of the compressor (psi or Bar).
In a normal automotive application you will require air, at regular intervals during the day, but rarely constantly. All our compressors are fitted with a pressure switch which on most models turns the compressor on at 7 Bar and off again at 10 Bar. Ideally you would want your compressor operating around 80% of the time. Any more than this and the motor unit could be damaged and any less you risk water condensation in the oil, which again is not good for the unit.
As already said, 10 bar (or 150psi) is normally adequate for most automotive applications. Commercial Tyres however can operate at 120 or even 130psi in which case a 13 bar compressor is required. The decision on the pressure output is a simple one. Think of what the highest pressure you will be inflating tyres to or operating tools at and then go for a pressure slightly above that.
The output required is again a simple calculation. Every piece of equipment you power by compressed air has a CFM rating. This describes how many CFM of air the tool requires to operate effectively. Let’s say for instance you have the following, potentially all happening at once;
Tyre Changer – 4 CFM
Air Tool – 4 CFM
Tyre Inflation – 2 CFM
This gives a total of 10 CFM. Therefore a compressor of around 12CFM FAD will be more than adequate for your situation. Alternatively, if you are a single operator and none of the above will be happening simultaneously you can use a 6-8 CFM compressor instead.
The size of the vessel does not directly correlate to the supply of compressed air. In the above example you could use a 20 CFM compressor which supplies air at a quicker rate than it is used without a vessel at all, although this is not recommended in practice. The only time vessel size over 200 litres become a requirement is if you require a large amount of air for a short period. A small 6cfm pump unit could fill a 500 litre vessel to 10 bar in about ½ hour so if you were using say 4000 litres of air in 2 minutes but only once an hour, you don’t need a big pump unit, just a big vessel.
It’s the pump units that increase the price of a compressor more than the vessel but getting the combination right is critical when selecting you compressor.
Other factors in selecting a compressor are more to do with workshop layout than performance. Weight, Dimensions, noise levels will all need to be considered in relation to where the compressor will sit. For more information view our “Different compressor types” article.