Free CARO Compressor Operations and Maintenance Questions and Answers — Questions and Answers
Question 1: An operator at a cold storage facility notices violent foaming in the compressor's oil sight glass immediately upon startup. This phenomenon quickly subsides. What is the most likely cause of this event?
- Excessive superheat at the compressor suction.
- Refrigerant migration into the crankcase during the off-cycle. (Correct answer)
- A malfunctioning high-pressure cut-out switch.
- Non-condensables present in the refrigeration system.
Correct answer: Refrigerant migration into the crankcase during the off-cycle.
This condition is a classic sign of a 'flooded start.' During the off-cycle, refrigerant can migrate to the coldest part of the system, which is often the compressor crankcase. There, it condenses and mixes with the oil. Upon startup, the sudden drop in crankcase pressure causes the liquid refrigerant to boil violently, creating foam.
Question 2: What is the primary function of a slide valve in a large screw compressor?
- To filter the compressor lubricating oil.
- To separate oil from the refrigerant gas.
- To provide a means of capacity control. (Correct answer)
- To regulate the motor's starting amperage.
Correct answer: To provide a means of capacity control.
The slide valve is a movable component within the screw compressor housing that changes the point where compression begins. By adjusting its position, it effectively alters the volume of refrigerant being compressed, allowing for smooth, continuous capacity control to match the cooling load.
Question 3: An operator hears a loud, rhythmic knocking sound from a reciprocating compressor and observes frost on the crankcase. Which of the following is the most probable and dangerous condition occurring?
- Oil pump cavitation.
- Excessive discharge superheat.
- Worn piston rings.
- Liquid refrigerant slugging. (Correct answer)
Correct answer: Liquid refrigerant slugging.
Liquid refrigerant slugging is the entry of liquid refrigerant into the compressor's cylinders. Since liquids are incompressible, the piston's attempt to compress the liquid can cause severe mechanical damage, including broken valves, connecting rods, or even a broken crankshaft. The knocking sound is a result of this hydraulic shock, and the frost indicates the presence of liquid refrigerant at or below its boiling point.
Question 4: Which of the following conditions is a common cause of compressor short cycling?
- A dirty and clogged condenser coil. (Correct answer)
- An oversized evaporator fan motor.
- A slightly undersized expansion valve.
- Low ambient temperature conditions.
Correct answer: A dirty and clogged condenser coil.
A dirty or clogged condenser coil impairs the system's ability to reject heat. This causes discharge pressure and temperature to rise until the high-pressure safety control trips, shutting off the compressor. After a short period, the pressure drops, the control resets, and the compressor starts again, leading to short cycling.
Question 5: The purpose of using unloaders on a multi-cylinder reciprocating compressor is to:
- Increase the compression ratio during high load.
- Reduce the starting torque and provide capacity control. (Correct answer)
- Ensure proper oil return to the crankcase.
- Prevent oil from leaving the compressor.
Correct answer: Reduce the starting torque and provide capacity control.
Unloaders work by holding the suction valves open on one or more cylinders. This prevents those cylinders from compressing gas, which significantly reduces the load on the motor during startup. It also serves as a method of capacity control by effectively reducing the compressor's pumping volume to match lower cooling demands.
Question 6: When performing a daily log check on an operating refrigeration compressor, which of the following is MOST critical to record and monitor?
- The brand and model number of the compressor.
- The ambient air temperature of the machinery room.
- Operating pressures, temperatures, and oil level. (Correct answer)
- The date of the last refrigerant filter-drier change.
Correct answer: Operating pressures, temperatures, and oil level.
Daily logs are essential for tracking the performance and health of a compressor. Recording key operating parameters like suction and discharge pressures, corresponding temperatures, and the oil level allows operators to identify trends and detect potential problems (like leaks, inefficiencies, or lubrication issues) before they become catastrophic failures.
Free CARO Compressor Operations and Maintenance Questions and Answers
An operator at a cold storage facility notices violent foaming in the compressor's oil sight glass immediately upon startup.
This phenomenon quickly subsides.
What is the most likely cause of this event?