The clearance space in a reciprocating compressor

• A. Has no effect on compressor capacity
• B. Increases capacity
• C. Reduces capacity
• D. Improves efficiency

Answer: (A)

Clearance space is the small volume in the cylinder that remains filled with gas after the discharge stroke. On the next suction stroke, this gas expands as the piston moves downward, occupying part of the cylinder that would otherwise draw in fresh gas. Because of this, less fresh gas is pulled into the cylinder each cycle, so the actual amount of gas delivered per unit time—the compressor’s capacity—drops. This is a loss in volumetric efficiency. The effect grows with a larger clearance relative to the swept volume and can also add undesirable re-work because the gas in the clearance must be re-compressed in the next cycle. So, the clearance space reduces capacity rather than having no effect, increasing capacity, or improving efficiency.