Many supermarket refrigeration systems use a parallel compressor rack system to refrigerate their cases. Its basic design consists of a bank of parallel-connected compressors and accessories mounted on a common base, piped to a common discharge suction and liquid header. A header is a piping arrangement in which one long length of large diameter pipe has multiple sections of small diameter pipe teed into the top. The smaller diameter pipe is typically connected to the top section of the larger diameter pipe.
The compressors could all be the same capacity or different capacities. When compressors of different capacities are used, the rack is referred to as an “uneven rack.”