Q: Radiation basics: Heat transfer by thermal radiation proceeds primarily from a

Introduction / Context: Thermal radiation is energy emitted by matter due to its temperature. All bodies above absolute zero emit radiation; net exchange between two bodies proceeds from the higher temperature body to the lower temperature one until radiative equilibrium is reached. Given Data / Assumptions: Two surfaces with different absolute temperatures. Line-of-sight or participating medium not strongly absorbing (for simplicity). View factors and emissivities define the net exchange. Concept / Approach: Each surface emits according to its emissive power; the cooler one also emits but less (for similar emissivity and area). Net radiative heat flow equals emitted minus absorbed energy, yielding a direction from hot to cold when temperatures differ. Step-by-Step Reasoning: 1) Hot body: higher emissive power.2) Cold body: lower emissive power.3) Net = emission_hot − absorption_cold in mutual exchange → direction from hot to cold. Verification / Alternative check: The Stefan–Boltzmann relation shows emissive power scales with T^4; higher T produces stronger emission, ensuring net flow from hotter to colder body. Why Other Options Are Wrong: Cold to hot violates the second law for spontaneous processes. “Smaller to larger” and “larger to smaller” confuse geometry with thermodynamic direction. Transparency/opacity does not set direction; temperature difference does. Common Pitfalls: Ignoring that both bodies emit; the net is dictated by temperature and radiative properties, not by size alone. Final Answer: hot body to a cold body

Q: Reversed Carnot refrigerator: The coefficient of performance (COP) in terms of absolute temperatures T1 (low) and T2 (high) is

Introduction / Context: The reversed Carnot cycle represents the upper theoretical limit of refrigeration performance operating between a cold reservoir at T1 and a hot reservoir at T2 (absolute temperatures). Its COP provides a benchmark for real systems. Given Data / Assumptions: Ideal, reversible cycle between T1 (evaporator temperature) and T2 (condenser temperature). No pressure drops or non-idealities. Temperatures in Kelvin (or Rankine). Concept / Approach: For a refrigerator, COP = desired effect / input = Q_L / W. For the Carnot refrigerator, Q_L / Q_H = T1 / T2 and W = Q_H − Q_L. Combine relations to obtain COP in terms of T1 and T2 only. Step-by-Step Derivation: 1) From reversibility: Q_L / T1 = Q_H / T2.2) Therefore Q_H = Q_L * (T2 / T1).3) Work W = Q_H − Q_L = Q_L * (T2 / T1 − 1) = Q_L * (T2 − T1) / T1.4) COP = Q_L / W = T1 / (T2 − T1). Verification / Alternative check: Dimensionless check: temperatures cancel to a pure number; as T2 approaches T1, COP → ∞, consistent with the limiting reversible case. Why Other Options Are Wrong: T2/(T2 − T1) is for a heat pump COP, not a refrigerator. (T2 − T1)/T2 or /T1 and T1/T2 are not the Carnot refrigerator COP. Common Pitfalls: Confusing refrigerator COP with heat pump COP; always confirm whether the numerator is Q_L or Q_H. Final Answer: COP = T1 / (T2 - T1)

Q: Capillary control in domestic refrigerators: The small-bore capillary tube meters refrigerant flow from the

Introduction / Context: Domestic refrigerators commonly use a fixed-orifice capillary tube as the expansion device. It meters liquid refrigerant and creates the pressure drop required between condenser (high side) and evaporator (low side). Understanding its placement clarifies system flow paths and diagnostics. Given Data / Assumptions: Standard single-evaporator domestic fridge with capillary tube. Condenser outlet provides subcooled liquid to minimize flash gas. Evaporator operates at low pressure to absorb heat. Concept / Approach: The capillary connects the high-pressure liquid line at the condenser outlet to the evaporator inlet. The long, narrow tube’s frictional resistance establishes the throttling pressure drop. Downstream, a two-phase mixture forms and evaporates, absorbing heat from the refrigerated space. Step-by-Step Reasoning: 1) Condenser → liquid refrigerant at high pressure.2) Capillary tube → throttling (isenthalpic approximation) to low pressure.3) Evaporator → liquid–vapour mixture absorbs heat and fully vaporizes. Verification / Alternative check: Exploded schematics of household refrigerators consistently show capillary from condenser outlet (receiver optional) to evaporator inlet. Why Other Options Are Wrong: (a) Mentions an “expansion valve”; capillary itself is the expansion device. (b) Thermostat is a control device, not a flow path. (c) Redundant with a separate expansion valve; not applicable here. Common Pitfalls: Ignoring the need for liquid at the capillary inlet; insufficient subcooling can cause flash gas and starve the evaporator. Final Answer: condenser to the evaporator

Q: Summer air-conditioning application: The process most commonly used to lower air temperature and remove moisture simultaneously is called

Introduction / Context: In hot and humid climates, summer comfort conditioning requires both sensible cooling (lower dry-bulb temperature) and latent cooling (moisture removal). Coil-based systems accomplish this by passing moist air over a cold surface below the air’s dew-point temperature. Given Data / Assumptions: Cooling coil surface temperature less than the entering air dew-point. Air flows at nearly constant pressure through the coil. Condensate drains remove condensed moisture. Concept / Approach: As warm, humid air contacts a cold coil, it cools sensibly toward the apparatus dew-point and simultaneously crosses the saturation curve, condensing moisture. On a psychrometric chart, the process line slopes down-left toward lower DBT and lower humidity ratio, embodying “cooling and dehumidification.” Step-by-Step Reasoning: 1) Lower coil surface temperature below entering air dew-point.2) Air cools sensibly until reaching saturation at the surface.3) Further cooling causes condensation → humidity ratio decreases.4) Leaving air is cooler and drier, improving thermal comfort. Verification / Alternative check: Psychrometric calculations using coil bypass factor show simultaneous reductions in DBT and humidity ratio when coil ADP is below entering air dew-point. Why Other Options Are Wrong: Humidification adds moisture, not removes it. Dehumidification alone omits the required cooling step. Heating and humidification is a winter process. Adiabatic saturation (evaporative cooling) increases humidity rather than removing it. Common Pitfalls: Mistaking evaporative coolers for dehumidifiers; evaporative devices raise humidity and are unsuitable for humid climates without desiccant stages. Final Answer: cooling and dehumidification

Q: Expansion devices in small vapour-compression systems Where is a capillary tube commonly used as the expansion device?

Introduction / Context: In vapour-compression refrigeration, the expansion device throttles high-pressure liquid refrigerant to the low-pressure evaporator. For small and medium capacities, the simplest device is a capillary tube. Knowing where capillary tubes are appropriate is key for design, troubleshooting, and exam questions. Given Data / Assumptions: Systems considered: domestic refrigerator, water cooler, and room air-conditioner. Typical capacities are relatively low (from a few hundred watts to a few tons of refrigeration). Steady operation with modest load variation. Concept / Approach: A capillary tube is a long, small-bore tube providing fixed flow resistance. The pressure drop depends on length, internal diameter, and refrigerant mass flow. It is inexpensive, has no moving parts, and works well when the evaporator load is fairly steady and when on-off or reciprocating compressors are used. Step-by-Step Solution: Identify applications that fit small, steady-capacity duty: domestic fridges, water coolers, room A/C.Recognize capillary tube advantages: low cost, simplicity, self-compensating during off-cycle due to pressure equalization.Conclude that all listed small systems routinely employ capillary tubes as standard expansion devices. Verification / Alternative check: Manufacturer service manuals for refrigerators, direct-expansion water coolers, and room A/C units routinely list capillary tubes in parts diagrams, confirming standard usage in these categories. Why Other Options Are Wrong: Singling out only one of the three would ignore the widespread practice across all these small-capacity systems. Very large chillers, however, typically use thermostatic or electronic expansion valves to cope with large load variations. Common Pitfalls: Confusing capillary tubes with thermostatic or electronic expansion valves; assuming capillary tubes can regulate superheat dynamically (they cannot). Final Answer: All of these

Q: Using waste heat effectively In which refrigeration system can waste heat be productively used to drive the cycle?

Introduction / Context: Industrial plants often have low-grade waste heat that can be harnessed to drive cooling. The selection of a refrigeration cycle that uses heat rather than mechanical work is the key to improving overall plant efficiency. Given Data / Assumptions: Waste heat is available as hot water, steam, or exhaust gases. Goal: use this heat to produce refrigeration. Systems compared: vapour compression, vapour absorption, and air cycles. Concept / Approach: Vapour absorption refrigeration (e.g., ammonia–water or lithium bromide–water) uses a thermal compressor (generator, absorber, solution pump) driven primarily by heat input. In contrast, vapour compression relies on mechanical compressors driven by shaft work. Air cycles (reverse Brayton) also require mechanical compression and are not heat-driven. Step-by-Step Solution: Identify the driver for each cycle: absorption → heat; compression/air → mechanical work.Where waste heat exists, absorption cycles can convert that into cooling effect by powering the generator.Hence, the correct choice is vapour absorption cycle. Verification / Alternative check: Combined heat and power (CHP) systems often integrate absorption chillers to utilize waste heat for chilled water production. Why Other Options Are Wrong: Vapour compression and air cycles require mechanical power; waste heat does not directly drive them. Thermoelectric systems are powered electrically, not by heat input. Common Pitfalls: Assuming that any “refrigeration system” can accept waste heat; in practice only absorption (and adsorption) systems are designed to be heat-driven. Final Answer: Vapour absorption cycle

Q: Psychrometrics — heating with dehumidification During a combined process of heating and dehumidification, how does the dry-bulb temperature of the air change?

Introduction / Context: Air-handling systems may heat air while also removing moisture (e.g., by desiccant dehumidifiers with a downstream heater). Understanding the direction of property changes is crucial for plotting on the psychrometric chart and sizing coils. Given Data / Assumptions: Simultaneous heat addition and moisture removal. No evaporative cooling effects overwhelming the heating. Standard atmospheric pressure conditions for the chart. Concept / Approach: Heating adds sensible energy to the air, raising its dry-bulb temperature. Dehumidification reduces humidity ratio. On the psychrometric chart, this is typically an inclined path up-and-left or vertical then right depending on process details; in any case, dry-bulb temperature rises when net heating is applied. Step-by-Step Solution: Recognize heating → T_db increases.Recognize dehumidification → humidity ratio w decreases.Combine effects: T_db increases while w decreases. Verification / Alternative check: Energy balance for moist air: Δh ≈ c_p,ma * ΔT_db + w * Δh_fg + h_vapour changes; with added sensible heat and reduced moisture, ΔT_db is positive. Why Other Options Are Wrong: Constant or decreasing dry-bulb contradicts heating; equality with wet-bulb happens only at saturation and is not an outcome of this general process; oscillation is not a defined thermodynamic trend. Common Pitfalls: Confusing this with cooling and dehumidification, where T_db falls; or with chemical dehumidification at constant T_db (requires nearly isothermal conditions). Final Answer: Increases

Q: Reading the p–h diagram On a pressure–enthalpy (p–h) chart for a refrigerant, the region to the left of the saturated liquid line represents which condition?

Introduction / Context: Pressure–enthalpy charts are indispensable for refrigeration analysis. Correctly identifying regions (subcooled liquid, two-phase mixture, superheated vapour) ensures accurate cycle plotting and state estimation. Given Data / Assumptions: Standard p–h chart below the critical point. Saturated liquid line and saturated vapour line form the two-phase dome. Concept / Approach: The saturated liquid line (left boundary of the dome) represents liquid at the verge of boiling. States to the immediate left are still liquid but at a temperature lower than saturation at that pressure, i.e., subcooled (compressed) liquid. To the right of the saturated vapour line is superheated vapour; under the dome is the wet region. Step-by-Step Solution: Locate saturated liquid line (left boundary of dome).Identify states left of this line as liquid with temperature below saturation → subcooled liquid.Therefore, the correct region is sub-cooled (compressed) liquid. Verification / Alternative check: Cycle plotting shows condenser outlet often in subcooled region to improve COP and avoid flash gas at the expansion device. Why Other Options Are Wrong: Wet vapour is under the dome; superheat is to the right of the saturated vapour line; transcritical requires crossing the critical point and is not determined simply by being left of the saturated liquid line. Common Pitfalls: Assuming any point left of the dome is invalid; it is a legitimate compressed liquid state. Final Answer: Sub-cooled (compressed) liquid region

Q: Methods of achieving undercooling (subcooling) Which practical method can be used to obtain liquid undercooling in a vapour-compression system?

Introduction / Context: Subcooling the liquid leaving the condenser reduces flash gas after expansion, boosting the net refrigerating effect and often improving COP. There are several practical ways to achieve subcooling. Given Data / Assumptions: Liquid at condenser outlet is at or near saturation temperature. Goal: reduce its temperature below saturation at the condensing pressure. Concept / Approach: Undercooling can be achieved by enhancing heat rejection at the condenser (more or colder cooling water) or by internal heat exchange where cold suction vapour cools the liquid line. All these methods remove sensible heat from high-pressure liquid to move it into the subcooled region on the p–h chart. Step-by-Step Solution: Option (a): more cooling water → greater heat transfer → lower liquid-line temperature.Option (b): colder cooling water → increased temperature difference → more subcooling.Option (c): liquid–suction heat exchanger → sensible heat transfer from liquid to suction vapour, providing subcooling while superheating suction slightly.Hence, any of the above methods can achieve undercooling. Verification / Alternative check: System logs show lower liquid-line temperature than saturation temperature at condensing pressure when subcooling is present. Why Other Options Are Wrong: Each of (a)–(c) is valid; rejecting them individually would be incorrect. Two-stage throttling does not inherently subcool the high-pressure liquid; it primarily reduces pressure in stages. Common Pitfalls: Over-subcooling can raise compressor discharge superheat via liquid–suction exchange and may reduce evaporator capacity if not balanced. Final Answer: Any one of the above

Question 1

Bell–Coleman (air) refrigerator comparison: For the same temperature range, a dense (closed) air system requires __________ power per tonne of refrigeration compared to an open air system.

Accepted Answer

Introduction / Context: Air-cycle refrigeration can operate as open or dense (closed) systems. For a fixed evaporator and condenser temperature range, practical implementations show the dense system achieving better performance (higher COP), which directly translates to lower shaft power per tonne of refrigeration (TR). Given Data / Assumptions: Identical temperature limits. Comparable component efficiencies. Closed system maintains elevated mean pressure and better recuperation. Concept / Approach: Power per TR is inversely related to COP: Power/TR = 211 kJ/min per TR / COP in consistent units. If COP increases in the dense system due to reduced irreversibilities, improved heat exchange, and better control, then required power per TR decreases. Step-by-Step Reasoning: 1) Dense system → higher density → smaller specific volume → reduced compressor work per unit cooling.2) Enhanced recuperation → larger refrigeration effect for a given work.3) Therefore COP_dense > COP_open → Power/TR_dense < Power/TR_open. Verification / Alternative check: Cycle analyses on T–s diagrams confirm lower specific work for the dense system, all else equal. Why Other Options Are Wrong: Same/indeterminate: contradicts established comparative practice. Higher: would imply COP is lower, which is not the typical case for dense systems. “Much higher at low pressure ratios only” is not a general truth. Common Pitfalls: Assuming ideal cycles yield identical COP; practical differences in heat-exchanger effectiveness and mixing losses matter. Final Answer: lower

Question 2

Radiation basics: Heat transfer by thermal radiation proceeds primarily from a

Accepted Answer

Introduction / Context: Thermal radiation is energy emitted by matter due to its temperature. All bodies above absolute zero emit radiation; net exchange between two bodies proceeds from the higher temperature body to the lower temperature one until radiative equilibrium is reached. Given Data / Assumptions: Two surfaces with different absolute temperatures. Line-of-sight or participating medium not strongly absorbing (for simplicity). View factors and emissivities define the net exchange. Concept / Approach: Each surface emits according to its emissive power; the cooler one also emits but less (for similar emissivity and area). Net radiative heat flow equals emitted minus absorbed energy, yielding a direction from hot to cold when temperatures differ. Step-by-Step Reasoning: 1) Hot body: higher emissive power.2) Cold body: lower emissive power.3) Net = emission_hot − absorption_cold in mutual exchange → direction from hot to cold. Verification / Alternative check: The Stefan–Boltzmann relation shows emissive power scales with T^4; higher T produces stronger emission, ensuring net flow from hotter to colder body. Why Other Options Are Wrong: Cold to hot violates the second law for spontaneous processes. “Smaller to larger” and “larger to smaller” confuse geometry with thermodynamic direction. Transparency/opacity does not set direction; temperature difference does. Common Pitfalls: Ignoring that both bodies emit; the net is dictated by temperature and radiative properties, not by size alone. Final Answer: hot body to a cold body

Question 3

Reversed Carnot refrigerator: The coefficient of performance (COP) in terms of absolute temperatures T1 (low) and T2 (high) is

Accepted Answer

Introduction / Context: The reversed Carnot cycle represents the upper theoretical limit of refrigeration performance operating between a cold reservoir at T1 and a hot reservoir at T2 (absolute temperatures). Its COP provides a benchmark for real systems. Given Data / Assumptions: Ideal, reversible cycle between T1 (evaporator temperature) and T2 (condenser temperature). No pressure drops or non-idealities. Temperatures in Kelvin (or Rankine). Concept / Approach: For a refrigerator, COP = desired effect / input = Q_L / W. For the Carnot refrigerator, Q_L / Q_H = T1 / T2 and W = Q_H − Q_L. Combine relations to obtain COP in terms of T1 and T2 only. Step-by-Step Derivation: 1) From reversibility: Q_L / T1 = Q_H / T2.2) Therefore Q_H = Q_L * (T2 / T1).3) Work W = Q_H − Q_L = Q_L * (T2 / T1 − 1) = Q_L * (T2 − T1) / T1.4) COP = Q_L / W = T1 / (T2 − T1). Verification / Alternative check: Dimensionless check: temperatures cancel to a pure number; as T2 approaches T1, COP → ∞, consistent with the limiting reversible case. Why Other Options Are Wrong: T2/(T2 − T1) is for a heat pump COP, not a refrigerator. (T2 − T1)/T2 or /T1 and T1/T2 are not the Carnot refrigerator COP. Common Pitfalls: Confusing refrigerator COP with heat pump COP; always confirm whether the numerator is Q_L or Q_H. Final Answer: COP = T1 / (T2 - T1)

Question 4

Capillary control in domestic refrigerators: The small-bore capillary tube meters refrigerant flow from the

Accepted Answer

Introduction / Context: Domestic refrigerators commonly use a fixed-orifice capillary tube as the expansion device. It meters liquid refrigerant and creates the pressure drop required between condenser (high side) and evaporator (low side). Understanding its placement clarifies system flow paths and diagnostics. Given Data / Assumptions: Standard single-evaporator domestic fridge with capillary tube. Condenser outlet provides subcooled liquid to minimize flash gas. Evaporator operates at low pressure to absorb heat. Concept / Approach: The capillary connects the high-pressure liquid line at the condenser outlet to the evaporator inlet. The long, narrow tube’s frictional resistance establishes the throttling pressure drop. Downstream, a two-phase mixture forms and evaporates, absorbing heat from the refrigerated space. Step-by-Step Reasoning: 1) Condenser → liquid refrigerant at high pressure.2) Capillary tube → throttling (isenthalpic approximation) to low pressure.3) Evaporator → liquid–vapour mixture absorbs heat and fully vaporizes. Verification / Alternative check: Exploded schematics of household refrigerators consistently show capillary from condenser outlet (receiver optional) to evaporator inlet. Why Other Options Are Wrong: (a) Mentions an “expansion valve”; capillary itself is the expansion device. (b) Thermostat is a control device, not a flow path. (c) Redundant with a separate expansion valve; not applicable here. Common Pitfalls: Ignoring the need for liquid at the capillary inlet; insufficient subcooling can cause flash gas and starve the evaporator. Final Answer: condenser to the evaporator

Question 5

Summer air-conditioning application: The process most commonly used to lower air temperature and remove moisture simultaneously is called

Accepted Answer

Introduction / Context: In hot and humid climates, summer comfort conditioning requires both sensible cooling (lower dry-bulb temperature) and latent cooling (moisture removal). Coil-based systems accomplish this by passing moist air over a cold surface below the air’s dew-point temperature. Given Data / Assumptions: Cooling coil surface temperature less than the entering air dew-point. Air flows at nearly constant pressure through the coil. Condensate drains remove condensed moisture. Concept / Approach: As warm, humid air contacts a cold coil, it cools sensibly toward the apparatus dew-point and simultaneously crosses the saturation curve, condensing moisture. On a psychrometric chart, the process line slopes down-left toward lower DBT and lower humidity ratio, embodying “cooling and dehumidification.” Step-by-Step Reasoning: 1) Lower coil surface temperature below entering air dew-point.2) Air cools sensibly until reaching saturation at the surface.3) Further cooling causes condensation → humidity ratio decreases.4) Leaving air is cooler and drier, improving thermal comfort. Verification / Alternative check: Psychrometric calculations using coil bypass factor show simultaneous reductions in DBT and humidity ratio when coil ADP is below entering air dew-point. Why Other Options Are Wrong: Humidification adds moisture, not removes it. Dehumidification alone omits the required cooling step. Heating and humidification is a winter process. Adiabatic saturation (evaporative cooling) increases humidity rather than removing it. Common Pitfalls: Mistaking evaporative coolers for dehumidifiers; evaporative devices raise humidity and are unsuitable for humid climates without desiccant stages. Final Answer: cooling and dehumidification

Question 6

Expansion devices in small vapour-compression systems Where is a capillary tube commonly used as the expansion device?

Accepted Answer

Introduction / Context: In vapour-compression refrigeration, the expansion device throttles high-pressure liquid refrigerant to the low-pressure evaporator. For small and medium capacities, the simplest device is a capillary tube. Knowing where capillary tubes are appropriate is key for design, troubleshooting, and exam questions. Given Data / Assumptions: Systems considered: domestic refrigerator, water cooler, and room air-conditioner. Typical capacities are relatively low (from a few hundred watts to a few tons of refrigeration). Steady operation with modest load variation. Concept / Approach: A capillary tube is a long, small-bore tube providing fixed flow resistance. The pressure drop depends on length, internal diameter, and refrigerant mass flow. It is inexpensive, has no moving parts, and works well when the evaporator load is fairly steady and when on-off or reciprocating compressors are used. Step-by-Step Solution: Identify applications that fit small, steady-capacity duty: domestic fridges, water coolers, room A/C.Recognize capillary tube advantages: low cost, simplicity, self-compensating during off-cycle due to pressure equalization.Conclude that all listed small systems routinely employ capillary tubes as standard expansion devices. Verification / Alternative check: Manufacturer service manuals for refrigerators, direct-expansion water coolers, and room A/C units routinely list capillary tubes in parts diagrams, confirming standard usage in these categories. Why Other Options Are Wrong: Singling out only one of the three would ignore the widespread practice across all these small-capacity systems. Very large chillers, however, typically use thermostatic or electronic expansion valves to cope with large load variations. Common Pitfalls: Confusing capillary tubes with thermostatic or electronic expansion valves; assuming capillary tubes can regulate superheat dynamically (they cannot). Final Answer: All of these

Question 7

Vapour-compression system conceptual check Which statement about performance and components is WRONG?

Accepted Answer

Introduction / Context: Understanding where useful cooling is produced and how operating temperatures influence performance is central to vapour-compression refrigeration. This question tests conceptual clarity about the evaporator, condenser, and the effects of subcooling and temperature levels on COP. Given Data / Assumptions: Standard single-stage vapour-compression cycle. Useful effect is refrigeration, i.e., heat absorbed from a cold space. Terms: evaporating temperature (at evaporator), condensing temperature (at condenser). Concept / Approach: In a refrigerator, the desired effect is at the evaporator: it absorbs heat from the space being cooled. The condenser rejects heat to the surroundings and is not the source of useful refrigeration. Performance depends strongly on the temperature lift (difference between condensing and evaporating temperatures). Subcooling liquid leaving the condenser reduces flash gas after throttling, increasing net refrigerating effect and COP. Step-by-Step Solution: Identify the useful heat transfer: heat absorbed in the evaporator is the refrigeration effect.Check each statement against fundamentals: performance varies with operating temperatures (true); Electrolux absorption systems are heat-driven with no mechanical compressor (true); subcooling increases COP (true); reducing condensing temperature improves COP (true).Therefore, the only wrong statement is that the condenser provides the useful effect. Verification / Alternative check: Energy balance: Q_evap is the desired cooling; Q_cond = Q_evap + W_comp is waste heat rejection for the basic cycle. Why Other Options Are Wrong (i.e., not the incorrect one): Options (a), (c), (d), and (e) are consistent with standard cycle analysis and observed practice; they reinforce how COP changes with operating temperatures and subcooling. Common Pitfalls: Confusing “useful” effect for a heat pump (where condenser heat is useful) with a refrigerator (where evaporator heat absorption is useful). Final Answer: In a vapour-compression cycle, the useful heat transfer occurs at the condenser.

Question 8

Using waste heat effectively In which refrigeration system can waste heat be productively used to drive the cycle?

Accepted Answer

Introduction / Context: Industrial plants often have low-grade waste heat that can be harnessed to drive cooling. The selection of a refrigeration cycle that uses heat rather than mechanical work is the key to improving overall plant efficiency. Given Data / Assumptions: Waste heat is available as hot water, steam, or exhaust gases. Goal: use this heat to produce refrigeration. Systems compared: vapour compression, vapour absorption, and air cycles. Concept / Approach: Vapour absorption refrigeration (e.g., ammonia–water or lithium bromide–water) uses a thermal compressor (generator, absorber, solution pump) driven primarily by heat input. In contrast, vapour compression relies on mechanical compressors driven by shaft work. Air cycles (reverse Brayton) also require mechanical compression and are not heat-driven. Step-by-Step Solution: Identify the driver for each cycle: absorption → heat; compression/air → mechanical work.Where waste heat exists, absorption cycles can convert that into cooling effect by powering the generator.Hence, the correct choice is vapour absorption cycle. Verification / Alternative check: Combined heat and power (CHP) systems often integrate absorption chillers to utilize waste heat for chilled water production. Why Other Options Are Wrong: Vapour compression and air cycles require mechanical power; waste heat does not directly drive them. Thermoelectric systems are powered electrically, not by heat input. Common Pitfalls: Assuming that any “refrigeration system” can accept waste heat; in practice only absorption (and adsorption) systems are designed to be heat-driven. Final Answer: Vapour absorption cycle

Question 9

Psychrometrics — heating with dehumidification During a combined process of heating and dehumidification, how does the dry-bulb temperature of the air change?

Accepted Answer

Introduction / Context: Air-handling systems may heat air while also removing moisture (e.g., by desiccant dehumidifiers with a downstream heater). Understanding the direction of property changes is crucial for plotting on the psychrometric chart and sizing coils. Given Data / Assumptions: Simultaneous heat addition and moisture removal. No evaporative cooling effects overwhelming the heating. Standard atmospheric pressure conditions for the chart. Concept / Approach: Heating adds sensible energy to the air, raising its dry-bulb temperature. Dehumidification reduces humidity ratio. On the psychrometric chart, this is typically an inclined path up-and-left or vertical then right depending on process details; in any case, dry-bulb temperature rises when net heating is applied. Step-by-Step Solution: Recognize heating → T_db increases.Recognize dehumidification → humidity ratio w decreases.Combine effects: T_db increases while w decreases. Verification / Alternative check: Energy balance for moist air: Δh ≈ c_p,ma * ΔT_db + w * Δh_fg + h_vapour changes; with added sensible heat and reduced moisture, ΔT_db is positive. Why Other Options Are Wrong: Constant or decreasing dry-bulb contradicts heating; equality with wet-bulb happens only at saturation and is not an outcome of this general process; oscillation is not a defined thermodynamic trend. Common Pitfalls: Confusing this with cooling and dehumidification, where T_db falls; or with chemical dehumidification at constant T_db (requires nearly isothermal conditions). Final Answer: Increases

Question 10

Reading the p–h diagram On a pressure–enthalpy (p–h) chart for a refrigerant, the region to the left of the saturated liquid line represents which condition?

Accepted Answer

Introduction / Context: Pressure–enthalpy charts are indispensable for refrigeration analysis. Correctly identifying regions (subcooled liquid, two-phase mixture, superheated vapour) ensures accurate cycle plotting and state estimation. Given Data / Assumptions: Standard p–h chart below the critical point. Saturated liquid line and saturated vapour line form the two-phase dome. Concept / Approach: The saturated liquid line (left boundary of the dome) represents liquid at the verge of boiling. States to the immediate left are still liquid but at a temperature lower than saturation at that pressure, i.e., subcooled (compressed) liquid. To the right of the saturated vapour line is superheated vapour; under the dome is the wet region. Step-by-Step Solution: Locate saturated liquid line (left boundary of dome).Identify states left of this line as liquid with temperature below saturation → subcooled liquid.Therefore, the correct region is sub-cooled (compressed) liquid. Verification / Alternative check: Cycle plotting shows condenser outlet often in subcooled region to improve COP and avoid flash gas at the expansion device. Why Other Options Are Wrong: Wet vapour is under the dome; superheat is to the right of the saturated vapour line; transcritical requires crossing the critical point and is not determined simply by being left of the saturated liquid line. Common Pitfalls: Assuming any point left of the dome is invalid; it is a legitimate compressed liquid state. Final Answer: Sub-cooled (compressed) liquid region

Question 11

Methods of achieving undercooling (subcooling) Which practical method can be used to obtain liquid undercooling in a vapour-compression system?

Accepted Answer

Introduction / Context: Subcooling the liquid leaving the condenser reduces flash gas after expansion, boosting the net refrigerating effect and often improving COP. There are several practical ways to achieve subcooling. Given Data / Assumptions: Liquid at condenser outlet is at or near saturation temperature. Goal: reduce its temperature below saturation at the condensing pressure. Concept / Approach: Undercooling can be achieved by enhancing heat rejection at the condenser (more or colder cooling water) or by internal heat exchange where cold suction vapour cools the liquid line. All these methods remove sensible heat from high-pressure liquid to move it into the subcooled region on the p–h chart. Step-by-Step Solution: Option (a): more cooling water → greater heat transfer → lower liquid-line temperature.Option (b): colder cooling water → increased temperature difference → more subcooling.Option (c): liquid–suction heat exchanger → sensible heat transfer from liquid to suction vapour, providing subcooling while superheating suction slightly.Hence, any of the above methods can achieve undercooling. Verification / Alternative check: System logs show lower liquid-line temperature than saturation temperature at condensing pressure when subcooling is present. Why Other Options Are Wrong: Each of (a)–(c) is valid; rejecting them individually would be incorrect. Two-stage throttling does not inherently subcool the high-pressure liquid; it primarily reduces pressure in stages. Common Pitfalls: Over-subcooling can raise compressor discharge superheat via liquid–suction exchange and may reduce evaporator capacity if not balanced. Final Answer: Any one of the above

Question 12

Freezing point comparison Relative to common halocarbon refrigerants, R-12 (dichlorodifluoromethane) has which characteristic for freezing point?

Accepted Answer

Introduction / Context: Refrigerant selection considers numerous properties: boiling point, critical temperature, miscibility with oil, toxicity, and freezing point. A very low freezing point helps avoid solidification in low-temperature applications and ensures reliable flow through capillaries and valves. Given Data / Assumptions: Comparison among typical pure refrigerants; R-12 historically used in domestic systems (legacy context). Freezing point is a substance property at atmospheric pressure and does not depend on condenser pressure. Concept / Approach: R-12 has an extremely low freezing point (around −157°C), lower than most common halocarbons traditionally used for comfort cooling and domestic refrigeration. Thus, within that family it is regarded as having a very low (i.e., “lowest”) freezing point, a favorable attribute for flow assurance. Step-by-Step Solution: Recognize the property: freezing point is intrinsic and notably low for R-12.Compare to other common refrigerants of similar era; R-12's freezing point is among the lowest.Therefore select “Lowest.” Verification / Alternative check: Property tables show R-12 with a much lower freezing point than, for example, R-22 or R-134a, confirming suitability for low-temperature lines without risk of solidification. Why Other Options Are Wrong: “Highest” contradicts data; “equal to ammonia” is incorrect because ammonia has different crystalline behavior; dependence on condenser pressure is irrelevant to the freezing point of a pure substance. Common Pitfalls: Confusing freezing point with boiling point or with the temperature glide of blends. Final Answer: Lowest

Question 13

Definition check in psychrometry The temperature indicated by a thermometer not affected by air moisture content is called:

Accepted Answer

Introduction / Context: Psychrometric properties are essential in HVAC design and analysis. Distinguishing between dry-bulb, wet-bulb, and dew-point temperatures is fundamental for chart reading and load calculations. Given Data / Assumptions: Standard atmospheric air mixture with water vapour. Thermometer measuring in free air without a wetted wick. Concept / Approach: Dry-bulb temperature (T_db) is the ordinary air temperature measured by a standard thermometer shielded from radiation and without evaporative effects. Wet-bulb temperature uses a wetted wick and airflow to register evaporative cooling. Dew-point temperature is the saturation temperature corresponding to the partial pressure of water vapour. Step-by-Step Solution: Identify measurement without moisture effects → dry-bulb.Recognize that wet-bulb requires a wetted wick and airflow.Dew-point is not directly measured by a dry thermometer; it is inferred from moisture content. Verification / Alternative check: On the psychrometric chart, dry-bulb is the horizontal axis, reaffirming its role as the basic temperature measure independent of immediate moisture interaction at the sensor. Why Other Options Are Wrong: Wet-bulb explicitly depends on evaporation; dew-point is a moisture property; adiabatic saturation temperature is a process temperature found by a specific apparatus. Common Pitfalls: Exposing dry-bulb thermometers to radiation or poor shielding leads to biased readings; ensure proper shielding and airflow. Final Answer: Dry-bulb temperature

Question 14

Wavelength of thermal radiation “The wavelength associated with peak emission depends only on temperature and not on the material.” Evaluate this statement.

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Introduction / Context: Radiative heat transfer involves spectral distributions that depend on temperature and surface properties. A common exam theme is differentiating material effects on emissive power from the temperature dependence of the peak wavelength. Given Data / Assumptions: Thermal radiation from opaque bodies in the infrared–visible bands. Interest in the wavelength of maximum emission (peak of the spectrum). Concept / Approach: Wien’s displacement law states that the wavelength at which the spectral emissive power is maximum varies inversely with absolute temperature: λ_max * T ≈ constant. While the magnitude of emission (emissivity) depends on material and surface condition, the position of the peak is governed primarily by temperature for a blackbody and closely follows temperature for many real surfaces. Step-by-Step Solution: Recognize the claim refers to “wavelength of the radiation emitted,” interpreted as peak wavelength.Apply Wien’s displacement: λ_max = b / T, where b is a constant.Material alters emissivity and spectral shape, but the dominant peak location tracks temperature; thus, the statement is effectively true in the sense of peak wavelength. Verification / Alternative check: As temperature increases, thermal glow shifts from infrared toward visible (red, then white), consistent with λ_max shifting to shorter wavelengths independently of the specific material. Why Other Options Are Wrong: Limiting validity to metals, vacuum, or gases is unnecessary; the temperature–peak relation is general. Declaring “False” confuses spectral emissivity magnitude (material-dependent) with λ_max (temperature-dependent). Common Pitfalls: Interpreting the statement to mean the entire spectrum is identical across materials; only the peak position follows Wien’s law strictly for blackbody reference, while real surfaces approximate it. Final Answer: True

Question 15

Location of the condenser in a refrigeration cycle A condenser operates on which side (pressure level) of a standard vapour-compression system?

Accepted Answer

Introduction / Context: Component placement by pressure level is core knowledge in refrigeration. Misidentifying low-side/high-side components leads to diagnostic errors and safety risks. Given Data / Assumptions: Standard single-stage vapour-compression system: evaporator, compressor, condenser, expansion device. No special economizers or intermediate vessels. Concept / Approach: The compressor raises vapour from low-pressure evaporator conditions to high-pressure discharge. The condenser receives this high-pressure, high-temperature vapour and rejects heat to condense it into high-pressure liquid. Only after throttling through the expansion device does the pressure drop back to the low side for the evaporator. Step-by-Step Solution: Trace the cycle: evaporator (low P) → compressor (raises P) → condenser (high P) → expansion device (drops P) → evaporator.Therefore, the condenser is a high-pressure component. Verification / Alternative check: Pressure gauges on service ports show high-side pressure at condenser outlet/liquid line and low-side pressure at evaporator/suction line. Why Other Options Are Wrong: Low-pressure side is the evaporator and suction line; “both” is impossible for a single component; the condenser is integral to the sealed circuit. Common Pitfalls: Confusing the role of receiver (also high-side) with condenser; or assuming subcooling changes pressure classification (it does not). Final Answer: High-pressure side

Question 16

Psychrometrics – identifying the correct term: The temperature indicated by a thermometer at the instant when the moisture present in air first begins to condense is called the dew-point temperature, not the wet-bulb temperature. State whether the g…

Accepted Answer

Introduction / Context: In air-conditioning and psychrometrics, three commonly used temperatures are dry-bulb temperature, wet-bulb temperature, and dew-point temperature. The question checks whether you can distinguish between dew-point and wet-bulb properly, because mixing these terms leads to serious design and control errors in HVAC calculations. Given Data / Assumptions: Air–water vapour mixture at near-atmospheric pressure. Standard definitions for psychrometric properties. Accurate, ventilated thermometers used for wet-bulb measurement with a properly wetted wick. Concept / Approach: Dew-point temperature is defined as the temperature at which air becomes saturated (relative humidity equals 100%) when cooled at constant moisture content, and condensation begins. Wet-bulb temperature is the equilibrium temperature attained by a wetted wick thermometer due to evaporative cooling into the surrounding airstream. These are different processes and usually yield different temperatures except at saturation. Step-by-Step Solution: Identify the phrase in the stem: “when the moisture present in it begins to condense.”Condensation onset during cooling at constant humidity ratio defines the dew-point temperature (T_dp).Wet-bulb temperature (T_wb) is reached by adiabatic saturation on a wetted wick; it reflects the air’s evaporative capacity, not the onset of condensation.Therefore the statement that this is “wet-bulb temperature” is false; the correct term is “dew-point temperature.” Verification / Alternative check: On a psychrometric chart, moving horizontally left at constant humidity ratio reaches the saturation curve at T_dp. In contrast, lines of constant T_wb are diagonally sloped; T_wb equals dry-bulb temperature only at 100% relative humidity. Why Other Options Are Wrong: Yes: Incorrect—this confuses dew-point with wet-bulb. Only at 100% relative humidity: Even then, the correct name at condensation onset is still “dew-point.” At saturation T_wb equals dry-bulb, but the definition remains distinct. Depends on barometric pressure only: Both temperatures depend on moisture content; pressure is not the sole deciding factor. Applicable only below 0°C: Dew-point can be above or below 0°C; freezing is not required. Common Pitfalls: Assuming wet-bulb indicates condensation; assuming dew-point equals wet-bulb at all states; ignoring the constant-humidity-ratio condition for dew-point measurement. Final Answer: No

Question 17

Leak detection for ammonia (R-717) refrigeration systems: Which method is specifically suitable for identifying ammonia leaks in practice?

Accepted Answer

Introduction / Context: Leak detection is a critical safety task in refrigeration plants. Different refrigerants require different techniques because their chemistry and safety classifications vary. Ammonia (R-717) is toxic and mildly flammable; it also has a sharp, penetrating odour, making it detectable at low concentrations, but reliable confirmation methods are still necessary. Given Data / Assumptions: System refrigerant: ammonia (R-717). Plant operating at typical industrial conditions. Technician has access to basic field tools for leak testing. Concept / Approach: Ammonia reacts with products of sulphur combustion to form white fumes (ammonium salts), so a gently smoking sulphur stick held near a suspected joint or valve will visibly indicate an ammonia leak. In contrast, a halide torch is intended for halogenated refrigerants (e.g., R-12, R-22) and is not appropriate for ammonia. Soap-and-water bubble testing can work generally, but it is a generic technique and not the primary textbook answer for ammonia-specific identification. Step-by-Step Solution: Recognize refrigerant: R-717 (ammonia).Match a detection method that exploits ammonia’s chemical behaviour: sulphur stick test yields visible white fumes near leaks.Rule out halide torch: useful for halogenated refrigerants, not for ammonia.Acknowledge that soap solution can show bubbles, but the classic ammonia-specific method is the sulphur stick. Verification / Alternative check: Industrial safety manuals and HVAC textbooks consistently list sulphur sticks or litmus paper (turns blue) for ammonia. Modern plants may also use fixed gas detectors, but the foundational field method remains the sulphur stick indication. Why Other Options Are Wrong: Halide torch: Not suitable for ammonia; designed for halogenated refrigerants. Soap-and-water only: Works in general, but the question asks the method used for ammonia—sulphur sticks are the classic targeted choice. All of these: Incorrect because halide torch is inappropriate for ammonia. Ultrasonic only: A possible modern tool but not the only valid or standard classroom answer. Common Pitfalls: Using the same leak detector for all refrigerants; ignoring chemical compatibility and safety guidance. Final Answer: Sulphur sticks (white fumes formation)

Question 18

During a purely sensible cooling process (cooling without condensation), which of the following holds true for the moist air stream?

Accepted Answer

Introduction / Context: Sensible cooling is a foundational psychrometric process used in HVAC coil analysis. When air is cooled without removing moisture, designers must know which properties change and which remain essentially unchanged to predict relative humidity shifts and to avoid unintended condensation. Given Data / Assumptions: No moisture removal (coil surface temperature above the dew-point of entering air). Air pressure near atmospheric and essentially constant. Well-mixed air at outlet; adiabatic external surroundings aside from the cooling coil. Concept / Approach: In sensible cooling, the humidity ratio w (and equivalently the partial pressure of water vapour p_w) stays nearly constant because no water is added or removed. As temperature drops, saturation vapour pressure p_ws(T) decreases, so relative humidity RH = p_w / p_ws increases. Wet-bulb temperature decreases since the air’s capacity to accept moisture increases as it gets cooler relative to its moisture content. Step-by-Step Solution: Assume no condensation: mass of water vapour in the air does not change.At nearly constant total pressure, p_w (and w) remain approximately constant.Lowering dry-bulb temperature reduces p_ws(T), thus RH increases (not constant).Wet-bulb temperature trends downward during sensible cooling (it does not increase). Verification / Alternative check: On a psychrometric chart, sensible cooling is a horizontal move to the left (constant w). Lines of constant RH slope upwards to the left, confirming RH rises. Lines of constant T_wb also slope such that T_wb falls with leftward movement at constant w. Why Other Options Are Wrong: Relative humidity remains constant: False; RH increases as temperature decreases at constant w. Wet-bulb temperature increases: Opposite of actual behaviour; T_wb decreases. Specific humidity increases: No moisture is added; w remains essentially constant. Humidity ratio increases sharply: Again, no moisture exchange means w stays constant. Common Pitfalls: Thinking RH is a conserved property; forgetting that RH is a ratio involving saturation pressure, which is temperature dependent. Final Answer: Partial pressure of water vapour remains approximately constant

Question 19

During cooling and dehumidification across a cooling coil (surface below the incoming dew-point), the specific humidity (humidity ratio) of the leaving air will:

Accepted Answer

Introduction / Context: Cooling and dehumidification is the standard process in comfort air-conditioning. When the coil surface temperature lies below the incoming air’s dew-point, moisture condenses on the coil fins, altering the air’s moisture content. Knowing how the specific humidity changes is essential for load calculations and condensate management. Given Data / Assumptions: Coil surface temperature lower than the entering air’s dew-point temperature. Steady flow, well-mixed outlet air, and negligible pressure change. Condensate drains away; no re-evaporation in the leaving airstream. Concept / Approach: Specific humidity (also called humidity ratio w) is the mass of water vapour per unit mass of dry air. When condensation occurs, liquid water is removed from the airstream, reducing the mass of vapour per unit dry air, so w decreases. This differs from sensible-only cooling, where w remains essentially constant. Step-by-Step Solution: Check coil condition: T_surface < T_dp,in → condensation begins.As air contacts the coil, vapour condenses to liquid; mass of vapour in the air decreases.Therefore w_out < w_in; the specific humidity decreases.Simultaneously, dry-bulb and wet-bulb temperatures drop; relative humidity may approach saturation depending on approach to apparatus dew-point. Verification / Alternative check: On the psychrometric chart, the state path slopes downwards and left toward the saturation curve, clearly showing a reduction in w. Why Other Options Are Wrong: Remain constant: That would be sensible cooling only (no condensation). Increase: Moisture is removed, not added. First increase then decrease: Not for a standard steady cooling/dehumidifying coil. Oscillate with coil load: Process description is steady; oscillation is irrelevant here. Common Pitfalls: Confusing relative humidity behaviour with humidity ratio; RH can rise even during sensible cooling, but w falls only when condensation occurs. Final Answer: decrease

Question 20

Thermal comfort principle: A human body feels thermally comfortable when the metabolic heat generation is equal to the __________.

Accepted Answer

Introduction / Context: Human thermal comfort is a balance problem. The body produces metabolic heat and must reject an equivalent amount to maintain a stable core temperature. HVAC design aims to support this balance through control of air temperature, humidity, air speed, and mean radiant temperature. Given Data / Assumptions: Steady metabolic rate appropriate to activity level. Clothing insulation is steady during the period considered. Environmental parameters are uniform around the occupant. Concept / Approach: The comfort condition is met when metabolic heat generation equals total heat dissipation from the body by convection, radiation, evaporation (sweating and respiration), and small conduction terms. If generation exceeds dissipation, heat is stored in the body and the person feels too warm; if dissipation exceeds generation, the person feels too cool. Step-by-Step Solution: Define metabolic heat M (W/m^2 or W).Define total heat loss Q_loss = Q_conv + Q_rad + Q_evap (+ minor terms).Comfort equilibrium condition: M = Q_loss (net storage S = 0).If M > Q_loss → S > 0 (body warms); if M < Q_loss → S < 0 (body cools). Verification / Alternative check: Thermal comfort models (e.g., PMV/PPD by Fanger) mathematically enforce this energy balance, incorporating clothing insulation and air movement. Neutral sensation corresponds to negligible net storage. Why Other Options Are Wrong: Heat stored in the human body: Storage indicates imbalance and discomfort if sustained. Sum of (a) and (b): Physically meaningless; you cannot add “dissipated” and “stored” to set a balance. Difference of (a) and (b): Also not the criterion; the correct condition is zero storage. Net radiant heat received: Only one component of loss/gain; not the full balance. Common Pitfalls: Ignoring evaporation’s importance in warm/humid climates; neglecting air speed effects on convective loss. Final Answer: heat dissipated to the surroundings

Heat Transfer, Refrigeration and Air Conditioning Questions