Question 25 3AE02 - Third Assistant Engineer (Alt)

The Correct Answer is D **Explanation for Option D (Close the "king" valve):** The "king" valve is the common term for the liquid line service valve, which is usually located at the outlet of the receiver (or the condenser if no receiver is present) in a typical vapor compression refrigeration system. When pumping down a system to test the low-pressure cut-out (LPC) switch while the compressor is running, the goal is to isolate the refrigerant charge in the high side (receiver/condenser) and then allow the compressor to evacuate the refrigerant from the low side (evaporator and suction line). By **closing the "king" valve (liquid line valve)**, the flow of liquid refrigerant to the expansion device and the evaporator is stopped. Since the compressor continues to run, it pulls down the pressure in the evaporator and suction line until the pressure drops below the setpoint of the low-pressure cut-out switch. When this setpoint is reached, the LPC switch opens, interrupting the control circuit and shutting off the compressor, thus confirming that the switch is functioning correctly. **Explanation for Incorrect Options:** **A) stop the compressor:** If the compressor is stopped, the pressure in the low side will quickly equalize with the high side, and the pressure will not drop low enough to trip the LPC switch (which is usually set very low, e.g., 20–30 PSIG or lower). The test requires the compressor to be running to actively pull a vacuum (or a very low pressure) on the low side. **B) secure the condenser:** Securing the condenser (e.g., stopping the fan or water flow) affects the head pressure (high side pressure) but does not directly isolate the low side or cause the suction pressure to drop rapidly enough to test the LPC switch. This action is related to testing the high-pressure cut-out switch, not the low-pressure switch. **C) stop the circulating pump:** This option is only relevant in chilled water systems or certain liquid cooling applications. Stopping the pump affects heat transfer but does not directly isolate the refrigerant circuit or initiate the rapid pressure drop necessary in the suction line to test the low-pressure cut-out switch.