FDNY z51 TEST QUESTIONS AND ANSWERS (VERIFIED AND WELL
ELABORATED ANSWERS) LATEST UPDATE 2025/2026
What is a superheated vapor? - CORRECT ANSWER - completely dry vapor
In electrical circuits the conductor is always what color? - CORRECT ANSWER
- Green or Bare
Unloaders are used for what? - CORRECT ANSWER - To increase capacity
Oil foaming in a crankcase is caused by what? - CORRECT ANSWER - A
sudden drop in crankcase pressure
How would you test for acid in the refrigerant? - CORRECT ANSWER -
Spraying liquid refrigerant from a condenser or receiver on to bond paper
What is the function of a cylinder bypass? - CORRECT ANSWER - To allow
operation at different capacities
The chilled water supply to the building is 55F. If the chiller is being used for
human comfort, should you be concerned about this, and why? - CORRECT
ANSWER - Yes. this temperature is too high for human comfort purpose
The packing gland of a centrifugal pump serves to: - CORRECT ANSWER - The
drip part of the packing gland gives evidence that there is water in the pump
,Capacity control of a reciprocating hermetic compressor is typically achieved
through - CORRECT ANSWER - Operation of the hot gas bypass valve
The most important reason to sub-cool the refrigerant in the condenser is to ensure
that: - CORRECT ANSWER - A pure liquid refrigerant will enter the metering
device
A refrigeration engineer is logging reading from a reciprocating system.
compressor discharge temperature have dropped significantly and the suction line
pressure is high. which of the following that may be causing this set of condition. -
CORRECT ANSWER - An expansion valve that is open to wide
A low side float valve is used to maintain: - CORRECT ANSWER - The level of
the refrigerant in the evaporator
Regarding fire safety, which ASHRAE refrigerant safety classification indicates no
flammability and lower toxicity? - CORRECT ANSWER - A1
All liquid has been recovered from a system to be overhauled, the system -
CORRECT ANSWER - Should not be opened until all vapor has been recovered
When a refrigerant engineer uses a portable compressor recovery unit to remove
vapor refrigerant from a refrigeration system the removal rate of the refrigerant
will gradually slow down because - CORRECT ANSWER - The vapor density is
decreasing
,You are the engineer running a 2000-ton (steam-driven turbine) centrifugal
refrigeration system, a refrigeration system supplies chilled water to a college
campus with multiple buildings. During the course of making your rounds, you
check the 9 cell cooling tower on the roof. You note the water level in all of the
cooling tower cells is very low. Which one of the following immediate action
would take? - CORRECT ANSWER - Add additional make up water to cooling
tower
A rooftop water cooling tower is connected to a water pump in the building
basement with a 12 inches diameter pipe, with the pump turn off, a pressure gauge
at the pumps inlet show a pressure of 140 PSIG the height of the water column, in
feet, would be closet to. - CORRECT ANSWER - 323
What device protects the steam turbine from excessive speed? - CORRECT
ANSWER - Governor assembly
A refrigeration compressor has failed to turn on. The refrigeration engineer on duty
verifies that the electrical circuit to the motor, including all switches and circuit
breakers, is complete according to recommended service practice, the next item to
be checked would likely be the: - CORRECT ANSWER - Temperature and
pressure controls
Refrigerants are characterized by their physical properties included flashpoint,
refrigeration effect, and others. Of the refrigerants in commercial and industrial the
highest latent heat and refrigerant effect is: - CORRECT ANSWER - Ammonia
On what characteristics is ASHRAE refrigerant safety group classification system:
- CORRECT ANSWER - flammability and toxicity
, Of the following, and example of an in-line refrigerant cleaning system that may
be used with a centrifugal compressor containing R123 refrigerant is: -
CORRECT ANSWER - Refrigerant recycling
Information about the physical and chemical properties of a hazardous material can
be found in: - CORRECT ANSWER - Material Safety Data Sheet
What is the function of the turbine auxiliary oil pump? - CORRECT ANSWER -
Establish oil pressure during turbine start up
The rupture disc for a low-pressure centrifugal refrigeration machine is normally
located on the: - CORRECT ANSWER - Compressor suction
What is the primary reason ribbed metal piping is used for electrical wires
connected to the refrigeration system shown below? - CORRECT ANSWER - It
allows for flexibility and accounts for vibrations
The most important reason to use at least two compressors in a cascade system is
to: - CORRECT ANSWER - Avoid very high compression ratio for each
compressor
The suction service valve of a reciprocating compressor is front seated and the
compressor runs until the low pressure cut-out shut the compressor off. When the
machine is stopped, the refrigeration engineer notices that the suction pressure
increase rapidly. The most probable cause of this pressure rise: - CORRECT
ANSWER - Malfunction of a compressor valve
ELABORATED ANSWERS) LATEST UPDATE 2025/2026
What is a superheated vapor? - CORRECT ANSWER - completely dry vapor
In electrical circuits the conductor is always what color? - CORRECT ANSWER
- Green or Bare
Unloaders are used for what? - CORRECT ANSWER - To increase capacity
Oil foaming in a crankcase is caused by what? - CORRECT ANSWER - A
sudden drop in crankcase pressure
How would you test for acid in the refrigerant? - CORRECT ANSWER -
Spraying liquid refrigerant from a condenser or receiver on to bond paper
What is the function of a cylinder bypass? - CORRECT ANSWER - To allow
operation at different capacities
The chilled water supply to the building is 55F. If the chiller is being used for
human comfort, should you be concerned about this, and why? - CORRECT
ANSWER - Yes. this temperature is too high for human comfort purpose
The packing gland of a centrifugal pump serves to: - CORRECT ANSWER - The
drip part of the packing gland gives evidence that there is water in the pump
,Capacity control of a reciprocating hermetic compressor is typically achieved
through - CORRECT ANSWER - Operation of the hot gas bypass valve
The most important reason to sub-cool the refrigerant in the condenser is to ensure
that: - CORRECT ANSWER - A pure liquid refrigerant will enter the metering
device
A refrigeration engineer is logging reading from a reciprocating system.
compressor discharge temperature have dropped significantly and the suction line
pressure is high. which of the following that may be causing this set of condition. -
CORRECT ANSWER - An expansion valve that is open to wide
A low side float valve is used to maintain: - CORRECT ANSWER - The level of
the refrigerant in the evaporator
Regarding fire safety, which ASHRAE refrigerant safety classification indicates no
flammability and lower toxicity? - CORRECT ANSWER - A1
All liquid has been recovered from a system to be overhauled, the system -
CORRECT ANSWER - Should not be opened until all vapor has been recovered
When a refrigerant engineer uses a portable compressor recovery unit to remove
vapor refrigerant from a refrigeration system the removal rate of the refrigerant
will gradually slow down because - CORRECT ANSWER - The vapor density is
decreasing
,You are the engineer running a 2000-ton (steam-driven turbine) centrifugal
refrigeration system, a refrigeration system supplies chilled water to a college
campus with multiple buildings. During the course of making your rounds, you
check the 9 cell cooling tower on the roof. You note the water level in all of the
cooling tower cells is very low. Which one of the following immediate action
would take? - CORRECT ANSWER - Add additional make up water to cooling
tower
A rooftop water cooling tower is connected to a water pump in the building
basement with a 12 inches diameter pipe, with the pump turn off, a pressure gauge
at the pumps inlet show a pressure of 140 PSIG the height of the water column, in
feet, would be closet to. - CORRECT ANSWER - 323
What device protects the steam turbine from excessive speed? - CORRECT
ANSWER - Governor assembly
A refrigeration compressor has failed to turn on. The refrigeration engineer on duty
verifies that the electrical circuit to the motor, including all switches and circuit
breakers, is complete according to recommended service practice, the next item to
be checked would likely be the: - CORRECT ANSWER - Temperature and
pressure controls
Refrigerants are characterized by their physical properties included flashpoint,
refrigeration effect, and others. Of the refrigerants in commercial and industrial the
highest latent heat and refrigerant effect is: - CORRECT ANSWER - Ammonia
On what characteristics is ASHRAE refrigerant safety group classification system:
- CORRECT ANSWER - flammability and toxicity
, Of the following, and example of an in-line refrigerant cleaning system that may
be used with a centrifugal compressor containing R123 refrigerant is: -
CORRECT ANSWER - Refrigerant recycling
Information about the physical and chemical properties of a hazardous material can
be found in: - CORRECT ANSWER - Material Safety Data Sheet
What is the function of the turbine auxiliary oil pump? - CORRECT ANSWER -
Establish oil pressure during turbine start up
The rupture disc for a low-pressure centrifugal refrigeration machine is normally
located on the: - CORRECT ANSWER - Compressor suction
What is the primary reason ribbed metal piping is used for electrical wires
connected to the refrigeration system shown below? - CORRECT ANSWER - It
allows for flexibility and accounts for vibrations
The most important reason to use at least two compressors in a cascade system is
to: - CORRECT ANSWER - Avoid very high compression ratio for each
compressor
The suction service valve of a reciprocating compressor is front seated and the
compressor runs until the low pressure cut-out shut the compressor off. When the
machine is stopped, the refrigeration engineer notices that the suction pressure
increase rapidly. The most probable cause of this pressure rise: - CORRECT
ANSWER - Malfunction of a compressor valve
