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Using Air Compressors

To a modeler, an air compressor exists for one thing only, to power our airbrushes† (or sometimes to air up our tires for our bicycles or cars to get to the hobby store). But, so much has been written about them or asked in forums about air compressors, that I hope this article will take my years of selling, using and fixing air compressors to help you fellow modelers. By the way, I am a novice modeler, at 56, and still intimidated by my airbrush. But, letís go forward and explain and de-mystify air compressors.

Air compressors compress air. They take in room air, or in very dusty locations, they can have an outside intake with a rain cover. So, from the very beginning, we have to pay attention to the intake filter. Now, this could be a piece of sponge or a pleated paper filter, or†even metal mesh.

Clean your filter! Remember, like a computer, GIGO, garbage-in garbage-out. Sponge intake filters can be cleaned in soap and water and THOROUGHLY dried (or replaced). Sometime a drop or two of oil squeezed into a clean sponge filter helps traps dust and dirt particles. Metal mesh intake filters can be washed and cleaned, if dried quickly to prevent rusting. Paper air intake filters need to be replaced, just like your automobile intake filter or your furnace filter.

Myth 1 - A dirty filter traps better because it closes up the gaps in the filter. Yes, but it makes your air compressor work harder and hotter and is not worth the trade-off. Air compressor salespeople love this one, because it sells more air compressors.

Now that the air has been taken into the compressor itself, we can look at the different kinds of compressors. Different style air compressors are rotary vane, rotary screw, reciprocating (with a piston or pistons moving back and forth) and diaphragm (a flexible fabric reinforced rubber piece that flexes in and out). Reciprocating compressors come in single-stage which are usually good to 100 PSI, or two-stage, with an intercooler between the stages, for pressures higher than 100 PSI. Then, you have the lubricated types (lubed) or the non-lubricated types (non-lube). The non-lubricated compressors use a diaphragm, if itís a diaphragm compressor pump, or Teflonģ piston rings if itís a reciprocating compressor pump. An advantage of non-lubes is that you canít have an oil problem. The disadvantage is that Teflon piston rings and diaphragms have a shorter life than a properly lubricated piston (reciprocating) compressor. The disadvantage of a lubed unit is carryover oil (see coalescing oil filters later in this article).

The lubricated compressors require oil to lubricated the moving parts, like the bearings or bushings on the crankshafts or the piston rings. Use the manufacturers recommended oil only. Hereís where intake dirt can damage the bearing surfaces or the walls of the cylinder or cylinders.

Air compressors that are lubed need to have the oil changed regularly, just like we should be doing with our automobiles. Iíve gone back to oil changes in my car every three months regardless of the miles, because Jiffy Lube bribes me with two free frozen custards with each oil change.

What do CFM and PSI and HP and gallons mean? CFM means Cubic Feet per Minute and indicates the volume of compressed air the unit produces, usually stated at 60 PSI. PSI is pounds per square inch and represents the pressure of the compressed air. HP is horsepower and this, of course, is the power of the compressor. Gallons refer to the capacity of the tank.

Myth 2 - Horsepower is king. This is a marketing ploy. The output or CFM at pressure is what counts. A 5 horsepower compressor can put out less usable CFM at pressure than a 3 horsepower compressor. Remember! CFM at pressure, not just CFM (another trick us scurrilous salespeople use).

What about a tank? Thatís your decision, but getting one is a good decision.†A tanks stores compressed air. It also dampens the pulsations caused by the pumping action of the compressor. If nothing else, it keeps the compressors from having to run continuously and gives your ears a break. It often allows the air to cool a bit and drop out some water before it goes down the line.

A tank should have a welded-on label from the ASME (American Society of Mechanical Engineers) or equivalent for the rest of the world. It assures that the tank was made in accordance with standards that make the tank safe. Big deal? Well, take the square inches of the surface of your tank and multiply that by your PSI (remember, pounds per square inch). You have a bomb in your modeling area!

Drain your tank regularly to prevent it from rusting out. When the hot compressed air expands into the tank, it cools and moisture drops out, just like a spray can of paint gets cold after using it.

Myth 3 - If the water drops out in the tank, then that should be sufficient to keep water out of my paint. Sorry, this is a common myth. The water leaving a tank is still fully saturated. Every time compressed air expands, even†after an expansion into a tank, and expansion into an air filter, it still is fully saturated and will drop out moisture the next time it expands (like coming out of your air-brush).

Now, itís time to leave the tank and head toward the finish line. Other than screw-in fittings, your choice is to use connectors; those pop/on-pop/off brass or silver colored devices.

Connectors have either a male, female or barbed end for, respectively, female, male or hose connections. Make sure the type and size match what you want to join it to.

The business ends of connectors are NOT the same. Make sure the parts that join together are the same profile. There are about three very common, but different profile connector ends on the market.

When you use threaded connectors, you shouldnít need to wrap the threads with Teflon tape, as the brass is soft enough for the male and female threads to seal to themselves. But, go ahead, for the few cents a roll of Teflon tape costs. Itíll help cut down the hissing and the work the compressor has to do. Remember, wrap the tape just a few times around the threads in the direction where screwing on the other piece helps push down the tapes, rather than unraveling it.

As the compressed air leaves the tank, you want to have a real compressed air filter in the line. These are made with a sintered brass filter (little brass balls heated and pushed together) inside a clear chamber, usually with a way to drain any moisture that collects at the bottom.

Myth 4 - A filter is not a water trap!. Once again, as the compressed air expands into the filter, moisture will drop out and should be drained. But, the air leaving the filter is still fully saturated.

The filter should go before the regulator or anything else in the line. The higher the pressure, the better a filter works. Filters need to be replaced periodically when the dirt clogs the spaces between the sintered brass balls. Many businesses use a pressure gauge on either side of the filter. When the pressure drops greater than 10 PSI, its time to change the filter element.

After the filter is where you want the pressure-regulating valve (PRV). Remember, the PRV, also called a pressure reducing valve, goes after the filter!

This is a biggie. You canít go from 50 or 100 PSI down to a steady 6 or 10 PSI with just one PRV.† PRVís canít handle it. Use two in a row. The first one should be set at about 5-10 PSI higher than the pressure you want your brush to operate. The second should be set exactly at the pressure you want your brush to operate. If you want a steady, reliable pressure, this is the only way to go. Air is made in pulses. Air pressure gauges and PRV's read and give AVERAGE pressures. Remember, the statistician that drowned crossing a river with an AVERAGE depth of three feet! (And he wasn't a midget!)

TIP 10
This tip is for modelers using lubricated compressors. Oil gets past the piston rings and loves to find your paint. Sometimes you think the problem is water, but it can be oil. The only solution is something called a coalescing oil filter. This is specifically designed to trap oil out of the system. Donít use one and you run the risk of what a fraction of a drop of oil can do to your otherwise perfect airbrushing.

TIP 11
If you are running a hard line, rather than a hose, from a lubricated compressor to where you paint, donít use plastic, even CPVC (hot water plastic pipe). It may handle the temperature, but the bypassed oil will weaken the plastic to destruction. Do you want your work area to look like a battle-wrecked diorama? Bite the†financial bullet and use black iron pipe or galvanized pipe.

TIP 12
Put drops (air lines pointing down) anywhere you want for other pneumatic tools, from impact wrenches to (please, low pressure) blowguns. If you have a tool that you want lubricated regularly, here is the place to put in a lubricator.

TIP 13
Have a drop at the lowest place in the line with a quarter turn valve. Moisture will collect here and you can crack the valve periodically to vent it.

Well, what do we do about the water?

Myth 5 - Air filters are water traps. Yes, they collect water, like an air compressor tank, but the air going downstream from them is still fully saturated and will drop out more water when the air expands through anything else, like your airbrush.

The only two ways to remove water from a compressed air system are to use a refrigerated air dryer, which is not economical unless you are using the compressor for instruments or other critical equipment needs, or a desiccant system. One types of desiccant system is the twin tower design where a reusable desiccant is heated in one tower, while the other tower full of desiccant is trapping the moisture. Then, when the working tower is saturated, the other tower takes over. This is too expensive for the hobbyist.

The other system has a desiccant that absorbs the water. This type has to be refilled with desiccant (if designed to do so) or replaced. Many modelers start experiencing problems, but are puzzled as to why. The culprit is probably a used up in-line desiccant water trap that is no longer able to take moisture out of the system.

I hope this is of some help. Now, if some of you want to help me get over my fear of the airbrush, weíll be even!

About the Author

About Jeffrey Winkel (Sealhead)

I am a middle-age newbie that bit off more than I can chew by working on my second diorama. It is a TRIORAMA or, in other words, three scenes from the movie Kelly's Heroes (on a turntable). I've been ten years on it. Putting it down, restarting, changing things as my knowledge and skills improve...


Thanks for the great article Sealhead!!! Ciao
JAN 08, 2004 - 02:51 PM
I just bought my compressor a few weeks back, and reading over this article has given me a bit more confidence that I made the correct decision with the compressor. Only question I have is that I thought my air regulator (that has a moisture trap in it) would catch any oil vapours coming from the compressor? Is this correct, or do I need to look into getting a coalescing oil filter?
JAN 08, 2004 - 03:24 PM
I am not aware of any filter (dirt filter) that also acts as a coalescing filter. Coalescing filters have to be changed periodically just like dirt filters. Again, the tip off is a pressure drop of more than 10 PSI across the filter. Sealhead (Kansas Sunflower)
JAN 12, 2004 - 09:34 PM
Ok, thanks Sealhead. I'll look into getting a oil filter then.
JAN 13, 2004 - 05:34 PM
Sealhead, hopefully you read this. I've been looking for a small desicant water trap to add to my system since reading this fine article, but the only moisture traps I've found specifically for airbrush use don't mention what type they are (and they look a lot like the inline air filter I already have). On the other end of the spectrum I have found the large industrial desicant traps, which would cost more than all the rest of my system, plus most of my accumulated kits. Do you, or anyone else, have any suggestions on a good, affordable moisture trap? Thanks in advance, Dave
FEB 07, 2004 - 05:17 PM
Thanks Jim. We recently got an air compresser, and it is worth the money. It sure beats Tamiya spraypaint cans, and from what I've heard, It sure beats canned air also.
FEB 13, 2004 - 09:48 PM
I appreciate the article, I just bought my first airbrush and I DO NOT LIKE THE CANNED AIR! Your article was a huge help! Thanks.
FEB 19, 2004 - 12:49 PM
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