How does a scroll compressor compress refrigerant vapor?
- Two scrolls spin in opposite directions, throwing gas to the outside
- A piston moves the gas past reed valves on each stroke of the crank
- A vane rotor sweeps gas around the inside of a round cylinder
- One scroll orbits against a fixed scroll, squeezing gas inward
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D. One scroll orbits against a fixed scroll, squeezing gas inward
A scroll set has one stationary and one orbiting spiral. Pockets of gas are trapped at the outer edge and pushed inward to the center discharge port as their volume shrinks. Neither scroll spins like an impeller.
Reference: Scroll compressor operation
A suction-cooled hermetic compressor is running on a system that is very low on charge. Why is the motor at risk?
- Too much liquid refrigerant floods back to the motor windings
- Too little cool suction vapor passes over the motor windings
- The head pressure climbs until the motor is overloaded
- The oil pump delivers too much oil to the motor bearings
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B. Too little cool suction vapor passes over the motor windings
These compressors depend on a full flow of cool suction gas to carry heat away from the motor. A low charge means low mass flow and high superheat, so the windings overheat. Head pressure and amp draw are usually low with a low charge, not high.
Reference: Compressor motor cooling; low charge effects
Why is short cycling hard on a refrigeration compressor?
- Each start lowers the head pressure until the valves fail to seat
- Each stop lets the condenser fill completely with liquid refrigerant
- Each stop drains the crankcase heater and leaves the oil too cold
- Each start heats the motor and pumps out oil that cannot return
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D. Each start heats the motor and pumps out oil that cannot return
Starting current is several times running current, so frequent starts overheat the windings, and oil carried out at each start does not get enough run time to come back. The result is burned motors and worn bearings.
Reference: Compressor operation; short cycling