February 24, 2016 by crew
Welcome back, folks, for the final article in our “Anatomy of an Air Compressor” series. In Part One we discussed the sorts of compressor available to the hobbyist and a took a high level overview of the operation of a compressor. In Part Two we looked at why a receiver is such a desirable feature, and how it improves the stability of your compressor’s pressure.
Today we shall conclude this series with a look at the various accessories you need to work with a compressor, and a take a brief overview of some routine maintenance.
When working with compressors and airbrushes, you’ll find a variety of connectors and line accessories available for most models. Many times these are classified by the thread type they use. For hobbyists there are three main thread types in use.
On most compressors you will find 1/4” BPT (British Pipe Thread) used for connections, as this is the standard size used for most air tools. However, airbrushes tend to use a smaller pitch thread 1/8” BPT, so it is common for a compressor to come with a 1/4″ to 1/8” BPT adaptor.
The third commonly found thread size is 5mm, often used by Badger brand airbrushes. Unless you use Badger airbrushes exclusively it makes sense to use a 1/8” BPT airline and use a 5mm to 1/8” adaptor to connect the airbrush.
As mentioned, there are various inline (i.e. they are mounted on the air line) accessories. Perhaps the most useful is a quick release connector, which allows you to quickly swap airbrushes and swill out the paint cup between colour changes. These are a two-part connector, featuring a male and female end, where you pull on the female bezel to release the male end.
The next most common accessory is a second moisture trap mounted on the airline (in addition to one mounted on the receiver). Not only does this give you additional protection from condensed water, it also extends the handle of the airbrush which can make it more comfortable to use for prolonged amounts of time.
Inline pressure regulators are also available, but in my experience they are quite fiddly and do not give as good a performance as the one mounted on the compressor.
The airline is the hose that connects the compressor to your airbrush. Like most things with a lead, buy as long an airline as you need. Two to three meters is generally the right length, as it gives you greater freedom where you position your compressor (but not too long, you don’t want to be tripping over it).
A word of warning though … avoid buying a very long airline (greater than five meters), especially if you don’t have a receiver on your compressor or you have a very small compressor. This is because an overly-long airline can lead to pressure drops and force your compressor to work harder to keep everything at the pre-set static pressure.
Airlines come with a variety of different connector sizes at either end, but the most common are 1/8” BPT at both ends, ranging to 1/4” BPT. Airlines tend to come with a braided protective sheath, which not only helps brace the hose so excess pressure doesn’t deform it, but also helps prevent snagging and knotting.
The moisture trap is often incorporated into the pressure regulator’s body, and is used to trap any moisture “squeezed out” of the compressed air. Water can also be present due to atmospheric humidity and from being condensed out during the compression process due to the heating caused by the compression of a gas.
It is vital to remove any moisture present in the compressed air before it gets to your airbrush. Otherwise the moisture will be blown at what you are painting and ruin the surface finish. For this reason, it is not uncommon to see a secondary moisture trap mounted on the handle of the airbrush, or just behind the female end of a quick release valve.
The check valve or one-way valve is used between the compressor and receiver to isolate the stored air in the receiver from the compressor. This part is sometimes internal and sometimes external, and not every compressor with a receiver has one. As mentioned in Part One, this is the sort of component which makes the difference between a “budget” compressor and a premium model.
The check valve prevents damage to the compressor by having the receiver’s stored pressure forced back through the compressor. This stops the pressure being too great for the motor to overcome when the pressure switch turns it on. If you have a cut-out pressure of 60 psi, then the motor has to generate over 60lbs of force to turn the piston.
By isolating the receiver’s pressure from the piston, you won’t stress the motor so much when it starts up, and the less you stress the motor the cooler it remains, and the longer it will last. Again, by reducing the heat generated by the motor, the greater the increase in the duty cycle.
Check valves also often incorporate a dump valve as well, this is used to release any pressure between the compressor and check valve when the compressor stops either by a pressure switch being engaged or by being turned off, this prevents any resistance to starting the compressor motor when its turned back on.
Maintenance required for most hobby compressors is minimal. Most are oil-less so you don’t have to top oil or anything like that. Most of what you have to do to ensure reliable operation consists of cleaning or replacing air filters and emptying moisture traps, as well as following any maintenance instructions in the manual.
The most common fault you may find in your compressor is an air leak. These can often be cured by tightening the connection and resealing the threads with PTFE tape. There are several methods of detecting a leak but the one I use most often is as follows.
Get some washing up liquid and an old large paint brush, put some washing up liquid in a suitable pot, then paint liberally around the connection you suspect is leaking. Now turn the compressor on and if its leaking you will see the washing up liquid you have applied starting to bubble.
To fix it, simply unscrew the connection, pick out any sealant of the threads with a tooth pick, then wrap the threads with PTFE tape in an anti-clockwise direction, ensuring the tape doesn’t cover the internal bore. By wrapping the tape anti-clockwise it tightens it up when you screw it in.
Now tighten the connection back down firmly, being careful to not cross-thread the connection. You don’t want to over-tighten the connection as airline fixtures tend to be made of brass so will sheer if you over tighten, and the thread will strip if its crossed. If the spanner has left a mark on my palm from tightening, then that’s about right.
Finally, reapply some washing up liquid and check for no bubbles. If you are still getting bubbles try more tape and cinching the connector down harder. If it still leaks following that, then you either need to buy a spare part as the old one is no longer air tight, or you need to get a specialist to look at it.
This concludes our series on the anatomy of an air compressor. Hopefully you now know what features are worth looking out for, what some of the key differences are between budget and high end models, and have a fair idea on what if any maintenance you need to conduct on your compressor.
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"For hobbyists there are three main thread types in use..."
"Hopefully you now know what features are worth looking out for, what some of the key differences are between budget and high end models..."