Single-stage reciprocating compressors have one piston to compress the air. Air goes from the piston cylinder to a storage tank. In a dual-stage reciprocating compressor, there is a second piston. After air is compressed in the first cylinder, it enters the second piston cylinder, where it undergoes another round of compression. There may be an intercooler between the two piston stages. After the air completes the second stage of compression, it goes through a cooler and then is sent to the storage tank.
Most piston-style compressors are single-stage, especially those used for home use or in smaller shops. Single-stage compressors are suitable for pressurizing air up to ~100 PSI, which is adequate for most pneumatic tools and basic shop operations. Dual-stage compressors can produce significantly more power. The dual-stage process allows these compressors to produce more air and produce it on a more continual basis. This makes them more appropriate for larger industrial applications. They are also considerably more expensive to buy and operate.
A dual stage air compressor is different from a duplex air compressor, which has two individual compressor pumps and motors mounted on a single tank. A duplex air compressor provides 100% back-up power or additional air power for larger jobs.