RSES Compressor Systems 3 — Questions and Answers
Question 1: What is the effect of increasing suction gas superheat on compressor volumetric efficiency?
- Increases volumetric efficiency by improving vapor density
- Has no measurable effect on volumetric efficiency
- Decreases volumetric efficiency due to lower vapor density (Correct answer)
- Increases volumetric efficiency by reducing re-expansion losses
Correct answer: Decreases volumetric efficiency due to lower vapor density
Higher suction superheat reduces refrigerant vapor density, so the compressor moves less mass of refrigerant per stroke, decreasing volumetric efficiency.
Question 2: A centrifugal compressor experiencing 'surge' will exhibit which symptom?
- Stable low noise with steady discharge pressure
- Rhythmic pressure oscillations and possible reverse flow (Correct answer)
- Gradual decrease in suction pressure with no discharge fluctuation
- Constant overcurrent draw on the motor
Correct answer: Rhythmic pressure oscillations and possible reverse flow
Surge occurs when the compressor cannot maintain sufficient flow to overcome discharge pressure, causing cyclical flow reversal and pressure oscillations.
Question 3: Which lubrication system type is most commonly used in hermetic compressors?
- External gear pump with oil cooler
- Splash or slinger disc system (Correct answer)
- Pressurized oil pump with oil separator
- Gravity-fed drip system
Correct answer: Splash or slinger disc system
Hermetic compressors typically use a splash or slinger disc that picks up oil from the sump and distributes it to bearings and moving parts.
Question 4: What does a high pressure differential across the compressor oil filter indicate?
- Adequate oil pressure for proper lubrication
- Clogged or restricted oil filter requiring service (Correct answer)
- Low oil viscosity from overheating
- Correct oil level in the crankcase
Correct answer: Clogged or restricted oil filter requiring service
A high differential pressure across the oil filter indicates it is clogged with contaminants, restricting oil flow and risking bearing damage.
Question 5: In a variable-speed compressor system, what is the primary benefit of reducing compressor speed at part load?
- Eliminates the need for a crankcase heater
- Reduces power consumption and improves part-load efficiency (Correct answer)
- Increases suction pressure to prevent evaporator freeze-up
- Allows higher discharge temperatures for better heat rejection
Correct answer: Reduces power consumption and improves part-load efficiency
Reducing compressor speed at part load lowers power draw proportionally and improves system efficiency by better matching capacity to actual cooling demand.
Question 6: What condition can cause a compressor motor to draw high amperage while producing insufficient cooling capacity?
- Refrigerant undercharge
- Broken or worn connecting rod
- Leaking discharge valves allowing hot gas to recirculate (Correct answer)
- Oversized expansion valve causing high suction pressure
Correct answer: Leaking discharge valves allowing hot gas to recirculate
Leaking discharge valves allow compressed hot gas to flow back into the cylinder, forcing the motor to work harder while reducing net refrigerant pumping capacity.
Question 7: Which refrigerant property most directly determines the required compressor displacement for a given cooling capacity?
- Latent heat of vaporization and specific volume of suction vapor (Correct answer)
- Thermal conductivity of the refrigerant liquid
- Global warming potential (GWP) of the refrigerant
- Critical temperature of the refrigerant
Correct answer: Latent heat of vaporization and specific volume of suction vapor
Compressor displacement is determined by the required mass flow rate, which depends on cooling capacity divided by latent heat, multiplied by the specific volume of suction vapor.
RSES Compressor Systems 3
What is the effect of increasing suction gas superheat on compressor volumetric efficiency?