Question 27 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is C ### Explanation for Option C ("the highest") The multi-box refrigeration system is operating under conditions that require maximum cooling capacity (all four evaporators are active and fully fed). A capacity-controlled compressor uses hydraulic unloading mechanisms (controlled by the oil pressure acting on the relay piston) to match the cooling capacity to the load demand. 1. **Hydraulic Unloading Principle:** In standard capacity control systems, the compressor cylinders are designed to default to the *unloaded* state (minimum capacity) via spring force when control oil pressure is absent or low. 2. **Loading the Compressor:** To activate (load) a cylinder and increase capacity, control oil pressure is applied to the hydraulic relay piston, overcoming the spring force and engaging the cylinder. 3. **Maximum Load:** Since all four evaporators are actively operating, the system requires 100% capacity. To achieve 100% capacity, all compressor cylinders must be fully loaded. 4. **Conclusion:** The control oil pressure acting on the relay piston must be maintained at its **highest** value to ensure sufficient force to keep the pistons in the fully loaded position against the opposing spring pressure, thus achieving maximum cooling output. ### Explanation for Incorrect Options **A) the lowest:** This pressure corresponds to the condition of minimum capacity. When the load drops, the control system bleeds off the control oil pressure. At its lowest value, the spring mechanism forces the relay piston into the "unload" position, significantly reducing compressor output. This contradicts the scenario of full load (four active evaporators). **B) at its mid-range:** Mid-range pressure would only be utilized if the system required intermediate capacity (e.g., 50% or 75% capacity), causing some of the cylinders to be unloaded while others remain loaded. Since the scenario demands maximum (100%) capacity, mid-range pressure is insufficient. **D) of no consequence as the lube oil is not used in the operation of the unloader:** This statement is factually incorrect for capacity-controlled compressors that use hydraulic relays. The operation of the capacity unloader mechanism is directly dependent on the *control oil pressure*, which is typically derived, regulated, and routed from the compressor's main lubrication oil system. Therefore, the oil pressure is central to the operation of the unloader.