FDNY Z-51 Refrigeration Exam Questions
with Verified Answers & Detailed
Rationales.
Question 1
What refrigerant is most often used in the bulb of a thermostatic expansion valve?
Correct Answer
The same refrigerant that is in the system
Question 2
A refrigeration system uses jet ejector to raise settled oil mixed with refrigerant to the
oil pump. If a restriction in the refrigerant flow to the jet ejector develops:
Correct Answer
There may be insufficient head to lift the oil
Question 3
When an expansion valve is operating properly, at the end of the evaporator will
contain:
Correct Answer
Only superheated vapor
Question 4
What device records the speed of the turbine's shaft?
Correct Answer
Tachometer
Page 1 of 220
,Question 5
When you are finished leak testing with R-22 and nitrogen the next step would be?
Correct Answer
Vent refrigerant and nitrogen to open air
Question 1: A Certificate of Fitness holder for Refrigerating System Operating
Engineer (Z-51) is inspecting a low-pressure centrifugal chiller using R-123.
The purge unit operates frequently, indicating air inleakage. What is the
primary operational hazard associated with air entering a low-pressure
refrigeration system?
A. Decreased head pressure and lower power consumption
B. Increased head pressure, elevated discharge temperatures, and reduced system
capacity
C. Evaporator freezing due to moisture absorption by the refrigerant
D. Instantaneous tripping of the high-pressure safety cutout
CORRECT ANSWER: B. Increased head pressure, elevated discharge
temperatures, and reduced system capacity
Rationale: Non-condensable gases such as air accumulate in the highest point of the
condenser. Because air cannot condense at normal operating refrigeration
temperatures, it occupies space within the shell, effectively reducing the active surface
area available for heat transfer. According to Dalton’s Law of Partial Pressures, the
total pressure in the condenser becomes the sum of the refrigerant pressure and the
partial pressure of the non-condensable gases. This raises the overall condensing head
pressure, increases the compressor work input (horsepower per ton), elevates
discharge temperatures, and significantly degrades system capacity and energy
efficiency.
Question 2: In accordance with the NYC Fire Code and Mechanical Code, high-
side pressure relief valves installed on a commercial vapor-compression
refrigeration system must discharge to which location?
A. Directly into the machinery room if an automatic ventilation system is active
B. Into the low-pressure evaporator shell via an internal bypass
C. To the outside atmosphere through dedicated piping terminated at an approved
height and location above ground level
D. Into a water storage vessel without requiring atmospheric venting
Page 2 of 220
,CORRECT ANSWER: C. To the outside atmosphere through dedicated piping
terminated at an approved height and location above ground level
Rationale: FDNY regulations and NYC Mechanical Code specify that pressure relief
devices protecting refrigerant-containing vessels must discharge safely to the outdoor
atmosphere. The discharge pipe must terminate outside the building at an approved
height (typically not less than 15 feet above adjoining ground level and at least 20
feet from windows, openings, or building air intakes) to prevent toxic or flammable
gas accumulation. Relief devices cannot discharge into occupied spaces or machinery
rooms due to asphyxiation, toxicity, and flammability risks.
Question 3: A reciprocating refrigeration compressor experiences severe liquid
slugging during startup. Which component is specifically designed to prevent
liquid refrigerant from entering the compressor suction inlet during operation?
A. Liquid receiver
B. Suction line accumulator
C. Thermostatic expansion valve (TXV)
D. Crankcase heater
CORRECT ANSWER: B. Suction line accumulator
Rationale: A suction line accumulator is installed in the suction line immediately
before the compressor inlet. Its primary function is to trap liquid refrigerant and oil
carried over from the evaporator, preventing liquid from reaching the compressor
cylinders. Liquid cannot be compressed; if liquid enters a reciprocating cylinder, it
causes mechanical failure, including broken valves, damaged wrist pins, and blown
cylinder heads. The accumulator allows liquid refrigerant to boil off safely while
returning vapor and metered amounts of oil to the compressor.
Question 4: An operator observes that a halide torch flame turns bright green
during a leak check on an older refrigeration system. What does this color
change indicate?
A. Presence of ammonia (R-717)
B. Presence of a halogenated refrigerant such as R-12 or R-22
C. Presence of carbon dioxide (R-744)
D. Absence of any refrigerant leak
CORRECT ANSWER: B. Presence of a halogenated refrigerant such as R-12 or R-
22
Page 3 of 220
, Rationale: A halide torch uses a copper reaction plate heated by an alcohol or
propane flame. When air containing a halogenated refrigerant (containing chlorine or
fluorine) is drawn through the sampling tube into the flame, the thermal reaction with
the copper plate produces a color shift. A light green flame indicates a small
halogenated leak, while a dark blue or purple flame indicates a severe leak. Halide
torches do not react with non-halogenated refrigerants like ammonia or carbon
dioxide.
Question 5: A thermostatic expansion valve (TXV) maintains a constant level of
what key operating parameter at the evaporator outlet?
A. Evaporator pressure
B. Condenser subcooling
C. Evaporator superheat
D. Compressor discharge temperature
CORRECT ANSWER: C. Evaporator superheat
Rationale: The primary function of a TXV is to meter liquid refrigerant into the
evaporator at a rate that matches the heat load while maintaining a constant
superheat at the evaporator outlet. It achieves this balance by sensing evaporator
outlet temperature via its remote sensing bulb and balancing that force against
evaporator pressure and internal spring tension. Maintaining proper superheat
ensures maximum heat absorption efficiency inside the evaporator coils while
guaranteeing that no unevaporated liquid refrigerant returns to damage the
compressor.
Question 6: In a large industrial refrigeration plant using Ammonia (R-717),
what ASHRAE safety classification applies to this refrigerant?
A. A1
B. A2L
C. B2L
D. B2
CORRECT ANSWER: D. B2
Rationale: ASHRAE Standard 34 categorizes refrigerants based on toxicity (Class A =
lower toxicity, Class B = higher toxicity) and flammability (Class 1 = no flame
propagation, Class 2L = lower flammability, Class 2 = moderate flammability, Class 3
= high flammability). Ammonia (R-717) carries a high toxicity rating and moderate
flammability rating, placing it strictly in Class B2 (or B2L under updated
Page 4 of 220
with Verified Answers & Detailed
Rationales.
Question 1
What refrigerant is most often used in the bulb of a thermostatic expansion valve?
Correct Answer
The same refrigerant that is in the system
Question 2
A refrigeration system uses jet ejector to raise settled oil mixed with refrigerant to the
oil pump. If a restriction in the refrigerant flow to the jet ejector develops:
Correct Answer
There may be insufficient head to lift the oil
Question 3
When an expansion valve is operating properly, at the end of the evaporator will
contain:
Correct Answer
Only superheated vapor
Question 4
What device records the speed of the turbine's shaft?
Correct Answer
Tachometer
Page 1 of 220
,Question 5
When you are finished leak testing with R-22 and nitrogen the next step would be?
Correct Answer
Vent refrigerant and nitrogen to open air
Question 1: A Certificate of Fitness holder for Refrigerating System Operating
Engineer (Z-51) is inspecting a low-pressure centrifugal chiller using R-123.
The purge unit operates frequently, indicating air inleakage. What is the
primary operational hazard associated with air entering a low-pressure
refrigeration system?
A. Decreased head pressure and lower power consumption
B. Increased head pressure, elevated discharge temperatures, and reduced system
capacity
C. Evaporator freezing due to moisture absorption by the refrigerant
D. Instantaneous tripping of the high-pressure safety cutout
CORRECT ANSWER: B. Increased head pressure, elevated discharge
temperatures, and reduced system capacity
Rationale: Non-condensable gases such as air accumulate in the highest point of the
condenser. Because air cannot condense at normal operating refrigeration
temperatures, it occupies space within the shell, effectively reducing the active surface
area available for heat transfer. According to Dalton’s Law of Partial Pressures, the
total pressure in the condenser becomes the sum of the refrigerant pressure and the
partial pressure of the non-condensable gases. This raises the overall condensing head
pressure, increases the compressor work input (horsepower per ton), elevates
discharge temperatures, and significantly degrades system capacity and energy
efficiency.
Question 2: In accordance with the NYC Fire Code and Mechanical Code, high-
side pressure relief valves installed on a commercial vapor-compression
refrigeration system must discharge to which location?
A. Directly into the machinery room if an automatic ventilation system is active
B. Into the low-pressure evaporator shell via an internal bypass
C. To the outside atmosphere through dedicated piping terminated at an approved
height and location above ground level
D. Into a water storage vessel without requiring atmospheric venting
Page 2 of 220
,CORRECT ANSWER: C. To the outside atmosphere through dedicated piping
terminated at an approved height and location above ground level
Rationale: FDNY regulations and NYC Mechanical Code specify that pressure relief
devices protecting refrigerant-containing vessels must discharge safely to the outdoor
atmosphere. The discharge pipe must terminate outside the building at an approved
height (typically not less than 15 feet above adjoining ground level and at least 20
feet from windows, openings, or building air intakes) to prevent toxic or flammable
gas accumulation. Relief devices cannot discharge into occupied spaces or machinery
rooms due to asphyxiation, toxicity, and flammability risks.
Question 3: A reciprocating refrigeration compressor experiences severe liquid
slugging during startup. Which component is specifically designed to prevent
liquid refrigerant from entering the compressor suction inlet during operation?
A. Liquid receiver
B. Suction line accumulator
C. Thermostatic expansion valve (TXV)
D. Crankcase heater
CORRECT ANSWER: B. Suction line accumulator
Rationale: A suction line accumulator is installed in the suction line immediately
before the compressor inlet. Its primary function is to trap liquid refrigerant and oil
carried over from the evaporator, preventing liquid from reaching the compressor
cylinders. Liquid cannot be compressed; if liquid enters a reciprocating cylinder, it
causes mechanical failure, including broken valves, damaged wrist pins, and blown
cylinder heads. The accumulator allows liquid refrigerant to boil off safely while
returning vapor and metered amounts of oil to the compressor.
Question 4: An operator observes that a halide torch flame turns bright green
during a leak check on an older refrigeration system. What does this color
change indicate?
A. Presence of ammonia (R-717)
B. Presence of a halogenated refrigerant such as R-12 or R-22
C. Presence of carbon dioxide (R-744)
D. Absence of any refrigerant leak
CORRECT ANSWER: B. Presence of a halogenated refrigerant such as R-12 or R-
22
Page 3 of 220
, Rationale: A halide torch uses a copper reaction plate heated by an alcohol or
propane flame. When air containing a halogenated refrigerant (containing chlorine or
fluorine) is drawn through the sampling tube into the flame, the thermal reaction with
the copper plate produces a color shift. A light green flame indicates a small
halogenated leak, while a dark blue or purple flame indicates a severe leak. Halide
torches do not react with non-halogenated refrigerants like ammonia or carbon
dioxide.
Question 5: A thermostatic expansion valve (TXV) maintains a constant level of
what key operating parameter at the evaporator outlet?
A. Evaporator pressure
B. Condenser subcooling
C. Evaporator superheat
D. Compressor discharge temperature
CORRECT ANSWER: C. Evaporator superheat
Rationale: The primary function of a TXV is to meter liquid refrigerant into the
evaporator at a rate that matches the heat load while maintaining a constant
superheat at the evaporator outlet. It achieves this balance by sensing evaporator
outlet temperature via its remote sensing bulb and balancing that force against
evaporator pressure and internal spring tension. Maintaining proper superheat
ensures maximum heat absorption efficiency inside the evaporator coils while
guaranteeing that no unevaporated liquid refrigerant returns to damage the
compressor.
Question 6: In a large industrial refrigeration plant using Ammonia (R-717),
what ASHRAE safety classification applies to this refrigerant?
A. A1
B. A2L
C. B2L
D. B2
CORRECT ANSWER: D. B2
Rationale: ASHRAE Standard 34 categorizes refrigerants based on toxicity (Class A =
lower toxicity, Class B = higher toxicity) and flammability (Class 1 = no flame
propagation, Class 2L = lower flammability, Class 2 = moderate flammability, Class 3
= high flammability). Ammonia (R-717) carries a high toxicity rating and moderate
flammability rating, placing it strictly in Class B2 (or B2L under updated
Page 4 of 220
