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 from the indoor air to the outdoor air, because heat always flows from a warmer body toward a cooler one.✓ Answer
   • B.It creates cold inside the evaporator and distributes that cold through the ductwork.
   • C.It converts indoor heat into cold energy that is stored in the refrigerant charge.
   • D.It destroys heat energy inside the compressor and discharges what is left outdoors.

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

   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 determines the capacity a system must deliver.
   • B.It reduces the total load, because the condensate carries heat down the drain and out of the building.
   • C.It adds latent load, because heat must be removed to condense water vapour into liquid even though that heat produces no drop in air temperature.✓ Answer
   • D.It adds sensible load, because pulling water out of the air 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 film of moisture on the pipe cools the sensing element.
   • B.Conduction, because heat passes by direct molecular contact between the pipe and the meter.
   • C.Radiation, because energy travels as electromagnetic waves and needs no material medium.✓ Answer
   • D.Convection, because air currents carry heat from the pipe to the 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; either one can be changed without affecting the other.
   • B.Raising the pressure lowers the temperature at which the refrigerant boils.
   • C.Saturation temperature is set by how many pounds of refrigerant are in the system, not by pressure.
   • D.Each saturation pressure corresponds to one specific temperature, and raising the pressure raises the temperature at which the refrigerant boils and condenses.✓ 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 remains entirely liquid and simply gets colder as it travels through the tubing.
   • B.It is compressed to a higher pressure so that it becomes able to absorb heat.
   • C.It absorbs heat from the air and boils from liquid to vapour, then picks up a little additional heat and leaves the coil as superheated vapour.✓ Answer
   • D.It condenses from vapour to liquid while giving up heat to the air passing over the coil.

   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 is then able to reject heat.
   • B.It raises the pressure and temperature of refrigerant vapour and keeps refrigerant circulating; it must receive vapour, because liquid entering it can cause slugging.✓ Answer
   • C.It condenses vapour into liquid before the refrigerant 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 heat, because it must also get rid of the heat of compression added by the compressor's work.✓ Answer
   • B.Which one is greater depends on the metering device and cannot be generalized.
   • C.The condenser rejects less heat, because part of the heat is lost through the insulated suction line.
   • D.The two are always exactly equal, because whatever heat goes in must come 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 suction accumulator.
   • C.The condenser and the liquid receiver.
   • D.The evaporator and the condenser.

   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, which shows the refrigerant leaving the evaporator is entirely vapour.✓ Answer
    • B.12°F of subcooling, which shows a solid column of liquid is present at that point.
    • C.92°F of superheat, found by adding the two readings together.
    • D.40°F of superheat, because the saturation temperature is itself the superheat value.

    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 at the condenser outlet.
    • B.There is 208°F of subcooling, obtained by adding the two values.
    • C.There is 12°F of subcooling, which indicates liquid refrigerant is reaching the metering device rather than flash gas.✓ Answer
    • D.There is no subcooling, because the liquid line must read higher than saturation for subcooling to exist.

    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 is present in the suction line and can flood back to the compressor.✓ Answer
    • B.The metering device is fully restricted and refrigerant flow has stopped.
    • C.The evaporator is being starved and the system is low on charge.
    • D.The compressor is receiving dry vapour with a comfortable margin of safety.

    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 measured at the condenser outlet and evaporator superheat at the metering device inlet.
    • B.Evaporator superheat is normally the higher of the two, because the coil outlet is the warmest point on the low side.
    • C.They are always identical, because superheat cannot change once the refrigerant has left the coil.
    • D.Total superheat, measured at the compressor inlet, is normally higher than evaporator superheat measured at the coil outlet, because the suction line picks up additional heat along the way.✓ 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; refrigerant flow through both is set only by compressor speed.
    • B.Both modulate to hold superheat steady, but the fixed orifice reacts faster to the change.
    • C.The valve closes down to protect the compressor while the fixed orifice opens wider.
    • D.The valve opens further to feed more refrigerant and hold superheat near its setting, while the fixed orifice cannot change its opening, so superheat climbs as the load rises.✓ 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.It is positioned by a controller acting on electronic temperature and pressure inputs, so it can hold superheat accurately across a wider range of operating conditions.✓ Answer
    • B.It has no moving parts and meters refrigerant purely by the size of a fixed orifice.
    • C.It responds only to head pressure and ignores conditions at the evaporator outlet.
    • D.It maintains a constant subcooling at the condenser outlet instead of a superheat at the evaporator outlet.

    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 systems with a thermostatic expansion valve are checked by subcooling.✓ Answer
    • B.Both types are checked by superheat, because superheat always reflects the amount of charge in the system.
    • C.Fixed-orifice systems are checked by subcooling, and systems with a thermostatic expansion valve are checked by superheat.
    • D.Both types are checked by reading suction pressure alone, with no temperature measurement needed.

    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 stacking in the condenser.
    • B.An undercharge or a restriction upstream in the liquid line, either of which starves the evaporator of refrigerant.✓ Answer
    • C.A condenser fan running faster than design, over-cooling the liquid.
    • D.An overfeeding metering device flooding the evaporator coil.

    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 the entire charge and weighing in a new one.
    • B.Only the undercharged system will show high superheat; a restriction leaves superheat normal.
    • C.The restricted system shows liquid backing up ahead of the restriction with adequate or high subcooling and a noticeable temperature drop across the restricted component, while the undercharged system shows low subcooling throughout the liquid line.✓ Answer
    • D.Only the restricted system will show a low suction pressure; an undercharge raises suction pressure.

    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 calculations; a single saturation value serves for both.
    • B.Use the bubble point for subcooling and the dew point for superheat.✓ Answer
    • C.Use the midpoint of the glide for both calculations.
    • 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 simply faster and helps cool the compressor during the process.
    • B.Because vapour charging cannot raise system pressure high enough to start the compressor.
    • C.Because the blend contains the system's oil, which will only transfer in the liquid phase.
    • D.Because the components of the blend boil at different rates, so drawing vapour would change the composition of what is left in the cylinder and of what enters the system.✓ 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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