Why should two compressors not be operated in parallel in a refrigeration system, unless designed for it?

The reasoning behind the correct answer focuses on the implications of operating two compressors in parallel without proper design. When compressors are intended to work in parallel, they must be designed to synchronize their operation. If not, lubricating oil can end up transferring between the compressors in an uncontrolled manner. This transfer can lead to one compressor becoming starved of lubrication while the other could be over-lubricated, potentially causing severe mechanical issues or even failure.

In contrast, the other options highlight potential issues but do not pinpoint the critical operational failure resulting from improper oil management. For instance, decreased efficiency may occur, but this effect is secondary to the more immediate risk posed by improper lubrication. Overpressure can certainly be an issue in a poorly designed system, but it is not specifically related to the parallels of compressor operation as it pertains to lubricant management. Likewise, while both compressors may fail, this is a broader consequence of several different failures, including but not limited to oil issues, rather than being a direct outcome of imbalance in lubrication. Understanding the critical role of lubricating oil in maintaining effective compressor performance underscores the importance of only operating compressors designed for parallel use.