Originally Posted by TargetIndy from PBN
Frequently Asked Questions--Compressor System
When purchasing a compressor, there's a lot more to consider than how much air it can push.
Compressors use a series of pistons, called stages, to increase the pressure of a given volume of air. High pressure compressors typically have 2-5 stages. When you look at a compressor, you'll notice that these pistons are different sizes. The largest piston is the first stage, taking atmospheric air and compressing it to a higher pressure. The higher pressure air is then passed to the next largest piston. The process is repeated until the air reaches the final stage (smallest piston). Most of the pressure increase occurs in the final stage, with some compressors increasing pressure at the final stage by as much as 4500 psi.
Perhaps one of the most over-looked items when purchasing a compressor is its duty rating. Simply put, a duty rating is the amount of time a machine can be run in a one hour period. A compressor with a 25% duty rating is only meant to be run 15 minutes out of every hour. Exceeding the duty rating of a compressor can greatly reduce its service life, due to overheating and other maintenance issues. Smaller, portable compressors typically have a lower duty rating.
Compressors are typically powered by one of three methods: electricity, diesel engines, or gasoline engines.
Each power system has its own advantages, and your application will help to determine which is best for your situation.
Electric compressors are usually powered by single-phase or three-phase, 220V. While three-phase motors use electricity more efficiently, not all locations have three-phase 220 available (particularly residential and rural areas). Single-phase 220 is typically available wherever there is access to electricity.
Easier to maintain than systems using an engine. System can be easily set up to automatically start/stop when pressure reaches a certain point. Can also be quieter than other options. No issues with exhaust.
Not a good option for mobile operations.
Most mobile units use a gasoline engine to supply power to the compressor. If a two-cycle or four-cycle engine is an option, go for the four-cycle.
Self-contained and very mobile. Cleaner than diesel
Dirtier than electric. Should only be used outdoors. Not as reliable as other options.
While similar in advantages as a gasoline engine, diesel is better suited for high-torque applications such as air compressors. The same amount of torque can be had from a smaller diesel engine than a larger gasoline engine. Diesel engines also have the advantage of being simpler to maintain (fewer parts than gasoline engines) and longer service lives.
Best option for larger compressors when electricity isn't available. Simpler design than gasoline engines means fewer items to fail.
Dirtiest system. Can be hard to start in cold weather applications (sub-zero).
Condensate Drain Systems
Most compressor systems come with some form of a condensate drain system (CDS). CDS's remove moisture from compressed air. Failure to remove moisture can damage your compressor and/or fill station components.
CDS's come in two types--manual and automatic. A manual system requires the operator to manually drain collected moisture. An automatic condensate drain system will drain water automatically. Having owned both types, I strongly recommend going with an automatic system. As a field or store operator, there will come a time when you'll forget or be too busy to maintain a manual system. An automatic system is cheap insurance compared to the cost of having a compressor repaired (my local repair place charges $72 per hour for labor).
Aside from regular maintenance, filters are the best protection of your investment that money can buy. Dirty, unfiltered air can damage compressors and air fill components. Air should be filtered at the intake point (prior to the first stage) and after compression. If bulk tanks are used for air storage, a filter should be placed downstream of the tanks to collect rust and other debris that can settle in the tanks. I also use particulate filters on the fill whips to prevent debris from being inadvertently transferred between the user's tank and the fill system.
CFM vs. SCFM (source: Wikipedia)
CFM is an often confusing term because it has no single definition that applies to all instances. In the most basic sense, CFM means cubic feet per minute. Sounds simple enough right? Unfortunately, air is a compressible gas. To further confuse the issue, a centrifugal fan is a constant CFM device or a constant volume device. This means that, provided the fan speed remains constant, a centrifugal fan will pump a constant volume of air. This is not the same as pumping a constant mass of air. Again, the fan will pump the same volume, though not mass, at any other air density. This means that the air velocity in a system is the same even though mass flow rate through the fan is not.
Standard Cubic Feet per Minute (SCFM) is a volumetric flow rate corrected to standard conditions of gas density, thus representing a precise mass flowrate. SCFM is volumetric flowrate at a “standardized” pressure, temperature, and relative humidity. However great care must be taken, as the "standard" conditions vary between definitions, and should therefore always be checked. The “standard” conditions are usually defined as 1 atmosphere (101325 pascals, 1.01325 bar, 14.7 psia) atmospheric pressure, some temperature (e.g., 68°F) depending on the "standard" used, and some relative humidity (e.g., 36%, 0%) depending on the "standard" used.
The temperature variation is the most important. In Europe, SCFM is normally defined as "Standard Temperature and Pressure" (STP), which is 0°C, 32°F. In the USA, however, ambient conditions are used as a basis, and different groups use 60°F, 68°F (20°C), 70°F and 77°F (25°C). SCFM defined in this way can be nearly 10% greater than the European values. For example, a mass flow of 1000 kg/hr is 455 SCFM defined at 0°C, 32°F but 489 SCFM defined at 68°F, 20°C, which is the most common US basis.