FDNY Z-51 WRITTEN EXAM |2026|2027
UPDATE|350 QUESTIONS AND VERIFIED
ANSWERS WITH DETAILED RATIONALES
|GRADED A+|ASSURED SUCCESS
Section 1: Refrigeration Fundamentals and Theory (Questions 1-50)
1. What is the primary function of the compressor in a vapor-compression
refrigeration system?
A. To absorb heat from the refrigerated space
B. To raise the pressure and temperature of refrigerant vapor
C. To remove heat from the refrigerant
D. To reduce the pressure of liquid refrigerant
Correct Answer: B
Rationale: The compressor's primary function is to draw low-pressure, low-
temperature refrigerant vapor from the evaporator and compress it, significantly
increasing both its pressure and temperature. This high-pressure, high-temperature
vapor is then discharged to the condenser where heat is rejected. Without the
compressor, the refrigerant would not circulate through the system.
2. Which component changes refrigerant from vapor to liquid in a
refrigeration system?
A. Compressor
B. Evaporator
C. Condenser
D. Expansion valve
,Correct Answer: C
Rationale: The condenser removes heat from the high-pressure, high-temperature
refrigerant vapor that has been compressed by the compressor. As heat is removed,
the refrigerant changes state from vapor to liquid through DE superheating,
condensing, and subcooling processes.
3. What is the primary function of the evaporator in a refrigeration system?
A. To increase refrigerant pressure
B. To absorb heat from the refrigerated space
C. To remove heat from the refrigerant
D. To sub cool the refrigerant
Correct Answer: B
Rationale: The evaporator absorbs heat from the space or product being cooled.
Low-pressure liquid refrigerant enters the evaporator and boils, changing to vapor
while absorbing latent heat from the surroundings.
4. What is the primary function of the expansion valve in a refrigeration
system?
A. To compress refrigerant vapor
B. To absorb heat from the refrigerated space
C. To reduce refrigerant pressure and regulate flow to the evaporator
D. To remove heat from the refrigerant
Correct Answer: C
Rationale: The expansion valve (metering device) reduces the pressure of liquid
refrigerant from the condensing pressure to the evaporating pressure and regulates
the flow of refrigerant into the evaporator to maintain proper superheat.
5. What is the fundamental purpose of a thermostatic expansion valve?
,A. To compress refrigerant vapor
B. To regulate refrigerant entering the evaporator while controlling superheat
C. To remove moisture from the refrigerant
D. To increase condenser pressure
Correct Answer: B
Rationale: The thermostatic expansion valve meters refrigerant into the evaporator
and responds to evaporator outlet conditions to maintain an appropriate superheat
level. This ensures the evaporator is properly fed while protecting the compressor
from liquid return.
6. What is "superheated vapor" in a refrigeration system?
A. Vapor mixed with liquid
B. Vapor above its boiling point temperature at given pressure
C. Liquid below freezing
D. Gas under vacuum only
Correct Answer: B
Rationale: Superheated vapor is vapor heated beyond its saturation temperature at
a given pressure. This condition ensures that no liquid refrigerant remains in the
vapor stream, protecting the compressor from potential damage.
7. Why is superheat measured at the evaporator outlet?
A. To verify that refrigerant entering the compressor contains sufficient
vaporization margin
B. To determine condenser subcooling
C. To measure oil viscosity
D. To determine the refrigerant's molecular weight
Correct Answer: A
Rationale: Outlet superheat indicates how much the refrigerant vapor has been
heated above its saturation temperature and helps prevent liquid refrigerant from
, reaching the compressor. This measurement is essential for proper system
operation and compressor protection.
8. What is the purpose of subcooling in a refrigeration system?
A. Ensure vapor enters compressor
B. Prevent refrigerant boiling in liquid line
C. Increase evaporator pressure
D. Raise suction temperature
Correct Answer: B
Rationale: Subcooling ensures refrigerant remains liquid in the line, improving
efficiency and preventing flash gas. A pure liquid refrigerant must enter the
metering device for proper flow control.
9. What does "superheated vapor" indicate about the refrigerant condition?
A. Vapor mixed with liquid
B. Vapor above its boiling point temperature at given pressure
C. Liquid below freezing
D. Gas under vacuum only
Correct Answer: B
Rationale: Superheated vapor is vapor heated beyond its saturation temperature at
a given pressure. This ensures no liquid refrigerant remains in the vapor stream.
10. Which gas law best describes pressure increase with temperature in a
sealed system?
A. Ohm's Law
B. Boyle's Law
C. Charles's Law
D. Gay-Lussac's Law
UPDATE|350 QUESTIONS AND VERIFIED
ANSWERS WITH DETAILED RATIONALES
|GRADED A+|ASSURED SUCCESS
Section 1: Refrigeration Fundamentals and Theory (Questions 1-50)
1. What is the primary function of the compressor in a vapor-compression
refrigeration system?
A. To absorb heat from the refrigerated space
B. To raise the pressure and temperature of refrigerant vapor
C. To remove heat from the refrigerant
D. To reduce the pressure of liquid refrigerant
Correct Answer: B
Rationale: The compressor's primary function is to draw low-pressure, low-
temperature refrigerant vapor from the evaporator and compress it, significantly
increasing both its pressure and temperature. This high-pressure, high-temperature
vapor is then discharged to the condenser where heat is rejected. Without the
compressor, the refrigerant would not circulate through the system.
2. Which component changes refrigerant from vapor to liquid in a
refrigeration system?
A. Compressor
B. Evaporator
C. Condenser
D. Expansion valve
,Correct Answer: C
Rationale: The condenser removes heat from the high-pressure, high-temperature
refrigerant vapor that has been compressed by the compressor. As heat is removed,
the refrigerant changes state from vapor to liquid through DE superheating,
condensing, and subcooling processes.
3. What is the primary function of the evaporator in a refrigeration system?
A. To increase refrigerant pressure
B. To absorb heat from the refrigerated space
C. To remove heat from the refrigerant
D. To sub cool the refrigerant
Correct Answer: B
Rationale: The evaporator absorbs heat from the space or product being cooled.
Low-pressure liquid refrigerant enters the evaporator and boils, changing to vapor
while absorbing latent heat from the surroundings.
4. What is the primary function of the expansion valve in a refrigeration
system?
A. To compress refrigerant vapor
B. To absorb heat from the refrigerated space
C. To reduce refrigerant pressure and regulate flow to the evaporator
D. To remove heat from the refrigerant
Correct Answer: C
Rationale: The expansion valve (metering device) reduces the pressure of liquid
refrigerant from the condensing pressure to the evaporating pressure and regulates
the flow of refrigerant into the evaporator to maintain proper superheat.
5. What is the fundamental purpose of a thermostatic expansion valve?
,A. To compress refrigerant vapor
B. To regulate refrigerant entering the evaporator while controlling superheat
C. To remove moisture from the refrigerant
D. To increase condenser pressure
Correct Answer: B
Rationale: The thermostatic expansion valve meters refrigerant into the evaporator
and responds to evaporator outlet conditions to maintain an appropriate superheat
level. This ensures the evaporator is properly fed while protecting the compressor
from liquid return.
6. What is "superheated vapor" in a refrigeration system?
A. Vapor mixed with liquid
B. Vapor above its boiling point temperature at given pressure
C. Liquid below freezing
D. Gas under vacuum only
Correct Answer: B
Rationale: Superheated vapor is vapor heated beyond its saturation temperature at
a given pressure. This condition ensures that no liquid refrigerant remains in the
vapor stream, protecting the compressor from potential damage.
7. Why is superheat measured at the evaporator outlet?
A. To verify that refrigerant entering the compressor contains sufficient
vaporization margin
B. To determine condenser subcooling
C. To measure oil viscosity
D. To determine the refrigerant's molecular weight
Correct Answer: A
Rationale: Outlet superheat indicates how much the refrigerant vapor has been
heated above its saturation temperature and helps prevent liquid refrigerant from
, reaching the compressor. This measurement is essential for proper system
operation and compressor protection.
8. What is the purpose of subcooling in a refrigeration system?
A. Ensure vapor enters compressor
B. Prevent refrigerant boiling in liquid line
C. Increase evaporator pressure
D. Raise suction temperature
Correct Answer: B
Rationale: Subcooling ensures refrigerant remains liquid in the line, improving
efficiency and preventing flash gas. A pure liquid refrigerant must enter the
metering device for proper flow control.
9. What does "superheated vapor" indicate about the refrigerant condition?
A. Vapor mixed with liquid
B. Vapor above its boiling point temperature at given pressure
C. Liquid below freezing
D. Gas under vacuum only
Correct Answer: B
Rationale: Superheated vapor is vapor heated beyond its saturation temperature at
a given pressure. This ensures no liquid refrigerant remains in the vapor stream.
10. Which gas law best describes pressure increase with temperature in a
sealed system?
A. Ohm's Law
B. Boyle's Law
C. Charles's Law
D. Gay-Lussac's Law
