Question 1

Foundations of radiative properties and equilibrium Which statement about radiative properties and thermal equilibrium is correct?

Accepted Answer

Introduction / Context: Radiative heat transfer depends on material properties: absorptivity (fraction absorbed), emissivity (effectiveness as an emitter compared to a black body), reflectivity, and transmissivity. Kirchhoff’s law provides a powerful relation between emission and absorption at thermal equilibrium. Given Data / Assumptions: Thermal equilibrium between a surface and its surroundings at a given wavelength and direction. Properties may be spectral and directional, but the law applies at matching wavelength and direction. Concept / Approach: Kirchhoff’s law states that, at thermal equilibrium, emissivity equals absorptivity for each wavelength and direction: ε_λ,θ = α_λ,θ. This does not mean the surface is a perfect absorber; it means its capability to emit relative to a black surface matches its capability to absorb. A black body is the special case where α = ε = 1. A grey body has spectral independence (ε and α constant with wavelength) but not necessarily unity. Step-by-Step Solution: Define absorptivity α: fraction of incident radiation absorbed.Define emissivity ε: emitted intensity relative to a black body at same T.Apply Kirchhoff’s law at equilibrium: ε = α (spectrally and directionally).Conclude option (b) is correct. Verification / Alternative check: Thermodynamic arguments show that if ε ≠ α at equilibrium, net radiation exchange would spontaneously drive a temperature change, violating equilibrium. Why Other Options Are Wrong: Grey body absorbing all radiation is the definition of a black body, not grey.Emissivity is not “energy absorbed/incident”; that is absorptivity.Visual black appearance does not guarantee black-body behavior across thermal wavelengths.Opaque bodies have τ = 0, but reflectivity need not be zero; α + ρ = 1 with ρ possibly large. Common Pitfalls: Confusing color with radiative property over the thermal spectrum; many shiny metals have low ε but high reflectivity in infrared, despite visual appearance. Final Answer: At thermal equilibrium, emissivity equals absorptivity.

Question 2

Leak detection for ammonia refrigerant When checking for leaks in an ammonia (NH3) refrigeration system using a burning sulphur stick, what visible indication confirms the presence of ammonia?

Accepted Answer

Introduction / Context: Safe and quick leak detection is essential in refrigeration maintenance. Ammonia (NH3) systems are common in industrial refrigeration due to high latent heat and zero ODP, but they require distinct detection methods because NH3 has strong odour, high affinity for acids, and reacts with certain combustion products. Given Data / Assumptions: Technician uses a burning sulphur stick near suspected joints. Airborne products include sulphur dioxide (SO2) and sulphur trioxide (SO3). Ammonia is a basic gas that forms salts with acidic oxides. Concept / Approach: When SO2/SO3 from the sulphur stick contacts ammonia, they react to form ammonium sulphite/sulphate aerosols. These salts appear as dense white fumes or “white smoke,” providing a clear visual indicator of NH3 presence. This simple field method supplements electronic detectors and soap solution tests. Step-by-Step Solution: Bring a gently smoking sulphur stick near the suspected leak (observe safety, ventilation, PPE).Watch for reaction of SOx with NH3 in the air around the leak.Observe formation of white fumes (ammonium salts) → confirms ammonia leakage.Mark and isolate the leak for repair; ventilate area. Verification / Alternative check: Soap bubble solutions can also identify leaks by visible bubbling. Electronic ammonia sensors provide quantitative detection and alarms, but the sulphur-stick method remains a classic qualitative check. Why Other Options Are Wrong: Blue flame or color changes are unrelated to NH3 detection with sulphur sticks.Green vapour trails or black droplets are not characteristic products of NH3–SOx interaction.Odour alone is not the recommended confirmation method due to safety and desensitization risks. Common Pitfalls: Holding an open flame too close to combustible materials, or using the method in confined, poorly ventilated spaces. Prefer non-ignition methods where flammability hazards exist, though NH3 itself has a relatively narrow flammability range. Final Answer: White smoke or fumes forming around the leak point

Question 3

Fundamental principle of heat transfer: Heat flows from one body to another only when there is a temperature difference between them. State whether this statement is correct.

Accepted Answer

Introduction / Context: Understanding when heat transfer occurs is foundational in thermodynamics and heat transfer. Heat is energy in transit due to a temperature difference. This question checks whether the learner recognizes that a temperature gradient is the driving potential for conduction, convection, and radiation. Given Data / Assumptions: Two bodies or control regions at different temperatures. No work interaction is being considered, only thermal energy transfer. Transport may occur by conduction, convection, or radiation. Concept / Approach: Heat transfer requires a nonzero temperature difference. If temperatures are equal, there is thermal equilibrium and net heat transfer is zero. This holds across all three modes: Fourier's law for conduction, Newton's law of cooling for convection, and the Stefan–Boltzmann relation for radiation all imply a driving potential tied to temperature. Step-by-Step Solution: Identify thermal states: T_hot and T_cold.Recognize driving potential: Delta T = T_hot - T_cold.If Delta T ≠ 0 → heat flows from higher to lower temperature.If Delta T = 0 → no net heat transfer (thermal equilibrium). Verification / Alternative check: At equal temperatures, thermodynamic equilibrium implies no spontaneous energy flow. Any microscopic exchanges cancel out to zero net transfer. Why Other Options Are Wrong: No: contradicts the definition of heat as energy in transit due to temperature difference. Only if solids: convection and radiation also require Delta T. Only when radiation is absent: radiation also ceases with no temperature difference between surfaces facing each other. Only in a vacuum: conduction and convection need matter; radiation needs a temperature difference regardless of medium. Common Pitfalls: Confusing heat (in transit) with internal energy (a stored property). Equal temperatures mean no net heat transfer even though internal energies may differ. Final Answer: Yes

Question 4

Refrigeration practice (ammonia system): During refrigerant charging in an ammonia vapour compression plant, should the charging connection be made at the compressor outlet (discharge) line?

Accepted Answer

Introduction / Context: Safe and effective charging of ammonia systems requires connecting at the correct side of the cycle. The question probes whether charging at the compressor discharge is appropriate. Given Data / Assumptions: Ammonia (R-717) vapour compression system. Normal charging operation (no emergency). Compressor with suction (low-pressure) and discharge (high-pressure) lines. Concept / Approach: Charging is typically done on the low-pressure side (suction line or receiver/liquid line with proper throttling and precautions) to avoid forcing refrigerant into the high-pressure discharge where pressures and temperatures are high. Introducing refrigerant at the discharge risks pushing liquid toward the compressor or over-pressurizing components. Step-by-Step Solution: Identify low side: evaporator outlet to compressor suction.Connect charging cylinder via manifold to the low side so the system draws refrigerant safely.Avoid discharge side connection to prevent charging against high pressure and potential liquid slugging. Verification / Alternative check: Field procedures and manufacturer service manuals specify suction-side charging for vapour charging; liquid charging uses receiver/liquid line with metering and system off as applicable, still not at compressor discharge. Why Other Options Are Wrong: Agree/Only for liquid charging/Only in vacuum/Only for small systems: discharge-line charging is not standard good practice due to high temperature/pressure and compressor safety concerns. Common Pitfalls: Feeding liquid directly into the suction without flashing can damage compressors. Use correct procedures (vapour at suction, or controlled liquid into receiver/liquid line) with proper throttling and monitoring. Final Answer: Disagree

Question 5

Refrigerants and altitude: Do water, alcohol, and ammonia provide the same refrigerating effect at different altitudes (i.e., varying ambient pressure)?

Accepted Answer

Introduction / Context: Refrigerating effect per unit mass depends on a refrigerant’s thermophysical properties (latent heat, specific heats) and operating pressures, which themselves are influenced by ambient conditions such as altitude. Given Data / Assumptions: Comparison among water, alcohol, and ammonia as working fluids. Different altitudes imply different ambient pressures and heat sink temperatures. Conventional vapour compression or absorption cycles. Concept / Approach: Refrigerating effect (per kg) is commonly approximated as the enthalpy difference between evaporator outlet and inlet. This depends on saturation temperatures/pressures and fluid properties, all of which vary distinctly for water, alcohols, and ammonia. Altitude alters condenser and sometimes evaporator pressures via ambient temperature/pressure changes, so the refrigerating effect is not identical across fluids or altitudes. Step-by-Step Solution: Note that at a given evaporator temperature, each refrigerant has a unique latent heat and specific heat.Ambient (altitude) changes condenser pressure/temperature, shifting cycle state points.Therefore, the enthalpy change in the evaporator differs among refrigerants and with altitude. Verification / Alternative check: Property charts (h–T or p–h) for ammonia vs. water vs. ethanol show markedly different latent heats and saturation curves, confirming unequal refrigerating effects. Why Other Options Are Wrong: Yes/conditional options: ignore fluid-specific property differences and cycle dependence on ambient conditions. Common Pitfalls: Assuming “same temperature lift” yields identical effects; property variability makes this false across different fluids. Final Answer: No

Question 6

Psychrometrics concept check: At 100% relative humidity, the wet-bulb temperature is ________ the dew-point temperature.

Accepted Answer

Introduction / Context: Wet-bulb temperature (WBT) and dew-point temperature (DPT) are fundamental air properties in HVAC design. Their relationship simplifies under saturation conditions (100% relative humidity). Given Data / Assumptions: Air–water vapor mixture at 100% relative humidity. Steady psychrometric conditions. Standard definitions of WBT and DPT. Concept / Approach: At 100% RH, the air is saturated. For saturated air, dry-bulb temperature, wet-bulb temperature, and dew-point temperature all coincide. Evaporative cooling potential is zero, so the wet-bulb cannot be lower than dry-bulb; likewise, the dew point equals the current air temperature. Step-by-Step Solution: Recognize 100% RH → saturated state.On a psychrometric chart, the state lies on the saturation curve.At this point, DBT = WBT = DPT. Verification / Alternative check: Using sling psychrometer logic: with no evaporation from the wet wick (air already saturated), WBT equals DBT. Dew point equals DBT at saturation. Why Other Options Are Wrong: Lower/higher/undefined/greater by a fixed value: contradict the saturation property relationship. Common Pitfalls: Confusing near saturation (e.g., 95% RH) with full saturation; at near saturation, WBT is slightly below DBT and DPT is slightly below DBT, but they are not equal. Final Answer: same as

Question 7

Thermal diffusivity definition: Given h = thermal diffusivity, rho = density, S = specific heat, and k = thermal conductivity, which expression is correct for thermal diffusivity?

Accepted Answer

Introduction / Context: Thermal diffusivity indicates how quickly a material's temperature field responds to thermal disturbances. It combines conductivity with thermal storage capacity (density and specific heat). Given Data / Assumptions: Symbols: h for thermal diffusivity (often denoted alpha), k for conductivity, rho for density, S for specific heat. Homogeneous, isotropic material. Concept / Approach: Thermal diffusivity is defined as the ratio of the ability to conduct heat to the ability to store heat: h = k / (rho * S)This has units of m^2/s and appears in the transient heat conduction equation, controlling the rate of temperature equalization. Step-by-Step Solution: Start from energy balance in a control volume and Fourier's law.Group terms to obtain the transient diffusion equation: dT/dt = h * (d^2T/dx^2 + ...).Identify h as k / (rho * S) with dimensions m^2/s. Verification / Alternative check: Dimensional analysis: k has W/(mK) = J/(smK); rhoS has J/(m^3K); dividing gives m^2/s, confirming the expression. Why Other Options Are Wrong: rhoS/k or krhoS: have incorrect units. (rho + S)/k: dimensionally inconsistent. k/rho: ignores specific heat, wrong units. Common Pitfalls: Confusing thermal diffusivity with thermal conductivity; high conductivity does not guarantee high diffusivity if heat capacity is also high. Final Answer: h = k / (rho * S)

Question 8

Heat exchangers: Compared with parallel flow, a counter-current (counter-flow) heat exchanger can transfer ________ total heat between the same two fluids (all else equal).

Accepted Answer

Introduction / Context: Flow arrangement strongly affects the temperature profiles and the log-mean temperature difference (LMTD) in a heat exchanger. Counter-current flow generally yields higher effectiveness than parallel flow given similar sizes and heat transfer coefficients. Given Data / Assumptions: Two-fluid, steady-state heat exchanger. Same overall heat transfer coefficient and area for both arrangements. No phase change (for simplicity). Concept / Approach: Total heat transfer rate Q is computed using Q = U * A * LMTDCounter-current flow offers a larger LMTD for the same inlet temperatures, because the temperature difference is maintained more uniformly along the length. Step-by-Step Solution: Compute LMTD for parallel: DeltaT1_par = Th_in - Tc_in; DeltaT2_par = Th_out - Tc_out.Compute LMTD for counter: DeltaT1_cnt = Th_in - Tc_out; DeltaT2_cnt = Th_out - Tc_in.Typically, LMTD_counter > LMTD_parallel → higher Q for counter-current. Verification / Alternative check: Effectiveness–NTU relations show higher maximum temperature approach on the cold end for counter-current, enabling greater heat recovery. Why Other Options Are Wrong: Less/same/zero: contradict LMTD and effectiveness analysis. Only latent heat: arrangement benefits both sensible and latent applications. Common Pitfalls: Assuming area or U can compensate equally; in design, counter-current is preferred when feasible for better performance and smaller area. Final Answer: more

Question 9

Applications: Natural-convection air-cooled condensers are most commonly used in which of the following systems?

Accepted Answer

Introduction / Context: Condensers reject heat from refrigeration systems. Air-cooled condensers may use either natural convection (no fan) or forced convection (with a fan). Selection depends on capacity and footprint. Given Data / Assumptions: Small domestic refrigerators use wire-on-tube condensers on the back or bottom. Water coolers and room ACs typically employ forced convection fans. Natural convection capacity is limited due to low air-side heat transfer coefficients. Concept / Approach: Natural convection provides small heat rejection rates appropriate for low-capacity systems. Domestic refrigerators are designed around modest loads, making passive wire-and-tube condensers practical, quiet, and reliable. Step-by-Step Solution: Compare required heat rejection with natural convection capability.Domestic refrigerators → low Q_cond → feasible without fans.Water coolers/room ACs → higher Q_cond → require forced convection or water-cooled systems. Verification / Alternative check: Product tear-downs show finned coils with fans in room ACs, while household fridges often rely on passive condensers. Why Other Options Are Wrong: Water coolers/room air conditioners: normally use fan-assisted condensers to meet higher heat loads. All of these/industrial chillers: overstate the capability of natural convection. Common Pitfalls: Confusing “air-cooled” with “natural convection.” Many air-cooled systems use fans (forced convection). Final Answer: Domestic refrigerators

Question 10

Air-conditioning design: The conditioned supply air delivered to a room must be capable of handling which types of internal loads?

Accepted Answer

Introduction / Context: HVAC systems must maintain both temperature and humidity to satisfy comfort and process requirements. This requires the supply air to address sensible (temperature-related) and latent (moisture-related) loads. Given Data / Assumptions: Typical comfort air-conditioning application. Loads include people, equipment, lighting, infiltration, and internal moisture sources. Supply air is conditioned to a state on the psychrometric chart capable of removing both heat and moisture. Concept / Approach: Total room load = sensible load + latent load. Sensible load changes dry-bulb temperature; latent load changes humidity ratio through condensation at the cooling coil. Adequate design chooses supply air temperature and humidity so that its enthalpy and humidity ratio allow removal of both components. Step-by-Step Solution: Compute room sensible heat (RSH) and room latent heat (RLH).Select supply air state below room air temperature and with a humidity ratio low enough to dehumidify.Ensure supply airflow rate satisfies both sensible and latent load constraints. Verification / Alternative check: On a psychrometric chart, the room sensible heat factor line and apparatus dew point selection ensure both temperature and moisture control. Why Other Options Are Wrong: Sensible only/latent only: ignores full comfort requirement. Neither or equipment-only: incomplete; ventilation is a separate requirement but does not replace thermal and humidity control. Common Pitfalls: Sizing by sensible load alone can lead to poor humidity control; oversizing airflow without dehumidification can worsen comfort. Final Answer: Both room sensible heat and latent heat loads

Question 11

Refrigeration troubleshooting: A high discharge-side pressure gauge reading on a refrigerant compressor most likely indicates which of the following?

Accepted Answer

Introduction / Context: Discharge pressure reflects condenser conditions. Elevated discharge pressure often signals poor heat rejection at the condenser, which can arise from several root causes. Given Data / Assumptions: Vapour compression system with water- or air-cooled condenser. Pressure measured at compressor discharge port. Steady operating load. Concept / Approach: High condensing pressure occurs when the condenser cannot reject heat effectively. Causes include insufficient cooling medium flow, high inlet cooling medium temperature, and fouling or scaling that increases thermal resistance. Each increases condensing temperature/pressure, raising discharge pressure. Step-by-Step Solution: Check condenser water flow; low flow → reduced heat transfer coefficient → higher condensing pressure.Check water inlet temperature; higher temperature → reduced temperature difference → higher condensing pressure.Inspect for fouling; dirty tubes/fins → higher resistance → higher condensing pressure. Verification / Alternative check: Compare approach temperature (condensing temperature minus cooling medium outlet temperature). Large approaches point toward condenser-side problems. Why Other Options Are Wrong: Any single item alone may be the cause, but the question asks for likely reasons collectively; therefore, all listed causes are valid contributors. Evaporator starved of refrigerant primarily affects suction pressure and superheat, not discharge pressure directly. Common Pitfalls: Adjusting expansion valve to “fix” high head pressure; this treats symptoms and can worsen system balance. Final Answer: All of these

Question 12

Reducing thermal resistance: Which choice correctly lists actions that reduce resistance in conduction, convection, and radiation?

Accepted Answer

Introduction / Context: Overall thermal resistance is the sum or combination of resistances from conduction, convection, and radiation. Reducing resistance increases heat transfer rate for a given temperature difference. This question checks practical levers for each mode. Given Data / Assumptions: Steady heat transfer across a composite path. Classical definitions: R_cond = L/(kA); R_conv = 1/(hA); radiation treated via an effective h_rad proportional to emissivity and temperature. Area A is considered fixed for comparison. Concept / Approach: To reduce conduction resistance, increase k or decrease thickness L. To reduce convection resistance, raise the convection coefficient h by increasing fluid velocity, turbulence, or cleaning surfaces. For radiation, increasing emissivity and operating temperature raises the effective radiative heat transfer coefficient, thereby lowering the associated “resistance.” Step-by-Step Solution: Conduction: R_cond = L/(kA) → lower L and/or higher k reduce R.Convection: R_conv = 1/(hA) → higher h via stirring/forced flow/clean surfaces reduces R.Radiation: q_rad depends on emissivity and T; higher emissivity and higher T increase the effective h_rad, reducing resistance. Verification / Alternative check: Compare heat transfer before and after modifications: increased h or k, or decreased L, yields higher q for the same Delta T, confirming reduced resistance. Why Other Options Are Wrong: Options a, b, and c each capture a correct strategy. The most complete answer is to apply all of them where applicable. None of the above: contradicts standard heat transfer relations. Common Pitfalls: Confusing radiation control for low-temperature systems where radiative contribution is small; at high temperatures, radiation dominates and emissivity changes have large effects. Final Answer: All of the above

Question 13

In winter air-conditioning practice, the supply air must be both warmed and given additional moisture to achieve indoor comfort. What is the name of the combined process used to increase air temperature and raise its humidity simultaneously?

Accepted Answer

Introduction / Context: Winter air-conditioning aims to condition outdoor cold and relatively dry air so that indoor spaces feel comfortable. The two key tasks are to raise the air temperature and to increase its moisture content so occupants do not feel dryness and static electricity. This question checks the foundational psychrometric concept and the correct naming of the process used in HVAC engineering. Given Data / Assumptions: The outdoor air is cold and has low absolute humidity in winter. Comfort targets usually require temperatures around 20–24°C and relative humidity about 30–50% depending on standards. The process is performed by heating coils and humidifiers within an air-handling unit. Concept / Approach: On a psychrometric chart, heating at constant humidity ratio moves horizontally to the right (temperature increases, moisture constant). Humidification at nearly constant dry-bulb temperature moves the state point upward (humidity ratio increases). In winter systems, both are applied: first heat to a comfortable temperature, then inject moisture (steam or water spray/atomization) to achieve the target humidity. The combined name in HVAC terminology is “heating and humidification.” Step-by-Step Solution: 1) Identify winter need: raise temperature and raise humidity.2) Heating coil adds sensible heat: temperature increases, humidity ratio unchanged.3) Humidifier adds moisture: humidity ratio increases; if steam is used, there can be slight sensible heating too.4) Therefore, the combined process is termed “heating and humidification.” Verification / Alternative check: A standard winter air-handling schematic shows an outdoor air damper, preheat coil (if needed), main heating coil, and a humidifier section. The psychrometric path is a rightward move due to heating followed by an upward or slightly slanted move due to humidification. This matches the named process exactly. Why Other Options Are Wrong: Cooling and dehumidification: A summer process, opposite of winter needs. Humidification only: Would not correct low temperature discomfort. Dehumidification only: Reduces moisture and is used for muggy summer air. Adiabatic saturation only: A theoretical process; real winter systems add heat and moisture with equipment. Common Pitfalls: Assuming raising humidity alone will improve comfort when air is still too cold; or confusing summer and winter processes on the psychrometric chart. Final Answer: Heating and humidification

Question 14

Among the refrigerants listed (ammonia, carbon dioxide, sulphur dioxide, and R-12), which one has the highest freezing point (that is, the least negative temperature at which it solidifies)?

Accepted Answer

Introduction / Context: Refrigerant selection considers many properties, including freezing point (solidification temperature). A higher freezing point (closer to 0°C) indicates the substance solidifies more easily and may limit low-temperature applications. This question checks knowledge of comparative freezing points for common historic/industrial refrigerants. Given Data / Assumptions: We compare typical reference freezing points: CO2 has a triple point at about -56.6°C, ammonia freezes near -77.7°C, sulphur dioxide near -75.5°C, and R-12 near about -157°C. “Highest freezing point” means the largest (least negative) temperature value. Concept / Approach: Rank the freezing points numerically. The substance that solidifies at the warmest temperature has the highest freezing point. Because CO2’s triple point is around -56.6°C, it lies significantly above the freezing points of ammonia and sulphur dioxide, and far above that of R-12. Step-by-Step Solution: 1) Note key data: CO2 ≈ -56.6°C, NH3 ≈ -77.7°C, SO2 ≈ -75.5°C, R-12 ≈ -157°C.2) Identify which is numerically highest (closest to zero): -56.6°C is greater than -75.5°C, -77.7°C, and -157°C.3) Therefore, CO2 has the highest freezing point among the listed options. Verification / Alternative check: CO2’s phase behavior is widely known in cryogenics and refrigeration: below its triple point temperature, it exists as solid (dry ice) and gas; liquid CO2 requires pressure above the triple point. This characteristic aligns with its comparatively higher freezing point. Why Other Options Are Wrong: Ammonia and sulphur dioxide: Both freeze at lower temperatures than CO2. R-12: Very low freezing point compared to CO2, hence not the highest. Propane: Not in the original list and has a much lower freezing point than CO2. Common Pitfalls: Confusing boiling point with freezing point, or overlooking the special triple point of CO2 that constrains its liquid region at atmospheric pressure. Final Answer: Carbon dioxide

Question 15

In a standard mechanical vapor-compression refrigeration system, the refrigerant in the liquid line between the condenser outlet and the expansion valve is predominantly in which phase?

Accepted Answer

Introduction / Context: Understanding the phase of refrigerant at different points in the vapor-compression cycle is essential for diagnostics, design, and safety. The line from the condenser to the expansion device is commonly called the liquid line. This question verifies the correct phase there in normal operation. Given Data / Assumptions: System is operating properly with a condenser that rejects heat to ambient. There may be a small amount of subcooling at the condenser exit to ensure pure liquid reaches the expansion device. No flash gas in the liquid line under normal conditions. Concept / Approach: In the condenser, refrigerant rejects latent heat and condenses from vapor to liquid. Additional sensible cooling below saturation temperature (subcooling) improves system reliability by preventing premature flashing before the expansion valve. Therefore, the correct description is “subcooled (or saturated) liquid.” Step-by-Step Solution: 1) Evaporator outlet is typically superheated vapor.2) Compressor raises pressure and temperature; condenser then rejects heat.3) At condenser exit, the fluid is liquid; a few degrees subcooling is desirable.4) Thus the liquid line carries subcooled (or at least saturated) liquid to the expansion device. Verification / Alternative check: Field practice uses sight glasses and subcooling measurements to confirm liquid quality at the expansion valve inlet. Presence of flash gas indicates insufficient subcooling or excessive pressure drop, confirming that the normal/ideal state is pure liquid. Why Other Options Are Wrong: Superheated vapor / dry saturated vapor: These occur at compressor outlet or evaporator outlet, not in the liquid line. Wet mixture: Undesirable in the liquid line; leads to maldistribution and expansion valve instability. Supercritical fluid: Requires pressures above the critical point (not typical for common HVAC conditions). Common Pitfalls: Confusing evaporator outlet superheat with liquid-line conditions; assuming two-phase flow is normal before the valve—it is not ideal. Final Answer: Subcooled (or saturated) liquid

Question 16

In an Electrolux (three-fluid absorption) refrigerator fired by a small gas burner, how is the coefficient of performance (COP) defined?

Accepted Answer

Introduction / Context: Absorption refrigeration systems are thermally driven; instead of mechanical compressor work, they use heat input to drive the cycle. The Electrolux refrigerator uses a gas flame with ammonia–water–hydrogen to achieve cooling. The coefficient of performance for such systems compares useful cooling to the thermal energy supplied. Given Data / Assumptions: Evaporator provides useful refrigeration effect Q_evap. Gas burner supplies heat Q_in to the generator. Neglect minor auxiliary work (no mechanical compressor). Concept / Approach: By definition for a heat-driven refrigerator, COP = desired effect / required input. Here, desired effect is heat absorbed in the evaporator (cooling), and required input is heat added by the burner to the generator. Therefore, COP = Q_evap / Q_in. Step-by-Step Solution: 1) Identify numerator (useful effect): heat removed in evaporator.2) Identify denominator (driving input): thermal energy from gas burner.3) Form the ratio: COP = Q_evap / Q_in.4) Select the option that matches this definition. Verification / Alternative check: Textbook definitions for absorption COP consistently use Q_evap divided by total heat supplied to the generator/absorber as the performance index, aligning with the chosen option. Why Other Options Are Wrong: Inverted or difference-based ratios do not represent COP and can give values < 0 or > 1 inappropriately. Pump work term is negligible and not the defining energy input for Electrolux systems. Common Pitfalls: Confusing mechanical COP definitions (based on compressor work) with thermally driven systems; mixing up which energy term belongs in the denominator. Final Answer: COP = (Heat absorbed by the evaporator) / (Heat supplied by the burner)

Question 17

In refrigeration terminology, the Freon family refers to which class of refrigerants?

Accepted Answer

Introduction / Context: “Freon” is a trade name historically used for a group of halogenated hydrocarbons widely applied as refrigerants and aerosol propellants. Knowing this classification is basic HVAC knowledge and helps avoid material incompatibility or environmental misapplication. Given Data / Assumptions: Freon examples include R-12, R-22, and various HFC blends. They are not the same as inorganic or pure hydrocarbon refrigerants. Concept / Approach: Freons are halo-carbons: carbon-based molecules containing halogens such as chlorine, fluorine, or both. The group spans CFCs, HCFCs, and later HFCs; environmental regulation has phased out many CFCs/HCFCs due to ozone depletion and global warming concerns. Step-by-Step Solution: 1) Identify trade name scope: branded halogenated hydrocarbons.2) Map to families: CFCs, HCFCs, and related HFCs used as Freon-branded products.3) Therefore, the correct category is halo-carbon refrigerants. Verification / Alternative check: Product literature and standards consistently list R-12 (CFC), R-22 (HCFC), and certain HFCs under the Freon brand, confirming the classification as halo-carbons. Why Other Options Are Wrong: Azeotropes are mixture types and may or may not be Freons. Inorganics (NH3, CO2) are not halo-carbons. Hydro-carbons (propane, isobutane) lack halogens; different family. Common Pitfalls: Using “Freon” generically for all refrigerants; it specifically refers to a halo-carbon family/brand. Final Answer: Halo-carbon refrigerants (CFCs/HCFCs/HFCs)

Question 18

In comfort cooling calculations, “sensible heat gain” of outdoor ventilation air is primarily due to which difference between the fresh air and the conditioned space air?

Accepted Answer

Introduction / Context: Cooling load calculations split total heat gain into sensible (temperature-related) and latent (moisture-related) components. Correctly attributing each portion is essential for sizing coils and selecting equipment. This question focuses on the sensible portion from ventilation (outdoor) air. Given Data / Assumptions: Sensible heat is associated with temperature change at essentially constant moisture content. Latent heat is associated with moisture addition/removal (change in humidity ratio). Ventilation air enters warmer than the indoor air in typical summer scenarios. Concept / Approach: Sensible heat gain depends on the dry-bulb temperature difference between the entering outdoor air and the indoor air. The formula commonly applied is Q_sensible = m_air * cp_air * (T_out − T_in), which makes temperature difference the driver for the sensible component. Step-by-Step Solution: 1) Define sensible heat: linked to temperature change, not moisture change.2) For ventilation air, compare outdoor dry-bulb to indoor setpoint.3) Use Q = m * cp * ΔT to quantify sensible gain.4) Conclude that temperature difference governs sensible heat gain. Verification / Alternative check: Psychrometric chart movements: a horizontal move represents sensible change (constant humidity ratio). This matches the temperature-difference explanation for sensible loads from outdoor air. Why Other Options Are Wrong: Humidity difference: Drives latent, not sensible, load. Velocity / Pressure / Elevation differences: Not primary factors in sensible load calculations for standard building HVAC. Common Pitfalls: Confusing sensible and latent components when ventilation air is both warmer and more humid; both may occur, but sensible is strictly linked to temperature difference. Final Answer: Temperature difference

Question 19

At 100% relative humidity (saturated air), the psychrometric temperatures coincide. Which statement correctly describes this condition?

Accepted Answer

Introduction / Context: Psychrometrics uses several temperature definitions: dry-bulb (ambient), wet-bulb (adiabatic saturation surrogate), and dew point (condensation onset). At saturation (100% relative humidity), these converge, simplifying analysis for cooling coils and evaporative processes. Given Data / Assumptions: Air–water vapor mixture is at thermodynamic equilibrium with liquid water. No measurement errors or radiation effects on thermometers. Concept / Approach: At saturation, the partial pressure of water vapor equals the saturation pressure at the mixture temperature. The adiabatic saturation line intersects the state point, so wet-bulb equals dry-bulb. Dew point equals the same temperature because the air is already saturated. This temperature is also the saturation temperature corresponding to that vapor pressure. Step-by-Step Solution: 1) At RH = 100%, air cannot hold additional moisture; any cooling causes condensation.2) Wet-bulb involves evaporation potential; at saturation, no further evaporation is possible, so wet-bulb = dry-bulb.3) Dew point is the temperature at which saturation occurs; when already saturated, dew point equals current temperature.4) Therefore, all four temperatures are equal. Verification / Alternative check: On the psychrometric chart, the state point at 100% RH lies on the saturation curve where DBT, WBT, and DPT converge, confirming the statement. Why Other Options Are Wrong: Options stating differences contradict saturation behavior. At saturation, none of the psychrometric temperatures remain distinct. Common Pitfalls: Confusing near-saturation (e.g., 95% RH) with exact saturation; small thermometer errors can also create apparent differences in practice. Final Answer: Wet-bulb temperature = Dry-bulb temperature = Dew point temperature = Saturation temperature

Question 20

A human body is surrounded by air hotter than the body temperature. Considering convection, what is the algebraic sign of the 'heat loss from the body to the surroundings' under this condition?

Accepted Answer

Introduction / Context: Convective heat transfer direction is driven by temperature difference between a surface and the surrounding fluid. Sign conventions matter in heat balance equations. The question asks for the sign of “heat loss from the body to the surroundings” when the surroundings are hotter than the body. Given Data / Assumptions: Surrounding air temperature T_air > body surface temperature T_body. Standard sign convention: heat loss from the body is positive when heat leaves the body. Concept / Approach: Newton’s law of convection: q_conv = h * A * (T_surface − T_fluid). If T_fluid > T_surface, then (T_surface − T_fluid) is negative, meaning heat flows from the fluid to the body. Thus, when we specifically refer to “heat loss from the body,” its value is negative because the body is actually gaining heat, not losing it. Step-by-Step Solution: 1) Write q_loss = h * A * (T_body − T_air).2) Since T_air > T_body, (T_body − T_air) < 0.3) Therefore, q_loss < 0, indicating negative heat loss (heat gain).4) Conclude the sign is negative for “heat loss from body to surroundings.” Verification / Alternative check: Energy balance on the body shows net heat entering by convection when ambient is hotter; the same sign logic is used in HVAC load analysis and human thermal comfort models under hot-air exposure. Why Other Options Are Wrong: Positive: Would imply body is hotter than air, which contradicts the condition. Zero: Only true when temperatures are equal or if h = 0 (not the case). Indeterminate / oscillatory: Unnecessary complexity; sign is fixed by temperature difference. Common Pitfalls: Reversing the temperature difference order or mixing up the definition of “loss” versus “gain.” Final Answer: Negative (body gains heat from surroundings)

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