313A Compressors 2 — Questions and Answers
Question 1: What is a variable-speed (inverter-driven) compressor and what is its main advantage?
- A compressor that uses two different speeds; advantage is lower cost
- A compressor that modulates its speed to match the load demand precisely; main advantage is improved energy efficiency and comfort (Correct answer)
- A compressor that alternates between heating and cooling modes
- A compressor with a variable oil pump
Correct answer: A compressor that modulates its speed to match the load demand precisely; main advantage is improved energy efficiency and comfort
A variable-speed compressor uses an inverter to adjust motor speed continuously, matching compressor capacity to the actual cooling or heating load. This eliminates the energy waste of constant on-off cycling, provides more precise temperature control, reduces noise at partial loads, and can improve efficiency by 30% or more.
Question 2: What is the function of the suction accumulator in a refrigeration system?
- To store excess refrigerant charge
- To trap liquid refrigerant and oil returning to the compressor, allowing only vapour to enter (Correct answer)
- To accumulate pressure for startup
- To filter contaminants from the suction gas
Correct answer: To trap liquid refrigerant and oil returning to the compressor, allowing only vapour to enter
A suction accumulator is a reservoir installed in the suction line before the compressor. It catches and holds liquid refrigerant and oil that might otherwise enter the compressor, protecting it from liquid slugging. A small metering orifice at the bottom gradually returns oil and small amounts of liquid to the compressor.
Question 3: What causes compressor short cycling and what are its effects?
- Running continuously; causes high efficiency
- Rapidly turning on and off due to low charge, thermostat issues, or high head pressure; causes increased wear, overheating, and premature failure (Correct answer)
- Operating at half speed; causes low noise
- Running only during peak demand; causes energy savings
Correct answer: Rapidly turning on and off due to low charge, thermostat issues, or high head pressure; causes increased wear, overheating, and premature failure
Short cycling is rapid, repeated starting and stopping caused by issues such as low refrigerant charge, faulty thermostat, oversized equipment, high head pressure, or electrical problems. It causes excessive heat buildup in the motor windings, increased mechanical wear on starting components, and significantly reduced compressor life.
Question 4: What is the compression ratio and how does it affect compressor performance?
- The ratio of oil to refrigerant
- The ratio of absolute discharge pressure to absolute suction pressure; higher ratios reduce efficiency and increase discharge temperature (Correct answer)
- The ratio of heating to cooling capacity
- The ratio of indoor to outdoor temperature
Correct answer: The ratio of absolute discharge pressure to absolute suction pressure; higher ratios reduce efficiency and increase discharge temperature
Compression ratio is calculated by dividing the absolute discharge pressure by the absolute suction pressure. As the ratio increases (due to higher head pressure or lower suction pressure), the compressor works harder, discharge temperatures rise, volumetric efficiency decreases, and energy consumption increases.
Question 5: What maintenance should be performed on a semi-hermetic compressor that cannot be done on a hermetic compressor?
- Refrigerant recovery only
- Internal service including valve plate inspection/replacement, bearing inspection, motor winding checks, and oil analysis (Correct answer)
- External cleaning only
- Electrical terminal tightening only
Correct answer: Internal service including valve plate inspection/replacement, bearing inspection, motor winding checks, and oil analysis
Semi-hermetic compressors have a bolted housing that can be opened for internal service, unlike welded hermetic units. Technicians can inspect and replace valve plates, check bearing condition, inspect motor windings for damage, analyze and change oil, and replace gaskets — extending the compressor's serviceable life.
Question 6: What is the purpose of an oil separator in a refrigeration system?
- To remove water from the oil
- To separate compressor oil from the hot discharge gas and return it to the compressor crankcase, preventing oil logging in the system (Correct answer)
- To mix oil with the refrigerant for better lubrication
- To cool the compressor oil before recirculation
Correct answer: To separate compressor oil from the hot discharge gas and return it to the compressor crankcase, preventing oil logging in the system
An oil separator is installed in the discharge line to separate compressor lubricating oil from the high-pressure refrigerant gas. It returns the oil directly to the compressor crankcase, preventing oil from travelling through the system and accumulating in the evaporator or condenser where it reduces heat transfer efficiency.
313A Compressors 2
What is a variable-speed (inverter-driven) compressor and what is its main advantage?