Reciprocating compressors have ______.

Reciprocating compressors are designed using a piston and cylinder mechanism to compress refrigerant. This fundamental design utilizes the linear motion of the piston within the cylinder to create a compression cycle. As the piston moves down, it creates a vacuum that allows refrigerant to enter the cylinder. When the piston moves back up, it compresses the refrigerant within the cylinder and then pushes it out into the discharge line.

The other options relate to different types of compressors. For example, the design that incorporates two scrolls that mesh together refers to scroll compressors, which operate on a different principle involving the movement of scrolls to compress the refrigerant. A rotor is characteristic of rotary compressors, which use a rotating mechanism for compression, differing from the linear motion of a reciprocating compressor. Lastly, a large fan is typically associated with air-moving equipment rather than the direct compression process used in reciprocating or other types of compressors. Thus, the correct choice highlighting the key components of reciprocating compressors is indeed the piston and cylinder.