Q: Control basis of a thermostatic expansion valve (TEV) A thermostatic expansion valve modulates refrigerant flow primarily in response to changes in which variable measured at the evaporator outlet?

Introduction / Context: Thermostatic expansion valves (TEVs or TXVs) are widely used to meter refrigerant into the evaporator while preventing liquid carryover to the compressor. Understanding their control feedback is essential for proper selection and troubleshooting. Given Data / Assumptions: A sensing bulb is strapped to the evaporator outlet (suction line) and filled with a charge. The valve experiences three forces: bulb pressure (opening), evaporator pressure (closing), and spring force (closing). Stable operation targets a set superheat, not just a fixed temperature or pressure. Concept / Approach: The TXV adjusts to maintain a set “superheat,” defined as the temperature rise above the saturation temperature corresponding to evaporator pressure at the outlet. The bulb temperature rises with superheat, increasing bulb pressure and opening the valve; low superheat reduces bulb pressure, closing the valve. Thus, the controlled variable is degree of superheat. Step-by-Step Solution: Measure suction temperature Ts at evaporator outlet and saturation temperature Tsat from suction pressure.Compute superheat SH = Ts − Tsat.Valve modulates to hold SH near a setpoint (e.g., 5–8 K).Stable SH ensures full evaporator utilization without risking liquid floodback. Verification / Alternative check: Manufacturer literature uniformly describes TEVs as “superheat controllers,” with bulb placement critical to accurate SH sensing. Why Other Options Are Wrong: Temperature only or pressure only: neither alone defines superheat; both are required to infer Tsat and compute SH.Condenser subcooling and discharge temperature are not the TXV control variables. Common Pitfalls: Relocating the bulb to a poor thermal contact point or after a suction-line heat exchanger can distort sensed SH and cause hunting. Final Answer: Degree of superheat at exit from the evaporator

Q: Prevost’s theory of exchanges in thermal radiation Evaluate the statement: Every body, regardless of its temperature, continuously emits radiation to and receives radiation from its surroundings at the same time.

Introduction / Context: Prevost’s theory of exchanges provides an intuitive basis for radiative heat transfer. It clarifies that emission and absorption are simultaneous processes and that net heat flow by radiation depends on temperature differences and surface properties. Given Data / Assumptions: Any real surface at finite temperature emits thermal radiation. The same surface can absorb incident radiation from its surroundings. Vacuum is not required; radiation propagates through transparent media as well. Concept / Approach: According to Prevost, all bodies emit radiation continuously as a function of their temperature and emissivity, and they also receive radiation from other bodies. Net radiative exchange equals emission minus absorption. A cooler body can still emit radiation; it simply absorbs more than it emits if surrounded by hotter surfaces, resulting in a net gain. Step-by-Step Solution: State emission: E_out = εσT^4 for an idealized diffuse–gray surface (conceptual form).State absorption: E_in depends on surroundings, view factors, and their temperatures.Net exchange q_rad = E_out − absorbed_incoming; sign determines heat gain or loss.Therefore, simultaneous emission and reception are always occurring. Verification / Alternative check: Kirchhoff’s law at equilibrium (ε = α) and the Stefan–Boltzmann relation support the continuous nature of emission and the role of absorption; at equilibrium net exchange is zero though two-way exchange persists. Why Other Options Are Wrong: “False” or restricted cases contradict the universal nature of radiative emission for T > 0 K.Polished metals and vacuum conditions are not prerequisites; they only alter amounts or paths of radiation. Common Pitfalls: Equating “net heat loss” with “no emission” when surroundings are hotter; remember both emission and absorption occur, and net direction depends on relative intensities. Final Answer: True

Q: Thermal diffusivity comparison: In general, the thermal diffusivity of gases is ________ that of liquids.

Introduction / Context: Thermal diffusivity indicates how quickly temperature disturbances propagate through a material. It is crucial for transient heat transfer analysis in fluids such as air (gas) and water (liquid). Given Data / Assumptions: Thermal diffusivity h (often denoted alpha) defined as h = k / (rho * cp). Typical properties at room temperature for common gases and liquids. No phase change; homogeneous fluids considered. Concept / Approach: For gases, density rho and specific heat capacity per unit volume rhocp are relatively small compared with liquids. Although thermal conductivity k of gases is lower than that of liquids, the much smaller thermal inertia (rhocp) in gases makes h larger for gases in many practical cases. Step-by-Step Solution: Start with h = k / (rho * cp).Compare orders of magnitude: k_gas < k_liquid but (rhocp)_gas ≪ (rhocp)_liquid.Because denominator drops faster for gases, h_gas tends to be greater than h_liquid.Therefore, temperature fields in gases adjust more rapidly for the same length scale. Verification / Alternative check: Typical values: h_air ~ 2.1e-5 m^2/s; h_water ~ 1.4e-7 m^2/s. The gas value is orders of magnitude higher, supporting the conclusion. Why Other Options Are Wrong: Equal to / approximately half / independent of phase: contradict property data trends. Less than: ignores the dominant effect of much larger volumetric heat capacity of liquids. Common Pitfalls: Confusing conductivity with diffusivity; a higher k does not guarantee higher h if rho*cp is also much higher. Final Answer: greater than

Q: Psychrometric chart reference: The “alignment circle” commonly marked on the comfort psychrometric chart corresponds to which dry-bulb temperature (DBT) and relative humidity (RH)?

Introduction / Context: Many psychrometric charts used in HVAC (e.g., comfort charts) include an “alignment circle” or reference point to help align scales or to denote a representative comfort condition. Knowing this reference aids quick estimations and chart navigation. Given Data / Assumptions: Standard comfort psychrometric charts used in HVAC education and practice. Alignment circle denotes a nominal comfort state near mid-range humidity. Units in SI (°C, % RH). Concept / Approach: Common charts mark a reference near 26 °C DBT and 50% RH (approximately 80 °F and 50% RH). This lies centrally in the comfort region and provides convenient alignment for scales like enthalpy or specific volume lines used in quick calculations. Step-by-Step Solution: Identify typical comfort zone: DBT roughly 22–26 °C and RH 40–60%.Select the central reference often used by chart publishers: 26 °C and 50% RH.Confirm that this point falls close to mid-range properties, simplifying alignment tasks. Verification / Alternative check: Cross-checking standard HVAC psychrometric charts shows a reference/marker near 26 °C, 50% RH; while exact labeling may vary by publisher, 26 °C and 50% RH is a widely used reference. Why Other Options Are Wrong: 20 °C combinations: Cooler than typical alignment reference used for comfort chart alignment. 26 °C at 60% RH: In comfort zone but not the commonly marked alignment circle. 24 °C, 50% RH: Reasonable comfort point but not the standard alignment marker. Common Pitfalls: Assuming all charts use identical markers; some charts are customized. For general exam purposes, 26 °C and 50% RH is the accepted reference. Final Answer: 26 °C DBT and 50% RH

Question 1

Cooling tower performance: To what theoretical limit can circulating water be cooled in a standard evaporative cooling tower operating with ambient air?

Accepted Answer

Introduction / Context: Cooling towers reject heat by partial evaporation of water into an airstream. Understanding the limiting cold-water temperature is essential for plant performance estimates and chiller selection. Given Data / Assumptions: Evaporative cooling tower operating at near-atmospheric pressure. Air–water contact allows heat and mass transfer (sensible and latent). Negligible heat sources other than process heat load. Concept / Approach: The fundamental limit is the entering air wet-bulb temperature (WBT). In an ideal tower with infinite size and perfect contact, the cooled water approaches the WBT asymptotically; it cannot be cooled below the WBT because equilibrium between water temperature and the adiabatic saturation temperature of the air defines the driving potential. Step-by-Step Solution: Define approach: approach = cold-water temperature − entering air wet-bulb temperature.In the ideal limit, approach → 0, so cold water → WBT.Therefore, the minimum achievable cold-water temperature equals the entering air wet-bulb temperature. Verification / Alternative check: Psychrometric analysis: adiabatic saturation process aligns with wet-bulb lines; at equilibrium, water temperature equals the air’s adiabatic saturation (wet-bulb) temperature. Why Other Options Are Wrong: Dew point: lower than WBT only at very dry conditions; towers cannot reach dew point by evaporative contact. Dry-bulb: generally higher than WBT; not the limiting temperature in evaporative cooling. “Ambient air temperature” is ambiguous and often equals dry bulb; it is not the limit. Common Pitfalls: Confusing dew point with wet bulb; dew point governs condensation processes, not evaporative cooling limits. Final Answer: Wet-bulb temperature of air

Question 2

In HVAC psychrometrics, what happens to the specific humidity (humidity ratio, kg of water vapour per kg of dry air) during a combined heating and humidification process of air?

Accepted Answer

Introduction / Context: In air-conditioning processes, “specific humidity” (also called humidity ratio) indicates the mass of water vapour per unit mass of dry air. Understanding how it changes during common processes such as heating, cooling, humidification, and dehumidification is crucial for load calculations and correct coil selection. Given Data / Assumptions: The process under consideration is heating combined with humidification. Air is contacted with a moisture source (steam injection or water spray) while sensible heat is added. Steady flow with negligible heat loss to surroundings outside the intended process. Concept / Approach: Heating changes the dry-bulb temperature (sensible effect). Humidification adds moisture to the air (latent effect), which increases the humidity ratio. On a psychrometric chart (for a fixed barometric pressure), humidification moves the state upward (higher humidity ratio), while heating moves it to the right (higher dry-bulb temperature). A combined heating-and-humidification step therefore tends to move diagonally up-right. Step-by-Step Solution: Define specific humidity (or humidity ratio) as w = m_vapour / m_dry_air.During humidification, m_vapour added > 0 while m_dry_air is essentially constant, so w must increase.Heating alone would change temperature but not w; adding humidification ensures w rises.Therefore, for heating + humidification, the net result is an increase in specific humidity. Verification / Alternative check: Trace the process on a psychrometric chart: starting state 1 moves horizontally right to state 2 (heating at nearly constant w), then moves vertically or diagonally up-right to state 3 (moisture addition). The vertical component confirms an increase in w. Why Other Options Are Wrong: Remains constant: only true for pure sensible heating or cooling with no moisture addition.Decreases: opposite of what humidification does.First decreases then increases: no inherent mechanism in standard heating + humidification for an initial decrease in w.Becomes zero: impossible unless all moisture is removed, which contradicts humidification. Common Pitfalls: Confusing relative humidity with humidity ratio. Relative humidity may rise or fall with heating depending on moisture addition, but humidity ratio increases whenever water vapour is added. Final Answer: Increases

Question 3

In HVAC terminology, humidification refers to adding moisture to air. If moisture is added in a way that the dry-bulb temperature does not change, is it still called humidification?

Accepted Answer

Introduction / Context: Humidification broadly means adding water vapour to an airstream. The dry-bulb temperature may rise, fall, or remain constant depending on how moisture is supplied (steam injection, adiabatic spray with water, or contact with wetted media). The definition focuses on moisture addition rather than a specific temperature change. Given Data / Assumptions: The process described adds moisture to air. Dry-bulb temperature is stated to remain unchanged. No requirement that relative humidity or enthalpy follow a particular path. Concept / Approach: Humidification is defined by an increase in humidity ratio (kg of water vapour per kg of dry air). Temperature behavior depends on the method: steam addition can raise DBT; adiabatic evaporative humidification typically lowers DBT; and carefully controlled steam or heat exchange can keep DBT nearly constant. All are valid humidification processes since the core attribute is increased moisture content. Step-by-Step Solution: Identify the key variable for humidification: humidity ratio w increases.Check the condition: DBT remains constant — this is a special case, not a disqualifier.Therefore, the statement that adding moisture without DBT change is humidification is acceptable.Conclusion: Agree with the definition in the general HVAC sense. Verification / Alternative check: On a psychrometric chart a constant-DBT humidification path moves vertically upward (w increases while DBT holds). This is physically feasible with appropriate control, e.g., steam at matched temperature or balanced heat exchange. Why Other Options Are Wrong: Disagree: incorrectly restricts humidification to temperature-changing paths.Conditions tied to “only if” are unnecessary; humidification is defined by added moisture, not by RH or wet-bulb behavior. Common Pitfalls: Assuming all humidification must be adiabatic (which typically decreases DBT). Steam humidification can increase or hold DBT depending on control. Final Answer: Agree

Question 4

Identify the refrigerant: Which common refrigerant has a normal boiling point closest to −10.5°C at approximately 1 bar?

Accepted Answer

Introduction / Context: Each refrigerant has a characteristic saturation temperature at a given pressure. Knowing approximate normal boiling points helps with quick identification tasks and sanity checks in design or troubleshooting. Given Data / Assumptions: Normal boiling point refers to saturation temperature at about 1 bar. Target value is approximately −10.5°C. We compare common historical refrigerants. Concept / Approach: Ammonia (R-717) boils near −33°C at 1 bar; R-12 near −29.8°C; carbon dioxide does not boil at 1 bar—it sublimates at −78.5°C; R-134a boils near −26.3°C. Sulphur dioxide has a normal boiling point close to −10°C (commonly quoted around −10.0 to −10.5°C), matching the query most closely. Step-by-Step Solution: List typical normal boiling points: NH3 ≈ −33°C; R-12 ≈ −29.8°C; SO2 ≈ −10°C; CO2 sublimation ≈ −78.5°C; R-134a ≈ −26°C.Compare with −10.5°C target.The closest and correct match is sulphur dioxide.Therefore, select SO2 as the correct answer. Verification / Alternative check: Consulting standard refrigerant property tables will confirm these typical values across reputable handbooks. Why Other Options Are Wrong: Ammonia/R-12/R-134a: all have significantly lower normal boiling points than −10.5°C.Carbon dioxide: does not have a liquid phase at 1 bar; it sublimates, so “boiling point” is inapplicable at 1 bar. Common Pitfalls: Assuming all older refrigerants boil near −30°C; SO2 is notably warmer at ~−10°C and was historically used in early systems despite toxicity concerns. Final Answer: Sulphur dioxide (SO2)

Question 5

Typical operating pressures for R-12 vapour-compression units For medium-temperature refrigerating units using R-12, which of the following is a representative condenser-side operating pressure?

Accepted Answer

Introduction / Context: Approximate operating pressures help technicians and students check gauges, anticipate component ratings, and identify faults. R-12 (legacy refrigerant) typically runs at moderate condenser pressures for comfort or commercial refrigeration conditions. Given Data / Assumptions: Ambient condenser conditions representative of moderate climates. Medium-temperature application (not deep-freeze). Single-stage, air-cooled condenser assumption. Concept / Approach: Condenser pressure corresponds to saturation pressure at typical condensing temperatures (say 35–45°C). For R-12 in this range, saturation pressures are commonly around 7–9 bar absolute. Evaporator pressures for above-zero evaporating temperatures are much lower (often 2–3 bar range), but the question focuses on condenser pressure. Step-by-Step Solution: Select a typical condensing temperature, e.g., 40°C.Read corresponding R-12 saturation pressure from tables: roughly ~8 bar abs.Compare options; 8 bar best matches normal condenser operation.Therefore, choose 8 bar as a representative value. Verification / Alternative check: Manufacturer pressure–temperature charts for R-12 corroborate that around 40°C the pressure is approximately 8 bar absolute. Why Other Options Are Wrong: 2 bar: more typical of evaporator side at mild evaporating temperatures.15 or 30 bar: far above standard R-12 condenser pressures; these are more aligned with high-pressure refrigerants or transcritical CO2 systems.0.8 bar: vacuum operation is not a normal condenser condition. Common Pitfalls: Confusing gauge with absolute pressure and forgetting temperature dependence; always cross-check with actual ambient and heat-rejection mode. Final Answer: 8 bar

Question 6

Effect of superheating in a vapour-compression refrigeration cycle How does adding suction superheat (beyond dry saturation at evaporator exit) generally affect the coefficient of performance (C.O.P.)?

Accepted Answer

Introduction / Context: Superheating in vapour-compression systems can occur intentionally in the evaporator or unintentionally in the suction line. Its effect on performance depends on how it influences refrigerating effect and compressor work. Given Data / Assumptions: Suction superheat occurs after the refrigerant is dry saturated at the evaporator exit. Pressure drops are modest; comparison made at the same evaporating and condensing pressures. Compressor is idealized with isentropic compression for the conceptual discussion. Concept / Approach: Superheating generally increases the refrigerant enthalpy at the compressor inlet, thereby increasing the refrigerating effect per kg (h1 − h4) because h1 rises while h4 (post-expansion enthalpy) is almost unchanged. Compressor work per kg (h2 − h1) also rises, but in many refrigerant–condition combinations the percentage increase in refrigerating effect is greater than the percentage increase in work, leading to a net increase in C.O.P. = (refrigerating effect)/(compressor work). Step-by-Step Solution: Define RE = h1 − h4 and W = h2 − h1.Introduce superheat: h1 increases; for similar pressure ratio, h2 increases but not as much proportionally.Compute C.O.P. = RE / W → often rises with moderate, controlled superheat.Conclusion: C.O.P. generally increases, especially when superheat is created in the evaporator adding useful capacity. Verification / Alternative check: Plot points on a p-h diagram for a common refrigerant (e.g., R-134a). Moving the suction point into the superheat region increases the area representing RE more than the increase in W for many practical cases. Why Other Options Are Wrong: No change: contradicts enthalpy changes.Decreases: can happen for extreme suction-line superheat with large pressure drops, but not as a general rule under controlled evaporator superheating.“Work to zero” is impossible; compression requires work. Common Pitfalls: Confusing evaporator superheat (beneficial capacity gain) with suction-line heat pickup (which may hurt C.O.P.). Proper design ensures controlled superheat at the evaporator outlet with minimal suction-line heat gain. Final Answer: Increases C.O.P.

Question 7

Seasonal effect on Carnot refrigerator performance Comparing seasons for the same refrigerated space temperature, how does the C.O.P. of a Carnot refrigerator in winter compare with its C.O.P. in summer?

Accepted Answer

Introduction / Context: The Carnot refrigerator offers a theoretical upper bound on performance: C.O.P._Carnot = TL / (TH − TL), where TL is the refrigerated space (cold reservoir) temperature and TH is the heat-rejection (hot reservoir) temperature. Seasonal ambient temperature shifts change TH for air-cooled systems, altering the C.O.P. Given Data / Assumptions: The refrigerated space temperature TL is the same in winter and summer. Heat is rejected to ambient air; thus TH is lower in winter and higher in summer. Idealized Carnot behavior used for comparison. Concept / Approach: Since C.O.P._Carnot = TL / (TH − TL), reducing TH (winter) decreases the denominator while TL stays fixed, thereby increasing the C.O.P. Conversely, a hotter ambient (summer) increases the denominator and reduces C.O.P. Hence the Carnot refrigerator is theoretically more efficient in winter. Step-by-Step Solution: Write the expression: COP = TL / (TH − TL).Hold TL constant; compare TH_winter < TH_summer.Compute qualitative change: denominator is smaller in winter → COP is larger.Conclude that the winter C.O.P. is higher than the summer C.O.P. Verification / Alternative check: Substitute representative values, e.g., TL = 273 K, TH_summer = 308 K vs TH_winter = 293 K. Then COP_summer = 273/(308−273) ≈ 7.8; COP_winter = 273/(293−273) ≈ 13.7, illustrating the increase. Why Other Options Are Wrong: Same or Lower: contradict the Carnot relationship when TH changes.“Unchanged if water-cooled”: even water-cooled systems experience seasonal drift unless the water is temperature-controlled; the theoretical statement remains tied to TH.Indeterminate: the direction is determinate under standard assumptions. Common Pitfalls: Mixing up C.O.P. for heat pumps vs refrigerators; a heat pump C.O.P. uses TH in the numerator but the same conclusion (improved performance at lower lift) holds qualitatively. Final Answer: Higher

Question 8

Control basis of a thermostatic expansion valve (TEV) A thermostatic expansion valve modulates refrigerant flow primarily in response to changes in which variable measured at the evaporator outlet?

Accepted Answer

Introduction / Context: Thermostatic expansion valves (TEVs or TXVs) are widely used to meter refrigerant into the evaporator while preventing liquid carryover to the compressor. Understanding their control feedback is essential for proper selection and troubleshooting. Given Data / Assumptions: A sensing bulb is strapped to the evaporator outlet (suction line) and filled with a charge. The valve experiences three forces: bulb pressure (opening), evaporator pressure (closing), and spring force (closing). Stable operation targets a set superheat, not just a fixed temperature or pressure. Concept / Approach: The TXV adjusts to maintain a set “superheat,” defined as the temperature rise above the saturation temperature corresponding to evaporator pressure at the outlet. The bulb temperature rises with superheat, increasing bulb pressure and opening the valve; low superheat reduces bulb pressure, closing the valve. Thus, the controlled variable is degree of superheat. Step-by-Step Solution: Measure suction temperature Ts at evaporator outlet and saturation temperature Tsat from suction pressure.Compute superheat SH = Ts − Tsat.Valve modulates to hold SH near a setpoint (e.g., 5–8 K).Stable SH ensures full evaporator utilization without risking liquid floodback. Verification / Alternative check: Manufacturer literature uniformly describes TEVs as “superheat controllers,” with bulb placement critical to accurate SH sensing. Why Other Options Are Wrong: Temperature only or pressure only: neither alone defines superheat; both are required to infer Tsat and compute SH.Condenser subcooling and discharge temperature are not the TXV control variables. Common Pitfalls: Relocating the bulb to a poor thermal contact point or after a suction-line heat exchanger can distort sensed SH and cause hunting. Final Answer: Degree of superheat at exit from the evaporator

Question 9

Prevost’s theory of exchanges in thermal radiation Evaluate the statement: Every body, regardless of its temperature, continuously emits radiation to and receives radiation from its surroundings at the same time.

Accepted Answer

Introduction / Context: Prevost’s theory of exchanges provides an intuitive basis for radiative heat transfer. It clarifies that emission and absorption are simultaneous processes and that net heat flow by radiation depends on temperature differences and surface properties. Given Data / Assumptions: Any real surface at finite temperature emits thermal radiation. The same surface can absorb incident radiation from its surroundings. Vacuum is not required; radiation propagates through transparent media as well. Concept / Approach: According to Prevost, all bodies emit radiation continuously as a function of their temperature and emissivity, and they also receive radiation from other bodies. Net radiative exchange equals emission minus absorption. A cooler body can still emit radiation; it simply absorbs more than it emits if surrounded by hotter surfaces, resulting in a net gain. Step-by-Step Solution: State emission: E_out = εσT^4 for an idealized diffuse–gray surface (conceptual form).State absorption: E_in depends on surroundings, view factors, and their temperatures.Net exchange q_rad = E_out − absorbed_incoming; sign determines heat gain or loss.Therefore, simultaneous emission and reception are always occurring. Verification / Alternative check: Kirchhoff’s law at equilibrium (ε = α) and the Stefan–Boltzmann relation support the continuous nature of emission and the role of absorption; at equilibrium net exchange is zero though two-way exchange persists. Why Other Options Are Wrong: “False” or restricted cases contradict the universal nature of radiative emission for T > 0 K.Polished metals and vacuum conditions are not prerequisites; they only alter amounts or paths of radiation. Common Pitfalls: Equating “net heat loss” with “no emission” when surroundings are hotter; remember both emission and absorption occur, and net direction depends on relative intensities. Final Answer: True

Question 10

Reading a pressure–enthalpy (p–h) diagram On a refrigerant p–h chart, which region corresponds to the superheated vapour state?

Accepted Answer

Introduction / Context: The pressure–enthalpy (p–h) diagram is the principal tool for visualizing vapour-compression cycles. Correctly identifying the single-phase regions avoids errors in reading states, enthalpy differences, and compressor work. Given Data / Assumptions: The two-phase region lies under the saturation dome bounded by the saturated liquid and saturated vapour lines. Single-phase liquid lies to the left of the saturated liquid line; single-phase vapour lies to the right of the saturated vapour line. Superheated vapour is single-phase vapour at temperatures above saturation for the given pressure. Concept / Approach: On the p–h chart, the saturated liquid line forms the left boundary of the dome and the saturated vapour line forms the right boundary. Any state to the right of the saturated vapour line is superheated vapour; any state between the lines is a liquid–vapour mixture; any state to the left of the saturated liquid line is subcooled (compressed) liquid. Step-by-Step Solution: Locate the saturation dome on the p–h diagram.Identify the right-hand boundary as the saturated vapour line.Recognize that all points to the right of this line represent superheated vapour.Thus, select “to the right of the saturated vapour line.” Verification / Alternative check: Cross-check with temperature–entropy (T–s) diagrams: superheat appears to the right of the saturated vapour line on T–s as well; both charts are consistent in topology. Why Other Options Are Wrong: Left of saturated liquid line: that is compressed liquid.Between the lines: two-phase mixture, not superheated.Exactly on the saturation lines: those are saturated states, not superheated. Common Pitfalls: Confusing superheat with subcooling; remember superheat applies to vapour above saturation, subcooling to liquid below saturation. Final Answer: To the right of the saturated vapour line

Question 11

Thermodynamic temperatures in saturated air For a saturated air–water vapour mixture (100% relative humidity), what is the relationship among dry-bulb temperature, wet-bulb temperature, and dew-point temperature?

Accepted Answer

Introduction / Context: In psychrometrics, the relation among dry-bulb (DBT), wet-bulb (WBT), and dew-point (DPT) temperatures depends on humidity level. Recognizing the equality at saturation (100% RH) is essential for interpreting charts and instruments. Given Data / Assumptions: Saturated air–water vapour mixture (RH = 100%). Steady-state conditions at a fixed barometric pressure. Thermometers and hygrometers are properly functioning. Concept / Approach: At saturation, the air is holding the maximum possible moisture at that temperature. The adiabatic evaporation that normally lowers a wetted-bulb thermometer cannot proceed because the air is already saturated; thus WBT equals DBT. Dew point equals the temperature at which condensation begins; in saturated air, the current temperature is already at the dew point, so DPT equals DBT as well. Therefore, all three temperatures coincide. Step-by-Step Solution: Define saturation condition: RH = 100%.Wet-bulb behavior: no net evaporation → WBT = DBT.Dew-point definition: temperature where saturation occurs for the current moisture content → DPT = current DBT at saturation.Therefore, DBT = WBT = DPT for saturated air. Verification / Alternative check: On the psychrometric chart, saturated states lie along the saturation curve where lines of constant DBT, WBT, and DPT intersect, confirming equality. Why Other Options Are Wrong: DBT higher than WBT or DPT: true only for unsaturated air (RH < 100%).WBT exceeding DBT: not physically achievable under standard definitions. Common Pitfalls: Assuming a small measurement error implies a fundamental difference. In perfect saturation, instrument readings should coincide; slight deviations are due to instrument accuracy or transient effects. Final Answer: Dry-bulb, wet-bulb, and dew-point temperatures are equal

Question 12

Refrigeration expansion devices: Why is a simple capillary tube generally not used in large-capacity refrigeration and air-conditioning systems?

Accepted Answer

Introduction / Context: Capillary tubes are fixed-orifice expansion devices widely used in small refrigerators and unitary systems. The question asks why this device is rarely selected for large-capacity plants, which typically prefer thermostatic or electronic expansion valves for control and efficiency. Given Data / Assumptions: Steady vapor-compression cycle with condenser, expansion device, evaporator, and compressor. Capillary tube is a long, small-diameter tube that produces a pressure drop mainly through friction. Large-capacity systems experience wide load variation and require modulation. Concept / Approach: The expansion device must match mass flow to evaporator load and maintain stable superheat. A capillary tube has a fixed flow characteristic; it cannot modulate in response to changing evaporator loads, ambient conditions, or compressor capacity. Thermostatic/EEV devices, in contrast, regulate flow using a sensing strategy to maintain superheat and avoid floodback. Step-by-Step Solution: Identify the control objective: maintain evaporator superheat within limits.Note the capillary tube has fixed Δp–flow behavior determined by length and diameter.For large systems with variable loads, a fixed orifice cannot adjust the mass flow → unstable superheat or poor capacity utilization.Therefore, capacity control is not possible, motivating the use of TEV/EEV. Verification / Alternative check: Compare part-load performance: TEV/EEV maintains target superheat over a range of condensing/evaporating pressures; a capillary does not, resulting in hunting, starved or flooded evaporator conditions. Why Other Options Are Wrong: Required pressure drop cannot be achieved: In practice, proper length/diameter can achieve Δp; not the main limitation. Material incompatibility: Copper is commonly compatible with many refrigerants and oils. High cost: Capillaries are cheap; cost is not the reason for avoidance. Cavitation in condenser: Not a design goal and not a reason. Common Pitfalls: Assuming longer capillaries solve control issues; length changes design Δp, not dynamic capacity control. Also, forgetting seasonal ambient swings that shift condensing pressure and hence mass flow through a fixed orifice. Final Answer: Capacity control is not possible with a plain capillary tube

Question 13

Thermal diffusivity comparison: In general, the thermal diffusivity of gases is ________ that of liquids.

Accepted Answer

Introduction / Context: Thermal diffusivity indicates how quickly temperature disturbances propagate through a material. It is crucial for transient heat transfer analysis in fluids such as air (gas) and water (liquid). Given Data / Assumptions: Thermal diffusivity h (often denoted alpha) defined as h = k / (rho * cp). Typical properties at room temperature for common gases and liquids. No phase change; homogeneous fluids considered. Concept / Approach: For gases, density rho and specific heat capacity per unit volume rhocp are relatively small compared with liquids. Although thermal conductivity k of gases is lower than that of liquids, the much smaller thermal inertia (rhocp) in gases makes h larger for gases in many practical cases. Step-by-Step Solution: Start with h = k / (rho * cp).Compare orders of magnitude: k_gas < k_liquid but (rhocp)_gas ≪ (rhocp)_liquid.Because denominator drops faster for gases, h_gas tends to be greater than h_liquid.Therefore, temperature fields in gases adjust more rapidly for the same length scale. Verification / Alternative check: Typical values: h_air ~ 2.1e-5 m^2/s; h_water ~ 1.4e-7 m^2/s. The gas value is orders of magnitude higher, supporting the conclusion. Why Other Options Are Wrong: Equal to / approximately half / independent of phase: contradict property data trends. Less than: ignores the dominant effect of much larger volumetric heat capacity of liquids. Common Pitfalls: Confusing conductivity with diffusivity; a higher k does not guarantee higher h if rho*cp is also much higher. Final Answer: greater than

Question 14

Psychrometric chart reference: The “alignment circle” commonly marked on the comfort psychrometric chart corresponds to which dry-bulb temperature (DBT) and relative humidity (RH)?

Accepted Answer

Introduction / Context: Many psychrometric charts used in HVAC (e.g., comfort charts) include an “alignment circle” or reference point to help align scales or to denote a representative comfort condition. Knowing this reference aids quick estimations and chart navigation. Given Data / Assumptions: Standard comfort psychrometric charts used in HVAC education and practice. Alignment circle denotes a nominal comfort state near mid-range humidity. Units in SI (°C, % RH). Concept / Approach: Common charts mark a reference near 26 °C DBT and 50% RH (approximately 80 °F and 50% RH). This lies centrally in the comfort region and provides convenient alignment for scales like enthalpy or specific volume lines used in quick calculations. Step-by-Step Solution: Identify typical comfort zone: DBT roughly 22–26 °C and RH 40–60%.Select the central reference often used by chart publishers: 26 °C and 50% RH.Confirm that this point falls close to mid-range properties, simplifying alignment tasks. Verification / Alternative check: Cross-checking standard HVAC psychrometric charts shows a reference/marker near 26 °C, 50% RH; while exact labeling may vary by publisher, 26 °C and 50% RH is a widely used reference. Why Other Options Are Wrong: 20 °C combinations: Cooler than typical alignment reference used for comfort chart alignment. 26 °C at 60% RH: In comfort zone but not the commonly marked alignment circle. 24 °C, 50% RH: Reasonable comfort point but not the standard alignment marker. Common Pitfalls: Assuming all charts use identical markers; some charts are customized. For general exam purposes, 26 °C and 50% RH is the accepted reference. Final Answer: 26 °C DBT and 50% RH

Question 15

Definition of 1 tonne of refrigeration (1 TR): What is the corresponding heat removal capacity expressed in kJ per minute?

Accepted Answer

Introduction / Context: “Tonne of refrigeration” is a legacy but still common capacity unit in HVAC&R. Converting 1 TR into consistent SI energy-rate units is essential for load calculations and equipment comparison. Given Data / Assumptions: By definition, 1 TR = heat required to freeze 1 short ton (2000 lb) of water at 0 °C into ice at 0 °C in 24 hours. Latent heat of fusion of water ≈ 334 kJ/kg. 1 short ton = 2000 lb ≈ 907 kg. Concept / Approach: Compute total heat removed and divide by time to obtain power. Then convert to kJ/min for direct comparison with the options. Step-by-Step Solution: Mass m ≈ 907 kg.Total heat Q_total = m * L_f ≈ 907 * 334 kJ ≈ 302,938 kJ.Time = 24 h = 24 * 60 = 1440 min.Rate Q_dot = Q_total / time ≈ 302,938 / 1440 ≈ 210.4 kJ/min.Therefore, 1 TR ≈ 210 kJ/min ≈ 3.517 kW. Verification / Alternative check: Using the well-known identity: 1 TR = 12,000 Btu/h ≈ 3.517 kW; 3.517 kW * 60 s/min ≈ 211 kJ/min, consistent with 210 kJ/min by rounding. Why Other Options Are Wrong: 21 and 351.7 kJ/min: 21 is a factor-of-10 error; 351.7 corresponds to 5.86 kW, not 1 TR. 420 and 620 kJ/min: Overstate the capacity (about 2 TR and 3 TR). Common Pitfalls: Confusing short ton and metric ton; assuming 1 TR equals 4 kW exactly (it does not). Final Answer: 210 kJ/min

Question 16

Property comparison: “The freezing point of carbon dioxide is equal to the boiling point of refrigerant R-12.” State whether this statement is correct.

Accepted Answer

Introduction / Context: Knowing basic thermophysical properties of common refrigerants and gases is important for safe design and selection. This statement compares the freezing point (or sublimation point) of CO₂ with the normal boiling point of R-12. Given Data / Assumptions: Carbon dioxide (CO₂) at 1 atm sublimates at about −78.5 °C and has a triple point near −56.6 °C. Refrigerant R-12 (dichlorodifluoromethane) normal boiling point at 1 atm is about −29.8 °C. Standard pressure comparison unless otherwise specified. Concept / Approach: Compare characteristic temperatures at standard conditions. The CO₂ freezing/sublimation and R-12 boiling points are very different, separated by tens of degrees Celsius, so they are not equal. Step-by-Step Solution: Identify CO₂ temperature: approximately −78.5 °C at 1 atm (sublimation).Identify R-12 boiling point: approximately −29.8 °C at 1 atm.Compare: −78.5 °C ≠ −29.8 °C → statement is false. Verification / Alternative check: Consult standard property tables: the large difference is consistent across reputable sources and is independent of unit system. Why Other Options Are Wrong: Conditional “correct” options presume pressure/mixture effects; the statement is absolute and thus incorrect. Common Pitfalls: Confusing sublimation (solid to gas) with melting/freezing or boiling points; overlooking pressure dependence of phase change temperatures. Final Answer: Incorrect

Question 17

Stefan–Boltzmann law: For an ideal black body, the total emissive power per unit area is directly proportional to which function of absolute temperature?

Accepted Answer

Introduction / Context: Thermal radiation from surfaces is central to high-temperature heat transfer. The Stefan–Boltzmann law provides the fundamental relation for the total emissive power of a black body. Given Data / Assumptions: Ideal black body surface (emissivity = 1). Absolute temperature T in Kelvin. Stefan–Boltzmann constant sigma is a universal constant. Concept / Approach: The law states: E_b = sigma * T^4 where E_b is the emissive power (W/m^2). The fourth-power dependence means small increases in temperature cause large increases in radiative heat emission. Step-by-Step Solution: Recognize black body assumption → emissivity = 1.Apply Stefan–Boltzmann equation: E_b proportional to T^4.Conclude the correct functional dependence is the fourth power. Verification / Alternative check: Compare with gray body: E = epsilon * sigma * T^4; the exponent remains 4; epsilon only scales magnitude. Why Other Options Are Wrong: T, T^2, T^3, log(T): none capture the steep growth rate observed and proven in radiation physics; only T^4 matches experiments and theory. Common Pitfalls: Confusing the Stefan–Boltzmann law with Wien’s displacement law (which relates peak wavelength and temperature). Final Answer: T^4 (fourth power of absolute temperature)

Question 18

Aircraft refrigeration: How many heat exchangers are employed in a standard boot-strap air-cooling system used for high-speed aircraft?

Accepted Answer

Introduction / Context: Air-cycle (reverse Brayton) refrigeration systems are common in aviation. The boot-strap arrangement improves cooling capacity at high flight speeds by using compressor-driven pressure rise and staged heat rejection. Given Data / Assumptions: High-speed aircraft with ram air available for heat rejection. Boot-strap system incorporates an auxiliary compressor driven by a turbine. Standard schematic includes primary and secondary heat exchangers. Concept / Approach: In the boot-strap system, ambient (ram) air first cools compressed bleed air in the primary heat exchanger. The air is then further compressed by an auxiliary compressor and cooled again in a secondary heat exchanger before expansion in a turbine to produce refrigeration. Step-by-Step Solution: Ram air provides sink for heat rejection.Primary heat exchanger: cools bleed air after main compression.Secondary heat exchanger: cools air after auxiliary compression (boot-strap stage).Air then expands in turbine to deliver low-temperature supply to cabin. Verification / Alternative check: System block diagrams consistently show two distinct heat exchangers labeled “primary” and “secondary.” Why Other Options Are Wrong: One HX: insufficient for staged cooling after both compression steps. Three or four HX: not standard for classical boot-strap; would be heavier and more complex. None: contradicts the need for rejecting compression heat. Common Pitfalls: Confusing boot-strap with simple air-cycle (one HX) or regenerative variants; diagrams differ by configuration. Final Answer: Two heat exchangers

Question 19

Logarithmic mean temperature difference (LMTD): Given inlet and outlet temperature differences Δt1 and Δt2 between hot and cold streams, the correct expression for the mean temperature difference is

Accepted Answer

Introduction / Context: The LMTD method is widely used in heat exchanger analysis to compute heat transfer rate when terminal temperatures are known. The right formula is essential for accurate design. Given Data / Assumptions: Two fluids exchanging heat with constant overall heat transfer coefficient U and area A. Terminal temperature differences Δt1 and Δt2 at the two ends of the exchanger. No phase change and steady state for simplicity. Concept / Approach: Since temperature difference varies exponentially along the length, the appropriate average is the logarithmic mean, not the arithmetic or geometric mean. Heat duty is Q = U * A * tm where tm = (Δt1 - Δt2) / ln(Δt1 / Δt2). Step-by-Step Solution: Write local heat transfer: dQ = U * dA * Δt(x).Integrate along length using boundary conditions to derive tm.Obtain tm = (Δt1 - Δt2) / ln(Δt1 / Δt2).Use Q = U * A * tm to compute duty. Verification / Alternative check: Limiting case Δt1 → Δt2 gives tm → Δt1 (using L’Hôpital’s rule), matching physical expectation of uniform Δt. Why Other Options Are Wrong: Arithmetic or geometric means do not satisfy the exponential temperature profile implied by energy balance. ln(Δt1 * Δt2) and (Δt1 - Δt2) * ln(Δt1 / Δt2) are dimensionally or mathematically incorrect. Common Pitfalls: Swapping Δt1 and Δt2 positions or using base-10 logarithm incorrectly (natural logarithm is standard in derivation). Final Answer: tm = (Δt1 - Δt2) / ln(Δt1 / Δt2)

Question 20

Fluid mechanics: The Reynolds number (Re) represents the ratio of which two characteristic forces in a flowing fluid?

Accepted Answer

Introduction / Context: The Reynolds number is a fundamental dimensionless parameter used to predict flow regime (laminar, transitional, or turbulent) in internal and external flows. It condenses geometry, velocity, and fluid properties into a single criterion. Given Data / Assumptions: Characteristic length L (e.g., pipe diameter D). Average flow velocity V. Kinematic viscosity nu or dynamic viscosity mu and density rho. Concept / Approach: Re is formed from the ratio of inertia to viscous effects. Using dynamic variables: Re = (rho * V * L) / mu or Re = V * L / nu. High Re implies dominance of inertia over viscous forces, favoring turbulence; low Re implies viscous forces dominate, favoring laminar flow. Step-by-Step Solution: Identify inertia term scale: ~ rho * V^2 acting over area, characteristic to fluid momentum.Identify viscous term scale: ~ mu * V / L reflecting internal friction.Form the ratio: (rho * V^2) / (mu * V / L) → (rho * V * L) / mu = Re.Interpretation: Re measures relative importance of momentum advection to viscous diffusion. Verification / Alternative check: Compare canonical thresholds: in a smooth circular pipe, laminar flow typically persists for Re < 2000, while Re > ~4000 indicates turbulence (with a transitional band between). Why Other Options Are Wrong: Convection/conduction ratio and kinematic viscosity/thermal diffusivity correspond to other dimensionless groups (e.g., Peclet or Prandtl numbers), not Reynolds. Elastic/pressure or buoyancy/gravity are unrelated to Re definition. Common Pitfalls: Confusing Pr = nu/alpha with Re; using inconsistent length or velocity scales leads to misclassification of flow regime. Final Answer: Inertia force to viscous force

Heat Transfer, Refrigeration and Air Conditioning Questions