The simplest way to explain the difference between a single stage compressor and dual or two stage compressor is the number of times that the air is compressed. In a single stage system the air is compressed once and in a dual stage the air is compressed twice.

 

In a single stage piston compressor the air is drawn into a cylinder and compressed in a single piston stoke to a pressure of approximately 120 PSI. Then it is sent to the storage tank. All rotary compressors are single stage.

In a dual stage compressor the first step is the same except that the air is not directed to the storage tank, the air is sent via an inter cooler tube to a second, smaller high pressure piston and compressed a second time and compressed to a pressure of 175 PSI. Then it is sent through the after cooler to the storage tank.

Customers sometimes confuse the fact that a piston compressor having two pistons with it being a two stage design. Most single stage pumps are twin cylinder design as this is easier to balance than a single piston. If the cylinders are the same outside diameter it is a single stage pump.

In a dual stage pump the first stage cylinder is always a larger diameter. Also a dual stage pump will always have an inter cooler tube or finned housing attached to the pump to cool the air before being compressed a second time.

This higher pressure allows greater storage in the tank. An 80 gallon receiver tank will hold 83 cubic feet at 100 PSI. The same tank holds 120 cubic feet at 150 PSI.

More importantly a dual stage compressor can easily maintain a header pipe pressure of 125 PSI. This is ideal in order to have 90 PSI at the end of the hose for the air tool inlet. A single stage compressor will let the header pressure drop below 90 PSI. This will lower the tool inlet pressure below 70 PSI affecting air tool performance.

source: doctorgun.custhelp.com/app/answers/detail/a_id/1576/~/what-is-the-difference-between-a-single-and-two-stage-compressor%3F