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.”
There are several advantages to a rack system, one of which is capacity control, since the whole rack of compressors provides the required capacity for the connected refrigerated cases as the case load reduces individual compressors or unloaders that can be cycled on and off to match the needed capacity. Only the compressors required to carry the load will operate. When compared to a system in which individual compressors are connected to individual refrigerated cases, this can be a tremendous energy savings.
Another advantage is redundancy. If one compressor fails, the other compressors normally can carry the load. A third major advantage is it allows for efficient latent defrosting of all its low-temperature cases. The defrosting can be selective with some units operating while others are in a defrost cycle. The operating units provide the necessary heat for the units that are in the defrost cycle.
For maximum efficiency, a bank of compressors should be connected to evaporators operating at similar temperatures. From an operational standpoint, cases of different evaporating temperatures can be connected to the rack; however, this causes a substantial energy loss.
When used in a typical supermarket environment, these systems produce refrigeration in the low or medium temperature range. A low-temperature rack maintains a suction temperature of minus 25?F with a possible satellite suction temperature of minus 33?. A medium-temperature rack maintains a suction temperature of 16° and a possible satellite suction temperature of 7?. An electronic rack controller is typically used to maintain a nearly constant suction pressure for the bank of compressors. As the suction pressure increases, more compressors are energized, and as the suction pressure falls, compressors are cycled off.
The actual design of a rack system will vary from store to store as the system is normally designed based on a required load and a refrigeration engineer’s design. Each designer can have his or her own method of how he or she wants the system to be controlled and how refrigerant is fed to each of the cases.
Some rack systems are built to cool a secondary refrigerant such as a glycol solution. The glycol solution is then pumped to the refrigerated cases instead of refrigerant. This system allows for much less refrigerant to be used. Piping to the cases becomes a little less critical, since oil return is not a factor.
Publication date: 9/1/2014