313A Compressors — Questions and Answers
Question 1: What are the three main types of compressors classified by their housing?
- Rotary, centrifugal, and axial
- Open (external drive), semi-hermetic (serviceable), and hermetic (welded/sealed) (Correct answer)
- Single-stage, two-stage, and multi-stage
- Low-pressure, medium-pressure, and high-pressure
Correct answer: Open (external drive), semi-hermetic (serviceable), and hermetic (welded/sealed)
Compressors are classified by housing type as: open (motor is external and connected by a shaft coupling, allowing motor service), semi-hermetic (motor and compressor in a bolted housing that can be opened for service), and hermetic (motor and compressor welded in a sealed shell, not field-serviceable).
Question 2: What is the primary function of compressor oil in a refrigeration system?
- To cool the refrigerant
- To lubricate moving parts, seal clearances, and help cool the compressor motor (Correct answer)
- To increase the pressure of the refrigerant
- To absorb moisture from the system
Correct answer: To lubricate moving parts, seal clearances, and help cool the compressor motor
Compressor oil lubricates bearings, pistons, and other moving parts, seals the clearances between components to maintain compression efficiency, and helps dissipate heat from the motor windings. The oil must be compatible with the specific refrigerant used in the system.
Question 3: What damage can liquid slugging cause to a reciprocating compressor?
- It causes the motor to overheat
- It can break valves, connecting rods, and the crankshaft because liquids are incompressible (Correct answer)
- It only causes minor noise issues
- It improves compressor efficiency
Correct answer: It can break valves, connecting rods, and the crankshaft because liquids are incompressible
Liquid slugging occurs when liquid refrigerant or oil enters the compressor cylinder. Since liquids are essentially incompressible, the compressor attempts to compress an incompressible fluid, resulting in extreme pressures that can break reed valves, bend connecting rods, crack pistons, and damage the crankshaft.
Question 4: What is a scroll compressor and what are its advantages?
- A compressor using rotating vanes; advantage is high capacity
- A compressor using two spiral-shaped scrolls (one fixed, one orbiting) that trap and compress gas; advantages include fewer moving parts, lower noise, and higher efficiency (Correct answer)
- A compressor using a piston and cylinder; advantage is simplicity
- A compressor using centrifugal force; advantage is very high capacity
Correct answer: A compressor using two spiral-shaped scrolls (one fixed, one orbiting) that trap and compress gas; advantages include fewer moving parts, lower noise, and higher efficiency
A scroll compressor uses two interleaving spiral-shaped scrolls — one stationary and one orbiting. Gas is trapped between the scrolls and compressed as it moves toward the centre. Advantages include fewer moving parts (no valves), quieter operation, higher efficiency, and better tolerance of liquid slugging compared to reciprocating compressors.
Question 5: What is the purpose of a crankcase heater on a compressor?
- To warm the refrigerant for faster startup
- To keep the compressor oil warm during off cycles, preventing refrigerant migration and liquid slugging on startup (Correct answer)
- To defrost the outdoor coil
- To preheat the supply air
Correct answer: To keep the compressor oil warm during off cycles, preventing refrigerant migration and liquid slugging on startup
A crankcase heater keeps the oil sump warm when the compressor is off. Without it, refrigerant can migrate to the cold compressor crankcase and mix with the oil. On startup, the sudden pressure drop would cause the liquid refrigerant to flash to vapour (foam), washing oil off bearings and causing potential liquid slugging.
Question 6: What indicates a faulty compressor valve in a reciprocating compressor?
- Normal operating pressures and temperatures
- Low discharge pressure, high suction pressure, reduced capacity, and higher than normal compressor amperage (Correct answer)
- Excessively low amperage
- Frost on the liquid line
Correct answer: Low discharge pressure, high suction pressure, reduced capacity, and higher than normal compressor amperage
A leaking compressor valve allows gas to bypass between the high and low sides. This results in: lower than normal discharge (head) pressure, higher than normal suction pressure, reduced cooling/heating capacity, and higher amperage as the compressor works harder to maintain pressure differential.
313A Compressors
What are the three main types of compressors classified by their housing?