Picking a compressor means treading a minefield of fraudulent claims. No matter what it says on the side of the tank, ALL consumer grade compressors are deceptively or fraudulently rated. I'm not suggesting they won't compress air or give good service. I'm saying you have to divide the available specs by a large BS factor get a compressor capable of fulfilling your requirements.
Ignorance will not only kill you but lead to you to squander money.
If you don't understand the basic physics of compressed air you're at the mercy of people who baffle you with an impressive technical vocabulary but who haven't a clue on how to spec out an air system. Be stubborn and skeptical. Compliant customers feed the fraud frenzy. Enough rant.
Here’s an introduction to home shop air compressors
A consumer grade air compressor is actually a unitized system consisting of a motor, a pump, tank, a pressure relief valve (sometimes called a pop-off valve) and a pressure switch. Often there’s a pressure regulator, an unloader, and some ancillary gadgets like a tank drain and a tank stop valve.
There are two basic compressor layouts, horizontal tank or vertical tank with the pump and motor mounted on a bracket welded on top of the tank. If you expect to move the compressor frequently, get the horizontal arrangement because of its low center of gravity. The vertical arrangement uses half the floor space.They’re intended to be moved in and left in place because they are so top heavy.
Compressors divide neatly into oilless and belt driven.
The oilless compressor pumps are directly coupled to the motor. Typically they are noisy, not particularly efficient, low first cost units designed for the occasional user where high duty cycle and longevity isn’t a major consideration. As the name “oillless” implies, there is no lubrication required. While they are simple and reliable, they are not intended for daily or commercial duty although a good many serve that exact purpose. 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 dust is abrasive or contains materials promoting deterioration of the pump through corrosion or seal deterioration the pump’s life will be shortened. When an oilless compressor pump dies it’s usually cheaper to replace the whole unit than fix it.
Belt driven compressors feature a separate induction motor driving a reciprocating compressor via a V belt reduction. Belt driven compressors are perceived as quieter, more efficient, and more durable than oilless and my experience has borne this perception out.
The belt driven compressor pump is built along the lines of an internal combustion engine where the crankshaft and other parts run in a sealed crankcase and 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. Examples of each have given reliable service for generations with little or no maintenance beyond oil replentishment.
The vulnerable part of any compressor pump is the valves. It’s generally a good idea to buy a valve and gasket kit when you buy the compressor. 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 setpoint. 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. Thus, the pressure relief valve is a safety device.
There’s been some horrific accidents attributed to pressure vessel failures. 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 150 PSI air released in 1/4 second is enough to shoot the entire compressor off like a rocket, smashing anything in its path. 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.
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 start 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 main function of the air tank is to serve as a reservoir, radiate the heat of compression, and to condense 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 requires a sheet metal label to be permanently welded to the exterior of any air tank sold in the US certifying its 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 somhow compensate for an undersized compressor. Not true: A large air tank gives you nothing more than 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 the length of the charge/discharge cycle.
The main enemy of air compressor receivers is water and the rust it causes. Air under pressure accelerates rust in 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 before and after use. Since the drain is always inconveniently located under the tank, most users pipe the drain line to a conveniently located valve and route the discharge outdoors or preferably down a plumbing vent.
Compressor pumps vibrate and the frequent charge/discharge cycles linked with internal rust pits sometimes cause tanks to fail through pinholing and/or metal fatigue. If the tank starts leaking through pinholes, chances are if you fix it 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.
This is a long way to convey a short message: if the tank leaks, replace it because it aint 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.
Induction motors are 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. A few 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.
Most any small oil-less compressor will serve a nailor, pump up the snow tires, and supply an occasional blast of air while lasting for a good many years. I have a heavy duty 23 CFM compressor I seldom use except for sandblasting. 99% of my compressed air is supplied by a 7 year old 1 HP Costco hot dog compressor.
As soon as you consider sprayguns and rotary air tools like a 4" sander, you instantly leave the 115 volt plug-in-the-wall-outlet compressor bracket.
Cheap import sanders are under-rated for air consumption. Furthermore any rotary air tool is VERY inefficient, even the expensive models used in industry. They typically require 5 HP of compressor power to generate 3/4 HP of air tool power. If an import sander spec says it requires 6 CFM at 90 PSI, count on 9 to 11 CFM of actual air consumption. If a 4" disk sander requires 9 CFM you need an 18 CFM compressor to run it, otherwise, you waste time waiting for the compressor to catch up.
According to traditional wisdom, you have to size a compressor to about double the largest air demand. Restating: to size a compressor, pick your air tool having the largest continuous demand (as opposed to a tool used in bursts) and double it to spec a compressor suited for your shop.
A three HP compressor is about the point where thermo-dynamic efficiency makes a two stage compressor economical. A two stage compressor pumps 20 to 30% more CFM per motor HP thanks to the heat of compression dissipated by the intercooler installed between the low pressure and high pressure cylinders. Add up the power savings over the 15 year working life of a two stage compressor compared to a single stage and you’ll find the 20% represents enough to pay for the two stage compressor several times over.
A two cylinder compressor is not necessarily a two stage compressor. The cylinders may be in a V configuration or side by side. In a two stage compressor a larger first stage cylinder takes atmospheric air and compresses it to about 1/3 the delivery pressure. The intermediate pressure air passes through the intercooler (the finned tube behind the pump flywheel) to be cooled by windage and into the second stage where it’s compressed to the delivery pressure. The first stage cylinder head will have a separate pressure relief valve. A common alternative design has two low pressure cylinders pumping through an intercooler into a third high pressure cylinder in a “W” configuration. In this design the low pressure cylinders are only slightly larger than the high pressure cylinder.
A two cylinder single stage compressor will have two side-by-side cylinders of equal size and no intercooler. Unscrupulous marketers may sometimes peddle a two cylinder single stage compressor as “two stage” so be alert if you find a “bargain”.
A consumer grade compressor run continuously will fail prematurely. A typical spraygun 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. A 5 HP 60 gallon vertical tank compressor occupies only a little more floor space than a 3 gal pancake but, because it’s nearly 6 feet high, it won't fit under the workbench.
Here's a list of applications and motor HP and electrical demand in ascending order:
Fill bicycle tires or run a nailor 1/2 to 1 HP (10 Amp @ 120 Volts)
Spray paint 2HP (9 Amp at 240 Volts)
General automotive use where air rachets and impact tools are employed 3 to 5 HP (12 to 22 Amps @ 240 volts
Running a blast cabinet 3 to 7.5 HP depending on nozzle diameter (12 to 33 amps @ 240 Volts)
Home Depot sells a good 5 HP two stage Ingersol Rand home duty compressor with an 60 gallon tank for $899. I regard it as a good buy for the home shop user (No plug intended).
The Sears oil-less two stage compressor is not suitable to power rotary air tools. While it is a true two stage compressor and will deliver 175 PSI, the Sears two stage compressor, if honestly rated, would be about 2 real HP. Once the Sears two stage is drawn down to cycling it won't quite keep up with an import 4" air sander under load (yes, I ran a test).
As a side issue, I use electric sanders and avoid the whole problem of large compressors and rotary air tools with their carried over oil and water sprayed on my projects. The electric 4" sanders have 115 volt 6 Amp motors which draw about 1/7 the juice of a 240 Volt 22 Amp compressor motor.
By the way and for what it's worth, most two stage compressors are set for 175 PSI service - too high for most air tools and shop service. If air is compressed much over the required line pressure, energy is wasted when when tank pressure is reduced to line pressure at the regulator. If you change out the motor pulley for one about 20% larger (calculate the actual diameter using Boyle's Law and common sense) and reset the pressure switch to kick in at 105 PSI and out at 125 PSI, you'll have extra delivery, lower duty cycle, cooler compressor operation, and lower power bills.
Any extra wear caused by higher pump speed is more than offset by the lower interstage and discharge pressures and lower head and reed valve temperatures.