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ITI/NCVT RAC Mechanic (AITT Theory) study materials
ITI/NCVT RAC Mechanic (AITT Theory) study resources

ITI/NCVT RAC Mechanic (AITT Theory) practice questions

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Question 1Trade safety, hand tools, measuring instruments and workshop practice

While charging a refrigerant cylinder in the workshop, which personal protective equipment is most important to protect the eyes from a sudden liquid refrigerant spray?

  1. Leather palm gloves
  2. Chemical splash goggles worn over the eyes Correct answer.
  3. Ear plugs
  4. A cotton dust mask

Explanation

Liquid refrigerant at atmospheric pressure boils violently and can splash into the eyes, causing cold burns and permanent damage. Chemical splash goggles that seal around the eyes are the specified protection, unlike open safety glasses. Gloves and masks protect other parts of the body and do not shield the eyes.

Question 2Trade safety, hand tools, measuring instruments and workshop practice

A fitter must loosen a rusted hexagonal bolt without damaging the flats. Which hand tool is the correct first choice?

  1. Chain pipe vice
  2. Pipe wrench with serrated jaws
  3. Six-point ring spanner of correct size Correct answer.
  4. Adjustable wrench opened slightly wide

Explanation

A six-point ring spanner grips all six flats of the bolt head, so the load is spread and the corners are not rounded off. Serrated pipe-wrench jaws and a loose adjustable wrench concentrate force on the corners and damage them. The pipe vice is a holding device, not a turning tool.

Question 3Trade safety, hand tools, measuring instruments and workshop practice

On a metric outside micrometer, the thimble has 50 divisions and one spindle revolution advances the spindle by 0.5 mm. What is the least count?

  1. 0.5 mm
  2. 0.01 mm Correct answer.
  3. 0.02 mm
  4. 0.05 mm

Explanation

Least count equals the spindle advance per revolution divided by the number of thimble divisions, so 0.5 mm divided by 50 gives 0.01 mm. This is the standard metric micrometer least count. The other values would need a different number of divisions or a different screw pitch.

Question 4Trade safety, hand tools, measuring instruments and workshop practice

Why is a bench vice fitted with soft jaw caps when holding a finished, polished copper tube?

  1. To increase the clamping force on the tube
  2. To spread the clamping load and prevent jaw serrations from marking the surface Correct answer.
  3. To insulate the tube from stray welding current
  4. To keep the vice screw lubricated

Explanation

Hard serrated jaws leave deep marks on soft polished surfaces and can ovalise thin tube. Soft caps such as lead or aluminium liners spread the load and grip without indentation. They do not increase clamping force or provide electrical insulation.

Question 5Trade safety, hand tools, measuring instruments and workshop practice

A vernier caliper reading shows main scale 24 mm, vernier coincidence at the 7th division, and least count 0.02 mm. What is the measured dimension?

  1. 24.14 mm Correct answer.
  2. 24.07 mm
  3. 31.00 mm
  4. 24.70 mm

Explanation

The vernier contribution is the coinciding division number multiplied by the least count, so 7 times 0.02 mm equals 0.14 mm. Adding this to the 24 mm main scale reading gives 24.14 mm. The other values come from misplacing the decimal or adding the division number directly.

Question 6Trade safety, hand tools, measuring instruments and workshop practice

Which property makes a try square unsuitable for checking the flatness of a machined surface?

  1. Its blade is too short to span long surfaces
  2. Its stock is hardened and cannot slide
  3. It is a squareness reference, not a surface-flatness gauge Correct answer.
  4. It has only one true right angle

Explanation

A try square checks the angle between two surfaces, usually 90 degrees, by comparing the blade with the work. Flatness requires a straight edge, surface plate or dial indicator sweep. Using a try square for flatness gives no meaningful reading.

Question 7Trade safety, hand tools, measuring instruments and workshop practice

In a workshop, the first action on discovering an electrical shock victim still in contact with a live conductor is to:

  1. Pull the victim away by the arm
  2. Pour water on the contact point
  3. Switch off the supply or isolate it safely Correct answer.
  4. Begin chest compressions immediately

Explanation

Rescuer safety comes first: the supply must be switched off or the victim separated using an insulated device before any touch. Touching the victim while the conductor is live can electrocute the rescuer as well. First aid and CPR follow only after the scene is safe.

Question 8Trade safety, hand tools, measuring instruments and workshop practice

Which file cut is most suitable for rapid removal of excess metal on a mild steel plate before finishing?

  1. Double-cut bastard file Correct answer.
  2. Single-cut dead smooth file
  3. Single-cut smooth file
  4. Double-cut dead smooth file

Explanation

A double-cut bastard file has coarse, crossing teeth that remove metal quickly while still leaving a workable surface. Smooth and dead smooth cuts are for finishing and polishing only. Single cut is used where a drawn, non-clogging cut is wanted.

Question 9Trade safety, hand tools, measuring instruments and workshop practice

A hacksaw blade is mounted with the teeth pointing forward, away from the handle. What is the effect during the normal forward cutting stroke?

  1. The blade cuts on the forward stroke as intended Correct answer.
  2. The blade cuts only on the return stroke
  3. The blade will not cut at all
  4. The blade teeth will break immediately

Explanation

Hacksaw teeth are set to cut on the forward stroke, away from the handle, so the blade must be mounted with teeth pointing forward. The return stroke clears the chips and repositions the blade. Mounting the blade with teeth reversed simply means it cuts on the pull stroke, which is not the intended technique.

Question 10Trade safety, hand tools, measuring instruments and workshop practice

Why is a centre punch given a sharp 90-degree point while a prick punch has a finer, steeper point?

  1. The centre punch is used only on wood
  2. The centre punch is hardened and the prick punch is soft
  3. The prick punch locates the layout line precisely, and the centre punch enlarges the mark to guide the drill Correct answer.
  4. The prick punch is struck with a hammer and the centre punch is pressed by hand

Explanation

A fine prick punch marks the intersection of layout lines accurately, then a 90-degree centre punch enlarges that dot so the drill point seats without wandering. Both are struck with a hammer and both are hardened. The difference lies in point angle and purpose, not material.

Question 11Trade safety, hand tools, measuring instruments and workshop practice

Which instrument would you select to check the internal diameter of a cylinder bore to a resolution of 0.01 mm?

  1. Outside micrometer
  2. Vernier height gauge
  3. Steel rule
  4. Inside micrometer or bore gauge Correct answer.

Explanation

An inside micrometer or a dial bore gauge is designed to measure internal diameters and can resolve 0.01 mm. An outside micrometer measures external dimensions and cannot enter a bore. A height gauge and a steel rule are not internal-diameter instruments.

Question 12Trade safety, hand tools, measuring instruments and workshop practice

A surface plate is used as a datum for marking out. Why must it be kept free of burrs, dirt and swarf?

  1. To prevent the plate from rusting overnight
  2. To keep the plate looking clean for inspection
  3. To improve the magnetic properties of the plate
  4. Because any raised particle tilts the work and destroys the datum accuracy Correct answer.

Explanation

A surface plate is a flat reference, so any trapped particle lifts one part of the workpiece and tilts it, making every height and marking measurement wrong. Cleanliness is therefore a metrological requirement, not cosmetic. Rust prevention and magnetic properties are unrelated to datum accuracy.

Question 13Trade safety, hand tools, measuring instruments and workshop practice

When using a portable electric angle grinder, the guard is removed by a previous user. What is the correct action?

  1. Reduce the disc speed to compensate
  2. Hold the grinder with one hand to stay clear of sparks
  3. Use the grinder carefully without the guard
  4. Fit the correct guard before starting any work Correct answer.

Explanation

The guard contains fragments if the disc bursts and shields the operator from sparks and debris. It must be refitted before the tool is used, and the disc must match the tool speed rating. Careful handling or reduced speed does not replace the guard.

Question 14Trade safety, hand tools, measuring instruments and workshop practice

A dial test indicator reading fluctuates by 0.03 mm while the spindle is stationary. Which cause should be investigated first?

  1. Incorrect least count of the dial
  2. Wrong colour of the dial face
  3. Loose clamping of the indicator or a worn stylus contact Correct answer.
  4. Excessive room lighting

Explanation

A stationary spindle should give a steady reading, so fluctuation points to mechanical instability such as a loose magnetic base, a loose stylus or a damaged plunger. Least count is fixed by the dial graduation and does not vary during a reading. Lighting and dial colour do not affect the pointer position.

Question 15Trade safety, hand tools, measuring instruments and workshop practice

A steel rule marked in millimetres is used at 20 degrees Celsius as the standard reference temperature. What does this standard temperature mean for precision measurement?

  1. Measurements are valid only at that temperature because steel expands when heated Correct answer.
  2. All workpieces must be cooled below 20 degrees Celsius
  3. Temperature has no effect on steel dimensions
  4. The rule must be stored at 20 degrees Celsius permanently

Explanation

Steel expands with rising temperature, so dimensions measured at other temperatures differ from the calibrated value. Precision measurement is therefore referred to a standard 20 degrees Celsius so results are comparable. The rule need not be stored at that temperature, and temperature clearly affects dimensions.

Question 16Trade safety, hand tools, measuring instruments and workshop practice

Which sequence correctly describes marking out a hole centre on a mild steel plate using a surface plate and height gauge?

  1. Drill the hole, then mark the centre with a punch
  2. Prick punch, then coat with layout dye, then set height gauge
  3. Scribe the line, then apply layout dye over it, then drill
  4. Coat with layout dye, set the height gauge to the datum height, scribe the line, then prick punch the intersection Correct answer.

Explanation

Layout dye is applied first so scribed lines are visible, then the height gauge is set from the datum and the line is scribed. The intersection is finally prick punched to seat the drill. Marking after drilling or dyeing over a scribed line defeats the purpose.

Question 17Basic electricity, electrical measuring instruments and control devices

In a split air conditioner's indoor unit, which electrical component is used to open and close the contacts supplying the compressor contactor coil?

  1. A PTC thermistor wired in series with the fan motor
  2. A relay or contactor driven by the room thermostat circuit Correct answer.
  3. A bimetal strip mounted on the evaporator coil
  4. A capacitor connected across the compressor winding

Explanation

Control devices such as relays and contactors use a small control current from the thermostat to switch a much larger compressor load current. The thermostat circuit energises the coil, which pulls in the contacts feeding the compressor. This separation of control and power circuits is the basic principle of relay-based switching in RAC equipment.

Question 18Basic electricity, electrical measuring instruments and control devices

A clamp meter is to be used to measure the running current of a hermetic compressor. Which precaution is essential for a valid reading?

  1. Set the meter to the highest resistance range first
  2. Disconnect the compressor earth lead before clamping
  3. Clamp only the single conductor carrying the current under test Correct answer.
  4. Clamp the whole three-core supply cable together

Explanation

A clamp meter works on the magnetic field produced by current in a conductor, so only one conductor must pass through the jaws. If the whole cable is clamped, the fields of the outgoing and return currents cancel and the reading is near zero. The earth lead must never be disconnected as a measuring step.

Question 19Basic electricity, electrical measuring instruments and control devices

In a single-phase capacitor-start induction motor used for a hermetic compressor, what is the function of the run capacitor in a permanent split capacitor arrangement?

  1. It limits the inrush current during defrost cycles
  2. It improves running power factor and torque by keeping the auxiliary winding current displaced in phase Correct answer.
  3. It discharges the start winding after the motor stops
  4. It supplies the initial starting torque only and is cut out by the centrifugal switch

Explanation

In a permanent split capacitor motor the capacitor stays in circuit with the auxiliary winding during running. It maintains a phase displacement between the main and auxiliary winding currents, which improves power factor and running torque. There is no centrifugal switch to remove it after starting.

Question 20Basic electricity, electrical measuring instruments and control devices

Which instrument connection is correct when a multimeter is used to check the continuity of a compressor overload protector removed from the circuit?

  1. Meter set to amps, probes across the two terminals of the protector
  2. Meter set to ohms, probes across the two terminals of the protector Correct answer.
  3. Meter in series with the supply while the compressor runs
  4. Meter set to volts DC, probes across the supply terminals

Explanation

Continuity testing is a resistance measurement made with the component isolated from the supply. The ohmmeter function is selected and the probes are placed across the terminals; a low reading indicates a closed path. Measuring resistance on a live circuit damages the meter and gives false readings.

Question 21Basic electricity, electrical measuring instruments and control devices

Which material is normally used for the heating element of a domestic electric room heater because of its high resistivity and oxidation resistance?

  1. Soft iron
  2. Silver
  3. Copper
  4. Nichrome alloy Correct answer.

Explanation

Heating elements need a material with high resistivity and the ability to withstand high temperature without rapid oxidation. Nichrome alloys are chosen because they form a protective oxide layer and have suitable resistance per unit length. Copper and silver have very low resistivity and are used for conductors, not heating elements.

Question 22Basic electricity, electrical measuring instruments and control devices

In an RAC control panel, what is the purpose of the NO (normally open) contact of a contactor?

  1. To remain closed until the coil is energised
  2. To close only when the coil is energised and open when it is de-energised Correct answer.
  3. To protect the coil against overvoltage
  4. To provide a permanent path for the control supply

Explanation

A normally open contact is open in the de-energised state and closes when the coil is energised. This allows the contactor to switch the load on only when the control circuit commands it. Contacts do not provide overvoltage protection for the coil.

Question 23Basic electricity, electrical measuring instruments and control devices

A technician measures 230 V across the supply terminals of a window air conditioner and 228 V across the compressor terminals while it is running. What does the small difference mainly indicate?

  1. A small voltage drop exists in the wiring and contacts between the two points Correct answer.
  2. The compressor winding is short-circuited
  3. The supply frequency is incorrect
  4. The voltmeter is connected in series with the load

Explanation

When current flows through wiring, terminals and contacts, a small potential difference appears across their resistance. The difference between supply voltage and terminal voltage is the normal voltage drop of the circuit path. A short circuit or gross fault would produce a much larger and more obvious disturbance.

Question 24Basic electricity, electrical measuring instruments and control devices

A single-phase motor draws 4 A at 230 V and has a power factor of 0.8. What is the approximate active power input?

  1. 184 W
  2. 1150 W
  3. 920 W
  4. 736 W Correct answer.

Explanation

Active power equals voltage multiplied by current multiplied by power factor: 230 × 4 × 0.8 = 736 W. The product of voltage and current alone gives apparent power in volt-amperes, not watts. Multiplying by the power factor converts apparent power to the real power consumed.

Question 25Basic electricity, electrical measuring instruments and control devices

A compressor motor winding reads 0 Ω between its terminals when tested with a digital multimeter. Which interpretation is most reasonable before condemning the motor?

  1. The reading may simply be below the resolution of the meter, so a low-range or milliohm test is needed Correct answer.
  2. The meter battery is flat because a good winding always reads several hundred ohms
  3. The winding is definitely shorted and the motor must be replaced
  4. The winding is definitely open circuit

Explanation

Large hermetic compressor windings often have a resistance of only a fraction of an ohm, which many handheld meters display as 0 Ω. Before condemning the winding, subtract the lead resistance or use a low-range instrument. An open winding would read infinite resistance, not zero.

Question 26Basic electricity, electrical measuring instruments and control devices

Which device is used to protect a compressor motor against sustained overload by sensing the current drawn and opening the control circuit?

  1. A room thermostat
  2. A current-operated overload relay with a bimetal element Correct answer.
  3. A solenoid valve coil
  4. A defrost timer

Explanation

An overload relay senses motor current through a heater or bimetal element and trips when the current stays above the setting for too long. The trip opens the control circuit and drops out the contactor. A thermostat responds to temperature, not to motor current.

Question 27Basic electricity, electrical measuring instruments and control devices

A PTC starting device is fitted to a small hermetic compressor. Why does the starting winding current fall after a few seconds of operation?

  1. The PTC element heats up and its resistance rises sharply, reducing current in the start winding Correct answer.
  2. The start winding is shorted out by the main winding
  3. The centrifugal switch opens because the motor has reached speed
  4. The run capacitor charges fully and blocks the start winding

Explanation

A PTC thermistor has a low cold resistance that rises steeply as it self-heats from the current passing through it. The increased resistance chokes the start winding current once the motor has accelerated. This replaces a mechanical centrifugal switch in many small compressors.

Question 28Basic electricity, electrical measuring instruments and control devices

In a three-phase compressor circuit, what is the purpose of a phase-failure or phase-sequence relay?

  1. To prevent the motor from running with a missing phase or reversed phase sequence Correct answer.
  2. To correct the power factor of the compressor motor
  3. To step down the control voltage to 24 V
  4. To measure the insulation resistance of the winding

Explanation

A phase-failure relay monitors the three supply phases and trips the control circuit if one phase is lost or if the sequence is reversed. Running a three-phase motor on two phases causes overheating and possible winding damage. Phase reversal would make the compressor rotate in the wrong direction.

Question 29Basic electricity, electrical measuring instruments and control devices

A digital multimeter set to the 10 A DC range is used to measure the current drawn by a 230 V AC compressor. What is the main problem with this practice?

  1. The meter will automatically convert the reading to RMS
  2. The meter will measure resistance instead of current
  3. The meter will read exactly half the true value
  4. The meter's AC current capability and fuse rating may be exceeded, and a series connection to a 230 V load is hazardous Correct answer.

Explanation

A current range must be suitable for the voltage and fault level of the circuit, and many handheld meters have a low breaking capacity on their current ranges. Connecting a meter in series with a mains load also exposes the user to line potential at the terminals. A clamp meter is the safer choice for compressor current.

Question 30Basic electricity, electrical measuring instruments and control devices

A capacitor used with a single-phase compressor motor is checked with an analog multimeter set to ohms. The needle deflects and then slowly returns toward infinity. What does this indicate?

  1. The capacitor is short-circuited
  2. The capacitor is charging from the meter's internal battery and is probably serviceable Correct answer.
  3. The capacitor has lost all its capacitance
  4. The capacitor is leaking electrolyte through the terminals

Explanation

The ohmmeter applies a small DC voltage, so a good capacitor draws charging current that falls as it charges. The needle swings toward zero and then drifts back toward high resistance. A capacitor that stays at zero is shorted, while one that never deflects is open.

Question 31Basic electricity, electrical measuring instruments and control devices

In a refrigeration control circuit, what is the function of a potential relay in a capacitor-start motor?

  1. It measures the supply voltage and protects against undervoltage
  2. It limits the current in the run winding by sensing temperature
  3. It provides a time delay for the defrost cycle
  4. It senses the voltage generated in the start winding and drops out the start capacitor at speed Correct answer.

Explanation

A potential relay coil is connected across the start winding, where the back-EMF rises as the motor accelerates. When the voltage reaches the relay's pickup value, the relay opens and removes the start capacitor from the circuit. This gives a reliable speed-related changeover without a centrifugal switch.

Question 32Basic electricity, electrical measuring instruments and control devices

A fuse in a 230 V control circuit keeps blowing immediately after replacement. What is the correct diagnostic approach?

  1. Ignore the fault because control fuses are not important
  2. Isolate the supply and test the circuit for a short or low-resistance path to earth or neutral Correct answer.
  3. Replace the fuse with a piece of wire to keep the unit running
  4. Fit a fuse with a higher current rating to stop nuisance blowing

Explanation

A fuse that fails immediately indicates a fault current path, such as a shorted coil, damaged insulation or a wiring error. The supply must be isolated and the circuit tested with an ohmmeter before any replacement. Increasing the fuse rating defeats the protection and risks fire.

Question 33Basic electricity, electrical measuring instruments and control devices

A 24 V control transformer in an air-conditioning unit has a primary rated for 230 V. A technician measures 24 V at the secondary with the load disconnected, but the voltage falls to 12 V when the contactor coil is connected. Which explanation fits best?

  1. The transformer is working normally because coils always halve the voltage
  2. The primary voltage has dropped to 115 V
  3. The contactor coil has infinite resistance
  4. The transformer secondary winding or its connections have excessive resistance or a poor joint Correct answer.

Explanation

A transformer that holds its open-circuit voltage but collapses under load has a high-resistance path in the secondary circuit, such as a corroded terminal or a partly open winding. The voltage is lost across that resistance as load current flows. A healthy transformer should hold close to its rated secondary voltage at rated burden.

Question 34Basic electricity, electrical measuring instruments and control devices

A three-phase compressor motor is protected by a thermal overload relay set to the nameplate full-load current. The relay trips repeatedly although current is within rating. Which cause is most consistent with this symptom?

  1. The compressor refrigerant charge is slightly low
  2. The contactor coil is energised
  3. The supply voltage is exactly nominal
  4. The overload relay ambient temperature compensation is faulty or the relay is mounted in a hot enclosure Correct answer.

Explanation

Thermal overload relays model motor heating and are affected by the ambient temperature around the relay. A faulty or uncompensated relay in a hot panel can trip even when motor current is normal. Checking the relay setting, ambient and mounting conditions is the appropriate first step.

Question 35Basic electricity, electrical measuring instruments and control devices

An insulation resistance test is to be carried out on a compressor motor winding. Which procedure is correct?

  1. Use a voltmeter between the winding and earth with the motor running
  2. Use an ordinary ohmmeter on the 200 Ω range with the supply connected
  3. Use a megger with the motor isolated and all electronic controls disconnected, and read the resistance after the test voltage stabilises Correct answer.
  4. Use a clamp meter around the earth conductor while the motor runs

Explanation

Insulation resistance is measured with a high DC test voltage applied between the winding and earth while the machine is isolated. Sensitive electronic controls must be disconnected to avoid damage from the test voltage. The reading is taken once the value stabilises, since capacitive charging causes it to rise initially.

Question 36Basic electricity, electrical measuring instruments and control devices

A contactor coil rated 230 V AC is found with a burnt coil after being connected to a 230 V DC supply. Which explanation is correct?

  1. The coil is designed for both supplies but the polarity was reversed
  2. The DC supply produces a higher voltage than the AC supply
  3. The contactor contacts welded because of the DC arc
  4. The AC coil has low DC resistance, so the DC current is much higher than the AC current and overheats the coil Correct answer.

Explanation

An AC coil relies on inductive reactance to limit its current, and its DC resistance is comparatively low. When DC is applied, only the resistance limits current, so the coil draws far more current and burns out. AC and DC coils are therefore not interchangeable.

Question 37Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

In a refrigeration cycle, the heat removed from the refrigerated space is absorbed by the refrigerant mainly through which process in the evaporator?

  1. Isenthalpic expansion through the capillary tube
  2. Adiabatic compression of superheated vapour
  3. Sensible heat rejection while the refrigerant cools as a liquid
  4. Latent heat absorption during evaporation at nearly constant pressure Correct answer.

Explanation

Inside the evaporator the refrigerant boils at a low pressure and absorbs mostly latent heat from the surroundings while its temperature stays nearly constant. This latent heat absorption is what produces the cooling effect. Compression and expansion occur elsewhere in the cycle and do not cool the space.

Question 38Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

A pressure gauge on a refrigerant cylinder reads 6 bar. If atmospheric pressure is taken as 1 bar, what is the absolute pressure of the refrigerant?

  1. 5 bar
  2. 7 bar Correct answer.
  3. 6 bar
  4. 0.6 bar

Explanation

Gauge pressure is measured above atmospheric pressure, so absolute pressure equals gauge pressure plus atmospheric pressure. Adding 6 bar and 1 bar gives 7 bar absolute. Refrigerant property tables and saturation charts are always based on absolute pressure.

Question 39Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

Which temperature scale has its zero point at absolute zero, where all molecular motion theoretically ceases?

  1. Fahrenheit
  2. Celsius
  3. Rankine
  4. Kelvin Correct answer.

Explanation

The Kelvin scale starts at absolute zero, so 0 K is the lowest possible temperature and no negative Kelvin values exist. Celsius and Fahrenheit both place their zero at arbitrary reference points. Gas-law calculations therefore use Kelvin.

Question 40Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

What is the standard instrument used to measure the relative humidity of air in an air-conditioning duct or room?

  1. Manometer
  2. Barometer
  3. Anemometer
  4. Hygrometer Correct answer.

Explanation

A hygrometer measures moisture content or relative humidity of air, using elements such as wet-and-dry bulb thermometers or a capacitive sensor. Barometers measure atmospheric pressure, anemometers measure air velocity, and manometers measure pressure difference.

Question 41Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

Heat transfer through the metal wall of a condenser tube, from the hot refrigerant to the cooling water, occurs mainly by which mode?

  1. Conduction through the metal, then convection into the water Correct answer.
  2. Radiation from the tube surface to the water
  3. Evaporation of the cooling water inside the tube
  4. Convection through the metal wall

Explanation

Heat passes through the solid tube wall by conduction and is then carried away from the inner surface by convection into the moving water. In heat exchangers this combined conduction-convection path is the normal mechanism. Radiation is negligible at these temperatures.

Question 42Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

In a vapour-compression system, which component raises the pressure and temperature of the refrigerant vapour before it enters the condenser?

  1. Compressor Correct answer.
  2. Expansion valve
  3. Receiver
  4. Evaporator

Explanation

The compressor draws low-pressure vapour from the evaporator and compresses it, raising both pressure and temperature so that it can be condensed by ambient air or water. The expansion valve lowers pressure, the evaporator absorbs heat, and the receiver stores liquid.

Question 43Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

Air at 30 °C dry-bulb and 20 °C wet-bulb is being conditioned. What does the wet-bulb reading indicate about the air?

  1. The saturation temperature of the refrigerant
  2. The dry-bulb temperature after cooling coil
  3. The dew point of the air exactly
  4. The temperature reached by evaporative cooling, reflecting moisture content Correct answer.

Explanation

The wet-bulb temperature is the temperature a wetted thermometer reaches when water evaporates into the air stream; the drier the air, the greater the evaporative cooling and the lower the wet-bulb reading. The gap between dry-bulb and wet-bulb therefore indicates how much moisture the air can still absorb.

Question 44Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

A room contains air at 25 °C dry-bulb with a dew point of 15 °C. At approximately what temperature will moisture begin to condense on a cooling coil surface?

  1. 15 °C Correct answer.
  2. 25 °C
  3. 10 °C
  4. 20 °C

Explanation

Condensation begins when a surface is cooled to the dew point temperature of the air, because at that temperature the air reaches saturation. Here the dew point is 15 °C, so moisture starts to deposit on any surface at or below that value. Cooling below it increases the condensation rate.

Question 45Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

According to Dalton's law as applied to moist air, the total pressure of the mixture equals which of the following?

  1. The product of the partial pressures of dry air and water vapour
  2. The partial pressure of dry air alone
  3. The sum of the partial pressures of dry air and water vapour Correct answer.
  4. The difference between the vapour pressure and the saturation pressure

Explanation

Dalton's law states that in a mixture of non-reacting gases the total pressure is the sum of the partial pressures each gas would exert alone. For moist air this means barometric pressure equals dry-air partial pressure plus water-vapour partial pressure. This relationship underpins psychrometric calculations.

Question 46Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

Sensible heat added to a refrigerant or to air produces which observable change?

  1. A reduction in total pressure only
  2. A change in temperature without a change of state Correct answer.
  3. An increase in moisture content only
  4. A change of state at constant temperature

Explanation

Sensible heat changes the temperature of a substance without altering its phase; it can be sensed by a thermometer. Latent heat, by contrast, causes a change of state at constant temperature. This distinction is central to evaporator and condenser analysis.

Question 47Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

A refrigerant enters an expansion device as saturated liquid at 40 °C and leaves at −5 °C. What is the primary purpose of this device in the cycle?

  1. To remove moisture from the refrigerant liquid
  2. To superheat the vapour before compression
  3. To increase refrigerant pressure before the evaporator
  4. To throttle pressure down so the refrigerant enters the evaporator at low temperature Correct answer.

Explanation

The expansion device throttles the refrigerant from condensing pressure to evaporating pressure, which simultaneously drops its saturation temperature so it can absorb heat in the evaporator. The process is essentially isenthalpic, with no work input. It does not raise pressure or superheat the vapour.

Question 48Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

If the absolute pressure of a fixed mass of gas is doubled while its absolute temperature is held constant, what happens to its volume?

  1. It becomes four times larger
  2. It remains unchanged
  3. It doubles
  4. It is halved Correct answer.

Explanation

Boyle's law states that at constant temperature the volume of a fixed mass of gas is inversely proportional to absolute pressure. Doubling the pressure therefore halves the volume. Using gauge pressure instead of absolute pressure here would give a wrong result.

Question 49Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

A sling psychrometer gives a dry-bulb reading of 35 °C and a wet-bulb reading of 35 °C. What does this indicate about the air?

  1. The air is superheated above its dew point
  2. The instrument has failed because readings must differ
  3. The air is fully saturated with water vapour Correct answer.
  4. The air is extremely dry

Explanation

When the wet-bulb reading equals the dry-bulb reading, no evaporative cooling is taking place because the air cannot absorb more moisture. This means the air is at saturation, with relative humidity of 100 percent and dew point equal to the dry-bulb temperature.

Question 50Thermodynamics, heat, temperature, pressure and psychrometry fundamentals

In a refrigeration system the condenser rejects heat to the surroundings. Which statement correctly describes the heat rejected relative to the heat absorbed in the evaporator?

  1. It is always smaller than the evaporator heat
  2. It equals the evaporator heat minus the work of compression
  3. It equals the evaporator heat plus the work of compression Correct answer.
  4. It is independent of the compressor work

Explanation

By the first law of thermodynamics applied to the cycle, the heat rejected at the condenser equals the heat absorbed at the evaporator plus the compressor work input. This is why the condenser must be larger than the evaporator for the same duty. The rejected heat is therefore always greater than the refrigerating effect.