Question 1

Absorption refrigeration: identify the refrigerant pairs  For aqua–ammonia and lithium-bromide–water absorption refrigeration systems, identify the refrigerants respectively. Choose the correct pair in the stated order.

Accepted Answer

Given systems Aqua–ammonia (NH₃–H₂O) Lithium bromide–water (LiBr–H₂O) Concept/Approach In aqua–ammonia systems, ammonia is the refrigerant and water is the absorbent. In LiBr–water systems, water is the refrigerant and lithium bromide is the absorbent. Final Answer Ammonia and water

Question 2

Thermal radiation law: fill in Stefan–Boltzmann law wording  Complete the statement: The total radiation from a black body per second per unit area is __________ the fourth power of its absolute temperature. Choose the correct phrase.

Accepted Answer

Concept/Approach Stefan–Boltzmann law: E = σT⁴ for a black body, where σ is the Stefan–Boltzmann constant. Interpretation The phrasing is that emissive power is proportional to the fourth power of absolute temperature (with σ as the constant of proportionality). Final Answer proportional to

Question 3

Dalton’s law of partial pressures: write the correct relation Let p_b be barometric pressure, p_a the partial pressure of dry air, and p_v the partial pressure of water vapour in a gas mixture at the same location. Cho…

Accepted Answer

Concept/Approach Dalton’s law for ideal gas mixtures: the total pressure equals the sum of the partial pressures of the components. Therefore pb = pa + pv. Common pitfall Subtracting or multiplying partial pressures has no basis in Dalton’s law. Final Answer pb = pa + pv

Question 4

Vapor-compression cycle component functions: where is heat rejected?  During the standard refrigeration cycle, identify the component in which the refrigerant rejects heat to the surroundings. Choose the correct component.

Accepted Answer

Cycle roles Evaporator: absorbs heat (QL) from the refrigerated space at low T and p. Compressor: adds work; raises pressure and saturation temperature. Condenser: rejects heat (QH) to ambient; vapor condenses to liquid. Expansion valve: throttles to low p, producing a low-temperature mixture. Final Answer Condenser

Question 5

Critical radius of insulation: effect on thermal resistance/heat flow  For cylindrical/spherical insulation with external convection, at the critical insulation radius the thermal resistance to heat flow is at what condition? Choose the correct cond…

Accepted Answer

Concept/Approach For a cylinder with external convection, the total resistance Rtot = ln(ro/ri)/(2πkL) + 1/(2πhroL). As outer radius ro increases from ri, conduction resistance increases but convection resistance decreases. Critical radius At ro = rc = k/h, dRtot/dro = 0 and Rtot is minimum; hence heat loss is maximum. Final Answer Minimum

Question 6

Free convection transition: identify the governing non-dimensional criterion  For natural convection over surfaces, transition from laminar to turbulent flow is governed by a critical value of which parameter(s)? Choose the most appropriate option.

Accepted Answer

Concept/Approach In free convection, buoyancy and viscous forces dominate. The transition criterion is commonly expressed in terms of the Rayleigh number, Ra = Gr × Pr. Implication Thus both Grashof number (buoyancy vs viscous) and Prandtl number (momentum vs thermal diffusivity) are involved. For many vertical surfaces, transition is observed around Ra ≈ 10⁹. Why others are incorrect Reynolds number governs forced convection, not natural convection. Grashof alone is incomplete; fluid thermal properties (Pr) also matter. Final Answer Prandtl number and Grashof number (i.e., Rayleigh number)

Question 7

Humidity ratio (specific humidity): choose the correct basis  Define humidity ratio/specific humidity as the mass of water vapour present in which reference amount? Choose the correct basis.

Accepted Answer

Definition Specific humidity (also called humidity ratio) ω is the mass of water vapour per unit mass of dry air: ω = mv / ma (kg/kg dry air). Why other options are incorrect Options A and B refer to absolute humidity (per unit volume). Option C uses total wet-air mass; standard ω uses mass of dry air as the reference. Final Answer 1 kg of dry air

Question 8

Concept check: dominant heat-transfer mode in solids  Evaluate the statement: “In the case of solids, heat transfer takes place according to radiation.” Decide whether the statement is correct or incorrect and choose the best justification.

Accepted Answer

Concept In solids, adjacent molecules/atoms transfer energy primarily by lattice interaction and electron transport — this is conduction. Radiation exists at all temperatures but is usually a secondary path compared with conduction inside a solid. Why the statement is incorrect Saying heat transfer in solids occurs “according to radiation” ignores the dominant conduction mechanism. Only at very high temperatures or across gaps does radiation become comparable or dominant. Common pitfall Confusing surface heat loss by radiation with internal heat transfer within the solid. Final Answer Disagree — conduction is the principal mode in solids.

Question 9

Definition identification: what correctly represents a heat pump?  Select the description/device that best characterizes a heat pump in thermodynamics. Choose the correct option.

Accepted Answer

Given/Concept A heat pump is a device that moves heat from a colder body to a hotter body by consuming external work. A refrigerator is a heat pump when its useful effect is heating the sink instead of cooling the source. Option analysis (A) Correct — a refrigerator viewed from the heating side is a heat pump.(B) Correct — it is a reversed heat engine.(C) Correct — this is the generic definition. Final Answer All of the above.

Question 10

Refrigeration cycle state point: condition of refrigerant after the compressor and before the condenser  Evaluate the statement about a vapor-compression system: “After leaving the compressor and before entering the condenser, the refrigerant is in …

Accepted Answer

Cycle reasoning Compressor raises pressure and temperature of the incoming vapor. The exit state is typically a superheated vapor at condenser pressure. The condenser then desuperheats, condenses, and (optionally) subcools the refrigerant. Final Answer Agree — the refrigerant is superheated vapor between compressor and condenser.

Question 11

Thermodynamic principle: which law dictates the direction of heat transfer?  Identify the correct fundamental law governing heat flow from high temperature to low temperature. Choose the correct option.

Accepted Answer

Concept The second law sets the direction of spontaneous processes: heat flows spontaneously from higher to lower temperature. Why others are wrong Zeroth law: defines thermal equilibrium/temperature.First law: energy conservation; no directionality.Kirchhoff's law: electrical circuits, not thermodynamic direction of heat flow. Final Answer Second law of thermodynamics.

Question 12

Radiation fundamentals: parameters controlling emitted thermal radiation  Which factors mainly affect the amount of thermal radiation emitted by a surface/body? Choose the correct option.

Accepted Answer

Concept Radiative emission depends strongly on temperature (∝ T⁴ for blackbodies) and on material/surface properties through emissivity (nature and surface finish). Final Answer All of these.

Question 13

Typical discharge temperature of ammonia (NH₃) in a vapor-compression system  Select the most representative temperature range for ammonia after compression and before entering the condenser. Choose the correct option.

Accepted Answer

Cycle insight Compressor discharge temperatures for NH₃ are typically high due to significant superheat at elevated pressure. Typical values Representative discharge: ~70–110°C depending on suction superheat, pressure ratio, and compressor type. Final Answer 70 to 110° C.

Question 14

Heat pump performance: COP magnitude relative to unity  Complete the statement about a heating-mode device: “The coefficient of performance (COP) of a heat pump is always ___ one (for normal operation).” Choose the correct completion.

Accepted Answer

Concept COPHP = QH/W = 1 + QL/W = 1 + COPRef > 1 for any functioning heat pump, since some of the delivered heat is the absorbed heat plus the input work. Final Answer greater than.

Question 15

Psychrometrics definition: degree of saturation / percentage humidity  Choose the most accurate definition among the options given. Select the correct statement.

Accepted Answer

Concept “Percentage humidity” (often used interchangeably with degree of saturation in basic MCQ contexts) compares actual vapor content to saturated vapor content at the same T and p, on a per volume basis. Option analysis (A) Absolute humidity (vapor density in a given volume of dry air).(B) Humidity ratio/specific humidity (mass basis per kg dry air).(C) Often used to describe relative humidity in mass-per-mass terms, but the classical “percentage humidity” definition aligns with (D). Final Answer The ratio based on equal volumes at the same T and p — option (D).

Question 16

Effect of liquid-line subcooling on refrigeration performance (vapor-compression cycle)  How does subcooling of the condenser outlet liquid affect the cycle's coefficient of performance (C.O.P.)? Choose the correct option.

Accepted Answer

Approach Subcooling increases the refrigerating effect (more liquid enthalpy drop through the evaporator) without increasing compressor work significantly. Result Net effect is a higher refrigeration effect per unit work, so the C.O.P. increases.

Question 17

Fourier's law of heat conduction: correct mathematical form  Given Q (heat rate), A (area normal to heat flow), dT (temperature difference), dx (thickness along heat flow), and k (thermal conductivity). Select the correct expression for one-dimensio…

Accepted Answer

Concept Fourier's law (1-D): heat flows from hot to cold in the direction of decreasing temperature; the negative sign enforces this directionality. Step-by-step Heat flux qx = -k (dT/dx).Multiply by area A to get heat rate: Q = qx A = - k A (dT/dx). Common pitfall Omitting the minus sign gives non-physical positive heat flow with positive temperature gradient.

Question 18

Heat transfer mode definition: identify radiation  The process of heat transfer from a hotter body to a colder body that travels in straight lines and does not require or disturb the intervening medium is called what? Choose the correct truth value …

Accepted Answer

Concept/Approach Heat can be transferred by conduction (requires material contact), convection (requires a fluid), and radiation (electromagnetic waves that can travel through vacuum). Why the statement is correct Radiative transfer proceeds in straight-line paths from a hot surface to a cooler one and does not require an intervening medium; it can occur through vacuum. Any intervening medium may absorb/reflect/emissively participate, but radiation itself does not require the medium. Common pitfalls Confusing radiation with convection; convection needs a moving fluid. Assuming radiation needs air; it does not. Final Answer True

Question 19

Cooling coil performance: effect of air velocity on by-pass factor  As the face velocity of air through a cooling coil increases, how does the coil's by-pass factor typically change? Choose the correct trend.

Accepted Answer

Given/Definitions By-pass factor (BPF) quantifies the fraction of air that effectively "bypasses" ideal contact with the coil surface (i.e., leaves above apparatus dew-point). Concept/Approach Higher air velocity reduces contact time and reduces heat/mass transfer effectiveness, so a larger fraction of air leaves closer to the inlet state → higher BPF. Therefore BPF increases as air velocity increases.

Question 20

Refrigerator coil frosting: effect on performance and energy use  What is the impact of frost formation on cooling coils inside a refrigerator on heat transfer and power consumption? Choose the correct effect.

Accepted Answer

Concept/Approach Frost acts as an insulating layer (low thermal conductivity), increasing the overall thermal resistance between the coil and air. Implications For the same cooling load, the compressor must run longer (or at higher duty), raising energy consumption; COP effectively degrades. Final Answer It increases power consumption.

Heat Transfer, Refrigeration and Air Conditioning Questions