What makes an air compressor 2 stage?

Thanks,
Bob

As ht says, it is like two compressors running in series with each other. The reason for multiple stages is to keep the final discharge temperature down to reasonable levels. Most single stage compressors are pretty much limited to around 125 psig. Certainly no more than about 150 psig due to extremely high temperatures generated at high compression ratios.

For most of the two stage machines of interest here, a final discharge pressure of about 175 psig is typical. The first stage piston is the largest and compresses the air from slightly sub-atmospheric to around 40 psig. The air usually then goes through some form of interstage cooling (finned tubing for instance) and is then introduced into the suction of the second stage smaller cylinder. Here it is compressed from near 40 psig to about 175 psig. The compression ratios for each stage are about equal. This process results in lower temperatures overall and a more efficient use of available horsepower for compression.

Rod