FIRST CLASS STATIONARY AIR CONDITIONING (636-LA) CONTRACTOR EXAM PRACTICE
EXAM QUESTIONS AND CORRECT ANSWERS LATEST
1. The primary role of a First Class Stationary Air Conditioning contractor is to:
A) Install residential split systems only
B) Design, install, service, and repair large commercial and industrial air conditioning and
refrigeration systems
C) Perform only electrical work on HVAC equipment
D) Clean air ducts
ANSWER: B
EXPLANATION: This license authorizes work on large, complex systems such as centrifugal
chillers, absorption chillers, industrial refrigeration, and extensive commercial HVAC
systems, often involving high-pressure refrigerants and complex controls.
2. For large commercial air conditioning systems, the refrigerant that is commonly used in
low-pressure centrifugal chillers is:
A) R-410A
B) R-134a
C) R-123
D) R-22
ANSWER: C
EXPLANATION: R-123 is a low-pressure refrigerant (evaporator pressure below
atmospheric) used in many older and some newer centrifugal chillers. It has a low ODP but
is being phased down. R-134a is used in positive-pressure centrifugals.
3. The ASHRAE safety classification for ammonia (R-717) is:
,A) A1 (Lower toxicity, no flame propagation)
B) B2 (Higher toxicity, flammable)
C) B1 (Lower toxicity, flammable)
D) A2 (Lower toxicity, flammable)
ANSWER: B
EXPLANATION: Ammonia (R-717) is Class B2: Higher toxicity and flammable. It requires
special safety considerations, ventilation, and is often used in industrial refrigeration
applications.
4. The maximum allowable concentration (MAC) for a refrigerant in an occupied space is
based on its:
A) Flammability
B) Toxicity and exposure limits (e.g., TLV-TWA)
C) Ozone Depletion Potential (ODP)
D) Global Warming Potential (GWP)
ANSWER: B
EXPLANATION: Threshold Limit Value - Time Weighted Average (TLV-TWA) is the
concentration a worker can be exposed to for 8 hours/day. Refrigerant safety groups (A vs.
B) are based on toxicity. B refrigerants have lower allowable concentrations.
5. The primary purpose of a "purge unit" on a low-pressure centrifugal chiller is to:
A) Remove oil from the refrigerant
B) Remove non-condensable gases (air and moisture) that have leaked into the refrigerant
side
C) Charge the system with refrigerant
D) Control compressor capacity
,ANSWER: B
EXPLANATION: Low-pressure chillers operate below atmospheric pressure. Air and
moisture can leak in, increasing head pressure and causing corrosion. The purge unit
compresses and cools these gases, condensing water and refrigerant, and vents the non-
condensables.
6. In a centrifugal chiller, the device that modulates cooling capacity by changing the angle
of inlet vanes is the:
A) Thermal expansion valve
B) Condenser water regulating valve
C) Pre-rotation vane (PRV) actuator
D) Slide valve
ANSWER: C
EXPLANATION: Inlet guide vanes (pre-rotation vanes) throttle the flow of refrigerant gas into
the impeller, controlling capacity from 100% down to about 10-20% without cycling.
Variable speed drives are also used.
7. "Surge" in a centrifugal compressor is a condition where:
A) The compressor operates at peak efficiency
B) Refrigerant flow reverses momentarily due to insufficient pressure difference between
discharge and suction, causing violent vibration
C) The condenser water temperature is too low
D) The evaporator load is perfectly matched
ANSWER: B
, EXPLANATION: Surge occurs when the system head is too high relative to the compressor's
capability (e.g., low load with cold condenser water). It causes flow reversal, noise, and can
damage bearings and impellers. It is prevented by hot gas bypass, variable speed, or
condenser water temperature control.
8. The "approach temperature" for a chiller evaporator is the difference between the:
A) Suction pressure and discharge pressure
B) Leaving chilled water temperature and the refrigerant evaporating temperature
C) Entering and leaving chilled water temperature (range)
D) Condensing temperature and ambient wet-bulb
ANSWER: B
EXPLANATION: Evaporator Approach = Leaving Chilled Water Temp - Refrigerant
Evaporating Temp. A higher than design approach (e.g., >2-4°F) indicates reduced heat
transfer due to fouled tubes, low refrigerant charge, or low water flow.
9. In a water-cooled condenser, the "approach temperature" is the difference between the:
A) Refrigerant condensing temperature and the leaving condenser water temperature
B) Entering and leaving condenser water temperature
C) Suction temperature and condensing temperature
D) Discharge temperature and ambient temperature
ANSWER: A
EXPLANATION: Condenser Approach = Refrigerant Condensing Temp - Leaving Condenser
Water Temp. A higher approach indicates fouled tubes, low water flow, or overcharge of
refrigerant.
10. The "lift" of a refrigeration compressor refers to the:
EXAM QUESTIONS AND CORRECT ANSWERS LATEST
1. The primary role of a First Class Stationary Air Conditioning contractor is to:
A) Install residential split systems only
B) Design, install, service, and repair large commercial and industrial air conditioning and
refrigeration systems
C) Perform only electrical work on HVAC equipment
D) Clean air ducts
ANSWER: B
EXPLANATION: This license authorizes work on large, complex systems such as centrifugal
chillers, absorption chillers, industrial refrigeration, and extensive commercial HVAC
systems, often involving high-pressure refrigerants and complex controls.
2. For large commercial air conditioning systems, the refrigerant that is commonly used in
low-pressure centrifugal chillers is:
A) R-410A
B) R-134a
C) R-123
D) R-22
ANSWER: C
EXPLANATION: R-123 is a low-pressure refrigerant (evaporator pressure below
atmospheric) used in many older and some newer centrifugal chillers. It has a low ODP but
is being phased down. R-134a is used in positive-pressure centrifugals.
3. The ASHRAE safety classification for ammonia (R-717) is:
,A) A1 (Lower toxicity, no flame propagation)
B) B2 (Higher toxicity, flammable)
C) B1 (Lower toxicity, flammable)
D) A2 (Lower toxicity, flammable)
ANSWER: B
EXPLANATION: Ammonia (R-717) is Class B2: Higher toxicity and flammable. It requires
special safety considerations, ventilation, and is often used in industrial refrigeration
applications.
4. The maximum allowable concentration (MAC) for a refrigerant in an occupied space is
based on its:
A) Flammability
B) Toxicity and exposure limits (e.g., TLV-TWA)
C) Ozone Depletion Potential (ODP)
D) Global Warming Potential (GWP)
ANSWER: B
EXPLANATION: Threshold Limit Value - Time Weighted Average (TLV-TWA) is the
concentration a worker can be exposed to for 8 hours/day. Refrigerant safety groups (A vs.
B) are based on toxicity. B refrigerants have lower allowable concentrations.
5. The primary purpose of a "purge unit" on a low-pressure centrifugal chiller is to:
A) Remove oil from the refrigerant
B) Remove non-condensable gases (air and moisture) that have leaked into the refrigerant
side
C) Charge the system with refrigerant
D) Control compressor capacity
,ANSWER: B
EXPLANATION: Low-pressure chillers operate below atmospheric pressure. Air and
moisture can leak in, increasing head pressure and causing corrosion. The purge unit
compresses and cools these gases, condensing water and refrigerant, and vents the non-
condensables.
6. In a centrifugal chiller, the device that modulates cooling capacity by changing the angle
of inlet vanes is the:
A) Thermal expansion valve
B) Condenser water regulating valve
C) Pre-rotation vane (PRV) actuator
D) Slide valve
ANSWER: C
EXPLANATION: Inlet guide vanes (pre-rotation vanes) throttle the flow of refrigerant gas into
the impeller, controlling capacity from 100% down to about 10-20% without cycling.
Variable speed drives are also used.
7. "Surge" in a centrifugal compressor is a condition where:
A) The compressor operates at peak efficiency
B) Refrigerant flow reverses momentarily due to insufficient pressure difference between
discharge and suction, causing violent vibration
C) The condenser water temperature is too low
D) The evaporator load is perfectly matched
ANSWER: B
, EXPLANATION: Surge occurs when the system head is too high relative to the compressor's
capability (e.g., low load with cold condenser water). It causes flow reversal, noise, and can
damage bearings and impellers. It is prevented by hot gas bypass, variable speed, or
condenser water temperature control.
8. The "approach temperature" for a chiller evaporator is the difference between the:
A) Suction pressure and discharge pressure
B) Leaving chilled water temperature and the refrigerant evaporating temperature
C) Entering and leaving chilled water temperature (range)
D) Condensing temperature and ambient wet-bulb
ANSWER: B
EXPLANATION: Evaporator Approach = Leaving Chilled Water Temp - Refrigerant
Evaporating Temp. A higher than design approach (e.g., >2-4°F) indicates reduced heat
transfer due to fouled tubes, low refrigerant charge, or low water flow.
9. In a water-cooled condenser, the "approach temperature" is the difference between the:
A) Refrigerant condensing temperature and the leaving condenser water temperature
B) Entering and leaving condenser water temperature
C) Suction temperature and condensing temperature
D) Discharge temperature and ambient temperature
ANSWER: A
EXPLANATION: Condenser Approach = Refrigerant Condensing Temp - Leaving Condenser
Water Temp. A higher approach indicates fouled tubes, low water flow, or overcharge of
refrigerant.
10. The "lift" of a refrigeration compressor refers to the:
