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
What is meant by the temperature split (TD) of an air-cooled condenser?
Leaving air temperature minus entering air temperature
Discharge line temperature minus liquid line temperature
Condensing temperature minus liquid line temperature
Condensing temperature minus entering air temperature
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D. Condensing temperature minus entering air temperature
Condenser split compares the saturated condensing temperature, taken from head pressure, with the air entering the coil. Condensing temperature minus liquid line temperature is subcooling, a different measurement.
Reference: Air-cooled condensers; condenser split
Why is the refrigerant charge on a capillary tube reach-in weighed in to the nameplate amount?
The receiver must be filled to exactly half its volume
There is no receiver, so the charge is critical
The capillary tube closes if the charge is too small
The compressor has no way to tolerate any subcooling
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B. There is no receiver, so the charge is critical
A cap tube system has no receiver to hold surplus liquid and no valve to regulate flow, so a few ounces too much or too little changes pressures and superheat noticeably. The charge is therefore weighed in.
During off-cycle defrost on a medium-temperature walk-in, what are the compressor and the evaporator fans doing?
Compressor running, evaporator fans off
Compressor off, evaporator fans off
Compressor off, evaporator fans running
Compressor running, evaporator fans running
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C. Compressor off, evaporator fans running
The fans keep moving above-freezing box air through the coil to melt the frost while no refrigerant is being evaporated. Stopping the fans is what is done on electric or hot gas defrost in a freezer, to keep heat out of the box.
Reference: Defrost methods; off-cycle defrost
Where is a moisture-indicating sight glass normally installed?
In the suction line after the evaporator
In the discharge line before the condenser
In the liquid line after the filter-drier
In the liquid line before the receiver
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C. In the liquid line after the filter-drier
Placed downstream of the drier, the sight glass shows whether solid liquid is reaching the expansion valve and whether the drier is keeping the refrigerant dry. Bubbles there can mean low charge, a restriction upstream or too little subcooling.
Reference: System accessories; liquid line sight glass