A two cylinder single acting vertical compressor has a suction pressure of 45 PSIG. The purpose of the shaft seal is to keep:

The primary function of the shaft seal in a compressor, like the two-cylinder single-acting vertical compressor described, is to prevent both refrigerant and oil from escaping from the internal components of the machine. As compressors operate under various pressures and contain moving parts, it is essential to maintain the integrity of the seal to ensure that the refrigerant used in the cooling cycle remains within the system and does not leak into the atmosphere. Additionally, the shaft seal helps keep the lubricating oil contained within the compressor to ensure lubrication of internal components. This dual function is crucial for the efficient and safe operation of the compressor, avoiding environmental contamination and maintaining performance.

Other potential reasons addressed in other choices have merit but do not encapsulate the full purpose of the shaft seal as comprehensively as the correct answer does. For example, while it is important to have oil circulating within the compressor, the seal’s primary role is not directly to facilitate circulation, but rather to contain it and ensure there are no leaks. Similarly, maintaining constant pressure is a function of the entire compressor system rather than a singular purpose of the shaft seal.