Choosing an Air Compressor

Dryers are probably the most over looked item when you make the jump to a shop sized air compressor. If your blowoff gun sprays like a garden hose when you first pull the trigger, or your grinder is blowing water from the exhaust port, you need a dryer. Wet compressed air is
the cause of many fish-eyes in paint jobs, clogged media blaster nozzles and frozen sanders. A good dryer can cost about half as much as your compressor. That can be a tough pill to swallow, but the air quality is worth it.

The two main dryer types are ‘refrigerated’ and ‘desiccant.’ A refrigerated dryer has a refrigeration compressor and a heat exchanger to cool the air, condense the water, remove it, then heat the air back up to the system. Refrigerated dryers come in two types which include ‘cycling’ and ‘non-cycling.’ A non-cycling dryer runs all the time and is the least expensive
option. They run like a pressure washer with a bypass valve. A cycling dryer runs like your home refrigerator. It cycles on to cool a secondary cooling fluid, then shuts off while the cooled fluid does the work of drying the air. Both of these dryers are the industry standard for most users and operate at a +38° F dewpoint. Dewpoint is defined as the temperature when the water in the air lines will condense. With a properly sized and maintained refrigerated dryer in an indoor environment, water in the air lines and tools will not be a problem. It should also be noted, when using a refrigerated dryer with a reciprocating
compressor, there needs to be an after-cooler on the compressor or built into the dryer. A reciprocating compressor without an aftercooler can discharge 200°+ F air. Refrigerated dryers work best when the air coming into them is 100° F or less.

The second type of dryer is a ‘desiccant’ dryer. These dry the air to -40° F dewpoint or better. They’re about twice the cost compared to refrigerated dryers. However, they provide very dry air, perfect in high end painting and laser cutting applications where any moisture is detrimental.

Even if you don’t buy a dryer, a filter is a good investment. It can drop out some of the moisture, and most of the oil and particulate matter from the air stream. It will protect your tools and paint jobs from contamination. For home compressors, the extra $50 dollars for a 3/8″ combination filter/regulator is money well spent. Just make sure you drain it and change the filter once in a while.

Above about 2 hp, you can’t just plug your compressor into a standard single phase, 15 Amp, 120 V wall outlet. From 3 hp to about 7.5 hp, you’ll be in the single phase, 220 V range—akin to what you’d need for an electric clothes dryer or kitchen range. Once you get to 7.5 hp and
up, it’s time to play with the big boys and get a three phase service. Then you can add that heavy duty welder or run that end mill you’ve always wanted. Three phase service comes mostly in 240 V. For 50 hp and up, three phase, 460 V is the standard.

Whether to put in hard pipe in your garage is another of those application questions we’re often asked. If you typically roll the compressor out to plug it in, it doesn’t make sense to have a bunch of hard pipe. But if the compressor is at the far end of your garage and you get tired tripping over your hoses all day and recoiling them every night, it’s time to put in some permanent pipe and hose reels. Don’t just loop the rubber hose over the rafters. The bends break down after a while and start to leak, robbing you of precious CFM. A long hose also causes a surprising amount of pressure drop. And, beware, when an old, long rubber hose breaks, it can turn into a pressurized whip with surprising force. And if you are gone for days when a hose breaks, your compressor may run non-stop and burn out the motor before you return. It’s better to get some hard pipe and run it where you need it most with ball valves in front of quick connects and have a drip leg at both ends with a petcock valve to drain out any condensation.

Never use PVC for hard air lines. Yes, it’s cheap and easy and says 150 psi rated; however, after time, the PVC can sag. If you bump it with anything, it can break. The hot temperatures of compressed air will break it down as will the compressor oils. And when PVC breaks, it can shatter, sending plastic shards everywhere.

Galvanized or black iron pipe is good if you have a threader to get the lengths right. Sweating copper tubing can make for a really nice installation. There are also aluminum piping systems available. They require expensive fittings, but the installation is quick and easy. You just cut the pipe to length, push the ends in the fitting and tighten them with a spanner wrench. It’s so easy to take apart and set up, you could take it with you when you move into a bigger shop.

It’s not really necessary to have downhill runs in your air lines, like you would for plumbing pipe. The air flow will keep any moisture or particulates moving through the system.