February 17, 2016 by crew
Greetings, fellow Beast of War subscribers. In Part One of this series we discussed the sorts of compressor available to the hobbyist and took a high level overview of the operation of a compressor.
Today we’ll look at why a receiver is a highly desirable feature of a compressor and why pressure stabilisation and regulation is important to miniature painters.
The receiver or air tank’s purpose is to receive and store the compressed air from the compressor, whilst not strictly necessary it is a highly desirable feature to have as I shall try to explain.
The receiver has several purposes, firstly it allows your compressor to work in a stop start manner rather than continuously whilst air is required, as the compressor runs until a preset pressure is reached inside the receiver then cuts off. Once the pressure drops below a preset minimum pressure, the compressor is switched back on.
This is advantageous as your compressor only runs to maintain pressure inside the receiver, which gives the compressor a greater duty cycle by only running when needed, and when not running allows it to cool down. The next major advantage is that it removes ‘pulsation’ from the output pressure and increases the stability of pressure.
What Is “Pulsation”?
Pulsation is inherent from the compression mechanism of a compressor, either piston or bellows based compressors suffer from this phenomenon. It is probably best explained with an analogy to a balloon.
When you inflate a balloon, your lungs are acting as an air compressor, and the balloon as a receiver. When you exhale you start off at the highest pressure you can generate and trail off to a lower pressure as you run out of puff, the transition from high to lower pressure is the pulse.
If you connect directly to output from a compressor, then the transitions from high to low pressure are present at the output of the air brush. This leads to inconsistent delivery of paint to your mini, while less of a problem when painting larger surface’s, it can be really noticeable on the small scales we deal with.
Just as it takes several puffs to inflate a balloon, a compressor takes some time to reach a working pressure, the higher the pressure the greater the number of strokes the piston takes to reach this pressure, being entirely possible that its unable to maintain that pressure with the air brushes valve fully open.
Without a receiver your compressor has to work harder to maintain this pressure which leads to increased wear and tear and the limits of the duty cycle being met sooner. You also can have dramatic drops in pressure when the airbrushes valve is opened vs. when its closed as you have nothing kept in reserve to meet demand.
By feeding your compressors output into a receiver it helps remove the pulsations present and increases the stability of the generated pressure. The pulsations are lost as each stroke of the compressor’s piston just tops up the pressure present the receiver. Which ensures a consistent pressure (between the cut in and out pressures) is always present at the regulator.
Finally, it can act as a water trap and helps in drying the compressed air, which due to the heat generated during compression, can lead to water condensing in the output. Most receivers have a drain plug for this reason, and should be regularly drained, as the last thing you want when spraying is water in the airline as it ruins any paint job.
If you already have a compressor without a receiver, there are aftermarket options available to hobbyists such as the ones made by Sparmax. There are also various examples of people using empty drinks bottles (I use a 3-litre Coca-Cola bottle on my scratch built compressor) as a receiver.
This is perhaps the most used part of any compressor, the regulator’s job is to regulate the output pressure and is an adjustable valve which lets you adjust the static (and working) pressure of the compressor. They often have a manometer (pressure gauge) to set precisely the output pressure.
It’s worth noting that regulators are directional, meaning that they have a high and low pressure side to them. This is usually marked with an arrow on the body, the direction of the arrow indicates the direction of airflow. You want to connect the airline to the side the arrow points to and the compressor to the other end.
When setting the pressure, it is vital to depress the trigger on your airbrush to check the working pressure as the static pressure will be higher than the working pressure, so if you set the pressure without opening the airbrushes air valve you will find that the manometer’s needle drops when you start using the airbrush, which can cause the paint to spatter.
Some regulators come with a bypass valve and some compressors come with a double regulator, this is really useful for cleaning, as it helps to clean with a higher pressure than you paint with as the added pressure can help dislodge grot and its quicker to spray a cupful of cleaner.
The pressure switch is the “brains” of the compressor, and simply turns the motor on and off based on the pressure in the receiver or piston head depending on compressor design. There are two sorts available, fixed and adjustable.
Fixed switches have a pre-set cut off and cut in pressure and are typically mounted directly to the receiver or to the piston head.
Adjustable switches allow you to adjust the maximum pressure before it cuts out, and on high end models also allow you to adjust the cut in pressure. This useful if you have a requirement for exact pressure, but is pretty much overkill for a hobbyist.
The safety or “pop off” valve is a safety critical device which is designed to pop open if for some reason the pressure switch didn’t engage. If you didn’t have one and your switch failed your compressor would in effect become a bomb, and could explode with dangerous consequences.
They often have a red collar on them with a split ring you can pull on to release pressure. It is a good idea to depressurise the receiver when you finish your painting session by pulling on the valve until you no longer hear a hiss.
This concludes the discussion on why you should have a receiver on your compressor, and the main pressure effecting parts of your compressor. In the final part of this series we shall be looking at the various accessories and ancillary items you need to get the most out of your compressor and airbrush, as well as some routine maintenance advice.
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"Without a receiver your compressor has to work harder to maintain this pressure which leads to increased wear and tear and the limits of the duty cycle being met sooner..."
"If you didn’t have one and your switch failed your compressor would in effect become a bomb, and could explode with dangerous consequences..."