Created: May 1997
Updated: April 15, 2020
My cousin has long worked as a mechanic, contractor, and equipment repair specialist. He has been a godsend over the years helping me with projects that I just can no longer do due to a bad leg. He used my big air compressor for a project at my home shop and during that effort it developed a knock. He advised me that I could fairly easily repair my unit with new or rebuilt parts, and he would be happy to do the rebuild for a reasonable fee. We discussed my wanting to upgrade to enough capacity to do the high-volume low pressure (HVLP) spraying I wanted to do. After doing some checking, he said upgrading the pump and motor would cost nearly as much as buying new, and he was not comfortable putting that larger equipment on my old tank. He said it was time to seriously consider buying a new bigger unit.
This effort started to upgrade my old air compressor. As with most things the more research, the more to consider. Just as with Cyclones & Dust Collection Research much of the available information provided by hobbyist suppliers proved to be advertising hype. Sorting through that hype took far more than my limited expertise and knowledge. Fortunately, I had lots of help. Michael O'Brien, PhD was my mentor and a senior UC Davis agricultural engineering professor who helped me buy my first compressor. Additionally, my next-door neighbor has owned and run an air compressor store for more than forty years. He reiterated much of the same information from Dr. O'Brien and shared lots more in bits and pieces. My cousin shared much as well. I augmented their advice with material shared on the Badger Pond, the Oak, and the Wood Central woodworking Internet forums.
That sharing left me with a problem. When my friend Forrest Addy saw this page, he said I did a good job, but was unhappy that much of this material was paraphrased from his copyrighted "Practical Machinist" publication. It was not and I have never even seen that publication which tells me that this information is pretty accurate and universal. Forrest has been an incredible resource in many areas for woodworkers and has helped me personally often and well. He consistently declines the few favors that others and I have tried to send in his direction. I did not want to offend him but did want to share. I took down this page, but soon had many asking me to put it back up because it was the one place where they and their friends could go for the advice needed to make an informed choice. I did a total rewrite and put this page back up. This page represents my thanks to so many including Forrest who helped me buy a really nice air compressor that has served me well for many years.
My first job was to determine the urgency of my need. My Craftsman air compressor after over 25 years of good service developed a bad knock. My retired friend who owned that air compressor business said it was not wise to rebuild mine. A tank that old will often have serious rust problems that can be dangerous. He responded to my dumb look saying a 125 PSI unit like mine won't fly around the shop too long if a major seam lets go and rarely will they fly more than about sixty feet. He then paused and said only once in a very great while will an old tank explode and make a big mess, but mostly they just develop pinholes from rust and leak air badly. He said I could probably get another year or more out of my compressor and many more than that from my tank by just keeping up the oil, draining the water regularly, and using it sparingly. He was right and I spent a couple of years at leisure looking for a replacement based on his advice.
My little home unit was a Sears that Dr. Mike O'Brien of UC Davis Agricultural Engineering Department helped me pick back in the late sixties. That particular unit was one of the many lines that Sears has custom built by better name suppliers. The one he helped me choose had a top-quality motor, one of the better brand name pumps, good safety features, and was well built. As with so much of his advice, that I now dearly and sorely miss, he was correct as that unit served me well for nearly three decades.
Perhaps I'm strange, but from the time when I left home in high school to become self-supporting, I'm terribly careful when it comes to buying anything as I want quality, reliability, and the assurance of long term serviceability. Both from personal experience and from being a quality control and purchasing engineer for the State of California I learned "the sweet taste of a good bargain vanishes quickly when you have to live with the bad taste of a poor-quality product".
I wanted a compressor that will carry me into retirement. I've used air compressors in the auto repair and body shops I've owned, used them when working as a machinist apprentice in the sixties, and used them both at home and at the university where I have taught engineering part time for twenty-eight years. In spite of all that use, I never paid much attention to them unless they broke. My philosophy was to simply buy top quality, set it up right and get on with whatever needed doing.
I then took that list to a longtime friend and prior neighbor who has long owned an air compressor store since his retirement as a machinist. My friend said he owed me considerably for my help on dust collection, so spent many hours in going over what considerations went into selecting a good air compressor. He crossed off the rumor, wrong, or poor advice. He added considerable material of his own as well as shared many stories backing up both what he read and what he contributed, including his own loss of a brand-new car. He went through his parts and repair inventory showing me the different kinds of pumps, showed me how a two stage works and the inter-cooler, plus the various valves, switches, drain systems, and many accessories that he carries in his store. He also shared with me many examples of ruined equipment such as pumps, oil-less pumps, belts, valves, and tanks that were not properly maintained or used beyond their design limits. He suggested I put that information on a web page similar to my dust collection as many could benefit.
- Air Compressor Basics
Small shop and home air compressors are fairly simple. A motor turns a piston pump. This pump takes in room pressure air then compresses it enough that it overcomes a backpressure valve to feed compressed air into an air tank. That backpressure valve keeps the pressurized air in the tank from leaking back into piston pump which would make moving that pump much harder. A pressure switch mounted on the tank turns the pump motor on and off. Most pressure switches can be adjusted to set the ideal pressure for your tank. Most smaller air compressors run at 125 pounds per square inch (PSI) of pressure. Most larger units can run up to about 175 PSI, but most adjust their larger units lower than this. I run mine at 125 PSI as that lets it run less time to get and stay full. A pressure relief valve on the tank (sometimes called a pop-off valve) keeps the tank from getting too much pressure. Some air compressors let you use that pressure relief valve to quickly empty the pressure from the tank after use. Most also have a tank valve (sometimes called a stop valve) that can be closed to keep the tank pressurized without having to keep your hose pressurized. You should never leave pressure in your hose, as hoses will fail if left with pressure for too long then go flying all all over. Most air compressors also have a tank drain to drain the water that condenses on the inside of the tank. Often there's a pressure regulator built into or mounted on the tank after that tank valve to set a fixed pressure for your hose. Most of the bigger and better quality air compressors use an unloader valve before the backpressure valve that opens on startup to let the motor come up to speed before the compressor starts to build pressure. Many people also add a dehumidifier to take the condensed water out of the line to keep from hurting your spaying efforts or putting water through your air tools.
The first thing I learned is picking a compressor based upon manufacturer sales literature is nothing short of foolish. Vendors have learned that truth in advertising now means they only have to prove their product can meet whatever they claim for an instant. This would be like me selling you my car and claiming that it gets 92 miles per gallon. If you look at the MPG gauge when coasting down a mountain, it actually gets that MPG, but in real use what I can count on is far less. The same sadly applies today to a good portion of the consumer goods being sold, particularly tools, motors, dust collectors, shop vacuums, and air compressors. I guess whichever firm can make the biggest sales claim sells the most units. If you don't take the time to learn about air compressors, then you just add to this problem! Worse, what you don't know about compressors will waste your money, can seriously damage your property and can kill you and others!
For compressors you have to adjust considerably to go from the advertised specifications to get a compressor that will meet your needs. Check the horsepower rating on the motor nameplate. If the plate is blank or says something like "special", then the motor rating will be totally different. If you multiply volts times amps then divide by 1100 (not 746) you will get a rough idea of the actual HP figure.
- Oil-less Versus Belt-Driven
Oil-less compressors as their name implies do not require lubrication. Actually all compressors require lubrication, but oil-less compressors get their lubrication from using teflon and other plastics that make a good seal without needing oil. Some wrongly call these direct drive compressors where the motor directly connects to the pump, but if you take them apart most instead still have some form of pulley and belt that connects the motor to the pump. Oil-less compressors almost always have the motor and pump all built into a single assembly. This makes for a simple and reliable compressor if used lightly. This also means most oil-less compressors cost more to repair than it does to replace.
Typically, oil-less air compressors are noisy, not very efficient, low cost entry-level units designed for the occasional user where high duty cycle and longevity isn't a major consideration. Oil-less compressors were never meant for continuous or high-volume use. They are best used where the pump runs more no more than about 20% of the time. The problem is simple, compressing air generates huge amounts of heat. Today you can buy fire starters that drop a little tinder into a long tube and all it takes to light this tinder is to quickly push down a plunger that compresses the air. If we use our oil-less compressors hard, this heat of compression keeps building until the unit grows way too hot and melts the plastic parts that seal our pumps. When the seals go, the pumps no longer produce enough pressure to fill the tanks. Using a paint sprayer, air tools, a sand blaster, or other high demand air tools will quickly build too much heat and ruin an oil-less compressor, sometimes in just a few minutes of continuous use. Worse, for oil-less compressors the incoming air passes through a fairly open filter that gets rid of the big stuff, but the fine dust goes right through. This means their crank assemblies, pistons, plastic piston rings and cylinders are exposed to ambient air and whatever dust it carries. If the air in your shop is damp or dirty with abrasive dust, metal dust or even sawdust, these materials will clog your filter and will rapidly wear out your pump. Worse, this same dirty air will shorten the life of your compressor through corrosion, wear, or seal deterioration.
Most small oil-less compressors can power a nail gun, pump up tires and supply an occasional blast of air. When used in this mode only, these units will last for years and can be very handy. Knowing I do a lot of fine work and finish nailing helping install molding and crown molding away from my big compressor, I wanted a portable unit for light duty on job sites. I did a lot of homework and checking with friends in construction. The choice came down to the Porter Cable pancake with Dewalt a good second choice a. I chose the Porter Cable pancake, like it and have used mine for years.
Belt-driven compressors use a separate induction motor driving a small pulley that turns a fan belt that turns a larger pulley wheel on a compressor pump. The better belt-driven compressors have vanes on that compressor drive pulley wheel so the wheel serves as a cooling fan to help get rid of the considerable heat generated when compressing air. Belt driven compressors are quieter, more efficient, and last longer. Belt driven compressor pumps are built like an internal combustion engine where the crankshaft and other pump parts run in a sealed crankcase. These pumps are either splash or pressure lubricated with oil. There is no particular advantage to a pressure-lubricated compressor over a splash-lubricated compressor provided they are properly designed. Often good quality belt-driven compressors will serve reliably for twenty or more years with little or no maintenance other than filter and oil changes.
Most consumer air compressors come with either one or two cylinders.
- Single Stage Air Compressor
All single cylinder units and most two-cylinder consumer grade air compressors are single stage air compressors. This means they directly compress the air very strongly in a single pass before it moves into the tank. Single stage air compressors are pretty much limited to a maximum tank pressure of about 125 PSI. Going higher can be done, but becomes so inefficient we end up spending a lot more for power that going with a two-stage compressor. Single stage compressors generate a huge amount of heat that can quickly build up and ruin a compressor pump in a big hurry. This is especially true for the oil-less units that rely on a Teflon or plastic seal in the cylinder that can overheat and melt making that compressor fail prematurely. You really do not want to have more than a two-horsepower single stage air compressor, because anything larger will build up too much heat and cause problems.
- Two Stage Air Compressor
A two-stage compressor is designed to permit continuous running and generate higher pressures without the heat problems from a single stage unit. Two stage compressors use multiple cylinders with the first one or two cylinders taking in atmospheric air and compressing it to about 1/3 the delivery pressure. That partially compressed air then passes through the inter-cooler (the finned tube behind the pump flywheel that has cooling vanes to act as a fan) to be air-cooled and into the second stage where it's compressed by a final cylinder to the delivery pressure. For two stage compressors, the first stage cylinder heads will have a separate pressure relief valve. Most two-stage compressors use two-cylinder units arranged in a V configuration or mounted side by side. In three cylinder designs the low-pressure cylinders are only slightly larger than the high-pressure cylinder and often arranged in a W configuration.
The result is two stage compressor pumps move 20 to 30% more CFM per motor HP thanks to the heat of compression dissipated by the inter-cooler installed between the low pressure and high-pressure cylinders. Add up the power savings over the 15 to 20 year working life of a two-stage compressor compared to a single stage and you'll find the savings in power alone will pay for upgrading to a two-stage compressor many times over. This much lower cost of operation couples with the lower heat making them last far longer to make two stage compressors much more desirable. Unfortunately, some unscrupulous vendors will try to sell their single stage compressors that have two side-by-side cylinders of equal size and no inter-cooler as "two stage" units, so be alert if you find a "bargain".
For what it's worth, most two stage compressors come set for 175 PSI service which is too high. Most air tools and shop requirements need no more than 125 PSI. If air is compressed much over the pressure you need, energy is wasted. You also just end up reducing tank pressure to line pressure at your regulator. My friend Forrest says there are significant advantages for most hobbyists to reduce that tank pressure. You can do so by resetting the pressure switch to kick in at 105 PSI and out at 125 PSI. He also recommends after you reduce the pressure to change out the motor pulley for one about 30% larger (calculate the actual diameter using Boyle's Law and common sense). These two changes will give you more air delivery, lower duty cycles, cooler compressor operation, and lower power bills. Any extra wear caused by higher pump speed is more than offset by the lower pressures and lower head and reed valve temperatures. Caution, if you don't reduce the tank pressure setting and go with the larger pulley, you will most likely quickly burn up your compressor motor.
- Horizontal Versus Vertical Compressors
There are two basic compressor layouts, horizontal tank or vertical tank, each with the pump and motor mounted on a bracket welded on top of the tank.
A 5 HP 60 gallon vertical tank compressor occupies only a little more floor space than a 3 gallon pancake, but because it's nearly 6 feet high, it won't fit under the workbench.
- Compressor Noise and Heat
Compressors make lots of noise and generate considerable heat. Generally, the more SCFM they move the greater the noise and heat. Once we go beyond the small portable units to power nail guns and tire pumps, the noise and heat often get so bad that many choose to put their compressors outside in dedicated well ventilated rooms or in heavily sound proofed areas.
The only way to reduce the amount of heat generated is to use a two-stage compressor then provide ample ventilation and cooling.
The noise can be addressed by picking a compressor configuration that makes less noise and then taking other steps to deal with the noise. A compressor powered by a 3450 RPM motor makes lots more noise than one powered by a 1750 RPM motor. The slower speed compressor will have to have much larger cylinders that work at roughly half the speed so generate far less noise which makes them the choice of many professionals. Most large compressors can be fitted with a muffler that also helps, but ultimately the best solution is to put these in their own enclosures with plenty of ventilation. Many woodworkers will put their large shop vacuums, air compressors and either dust collectors or cyclones all in their own separate attached outdoor room to address noise and cooling issues.
- Compressor Valves
The valves represent the most vulnerable part of any compressor pump. It's generally a good idea to buy a valve and gasket kit when you buy the compressor. My friend Forrest says, "You'll need them ten years in the future on Christmas Eve when the compressor dies just before you need to apply the final coat of lacquer on the blanket chest intended for your about-to-be-married granddaughter". If a belt driven compressor dies, any part of it including the motor and the pump can be readily replaced with standard items for lower cost than replacing the whole unit.
- Compressor Pressure Sensor Switch
The pressure switch senses the tank pressure and shuts off the power when it reaches the set-point. The set-point and the differential are usually separately adjustable. The set point (PSI to turn off the compressor) is adjusted to 150 PSI, for example, and the differential is adjusted to turn the compressor on at 20 or 30 lb. below the set-point. Thus, it cycles, turning on at 120 PSI and shutting off at 150.
- Pressure Relief Valve
The pressure relief (pop-off) valve is a safety device designed to open when the tank pressure exceeds its safe working pressure, blowing down the pressure to a safe level, then automatically closing. If the pressure switch failed closed, it's conceivable the unit would keep on pumping until the tanks bursts.
- Check Valve
The check valve prevents tank pressure from flowing back to the pump. Its function is often combined with the unloading valve. The unloading valve relieves trapped pump discharge so when the compressor starts it doesn't have to work against tank pressure. When the compressor comes up to speed the unloading valve directs pump pressure to the tank. The PPSSsssst you hear when the compressor shuts off is the unloading valve - well - unloading.
- Air Tank
The air tank serves as a reservoir, radiates the heat of compression, and condenses the water entrained in the compressed air. The tank is a pressure vessel whose manufacture and testing is controlled by UL procedures similar to steam boilers and compressed gas cylinders. US Dept of Commerce regulations require a sheet metal label to be permanently welded to the exterior of any air tank sold in the US. This label certifies the tank service, safe pressure, hydrostatic test pressure, and other data including the alloy and gage of the sheet metal used for the shell and heads.
A common belief is that a large tank (actually, "receiver") is advantageous and will somehow compensate for an undersized compressor. Not true. Unless you have a very big tank, all a larger tank provides is a few extra seconds of surge capacity for short term, high demand tools like impact wrenches. As soon as the compressor kicks in, it's only the compressor delivery that runs the tool. The size of the tank determines how long it takes your compressor to fill it and how long for your tools to empty it.
There have been some terrible accidents from air compressor tank failures. Many of these are old grandpa stories but enough are true that wise people take careful care of their pressure tanks. The energy of the pressurized air is something like a weak bomb. Ductile or fatigue failure of the shell may be sudden and the reaction of a large volume of highly compressed air released in 1/4 second is enough to shoot the entire compressor off like a rocket, smashing anything breakable in its path.
Although some explosions are caused by failure of the cut-off switch coupled with failure of the pressure relief valve, most explosions are caused by rust. Be sure the pressure relief valve on your compressor is exercised once a year and that nothing is allowed to interfere with its proper operation. Still, the main enemy of an air compressor tank is water and the rust it causes. Hot air under pressure accelerates rusting a bare steel tank. Frequent draining of accumulated water is the best protection against rust. While it's not necessary to blow down the tank completely after every use, accumulated water should be drained at the end of each day of use. Since the drain is always inconveniently located under the tank, most commercial compressor users pipe the drain line to a conveniently located valve and route the discharge outdoors or preferably down a plumbing drain.
Compressor vibration, frequent charge/discharge cycles, high temperatures, and lots of moisture can quickly lead to internal rust and sometimes cause tanks to fail through pin holes and/or metal fatigue. If the tank starts leaking through pinholes chances are if you fix one another will be along soon. Pinhole leaks are like cockroaches. If you find one there are a thousand others, waiting, and the tank interior will be dotted with almost rusted through places. The one leak your find is only the first. If you see a streak of rust along a line starting from a weld or seam in the tank's construction, you most likely are looking at the beginnings of metal fatigue. This can be a dangerous condition because the final stages of fatigue failure can be very rapid if not explosive. For what it is worth, this is why many better air compressor tanks are painted a light color, so we can quickly see if rust is developing. My friend that sold these units said his firm always repainted refurbished units with a light color for this very reason!
This is a long way to convey a short message: if the tank leaks, replace it because it is not worth fixing. They aren't that expensive (compared to a new belt driven compressor) and most replacements have a universal frame to mount your pump and motor on and a plethora of welded-in connections.
Most large air compressors are driven by induction motors that are probably the most reliable component in an air compressor, but they are not bullet proof. It's important that their fans and air inlets are vacuumed (not blown) free of dust and lint. Likewise, you need to check the duty cycle and not run your motor beyond its rating. Many inexpensive compressor motors are rated for only a 20% duty cycle. This means running it for more than twelve minutes an hour can hurt the motor! Some of the small pancake compressors are driven by a series wound motor. If you find it necessary to replace the brushes, you may find it maddening to get at them. Pay close attention to disassembly order.
My friend who owned the compressor shop gave me a real eye opener in terms of hoses. Unlike catastrophic tank failures that are pretty rare, hose failures are common. He said one of the worst examples happened in his own garage. He had just bought a new car and had left his hose heaped in a pile next to his compressor when he stopped painting and went for lunch. The hose ruptured near the end and went flying all over the shop. The result left his pretty new car looking as though it had been beat with a hammer all over! Fortunately, his insurance company totaled his vehicle and let him go get another. His recommendation is to always install and use a good shutoff valve right at your compressor and release the hose pressure any time you leave your shop. He said turning off the valve and releasing the pressure is inconvenient, but an unpressurized hose lasts far longer and these steps would have saved his new car. He also recommended that we buy good quality hose, an automatic hose reel to keep it wound up, and to replace any hose that becomes worn, frayed or that gets badly kinked!
As with any worthwhile project, I first defined my needs and my wants. In this case I wanted a top quality compressor for personal use, but would use it for a host of activities that I knew little about. I made a list of my requirements and what I knew of air compressors. I then went to the on-line web sites for the more respected air compressor makers and added to my list those features that they felt made their products superior. I then added to that growing list information from the better air compressor advice posts from the Badger Pond, the Oak, and Wood Central woodworking forums. My list ended up having lots of features, selection information and care information. I ended up with a many dozen page disorganized list with considerable contrary advice. I arranged all by topic and rewrote much so it made more sense. I then called upon my friend in the air compressor business to resolve most of the concerns. The ones that still left me confused I got help from Forrest Addy to resolve, plus I incorporated his emailed responses.
I first had to determine how much compressor I needed. I do mostly woodworking, some painting, and a little metal work. I own more than my fair share of tools, including stuff left over from a period where I did some professional auto body work. I have some nice air sanders, light auto shop tools, pneumatic wrenches, air powered metal saw, air powered tin snips, a small and large sandblaster, and a quite a few paint guns. I also wanted to learn how and get more into HVLP painting, so spoiled myself with both an HVLP touchup and larger spray gun. I looked over the literature for each of my air tools and hoped for air tools making a list of what they needed in terms of pressure, cubic feet per minute (CFM) and what the duty cycles were on my tools. I learned the hard way years ago that if a random orbit sander maker says their unit needs 6 CFM at 90 PSI, it is really going to need about 50% more meaning about 10 CFM airflow. If a 4" disk sander requires 9 CFM you need an 18 CFM compressor to provide enough air, otherwise, you waste time waiting for the compressor to catch up. According to traditional wisdom, I sized my compressor at about double the largest air demand to make sure I got an appropriate compressor for my shop. All of my tools were 100% duty cycle meaning they could run as long as I wanted. Some tools, especially motors used in consumer air compressors need to only be run a percentage of the time or they will overheat then burn up.
My friend that owned the air compressor shop looked over my list of tools and their air requirements with me. As soon I mentioned I had spray guns and other air tools like my in-line sander, sand blaster, and a rotary sander, he said I had left the 115-volt plug-in-the-wall-outlet compressor bracket. He said all my air tools were under-rated for air consumption and very inefficient, even my expensive industrial tools. He said I need about 5 HP of compressor power to generate 1 HP of air tool power!
My friend Forrest added to this saying I should seriously consider giving up on my pneumatic sanders left over from ancient days when I used to do a little professional automotive body work. He said he uses electric sanders and avoids the whole problem of large compressors and rotary air tools with their carried over oil and water sprayed on his almost ready to paint projects. The electric 4" sanders have 115 volt 6 Amp motors that draw about 1/7 the juice of a 240-volt 22 Amp compressor motor.
Here's a list of applications and motor HP and electrical demand in ascending order:
Fill bicycle tires or run a nailer 1/2 to 1 HP (10 Amp @ 120 Volts)
Using a detail spray gun 2HP (9 Amp at 240 Volts) Because of its considerably reduced need for airflow, this is what most hobbyists should consider instead of buying a large industrial type gun.
General automotive use where air ratchets and impact tools are employed 3 to 5 HP (12 to 22 Amps @ 240 volts
Running a blast cabinet to do sand and bead blasting requires 3 to 7.5 HP depending on nozzle diameter (12 to 33 amps @ 240 Volts)
A typical spray gun requires 5 to 8 CFM. Doubling the largest rating equals 16 CFM. That requires a real 5 HP two-stage compressor whose induction motor draws 22 Amps @ 240 Volts.
My HVLP spray guns require 9 to 11 CFM which can easily exceed even the capacity of that 5 HP two-stage compressor!
Based upon my set of tools, my friends said I had two choices. I could buy at least a 60-gallon tank with not less than a real 5 HP motor powering a two-stage compressor that will deliver at least 11 CFM at 90 PSI. Alternatively, I could buy a smaller unit and keep using patience as I did with my Craftsman while waiting for recovery and make sure I gave it time to cool down after use. The cost difference is about double, I decided on a quality smaller unit, but not too small.
Here is the list of things they told me and that I already knew to consider in my air compressor purchase:
Buy an oil bath unit because the oil-less tend to have heating problems that would not be good for the amount of spraying and sanding that I do, plus they are much noisier.
They said there are two compressor preferences. Many prefer big slow pumps as they will wear forever. Others prefer midsized units with a fast motor to permit quick recovery. My friend with the air compressor shop prefers the slow movers. He recommends using top quality synthetic oil that you change as needed, depending on use, but not less than once a year. He uses the Ingersoll Rand synthetic oil because it is readily available at Home Depot.
I also needed to decide if I wanted a stationary compressor or one that I could move around. For portability most like a horizontal under 40-gallon tank or one with a short squat vertical tank and wheels. The tall ones were reported in the forums as having too high of a center of gravity so are dangerous for one person to try to move alone. We need two people to move or install the larger vertical tank air compressors. Many like the vertical tanks because they use less floor space. The horizontal tanks often will fit nicely under a bench or work table. The big ones should be moved in and left in place because they are so heavy. All need access and plenty of airflow so they can stay cool.
Buy one with a motor rated for continuous duty, preferably a good American made motor with 100% duty cycle rating. The huge airflows needed for my sanding and painting makes me an infrequent but very hard user. Hard use is something many inexpensive imported limited duty motors were never meant to handle. You can do bigger jobs with the smaller units, but they will force you to pause often to refill your air tank. Unfortunately, you also must pause at least every half hour to let your unit cool or it will build enough heat to self-destruct.
Most need at least a 30-gallon tank, anything less is going to not have enough reserve to do much more than fill tires and run impact wrenches. If you are going to paint you need at least a 60-gallon tank and for me to run my HVLP and larger air driven sanding tools I needed at least an 80-gallon tank. You must have enough tank capacity or you will have constant delays to recharge your tanks.
Find a unit with an easily accessible drain line and replace it with a cable pull drain. Apparently, until you get in the hundred plus dollar range, the automatic drains are not worth bothering with.
Buy one with an oil sight gauge that makes it easy to check the oil every time the unit is used, then make sure you check it each time!
Buy one that makes it easy to change the oil. Many have the drain plugs setup so all you can do is make a huge mess draining your oil all over your compressor. He said he often replaced drain plugs with an oil drain line that he made up with a valve. This let the end of the oil drain go into a bucket for an oil change without giving the compressor an oil bath.
Buy one with a built in muffler or that can add one later.
If I buy an upright, go to an air compressor store and buy four of the rubber or neoprene and cork vibration pads to set that unit on to significantly reduce the noise level.
Make sure the unit has or buy a dual set of gauges, one for the tank pressure, and the other for the line pressure. Put another gauge on fittings so it can sit right before my paint spray guns, especially the HVLP unit.
Make sure to get one with or add a line turnoff valve at the tank and use it, as it is bad news to leave the hoses pressurized. Always let the line pressure out of the hose after use.
Buy all my air fittings at once from the same maker as mixing and matching always leads to leaks and frustration. They and many others said don't buy them from my Tools By the Pound store. Sticking with a popular brand name helps, as fittings can be added later and generally work without leaking.
Install a water filter with built in drain to protect my spraying. The size of that filter depends on how humid each area is. In my area, a tiny one is ample except in the wet winters.
Although there are many ways to plumb a shop for air, both of my experts recommended the same thing. Start with a heavy flex hose that connects the air compressor to wall mounted copper pipe. That flex hose takes care of the vibration of the compressor. Run the copper pipe up (or in) the wall and across the ceiling to a pull-down self-winding air hose reel located in the center of my shop. One recommended also running a line to my workbench and using one of those self-winding curled little 6' hoses. Here is a site that gives a good idea on how to setup your air compressor lines and avoid water problems.
Copper pipe comes as either flexible tubing or rigid pipe. The flexible tubing is not suitable for an airline. The heaver "L" gauge rigid pipe is what I recommend but you can get by with the lighter "M" gauge. David Craig pointed out to me that the authority on copper pipe is http://www.copper.org/publications/pub_list/pdf/copper_tube_handbook.pdf and their Table 3 shows either the "L" or "M" rigid pipe in 1/2" diameter will work fine. My air compressor expert agrees. For sure use 1/2" pipe, as the smaller diameter pipe adds a huge amount of resistance and reduces airflow.
My friends recommended using sweated fittings to plumb the airlines and feed the hose reel. Don't forget particle and water traps at every service drop. You might want to look at this Copper Soldering Website for information on copper piping and how to solder.
Both said PVC works and is plenty strong, but not something either would use. When this stuff gets old or cold enough it becomes so brittle it can easily explode.
Install your air lines so that they drain any water to your vertical run(s). At the bottom of each vertical run, put in an inexpensive finger operated drain to get rid of any condensation, then regularly drain those pipes (the box stores sell the inexpensive CH units).
Buy a nice self-winding hose reel and mount it centrally on the shop or garage ceiling. The hose reel is not just a convenience it is also an important safety item as it keeps the hose under control if it should rupture.
Always buy a good quality air hose of at least 3/8" interior diameter with at least a 300 PSI rating. Make sure you limit your air hose to about 25 feet as longer lengths really degrade the airflow. If you really do need a long hose, seriously consider moving up to 1/2".
- Making a Choice
After all my research I wanted a 5 hp two-stage 60 to 80-gallon Quincy commercial compressor with the same sized Ingersoll Rand as my second choice. My budget forced another direction and left me considering a big Campbell Hausfeld from Home Depot, a big two stage oil-less unit from Sears, a mid-sized Ingersoll Rand from HD or Harbor Freight, a Coleman Black Max from Sam's Club, or the similar unit from Costco. They all looked pretty much the same, so I again called for help, plus asked for help on the few woodworking forums I follow.
I got a lot of advice and some very strong opinions on a few units.
One fellow said he was suing one well known vendor after having bought their oil-less two stage compressor. He says it is a true two stage compressor and will deliver 175 PSI, but not at all suitable to power paint guns or rotary air tools both because it cannot handle the required continuous duty and its performance is far below what the vendor rating. As a result, once the tank gets drawn down, the compressor will not keep up, so it burned up. Many others made similar comments about their single stage consumer units saying they simply burned up the first time they used them hard.
The advice from those who seem to best know their stuff was if you can afford it, you should buy at least a 5 hp two-stage Quincy. These are known to run at slower speeds and outlast the competition. Most gave the same 5 hp two-stage 60- or 80-gallon tanked Ingersoll Rand as a second choice. All cautioned me to stay away from the near identically looking single stage units as these run far hotter, do not last nearly as long and cost far more electricity to operate.
These same knowledgeable people said if budget is a problem then the next best choice is to buy one of the Ingersoll-Rand Garage Mate units carried by many firms. Although these are single stage compressors, my friend Forrest recommends the Garage Mate as the best value he has seen for the home shop user (no plug intended). My local Home Depot used to carry the Garage Mate compressors with either horizontal or vertical tanks with either a smaller or larger tank size. I found many available sources, some that sell with no local sales tax or shipping charges.
I looked one over and decided the upright version of the larger tanked Garage Mate was right for me. With a big Spring project, I opened my wallet, saw a few moths fly out, and went to my local Home Depot. There I found Home Depot had these units on a great sale, but they were sold out! We have five Home Depot stores within fifteen miles, so I did some running around, but none had the air compressor I decided to buy. After lots of calls I found a distant Home Depot had one. In my running around I did find a near new 80 gallon 5 hp two stage Ingersoll-Rand vertical air compressor for sale at a ridiculously low price. Someone returned that unit to Home Depot after minimal use. Home Depot had it their discount pile in a different part of the store and it had been marked down repeatedly to a price of about the same as an Ingersoll-Rand Garage Mate. I bought it with their full warranty! It only takes a little more room than the Garage Mate vertical I was looking to buy. It did take two strong men and a van to get home and set up. It runs like a champ!