A catalog of features and specifications to look for when
choosing a job-site air compressor
There’s a huge number of small compressors on the
market. That’s because portable air compressors are
used for everything from running jackhammers and sand blasters
to air wrenches and paint guns. They’re used by nearly
every industrial trade, as well as by homeowners.
However, in our industry — residential construction
— most compressors are enlisted into active duty for
running nail guns on site. With this in mind, I will focus here
on directing builders and remodelers towards the features and
specifications to look for on smaller hand-carry models (those
with 11/2- to 4-gallon tanks and electric motors in the 3/4- to
2-hp range), and on typical wheelbarrow models (those with
8-gallon tanks and 11/2- to 5-hp electric motors or 4- to 8-hp
Portable compressors come in a variety of different
configurations. Single tank designs include the pancake (top
left) with a single oval-shaped tank, and the more conventional
shop-style design (top right). Twin-tank models come with
horizontal tanks or a vertical stack (bottom left &
bottom right). These are differences of convenience, not of
compressor is a very simple machine. It’s essentially
a piston under power that pushes air into a tank. Simple valves
— no more than flaps that open or close in one
direction — allow air into the cylinder on the
piston’s downstroke. As the piston returns on the
upstroke, this trapped air is forced into a storage tank. Each
piston stroke sends another gulp of air into the tank until the
tank reaches a maximum operating pressure (about 120 psi on
portables). At this maximum, a pressure switch turns the pump
off. The faster the piston moves, the faster pressure builds.
The time from when a compressor kicks on to when it reaches
maximum operating pressure and cuts out is the recovery
As the pressure in the tank builds, the piston must have
enough force to push against the increasing backpressure.
Piston rings provide a seal to prevent air from blowing back
through the cylinder. But as a compressor wears, this seal
breaks down, and it takes longer and longer for pressure to
build in the tank.
For job-site compressors, the recovery time can be from 10
to 30 seconds. Any machine that takes longer than this is
underpowered or in need of a rebuild.
Nearly all hand-carry portable
compressors made for running nailers (those with a maximum
operating pressure set at around 120 psi) have simple
single-stage air pumps, with a single cylinder and piston as
described above. Some larger wheelbarrow models (as well as
many shop-sized stationary rigs) have a twin-piston pump, known
as a two-stage compressor. This type of pump pushes air from a
large cylinder into a smaller cylinder, where it is further
compressed by a second, smaller piston. Between the two
cylinders (compression stages) the air passes through an
intercooler — a pipe with cooling fins. Compressing
air causes it to heat up and expand, so the cooler the air, the
more easily it is compressed.
Two-stage compressors are used whenever a system needs
operating pressures above 150 psi. While this is excessive in a
portable, a two-stage pump will be able to compress air in
large storage tanks with quicker recovery times. If you need
lots of air for running many framing or roofing nailers at
once, opt for a two-stage pump.
Oil-less vs. oil-bath.
Less than a decade ago, almost all job-site compressors were
built with oil-bath pumps. As the piston crank in these pumps
turns through a pool of oil, the oil splashes up on the
cylinder walls. The bottom-most ring on the piston is
corrugated to capture this oil and spread it around. This not
only lubricates the piston, but provides an airtight seal
around the upper two piston rings.
Oil-bath compressor pumps are typically built with cast-iron
cylinder heads that can be machined to close tolerances. As
long as the pump is topped off with oil (and kept near level so
the piston properly splashes in its bath), an oil-bath model
will provide years of trouble-free service. It’s not
uncommon for a well-maintained oil-bath compressor to run 8 to
10 years before needing an overhaul, even with continuous
In the last decade, oil-less compressors have taken a
foothold in the compressor market. These pumps have a piston
ring impregnated with Teflon (or similar type of material).
This "self-lubricating" piston rides against an aluminum
cylinder. Oil-less pumps don’t have a crank shaft. The
piston rides on an eccentric wheel that comes straight off the
motor shaft. And the piston arm doesn’t have a wrist
pin. Instead, the arm rises through the cylinder at an angle,
so the piston wobbles from side to side as it pumps. By design,
oil-less pumps are built to looser tolerances than oil-bath
pumps, and need to be rebuilt more often. Most are rated for
about 2,000 hours of operation.
Oil-less compressors can be rebuilt fairly easily, using
kits provided by the manufacturer. The first time I did it, it
took me about 40 minutes. Once I knew what I was doing, I got
it down to about 20 minutes. Each time, the parts cost about
$45. Rebuilding an oil-bath unit, however, is a job for a tool
repair shop. It’s a messy job, on account of all the
oil, and in most cases, the cylinders will need honing.
It’s a 1- to 2-hour job at a cost of $25 to $35 per
hour. New rings will cost about $25 to $30 per cylinder and new
bearings about $35.