This is an excellent question because the wrong air compressor can cost you big bucks and cause untold headaches.  Each type of compressor (reciprocating, rotary screw, and rotary vane are the primary types) has its own “pros and cons”, but it is not difficult to determine the most appropriate type for your application once you know a little about each one. Reciprocating, or piston, air compressors are the most familiar.  An industrial quality machine will last a long time with routine maintenance.  Reciprocating compressors are also very efficient because they only run when there is a demand for compressed air.  When there is no demand, they turn off, saving wear and tear on the machine and, most importantly, saving on your power bill.  In fact, most of these machines must turn off regularly to cool.  An ideal on/off ratio is 60/40.  Reciprocating air compressors are available in a wide range of sizes from fractional horsepower up to hundreds of horsepower.  A reliable rule of thumb is that each horsepower will produce four cubic feet per minute (4 cfm) of compressed air at 120-150 pounds per square inch (psi) pressure. Rotary screw air compressors run continuously, producing a steady flow of compressed air.  They are suited for applications that require a constant supply of compressed air that would never give a reciprocating machine rest.  However, if the air demand decreases, a rotary screw machine will produce compressed air that is not needed.  This inefficiency will show up in your power bill.  Sometimes a rotary screw machine is used to provide a base amount of compressed air for which there is a constant demand and a reciprocating machine is used to “trim” the system, operating only when demand exceeds the capacity of the rotary screw.  This arrangement can be very efficient.  Also, newer technology variable speed motors can reduce the power consumption of rotary screw machines when air demand diminishes.  Rotary screw compressors are available in the range of five horsepower up.  Each horsepower