Bill Pentz Aircompressor Purchasing Advice

Small shop and home air compressors are fairly simple. A motor turns a pump that feeds air through a backpressure valve into an air tank. That backpressure valve keeps the compressor from having to work against the already compressed air. The motor is turned on and off by a pressure switch mounted on the tank. A pressure relief valve (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 sealed to keep the tank pressurized without having to keep your hose pressurized. This is something you always should use if you leave your tank pressurized! 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 bigger and better quality air compressors use an unloader 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, 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 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 compressors as their name implies do not require lubrication. They generally have a motor directly driving the pump. This makes for a simple and reliable compressor if used lightly. Typically they 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. Most small oil-less compressors will 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. However, they are not intended for steady use where the pump runs more than about 20% of the time in spite of many trying to use them that way. When you start using a paint sprayer, air tools, a sand blaster, or other high demand air tools, you can quickly ruin an oil-less compressor in a matter of hours, as they were never meant for continuous or high-volume use.

For oil-less compressors the incoming air passes through a rudimentary filter but their crank assemblies and the bottoms of the pistons and cylinders are exposed to ambient air and whatever dust it carries. If the air in your shop is damp or dirty with abrasive or other particles like sawdust, these materials will clog your filter and shorten the life of your pump through corrosion, wear, or seal deterioration. It generally costs more to repair an oil-less compressor pump than it does to replace it.

Belt-driven compressors use an induction motor driving a fan belt that turns a larger wheel on a compressor. The better belt-driven compressors have vanes on that compressor drive wheel so it also 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 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.

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. Doing so generates a huge amount of heat that can quickly overheat and ruin a compressor 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. A three HP compressor is about the point where thermodynamic efficiency makes a two-stage compressor far more economical.

A two-stage compressor is designed to permit continuous running 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. 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. For two stage compressors, the first stage cylinder heads will have a separate pressure relief valve.

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 makes 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 for most air tools and shop uses that need 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 changing out the motor pulley for one about 30% larger (calculate the actual diameter using Boyle's Law and common sense). These two changes is 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.

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.

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.

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.

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.

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.

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 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's a thousand others, waiting. The interior of the tank 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 larger 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 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!