CARO - Certified Assistant Refrigeration Operator Compressor Operation and Maintenance Questions and Answers — Questions and Answers
Question 1: An operator notices excessive vibration and a new, unusual noise coming from a reciprocating compressor during operation. Which of the following is the most likely immediate cause for this issue?
- Clogged oil filter
- Slugging of liquid refrigerant (Correct answer)
- Incorrect oil viscosity
- Loose mounting bolts
Correct answer: Slugging of liquid refrigerant
Slugging occurs when liquid refrigerant enters the compressor's cylinder. Since liquids are incompressible, this causes a sudden, violent hydraulic shock, leading to severe vibration, loud knocking noises, and potentially catastrophic damage to internal components like valves, pistons, and connecting rods.
Question 2: During a routine maintenance check on a screw compressor used in an ammonia refrigeration system, an operator finds that the oil in the separator is milky and foamy. What is the most probable cause of this condition?
- The oil is contaminated with moisture/water. (Correct answer)
- The compressor has been running unloaded for too long.
- The oil filter is completely bypassed.
- Non-condensable gases are present in the system.
Correct answer: The oil is contaminated with moisture/water.
In ammonia refrigeration systems, oil that appears milky or foamy is a classic sign of moisture contamination. Water emulsifies with the refrigeration oil, creating this appearance. This contamination reduces the oil's lubricating properties and can lead to corrosion and component failure.
Question 3: Which of the following routine maintenance tasks is most critical for ensuring the long-term health and efficiency of a reciprocating compressor's moving parts?
- Calibrating pressure gauges annually
- Wiping down the compressor housing weekly
- Regularly checking and changing the crankcase oil and filters (Correct answer)
- Repainting corroded exterior surfaces
Correct answer: Regularly checking and changing the crankcase oil and filters
Proper lubrication is essential for the longevity of any reciprocating compressor. Regularly checking the oil level and changing the oil and filters according to the manufacturer's schedule prevents premature wear of critical components like pistons, cylinders, and bearings by ensuring they are properly lubricated and cooled.
Question 4: An operator needs to perform maintenance that requires opening up a compressor to the atmosphere. What is the critical first step in the pump-down procedure before any components are loosened?
- Close the suction and discharge service valves.
- Disconnect the main power supply using lockout/tagout procedures.
- Attach a gauge manifold to the service ports.
- Slowly throttle the suction stop valve to lower crankcase pressure. (Correct answer)
Correct answer: Slowly throttle the suction stop valve to lower crankcase pressure.
Before isolating the compressor, it's crucial to run it and slowly close the suction stop valve. This process, known as pumping down the compressor, safely removes most of the refrigerant from the crankcase. Doing this slowly prevents oil from foaming violently and being carried out of the compressor, which can happen if the pressure is dropped too quickly.
Question 5: A screw compressor is consistently tripping on its high-temperature safety cutout. The condenser appears to be operating normally, and the refrigerant charge is correct. Which of the following maintenance issues is the most likely cause?
- A malfunctioning capacity slide valve
- A dirty or clogged oil cooler/filter (Correct answer)
- Excessive non-condensable gases
- A partially closed suction service valve
Correct answer: A dirty or clogged oil cooler/filter
In a screw compressor, oil is used not only for lubrication but also for cooling and sealing the rotors. If the oil cooler or oil filter is dirty or clogged, oil flow will be restricted, and its cooling efficiency will be greatly reduced. This leads to the compressor overheating and tripping the high-temperature safety control.
Question 6: What is the primary purpose of an oil separator in an industrial refrigeration system?
- To filter out metal particles from the compressor oil.
- To remove liquid refrigerant that has mixed with the oil.
- To cool the oil before it is reinjected into the compressor.
- To remove entrained oil from the discharge gas and return it to the compressor. (Correct answer)
Correct answer: To remove entrained oil from the discharge gas and return it to the compressor.
A small amount of lubricating oil inevitably leaves the compressor with the hot discharge gas. The oil separator's function is to remove these oil droplets from the high-pressure refrigerant vapor stream and return the collected oil to the compressor crankcase to ensure it remains properly lubricated.
