Refrigeration Cycle Quiz

Ten questions on the vapor-compression cycle and what the refrigerant does in each component. Pick an answer to see whether you are right and why. Related reading: The Code: charging.

1. What are the four main components of a basic vapor-compression refrigeration cycle?

   1. Compressor, condenser, metering device and evaporator
   2. Compressor, blower, filter and thermostat
   3. Condenser, accumulator, filter drier and sight glass
   4. Evaporator, reversing valve, contactor and capacitor

   Those four do the work of moving heat. Filter driers, accumulators and reversing valves are accessories or heat pump additions.

2. Refrigerant leaving the compressor is:

   1. A low-pressure, low-temperature liquid
   2. A high-pressure, high-temperature superheated vapor
   3. A low-pressure saturated mix
   4. A high-pressure subcooled liquid

   The compressor raises the pressure and temperature of the vapor, so the discharge line carries hot, superheated vapor to the condenser.

3. The condenser's job is to:

   1. Absorb heat from the conditioned space
   2. Reject heat so the refrigerant condenses from vapor to liquid
   3. Drop the refrigerant's pressure
   4. Store refrigerant during heating mode

   In the condenser the refrigerant desuperheats, condenses and subcools, giving its heat up to the outdoor air.

4. What does the metering device do?

   1. Raises the refrigerant's pressure
   2. Filters out moisture
   3. Drops the pressure of the liquid so it can boil in the evaporator at a low temperature
   4. Measures airflow

   Fixed orifices, TXVs and EEVs create the pressure drop between the high side and the low side. Some liquid flashes to vapor, which cools the rest.

5. In the evaporator, the refrigerant:

   1. Absorbs heat as it boils from liquid to vapor
   2. Gives off heat as it condenses
   3. Is compressed
   4. Becomes a subcooled liquid

   Boiling at low pressure and temperature lets the refrigerant absorb heat from the air passing over the coil. That is the cooling effect.

6. Which line normally carries subcooled liquid?

   1. The suction line
   2. The discharge line
   3. The liquid line, from the condenser to the metering device
   4. The equalizer line

   Liquid leaving the condenser has cooled below its condensing temperature, which keeps it fully liquid on its way to the metering device.

7. Flash gas just after the metering device is caused by:

   1. A refrigerant leak
   2. Some liquid boiling off as the pressure drops, which cools the remaining liquid
   3. Non-condensables in the system
   4. An overcharged condenser

   When the pressure drops, part of the liquid boils instantly and cools the rest down to the evaporator's saturation temperature. That is normal. Flash gas in the liquid line before the metering device is a problem, usually a restriction or low charge.

8. On a heat pump in heating mode, the indoor coil acts as the:

   1. Evaporator
   2. Condenser
   3. Metering device
   4. Accumulator

   The reversing valve sends hot discharge gas to the indoor coil, which rejects heat into the house as the refrigerant condenses.

9. Which component lets a heat pump switch between heating and cooling?

   1. The contactor
   2. The reversing valve
   3. The accumulator
   4. The filter drier

   The reversing valve changes the direction of refrigerant flow, swapping which coil acts as the condenser and which acts as the evaporator.

10. Why must the refrigerant entering the compressor be vapor, not liquid?

    1. Liquid barely compresses, and liquid floodback can damage the compressor
    2. Liquid would make the condenser too cold
    3. The thermostat cannot sense liquid
    4. Vapor carries more oil

    Compressors are built to compress vapor. Liquid in the compression chamber can break valves and wash oil off the bearings. Superheat at the compressor is the protection.