Effect of clearance on work and power in a single-stage, single-acting reciprocating compressor Consider a single-stage, single-acting reciprocating air compressor operating between fixed suction and delivery pressures. The presence of clearance volume affects volumetric efficiency. Does it affect the work done on the air and the power required for compressing a given mass of air?

Introduction / Context:
Clearance volume—dead space that remains when the piston is at top dead center—allows trapped high-pressure air to expand during the next suction stroke. It primarily reduces volumetric efficiency, but its influence on indicated work and shaft power depends on how we define the duty (fixed speed vs fixed mass flow rate).

Given Data / Assumptions:

• Single-stage, single-acting compressor with fixed suction and delivery pressures.
• Same polytropic exponent for compression and re-expansion in the clearance pocket.
• Comparison on the basis of delivering a specified mass flow rate of free air.

Concept / Approach:
For a given pressure range, the indicated work per kilogram compressed is essentially independent of clearance if compression and re-expansion follow the same index; the p–v loop area for one kilogram is unchanged. However, clearance reduces the mass inducted per cycle (volumetric efficiency drops). To deliver the same mass flow, the compressor must run at higher speed or use a larger cylinder, which increases total power. Therefore, in practical plant terms (fixed required delivery), clearance does affect the power required.

Step-by-Step Solution:

Verification / Alternative check:
Indicator diagrams with and without clearance show similar per-kg loop areas but reduced trapped-mass throughput per cycle; plant power meters confirm higher power to achieve the same flow when clearance losses are significant.

Why Other Options Are Wrong:

Categorically stating “no effect” ignores the operational objective of delivering a specified mass rate; in practice, more cycles are needed and power rises.

Common Pitfalls:
Confusing “work per kg” (nearly unchanged) with “power for a given delivered flow” (increases because volumetric efficiency drops). Exam questions sometimes focus on the latter practical viewpoint.

Final Answer:

True