Anatomy Of An Air Compressor Part One – Introduction

February 10, 2016 by crew

Greetings, fellow Beast of War subscribers. Thinking of making that big jump into air brushing?

Airbrushing #1

I am BoW community member @nakchak, and in this article series I hope to shed some light on the anatomy of a compressor, what each part does, and hopefully help you be better informed when you come to buy one.

Explanation Of Terms

The following terms will be used throughout this series…

  • Receiver – also known as an air tank, stores compressed air and smooths the pulsations from compression from the output pressure.
  • Static Pressure – this is the output pressure when your airbrushes valve is closed.
  • Working Pressure – also known as operating pressure this is the pressure available when your airbrush valve is open.
  • Duty Cycle – the amount of time your compressor can run for without having to be left to cool. This essentially dictates how long you can be airbrushing before you have to let your compressor cool down. If your compressor has a 30-minute duty cycle then it means for every thirty minutes the motor runs for, you need to give it thirty minutes to cool down. In effect you get half an hour of painting time.
  • PSI – Pounds per Square Inch a measure of pressure, other scales are sometimes used such as bar, but most tutorials refer to psi so shall I.

Types Of Compressor

In the hobby market there are effectively three sources of compressed air for hobbyists to use…

The first method is to use cans of propellant (Tinned Wind), These are cheap cans of aerosol propellant and come with most starter sets without a compressor. These are a sure fire path to frustration, whilst they can certainly power your airbrush, they suffer from two major problems, firstly the pressure is hard to regulate and hitting and exact PSI is nigh on impossible.

Airbrushing #2

Secondly and more fundamental is the cooling caused by using the propellant (think ice on a propane tank in the middle of summer), as the can cools, the pressure inside drops off. This means you crank open the valve some more to maintain pressure and thus it cools some more. Eventually you have the valve fully open and still your airbrush is spattering. All you can then do is swap cans or wait for the cold one to warm up.

The second common option is to use a micro compressor; these are generally cheap (£45 – 60) direct from Hong Kong eBay bargains. Micro compressors generally use a bellows-type compressor, which works in a very similar manner to aquarium air pumps. Often sold for nail art, they are small palm-sized units and are quite good if you are travelling a lot, but suffer from some fairly hefty drawbacks as well.

They tend to have a low working pressure (< 30psi) due to the compression mechanism used and the lack of a receiver means they suffer from pulsation of the output pressure. They also tend to run continuously, so are prone to wearing out and getting warm with use.

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They are however quite quiet in operation and take up very little space in storage, so can be easily chucked in a rucksack if you paint on the go, and can be quite useful as a backup air source in case your main compressor fails.

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The third and perhaps most common are piston based compressors these are the sort you most often see in starter sets and on sale in hobby shops. They are miniature versions of workshop compressors (these can also be used without issue, just the noise means you would be unlikely to not be banished to the shed) with a small footprint, which vary in price from £45  to…well, the sky is the limit based on quality.

The cheapest models tend to not have a receiver but mid and top range models do. The main difference between bottom and top range is the quality of ancillary the components (regulators, water traps and pressure switch) used, the engineering and the materials used, as a result of this the duty cycle varies greatly between budget and premium models.

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Cheap compressors often have a thirty-minute duty cycle (i.e. thirty minutes of the motor running) which must be observed otherwise the piston can warp in the bore and write off the compressor as well as burn out the motor, if either happen you will need a new compressor. More expensive models tend to have a higher duty cycle with top of the range being rated for continuous use.

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The main drawbacks with piston based compressors are the noise they produce in operation, especially if you don’t have a receiver, as the compressor runs as you use your airbrush, and the heat generated by compressing the air and the friction of a piston running in a bore, which is why most piston based compressors have cooling fins on the piston housing, and require a cool down period.

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There is a fourth method air generation; the DIY approach, which allows you to make a virtually silent compressor for under £100 but is beyond the scope of this article.

Operation

All air compressors share the same basic function, to take in atmosphere (aka air), compress its volume and release it to a device to do some work. Work being powering a tool, or transferring air into a container like a tyre.

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The following assumes a piston based compressor with a receiver; When the compressor is turned on and the receiver is empty/at one atmosphere pressure inside, the motor will run driving the piston until a cut off pressure is reached which disengages the pressure switch and cuts the motor. At this point the pressure inside the receiver is at the threshold pressure (usually about 60 – 80psi on airbrush compressors).

As air is let out of the receiver by opening the air valve on your airbrush, via the pressure regulator, the pressure inside the receiver drops, until it reaches a point where the cut in pressure (40-50psi) is reached, which re-engages the pressure switch and restarts the compressors motor, which runs again until the cut out pressure is reached.

When air exits the receiver its generally at too high a pressure to be useable for painting, so has its flow restricted by an adjustable regulator that allows the user to set the pressure from 0psi to the cut out pressure of the pressure switch (you cannot exceed the cut out pressure, as that’s the limit of compression set by the manufacturer).

Some compressors come with an output direct from the receiver, which is used as a high pressure line, and is often used to aid cleaning and paint changes.

The operation of a receiver-less compressor is virtually the same other than it’s the pressure in the piston bore which cuts in and out the motor, and as it’s such a small volume opening and closing the airbrushes air valve is all that’s needed to start and stop the motor.

A micro compressor tends to use the speed of the bellows operation to dictate the pressure generated, some run continuously like an aquarium air pump, others have a pressure switch. They all tend to offer pressure regulation by adjusting the speed of the motor and so offer very little in the way fine of pressure adjustment.

More To Come…

This concludes the first part of this series. In the next instalment we shall look at why a receiver is highly recommended on an air compressor and why pressure stabilisation and regulation should be considered before buying a compressor.

Tim Chubb

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"In the hobby market there are effectively three sources of compressed air for hobbyists to use..."

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"There is a fourth method air generation; the DIY approach, which allows you to make a virtually silent compressor for under £100 but is beyond the scope of this article..."