A centrifugal compressor works differently from the reciprocating and scroll compressors used on high-pressure equipment. How does it raise the pressure of the vapor?
Correct Answer
A) It accelerates vapor outward with a spinning impeller and converts that speed into pressure
A centrifugal compressor is a dynamic machine. An impeller spins the vapor outward at high speed, and the diffuser around it slows that flow down and converts the velocity into pressure. Nothing is trapped and nothing is squeezed. That is why these machines suit the very large flows and modest pressure rises a low-pressure chiller needs, and why they surge instead of continuing to pump when suction pressure falls away.
Why This Is the Correct Answer
Dynamic compression works by giving the gas velocity and then recovering it as pressure in a diffuser. It handles enormous volumes of low density vapor efficiently, which is exactly what a machine running on a low-pressure refrigerant produces, but it depends on having gas to accelerate, so its behavior collapses at very low suction pressure.
Why the Other Options Are Wrong
Option B: It traps a fixed volume of vapor in a cylinder and reduces that volume with a moving piston
That describes a reciprocating compressor, which is a positive displacement machine and behaves quite differently at low suction pressure.
Option C: It squeezes vapor between two interleaved spirals as one of them orbits inside the other
That describes a scroll compressor, common on small high-pressure equipment and not used on low-pressure chillers.
Option D: It carries vapor along in the pockets between two meshing rotors of unequal lobe count
That describes a screw compressor, which is used on high-pressure chillers but not on low-pressure machines.
Memory Technique
A recip squeezes, a scroll winds, a screw carries, a centrifugal throws.
Reference Hint
A centrifugal compressor raises pressure by accelerating vapor with an impeller and recovering the velocity in a diffuser.
More epa-608-type-3 Questions
Some jobs on a chiller count as a major repair under the refrigerant rules, which changes what a technician is allowed to do with a badly leaking machine. Which job falls into that category?
An owner asks how many pounds of refrigerant the rules allow to be left inside a chiller once the recovery has been signed off. What is the correct answer to give him?
The motor of a hermetic centrifugal chiller sits inside the pressure boundary rather than outside it on a shaft. What follows from that construction?
An oxygen monitor is used to decide whether a chiller machine room is safe to enter after a refrigerant release. Below what oxygen reading is the atmosphere treated as oxygen deficient?
A low-pressure chiller's rupture disc has a discharge line running from it. Where does that line have to terminate?
A refrigerant vapor detector in a chiller machine room reaches its alarm level. Besides sounding, what is that alarm expected to do?
Which piece of equipment requires a technician to hold Type III certification before it may be serviced?
What property of the refrigerant is what actually places an appliance into the low-pressure category?
During normal operation, what condition exists inside the evaporator of a running low-pressure centrifugal chiller?
In what unit is the required final recovery vacuum for a low-pressure appliance normally expressed?
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