Question 19 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is B. ### Explanation of why Option B is correct: When the suction or discharge valves in a reciprocating air compressor fail to properly seat (i.e., they leak), the efficiency of the compressor dramatically decreases. 1. **Lower Volumetric Efficiency (Lower Displacement Capacity):** If a discharge valve leaks, high-pressure air from the receiver leaks back into the cylinder during the compression and suction strokes. If a suction valve leaks, some air drawn in during the suction stroke is forced back out to the suction line during the compression stroke. In either case, the actual volume of compressed air delivered to the receiver per stroke (the effective displacement capacity) is reduced. The compressor has to do more work for the same volume of delivered air, or rather, it delivers less air for the work it is doing. 2. **Longer Running Periods:** Since the compressor is now delivering less actual compressed air (lower volumetric efficiency) per unit of time, it takes longer to increase the pressure in the receiver tank from the cut-in pressure to the cut-out pressure (the operating cycle). Therefore, the time the compressor must run to satisfy the air demand or reach its set cut-out pressure is **longer**. Thus, the compressor runs longer while delivering less air efficiently, leading to **longer running periods at lower displacement capacity** between operating cycles. ### Explanation of why other options are incorrect: * **A) The compressor would have longer running periods at higher displacement capacity between operating cycles.** * **Incorrect:** Valve leakage fundamentally lowers the volumetric efficiency, meaning the *actual* capacity (displacement) for delivering usable air is *lower*, not higher. While the running period is longer (correct), the capacity is lower (incorrect). * **C) The compressor would have shorter running periods at lower displacement capacity between operating cycles.** * **Incorrect:** While the displacement capacity is lower (correct), the running periods would be *longer*, not shorter, because the compressor has to run for more time to make up the deficit caused by the inefficiency. * **D) The compressor would have shorter running periods at higher displacement capacity between operating cycles.** * **Incorrect:** Both parts are incorrect. The running period would be *longer* due to inefficiency, and the effective displacement capacity would be *lower* due to the leaking valves.