ATP Refrigeration Systems & Components 3 — Questions and Answers
Question 1: A scroll compressor differs from a reciprocating compressor in that it:
- Uses pistons and connecting rods to compress refrigerant
- Compresses refrigerant using two interleaved spiral scrolls without suction valves (Correct answer)
- Relies on a rotating vane to create variable displacement
- Requires oil injection for internal sealing at all operating speeds
Correct answer: Compresses refrigerant using two interleaved spiral scrolls without suction valves
Scroll compressors use one fixed and one orbiting spiral scroll to progressively compress refrigerant pockets toward the center discharge port, eliminating the need for suction valves and reducing noise and vibration.
Question 2: Which compressor type is most commonly used in large commercial refrigeration systems requiring capacity modulation through cylinder unloading?
- Scroll compressor
- Rotary vane compressor
- Semi-hermetic reciprocating compressor (Correct answer)
- Centrifugal compressor
Correct answer: Semi-hermetic reciprocating compressor
Semi-hermetic reciprocating compressors are widely used in commercial refrigeration because they allow cylinder unloaders to be installed, enabling staged capacity reduction without stopping the compressor.
Question 3: Oil logging in an evaporator coil is most likely caused by:
- Excessive refrigerant charge in the system
- Insufficient refrigerant velocity to carry oil through the coil back to the compressor (Correct answer)
- High ambient temperature increasing condenser pressure
- A failed discharge valve in the compressor
Correct answer: Insufficient refrigerant velocity to carry oil through the coil back to the compressor
If refrigerant velocity in the evaporator is too low, oil cannot be carried through the coil and returns to the compressor, reducing heat transfer efficiency and starving the compressor of lubrication.
Question 4: When a hermetic compressor is found to have a grounded motor winding, the refrigerant and oil in the system are likely to be:
- Unaffected because the motor is isolated from the refrigerant circuit
- Contaminated with acidic byproducts from the burned winding insulation (Correct answer)
- Slightly overcharged due to refrigerant migration during the burnout
- Too cold to support the chemical reaction that causes acid formation
Correct answer: Contaminated with acidic byproducts from the burned winding insulation
A motor burnout releases acidic decomposition products (hydrofluoric and hydrochloric acid) into the refrigerant and oil, requiring an acid cleanup procedure including filter-drier replacement and oil flushing.
Question 5: A refrigeration technician notices a compressor is short-cycling on the low-pressure cutout. Which of the following is the MOST likely cause?
- Overcharge of refrigerant
- Dirty or restricted air-cooled condenser coil
- Low refrigerant charge or a restricted liquid line filter-drier (Correct answer)
- Compressor valve failure allowing high-side gas to bleed back
Correct answer: Low refrigerant charge or a restricted liquid line filter-drier
Low refrigerant charge or a plugged filter-drier starves the evaporator, causing suction pressure to drop below the low-pressure cutout setpoint and cycling the compressor off prematurely.
Question 6: What is the function of a crankcase heater on a refrigeration compressor?
- To warm the refrigerant vapor before compression to improve efficiency
- To prevent refrigerant migration into the compressor oil during the off cycle (Correct answer)
- To heat discharge gas reducing condensing pressure at startup
- To maintain oil viscosity during high-load summer operation
Correct answer: To prevent refrigerant migration into the compressor oil during the off cycle
Crankcase heaters keep the oil warm during the off cycle, preventing refrigerant from migrating into and dissolving in the crankcase oil, which would cause oil foaming and inadequate lubrication at startup.
Question 7: In a two-stage refrigeration system, an intercooler between the low-stage and high-stage compressors is used to:
- Add superheat to the suction gas entering the low-stage compressor
- Desuperheat and partially cool the gas between compression stages to improve efficiency (Correct answer)
- Separate oil from the refrigerant vapor leaving the low-stage compressor
- Measure refrigerant enthalpy at the intermediate pressure point
Correct answer: Desuperheat and partially cool the gas between compression stages to improve efficiency
The intercooler removes heat of compression from the low-stage discharge gas before it enters the high-stage compressor, reducing the work of the second stage and improving overall system COP.
ATP Refrigeration Systems & Components 3
A scroll compressor differs from a reciprocating compressor in that it: