For these reasons, let’s examine the air compressor
itself and review what you can do to make your existing unit run
cooler and last longer.
The tank on an air compressor is a pressure
vessel, so it’s vitally important that the welds that hold together
the tank are correctly done – and they are if they’re performed
to the appropriate welding code. The code for air pressure vessels
is rigidly proscribed according to the American Society of Mechanical
Engineers (ASME) standard. And after a tank is built, it’s tested
and/or inspected to make sure it complies with the standard. You
know those great big plugs in the tank sides? They aren’t there
for you to connect 5-inch diameter piping to; they’re inspection
holes. Someone actually pulled those plugs and inspected the welds
inside the tank. Having passed the tests, the tank has a metal
inspection tag tack welded on the outside, providing permanent
proof that the welds that form the tank met code. (Low-cost compressors
advertised in the Sunday supplements are seldom ASME certified.)
We rate the size of air-receiver tanks in
gallons when used with a compressor. The most common sizes in
our industry are 20-, 30-, 60-, 80- and 120-gallon tanks. Larger
tanks provide a small cushion of extra air for temporary bursts
in demand and serve to cool the compressed air temperature as
Tanks have their mounting feet welded on to
form either a vertical or horizontal unit. Vertical tanks offer
several advantages. The vertical orientation allows the moisture
in the cooling air to fall further, so, all else being equal,
a vertical tank provides slightly drier air at the discharge.
Vertical tanks also use less floor space. On the other hand, vertical
tanks are difficult to transport because air compressors with
the motor and pump mounted on the narrow end of a tank are very
top heavy. If you must move a compressor to another location,
a horizontal tank slides right in the back of a pickup truck,
and you can drive normally to your destination. A vertical tank,
however, needs to be adequately tied down.
Once in place in the shop, tanks will ideally
be mounted on special pads that absorb some of the natural vibration
of the unit. These pads are often a sandwich of cork and rubber
that bolt directly to the welded-on tank feet. A vibration hose
is a steel-braided, flexible hose that connects the tank to the
wall piping. This quiets the echo in the air lines by further
isolating the compressor’s vibration.
The electric motor is the power source that
causes the compressor pump to run. A helpful rule of thumb is
that for every one horsepower (1 hp) on the electric motor, the
pump should displace about four cubic feet of air (4 cfm). By
this reasoning, a 5-hp compressor will displace about 20 cfm,
and a 10-hp motor will drive approximately a 40-cfm pump. When
you read an ad in the Sunday paper about the super-cheap compressor
at the local farm supply store, look at the displacement per horsepower.
Their “5-hp” unit often only produces 10 or 12 cfm of
air. By the industry standard rule of thumb, this is only a 2-
to 3-hp outfit.
Specifications on electric motors include
their voltage, their cycles and their phases. Written volts/cycles/phases,
the markings look like this: “220/60/1” or “230/60/3”
or “110/60/1” or even “440/60/3.” The voltage
varies with what’s produced by your local utility, while the cycles
are always “60” in the United States. (This is why your
toaster or shaver often won’t run in another country.) The last
marking for phases signifies either single-phase or three-phase
Single-phase electricity is household current.
If you have an electric stove or electric dryer at your house,
you have 220-volt/single-phase current. Three-phase electricity
is an industrial current, so even if three-phase power passes
by on the pole outside your building, you’ll have to qualify as
a sufficiently large user for the power company to bring it down
to your shop. Three-phase electric motors are cheaper to buy and
cheaper to run, if you can always be assured of getting three-phase
current. By this same token, resale on used three-phase compressors
is pretty slim because relatively few people have access to the
Most customers describe an air compressor
as a “5-hp, 80-gallon unit” or a “7 1/2-hp, 120-gallon
outfit.” While this says a little about the motor and a little
about the tank, it says nothing about the main component of the
compressor: the pump.
Piston-driven compressors (the only type covered
here) are described by the number of cylinders they have and by
how many times the air is compressed. Counting cylinders is easy:
simply count the number of jugs/pistons. Automotive compressors
are either one-, two-, three- or four-cylinder units.
Not quite so obvious is how many times the
pump compresses the air, or the number of “stages.”
It’s possible, though, to tell the difference by looking at the
pump. On a two-stage compressor, the second cylinder is approximately
one-half the diameter of the first cylinder. It’s also possible
to spot a single-stage or a two-stage compressor by reading the
printed specifications. A single-stage compressor will develop
a maximum pressure of about 125 psi, while a two-stage compressor
will develop about 175 psi maximum pressure.
A two-stage takes the air into the large cylinder
at atmospheric pressure (14.7 psi) and compresses it to about
40 psi in the first stage. It then passes the air at 40 psi to
the smaller cylinder, where the air is compressed again to the
approximate 175-psi maximum. (How are really high air pressures
compressed for scuba divers? With a multi-stage compressor, in
which each cylinder is approximately half the size of the preceding
There’s a difference, however, between the
volume of air that’s displaced by the piston rushing up and down
in the cylinder and the volume of air that’s actually available
for use at the tank outlet. Displaced air is a mathematical calculation
without allowance for heat friction or other losses: Cylinder
bore x piston stroke x pump rpm yields the theoretical volume
of air compressed in one minute (cubic feet per minute).
But because determining the actual amount
of delivered air at the pump outlet takes more than a mathematical
equation, the compressor manufacturer has to provide the information,
which is dependent on such things as the style of intake and exhaust
valves, bearings, and connecting-pipe shape and diameter. The
delivered air volume is also pressure related, meaning that as
pressure per square inch goes up, volume delivered in cfm goes
down. For example, if a compressor displaced 20 cfm of air, it
might deliver 16 cfm at 100 psi. That same compressor would deliver
more air at a lower pressure, i.e. 18 cfm at 40 psi. (That dirt
cheap compressor in the Sunday paper is often advertised at the
highest delivered volume but at the lowest possible pressure.
And this is OK as long as your air use takes place at the quoted
Dividing the actual delivered air volume by
the total air volume displaced is a good way to compare one brand
of compressor to another. If the compressor displaced 20 and delivered
16 cfm, it would be 80 percent efficient (16 cfm divided by 20
cfm equals .80). Volumetric efficiency for automotive compressors
is usually above 70 percent and, naturally, higher numbers are
representative of a better pump.
Another important measure of the quality of
the compressor pump is the actual revolutions per minute (rpm)
that the pump spins to compress the air. Low numbers like 650
rpm will make the pump last longer than a high rpm like 1,500.
Given the choice of two compressors with identical air volumes,
the one with the slower running pump should last longer. Much
like driving your car at either 65 mph or 150 mph, hopefully the
slower speeds would stress the engine less and help it to run
Other desirable features when shopping for
a new compressor include: multiple piston rings per piston to
keep good compression and serve to prevent any oil blow-by; honed
cylinders for a smooth-running unit; a positive oiling system
for good lubrication; and good bearings, which make any piston
device last longer.
Other unique components to an air compressor
often leave people confused. To clarify any confusion, what follows
is a brief description of some of the special parts that make
the compressor work:
A pressure switch is a pneumatically operated
electric switch for starting and stopping the electric motor.
These devices have both an electrical connection and an air line
connection. When the pressure in the tank falls because of use,
the pressure switch reads the drop-in air pressure and closes
the connection that makes the electric current pass to the motor.
Typical cut-in, cut-out pressures are 140 psi for the pressure
switch to turn on the electric motor and 175 psi in the tank to
close the switch and shut off the current. Because the connecting
points inside this switch can be easily contaminated by all the
sanding dust in a body shop, periodically take off the cover and
clean it with electric contact cleaner.
- A pop-off valve is a safety device that prevents the
unit from exploding if the pressure switch fails. Imagine that
the pressure switch breaks, and the pump continues to run past
the 175 psi maximum pressure. At some point, the pump will fail
because it can’t compress any more air against the huge pressure
in the tank, or the welds that hold the tank together will rupture
from the high pressures and the tank will explode. Before either
of these catastrophes can occur, the pop off blows and bleeds
the excess pressure harmlessly off into the shop.
Some pop-off valves have a small ring just the size of a finger
attached to the end. This is a method to test the functioning
of the pop-off valve. Feel free to grab on and pull the ring next
time you and a tech are standing around the compressor. (Don’t
tell him your plans and watch him leap into the air in surprise
as 175 psi blows off in a loud burst. Good fun as long as he doesn’t
have a heart attack!)
- A check valve is a one-way valve. On an air compressor,
it will allow the compressed air to flow into the receiver tank
but won’t allow it to pass backward up to the cylinder. These
valves are often located right where the pump discharge meets
the air tank.
- An unloader isn’t too common in our industry. These
are more popular in an industrial environment, where the demand
for compressed air is constant except during coffee breaks and
lunch time. An unloader simply holds open the intake valves while
the motor and the flywheel continue to run. When it’s time to
go back to work, the valves close and the pump once again compresses
- A motor starter is a switch to provide overload protection
to the electric motor. Rather than overwhelm the motor with the
huge amperage needed to get started, a magnetic starter feeds
in the current gradually. (Instead of starting a big rock rolling
down a hill by whacking it once – really hard – with a fence post,
you start the rock rolling by giving it a steady push, slowly
- A centrifugal pressure release (CPR) bleeds
the pressure off the top of the cylinder head so the pump will
start easier on the next demand. Without this pressure bleed-off,
the pump would have to begin each time by pushing against the
175 psi that was left on top of the piston. When you hear the
compressor stop, followed by a “psssst,” that’s the
CPR at work.
Preventing Air Line Moisture
Paint work gets ruined when compressed air still has moisture
in it by the time it gets to the spray gun. Moisture comes in
three forms: solid (ice), liquid and vapor. The difference, of
course, is the temperature. Hot air holds more moisture vapor
than cool air does – which is why humidity is so bad in the summer,
and air is dry enough to cause a static spark in the winter.
The air compressor generates its own heat by virtue of the pistons
sliding up and down, rubbing metal against metal. The head temperature
on an overworked compressor can reach 350 degrees F, and air this
hot will hold lots of water vapor. The “typical” discharge
temperature out the side of the tank is often about 110 degrees
Once the air cools down enough, some of the water vapor will turn
to liquid. A helpful rule is that for every 20 degrees F you cool
the compressed air, one half of the moisture vapor will turn to
liquid. And once the moisture is in liquid form, it’s easy to
trap. Ordinary moisture traps (which currently hang on your shop
walls) simply knock the liquid water out of the compressed air
by smashing it against a baffle plate inside the trap. All the
technician has to do then is drain this liquid water out of the
trap at regular intervals.
Note: If the effluent that runs out of your trap is white, your
compressor is pumping oil. The white liquid is really oil droplets
riding along on the back of water droplets. Solve the oil problem
by repairing the pump.
The compressor manufacturer also takes several steps to ensure
that the air temperature cools off as much as possible. These
“aftercoolers” are the finned piping that carries the
air from the pump to the tank. Most models pass the air piping
between the pump and the flywheel, which acts like a fan blade.
The spinning flywheel cools the pump itself and any piping that’s
in its path. As the air cools, some of the moisture turns to liquid
and can be trapped out easily.
Besides the steps the manufacturer takes to decrease moisture,
there are also several things you can do.
- Installation – How the compressor is installed will
have a big effect on how much moisture will remain in the air
line. For example, it helps to keep the flywheel at least 1 foot
from the wall for air movement. It’s safer with the flywheel toward
the wall (no fingers inside the belt guard), but it’s better cooling
if the flywheel can face out because it can take in more air.
Ultimately, it’s best to place an air compressor where it’s easy
to service – or no one will service it! On many compressors, if
you face the flywheel away from the wall, the oil is difficult
to change because the drain plug now faces the wall.
- Piping – Piping your shop correctly will do as much
as anything to prevent water problems in your paint work. The
trick is to get the air to cool down and the moisture to turn
to liquid. By this reasoning, high-pressure copper pipe (green
stripe, K Code) does the best job because it dissipates heat so
well. But because it’s relatively expensive, some shop owners
can’t see the value.
The next best solution for shop piping is to use galvanized pipe,
which won’t rust inside. A common, and less expensive, choice
is to use black gas pipe. The initial cost is low, but the rust
that forms inside the pipe will eventually blow down stream and
could cause problems. Plastic pipe isn’t a particularly good choice
either. It won’t rust, but plastic is a great insulator, so the
air doesn’t cool off much.
- Maintenance – In most shops, no one pays any attention
to the compressor until the day it quits working or the moisture
problem becomes so bad that cars need to be repainted frequently.
Simply changing the oil at regular intervals (if you can’t remember
the last time it was changed, now would be good) helps to keep
the internal parts well lubricated. Also, low oil in the crankcase
and sticky valves can cause the pump to work harder and run hotter.
So will a clogged intake filter.
Keeping the cooling fins of the aftercooler or the fins that are
cast into the pump clean and free of a dirt/grease build-up will
make the pump run cooler, while keeping the pump itself clean
allows the cooling fins to do their job of dissipating heat.
An annual look at the valves would also be time well-spent – particularly
if your compressor intake isn’t vented outside. All that sanding
dust that fills the office and ruins the paint work will also
wreck compressor valves. Keeping the intake filter clean is easy
and makes a big difference in how hard the pump has to work to
produce the air. Daily maintenance should include draining the
compressor tank and the moisture traps and opening the air valve
on the drain legs (the last takeoff on an overhead line) to remove
the liquid water that’s already trapped.
- Window fan – One cheap trick that may help your shop’s
moisture problem on the hottest days of the summer is to simply
cool the compressor pump with a window fan. When the intake air
is already 100 degrees F and full of water vapor, by the time
the compressor heats it up some more, the water problem is enormous.
If you can’t afford to change all the piping to large-diameter
copper or you don’t have a couple grand for a refrigerated or
desiccant dryer, simply open the doors and windows in the compressor
room to get some cooler air inside and then just take a window
fan (or two) and point it at the pump. Remember, for every 20
degrees F you cool the air, half the water vapor turns to liquid
and can be easily trapped out. It’s not as good as a permanent
solution, but it’s better than nothing!
Desiccant and Refrigerated Air Dryers
Running the compressor all day, every day makes for hot discharge
temperatures, and hot moisture vapor will blow right through a
conventional moisture trap. So, while the tips discussed above
are helpful, often, more needs to be done.
There are two basic ways to attack the moisture vapor in a paint
shop: a desiccant drier and a refrigerated air drier. And both
will dry the air to the point necessary for trouble-free painting.
Which method is better for your shop? This is a decision you need
to make with the respective vendors.
- Refrigerated air dryers – Remember that moisture in
the air is easy to trap if it’s turned liquid. A refrigerated
air drier is simply an air-conditioning compressor and a regular,
old moisture trap together in the same box. The air comes in at
about 110 degrees F and is immediately cooled to about 30 degrees
F. Virtually all the moisture turns from vapor to liquid and is
trapped out by a simple, smashing, baffle-type trap.
- Desiccant dryers – A desiccant drier has at least three,
and sometimes four, parts. The first stage is a standard moisture
trap that knocks out the water that’s already liquid, and the
second stage is an oil coalescer that collects any oil blow-by
before it can ruin the desiccant in the third stage. (Imagine
taking a sponge from the kitchen sink and picking up motor oil
with it. Even if you wring it out when you’re done, it won’t ever
pick up much water again.) The third stage is a chamber that holds
desiccant beads of some design. The desiccant absorbs or holds
the water even in vapor form. Once saturated with water, the desiccant
must be replaced or dried out and renewed by heating the beads
to drive off the moisture. Some units offer a fourth-stage filter
that traps any tiny dust particles that are the result of the
desiccant beads deteriorating through use.
To a Long and Healthy Life
If it weren’t for your air compressor, you’d be out of business.
So, obviously, it pays to pay some attention to this misunderstood
and often neglected unit.
By understanding how an air compressor works, what your shop
needs in an air compressor and what you can do to increase a compressor’s
life expectancy, you’ll ensure that your air compressor gives
you many years of trouble-free service – and many years of problem-free
Mark Clark, owner of Clark Supply Corporation in Waterloo,
Iowa, is a contributing editor to BodyShop Business.