313A Compressors & Condensers 1 — Questions and Answers
Question 1: What type of compressor uses pistons driven by a crankshaft to compress refrigerant?
- Scroll compressor
- Centrifugal compressor
- Reciprocating (piston) compressor (Correct answer)
- Screw compressor
Correct answer: Reciprocating (piston) compressor
A reciprocating compressor uses pistons connected to a crankshaft that move up and down in cylinders to compress refrigerant vapour, and is one of the most common types in commercial refrigeration.
Question 2: What is the main advantage of a scroll compressor over a reciprocating compressor?
- Lower initial purchase cost
- Fewer moving parts, quieter operation, and higher efficiency at part load (Correct answer)
- Ability to handle liquid slugging without damage
- Higher compression ratios suitable for industrial refrigeration
Correct answer: Fewer moving parts, quieter operation, and higher efficiency at part load
Scroll compressors have fewer moving parts than reciprocating compressors, resulting in smoother, quieter operation and generally higher energy efficiency, particularly at part-load conditions.
Question 3: What is 'liquid slugging' in a compressor and why is it dangerous?
- Oil foaming in the crankcase that starves bearings of lubrication
- Liquid refrigerant entering the compressor cylinders, which can cause mechanical damage due to incompressibility of liquids (Correct answer)
- A buildup of sludge on compressor valves reducing efficiency
- Excessive oil circulation through the system causing liquid hammer in pipes
Correct answer: Liquid refrigerant entering the compressor cylinders, which can cause mechanical damage due to incompressibility of liquids
Liquid slugging occurs when liquid refrigerant enters the compressor, and because liquids cannot be compressed, the compressor valves and pistons can be severely damaged or destroyed.
Question 4: What is the purpose of a crankcase heater on a refrigeration compressor?
- To keep the oil warm for easier starting in cold weather and to prevent refrigerant migration into the crankcase oil (Correct answer)
- To heat the discharge gas before it enters the condenser
- To maintain minimum ambient temperature around the compressor
- To prevent ice formation on the compressor body in low-temperature applications
Correct answer: To keep the oil warm for easier starting in cold weather and to prevent refrigerant migration into the crankcase oil
A crankcase heater prevents refrigerant from migrating and condensing in the compressor oil during off-cycles, which would cause foaming and oil dilution on startup, potentially leading to bearing failure.
Question 5: In a hermetic compressor, how is the motor cooled?
- By an external fan mounted on the compressor shell
- By the suction refrigerant vapour passing over the motor windings (Correct answer)
- By a separate water-cooling circuit connected to the shell
- By oil circulated through an external heat exchanger
Correct answer: By the suction refrigerant vapour passing over the motor windings
In a hermetic compressor, the motor and compressor are sealed in the same shell, and the cool suction vapour returning from the evaporator passes over the motor windings to carry away heat.
Question 6: What is compression ratio in a refrigeration compressor?
- The ratio of refrigerant mass in the discharge line to suction line
- The ratio of absolute discharge pressure to absolute suction pressure (Correct answer)
- The ratio of compressor displacement to cylinder volume
- The ratio of compressor motor watts to cooling capacity in watts
Correct answer: The ratio of absolute discharge pressure to absolute suction pressure
Compression ratio is calculated by dividing the absolute discharge pressure (kPa absolute) by the absolute suction pressure (kPa absolute), and high compression ratios reduce compressor efficiency and capacity.
313A Compressors & Condensers 1
What type of compressor uses pistons driven by a crankshaft to compress refrigerant?