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WorksheetsUnit 13 - HVAC
Total questions: 22
Worksheet time: 11mins
What type of classification applies to a hermetic compressor?
Mechanical design
Cooling mechanism
Displacement
Physical enclosure
Which of the following compressors is classified by mechanical design?
Ultralow
Positive displacement
Centrifugal
Open
How does a dynamic displacement compressor cause an increase in refrigerant pressure?
By pushing the gas out of a space with a piston
By creating a surge in the compressor
By blocking the discharge port of the compressor
By speeding the refrigerant up to a high velocity
Which of the following statements BEST describes the difference in the way positive displacement compressors and dynamic displacement compressors move the refrigerant?
A positive displacement compressor pulls the refrigerant out of a space, whereas a dynamic displacement compressor pushes it out
A positive displacement compressor pushes the refrigerant out of a space, whereas a dynamic displacement compressor pulls it out
A positive displacement compressor pushes the refrigerant out, whereas a dynamic displacement compressor throws it out
A positive displacement compressor throws the refrigerant out, whereas a dynamic displacement compressor pushes it out
Which of the following describes a similarity of open compressors and semi-hermetic compressors?
The valve plates of both types of compressor are field serviceable.
The motors of both types of compressors use a shaft seal that is subject to forming leaks.
The motors of both types of compressors are enclosed in a steel shell.
The motors of both types of compressors are separate from the compressor.
The motors of both types of compressors are separate from the compressor.
They have a lower sound level
They have motors that are separate from the compressor
They use a shaft seal to prevent refrigerant leakage
They can be made in larger sizes
Which type of compressor uses an orbital component to trap pockets of refrigerant and decrease the size of those pockets in order to increase the pressure and temperature of the refrigerant?
Screw compressor
Reciprocating compressor
Scroll compressor
Rotary compressor
Which of the following BEST describes the operation of a centrifugal compressor?
They use a crankshaft and a rod to move pistons up and down in a cylinder to compress refrigerant
They use one fixed scroll and one orbital scroll to capture pockets of refrigerant and gradually decrease the size of those pockets
They use an impeller to throw refrigerant into a snail shell-shaped scroll that directs the refrigerant toward the compressor
They use a rotor that turns within a cylinder and progressively compresses the gas in the rotor spaces
Which of the following best describes the method typically used to cool an air-cooled compressor?
The condenser fan airflow blows over the compressor
The refrigerant gas is passed over the motor before it enters the compressor
The condenser water line is wrapped around the condenser fan blades
Outside air is blown over the condenser water line
Why can a refrigerant-cooled semi-hermetic compressor operate at a lower superheat than compressors cooled by other methods?
Because outside air is pulled into the compressor
Because a dedicated fan is not needed to cool the compressor
Because the refrigerant goes over the motor before entering the cylinders
Because the temperature and pressure of the system are higher than in other systems
Which of the following statements about low evaporator temperature compressors is TRUE?
More refrigerant enters the cylinder for each compression cycle
Warmer air or water enters the condenser, decreasing heat-removal capacity
Cooler air or water enters the condenser, increasing heat-removal capacity
Less refrigerant enters the cylinder for each compression cycle
Which of the following statements BEST generalizes the relationship between evaporator temperature and horsepower for normal condenser operating temperature ranges?
The lower the evaporator temperature, the less horsepower required to produce the same BTUH capacity
Evaporator temperature and horsepower required to produce BTUH capacity are unrelated to each other
The lower the evaporator temperature, the more horsepower required to produce the same BTUH capacity
The higher the evaporator temperature, the more horsepower required to produce the same BTUH capacity
What is the purpose of capacity control?
To cool the compressor motor by passing refrigerant gas over it on its way to the cylinders
To match the size and type of motor to the type of work the compressor must do
To minimize the energy used by making compressor capacity match load
To allow the compressor to start and stop while running at full capacity
Which of the following describes a method of cylinder unloading used in capacity control for reciprocating compressors?
Holding the suction valves shut
Bypassing the cylinder discharge to the suction manifold
Opening up the suction inlet
Activating an internal solenoid
What is the purpose of the cup in in a splash system for lubricating a compressor?
It throws the oil out by centrifugal force
It catches oil and drains it to the crankshaft
It slings the oil around the inside of the compressor
It picks oil up in the crankcase and forces it down the crankshaft
Which of the following assumptions can you make if a compressor is operating on low oil?
The oil is trapped somewhere in the system
The oil has evaporated from the system
The oil has leaked from the system
The oil has burnt off somewhere in the system
How is compression ratio calculated?
By dividing absolute discharge pressure by absolute suction pressure
By dividing gauge discharge pressure by gauge suction pressure
By dividing absolute suction pressure by absolute discharge pressure
By multiplying absolute discharge pressure by absolute suction pressure
If gauge pressure reads 190 psig, which of the following is the most reasonable estimate of absolute pressure?
170 psig
175 psig
205 psig
210 psig
Which of the following is one common cause of liquid refrigerant floodback?
Expansion valve underfeeding because of a dead bulb charge.
Operating with an unusually high evaporator load
Dirty or iced evaporator coils
Refrigerant restriction in the system
A discharge line temperature above 220°F would be indicative of which of the following problems?
Liquid slugging
Flooded starts
Compressor overheating
Oil dilution
What is the FIRST step in any compressor changeout?
Recover the system refrigerant
Use an electric meter to test for voltage at the unit
Disconnect and lock out power to the unit
Discharge the compressor and run capacitor
When should you remove the caps or plugs holding the nitrogen charge in a new compressor during a compressor changeout?
After mounting the new compressor
After recovering the system refrigerant
After nitrogen brazing the new compressor lines
After cutting the lines to the old compressor
