Question 1

Psychrometric chart reading: The inclined, non-uniformly spaced straight lines on a standard psychrometric chart represent constant wet-bulb temperature (WBT). State whether this statement is correct.

Accepted Answer

Introduction / Context: Psychrometric charts are essential tools in HVAC engineering for visualizing moist-air properties. Each family of lines corresponds to a specific property. Recognizing which lines represent the wet-bulb temperature helps engineers quickly estimate processes like adiabatic saturation, evaporative cooling, and cooling with dehumidification. Given Data / Assumptions: Standard SI psychrometric chart for atmospheric pressure near 1 atm. Families of lines: dry-bulb temperature (DBT), wet-bulb temperature (WBT), relative humidity (RH), humidity ratio (w), enthalpy (h), and specific volume (v). Graphical conventions follow widely used HVAC charts. Concept / Approach: On most charts, constant DBT lines are vertical; constant humidity ratio lines are horizontal; constant RH lines are curved; constant enthalpy and constant WBT lines are inclined and nearly coincident. The WBT lines are drawn as straight, slanted lines that are not uniformly spaced because the psychrometric relationships are nonlinear. Step-by-Step Solution: Identify the vertical axis (humidity ratio) and horizontal axis (dry-bulb temperature).Observe the family of slanted straight lines labeled with values such as 15 °C WB, 20 °C WB, etc.Note the spacing varies with DBT and RH due to nonlinear moist-air behavior; hence, lines are non-uniformly spaced.Conclusion: These inclined straight lines represent constant wet-bulb temperature. Verification / Alternative check: Trace an adiabatic saturation (constant WBT) process line during evaporative cooling; it should follow one of these inclined lines, confirming identification. Why Other Options Are Wrong: Disagree / conditional options: WBT lines exist over the normal chart domain, not limited to specific RH or subfreezing ranges. Common Pitfalls: Confusing constant enthalpy lines with constant WBT lines; in many charts they coincide or are almost coincident, but they are explicitly labeled—always read the legend. Final Answer: Agree

Question 2

Coefficient of performance (COP) of a heat pump operating on the reversed Carnot cycle: choose the correct expression in terms of absolute temperatures.

Accepted Answer

Introduction / Context: The reversed Carnot cycle is an idealized benchmark for heat pumps and refrigerators. Understanding the coefficient of performance (COP) formulas helps compare real systems to the theoretical maximum performance. Given Data / Assumptions: Heat pump moves heat from a cold reservoir at Tc (K) to a hot reservoir at Th (K). Ideal (reversed Carnot) processes; no irreversibilities. Temperatures are absolute (Kelvin). Concept / Approach: For a reversed Carnot heat pump, the COP is defined as desired effect (heat delivered to hot space) divided by net work input. The Carnot relation gives a simple temperature-based expression. Step-by-Step Solution: For a heat pump: COP_HP = Qh / W.For reversed Carnot: Qh / Qc = Th / Tc and W = Qh - Qc.Therefore COP_HP = Qh / (Qh - Qc) = Th / (Th - Tc).Hence, the correct expression is COP_HP = Th / (Th - Tc). Verification / Alternative check: Refrigerator COP_RF = Qc / W = Tc / (Th - Tc); heat-pump COP exceeds refrigerator COP by 1 since COP_HP = COP_RF + 1. Why Other Options Are Wrong: COP_HP = Tc / (Th - Tc) is for a refrigerator, not a heat pump. (Th - Tc)/Th and Th/Tc are not COPs for Carnot heat pumps. 1/(Th - Tc) lacks units consistency and temperature scaling. Common Pitfalls: Using Celsius instead of Kelvin; mixing up heat pump and refrigerator COP expressions. Final Answer: COP_HP = Th / (Th - Tc)

Question 3

Cold storage at −2 °C (271 K) with ambient 40 °C (313 K): heat leakage is 30 kW. The actual COP of the plant is one-fourth of the ideal (Carnot) COP between these temperatures. Calculate the required input power to drive the plant.

Accepted Answer

Introduction / Context: Refrigeration problems often benchmark real systems against the reversed Carnot cycle. Here we compare an actual plant to the ideal COP to determine the shaft power needed for a specified cooling load. Given Data / Assumptions: Tc = −2 °C = 271 K (cold space). Th = 40 °C = 313 K (ambient sink). Cooling load Q_L = 30 kW (heat leakage to be removed). Plant COP_actual = (1/4) * COP_Carnot, same temperature limits. Concept / Approach: For a Carnot refrigerator, COP_Carnot = Tc / (Th − Tc). The actual COP is a fraction of this. Once COP_actual is known, power input follows from W = Q_L / COP_actual. Step-by-Step Solution: Compute temperature difference: Th − Tc = 313 − 271 = 42 K.COP_Carnot = Tc / (Th − Tc) = 271 / 42 ≈ 6.452.COP_actual = (1/4) * 6.452 ≈ 1.613.Required power W = Q_L / COP_actual = 30 / 1.613 ≈ 18.6 kW. Verification / Alternative check: If the plant had ideal performance (COP ≈ 6.45), the power would be ≈ 4.65 kW. One-fourth of this COP multiplies power by four, giving ≈ 18.6 kW—consistent with the calculation. Why Other Options Are Wrong: 1.86, 3.72, 7.44 kW: These correspond to dividing 30 kW by larger COPs than the stated degraded COP; they underestimate the needed power. 24.0 kW: Overestimates the power; would imply an even poorer COP than specified. Common Pitfalls: Using Celsius in the COP_Carnot formula; forgetting that a lower actual COP raises required power for the same cooling load. Final Answer: 18.6 kW

Question 4

On a standard psychrometric chart, which family of lines represents constant dry-bulb temperature (DBT)?

Accepted Answer

Introduction / Context: The psychrometric chart is a graphical map of moist-air properties. Correctly identifying the DBT lines is fundamental for plotting air-conditioning processes such as sensible heating/cooling and mixing. Given Data / Assumptions: Standard atmospheric-pressure psychrometric chart. Axes: DBT (°C) on the abscissa and humidity ratio (kg/kg dry air) on the ordinate. Other properties drawn as families of lines. Concept / Approach: By convention, the dry-bulb temperature increases to the right on the horizontal axis. Therefore, constant DBT is represented by vertical lines. Because DBT scale is linear, these vertical lines are uniformly spaced. Step-by-Step Solution: Locate the horizontal axis labeled in °C (DBT).Observe that moving right increases DBT; staying at a constant DBT implies a vertical movement.Hence, constant DBT lines are vertical and, due to linear scaling, are evenly spaced. Verification / Alternative check: Plot a simple sensible cooling process (no moisture change): it travels horizontally left along a line of fixed humidity ratio, crossing several vertical constant-DBT lines. Why Other Options Are Wrong: Horizontal lines: represent constant humidity ratio. Curved lines: correspond to constant relative humidity. Inclined lines: typically represent constant wet-bulb temperature/enthalpy. Common Pitfalls: Confusing constant enthalpy (inclined) with constant DBT; forgetting that DBT is the x-axis variable. Final Answer: Vertical and uniformly spaced lines

Question 5

Ammonia vapour-compression system: The liquid line temperature rise between the condenser outlet and the expansion valve should be kept as low as possible. State whether this is advisable.

Accepted Answer

Introduction / Context: In vapour-compression refrigeration, the liquid leaving the condenser should reach the expansion device with minimal heat pickup. Excessive liquid-line temperature rise increases flash gas and reduces evaporator effectiveness. Given Data / Assumptions: Working fluid: ammonia (NH3), but the reasoning holds for most refrigerants. Condenser outlet provides subcooled liquid. Liquid line transfers refrigerant to the expansion device. Concept / Approach: Any heat gained in the liquid line partially cancels condenser subcooling. At the throttling device, higher inlet temperature promotes flash gas formation, reducing the mass of liquid available to evaporate and thus lowering net cooling capacity. Step-by-Step Solution: Goal: maximize liquid quality at the expansion device to reduce flash gas.Minimize heat pickup in the liquid line by insulation, short routing, and avoiding hot zones.If liquid subcooling is preserved, the evaporator receives more liquid refrigerant, increasing capacity and stability. Verification / Alternative check: Compare two cases: with and without liquid-line heat gain. The case with lower line temperature yields higher mass of liquid after throttling and better evaporator utilization. Why Other Options Are Wrong: “No” and conditional options: Good practice is to keep the liquid line as cool as practical regardless of ambient; it is not limited to capillary-tube systems. Common Pitfalls: Confusing beneficial condenser subcooling (inside the condenser) with unwanted liquid-line heat pickup; both affect inlet state to the expansion device but in opposite directions. Final Answer: Yes

Question 6

Refrigerant carbon dioxide (R-744): It is not widely used in conventional systems primarily because of its __________ power requirements per tonne of refrigeration and high operating pressures.

Accepted Answer

Introduction / Context: CO₂ (R-744) has attractive environmental credentials (ODP = 0, very low GWP for transcritical venting applications), but its thermodynamic characteristics impose design and efficiency challenges compared to many halocarbon refrigerants in traditional cycles. Given Data / Assumptions: CO₂ operates at very high pressures, especially in transcritical cycles. Comparison is to typical HFC/HFO cycles at similar temperature lifts. Metric of interest: power per tonne of refrigeration (kW/TR). Concept / Approach: At common ambient temperatures, CO₂ systems often run transcritically with a gas cooler rather than a condenser. The absence of a condensation plateau and high pressure levels lead to higher compressor discharge temperatures and significant compressor power for a given cooling capacity. Step-by-Step Solution: Consider a medium-temperature application at 35–40 °C ambient.CO₂ discharge pressures can exceed 90 bar; optimal gas-cooler outlet temperature control is critical.For the same cooling load, compressor input tends to be higher than many conventional refrigerants unless advanced cycles (parallel compression, ejectors) are used. Verification / Alternative check: Performance maps show improved efficiency in cooler climates or with system enhancements; however, baseline systems typically exhibit higher kW/TR than legacy HFCs at warm ambients. Why Other Options Are Wrong: Low/moderate/negligible/variable: Do not match the general baseline behavior; “high” is the key reason cited alongside high operating pressures. Common Pitfalls: Assuming CO₂ is always inefficient; in cold climates or with modern architectures, efficiency can be competitive. The question asks about general non-use historically. Final Answer: high

Question 7

Moist-air mixtures: “The total pressure of moist air equals the sum of the partial pressure of dry air and the partial pressure of water vapour.” This statement is known as:

Accepted Answer

Introduction / Context: Psychrometrics treats moist air as an ideal mixture of dry air and water vapour. The relationship between total pressure and constituent partial pressures underpins property calculations and chart construction. Given Data / Assumptions: Ideal-gas behavior for each component at HVAC conditions. Mixture of dry air and water vapour in equilibrium at a given temperature and total pressure. Concept / Approach: Dalton’s law states that the total pressure of a gaseous mixture equals the sum of the partial pressures of the components, each evaluated as if it alone occupied the entire volume at the mixture temperature. Step-by-Step Solution: Let p = total pressure; p_a = partial pressure of dry air; p_v = partial pressure of water vapour.By Dalton: p = p_a + p_v.Use ideal-gas relations for each component to compute humidity ratio and relative humidity as needed in HVAC analysis. Verification / Alternative check: Compare computed humidity ratio from p_v with chart values; they match when Dalton’s law holds (typical at low to moderate pressures). Why Other Options Are Wrong: Kinetic theory and Avogadro’s hypothesis are foundational but do not directly state this additive pressure law; Newton’s law is unrelated; Clapeyron’s equation is a thermodynamic relation for pure substances. Common Pitfalls: Applying Dalton’s law at very high pressures where real-gas effects are significant; in HVAC ranges, the ideal assumption is valid. Final Answer: Dalton’s law of partial pressures

Question 8

Comfort air-conditioning: Select the most commonly accepted comfort condition from the options (DBT = dry-bulb temperature; RH = relative humidity).

Accepted Answer

Introduction / Context: Comfort conditions define the target state for HVAC design. While exact preferences vary, guidelines (ASHRAE-like recommendations) identify a zone typically around 21–25 °C and 40–60% RH for sedentary activity and normal clothing. Given Data / Assumptions: Standard comfort for indoor spaces at light activity. Neutral thermal sensation near 22–24 °C, moderate RH. Air velocity low to moderate. Concept / Approach: Excessively high RH (near saturation) impairs evaporative cooling from skin, while very low RH causes dryness. A mid-range RH around 60% with a DBT near 22 °C is widely cited as comfortable for many occupants. Step-by-Step Solution: Scan options for mid-range RH (40–60%) and moderate DBT (≈22–24 °C).22 °C, 60% RH falls squarely in common comfort zones.Options with 100% or 80% RH are outside typical comfort recommendations. Verification / Alternative check: Comfort charts place operative temperature with RH in a “comfort polygon,” in which 22 °C and 60% RH are acceptable for many conditions. Why Other Options Are Wrong: 25 °C at 100% RH: Stuffy and uncomfortable due to no evaporative cooling. 20 °C at 80% RH or 18 °C at 70% RH: Too cool and too humid for typical comfort. 25 °C at 40% RH: Potentially acceptable for some, but 22 °C at 60% RH is more canonical in basic theory questions. Common Pitfalls: Assuming a single fixed point applies to all climates and clothing; real design uses comfort ranges and adaptive models. Final Answer: 22 °C DBT and 60% RH

Question 9

Interpreting wet-bulb temperature: A low measured wet-bulb temperature generally indicates very __________ relative humidity.

Accepted Answer

Introduction / Context: Wet-bulb temperature (WBT) reflects the combined effect of temperature and humidity through evaporative cooling. It is vital for assessing comfort, cooling tower performance, and evaporative cooler effectiveness. Given Data / Assumptions: Psychrometric relationships at standard atmospheric pressure. WBT is measured with a wetted wick over a thermometer bulb in moving air. Concept / Approach: When air is dry (low RH), evaporation from the wet wick is strong, causing a large temperature drop between dry-bulb temperature (DBT) and WBT. Thus, a low WBT relative to DBT signals low humidity. Conversely, at high RH, evaporation is limited and WBT approaches DBT. Step-by-Step Solution: Compare DBT and WBT: a large DBT−WBT gap means strong evaporative potential → dry air.If WBT nearly equals DBT, air is near saturation → high humidity.Therefore, a low WBT reading (relative to DBT) indicates low RH. Verification / Alternative check: Use the sling psychrometer: compute RH via tables from DBT and WBT; large depression corresponds to low RH values. Why Other Options Are Wrong: High or fixed RH values contradict psychrometric tables; pressure effects are minor at normal altitudes for this qualitative inference. Common Pitfalls: Interpreting absolute WBT without reference to DBT; always consider the depression (DBT−WBT). Final Answer: low

Question 10

In HVAC chemical dehumidification (using hygroscopic absorbers or adsorbers), which of the following changes correctly describe the air-state outcomes as moisture is removed?

Accepted Answer

Introduction / Context: Chemical dehumidification removes moisture from air by exposing it to hygroscopic materials (desiccants) such as silica gel, lithium chloride, or solid desiccant wheels. Unlike cooling-based dehumidification, this process is exothermic and usually warms the air while reducing its moisture content. Given Data / Assumptions: Process is a chemical (desiccant) dehumidification step, not cooling coil dehumidification. Air loses moisture to the desiccant; heat of adsorption/absorption raises air temperature. Psychrometric properties of interest: dry-bulb temperature, wet-bulb temperature, and dew point temperature. Concept / Approach: When moisture is removed at nearly constant enthalpy input from adsorption heat, the humidity ratio decreases. Dry-bulb temperature typically increases because the exothermic process adds sensible heat to the air stream. Lower moisture content shifts both dew point and wet-bulb temperatures downward, since both are functions of moisture content at a given pressure. Step-by-Step Solution: 1) Moisture transfer: water vapor moves from air to desiccant → humidity ratio decreases.2) Exothermic release: heat of sorption warms the air → dry-bulb temperature increases.3) With less vapor, saturation tendency reduces → dew point temperature decreases.4) Wet-bulb temperature, linked to adiabatic saturation and moisture content, also decreases. Verification / Alternative check: Plot the initial and final states on a psychrometric chart: the final state shows lower humidity ratio, a lower dew point, a lower wet-bulb, and a higher dry-bulb temperature as commonly observed with desiccant wheels (before any post-cooling stage). Why Other Options Are Wrong: (a), (b), (c) each capture only one aspect; chemical dehumidification causes all three listed changes. (e) Incorrect because wet-bulb also changes due to altered moisture content, and dry-bulb typically rises. Common Pitfalls: Confusing chemical dehumidification with cooling coil dehumidification (cooling and dehumidifying), where dry-bulb temperature generally decreases. Also, assuming wet-bulb stays constant is a misconception from evaporative processes, not chemical drying. Final Answer: All of these

Question 11

In a standard vapor-compression refrigeration cycle, the highest refrigerant temperature occurs at which point of the cycle?

Accepted Answer

Introduction / Context: Understanding state points in a vapor-compression cycle is foundational in refrigeration and air-conditioning. Identifying where the refrigerant reaches its highest temperature helps with diagnosing discharge overheating, sizing condensers, and ensuring compressor safety. Given Data / Assumptions: Cycle components: evaporator → compressor → condenser → expansion device. Compression is non-isothermal; it raises both pressure and temperature. No abnormal subcooling/flash-gas issues overriding basic trends. Concept / Approach: The compressor takes low-pressure vapor from the evaporator and compresses it to a high-pressure, high-temperature superheated vapor. This discharge state is the cycle’s maximum temperature. The condenser then rejects heat, decreasing refrigerant temperature toward saturation and subcooling levels. Step-by-Step Solution: 1) Evaporator exit: low-pressure superheated vapor at relatively low temperature.2) Compressor: adds work → temperature rises sharply.3) Condenser: rejects heat at nearly constant pressure → temperature falls.4) Expansion device: throttling reduces temperature further before evaporator. Verification / Alternative check: On a T–s diagram, the peak temperature is at the end of the compression line (compressor discharge) before the condensation curve. Why Other Options Are Wrong: (a) Evaporator outlet is the coolest vapor state, not the hottest. (c) In the condenser, temperature is dropping as heat is rejected. (d) After throttling, temperature decreases substantially. (e) Temperature varies throughout the cycle. Common Pitfalls: Confusing “after evaporation” with “after compression.” The maximum temperature is not at evaporator exit; it is at compressor discharge. Final Answer: Immediately after the compressor (compressor discharge, before the condenser)

Question 12

On a standard psychrometric chart, which of the following statements about dew-point temperature (DPT) lines is correct?

Accepted Answer

Introduction / Context: The psychrometric chart is a graphical tool representing moist-air properties. Correctly identifying families of lines (dry-bulb, wet-bulb, humidity ratio, dew point) is essential for tracing HVAC processes like cooling, heating, humidification, and dehumidification. Given Data / Assumptions: Standard sea-level psychrometric chart representation. Dew-point temperature at a given pressure is a direct function of humidity ratio. Concept / Approach: Dew-point temperature corresponds to the temperature at which moist air becomes saturated at its prevailing water vapor partial pressure. On common charts, humidity ratio w is plotted on the vertical axis, and lines of constant w are horizontal and equally spaced. Because DPT is a one-to-one function of w at fixed pressure, constant DPT lines coincide with constant humidity-ratio lines—hence horizontal and evenly spaced. Step-by-Step Solution: 1) Recognize constant w lines are horizontal, equally spaced.2) For fixed barometric pressure, DPT is a function of w alone.3) Therefore, constant DPT lines align with constant w lines → horizontal and uniformly spaced. Verification / Alternative check: Inspect any standard chart: DPT scale marks align with constant humidity-ratio horizontal grids; spacing is uniform on most charts for the typical w-axis. Why Other Options Are Wrong: (b) Non-uniform spacing is not typical for w/DPT families. (c) Enthalpy lines (often slanted) differ from horizontal DPT/w lines. (d) Vertical lines correspond to constant dry-bulb temperature, not DPT. (e) DPT lines do not coincide with enthalpy lines. Common Pitfalls: Confusing wet-bulb/enthalpy lines (slanted) with humidity-ratio/dew-point lines (horizontal). Final Answer: DPT lines are horizontal and uniformly spaced

Question 13

In aircraft environmental control, a simple air (open) cooling system is best suited for which flight speed regime?

Accepted Answer

Introduction / Context: Air-cycle refrigeration (simple, bootstrap, regenerative) is used in aircraft. The simple air system relies on ram air compression and turbine expansion, making its performance closely tied to flight speed and altitude conditions. Given Data / Assumptions: System: simple (open) air-cycle cooler without elaborate regeneration or multiple expansion stages. Ram effect increases with flight speed. Ambient at altitude provides low temperatures aiding heat rejection. Concept / Approach: At high flight speeds, ram compression raises air pressure before the turbine, improving cycle effectiveness. The high-speed slipstream also enhances heat exchanger performance. At low speeds or on ground, there is minimal ram effect; simple systems deliver poor COP and limited capacity, prompting use of more complex cycles or ground air carts. Step-by-Step Solution: 1) Recognize that ram pressure rise ∝ dynamic pressure ∝ velocity^2.2) Higher ram → greater temperature drop across turbine for a given expansion.3) High-speed airflow boosts heat exchanger (precooler/intercooler) effectiveness.4) Therefore, simple air systems are preferable at higher flight speeds. Verification / Alternative check: Performance maps show increased net refrigeration at cruise; on-ground performance deteriorates without supplemental fans or bleed strategies. Why Other Options Are Wrong: (a) Low speeds provide weak ram effect; system is inefficient. (c) Hovering is rotorcraft-specific and does not provide useful ram for fixed-wing air-cycle systems. (d) Taxiing is slow; again poor performance. (e) Stationary operation relies on auxiliary equipment; not the intended regime for a simple air cycle. Common Pitfalls: Assuming an air-cycle system performs equally well on ground; it is highly speed-dependent. Final Answer: High flight speeds

Question 14

Among common historical refrigerants (e.g., ammonia, carbon dioxide, sulphur dioxide, R-12), does sulphur dioxide (SO₂) have the lowest normal boiling point?

Accepted Answer

Introduction / Context: Boiling point at standard pressure is a key selection factor for refrigerants. Lower normal boiling point generally indicates stronger cooling potential at low evaporator temperatures, though toxicity, flammability, and environmental impact also matter. Given Data / Assumptions: Comparison uses approximate normal boiling points at 1 atm. Typical references: NH₃ (ammonia), CO₂, SO₂, and R-12. Concept / Approach: SO₂ has a normal boiling point near −10 °C, whereas ammonia is about −33 °C and R-12 about −30 °C. Carbon dioxide does not have a normal liquid boiling point at 1 atm (it sublimates at about −78.5 °C), and its critical temperature is near 31 °C; thus, at 1 atm it exists as either gas or dry ice, not a liquid. Therefore, SO₂ is not the lowest among these; in fact, it is comparatively high (warmer) among traditional refrigerants. Step-by-Step Solution: 1) Note SO₂ boiling point ≈ −10 °C.2) Compare with NH₃ (≈ −33 °C) and R-12 (≈ −30 °C) → these are lower (colder).3) Consider CO₂ behavior at 1 atm (sublimation at −78.5 °C) → effectively a much lower temperature benchmark. Verification / Alternative check: Data tables corroborate NH₃ and R-12 having lower boiling points than SO₂ at standard pressure; CO₂ does not boil at 1 atm, indicating even stronger low-temperature behavior via sublimation reference. Why Other Options Are Wrong: (a) Incorrect: SO₂ is not the lowest. (c) Boiling points differ widely. (d) While boiling point depends on pressure, standard comparisons use 1 atm. (e) Altitude changes pressure but does not invert the ordering cited. Common Pitfalls: Confusing SO₂’s historical use with superior low-temperature properties; toxicity and corrosion issues limited its application. Final Answer: No, SO₂ does not have the lowest boiling point

Question 15

Which material listed typically exhibits the lowest thermal diffusivity (α = k / (ρ * c)) and therefore heats up/ cools down the slowest in depth?

Accepted Answer

Introduction / Context: Thermal diffusivity α indicates how quickly a material’s temperature field adjusts to a change in heat input. Low α means the material is sluggish in transmitting thermal disturbances, useful for insulation or damping temperature fluctuations. Given Data / Assumptions: α = k / (ρ * c), where k is thermal conductivity, ρ is density, and c is specific heat. Comparing common engineering materials: metals versus elastomers. Concept / Approach: Metals have high k and moderate ρ and c, yielding high α. Polymers and elastomers have low k and often moderate c, producing very low α. Rubber, with very low conductivity, is a classic example of low thermal diffusivity material; it resists rapid internal temperature changes. Step-by-Step Solution: 1) Metals (iron, aluminium, copper) → high k → high α.2) Lead has relatively lower k among metals but still higher α than rubber.3) Rubber → very low k and modest ρ and c → lowest α among the options. Verification / Alternative check: Typical α (m^2/s): aluminium ~ 8.5e−5, copper ~ 1.1e−4, iron ~ 2–4e−5, lead ~ 1.2e−5, rubber ~ 1e−7 to 1e−8, confirming rubber’s minimal diffusivity. Why Other Options Are Wrong: Metals conduct heat well and quickly diffuse thermal changes. Lead is lower than many metals but still above rubber. Common Pitfalls: Confusing low conductivity with low heat capacity; diffusivity combines both and is the appropriate metric for transient heat penetration. Final Answer: Rubber

Question 16

Heat exchanger effectiveness (ε) is defined how, in terms of actual heat transfer relative to the thermodynamic maximum possible?

Accepted Answer

Introduction / Context: Effectiveness–NTU analysis allows heat exchanger performance evaluation independent of outlet temperatures. The definition of effectiveness anchors all derived relations across flow arrangements and capacity-rate ratios. Given Data / Assumptions: Two-stream heat exchanger with capacity rates C_hot and C_cold. Maximum possible heat transfer is limited by Cmin and the inlet temperature difference. Concept / Approach: Effectiveness ε quantifies how closely an exchanger approaches its theoretical limit: ε = q_actual / q_max, where q_max = Cmin * (T_h,in − T_c,in). This definition is universal across LMTD and NTU methods; specific formulas like ε(NTU, C_r) depend on flow arrangement (parallel, counter, cross-flow) and are derived from this definition. Step-by-Step Solution: 1) Identify Cmin = min(C_hot, C_cold).2) Compute q_max = Cmin * (T_h,in − T_c,in).3) Actual transfer q_actual depends on UA and flow configuration.4) Define ε = q_actual / q_max (dimensionless, 0 to 1). Verification / Alternative check: For a given NTU = UA / Cmin and capacity ratio C_r = Cmin / Cmax, tables provide ε as a function ε(NTU, C_r), consistently rooted in ε = q_actual / q_max. Why Other Options Are Wrong: (b) Inverts the definition. (c) UA/Cmax is NTU only for a specific normalization; not ε. (d) Is one special-case formula, not the general definition. (e) Cmin/Cmax is capacity ratio, not effectiveness. Common Pitfalls: Mixing up NTU and ε or assuming a single ε equation applies to all configurations. Final Answer: ε = actual heat transfer / maximum possible heat transfer

Question 17

Select the correct SI unit for the overall heat transfer coefficient U in convection–conduction composite analysis.

Accepted Answer

Introduction / Context: The overall heat transfer coefficient U aggregates conduction and convection resistances into a single parameter for heat exchanger and wall calculations. Using correct units ensures dimensional consistency when computing heat duties from temperature differences. Given Data / Assumptions: General heat-transfer relation: q = U * A * ΔT_lm (or q = U * A * ΔT for simple cases). q measured in watts (W), area A in m^2, temperature difference in kelvin (K). Concept / Approach: Rearranging q = U * A * ΔT shows U must carry units that convert A * ΔT to power: U = q / (A * ΔT). Therefore, U has SI units W / (m^2 * K) = W/m^2·K. Step-by-Step Solution: 1) Start with q = U * A * ΔT.2) Substitute dimensions: [q] = W, [A] = m^2, [ΔT] = K.3) Solve: [U] = W / (m^2 * K) = W/m^2·K. Verification / Alternative check: For conduction only, the analogous coefficient k uses W/m·K; including area-normalized convection terms on both sides yields U in W/m^2·K. Why Other Options Are Wrong: (b) Missing temperature dimension. (c) Units of thermal conductivity k, not overall U. (d) Incomplete units. (e) K/W is overall thermal resistance per unit area inverted, not U. Common Pitfalls: Confusing U with k or h; h and U both use W/m^2·K, while k uses W/m·K. Final Answer: W/m^2·K

Question 18

In psychrometric terms, the humidity ratio (also called specific humidity or moisture content) is defined as the mass of water vapor present per kilogram of dry air. Is this definition correct?

Accepted Answer

Introduction / Context: Psychrometrics uses the humidity ratio to quantify moisture in air-conditioning calculations. It is critical for coil sizing, dehumidification strategies, and calculating latent loads in HVAC systems. Given Data / Assumptions: Standard psychrometric definitions at typical barometric pressures. Humidity ratio often denoted by w. Concept / Approach: The humidity ratio is defined as w = m_vapor / m_dry_air. Because the basis is per kilogram of dry air, w remains well-defined across a wide range of conditions and is independent of relative humidity’s percentage expression. This contrasts with specific volume or enthalpy, which depend on the total moist-air mixture properties. Step-by-Step Solution: 1) Write the definition: w = mass of water vapor divided by mass of dry air.2) Note that w is typically expressed in kg_vapor/kg_dry_air.3) Use w to compute latent loads and to plot states on the psychrometric chart (vertical axis is commonly humidity ratio). Verification / Alternative check: On the psychrometric chart, moving vertically changes w; calculations of dew point and mixing lines rely on this definition. Why Other Options Are Wrong: (b) Incorrect; the given definition is standard. (c) and (d) add unnecessary conditions; the definition holds broadly. (e) Per kilogram of moist air would be mass fraction of vapor, not the standard humidity ratio. Common Pitfalls: Confusing mass fraction (per moist air) with humidity ratio (per dry air) or mixing absolute humidity with relative humidity. Final Answer: True

Question 19

Refrigerants in household refrigerators (legacy context) Which refrigerant has been widely used historically in domestic refrigerators?

Accepted Answer

Introduction / Context: Domestic refrigerators have used several refrigerants over the last century. Understanding which fluids were common in home appliances helps students link thermodynamic properties (pressure levels, toxicity, flammability) to system design and safety standards. Given Data / Assumptions: Question asks about the refrigerant widely used in household refrigerators in traditional/legacy contexts. Focus is on common mass-market units rather than special laboratory or industrial systems. Environmental phase-outs (e.g., of CFCs) do not change what was historically most prevalent. Concept / Approach: R-12 (a CFC) was historically the dominant choice in domestic refrigerators for many decades because it is noncorrosive, nonflammable, and compatible with mineral oils, with moderate pressures that suit small hermetic compressors. Although R-12 has now been phased out due to ozone depletion concerns and replaced largely by R-134a or newer low-GWP blends, the historical answer remains R-12. Step-by-Step Solution: Identify legacy domestic practice: small hermetic compressors + safe, stable refrigerant.Match properties: R-12 offered low toxicity, low discharge temperature, and good oil miscibility.Therefore, the historically widespread domestic refrigerant is R-12. Verification / Alternative check: Manufacturer literature and refrigeration handbooks list R-12 for domestic units before environmental regulations led to R-134a transitions. Why Other Options Are Wrong: Ammonia is efficient but toxic and corrosive to copper; rarely used in home fridges. CO2 uses very high pressures and is uncommon in legacy domestic units. SO2 was used early on but was replaced due to toxicity. R-22 is mainly for air-conditioning, not typical household refrigerators. Common Pitfalls: Confusing modern replacements (like R-134a) with the historically “widely used” fluid specified by older curricula. Final Answer: R-12 (dichlorodifluoromethane)

Question 20

Deep freezers — why R-12 was preferred over R-22 (legacy systems)? Select the most appropriate reason set for preferring R-12 over R-22 in deep-freeze applications (historical context).

Accepted Answer

Introduction / Context: Refrigerant selection for deep freezers balances thermodynamic performance, compressor mechanics, oil return, and safety. Historically, certain CFCs offered an advantageous combination that made them popular in low-temperature domestic and commercial units. Given Data / Assumptions: Legacy (pre phase-out) comparison of R-12 and R-22. Deep-freeze conditions (very low evaporator temperatures). Hermetic/small compressors typical of domestic or light commercial freezers. Concept / Approach: R-12 operated at lower discharge pressures than R-22 for comparable evaporator/condensing conditions, reducing compressor stress. It also provided good oil miscibility across a broad temperature range, important for reliable oil return. Together these factors often produced higher COP in small systems at deep-freeze conditions compared with R-22. Step-by-Step Solution: Check operating pressures: R-12 < R-22 at similar conditions → less mechanical load.Check oil return: R-12’s miscibility improves lubrication reliability.Net performance: Under many legacy designs, R-12 yielded higher COP.Hence, all listed advantages are valid. Verification / Alternative check: Historical compressor application charts and service manuals reflect lower pressures and good oil behavior with R-12 in low-temp applications. Why Other Options Are Wrong: Single reasons (a), (b), or (c) are incomplete; the preference arose from the combination. Common Pitfalls: Applying current environmental rules retroactively to historical selection; the question focuses on engineering reasons, not regulatory status. Final Answer: all of the above

Heat Transfer, Refrigeration and Air Conditioning Questions