20 Science & Refrigeration Cycle Practice Questions & Answers

1. 1. A homeowner says the air conditioner "makes cold and blows it into the house." What is the technically correct description of what the equipment actually does?

   • A.It moves heat out of the indoor air and releases it outdoors.✓ Answer
   • B.It creates cold inside the evaporator and blows it through the ducts.
   • C.It converts indoor heat into cold energy stored in the refrigerant.
   • D.It destroys heat energy inside the compressor as the vapour is squeezed.

   Energy is neither created nor destroyed, and there is no such thing as "cold energy." A refrigeration system lowers indoor temperature by transporting heat out of the space: the refrigerant in the evaporator is held colder than the indoor air, so heat flows from the air into the refrigerant, and that heat is later rejected outdoors.

   Source: NATE Core knowledge areas, Using Basic Science — mechanics and conservation of energy; direction of heat flowReport a problem with this question

2. 2. Which statement correctly distinguishes sensible heat from latent heat?

   • A.Both of them raise the temperature, but latent heat acts only on liquids and sensible heat only on gases.
   • B.Sensible heat changes the temperature with no change of state; latent heat changes the state with no change in temperature.✓ Answer
   • C.Sensible heat changes the state with no temperature change; latent heat changes the temperature with no state change.
   • D.Sensible heat is measured in BTU, while latent heat is measured only in degrees of temperature rise instead.

   Sensible heat is the heat you can sense on a thermometer: it raises or lowers temperature while the substance stays in the same phase. Latent heat is "hidden" heat absorbed or released during boiling, condensing, melting or freezing, and while that change of state is happening the temperature holds steady. Both are quantities of heat and both are measured in BTU.

   Source: NATE Core knowledge areas, Taking Temperature and Humidity Measurements — latent heat and sensible heatReport a problem with this question

3. 3. On a hot, humid afternoon the indoor coil is condensing a large quantity of water out of the return air. How does that moisture removal affect the load the system must handle?

   • A.It has no effect on the load, because only dry-bulb temperature sets capacity.
   • B.It reduces the total load, because the condensate carries heat down the drain.
   • C.It adds latent load, because heat must be removed to condense the water vapour.✓ Answer
   • D.It adds sensible load, because removing water is what lowers the dry-bulb temperature.

   Condensing water vapour releases its latent heat of vaporization into the coil, and the system must remove that heat just as it removes sensible heat. In humid weather the latent portion can be a large share of the total load, which is why a system may satisfy the thermostat on dry-bulb temperature and still leave the space feeling clammy.

   Source: NATE Core knowledge areas, Taking Temperature and Humidity Measurements — fundamentals of humidity; latent and sensible heatReport a problem with this question

4. 4. A technician points an infrared thermometer at a suction line and reads its surface temperature without touching it. Which mode of heat transfer does that instrument depend on?

   • A.Evaporation, because the moisture film on the pipe cools the sensor.
   • B.Conduction, because heat passes by direct contact between pipe and meter.
   • C.Radiation, because the energy travels as waves and needs no medium.✓ Answer
   • D.Convection, because moving air currents carry heat from pipe to sensor.

   Conduction requires direct contact, convection requires a moving fluid, and radiation travels as electromagnetic energy through space. An infrared thermometer senses radiated energy, which is why it reads only the surface temperature of an object, is affected by the surface's emissivity, and cannot measure the temperature of the air itself.

   Source: NATE Core knowledge areas, Taking Temperature and Humidity Measurements — conduction, convection and radiation; infrared thermometersReport a problem with this question

5. 5. For a pure refrigerant sitting at saturation, with liquid and vapour present together, what is the relationship between pressure and temperature?

   • A.Pressure and temperature are independent, so either one can change alone.
   • B.Raising the pressure lowers the temperature at which the refrigerant boils.
   • C.Saturation temperature is fixed by the pounds of charge, not by the pressure.
   • D.Each saturation pressure matches one temperature, so raising the pressure raises it.✓ Answer

   At saturation the pressure and temperature of a pure refrigerant are locked together in a fixed one-to-one relationship, which is exactly what a pressure-temperature chart tabulates. The whole cycle is built on it: the compressor raises pressure so the refrigerant will condense at a temperature above outdoor ambient, and the metering device drops pressure so it will boil at a temperature below indoor air.

   Source: NATE Core knowledge areas, Using Basic Science — temperature versus pressure and the saturation relationshipReport a problem with this question

6. 6. What happens to the refrigerant inside a properly operating evaporator coil?

   • A.It stays entirely liquid and only gets colder as it travels the tubing.
   • B.It is compressed to a higher pressure so that it can then absorb heat.
   • C.It absorbs heat from the air and boils from liquid into vapour.✓ Answer
   • D.It condenses from vapour to liquid and gives up heat to the air.

   The evaporator is the heat-absorbing component of the low side. Low-pressure liquid entering from the metering device boils at a temperature below the air temperature, so heat flows from the air into the refrigerant as latent heat. Once the last liquid has boiled off, the remaining vapour continues absorbing sensible heat, which is the superheat measured at the coil outlet.

   Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — basic refrigeration circuitReport a problem with this question

7. 7. Which statement best describes the compressor's job in the refrigeration circuit?

   • A.It cools the refrigerant so that the condenser can then reject heat.
   • B.It raises the pressure and temperature of refrigerant vapour.✓ Answer
   • C.It condenses the vapour into liquid before it reaches the condenser.
   • D.It meters liquid refrigerant into the evaporator at a controlled rate.

   The compressor is a vapour pump. Compressing the vapour raises its pressure and, with it, its saturation temperature, so the refrigerant can later condense at a temperature above the outdoor air. Vapour compresses but liquid does not, so liquid returning down the suction line can damage valves and pistons — which is the practical reason superheat is verified.

   Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — basic refrigeration circuitReport a problem with this question

8. 8. Comparing the heat a condenser rejects with the heat the evaporator absorbs in the same operating system, which statement is correct?

   • A.The condenser rejects more, because it also sheds the heat of compression.✓ Answer
   • B.Which one is greater depends on the metering device and cannot be generalized.
   • C.The condenser rejects less, because heat is lost through the suction line.
   • D.The two are always exactly equal, because the heat in must equal the heat out.

   Total heat of rejection equals the heat absorbed in the evaporator plus the work energy the compressor puts into the refrigerant. Because that compressor work becomes heat in the discharge vapour, the condenser always has more heat to shed than the evaporator picked up, which is why a condenser is sized for more than the evaporator load.

   Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — dynamic analysis of temperatures and pressures in the refrigerant circuitReport a problem with this question

9. 9. Which two components divide a mechanical refrigeration system into its high-pressure side and its low-pressure side?

   • A.The compressor and the metering device.✓ Answer
   • B.The filter-drier and the accumulator.
   • C.The condenser and the liquid receiver.
   • D.The evaporator and the condenser coil.

   Pressure changes only where work is added or where flow is throttled. The compressor raises pressure at its discharge and the metering device drops it at its outlet, so the high side runs from compressor discharge through the condenser and liquid line to the metering device inlet, and the low side runs from the metering device outlet through the evaporator and suction line back to the compressor.

   Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — basic refrigeration circuit componentsReport a problem with this question

10. 10. A technician reads a suction pressure whose saturation temperature is 40°F, and measures the suction line at that same point at 52°F. What is the correct interpretation?

    • A.12°F of superheat, so the refrigerant leaving the coil is all vapour.✓ Answer
    • B.12°F of subcooling, so a solid column of liquid stands at that point.
    • C.92°F of superheat, obtained by adding the two readings together.
    • D.40°F of superheat, because the saturation temperature is the superheat.

    Superheat is the actual line temperature minus the saturation temperature for the measured pressure, so 52°F minus 40°F equals 12°F. Any measurable superheat proves the refrigerant has boiled completely and the vapour has then risen above saturation temperature, which is what keeps liquid out of the compressor.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — superheatReport a problem with this question

11. 11. A liquid-line pressure corresponds to a saturation temperature of 110°F, and the liquid line at the same point measures 98°F. Which statement is correct?

    • A.There is 12°F of superheat measured at the condenser outlet.
    • B.There is 208°F of subcooling, found by adding the two values.
    • C.There is 12°F of subcooling in the liquid line at that point.✓ Answer
    • D.There is no subcooling, since the line must read above saturation.

    Subcooling is the saturation temperature for the measured liquid-line pressure minus the actual liquid-line temperature: 110°F minus 98°F equals 12°F. Liquid cooled below its saturation temperature will not flash into vapour in the line, so subcooling is the evidence that a solid column of liquid, not a mixture of liquid and bubbles, is feeding the metering device.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — subcoolingReport a problem with this question

12. 12. A suction-line temperature exactly equal to the saturation temperature for the measured suction pressure means zero superheat. What does that tell the technician?

    • A.The refrigerant is still saturated, so liquid can flood back to the compressor.✓ Answer
    • B.The metering device is fully plugged and refrigerant flow has stopped.
    • C.The evaporator is starved of refrigerant and the system is undercharged.
    • D.The compressor is receiving dry vapour with a safe margin of protection.

    As long as liquid and vapour coexist, the mixture stays at saturation temperature no matter how much more heat is added, so the temperature cannot rise above saturation until the last of the liquid has boiled. Zero superheat therefore means boiling is not finished inside the coil, and unboiled liquid can travel down the suction line to the compressor.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — superheatReport a problem with this question

13. 13. How do evaporator superheat and total superheat relate to one another on the same operating system?

    • A.Total superheat is read at the condenser outlet and evaporator superheat at the valve.
    • B.Evaporator superheat is normally the higher of the two on a running system.
    • C.They are always identical, because superheat cannot change past the coil outlet.
    • D.Total superheat at the compressor inlet normally exceeds evaporator superheat.✓ Answer

    Evaporator superheat is read at the coil outlet and shows what the coil itself is doing; total superheat is read at the compressor inlet and shows what the compressor actually receives. Because the suction line absorbs heat from its surroundings between those two points, total superheat is normally the larger number, and confusing the two leads to wrong conclusions about the charge.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — superheatReport a problem with this question

14. 14. The indoor load rises sharply on a hot afternoon. How does a thermostatic expansion valve respond differently from a fixed-orifice metering device?

    • A.Neither responds to load; flow through both is set by compressor speed.
    • B.Both modulate to hold superheat steady, but the orifice reacts faster.
    • C.The valve closes down to protect the compressor while the orifice opens.
    • D.The valve opens further to hold superheat; the fixed orifice cannot.✓ Answer

    A thermostatic expansion valve senses the temperature and pressure of the refrigerant leaving the coil and modulates its port to hold superheat near a set value across a wide range of loads. A fixed orifice or capillary tube has one fixed passage; its flow varies only with the pressure difference across it, so it cannot match a changing load and its superheat swings with load and charge.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — metering devicesReport a problem with this question

15. 15. How does an electronic expansion valve differ in principle from a conventional thermostatic expansion valve?

    • A.A controller positions it from electronic temperature and pressure inputs.✓ Answer
    • B.It has no moving parts and meters only through a fixed orifice size.
    • C.It responds only to head pressure and ignores conditions at the coil outlet.
    • D.It senses the coil outlet with a bulb and capillary tube, like a TXV.

    Both valve types control superheat, but a thermostatic valve is a self-contained mechanical device balancing bulb pressure against spring and evaporator pressure, while an electronic valve is driven by a controller reading sensors. Because the controller can act on the readings continuously and change its target, the electronic valve holds superheat more tightly and over a wider range of loads and evaporating temperatures.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — metering devicesReport a problem with this question

16. 16. Which statement correctly matches the charge-verification method to the metering device?

    • A.Fixed-orifice systems are checked by superheat, and TXV systems by subcooling.✓ Answer
    • B.Both types are checked by superheat, which always reflects the charge.
    • C.Orifice systems are checked by subcooling, and TXV systems by superheat.
    • D.Both types are checked by suction pressure alone, with no thermometer.

    A thermostatic expansion valve modulates to hold superheat near its setpoint over a range of charge levels, so superheat says little about charge on such a system; subcooling, which reflects the liquid inventory stacked in the condenser, is the meaningful indicator. A fixed orifice cannot modulate, so its superheat moves directly with the charge and is the appropriate measurement, taken against the manufacturer's chart for the measured outdoor dry bulb and indoor wet bulb.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — superheat and subcoolingReport a problem with this question

17. 17. A cooling system is running with high superheat and low subcooling. Assuming airflow and coil cleanliness have already been verified as normal, what does that combination point to?

    • A.An overcharge, with excess liquid refrigerant stacking in the condenser.
    • B.An undercharge or a liquid-line restriction starving the evaporator.✓ Answer
    • C.A condenser fan running fast and over-cooling the liquid refrigerant.
    • D.An overfeeding metering device flooding the evaporator with liquid.

    Low subcooling means there is not enough liquid inventory reaching the metering device, and high superheat means the evaporator runs out of liquid early and finishes the coil with vapour. Too little refrigerant in the system and a restriction that limits liquid flow both produce that same starved pattern, which is why the next step is to look for the pressure or temperature drop that identifies a restriction.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — superheat and subcooling diagnosticsReport a problem with this question

18. 18. Two systems both show low suction pressure: one is low on charge, the other has a partially plugged liquid-line filter-drier. Which observation best separates the two conditions?

    • A.The two cannot be told apart without recovering and weighing the charge.
    • B.Only the undercharged system will show high superheat; a restriction leaves it normal.
    • C.The restricted system keeps normal subcooling with a temperature drop at the drier.✓ Answer
    • D.Only the restricted system shows low suction pressure; an undercharge raises it.

    Both faults starve the evaporator, so both drive suction pressure down and superheat up. The difference is where the refrigerant is: a restriction dams liquid upstream, so subcooling stays adequate or rises and the pressure drop across the drier shows up as a temperature drop, sometimes with sweating or frost. An undercharged system simply has too little refrigerant anywhere, so subcooling is low along the whole liquid line.

    Source: NATE CHP-5 HVAC Fundamentals knowledge areas, heat transfer and the basic cooling cycle — dynamic analysis of temperatures and pressures in the refrigerant circuitReport a problem with this question

19. 19. A system is charged with a zeotropic blend that has appreciable temperature glide. Which saturation temperature should be used for each calculation?

    • A.Glide does not affect these readings; one saturation value serves both.
    • B.Use the bubble-point temperature for subcooling and the dew point for superheat.✓ Answer
    • C.Use the midpoint of the glide range for both superheat and subcooling.
    • D.Use the dew point for subcooling and the bubble point for superheat.

    In a zeotropic blend the components boil and condense over a range of temperatures rather than at a single point. The bubble point is where the last vapour condenses into liquid, so it is the reference for subcooling; the dew point is where the last liquid finishes boiling into vapour, so it is the reference for superheat. Using the wrong end of the glide shifts every reading by the amount of the glide.

    Source: Standard refrigeration cycle theory — zeotropic blend temperature glide, bubble point and dew pointReport a problem with this question

20. 20. Why must a zeotropic refrigerant blend be removed from the cylinder as a liquid when charging a system, rather than drawn off as vapour?

    • A.Because liquid charging is faster and helps cool the compressor.
    • B.Because vapour charging cannot raise pressure enough to start the unit.
    • C.Because the blend carries the system's oil, which moves only as liquid.
    • D.Because drawing vapour would change the composition of the blend.✓ Answer

    A zeotropic blend is a mixture of refrigerants with different boiling points, so vapour leaving the cylinder is richer in the more volatile component. Withdrawing vapour therefore fractionates the blend and neither the cylinder nor the system ends up with the intended mixture, which changes pressures, capacity and glide. Liquid is withdrawn instead, and it is metered or throttled so the compressor never receives a slug of liquid.

    Source: Standard refrigeration cycle theory — zeotropic blends, fractionation and liquid charging practiceReport a problem with this question

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