FDNY Z-51 REFRIGERATING SYSTEM OPERATING ENGINEER WRITTEN EXAMINATION
2026 COMPREHENSIVE PREPARATION GUIDE WITH VERIFIED ANSWERS AND
RATIONALES
SECTION 1: REFRIGERATION CYCLE FUNDAMENTALS AND THERMODYNAMICS
1. The efficiency of the refrigerant cycle is most significantly affected by the operation
of the condenser. A more efficient centrifugal compressor operation results from:
A. Raising the condensing pressure
B. Lowering the condensing pressure
C. Increasing the evaporator temperature
D. Decreasing the compressor speed
Answer: B. Lowering the condensing pressure
Rationale: Lowering the condensing pressure reduces the compression ratio, allowing the
compressor to move refrigerant more efficiently with less work input. This directly improves
the coefficient of performance (COP) and overall system efficiency.
2. In most semi-hermetic compressors, the suction vapor is used to:
A. Lubricate the bearings
B. Cool the motor windings
C. Heat the crankcase oil
D. Increase the discharge pressure
Answer: B. Cool the motor windings
Rationale: Suction vapor passes over the motor windings in semi-hermetic compressors,
absorbing heat and maintaining safe operating temperatures while also becoming slightly
superheated before entering the compression chamber.
3. The amount of refrigerant that flows through the orifices of an economizer is
determined by the:
A. Evaporator load
B. Pressure differential across the orifices
C. Condenser water temperature
D. Compressor discharge pressure
Answer: B. Pressure differential across the orifices
,Rationale: The economizer functions based on the pressure difference between the flash
chamber and the compressor's intermediate pressure port, which regulates the flow
through its orifices.
4. When an expansion valve is operating properly, at the end of the evaporator will
contain:
A. Only saturated liquid
B. Only superheated vapor
C. A mixture of liquid and vapor
D. Subcooled liquid
Answer: B. Only superheated vapor
Rationale: Proper expansion valve operation ensures all liquid refrigerant has evaporated
before reaching the end of the evaporator coil, providing superheated vapor to protect the
compressor from liquid slugging.
5. The heat removed from the refrigerant in a condenser is:
A. Only sensible heat
B. Only latent heat
C. Sensible and latent heat
D. Neither sensible nor latent heat
Answer: C. Sensible and latent heat
Rationale: The condensing process removes both latent heat (during the phase change
from vapor to liquid) and sensible heat (cooling the liquid below its saturation
temperature).
6. Liquid refrigerant that has passed through a metering device is classified as:
A. High-pressure liquid
B. Low-pressure saturated liquid
C. Subcooled liquid
D. Low-pressure vapor
Answer: B. Low-pressure saturated liquid
Rationale: After passing through the metering device, the refrigerant is at low pressure and
is in a saturated condition, meaning it is a mixture of liquid and flash gas ready to absorb
heat in the evaporator.
7. A refrigeration engineer verifies that a condenser pump is cavitating. The most likely
cause for this condition is:
A. A faulty cooling tower water level controller
B. Excessive pump speed
C. Clogged impeller
,D. High discharge pressure
Answer: A. A faulty cooling tower water level controller
Rationale: Cavitation occurs when insufficient water reaches the pump suction. A faulty
water level controller can allow the cooling tower sump to run dry, causing the pump to
operate with inadequate net positive suction head (NPSH).
8. The most important reason to use at least two compressors in a cascade system is
to:
A. Avoid the need for a water-cooled condenser
B. Divide the cooling load between the compressors
C. Accomplish higher temperatures
D. Avoid very high compression ratio for each compressor
Answer: D. Avoid very high compression ratio for each compressor
Rationale: Cascade systems use multiple compressors with different refrigerants to
achieve very low temperatures. Each compressor handles a reasonable compression ratio
rather than one compressor attempting an extreme ratio.
9. Under normal operating conditions, the refrigerant enters the compressor as a:
A. High-pressure liquid
B. Low-pressure superheated vapor
C. High-pressure vapor
D. Low-pressure saturated liquid
Answer: B. Low-pressure superheated vapor
Rationale: Proper system operation requires that refrigerant entering the compressor be
superheated to ensure no liquid droplets enter, which would cause mechanical damage
through slugging.
10. The compression ratio of a refrigeration system is calculated by:
A. Evaporator pressure divided by condenser pressure
B. Condenser pressure divided by evaporator pressure (absolute pressures)
C. Discharge temperature divided by suction temperature
D. Suction pressure minus discharge pressure
Answer: B. Condenser pressure divided by evaporator pressure (absolute pressures)
Rationale: Compression ratio is a critical indicator of compressor efficiency and is
determined by dividing absolute discharge pressure by absolute suction pressure. Higher
ratios indicate increased work requirements.
11. Subcooling in a refrigeration system is defined as:
A. Cooling refrigerant vapor below its saturation temperature
B. Cooling refrigerant liquid below its saturation temperature
, C. Removing moisture from the refrigerant
D. Reducing the refrigerant pressure
Answer: B. Cooling refrigerant liquid below its saturation temperature
Rationale: Subcooling represents the temperature difference between the actual liquid
temperature and its saturation temperature at the same pressure, ensuring no flash gas
forms before the expansion device.
12. Superheat in a refrigeration system is defined as:
A. Heating refrigerant liquid above its saturation temperature
B. Heating refrigerant vapor above its saturation temperature
C. Increasing the system pressure
D. Removing heat from the evaporator
Answer: B. Heating refrigerant vapor above its saturation temperature
Rationale: Superheat is the additional heat added to refrigerant vapor after it has
completely evaporated, raising its temperature above the saturation point at that pressure.
13. The coefficient of performance (COP) of a refrigeration system is the ratio of:
A. Heat rejected to work input
B. Heat absorbed to work input
C. Work input to heat absorbed
D. Condenser capacity to evaporator capacity
Answer: B. Heat absorbed to work input
Rationale: COP measures the efficiency of the refrigeration cycle, representing the amount
of cooling provided (heat absorbed at the evaporator) per unit of work input to the
compressor.
14. A higher compression ratio in a refrigeration system results in:
A. Increased system efficiency
B. Decreased compressor discharge temperature
C. Reduced volumetric efficiency
D. Lower power consumption
Answer: C. Reduced volumetric efficiency
Rationale: Higher compression ratios increase the clearance volume losses, reducing the
amount of refrigerant the compressor can move per stroke, thus decreasing volumetric
efficiency.
15. Non-condensable gases in a refrigeration system will:
A. Increase system efficiency
B. Decrease condensing pressure
C. Increase condensing pressure and reduce efficiency
2026 COMPREHENSIVE PREPARATION GUIDE WITH VERIFIED ANSWERS AND
RATIONALES
SECTION 1: REFRIGERATION CYCLE FUNDAMENTALS AND THERMODYNAMICS
1. The efficiency of the refrigerant cycle is most significantly affected by the operation
of the condenser. A more efficient centrifugal compressor operation results from:
A. Raising the condensing pressure
B. Lowering the condensing pressure
C. Increasing the evaporator temperature
D. Decreasing the compressor speed
Answer: B. Lowering the condensing pressure
Rationale: Lowering the condensing pressure reduces the compression ratio, allowing the
compressor to move refrigerant more efficiently with less work input. This directly improves
the coefficient of performance (COP) and overall system efficiency.
2. In most semi-hermetic compressors, the suction vapor is used to:
A. Lubricate the bearings
B. Cool the motor windings
C. Heat the crankcase oil
D. Increase the discharge pressure
Answer: B. Cool the motor windings
Rationale: Suction vapor passes over the motor windings in semi-hermetic compressors,
absorbing heat and maintaining safe operating temperatures while also becoming slightly
superheated before entering the compression chamber.
3. The amount of refrigerant that flows through the orifices of an economizer is
determined by the:
A. Evaporator load
B. Pressure differential across the orifices
C. Condenser water temperature
D. Compressor discharge pressure
Answer: B. Pressure differential across the orifices
,Rationale: The economizer functions based on the pressure difference between the flash
chamber and the compressor's intermediate pressure port, which regulates the flow
through its orifices.
4. When an expansion valve is operating properly, at the end of the evaporator will
contain:
A. Only saturated liquid
B. Only superheated vapor
C. A mixture of liquid and vapor
D. Subcooled liquid
Answer: B. Only superheated vapor
Rationale: Proper expansion valve operation ensures all liquid refrigerant has evaporated
before reaching the end of the evaporator coil, providing superheated vapor to protect the
compressor from liquid slugging.
5. The heat removed from the refrigerant in a condenser is:
A. Only sensible heat
B. Only latent heat
C. Sensible and latent heat
D. Neither sensible nor latent heat
Answer: C. Sensible and latent heat
Rationale: The condensing process removes both latent heat (during the phase change
from vapor to liquid) and sensible heat (cooling the liquid below its saturation
temperature).
6. Liquid refrigerant that has passed through a metering device is classified as:
A. High-pressure liquid
B. Low-pressure saturated liquid
C. Subcooled liquid
D. Low-pressure vapor
Answer: B. Low-pressure saturated liquid
Rationale: After passing through the metering device, the refrigerant is at low pressure and
is in a saturated condition, meaning it is a mixture of liquid and flash gas ready to absorb
heat in the evaporator.
7. A refrigeration engineer verifies that a condenser pump is cavitating. The most likely
cause for this condition is:
A. A faulty cooling tower water level controller
B. Excessive pump speed
C. Clogged impeller
,D. High discharge pressure
Answer: A. A faulty cooling tower water level controller
Rationale: Cavitation occurs when insufficient water reaches the pump suction. A faulty
water level controller can allow the cooling tower sump to run dry, causing the pump to
operate with inadequate net positive suction head (NPSH).
8. The most important reason to use at least two compressors in a cascade system is
to:
A. Avoid the need for a water-cooled condenser
B. Divide the cooling load between the compressors
C. Accomplish higher temperatures
D. Avoid very high compression ratio for each compressor
Answer: D. Avoid very high compression ratio for each compressor
Rationale: Cascade systems use multiple compressors with different refrigerants to
achieve very low temperatures. Each compressor handles a reasonable compression ratio
rather than one compressor attempting an extreme ratio.
9. Under normal operating conditions, the refrigerant enters the compressor as a:
A. High-pressure liquid
B. Low-pressure superheated vapor
C. High-pressure vapor
D. Low-pressure saturated liquid
Answer: B. Low-pressure superheated vapor
Rationale: Proper system operation requires that refrigerant entering the compressor be
superheated to ensure no liquid droplets enter, which would cause mechanical damage
through slugging.
10. The compression ratio of a refrigeration system is calculated by:
A. Evaporator pressure divided by condenser pressure
B. Condenser pressure divided by evaporator pressure (absolute pressures)
C. Discharge temperature divided by suction temperature
D. Suction pressure minus discharge pressure
Answer: B. Condenser pressure divided by evaporator pressure (absolute pressures)
Rationale: Compression ratio is a critical indicator of compressor efficiency and is
determined by dividing absolute discharge pressure by absolute suction pressure. Higher
ratios indicate increased work requirements.
11. Subcooling in a refrigeration system is defined as:
A. Cooling refrigerant vapor below its saturation temperature
B. Cooling refrigerant liquid below its saturation temperature
, C. Removing moisture from the refrigerant
D. Reducing the refrigerant pressure
Answer: B. Cooling refrigerant liquid below its saturation temperature
Rationale: Subcooling represents the temperature difference between the actual liquid
temperature and its saturation temperature at the same pressure, ensuring no flash gas
forms before the expansion device.
12. Superheat in a refrigeration system is defined as:
A. Heating refrigerant liquid above its saturation temperature
B. Heating refrigerant vapor above its saturation temperature
C. Increasing the system pressure
D. Removing heat from the evaporator
Answer: B. Heating refrigerant vapor above its saturation temperature
Rationale: Superheat is the additional heat added to refrigerant vapor after it has
completely evaporated, raising its temperature above the saturation point at that pressure.
13. The coefficient of performance (COP) of a refrigeration system is the ratio of:
A. Heat rejected to work input
B. Heat absorbed to work input
C. Work input to heat absorbed
D. Condenser capacity to evaporator capacity
Answer: B. Heat absorbed to work input
Rationale: COP measures the efficiency of the refrigeration cycle, representing the amount
of cooling provided (heat absorbed at the evaporator) per unit of work input to the
compressor.
14. A higher compression ratio in a refrigeration system results in:
A. Increased system efficiency
B. Decreased compressor discharge temperature
C. Reduced volumetric efficiency
D. Lower power consumption
Answer: C. Reduced volumetric efficiency
Rationale: Higher compression ratios increase the clearance volume losses, reducing the
amount of refrigerant the compressor can move per stroke, thus decreasing volumetric
efficiency.
15. Non-condensable gases in a refrigeration system will:
A. Increase system efficiency
B. Decrease condensing pressure
C. Increase condensing pressure and reduce efficiency
