Air compressors

Capt. Mike

Near & dear to every home mechanic is the day he graduates from hand tools to air tools. Nothing can replace the air impact wrench and that just leads to an insatiable appetite for more air tools. Air tools are generally superior and stronger than electric versions. And having compressed air for everything from blowing up tires to blowing off dirt can’t be beat.

Air compressors come in almost every imaginable combination. 110v; 220v, gas & even LPG. They also break down into single stage vs. dual stage and many tank sizes. There is one to fit every Wannabe mechanic but I’ll start with the caveat to decide what size you need, double it, then finally buy one at least double that! You will grow into and probably still grow out of whatever you pick.

A single stage compressor has one compression stroke. Picture a typical car cylinder. It will compress the amount of air in the volume of the cylinder at BDC into the amount at TDC, i.e. the compression ratio. A 2-stage then takes the compressed output of the first stage and recompresses it. A quart is compressed into a cup, the cup is compressed into a ¼-cup sort of thing.

First, the number game. It is just that, a game! The numbers make little sense and most of it is Madison Ave. hype. Horsepower is a common term. It’s useless except that it takes more HP to run bigger pumps. An old 2 HP will probably compress close to today’s 4-5 HP – they’ve played with the numbers to make them sound better. Ditto the output. About the only number that stands up for comparison is the SCFM – Standard Cubic Feet per Minute. That spec will be at a certain pressure so X SCFM at 90 PSI will be a true generating comparison between compressors.

The small compressors around 1-2 HP will probably put out 3-6 SCFM at 90 psi; middle range 2-4 HP, 6-8 SCFM; the big 5+ HP 2-stage will break 15 and may go beyond 20 SCFM.

110v compressors will plug in most anywhere, are smaller in size & weight, and can handle the most common shop tools like impact wrenches and air hammers. Typically they peak at about 2 HP. Bear in mind that the typical 110v air compressor may take 15-20 amps and max out the typical household wall circuit -- which you are usually sharing with other appliances. A dedicated circuit is recommended.

To go higher you need 220v. In 220v, be sure you watch for single- vs. 3-phase AC. Few homes are wired for 3-phase. 3-phase is found in the super heavy-duty compressors and probably not necessary in any but commercial shops. Also watch your start-up amperage. Some motors take about double their running power to start; others use devices like ‘magnetic starters’ to reduce that initial load. Your shop circuit has to be sized and fused for start-up draw, not running draw. Again, plan on a dedicated circuit.

There are three pressures one should consider. 90 psi seems to be a common denominator for most air tools so check your compressor output against that standard. Paint guns and small bead-blasters typically run lower, so the 40 psi output might be sufficient for you if its dedicated to that service. A friend runs a paint booth with a little dedicated one instead of his big shop compressor as it's easier to keep clean, filtered and contaminate free. Finally, in 2-stage compressors, 175 psi output is the common denominator. Sufficient SCFM at one pressure will always give it to you at a lower pressure. 10 SCFM at 90 psi will guarantee 10 SCFM at 40.

Air tools use air in different ways. An impact wrench will use a built-up shot of air to make the tool ‘strike’. It’s a single, timed release (though repeated often.) Each release only uses a small shot of air. Thus these type of tools are quite happy with small compressors.

Vane type motors, common in buffers, drills and die-grinders, use air in a full flow manner, much like blowing across a windmill. In these, the air flow can be quite high and, if it exceeds the output of the compressor, will quickly drain down the compressor tank. Thus the compressor that can handle your big ½” drive impact wrench may bog down trying to keep up with a little die-grinder. Size of the tool is not the governing factor!

Tools are equally nebulous in their use of the air consumption numbers. A typical impact wrench will be 3-4 SCFM. But I have a die grinder, also rated at 4 SCFM, that will gradually drain down my 7.4 SCFM @ 90 psi compressor. My BIG grinder is good for about 5 minutes before it slows down. They didn't publish consumption specs 30 years ago, but the equivilent in a new catalog says 9 SCFM. Go figure! I'd guess closer to 15 SCFM. See why I say get 4X as big as you think you need!

Tank size is a measure of your reserve capacity. It allows you to run cooler (compressing air heats it up) and helps control moisture (hot air holds more moisture). It’s also the buffer between consumption and pumping, so a large tank allows your compressor more rest & cool-down time between runs.

Tank size on a 1-2 HP will typically be 12-20 gallons. Some construction site compressors will have even smaller tanks but they are designed for low volume consumers like nailers & staple guns. They are not adequate for even a home shop. The 2-5 HP will usually be in the 20-30 gallons size. This is adequate for most tools, but can still be marginal for sandblasting and high-consumption air-vane motors like a heavy duty grinder.

Most of the 5 & up HP units are now on 60-80 gallon tanks, many of which are vertical to save shop floor space. Horizontal tank units are often on wheels and have some portability -- not so a vertical.

Construction varies with price & capacity. Most shop units are still piston pumps. Newer designs, still not widely used in home-shop environments, are rotary-screw & centrifugal. All are available in an ‘oil-less’ design but the old oil-filled crankcase piston pump is still the mainstay and is all I'll address here.

Construction on the cheaper units is all aluminum; middle quality is cast-iron cylinders; and the best are cast iron cylinders AND heads. Take a magnet with you -- I've found many advertised as "cast iron" but having aluminum heads. Another guide to quality is pump RPM. The better pumps do their job at lower RPMs. Like an engine, durability is a question of how hard the pump has to work.

I’m not going to get into recommending brands or models. There are many models & prices at discount distributors like Northern Tool and even Sears. Lowes, Home Depot & Menards carry all sizes. You can even find some at Sam's Club or Costco. TP Tools specializes in the larger heavy duty models. You can also find all sizes at industrial and agricultural supply stores.

Like an engine, compressors require service and occasional repairs. Pays your money and takes your choice. You usually get what you pay for so the bargains may not be bargains after all! I recommend the largest you can possible afford.


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Capt. Mike

Controls & piping

Your air compressor will only perform as well as the final air at the tool. Output at the compressor is controlled by the pump capacity and the built-in regulator. It will have a cut-in/cut-out range so the tank will drain to a pre-set pressure, then the pump will run until it’s back to maximum pressure. A 2nd gauge (1st may be missing on cheap models) will be controlled by the regulator and is the air pressure put into the outlet. This is the “throttle” and where you control the pressure for the tool and use desired.

Once the air comes out of the tank, the pressure starts a rapid downhill trip. The length and size of the hose are the first factors. Consider the hose a combination of a constrictor and another tank to fill. A narrow hose -- ¼” is common -- will throttle the air flow to your tool. The larger hoses, 3/8” and ½”, allow better flow. But the internal size and length also comprise a 2nd little tank. The space inside the hose has to be filled to the tank pressure BEFORE any is passed on to the tool. I’m not a math major, but consider how much water you dump when you drain your garden house. Same principal for air, except that it’s compressed so is the amount is many times the internal volume of the hose. Lost before you use the tool. Thus the need for a combination of the largest hose, the shortest distance AND the larger compressor and tank capacities. That 400 ft-lb. torque wrench will be lucky to pull 200 ft-lb. at the end of 100’ of ¼” hose.

I’ve mentioned the regulator on the compressor, but most of us like the noisy compressor out of sight & sound. Thus it’s often put in a back corner or outside the work bay itself. Here, it’s advisable to pipe the air into a separate regulator in the shop. Run the pump at capacity and regulate nearer the tool. This minimizes the above losses. There are also mini regulators for attachment at the tool, but without gauges, their effectiveness is questionable.

While we’re talking regulators, consider the previous post’s warning of moisture. It IS there and does no good to the tools and much harm to things like painting and sandblasting. Thus you need a filter and moisture trap as close to the application as possible. A combination filter-regulator-oiler is a wise investment. You will then want a 2nd filter for attaching close to the tool end when super-dry air is needed (painting). You may even want a 3rd mini-filter at the sprayer. I have a filter on a stand and quick-connects so I can hook up my standard 50’ shop hose, then keep a 2nd 15’ “clean & dry” hose for those uses where I need clean & dry air. That 2nd hose is never used without the filter or for oiled air. Drain your filter frequently and before any ‘dry air’ use.

I mention oiled air because, like anything mechanical, air-tools require lubrication. The simplest and least satisfactory method is to put a shot of air-tool oil in the tool air inlet before use. That guarantees a big blow out of oil out the exhaust at the beginning, and then loss of lubrication after using the tool for a short period. There are small, at the tool, oilers that do quite well. You fill these before use and they will typically last several days of average shop use. The best method is the adjustable oiler in the line, usually right after the regulator. These devices inject a measured amount of oil into the air based on the air flow. Typically they hold several ounces of air-tool oil and will last months. Most can adjust the amount. Use ONLY air-tool oil; do not use motor or household oil.

The downside of an oiler at the regulator is that it puts oiled air into the hose, contaminating it for dry use. It also means you have oiled air even when you don’t want it. Two cures -– have a separate dry hose, or use the 2nd filter mentioned above. If you run dry air through the hose for a minute or two, the 2nd filter will usually catch any contaminates. To achieve this 'dry supply', I installed a T with quick-connects between my regulator and the oiler. Thus I can plug the hose into the filtered and regulated dry air ahead of the oiler, or plug into the oiled air for tool use.


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Capt. Mike

Compressor maintenance

Piston compressors are like miniature engines. They have a crankcase, pistons, & rods, and run in oil. Instead of valves and a cam, they usually use flapper valves, which are a type of check-valve, to allow them to take air in one port, and then exhaust it out the other to the tank. Pretty much maintenance free except for the need to change oil. Since there is no combustion by-product but lots of moisture, use an oil specifically formulated for compressors. Pick a weight by the compressor manufacturer’s recommendations for your temperatures. DO NOT use an automotive or general purpose oil. Change at least once per year.

Because piston compressors have clearances and ring gaps, they will pick up small amounts of oil from the crankcase. This is pumped out with the compressed air and causes no harm to the compressor. Check oil level at regular intervals. The amounts vary with compressor condition but many will go between annual changes without needing additional oil.

Compressors take in outside air to compress, so it will have the same contamination and dirt as the environment it’s in. Dust, pollen, little critters and fumes. NEVER let your compressor intake flammable vapors like paint or cleaners. Clean or change filters frequently. Whatever it takes in will end up in the tank and your air supply.

Most compressors use belts. Like the V-belt in a car, they must be in good condition, in alignment and properly tensioned. Change at the first signs of wear, cupping, or cracking. Do NOT use belt dressings -- they do more harm than good. Since many compressors use plastic belt housings and guards, a broken belt can cause considerable damage – don’t push your luck.

The tank will accumulate moisture and oil as mentioned above. Each tank will have a moisture drain. How much and how long between draining depends on your conditions and amount of use. I drain mine frequently, before any ‘dry air’ use and again after a heavy session like bead-blasting where I know I’ve really heated things up.

[Addendum 12-29-05:] When my old compressor blew, I 'borrowed' the little 1HP I had given my Dad about 20 years ago. Needless to say, when he became handicapped, maintenance stopped. The compressor sat in the shed mostly for the neighborhood kids' bicycle tires. I brought it home to do a maintenance round. What drained out of the tank was a wake-up call for anyone that doesn't think maintenance is important. What came out was also the rust that had been holding the tank together. It started leaking from a rust-out at the bottom of the tank. All the rust was on the INSIDE; the tank looked good. Net result -- a compressor that still had a good pump & motor, shot.

One thing that will both reduce moisture AND improve compressor life is to provide it with plenty of cooling air. Compressors are air-cooled and typically stuck in a corner of the shop with little fresh air flow. A fan, blowing across the compressor pump, will do wonders. I always have my fan on low when the compressor is on and switch to high for heavy sessions.


:cool: Not a maintenance item per se but a caveat. Your compressor has an automatic shut-off so is rarely running at the end of a shop session. It’s pumped up and ready to go. BUT . . . turn your compressor off with either it’s built-in switch or shutting off the power. If a hose or fitting lets go while on automatic, your compressor will run continuously without your knowing. This is likely to overheat the compressor and cause serious damage. No compressor is ‘continuous use’; they all except to cycle on & off, cooling in between.

[Addendum 12-29-05:] I bought a big, commercial-grade compressor earlier this year. Included in the directions was the warning NOT to use the pressure switch as a shut-off switch, but to install one in the power supply. My old small compressor allowed the pressure switch to be used, but the new big ones don't. However, seeing the operation (actually blocking moveable contacts), I can see the reasoning. Better to use a true dedicated electrical switch.

Since the big compressors require hard-wiring, I put a disconnect switch on the wall right next to the receptical. I had an electrician wire and install a proper UL certified 50A receptical & plug on the compressor lead so I could unplug for maintenance. Thus I'm not truly hard-wired, but this receptical easily handles the compressor draw and meets code.
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Capt. Mike

I was in Lowe's the other day and noticed a display of new air compressors, I forget the brand but it was a major, well known. A 7½ HP, 2-stage with 80 gal. tank was only $800 -- a good price for a name brand. Except . . .!!!

:p Plastered on the tank was a huge decal reading Cast Iron. Thus why I thought the price good. But being of the suspicious persuasion and not believing in free lunch, I picked up a magnet tool off the nearby display and laid it on the cylinder head. It fell off! Cast iron my fanny!

I did find the magnet would stick to the compressor block, so what I deduce is that it had cast iron block and aluminum heads. Since it's the heads that are the weak point for damage and distortion, the fancy Cast Iron decal is a misleading come-on. If they'll cheat there, I'd suspect other short-cuts. When you're compressor shopping, take your magnet with you. You may chose aluminum -- per the post above -- but understand the huge difference in quality and expected price.
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Capt. Mike

Compressor Piping

Above, I alluded to hard-piping the shop for air compressors. Well, I'm making progress. Here's some things I've learned over time.

First, the distance run, pressure and amount of use should have considerable effect on your choice.

Plastic pipe has so many variables I don't know how one would keep track. There is PVC, CPVC & ABS in several 'schedules'. I've seen systems where it melted. I've also been told never to use it without an after-cooler as compressed air can get pretty hot! I couldn't tell you one piping from another and then understand each type needs special glues to hold that type together. I've been told that Schedule 80 can handle 200 psi pressures (most home air compressor systems peak at 175) but presume the necessary fittings and glue have to match. I would be concerned that the psi rating is assuming 'cold' and that the very high heat of compressed air in heavy use may reduce that significantly down into the compressor pressures. Then there is the transition to the metal fittings on both ends.

My feelings on copper are about the same. Many quality specs and I've seen warnings on using only the correct solder for air pressure. I've also read the heat can change the anneal.

Within the piping system are necessary metal fittings for the various valves, drains and pressure releave valves. Typically there will be filters, regulators and perhaps oilers in the piping. All of these require a transition, compounded when changing material types as well.

The commercial shops and the air-compressor manufacturers will all recommend steel as the best. "Black iron" piping is their choice. That's the route I chose. My thought was I'm more comfortable with metal work and the installation would be permanent and maintenance-free.

All that's true, but it is a lot of work. TP Tools, where I bought my compressor, sells kits, fittings and supplies. [Use the link to view a typically installation.] I chose their kits for the main lift line from compressor to get to the ceiling-hieght distrubtion line and again for my primary drop feed with the more exotic filters & regulators for my bead blaster, where moisture would be crippling.

I then went to a steel products supply place for the pipe and a plumbing supply dealer for the rest of the fittings. They were considerably cheaper than mail order with the high shipping weight. I also ended up making about 4 more trips for 'forgots' and fittings the I couldn't fabricate from the pipe stock. I dare say I'll have over $400 in the piping, not including regulators & filters. But that includes 4 drops with ball valves and drains. I'm using ¾" for my distribution line and ½" for my drops.

The fitting in steel pipe requires even more exact measurements than plastic. Plastic allows some movement in or out of the fittings before the glue sets. Not so steel. It must not only be exact, it must include the amount of thread going into the next fitting EXACTLY. Then there is the added problem that, since the threads are tapered, one must be quite exact in the direction of the next fitting and still be air tight. Finally, the added weight means the stand-offs for mounting to the wall need to be heavier and more exact.

I bought an inexpensive pipe threading die set and already had a cut-off chop-saw for metal. It was quite an experience and I had great help and experience from a friend that used to oversee construction & maintenance of our school system. Plus he'd already piped his shop with schedule 80.

I was surprised how hard it is to thread pipe. Lots of cutting oil! My threader is manual and is the rachet-wrench style. I found it impossible to thread anything shorter than 6" so bought any nipples I needed shorter than that. One must have a coupler at each compressor rise or line drop as that is where all breakdowns of the system must occur. They allow direct matching length connections as the connection threading is independent of the pipe. These and the ball valves are the most expensive of the fittings. The nipples, T's, 90°'s and couplings are relatively inexpensive.

Anyway, I'm proud to say we're piped -- at least my two main drops. One is to feed the bead blaster so has the most effective regulator, water trap and a filter. The other is my older main feed to the shop with a quality filter, regulator and an in-line air-tool oiler. I'll post pictures on the Tech Drawings site in the <My Photos> album.

The horizontal distrubition line is already T'ed & capped so I can run additional drops. I will be adding a drop in the back bay near the door so I can take air out to the gravel pad where I do a lot of my bigger sandblasting and spray painting jobs.

I'll have another drop at the front corner of the main bay which will both be closer to the work bench and from which I can also run air out to my parking pad for jobs I can't get into the shop without having to run a hose all the way from the shop back wall.

It's been a big project but I'm pleased with the results so far. The one overriding warning is to install a shut-off switch for the compressor and a shut-off air valve in the line leaving the compressor. There are often leaks in tool hoses and fittings that, by themselves, are acceptable but left unattending overwork the compressor. 2nd, is any break (far more likely in hoses & PVC systems) will cause the compressor to run continuously and quickly overheat it, causing damage. Shut off air to the shop at the compressor when not in use and also shut off the electric supply to the compressor at night or any long absences.


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Capt. Mike

Warning -- compressor explosion!

A friend recently forwarded me a Power Point set of slides on a home compressor that exploded. It was one of the wheeled version with a horizontal tank -- I'd guess about 1-2 HP. The tank is ruptured and literally butterflied open. Parts flew everywhere. Debris put a hole in the garage ceiling; several in the walls, damaged other items in the garage, and even dented a car parked in the drive. The force was so powerful it pulled the AC cord out and it whipped with such force as to slice open a jug of antifreeze like with a knife.

The cause, according to the lead-in for the file, was failure to regularly drain moisture from the tank, causing a failure along a tank weld seam. I've posted 2 pics on the Tech Drawings link in the <My Photo Album> folder.

Regardless of cause (source and conclusion unverified) it does point out the potential of danger of any system having high-pressures. Thus my earlier warnings to turn the compressor off when not in use, drain the tank regularly, and de-pressurize the lines. Also, routinely check your pressure cut-off switch and pressure relief valve to be sure the compressor operates within the designed pressure range. Never put in a higher cut-off or pressure relief.

Inspect your tank regularly. Is the drain water excessively rusty? Any signs of rust on the tank like pitting or maybe bubbles coming through from inside out? Fittings, regulator and valves clean and rust-free? Pressure relief tested? Some of the damage was sufficient to be fatal if the parts had hit a person. Think about it!


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Yes indeed it does happen!

I had a portable Emglo dual tank compresor tank fail once. Not as catastophic as described above, but,,,

I was trimming out a house and had the tank sitting on a freshly laid white carpet. The rusty water and debris destroyed the carpet. Drain daily!


[Moderator Note: Emglo is a division of DeWalt.]

Capt. Mike

Compressor oil drain

Most compressors (oil & piston type) have a rather crude oil fill & drain system. Much like a lawn mower engine with a drain plug on the side of the sump and a fill cap on the side of the engine. The usual method is to make every attempt to catch the oil, fail, and then watch it run down the side of the compressor onto the shop floor. Then wipe up the mess on both. The vertical big compressors are even worse as their drain is up around chest level.

:p I fought it with my old 2HP and horizontal tank. I'd fabricated a tray that let the oil run clear to a drain pan as long as I didn't look at it crosswise. But the new compressor is the 5HP/60gal. vertical tank. After my 2nd compressor oil change on it, I decided I had enough of holding the catch pan for 20 minutes until it stopped dripping, or terminal cramps, whichever came first. My Champion compressor at least has a stub pipe sticking out about 4"; many just open flush with the sump. I decided a drain valve and take-away hose was necessary.

I attached a 90º elbow to the end of the sump drain. If yours doesn't have the extension, you can get a pipe nipple of the appropriate size to get the elbow clear of the compressor. I then added a ball valve and a hose barb, to which I attached a length of 3/8" ID hose to take the oil down to floor level. I even put a removable plug in the end so there wouldn't be any drips. Now I can drain oil by just opening the valve and having a catch pan sitting on the floor fed by the hose.

Caution: Since it would be possible to drain the oil accidentally by just opening the valve, I installed a seal. Nothing exotic. I just attached a loop above it to the compressor and drilled the handle for a ring. Now I install a wire tie and cinch up tight so the valve can't be moved without cutting the wire tie. I'll have to install a new tie every change, but what the heck.

I picked up some self-adhesive wire tie mounts to space out down the side of the tank and wire-tie the drain line to it for neatness. Cost, about $20 for the valve (brass), fittings & hose.


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