Question 1

Thermal radiation – Identify the law  “The ratio of emissive power to absorptive power is the same for all bodies at a given temperature and equals the emissive power of a perfect black body.” This statement refers to which law?

Accepted Answer

Introduction / Context: Radiative heat transfer relies on several cornerstone laws. Correctly identifying them is essential for solving enclosure radiation, surface properties, and spectral distribution problems. Given Data / Assumptions: Thermal radiation at thermodynamic equilibrium (same temperature and surroundings). Emissive and absorptive powers are per unit area for a given wavelength band or total, as context requires. Concept / Approach: Kirchhoff’s law states that, at a given temperature and wavelength under equilibrium, a body’s emissivity equals its absorptivity. In its total form, the ratio of total emissive power to absorptive power is the same for all bodies and equals the blackbody emissive power. Step-by-Step Solution: Recognize the equality between emission and absorption characteristics at equilibrium.Map the described ratio to the classical statement of Kirchhoff’s law.Conclude the named law is Kirchhoff’s law. Verification / Alternative check: Other laws: Stefan–Boltzmann relates total emissive power of a blackbody to T^4; Wien gives spectral peak–temperature relation; Planck gives full spectral distribution; Lambert addresses angular dependence. None describe the absorptivity–emissivity equivalence. Why Other Options Are Wrong: Stefan’s, Wien’s, Planck’s, and Lambert’s laws address different aspects (magnitude, spectrum, or directionality) and do not equate emission to absorption. Common Pitfalls: Confusing “emissive power” (W/m^2) with “emissivity” (dimensionless), and overlooking that Kirchhoff’s law is rigorously valid under thermal equilibrium. Final Answer: Kirchhoff's law

Question 2

Refrigerant leak detection – Freon (halocarbon) systems  Which method is traditionally used to detect leaks in systems using Freon-type (halocarbon) refrigerants?

Accepted Answer

Introduction / Context: Leak detection is essential for safety, performance, and environmental compliance in refrigeration systems. Different refrigerants require different detection methods because their chemical interactions with test media vary. Given Data / Assumptions: System uses a halocarbon (e.g., R-12, R-22, R-134a family historically termed “Freon”). Traditional field methods considered (modern electronic detectors also exist). Concept / Approach: The classic method for halocarbon leaks is the halide torch. The torch flame changes color in the presence of halogenated refrigerant vapors drawn across a heated copper element, signaling a leak. Step-by-Step Solution: Identify refrigerant family: halocarbon (Freon-type).Recall detection methods: halide torch is specific to halogens and historically standard.Select the method best matched to Freon: halide torch. Verification / Alternative check: Soap-bubble tests can reveal any gas leak by bubble formation but provide no refrigerant specificity and are less sensitive for tiny leaks. Sulphur sticks react with ammonia, not with halocarbons; therefore they are unsuitable for Freon leaks. Why Other Options Are Wrong: Sulphur sticks: used for ammonia (white fumes), not for halocarbons. Soap-and-water only: generic but not the traditional or most sensitive method for Freon. “All of these”: incorrect because sulphur sticks do not indicate halocarbon leaks. Acetylene flame color test: not a recognized safe practice for refrigerant detection. Common Pitfalls: Using sulphur sticks on halocarbon plants; ignoring modern electronic leak detectors which are now preferred for sensitivity and safety. Final Answer: Halide torch

Question 3

Air-cooled condenser design – Typical temperature difference  Most air-cooled condensers are designed to operate with approximately what condensing-to-air temperature difference (approach)?

Accepted Answer

Introduction / Context: The temperature difference between condensing refrigerant and leaving ambient air (often called the “TD” or approach) is a key sizing parameter for air-cooled condensers. It affects coil surface area, fan power, and system efficiency. Given Data / Assumptions: Typical comfort-cooling or light industrial HVAC conditions. Reasonable ambient design temperatures and airflow rates. Standard finned-tube air-cooled condensers. Concept / Approach: Design practice often targets a temperature difference around 14 °C (about 25 °F). This balances coil cost and size against fan energy and compressor discharge pressure. Much lower TD requires oversized coils and fans; much higher TD raises condensing temperatures, hurting COP. Step-by-Step Solution: Define TD ≈ T_condensing − T_ambient (leaving air or outdoor ambient as specified).Adopt common design guidance: TD ≈ 14 °C for many packaged and split systems.Select the closest nominal value among choices: 14 °C. Verification / Alternative check: Manufacturers’ rating catalogs show nominal ARI/ASHRAE conditions that imply condenser approaches in the 10–15 °C range for many standard units, with 14 °C a widely cited rule-of-thumb. Why Other Options Are Wrong: 5 °C or 8 °C imply very large coils/fans and higher first costs. 22 °C significantly elevates condensing temperature, reducing efficiency and compressor life under design conditions. 2 °C is impractically low for typical air-cooled designs. Common Pitfalls: Confusing “approach” with subcooling; comparing water-cooled versus air-cooled figures (water-cooled typically use much smaller approaches). Final Answer: 14°C

Question 4

Definition check – What is a heat exchanger?  Confirm whether the statement “A device used for transferring heat from one fluid to another” correctly defines a heat exchanger.

Accepted Answer

Introduction / Context: Heat exchangers are ubiquitous in HVAC, power, chemical processing, and refrigeration. A clear definition underpins proper selection among shell-and-tube, plate-and-frame, finned-tube, condensers, evaporators, and regenerative devices. Given Data / Assumptions: Two or more fluids at different temperatures exchange heat. Heat transfer may be with or without phase change (sensible or latent). Fluids can be kept separate (indirect contact) or mixed (direct contact towers). Concept / Approach: A heat exchanger is any device facilitating heat transfer between fluids separated by a wall (indirect contact) or in direct contact without significant external work. The definition does not mandate a particular flow arrangement or phase change. Step-by-Step Solution: Match the statement to standard definitions.Recognize inclusivity: condensers, evaporators, economizers, oil coolers, air heaters, and cooling towers fit.Therefore the statement is correct. Verification / Alternative check: Texts define heat exchangers broadly; specialized names (condenser, boiler) are specific applications but still heat exchangers by function. Why Other Options Are Wrong: Limiting to direct contact, requiring phase change, or restricting to counter-flow excludes many legitimate devices. Common Pitfalls: Equating “heat exchanger” only with shell-and-tube; forgetting that regenerative and recuperative devices are included; mixing “heater” and “exchanger” terminology. Final Answer: True

Question 5

Psychrometrics – Relative humidity trend  How does relative humidity change as air becomes drier (i.e., moisture content decreases at roughly constant temperature)?

Accepted Answer

Introduction / Context: Relative humidity (RH) is a key comfort and process variable defined as the ratio of the partial pressure of water vapor in air to the saturation vapor pressure at the same temperature. Understanding its qualitative behavior is crucial for HVAC control and drying operations. Given Data / Assumptions: Air temperature roughly constant during the comparison. “Drier” means a reduction in moisture content (humidity ratio). Concept / Approach: At fixed temperature, saturation pressure is fixed. Reducing actual water-vapor partial pressure lowers RH because RH = p_v / p_vs. Thus, as air loses moisture while temperature remains about the same, RH declines. Step-by-Step Solution: Hold temperature T approximately constant ⇒ p_vs(T) is constant.Decrease moisture content ⇒ p_v drops.Compute RH = p_v / p_vs ⇒ RH decreases. Verification / Alternative check: On a psychrometric chart at constant DBT, moving left (lower humidity ratio) also lowers RH curves, confirming the trend. Why Other Options Are Wrong: Increases/constant: contradict the definition at constant temperature. “First increases then decreases”: no such behavior absent temperature change. “Cannot be inferred”: it can be, given the stated dryness. Common Pitfalls: Confusing RH with absolute humidity; ignoring the effect of temperature changes—warming air without adding moisture reduces RH, while cooling raises RH even if absolute moisture is unchanged. Final Answer: decreases

Question 6

Dimensionless numbers – Identify Re·Pr  The product of the Reynolds number and the Prandtl number is known as which dimensionless group?

Accepted Answer

Introduction / Context: Dimensionless numbers condense complex transport phenomena into ratios that govern similarity and scaling. Recognizing relationships among Reynolds (Re), Prandtl (Pr), Stanton (St), Peclet (Pe), and others is foundational for convective heat transfer analysis. Given Data / Assumptions: Standard definitions: Re = inertia/viscous, Pr = momentum diffusivity/thermal diffusivity. Single-phase convection in ducts or external flows. Concept / Approach: Peclet number measures the relative importance of advection to thermal diffusion: Pe = Re * Pr. It features prominently in convection–diffusion equations and in assessing axial conduction effects in heat exchangers. Step-by-Step Solution: Write Re = ρ * V * L / μ and Pr = μ * c_p / k.Multiply: Re * Pr = (ρ * V * L / μ) * (μ * c_p / k) = ρ * V * L * c_p / k.Recognize Pe = ρ * c_p * V * L / k = Re * Pr. Verification / Alternative check: From convection correlations: Nu = f(Re, Pr). Also, Stanton number satisfies St = Nu / (Re * Pr), highlighting Pe in the denominator if rearranged. Why Other Options Are Wrong: Stanton number is Nu/(RePr), not RePr itself. Biot number compares internal to external resistance (h * L / k_s), unrelated to RePr. Grashof pertains to buoyancy in natural convection. Nusselt is a heat transfer coefficient group hL/k, not a product of Re and Pr. Common Pitfalls: Interchanging St and Pe; forgetting that Pr links momentum and thermal diffusivities, so Pe collapses convective–diffusive transport into one group. Final Answer: Peclet number

Question 7

In refrigeration engineering, what is the normal atmospheric-pressure boiling point of ammonia (NH3) used as a refrigerant?

Accepted Answer

Introduction / Context: Ammonia (NH3) is a classic refrigerant used in industrial refrigeration because of its high latent heat and excellent thermodynamic performance. Knowing its saturation (boiling) temperature at standard atmospheric pressure is a foundational property needed for cycle calculations, selection of evaporating pressures, and safety considerations. Given Data / Assumptions: Pressure is approximately 1 atm (101.325 kPa). Boiling point refers to saturation temperature where liquid and vapour coexist. Pure anhydrous ammonia is considered (no mixture effects). Concept / Approach: The boiling point at a specified pressure is the saturation temperature at which the liquid begins to change phase to vapour. For refrigerants, property tables or p–T charts provide this value. Engineers use these data to pick evaporator temperatures and to estimate compressor suction conditions. Step-by-Step Solution: Look up NH3 saturation properties at 1 atm → saturation temperature is about -33.3°C.Interpretation: At -33.3°C and 1 atm, ammonia begins to boil; below this, it remains liquid (if present) and above, it becomes vapour (assuming open system at 1 atm).Application: If an evaporator is at -20°C, the corresponding saturation pressure must be above 1 atm, found from tables; this affects compressor suction pressure. Verification / Alternative check: Cross-check multiple standard references (refrigerant tables or ASHRAE data) which consistently list ~ -33°C at 1 atm for ammonia. Why Other Options Are Wrong: -10.5°C and -30°C are too warm; ammonia would still be saturated vapour at 1 atm around these temperatures. -77.7°C is closer to some cryogenic substances, not ammonia. 0°C is the freezing/boiling reference for water, not NH3. Common Pitfalls: Confusing boiling point at 1 atm with saturation temperature at the system’s operating pressure; in closed cycles the boiling point shifts with pressure. Final Answer: -33.3°C

Question 8

Bell–Coleman (air) refrigeration: For the same temperature range, does a dense (closed) air system achieve a higher coefficient of performance than an open air system?

Accepted Answer

Introduction / Context: The Bell–Coleman or reversed Brayton (air) cycle can be implemented as an open system (ship and aircraft cooling) or as a dense/closed system. Understanding the performance difference helps in selecting configurations for refrigeration and air-conditioning applications where air is the working fluid. Given Data / Assumptions: Same evaporator and condenser (hot/cold) temperature limits for comparison. Similar component efficiencies. Closed (dense) system avoids mixing with ambient air. Concept / Approach: In a dense (closed) system, the working air circulates at elevated mean pressures, allowing better heat transfer in compact heat exchangers and reducing throttling/mixing losses associated with open cycles. The closed system also allows more effective recuperation and control of superheat/subcooling, generally improving COP = refrigeration effect / net work input. Step-by-Step Solution: 1) Open system incurs irreversibilities by ingesting and rejecting to ambient each pass.2) Dense system maintains higher density → smaller specific volume → reduced compressor size for same capacity.3) Better recuperation and approach temperatures increase refrigeration effect per unit work.4) Hence, for equal temperature limits, the dense system attains higher COP. Verification / Alternative check: Simplified Brayton cycle analysis on T–s or p–h coordinates shows lower specific work and improved heat-exchanger effectiveness in the closed arrangement. Why Other Options Are Wrong: Disagree: ignores controllability and reduced losses in closed systems. “Only at very high pressure ratios” or “only with intercooling and reheating” are too restrictive; benefits exist across typical ranges. “Indeterminate…”: while exact COP needs data, the qualitative comparison is well-established. Common Pitfalls: Assuming identical COPs because the ideal Brayton diagrams look similar; practical losses differ significantly between open and closed arrangements. Final Answer: Agree

Question 9

During humidification of air by steam injection at constant pressure (typical HVAC process), how does the dry-bulb temperature (DBT) of the air change?

Accepted Answer

Introduction / Context: Humidification processes in air-conditioning can be achieved by steam injection or by adiabatic water spray (evaporative cooling). The direction of change of dry-bulb temperature (DBT) depends on the method. This question targets steam humidification at essentially constant pressure, widely used for precise humidity control. Given Data / Assumptions: Process: direct steam injection into the airstream. Pressure is approximately constant (duct/room pressure). No significant heat loss to surroundings during injection. Concept / Approach: When saturated steam is injected, its latent and sensible energy are transferred to the air–vapour mixture. Both humidity ratio and mixture enthalpy rise. On the psychrometric chart, steam injection moves the state up and to the right, increasing moisture content and DBT simultaneously (unless specifically arranged for adiabatic saturation with water only). Step-by-Step Solution: Start: air at DBT1, humidity ratio w1.Inject steam: add mass of water vapour Δm_v and energy Δh to the air.Result: new state (DBT2, w2) with w2 > w1 and DBT2 > DBT1 due to added sensible heat from steam.Conclusion: DBT increases during steam humidification. Verification / Alternative check: Compare to evaporative (adiabatic) humidification where adding liquid water causes DBT to drop toward wet-bulb; the opposite trend confirms that steam injection increases DBT. Why Other Options Are Wrong: Remains constant: true only for specific adiabatic processes with water spray, not steam injection. Decreases: that describes evaporative cooling, not steam humidification. Equality to wet-bulb/dew-point: not a general outcome of steam injection. Common Pitfalls: Mixing up steam humidification with evaporative cooling; always identify the humidification method before predicting DBT changes. Final Answer: Increases

Question 10

In a standard vapour-compression refrigeration cycle, what is the thermodynamic state of the refrigerant immediately after passing through the condenser (i.e., at the condenser outlet before the expansion device)?

Accepted Answer

Introduction / Context: Recognizing refrigerant states at key points in the vapour-compression cycle is vital for sizing heat exchangers and protecting the compressor. After compression, the hot superheated vapour enters the condenser, where it is first desuperheated and then condensed to liquid before throttling. Given Data / Assumptions: Ideal cycle description without deliberate subcooling. Condenser rejects heat to ambient until condensation is complete. Pressure is approximately constant across the condenser. Concept / Approach: Within the condenser, three regions may exist: desuperheating, condensation (phase change at essentially constant temperature for a pure refrigerant), and sometimes subcooling. The minimum requirement before the expansion device is liquid at saturation; subcooling is beneficial but not mandatory in the idealized description. Step-by-Step Solution: 1) Compressor discharge is superheated vapour at high pressure.2) Desuperheating region lowers temperature to the saturation temperature at condenser pressure.3) Phase change region removes latent heat → saturated liquid at outlet (ideal case).4) Optional subcooling further lowers liquid temperature below saturation. Verification / Alternative check: On a p–h diagram, the condenser outlet lies on the saturated liquid line (or slightly to the left if subcooled). The expansion valve requires liquid to avoid flash-gas before the evaporator inlet. Why Other Options Are Wrong: Wet vapour and dry saturated vapour describe states before full condensation. Superheated vapour is the condenser inlet, not outlet. Subcooled liquid may occur by design, but “always subcooled” is not universally true. Common Pitfalls: Confusing the condenser outlet with the compressor discharge, or assuming subcooling without checking system design. Final Answer: Saturated liquid

Question 11

Fourier’s law of heat conduction: For which class of problems does the basic one-dimensional formulation q = -k * A * dT/dx directly give the heat flow with area A and gradient dT/dx?

Accepted Answer

Introduction / Context: Fourier’s law is the foundation of conduction analysis. While the vector law applies generally, the familiar textbook expression q = -k * A * dT/dx (with a single spatial gradient) assumes one-dimensional heat flow and constant properties along the direction considered. Given Data / Assumptions: Steady or differential 1-D conduction along x. Heat flux normal to area A. Thermal conductivity k treated as uniform or known function of T. Concept / Approach: In 1-D conduction, temperature varies along only one coordinate. The heat rate is proportional to area, thermal conductivity, and temperature gradient. For 2-D/3-D fields, the full vector form q⃗ = -k ∇T and governing PDEs (Laplace/Poisson) must be solved; direct use of the simple product form with a single gradient is insufficient. Step-by-Step Explanation: 1) Assume 1-D slab: T = T(x); then q = -k * A * dT/dx.2) Integrate over thickness L with boundary temperatures T1 and T2 to get Q = k * A * (T1 - T2) / L (for constant k).3) For long cylinders/spheres with small radial gradients across thickness, approximate 1-D forms are used with appropriate areas. Verification / Alternative check: When lateral heat spreading or fins cause 2-D fields, resort to the general form and energy equations; the simple 1-D expression becomes an approximation. Why Other Options Are Wrong: (a) “Irregular surfaces only” is unrelated to dimensionality. (b) Non-uniform surface temperatures often lead to multi-D fields. (d) Two-dimensional problems require solving PDEs; the 1-D formula is not directly applicable. (e) Radiation is governed by Stefan–Boltzmann type laws, not Fourier’s law. Common Pitfalls: Applying 1-D conduction formulas to fins or heat-spreading plates without verifying dimensionality. Final Answer: One-dimensional conduction cases (through plane walls, long cylinders approximated 1-D)

Question 12

In a shell-and-tube heat exchanger, what is the principal purpose of providing baffles on the shell side?

Accepted Answer

Introduction / Context: Shell-and-tube exchangers are the workhorses of process and HVAC industries. Baffles are structural and hydraulic elements on the shell side that influence heat-transfer coefficients, pressure drop, and mechanical integrity of the tube bundle. Given Data / Assumptions: Single-phase shell-side flow. Segmental or double-segmental baffle arrangement. Normal design objectives: high U with acceptable pressure drop and vibration control. Concept / Approach: Baffles direct the shell-side fluid across the tube bundle in a zig-zag path, increasing cross-flow velocity and turbulence, which elevates the shell-side heat-transfer coefficient. They also physically support tubes, reducing vibration risks, and minimize stagnant zones and by-pass streams that would otherwise reduce effectiveness. Step-by-Step Reasoning: 1) Flow guidance: baffles force cross-flow, enhancing h_shell.2) Support: baffle spacing limits unsupported tube spans to resist flow-induced vibration.3) Stagnation control: proper cut and spacing reduce dead zones and by-pass, improving temperature cross. Verification / Alternative check: Design correlations (e.g., Bell–Delaware method) explicitly include baffle spacing and leakage effects in predicting shell-side h and pressure drop. Why Other Options Are Wrong: Options (a), (b), and (c) each state true but partial roles; hence the most complete answer is “All of these”. (e) Baffles increase, not minimize, pressure drop; designers balance heat transfer and pumping power. Common Pitfalls: Over-tight baffle spacing: while it raises heat transfer, it may cause excessive pressure drop and vibration; design must optimize. Final Answer: All of these

Question 13

Psychrometrics check: “Dew-point temperature is the temperature at which moisture in the air just begins to condense.” Evaluate the statement.

Accepted Answer

Introduction / Context: Dew-point temperature (DPT) is a key property in air-conditioning, drying, and condensation control. It defines when condensation will occur on a surface and guides coil surface temperature selection in cooling/dehumidification. Given Data / Assumptions: Fixed total pressure (near atmospheric) during consideration. Air–vapour mixture with partial pressure of water vapour p_v. Pure water condensation assumption. Concept / Approach: The dew-point temperature is the saturation temperature corresponding to the partial pressure of water vapour in the mixture. When air is cooled at constant moisture content (no removal yet), reaching DPT means the air becomes saturated and any further cooling causes condensation. Step-by-Step Explanation: 1) Determine p_v from humidity ratio or RH.2) From saturation tables, find T_sat such that p_sat(T_sat) = p_v.3) That temperature is the dew-point; below it, liquid water will form on surfaces. Verification / Alternative check: On a psychrometric chart, move horizontally left (constant humidity ratio) to intersect the saturation curve; the temperature at that point is the dew-point. Why Other Options Are Wrong: “Incorrect” conflicts with the thermodynamic definition. “Only at 100% RH” confuses condition with definition; DPT is defined for any state and equals DBT only at 100% RH. Below 0°C or pressure specifics are not necessary to define the concept, though they influence numeric value. Common Pitfalls: Assuming dew-point equals wet-bulb; they coincide only at saturation. Surface temperatures must be kept above DPT to avoid condensation. Final Answer: Correct

Question 14

Refrigeration performance metric: The heat-rejection factor (HRF) is defined as the ratio of heat rejected in the condenser to the refrigeration effect. Which expression is correct in terms of coefficient of performance (COP)?

Accepted Answer

Introduction / Context: Designing condensers and cooling towers requires an estimate of total heat rejection. The heat-rejection factor (HRF) links the condenser duty to the evaporator load (refrigeration effect), using the coefficient of performance (COP) of the refrigeration machine. Given Data / Assumptions: Refrigeration effect per unit mass = q_L. Work input per unit mass = w. Condenser heat rejected per unit mass = q_H = q_L + w. Concept / Approach: By definition, COP = q_L / w. The heat-rejection factor is HRF = q_H / q_L. Substitute q_H = q_L + w and express w in terms of COP to obtain HRF in a simple algebraic form. Step-by-Step Derivation: COP = q_L / w ⇒ w = q_L / COPq_H = q_L + w = q_L + q_L / COP = q_L * (1 + 1/COP)HRF = q_H / q_L = 1 + 1 / COP Verification / Alternative check: For a COP of 4, HRF = 1 + 0.25 = 1.25. This matches practical rules of thumb that condenser load is about 1.2–1.3 times the cooling load for moderate COPs. Why Other Options Are Wrong: 1 - 1/COP would predict condenser duty smaller than evaporator load, which is physically impossible. 1 + COP or the fractional forms offered do not arise from the energy balance q_H = q_L + w. Common Pitfalls: Confusing HRF with heat-rejection ratio expressed as a percent or confusing COP based on different definitions (cooling or heating mode). Final Answer: HRF = 1 + 1 / COP

Question 15

Critical insulation thickness for a cylindrical cable: An aluminium-conductor cable (in air with h = 6 W/m^2K) is insulated with rubber of thermal conductivity k = 0.15 W/m·K. What is the critical thickness (radius) of insulation?

Accepted Answer

Introduction / Context: For cylindrical systems (wires, pipes), adding insulation does not always reduce heat loss. Up to a certain “critical radius,” adding insulation can increase external surface area faster than thermal resistance, increasing heat transfer. Correctly computing the critical insulation radius is crucial to avoid counter-intuitive performance. Given Data / Assumptions: External convection coefficient, h = 6 W/m^2K (air). Insulation thermal conductivity, k = 0.15 W/m·K (rubber). Cylindrical geometry, external convection to air. Concept / Approach: For a cylinder with external convection, the critical insulation radius r_c is given by r_c = k / h. If insulation thickness is increased to this radius, heat loss is maximized; beyond it, added insulation reduces heat loss as expected. Step-by-Step Solution: Formula: r_c = k / hInsert values: r_c = 0.15 / 6Compute: r_c = 0.025 mConvert to mm: 0.025 m = 25 mm Verification / Alternative check: Dimensional check: k (W/m·K) divided by h (W/m^2·K) → m, consistent with a length scale; typical values for air convection and rubber indeed yield a few centimeters. Why Other Options Are Wrong: 40 mm, 160 mm, 800 mm are not consistent with r_c = k/h for the given numbers. 2.5 mm is an order of magnitude too small (would require h ≈ 60 W/m^2K). Common Pitfalls: Confusing critical thickness for cylinders with that for spheres (r_c = 2k/h) or thinking any insulation always reduces heat loss; for small wires in low h environments, initial insulation can increase heat loss until r > r_c. Final Answer: 25 mm

Question 16

Conduction through a plane wall: How does the total heat flow rate depend on area, temperature difference, and thickness, according to elementary steady 1-D conduction relations?

Accepted Answer

Introduction / Context: Fourier’s law for steady, one-dimensional conduction through a homogeneous plane wall leads to a simple proportionality that is widely used for first-cut thermal sizing of walls, insulations, and heat exchanger plates. Given Data / Assumptions: Plane wall of thickness L and area A. Constant thermal conductivity k. Steady state with boundary temperatures T_hot and T_cold. Concept / Approach: For 1-D conduction, q = -k * A * dT/dx. With linear temperature distribution and constant k, integration yields Q = k * A * (T_hot - T_cold) / L. This shows direct proportionality to area and temperature difference, and inverse proportionality to thickness; it also highlights dependence on material via k. Step-by-Step Solution: Start with q" = -k * dT/dx.Integrate across thickness L with ΔT = T_hot - T_cold.Obtain Q = k * A * ΔT / L.Interpret proportionalities directly from the expression. Verification / Alternative check: Compare two walls of the same material: doubling area doubles Q; doubling thickness halves Q; doubling ΔT doubles Q. Experimental results match these trends in the linear regime. Why Other Options Are Wrong: (a), (b), and (c) each state a true but partial relationship; the complete statement is “All of the above”. (e) is false—material property k is central to conduction. Common Pitfalls: Applying this 1-D formula to multilayer walls without using the appropriate series thermal resistance model; neglecting contact resistances and temperature-dependent k. Final Answer: All of the above

Question 17

Refrigeration safety and liquid carryover At the suction line of a vapour-compression system, an accumulator is installed to collect liquid refrigerant and prevent it from reaching which component?

Accepted Answer

Introduction / Context: A suction line accumulator is a protective device widely used in vapour-compression refrigeration and heat pump systems. Its purpose is to prevent liquid refrigerant from entering the compressor, which would otherwise cause severe mechanical damage (liquid slugging) and dilute the lubricating oil. Given Data / Assumptions: Single-stage vapour-compression system with evaporator, compressor, condenser, and expansion device. Accumulator is located in the suction line (low pressure side) just upstream of the compressor. Objective: identify the component being protected from liquid ingress. Concept / Approach: Compressors are designed to compress vapour, not liquid. Because liquids are nearly incompressible, any significant liquid entrainment reaching compressor cylinders or scroll/rotary elements can lead to broken valves, bent rods, or catastrophic failure. The accumulator acts as a reservoir that traps liquid and meters only vapour (and a controlled oil mist) forward. Step-by-Step Solution: Recognize the accumulator’s position: suction line before compressor.Understand function: separate liquid from vapour and return only vapour to compressor.Therefore, the component protected is the compressor. Verification / Alternative check: Service manuals specify accumulators on systems with risk of floodback (e.g., heat pumps in heating mode, low ambient cooling). The protective device is never placed to shield the condenser or expansion valve from liquid, since those components routinely handle liquid. Why Other Options Are Wrong: Evaporator handles boiling liquid by design; condenser receives superheated vapour and rejects heat to produce liquid; expansion valve meters liquid to the evaporator; receiver stores liquid at high side pressure. None require an upstream accumulator. Common Pitfalls: Confusing receivers (high-side liquid storage) with accumulators (low-side vapour protection). Also assuming sight glass dryness guarantees no liquid floodback. Final Answer: Compressor

Question 18

Psychrometrics — sensible heating During a purely sensible heating process of air (no moisture added or removed), what happens to the dry-bulb temperature?

Accepted Answer

Introduction / Context: Sensible heating is a foundational psychrometric process used in space heating and air-handling units. Understanding how state points move on the chart helps with coil selection and energy calculations. Given Data / Assumptions: No moisture is added or removed (humidity ratio remains constant). Air is treated as an ideal gas–vapour mixture at low to moderate pressures. Process is “sensible” only: enthalpy rise is due to temperature increase of the moist air. Concept / Approach: On a psychrometric chart, constant humidity ratio corresponds to a horizontal line. Sensible heating shifts the state point horizontally to the right, increasing dry-bulb temperature while keeping specific humidity unchanged. Step-by-Step Solution: Identify variables: dry-bulb temperature T_db, humidity ratio w.For sensible heating: Δw = 0 and ΔT_db > 0.Therefore, T_db must increase. Verification / Alternative check: Energy balance for a control volume: Q̇_sensible = m_dot * c_p,ma * (T_out − T_in), with c_p,ma approximately constant, giving a direct proportionality between heat added and increase in T_db. Why Other Options Are Wrong: Constant or decreasing T_db contradicts the definition of sensible heating. The dry-bulb does not automatically equal wet-bulb except at saturation, which is not implied here. Common Pitfalls: Confusing “sensible heating” with “reheating after dehumidification,” where humidity changes first and then T_db is raised; here we specify no moisture change throughout. Final Answer: Increases

Question 19

Reversible device reconfigured as a refrigerator A reversible heat engine has an ideal thermal efficiency of 30%. If the same device operates reversibly as a refrigerator between the same two temperature levels, what will be its coefficient of perfo…

Accepted Answer

Introduction / Context: Carnot relations connect the efficiency of a reversible heat engine with the performance of its corresponding reversible refrigerator or heat pump operating between the same two thermal reservoirs. Given Data / Assumptions: Reversible (Carnot) operation between T_H and T_L. Thermal efficiency of the engine η = 0.30. All other conditions (reservoir temperatures) unchanged when running as a refrigerator. Concept / Approach: For a Carnot engine, η = 1 − (T_L / T_H). For a Carnot refrigerator, C.O.P._R = T_L / (T_H − T_L). We first find the temperature ratio from the efficiency, then compute C.O.P. Step-by-Step Solution: η = 1 − (T_L / T_H) = 0.30 ⇒ T_L / T_H = 1 − 0.30 = 0.70.C.O.P._R = T_L / (T_H − T_L) = (T_L / T_H) / (1 − T_L / T_H).Substitute: C.O.P._R = 0.70 / (1 − 0.70) = 0.70 / 0.30 = 2.333…Rounded to two decimals, C.O.P. ≈ 2.33. Verification / Alternative check: Use the relation (C.O.P.)_HP = (C.O.P.)_R + 1 to see that the heat pump C.O.P. would be 3.33; this is consistent with η and the same temperature pair. Why Other Options Are Wrong: 1.33 and 4.33 correspond to different temperature ratios; 3.33 is the heat pump value, not the refrigerator; 0.70 is the temperature ratio, not a C.O.P. Common Pitfalls: Confusing engine efficiency η with C.O.P., and mixing the refrigerator and heat pump expressions. Final Answer: 2.33

Question 20

State of refrigerant in the basic vapour-compression cycle In an ideal analysis, the refrigerant is a dry saturated vapour at which location in the cycle?

Accepted Answer

Introduction / Context: Idealized vapour-compression cycle analysis often assumes specific refrigerant states at key nodes to avoid complexities like wet compression or excessive superheat. Given Data / Assumptions: Ideal single-stage cycle with isentropic compression and throttling expansion. Negligible pressure drops in lines and heat exchangers. Compressor handles vapour only (no liquid). Concept / Approach: To avoid wet compression, the suction condition is set as dry saturated vapour (or slightly superheated in practice). After compression the vapour is superheated; the condenser then desuperheats and condenses it to saturated liquid. Step-by-Step Solution: Compressor suction: dry saturated vapour (ideal) → safe for compression.Compressor discharge: superheated vapour at high pressure.Condenser outlet: saturated or subcooled liquid.Expansion device outlet: low-quality mixture entering evaporator. Verification / Alternative check: p–h and T–s diagrams in textbooks show state 1 at the saturated vapour line (or slightly to the right if superheated) for an idealized cycle. Why Other Options Are Wrong: Before the condenser the fluid is generally superheated; after the condenser it is liquid; after the expansion device it is a low-quality mixture, not dry vapour. Common Pitfalls: Assuming “before the condenser” equals saturated vapour; remember compression adds superheat that must be rejected before condensation begins. Final Answer: Before entering the compressor (compressor suction)

Heat Transfer, Refrigeration and Air Conditioning Questions