313A Compressors 2

Question 1

What is a variable-speed (inverter-driven) compressor and what is its main advantage?

Accepted Answer

A compressor that modulates its speed to match the load demand precisely; main advantage is improved energy efficiency and comfort

Answer

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?

Accepted Answer

To trap liquid refrigerant and oil returning to the compressor, allowing only vapour to enter

Answer

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?

Accepted Answer

Rapidly turning on and off due to low charge, thermostat issues, or high head pressure; causes increased wear, overheating, and premature failure

Answer

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?

Accepted Answer

The ratio of absolute discharge pressure to absolute suction pressure; higher ratios reduce efficiency and increase discharge temperature

Answer

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?

Accepted Answer

Internal service including valve plate inspection/replacement, bearing inspection, motor winding checks, and oil analysis

Answer

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.

Red Seal HVAC 313A Practice Test

Red Seal HVAC 313A Practice Test

313A Compressors 2