Saturated Refrigerant Quiz

Ten questions on saturation and the pressure-temperature relationship. Pick an answer to see whether you are right and why. Related reading: The Code: pressure and temperature.

1. A refrigerant is saturated when:

   1. It is entirely vapor, above its boiling point
   2. Liquid and vapor exist together at the same temperature and pressure
   3. It has been cooled below its condensing temperature
   4. It contains no oil

   Saturation is the state where liquid and vapor coexist in equilibrium. At saturation, pressure and temperature are locked together, which is exactly what a PT chart describes.

2. Where in a running air conditioning system is the refrigerant normally saturated?

   1. In the suction line at the compressor
   2. In the liquid line at the metering device
   3. Inside the evaporator and condenser coils, where it is boiling or condensing
   4. In the discharge line

   Boiling in the evaporator and condensing in the condenser both happen at saturation. The suction line is superheated, the liquid line is subcooled and the discharge line is highly superheated.

3. R-410A in the evaporator reads about 118 psig. On a PT chart that corresponds to a saturation temperature of about:

   1. 20°F
   2. 40°F
   3. 55°F
   4. 70°F

   R-410A boils at about 40°F at roughly 118 psig. That saturation temperature is the starting point for calculating superheat.

4. What happens if a gauge reading is converted using the PT chart for the wrong refrigerant?

   1. Nothing, because all refrigerants share one PT chart
   2. The saturation temperature, and every superheat and subcooling value calculated from it, will be wrong
   3. Only the high side reading is affected
   4. The gauge reads zero

   Each refrigerant has its own pressure-temperature relationship. Use the scale or setting for the refrigerant actually in the system, as shown on the data plate.

5. Superheat is calculated by:

   1. Subtracting the evaporator saturation temperature from the actual suction line temperature
   2. Subtracting the suction line temperature from the outdoor temperature
   3. Subtracting the liquid line temperature from the condenser saturation temperature
   4. Adding the evaporator and condenser saturation temperatures

   Superheat is how far the vapor has warmed above its saturation temperature after all the liquid has boiled off. Option C describes subcooling.

6. Subcooling is the difference between:

   1. Return air and supply air temperature
   2. The condenser saturation temperature and the actual liquid line temperature
   3. The suction line temperature and the evaporator saturation temperature
   4. Indoor wet bulb and outdoor dry bulb

   Subcooling measures how far the liquid has cooled below its condensing temperature. It shows how much liquid is stacked up in the condenser.

7. A system has been off long enough for everything to settle at 80°F, and liquid refrigerant is still present. The pressure should read:

   1. About the saturation pressure for 80°F
   2. Zero psig
   3. Half the running head pressure
   4. The pressure stamped on the cylinder

   With liquid present and temperatures equalized, pressure settles at the saturation pressure for that temperature. A reading well below it suggests little or no liquid is left in the system.

8. Raising the pressure on a saturated refrigerant does what to its boiling point?

   1. Lowers it
   2. Raises it
   3. Has no effect
   4. Makes it impossible to boil

   Higher pressure means a higher boiling point. That is how the compressor lets refrigerant condense at outdoor temperatures and boil at indoor ones.

9. A refrigerant blend with temperature glide, such as R-454B, boils:

   1. At exactly one temperature at a given pressure
   2. Across a small range of temperatures at a given pressure
   3. Only when the compressor is off
   4. At a higher temperature as the pressure drops

   Glide means the blend boils across a range of temperatures at constant pressure. Use dew point values for superheat and bubble point values for subcooling. The glide of R-454B is small, but it is there.

10. Which reading confirms that the refrigerant leaving the evaporator is completely vapor?

    1. Any positive superheat
    2. Zero subcooling
    3. A suction pressure above 100 psig
    4. A frosted suction line

    Positive superheat means the vapor is warmer than its saturation temperature, which can only happen once all the liquid has boiled. Zero superheat means liquid may still be present and heading for the compressor.