Air compressor selection and info


Elite Cafe Member
Mar 25, 2007
Florida USA
Thread starter #1
Well, I don't know anything about the engraving equipment y'all are talking about but I do know controls & compressors.

First off, short cycling on a compressor will subject it to rapid wear on all the moving parts and it becomes costly on the utility side because on every start cycle you have a high electrical in-rush current - since the electric bill is calculated by the amount of current you use, every start cycle is costing you additional money.

Because of the in-rush current, every start cycle generates more heat within the electrical components shortening thier normal life span. You also have mechanical shock loading on moving parts on each start cycle shortening their life span as well. For these reasons, it is always best to have a compressed air system with as much storage volume as possible. You can increase storage by adding tank(s) but please keep in mind that each additional tank must be equipped with a properly sized and rated automatic safety pressure release valve and each tank must be rated at or above the maximum operating pressure of the system. If your compressor is short cycling, you may or may not have a control issue depending on the amount of system storage volume you have available. Generally, small recip compressors are set with about a 30 psig differential meaning that if it cut-out at say 120 psig, it should engage somewhere around 90 psig. If the unit is cycling with less than a 30 psig differential, then you have a control problem and most small units are equipped with a non-adjustable control switch and the entire unit must be replaced with an OEM spec unit because most are built brand-specific and you cannot obtain parts from anywhere other than the OEM. This is why I stress the point that when buying a compressor, make the initial investment for a high quality professional unit and it will save you more than the cost difference in the long run.

Air leaks are another major killer of compressed air systems. If you're system is loosing more than 10 psig over a 24 hour period, the leaks need to be stopped.

Moisture and heat within the compressed air are the two biggest enemies of air powered equipment. If you do not run an after-cooler/moisture separator system between the compressor head and storage tank, adding additional storage tank(s) will definitely help because the air will have more time to cool allowing more moisture to drop out in the tank rather than being pushed through the lines but this does not remove the need to use a high quality in-line moisture separator between the supply and tool(s). Fegarex also brings up an excellent point on using "multi-staged regulation" of the supply air pressure. Primary regulation should drop your main supply pressure to within 20% of the secondary regulation set point.

As for the secondary control system, you have several options and the least costly would be to install a "Start/Stop" station that will not require modifications to the plug & play equipment. - - Since I'm not familiar with the graver unit in question, I can only "assume" at this point and if my assumption is incorrect I hope someone will correct me - - I'm assuming the graver unit operates on standard 120Vac utility electrical power and has an "ON/OFF" switch so that if you leave that switch in the "ON" position and plug the unit in, it begins to operate. Provided this is the case, all you will need is a simple electrical enclosure that you can mount in a safe location within proximity of the graver unit that is large enough to house a properly sized SPST switch and a duplex outlet. The outlet will be controlled by the switch and the graver unit will plug into one side of the duplex. The you will need a solenoid valve rated for "gas" or "liquid/gas" with a 120Vac coil and the port sized correctly to the air supply line. (These are available at industrial suppliers like MSC, Granger, McMaster, ect. and always suggest getting Red Hat brand simply because they are quality proven in my experience) Properly and safely wired the solenoid valve will plug into the other side of the duplex outlet controlled by the switch. When you apply 120Vac power to the outlet via the switch, it will energize both the graver unit and the solenoid valve to turn on the air supply. Using a single switch control will prevent you from turning off either electrical or air supply to the unit independently helping to prevent damage to the unit. The solenoid valve will cost you around $25 and total cost for this set-up will likely be less than $40 in parts provided you can do the work correctly yourself - never guess when messing with electrical, it has no problem with killing you dead without remorse or warning! Same goes for compressed air systems, the entire thing is a potential bomb just waiting to explode and if you don't know what you're doing, please by all means hire a professional!! You can get as fancy as you want with control system by adding indicator lamps or using one control station to run not only the graver & air supply but also the compressor, air drier & oilers as well but it must all be done correctly and safely.

Selecting the right air compressor is not difficult if you have the information you need, if you don't know what to buy and why, chances are you'll end up spending a lot of money over the long run. A high quality air compressor that is properly maintained will easily last you a lifetime.

Let's get the technical terms out of the way first.
ACFM / ICFM - Actual / Intake Cubic Feet Per Minute:
This is the amount of air the compressor is capable of drawing into the intake expressed in cubic feet of air at standard atmospheric pressure which for the case of discussion is rounded off to "14.7 PSI" or "1 Atmosphere" with the actual calculation figure being 14.69594878 PSI.

SCFM - Standard Cubic Feet Per Minute:
This is used to identify the amount of air consumed by pneumatic equipment and to show the delivery rate of an air compressor, in both cases SCFM will be listed in relation to a specific pressure. Example: "13.5 SCFM @ 90 PSIG". Even though it is incorrect, most people drop the "S" when giving ratings for pneumatic equipment so if you see something like "4 CFM @ 60 PSI" this is just the lazy or misinformed persons way of expressing "4 SCFM @ 60 PSIG".

*Note: Both ACFM and SCFM are affected by the density, temperature, humidity content and atmospheric pressure of the air at the given location of the compressor - If you live in a high altitude region, add about 10% to your usage requirements and you'll be fine.

PSIG - Pounds per Square Inch Gauge: In most cases people tend to drop the "G" off the end of the abbreviation but engineers will get picky about doing such. The "G" is used to show that the pressure is taken with a gauge and is therefore already corrected for atmospheric pressure. Dropping the "G" signifies you are speaking in terms of absolute pressure and I'm not going to get into that discussion.

Vac - Volt AC:
Voltage of an alternating current circuit that is rounded off to the RMS (Root Mean Square) value such as "120Vac". If you want to get picky, the actual peak voltage is 167 volts ... another thing we don't need to get into. Standard wall outlets are "AC" circuits.

Vdc - Voltage DC:
Voltage of a direct current circuit. You won't usually find DC circuits on air compressors unless they are microprocessor controlled. DC is the type of electrical current your car runs on.

Hz - Hertz:
This is the unit of measure for the frequency of voltage and current reversal in an AC electrical circuit. Here in the USA the standard is "60Hz" or "60 cycle" and in Europe the standard is "50Hz" or "50 cycle". You must NOT run electrical devices on the wrong frequency.

This indicates the number of “phases” as they relate to AC electrical systems. Normal household utility supply is “single phase”, do not confuse “phase” with “voltage”. A normal 120Vac household outlet has one “hot” wire and one “common” wire … however, household items like well pumps and clothes driers normally run on 240Vac and will have two “hot” wires going to them, despite the second “hot” wire, it is still a “single phase” device. Three phase utility power is normally limited to use in commercial application and devices identified as “3ph” (3 phase) will require three “hot” wires. Motors designated as 1ph or 3ph cannot be interchanged, they must operate on the correct phasing.

*NOTE: Some motors are built in a manner as to allow them to be used on different voltages (not different phases). You will generally find smaller single phase motors rated for use on 120/240Vac, this means you can run them on either voltage but the internal wiring / connections must be changed to match the supply voltage. Anytime you can have the option of using a higher voltage to operate a device, use it. When you double the supply voltage, you cut the amperage draw in half allowing switches and outlet to run cooler and it makes it easier on the motor too.

Soft Start:
These are becoming more common on larger compressors. They are an electronic device that varies the voltage and or frequency delivered to the motor(s) during the start-up cycle. The two primary functions of these devices is to eliminate high current start-up loading on the electric utility system and to eliminate shock loading on the mechanically driven parts.

ASTM - American Society for Testing and Materials:
You'll find a rating on the reservoir (tank) such as "ASTM 250 PSI MAX". What this means is that the tank is rated for a working pressure not to exceed 250 PSIG. This tells you that you cannot install a reservoir into a compressed air system that is not rated at or above the maximum operating pressure of that system.

Hp - Horse Power
Unit of work for a given mechanical or electrical device equal to 746 Watts

* NOTE - Small compressors from the big box stores often have a sticker reading "xHp" and this is false advertising I won't get into the details ... what you need to do is find the motor information tag on the electric motor and look in the little block marked "Hp" and this will tell you the actual Hp rating of the motor itself. Also on this tag you will find the voltage(s) and frequency required to operate the motor and a block marked "LR" or "Locked Rotor", this one tells you the absolute maximum current that will be drawn by that motor if it gets jammed up and cannot turn. This is a valuable piece of information because it tells you what size fuse/overload you must use to the protect the equipment from damage.

Difference between "fuse" and "circuit breaker". A "circuit breaker" protects an electrical circuit against a short to ground. A fuse protects an electrical circuit against an over-current condition. A breaker is designed to protect the wiring, a fuse is required to protect electrical/electronic devices.

To calculate the size compressor you need, you must figure out the maximum in air flow load you will place on the compressor at any given time. You do this by adding up all the tools you will be running on the compressed air at any given time or if you only use one tool at a time, use the one that draws the most load. Once you know your maximum draw, increase it's consumption value by at least 50% and the pressure by 30% - thus, if you're largest draw tool requires 10 SCFM @ 90 PSIG, you want a compressor that will deliver at least 15 SCFM @ 125 PSIG. You want the compressor capable of delivering considerably more volume and pressure than your tool(s) require otherwise the compressor will be taxed too heavily and this will lead to pre-mature failure of the unit.

I explained air storage volume in the other post but just to reinforce my statements, having no less than double the amount of storage volume is considered to be the minimum when designing compressed air systems.

Pipe and hose. Do NOT use PCV, ABS or CPVC for compressed air systems!!! Plastic cannot handle the combination of heat and pressure within compressed air systems and normal oil within the air stream will cause degradation of the plastic resulting in catastrophic failures. Trust me on this one, I have an acquaintance who did not heed this warning and he is now blind in one eye and endured numerous reconstructive operations to put his face and hands back together following a PVC fitting attached to his compressed air system exploding in his face when he attempted to plug a hose in. All compressed air piping should be made from a minimum of schedule 40 galvanized steel for systems operating at or below 175 PSIG maximum. All fittings, valves and other items exposed to the compressed air must also be rated for use in "gas" systems and have a working pressure rating higher than the maximum system operating pressure.

"Splash lube" means that the crankshaft within the compressor unit splashes the oil around to lubricate the internal parts when the unit is operating. Pressure lube means there is a pump within the unit that supplied the lubrication oil under pressure to the internal parts. On a quality unit, either lubrication system is fine.

Oil-less or oil-free compressors are extremely loud, annoying and operate at much higher speeds. Not only will one of these things drive you insane, it won't last very long anyway. These are extremely poor choices for anything other than a unit you will use once a year to air up some tires. Slow speed oil lubricated compressor units are a far better choice all-around no matter how you stack it.

Single Stage:
This means the compressor pump draws in the air and compresses it to working pressure in one piston stroke.

Two Stage:
This means the compressor has one large piston that draws in the air and compresses it to a value less than working pressure. The air then goes to a second smaller piston that compresses to full working pressure. Two stage pumps tend to run cooler and provide higher flow volumes at higher pressures.

Condensate Drain:
Manual or automatic drain valve on, or plumbed to, the bottom of the reservoir and or after-cooler to allow removal of condensed water vapor from the air system.

Device that cools the compressed air coming from the pump so moisture condenses out before it goes into the reservoir.

More notes:

Never connect a compressor with rigid piping to a secondary storage reservoir or distribution piping system, always make the attachment using a high quality reinforced flexible line. This will eliminate vibration & noise transfer from the compressor unit through the rigid piping reducing the annoyance level and making for safer system since the piping and other attached devices will not be subjected to unnecessary stresses.

Pipes, hoses and fitting will restrict air flow and pressure to tools, just like the reservoirs and compressors, oversize everything! Bigger is definitely better! When installing rigid piping, the distribution system must be correctly designed to provide sufficient air volume and must allow for contamination traps and drains within the distribution system itself. Always install plenty of valves and drains so that branch circuits can be quickly isolated and secured in case of emergency and to allow work on the system without having to drain the entire system.

The compressor itself needs to operate in as clean an environment as possible and it must have plenty of ambient air-flow so it can cool itself. You cannot stick the compressor in an un-ventilated closet and expect it to last, nor can you place it where it’s exposed to adverse environmental conditions. Absolutely avoid placing an air compressor anywhere it can be exposed to flammable or explosive materials including vapors. Compressors draw in ambient air and if the air is contaminated with hazardous materials, they will be drawn into the air system. There have been documented cases of air compressors exploding because hazardous materials were drawn into the pump and or the electrical components caused ignition of vapors in the ambient air. SAFETY FIRST!!!

FYI – Air compressor manufacturers will provide you with free help to select the proper compressor for your application but they will not design a distribution system for you. Distribution systems, even simple ones, need to be designed correctly and for this you will need to consult a professional that has been specifically trained on compressed air systems. The local plumber may be capable of installing the piping but it’s highly doubtful he/she has been properly trained on compressed air systems, they are very different from water systems.


:::Pledge Member:::
::::Pledge Member::::
Mar 16, 2007
Plainfield, Illinois
Thanks Mark -- Great information in a small package!!

I offer some clarification on two points:

Generally single phase motors can be used on three phase power systems. This should not be an issue for most engravers unless they are working in industrial or heavy commercial locations. Watch the voltage -- 120V motors can be used on 120/208V wye or 120/240V delta systems. 240V single phase motors should not be operated at 208V unless it is labeled for that voltage -- the motor may have a specific connection for operation at 240V and a different connection for 208V.

Fuses and circuit breakers perform the same function, de-energizing the circuit when their rating is exceeded by either overloads or short circuit conditions.

Circuit breakers generally found in the home or small businesses are thermal-magnetic. The "thermal" element is responsive to overload conditions and effectively delays the trip of the circuit breaker for brief overloads ( allows motor starting ) but trips for sustained overloads. The magnetic element operates very quickly to interrupt short circuits. There are also GFI circuit breakers that contain thermal-magnetic elements as well as a sensitive ground fault trip device.

Fuses come in a variety of styles that have different applications. The two most often seen in residences are single element fuses and dual element fuses. Single element fuses are standard speed or time delay -- standard speed are relatively quick to blow for overloads or short circuits, time delay are quick for short circuits but take a little longer to operate for overloads (hence the "time-delay" label). Generally if the fuse does not have a label identifying it as any particular style it is a "standard speed" fuse. Single element fuses can be damaged by repeated inrush currents from motor starting that are near but below the operating point of the fuse.

I generally recommend dual element fuses for heavy motor starting loads -- air compressors would fit in that category. Dual element fuses are constructed to allow surges on motor starting to be carried without degrading the fuse but allow the fuse to blow for sustained overloads or short circuits. The fuse will be clearly labeled as a dual-element fuse.