Reciprocating compressors double acting piston

Figure 3-72. Double-acting (crosshead-type) reciprocating piston compressor [4],...

Figure 29.1 is a simplified sketch of a cylinder of a reciprocating compressor. The cylinder is shown as a single-acting cylinder. Typically cylinders are double-acting, meaning that there are valves on both ends of the cylinder and that the piston is compressing gas, in turn, on both ends of the cylinder. 

For a reciprocating compressor, at end of a discharge stoke, the piston does not travel all the way to the end of the cylinder. The volume between the piston and the cylinder end at end of the discharge stoke is called the clearance volume. For a double-acting cylinder, there are two clearance volumes, one at the cylinder crank end and the other at the cylinder head end. 

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