Q: Pulverised coal firing in furnaces: Which advantage best explains why finely pulverised coal improves furnace operation and fuel economy?

Introduction / Context: Pulverised coal (PC) firing grinds coal to fine particles and conveys it with primary air into the burner. The very high surface area accelerates devolatilisation and char burnout, enabling compact, efficient furnaces with good load response and fuel flexibility. Given Data / Assumptions: Coal is milled to fine powder and injected with air. Proper burner design ensures good mixing and flame stability. Appropriate slagging/ash handling is implemented. Concept / Approach: Fine particles heat rapidly and react quickly, raising flame temperature locally and improving thermal efficiency. The improved burnout accommodates a broader range of coals (including higher-ash) while careful staging and temperature control can also handle lower ash-fusion temperatures by limiting local hot spots and maintaining suitable slag viscosity. Step-by-Step Solution: Increased specific surface area → faster kinetics → higher burnout efficiency → lower unburnt carbon.Better mixing and controllability → improved heat transfer and reduced fuel consumption per unit output.Optimised operation broadens acceptable coal specifications (ash content and fusion characteristics). Verification / Alternative check: Heat balance comparisons show reduced excess air requirement and lower loss-on-ignition in fly ash for PC systems versus stoker firing. Why Other Options Are Wrong: Options (a), (b), and (c) each state true benefits; choosing only one ignores the full scope of PC firing advantages. The combined statement reflects industry practice. Common Pitfalls: Assuming any coal can be burned without considering slagging/fouling limits and burner aerodynamics; PC systems still require proper selection and tuning. Final Answer: all (a), (b) and (c).

Q: Salt bath furnace heat-transfer mechanism: When a workpiece is immersed in a molten salt bath for heating, which mode of heat transfer to the charge dominates?

Introduction / Context: Salt bath furnaces heat parts by immersing them in molten salts. These units are valued in heat treatment for rapid, uniform heating and minimal oxidation compared to direct-fired or gaseous atmospheres. Given Data / Assumptions: Part is submerged in molten salt at controlled temperature. Bath is agitated naturally (buoyancy) or mechanically. Surface oxide formation is limited by exclusion of air at the metal surface. Concept / Approach: Heat transfer from the salt to the part occurs primarily via convection in the liquid boundary layer coupled with some conduction within the part. Because the surrounding medium is a liquid with high heat capacity and good thermal contact, convective heat transfer coefficients are significantly higher than for gases, and radiation is comparatively less important at typical salt bath temperatures. Step-by-Step Solution: Identify surrounding medium: molten salt (liquid).In liquids, forced or natural circulation dominates surface heat transfer → convection.Conduction is dominant inside the solid; radiation is secondary at these conditions. Verification / Alternative check: Empirical heat-up times correlate with liquid-side convection coefficients rather than radiative exchange equations. Why Other Options Are Wrong: Conduction: governs internal temperature equalisation, not the external surface transfer.Radiation: smaller contribution compared to vigorous liquid convection.None of these: incorrect because convection is clearly dominant. Common Pitfalls: Confusing overall heating (including in-part conduction) with the primary surface mechanism from bath to part. Final Answer: convection

Q: Pusher-type reheating furnaces in steel mills: These continuous furnaces are typically used for bringing which product to rolling temperature before the mill?

Introduction / Context: Reheating furnaces raise semi-finished steel products to a uniform temperature suitable for hot rolling. Pusher-type furnaces advance the stock on skid rails or hearths by periodically pushing the charge forward through preheat, heating, and soaking zones. Given Data / Assumptions: Continuous, longitudinal furnace with zones and flue gas flow. Material advanced by mechanical pusher. Common steel mill practice. Concept / Approach: While walking-beam and walking-hearth designs are widely used today, classic pusher furnaces have long handled slabs and billets. Among the listed forms, slabs are the archetypal feed to pusher furnaces prior to hot strip or plate mills. Coils and sheets are usually heated in batch or bell furnaces; ingots historically used soaking pits or different furnace arrangements. Step-by-Step Solution: Identify common semi-finished shapes: slabs, blooms, billets.Match to furnace type: pusher → slabs/billets (most typically slabs for strip/plate mills).Select “slabs.” Verification / Alternative check: Process flowsheets show slab casters feeding slab yards, then pusher or walking-beam reheating into hot strip mills. Why Other Options Are Wrong: Ingots: more often soaked vertically in pits.Steel coils/sheets: heated in different equipment (batch bell or continuous annealing/processing lines), not pusher reheaters. Common Pitfalls: Assuming any steel form can be reheated in any furnace; geometry and handling dictate furnace choice. Final Answer: slabs

Question 1

Furnace atmospheres and process selection: In which of the following high-temperature units is a deliberately reducing atmosphere (oxygen-deficient, carbon-rich gases that promote reduction reactions) normally maintained during standard operation?

Accepted Answer

Introduction / Context: Industrial furnaces and reactors operate with different gas atmospheres depending on the chemistry needed. A reducing atmosphere is oxygen-lean and contains species such as CO and H2 that remove oxygen from ores or surfaces. Recognizing which equipment routinely runs under reducing conditions is a foundational concept in furnace technology and extractive metallurgy. Given Data / Assumptions: “Reducing atmosphere” means an atmosphere that favors reduction (e.g., CO + Fe2O3 → CO2 + Fe). Typical operations and purposes of each listed unit are considered. We assume normal, steady operation rather than exceptional or transient conditions. Concept / Approach: The blast furnace (for hot-metal/iron production) operates with coke and injected fuels to generate CO and H2 in the shaft. These gases reduce iron oxides while ascending through the burden. By contrast, units such as soaking pits or basic oxygen converters are not operated to maintain reducing gas compositions; their goals and gas management differ substantially. Step-by-Step Solution: List each unit’s typical atmosphere and objective.Blast furnace: sustained reducing gas stream (CO, H2) for ore reduction → reducing atmosphere.Calcination kiln: often near neutral/oxidizing for limestone; reduction is not the aim.Soaking pit: aims at temperature uniformity of steel ingots/billets; generally uses controlled but not strongly reducing gases.L.D. converter: blown with high-purity oxygen to oxidize impurities; atmosphere is highly oxidizing.Therefore, the correct selection is “Blast furnace”. Verification / Alternative check: Standard ironmaking texts describe the counter-current flow of reducing gases in the blast furnace, with gas analysis showing high CO and some H2 at the top. This is the canonical example of a reducing furnace atmosphere in large-scale industry. Why Other Options Are Wrong: Calcination kiln: designed mainly for decomposition (e.g., CaCO3 → CaO + CO2); strong reduction is not required.Soaking pit: atmosphere control focuses on scale minimization and heating quality, not on bulk chemical reduction.L.D. converter: explicitly oxidizing to remove C, Si, P, and Mn from hot metal.Electric arc ladle refining station: typically inert/neutral for cleanliness and trimming, not strongly reducing as a rule. Common Pitfalls: Assuming “high temperature” automatically means “reducing.” The gas composition and the process objective—not temperature alone—define whether the atmosphere is reducing or oxidizing. Final Answer: Blast furnace

Question 2

Furnace heat accounting visualisation: Which diagram is conventionally used to present a furnace heat balance in a clear, proportional, and diagrammatic form?

Accepted Answer

Introduction / Context: Energy management in furnaces requires a concise visual of where input heat goes: to the stock, to walls, to flue gases, and to losses. Engineers commonly use a specific flow-proportional diagram to depict this distribution when auditing or optimizing furnace performance. Given Data / Assumptions: The question asks for a “diagrammatic heat balance,” i.e., a plot where arrow widths scale with heat quantities. We focus on standard industrial practice in thermal systems. Concept / Approach: A Sankey diagram shows energy or mass flows with arrows whose widths are proportional to flow magnitudes. For furnaces, this directly communicates how much heat is used productively and how much is lost via flue gas, wall losses, openings, and radiation, enabling quick identification of improvement opportunities. Step-by-Step Solution: Identify the diagram type that scales arrow thickness with quantity: the Sankey diagram.Relate to furnace use: input fuel energy splits into useful heating and various losses; Sankey captures this.Select the correct option accordingly. Verification / Alternative check: Energy audit literature consistently illustrates furnace and boiler heat balances using Sankey diagrams because they intuitively display proportional flows and losses. Why Other Options Are Wrong: Cox/Ostwald charts: not standard for heat balance flow visualization.Mollier (h-s) chart: used for steam/water thermodynamic properties, not for heat balance depiction.None of these: incorrect because Sankey diagram is precisely the established tool. Common Pitfalls: Confusing property charts (state diagrams) with flow accounting charts; the former describe thermodynamic states, while Sankey diagrams portray energy distribution. Final Answer: Sankey diagram

Question 3

Controlling artificial draught in furnaces: A mechanically produced (fan-driven) draught in a fired furnace can be regulated using which of the following methods?

Accepted Answer

Introduction / Context: Artificial (mechanical) draught is created by forced-draft and/or induced-draft fans in furnaces, boilers, and kilns to supply combustion air and evacuate flue gases. Effective control of this draught is essential for stable flames, appropriate excess air, and safe operation across varying loads. Given Data / Assumptions: A fan is installed to create or augment draught. We consider practical, industry-standard control actions available to operators or control systems. Concept / Approach: Fan-based draught can be controlled via multiple levers: (i) speed control (e.g., variable frequency drive) changes fan head and flow; (ii) dampers adjust system resistance and flow distribution; (iii) variable-pitch blades (on suitable fans) alter the fan characteristic and delivered flow/pressure at a given speed. Step-by-Step Solution: Identify all legitimate, commonly used mechanisms for regulating flow/pressure.Fan speed, dampers, and pitch change are all established and effective methods.Therefore, “All (a), (b) and (c)” is correct. Verification / Alternative check: Fan control strategies in boiler/furnace practice often combine VFD speed control with inlet or outlet dampers to balance multi-zone flows, confirming all three approaches are valid. Why Other Options Are Wrong: Single options (a, b, or c) are incomplete; industry uses all of them where available.Grate modifications: relevant only to stoker systems and not a primary draught control for fan-driven systems. Common Pitfalls: Believing dampers alone are sufficient. While dampers work, VFD speed control often saves power and stabilizes control—best practice frequently combines methods. Final Answer: All (a), (b) and (c)

Question 4

Combustion chamber sizing in industrial furnaces: The required size (volume) of a furnace combustion chamber primarily depends on which factors?

Accepted Answer

Introduction / Context: Combustion chamber sizing is vital to ensure complete combustion, proper flame development, acceptable residence time, and uniform heat flux to the load. Undersized chambers risk flame impingement and CO formation; oversized chambers can be inefficient and costly. Given Data / Assumptions: We consider a conventional fired furnace or process heater. Fuel type, preheat, and mixing arrangements are design variables. Target emissions and completeness of combustion are to be satisfied. Concept / Approach: Key drivers include: (i) heat release rate (kW per unit volume) which sets necessary volume for safe flame development; (ii) preheating fuel/air accelerates kinetics and can reduce required volume; (iii) efficient mixing ensures rapid, uniform combustion, affecting required residence time and thus chamber size. Step-by-Step Solution: Relate heat release rate to chamber volume: higher rates generally need larger volume or advanced burners.Account for preheat: hotter reactants shorten ignition delay and reaction time, permitting more compact chambers.Assess mixing: good mixing reduces unburned pockets and stabilizes shorter flames.Hence all three factors jointly determine sizing → choose “All (a), (b) and (c)”. Verification / Alternative check: Burner and heater design guidelines tie allowable heat release (per m^3), reactant preheat, and mixing intensity to the recommended firebox volume, confirming the combined influence. Why Other Options Are Wrong: Any single factor alone is insufficient; real designs balance all three.Refractory thickness affects durability/heat loss, not primary chamber volume sizing. Common Pitfalls: Overlooking the impact of high air preheat: it can significantly reduce required volume but increases NOx potential, so designs must also address emissions control. Final Answer: All (a), (b) and (c)

Question 5

Pot furnaces (regenerative or recuperative): A pot furnace, which may be equipped with regenerators or recuperators for heat recovery, is primarily used in the manufacture of which product?

Accepted Answer

Introduction / Context: Pot furnaces are traditional melting units where material is contained in “pots” (crucibles) within a heated chamber. In thermal-intensive industries, heat recovery via regenerators or recuperators is often added to improve fuel economy. Recognizing the typical product associated with pot furnaces supports equipment selection knowledge. Given Data / Assumptions: “Pot furnace” refers to a glassmaking furnace type where glass batches are melted in ceramic pots. Regenerative/recuperative add-ons improve efficiency by preheating combustion air. We seek the industry most closely linked with pot furnaces. Concept / Approach: Historically and in small-scale or specialty production, glass is melted in pot furnaces to produce different compositions or colors in separate pots. While many modern plants use large tank furnaces, pot furnaces remain relevant for specialty glass and lower-throughput operations. Step-by-Step Solution: Identify the canonical application: glass melting in ceramic pots.Exclude other industries: stainless steel and refractories typically use other furnace types and forming routes.Select “Glass”. Verification / Alternative check: Glassmaking literature references pot furnaces as a distinct class for art glass and specialty production, often with regenerative/recuperative firing systems for energy efficiency. Why Other Options Are Wrong: Stainless steel: produced via primary steelmaking and secondary metallurgy routes, not pot furnaces.Potteries/refractory bricks: use kilns for firing formed ware, not pot furnaces for melting.Quicklime: produced by rotary/shaft kilns, not pot furnaces. Common Pitfalls: Confusing “pottery kilns” with “pot furnaces”; the former fire clay articles, whereas the latter melt glass batch inside pots. Final Answer: Glass

Question 6

Identifying a rotary furnace type: Which of the following units is classified as a rotary furnace commonly used in high-temperature processing?

Accepted Answer

Introduction / Context: Rotary furnaces are rotating, refractory-lined vessels that process solids continuously with controlled residence time and heat transfer. Identifying which equipment belongs to this category helps in selecting the right technology for calcination, roasting, and other thermal treatments. Given Data / Assumptions: The “rotary” descriptor refers to a cylindrical shell rotating on its axis. Lime kilns are widely constructed as rotary kilns for calcining limestone. Other listed items are not typically rotary by design. Concept / Approach: In a rotary lime kiln, limestone moves slowly through a rotating tube, heated by a flame at one end, producing quicklime with controlled residence time and temperature profile. Soaking pits and many annealing furnaces are stationary enclosures; hot-blast stoves are regenerative heaters with checker bricks, also stationary. Step-by-Step Solution: Identify which option is commonly a rotating cylinder: lime kiln.Confirm others: soaking pits, stoves, and many annealing furnaces are not rotary.Select “Lime kiln”. Verification / Alternative check: Standard process equipment catalogs categorize rotary kilns prominently for lime, dolime, cement clinker, and more, affirming the choice. Why Other Options Are Wrong: Soaking pit: stationary heating pit.Stoves: stationary regenerative heaters.Annealing furnace: typically stationary box, bell, or continuous type, not rotary. Common Pitfalls: Assuming any furnace with moving material is “rotary.” Conveyance can be via walking beams, rollers, or cars without a rotating shell. Final Answer: Lime kiln

Question 7

Regenerator vs. recuperator trade-offs: Compared to recuperators for the same heat-recovery duty, regenerators typically… (select the most accurate statement)

Accepted Answer

Introduction / Context: High-temperature furnaces often reclaim heat from flue gases using either recuperators (continuous exchangers) or regenerators (cyclic storage and release). Understanding the engineering trade-offs between these technologies helps in choosing equipment that meets temperature, footprint, and budget constraints. Given Data / Assumptions: Same overall duty (air preheat target) is considered. Typical industrial designs: metallic/ceramic recuperators vs. checker-brick regenerators. We compare general tendencies, not edge-case custom designs. Concept / Approach: Regenerators use large thermal masses (checkerwork) and periodic flow reversal to achieve very high preheat levels, especially at extreme furnace temperatures. This typically entails heavy structures, valves, and foundations, increasing capital cost relative to many recuperators. They are not lighter/compact, and they store a larger, not smaller, quantity of heat per cycle. Step-by-Step Solution: Evaluate each statement against common practice.(a) “Store smaller heat”: incorrect—regenerators store substantial heat in checker bricks.(b) “Lighter & compact”: incorrect—regenerators are generally bulkier.(c) “Higher initial cost”: generally true due to size, refractory content, and valve systems.Therefore select option (c). Verification / Alternative check: Design handbooks and vendor literature consistently note regenerators’ higher capital cost but excellent high-temperature performance versus recuperators, validating the choice. Why Other Options Are Wrong: (a) Opposite of typical behavior.(b) Opposite; regenerators are not typically compact.(d) Cannot be true because (a) and (b) are false.(e) Regenerators often achieve higher, not lower, air preheat at a given duty. Common Pitfalls: Confusing “continuous” with “better.” While recuperators are simpler, regenerators shine at the very high end of temperature and preheat requirements. Final Answer: Involve higher initial cost

Question 8

Maintaining an oxidising furnace atmosphere: To sustain an oxidising atmosphere during fired operation, a furnace should be operated with…

Accepted Answer

Introduction / Context: Process metallurgy and heat treatment often require control over furnace atmosphere to influence reactions like oxidation, decarburization, or scaling. An oxidising atmosphere contains free oxygen that promotes oxide formation on metals or complete combustion of combustibles. Given Data / Assumptions: We consider steady combustion of hydrocarbon or carbonaceous fuels. Excess air is the fraction of air supplied over the theoretical stoichiometric amount. Flue-gas indicators such as O2, CO, and CO2 reflect the achieved atmosphere. Concept / Approach: Operating with excess air ensures residual oxygen remains after the fuel is burned, producing an oxidising environment within the furnace. Fuel-rich conditions (deficient air) elevate CO and unburned hydrocarbons, which are reducing. While CO2 increases with complete combustion, CO2 alone is not the control lever; excess O2 is the essential criterion. Step-by-Step Solution: Target an O2 reading above zero in the flue gas.Achieve this by supplying more than stoichiometric air—i.e., excess air.Avoid elevated CO, which signifies reducing/oxygen-deficient combustion. Verification / Alternative check: Combustion tuning practice sets an O2 trim setpoint (e.g., a few percent O2 dry) to maintain an oxidising atmosphere with acceptable efficiency and safety. Why Other Options Are Wrong: Less excess air: promotes reducing conditions.More CO: characteristic of reducing/poorly mixed flames.“More CO2 at any cost”: CO2 level alone is not a guarantee of oxidising conditions; oxygen availability is decisive.Reduced air preheat: unrelated to oxidising/reducing balance. Common Pitfalls: Chasing high CO2 numbers without monitoring O2 can lead to unstable combustion; using an O2 trim ensures true oxidising operation. Final Answer: More of excess air (oxygen present above stoichiometric requirement)

Question 9

Fuel preheating practices in furnace operation: In standard industry practice, which of the following fuels are generally not preheated before entry to the furnace (air may be preheated separately)?

Accepted Answer

Introduction / Context: Preheating combustion air is a cornerstone of furnace efficiency. However, preheating the fuel itself depends on fuel phase, safety, and practicality. Understanding which fuels are commonly preheated and which are not helps avoid unsafe practices and focus on the largest efficiency gains. Given Data / Assumptions: “Standard practice” refers to typical industrial furnaces. Air preheat via recuperators/regenerators is common. Heavy fuel oils are often warmed for viscosity control, but gaseous and solid fuels are generally not preheated before the burner/grate. Concept / Approach: Liquid heavy oils require heating to reduce viscosity and enable proper atomization—this is fuel preheat. In contrast, gaseous hydrocarbon fuels are usually supplied at ambient temperature for safety and equipment simplicity; heating gas fuels poses explosion risks and adds limited benefit. Solid fuels (e.g., coal) are not practically preheated inline; instead, their drying or devolatilization occurs in the combustion zone or upstream processing, not as a controlled preheat into the furnace chamber. Step-by-Step Solution: Identify fuels typically preheated: heavy oils for atomization.Identify fuels typically not preheated: solids and gaseous fuels.Therefore, choose “Both (a) and (b)”. Verification / Alternative check: Burner and furnace manuals specify oil heaters, but gas trains omit fuel preheaters; solid fuel systems address drying/volatiles via combustion air distribution and bed design. Why Other Options Are Wrong: (a) only or (b) only: incomplete; both categories are generally not preheated.Neither (a) nor (b): incorrect, as industry practice avoids routine preheating of both solid and gaseous fuels.“Only heavy fuel oils”: this is the category that is preheated, not “not preheated.” Common Pitfalls: Confusing air preheat with fuel preheat. Air is widely preheated; fuel preheat is limited mainly to viscous liquid fuels for atomization, not to gas or solid fuels. Final Answer: Both (a) and (b)

Question 10

Optimising excess air on gas-fired furnaces: For a gaseous-fuel-fired furnace operating efficiently with complete combustion and minimal losses, the ideal oxygen percentage measured in the flue gas is typically about…

Accepted Answer

Introduction / Context: Excess air in combustion ensures completeness but also carries heat up the stack. The flue-gas oxygen reading is a convenient proxy for excess air. The objective is to maintain enough O2 to avoid CO/soot while keeping it low enough to limit sensible stack losses. Given Data / Assumptions: Gaseous fuel (well-mixed, clean-burning) is being fired. Modern burners and controls allow reasonably tight excess-air management. Target is efficient, safe operation with negligible CO. Concept / Approach: For gas-fired furnaces, typical economical flue-gas O2 setpoints often lie in the low single digits. Values below about 2% risk instability or CO formation under disturbances, while values near 5% strike a practical balance between complete combustion and reduced stack loss. Higher O2 levels correspond to more excess air and efficiency penalties. Step-by-Step Solution: Consider safety and completeness: maintain some positive O2 margin.Minimize heat loss: avoid unnecessarily high O2 which increases nitrogen ballast and stack loss.Industry practice converges near a few percent O2; hence “less than 5%” is the most appropriate target band. Verification / Alternative check: Combustion tuning guides for process heaters and boilers frequently recommend O2 control between about 2% and 4% (dry) for natural gas when conditions allow, aligning with “< 5%”. Why Other Options Are Wrong: < 2%: can be risky, leaving little margin before CO appears.< 8% or < 10%: too loose; allows avoidable efficiency loss.~12% or higher: indicates excessive excess air and significant stack loss. Common Pitfalls: Setting a fixed O2 value without considering draft, load swings, or burner condition; O2 trim systems can maintain the chosen band dynamically. Final Answer: < 5%

Question 11

Effect of sulphur dioxide (SO₂) in furnace flue gases on ferrous alloys: In an industrial heating furnace atmosphere containing SO₂, how does sulphur attack typically manifest on steels and other ferrous metals at elevated temperature?

Accepted Answer

Introduction / Context: Sulphur-bearing combustion products such as sulphur dioxide (SO₂) and, under some conditions, sulphur trioxide (SO₃) are common in furnaces firing high-sulphur liquid or solid fuels. These species can degrade ferrous materials by accelerating oxidation, promoting hot corrosion, and weakening the metal microstructure. Understanding the combined effects is essential for selecting fuels, refractories, and protective atmospheres in reheating and heat-treatment furnaces. Given Data / Assumptions: Furnace processing steels at high temperature. Flue gas contains SO₂ from fuel-bound sulphur. Normal excess air operation, with typical oxygen present. Concept / Approach: At elevated temperature, SO₂ participates in reactions that increase oxide scale growth on iron and steel. Sulphur also diffuses to grain boundaries where it forms low-melting sulphides or segregates, reducing cohesion and increasing embrittlement risk. These effects are more pronounced in low-carbon and some nickel-bearing alloys where grain boundary chemistry is sensitive. Step-by-Step Solution: Recognize that SO₂ raises oxidation potential in concert with O₂ → faster scale formation (option a).Consider sulphur segregation and sulphide formation at high temperature → loss of ductility and hot shortness (option b).Acknowledge intergranular attack mechanisms → pronounced grain boundary degradation in certain steels (option c). Verification / Alternative check: Plant evidence includes thicker, friable scale; increased metal loss; and intergranular fracture features on metallography following exposure to sulphur-laden gases. Why Other Options Are Wrong: Each individual statement describes a real SO₂ effect; selecting only one underestimates the combined degradation. The comprehensive, correct description is that all occur together. Common Pitfalls: Assuming sulphur effects are negligible at moderate sulphur levels; even ppm-level sulphur can matter at high temperature and long residence times. Final Answer: all (a), (b) and (c).

Question 12

Pulverised coal firing in furnaces: Which advantage best explains why finely pulverised coal improves furnace operation and fuel economy?

Accepted Answer

Introduction / Context: Pulverised coal (PC) firing grinds coal to fine particles and conveys it with primary air into the burner. The very high surface area accelerates devolatilisation and char burnout, enabling compact, efficient furnaces with good load response and fuel flexibility. Given Data / Assumptions: Coal is milled to fine powder and injected with air. Proper burner design ensures good mixing and flame stability. Appropriate slagging/ash handling is implemented. Concept / Approach: Fine particles heat rapidly and react quickly, raising flame temperature locally and improving thermal efficiency. The improved burnout accommodates a broader range of coals (including higher-ash) while careful staging and temperature control can also handle lower ash-fusion temperatures by limiting local hot spots and maintaining suitable slag viscosity. Step-by-Step Solution: Increased specific surface area → faster kinetics → higher burnout efficiency → lower unburnt carbon.Better mixing and controllability → improved heat transfer and reduced fuel consumption per unit output.Optimised operation broadens acceptable coal specifications (ash content and fusion characteristics). Verification / Alternative check: Heat balance comparisons show reduced excess air requirement and lower loss-on-ignition in fly ash for PC systems versus stoker firing. Why Other Options Are Wrong: Options (a), (b), and (c) each state true benefits; choosing only one ignores the full scope of PC firing advantages. The combined statement reflects industry practice. Common Pitfalls: Assuming any coal can be burned without considering slagging/fouling limits and burner aerodynamics; PC systems still require proper selection and tuning. Final Answer: all (a), (b) and (c).

Question 13

Salt bath furnace heat-transfer mechanism: When a workpiece is immersed in a molten salt bath for heating, which mode of heat transfer to the charge dominates?

Accepted Answer

Introduction / Context: Salt bath furnaces heat parts by immersing them in molten salts. These units are valued in heat treatment for rapid, uniform heating and minimal oxidation compared to direct-fired or gaseous atmospheres. Given Data / Assumptions: Part is submerged in molten salt at controlled temperature. Bath is agitated naturally (buoyancy) or mechanically. Surface oxide formation is limited by exclusion of air at the metal surface. Concept / Approach: Heat transfer from the salt to the part occurs primarily via convection in the liquid boundary layer coupled with some conduction within the part. Because the surrounding medium is a liquid with high heat capacity and good thermal contact, convective heat transfer coefficients are significantly higher than for gases, and radiation is comparatively less important at typical salt bath temperatures. Step-by-Step Solution: Identify surrounding medium: molten salt (liquid).In liquids, forced or natural circulation dominates surface heat transfer → convection.Conduction is dominant inside the solid; radiation is secondary at these conditions. Verification / Alternative check: Empirical heat-up times correlate with liquid-side convection coefficients rather than radiative exchange equations. Why Other Options Are Wrong: Conduction: governs internal temperature equalisation, not the external surface transfer.Radiation: smaller contribution compared to vigorous liquid convection.None of these: incorrect because convection is clearly dominant. Common Pitfalls: Confusing overall heating (including in-part conduction) with the primary surface mechanism from bath to part. Final Answer: convection

Question 14

Pusher-type reheating furnaces in steel mills: These continuous furnaces are typically used for bringing which product to rolling temperature before the mill?

Accepted Answer

Introduction / Context: Reheating furnaces raise semi-finished steel products to a uniform temperature suitable for hot rolling. Pusher-type furnaces advance the stock on skid rails or hearths by periodically pushing the charge forward through preheat, heating, and soaking zones. Given Data / Assumptions: Continuous, longitudinal furnace with zones and flue gas flow. Material advanced by mechanical pusher. Common steel mill practice. Concept / Approach: While walking-beam and walking-hearth designs are widely used today, classic pusher furnaces have long handled slabs and billets. Among the listed forms, slabs are the archetypal feed to pusher furnaces prior to hot strip or plate mills. Coils and sheets are usually heated in batch or bell furnaces; ingots historically used soaking pits or different furnace arrangements. Step-by-Step Solution: Identify common semi-finished shapes: slabs, blooms, billets.Match to furnace type: pusher → slabs/billets (most typically slabs for strip/plate mills).Select “slabs.” Verification / Alternative check: Process flowsheets show slab casters feeding slab yards, then pusher or walking-beam reheating into hot strip mills. Why Other Options Are Wrong: Ingots: more often soaked vertically in pits.Steel coils/sheets: heated in different equipment (batch bell or continuous annealing/processing lines), not pusher reheaters. Common Pitfalls: Assuming any steel form can be reheated in any furnace; geometry and handling dictate furnace choice. Final Answer: slabs

Question 15

Decarburisation during high-temperature exposure of steel: Which statements correctly describe the mechanism and conditions that favour decarburisation in furnaces?

Accepted Answer

Introduction / Context: Decarburisation is the loss of carbon from the surface layers of steel when heated in oxidising or certain neutral atmospheres. It alters surface hardness and strength, and is critical in heat treatment, forging, and hot rolling operations. Given Data / Assumptions: Steel heated above critical temperatures for significant time. Furnace atmosphere contains oxidising species (e.g., CO₂, H₂O, O₂). No protective coatings or inert atmospheres are used. Concept / Approach: At temperature, carbon in austenite and iron carbide (Fe₃C) reacts with oxidisers to form CO or CO₂, reducing surface carbon content. This changes phase transformation behaviour and microstructure upon cooling, affecting properties like hardness and wear resistance. The reaction rate rises with CO₂ and H₂O partial pressure and time-at-temperature. Step-by-Step Solution: Mechanism: C (dissolved/Fe₃C) + oxidiser → CO/CO₂ (surface carbon loss).Microstructural impact: altered case depth, softer ferritic surface after transformation → property change.Atmosphere effect: higher CO₂ or H₂O promotes decarburisation; reducing atmospheres suppress it. Verification / Alternative check: Metallography shows a decarburised layer with reduced pearlite/martensite content near the surface; hardness profiles confirm lower surface hardness. Why Other Options Are Wrong: Each individual statement is true; the combined option summarises the mechanism (a), consequence (b), and promoting condition (c). Common Pitfalls: Confusing decarburisation with oxidation scale. They often coexist, but decarburisation specifically refers to carbon loss in the metal, not only oxide formation. Final Answer: all (a), (b) and (c).

Question 16

Determinants of heating capacity in a continuous reheating furnace: Which combination of furnace and stock properties most strongly governs throughput capability?

Accepted Answer

Introduction / Context: Throughput in a reheating furnace depends on how quickly heat can be delivered to and absorbed by the stock. Radiative heat transfer dominates at high temperature, but geometry and surface properties also strongly influence effective heat flux and uniformity. Given Data / Assumptions: Continuous furnace with preheat, heating, and soak zones. Predominant radiative exchange between walls/flames and stock. Steel slabs or billets with known surface condition. Concept / Approach: Heat transfer rate scales with radiative exchange area and temperature difference (approx. proportional to T^4 in radiation correlations) and is modified by emissivity. The wall-to-stock surface ratio affects how much radiating surface is available per unit of charge surface; larger hearth area and higher furnace temperature raise potential heat input, while higher stock emissivity improves absorption. Step-by-Step Solution: Relate heating capacity to available radiant area (hearth area) and furnace temperature level.Include material emissivity: higher emissivity → better absorption → faster heating.Geometry ratio (wall/stock surfaces) determines view factors and net exchange → impacts capacity. Verification / Alternative check: Furnace models and performance tests show capacity gains when emissivity coatings are used or when higher wall temperatures and improved view factors are achieved. Why Other Options Are Wrong: Each single factor matters, but capacity is the combined outcome; hence, selecting only one underrepresents reality. The comprehensive option reflects all governing influences. Common Pitfalls: Ignoring emissivity changes due to scale or surface condition; these can measurably shift heating rates. Final Answer: all (a), (b) and (c).

Question 17

Burner mixing classification in furnaces: When the fuel and the combustion air are fully mixed before ignition at the burner head, the device is called a ________ burner.

Accepted Answer

Introduction / Context: Burners are commonly classified by where and how fuel and oxidant mix. This affects flame stability, emissions, turndown capability, and safety. In many low-pressure gas systems, full mixing before the flame front produces characteristic premixed flames. Given Data / Assumptions: Fuel and air are mixed upstream of the flame. Ignition occurs at or near the burner tile/port. Stable, typically blue, premixed flame. Concept / Approach: A premix burner forms a combustible mixture ahead of the nozzle, contrasted with diffusion (non-premixed) burners where fuel and air meet and mix at the flame. Outside mixing type is a generic description akin to diffusion. Rotary refers to mechanical atomisation/dispersion and does not by itself define premixing. Step-by-Step Solution: Identify key phrase: “mixed ahead of the burner.”Map to standard terminology: premix burner.Exclude alternatives that imply mixing at flame or different mechanisms. Verification / Alternative check: Premix gas burners in ovens and kilns show distinct flame speed limits and require flashback protection, consistent with the definition. Why Other Options Are Wrong: Outside mixing/diffusion: mixing at the flame, not ahead of the burner.Rotary: relates to atomisation, not necessarily premixing. Common Pitfalls: Confusing premix with staged or partially premixed designs; here the statement indicates full premixing. Final Answer: premix

Question 18

Fuel savings with preheated combustion air: A soaking-pit furnace firing furnace oil uses preheated air at 400°C instead of ambient air, with flue gas leaving at 1200°C. Approximately what percentage saving in furnace oil is expected compared to usi…

Accepted Answer

Introduction / Context: Preheating combustion air recovers sensible heat from flue gases, reducing the additional fuel needed to reach required flame and furnace temperatures. Soaking pits and reheating furnaces commonly use recuperators or regenerators to preheat air, delivering noticeable fuel savings. Given Data / Assumptions: Air preheat temperature: 400°C (significant above ambient). Flue gas exit temperature: 1200°C. Same production rate and final stock temperature. Concept / Approach: Fuel saving is roughly proportional to the fraction of the theoretical air and gas sensible heat supplied by preheat rather than by fuel. Typical industrial data for heavy oil firing indicate about 15–25% fuel reduction when air is preheated to several hundred degrees Celsius with high flue gas temperatures. The value of about 20% is a widely cited engineering estimate for this temperature pairing. Step-by-Step Solution: Recognize that preheat reduces enthalpy rise required from fuel.Consult standard rules-of-thumb linking air preheat to fuel savings.At 400°C preheat with 1200°C stack, choose ~20% saving. Verification / Alternative check: Simplified heat balance comparing adiabatic flame temperatures with and without preheat supports a 15–25% band, placing 20% near the midrange. Why Other Options Are Wrong: 5%: underestimates typical savings for 400°C preheat.40% or 60%: overly optimistic for the given conditions and would require much higher preheat or heat recovery extent. Common Pitfalls: Assuming savings are linear with preheat at all ranges; diminishing returns occur as preheat approaches flue gas temperature. Final Answer: 20

Question 19

How a resistance furnace generates heat: Identify the fundamental heating mechanism inside an electric resistance furnace.

Accepted Answer

Introduction / Context: Electric furnaces are grouped by how they transform electrical energy into heat. Resistance furnaces rely on ohmic heating of elements, arc furnaces on high-current arcs, and induction furnaces on electromagnetic induction and skin effect within the charge. Given Data / Assumptions: Standard resistance elements (e.g., Kanthal, SiC, MoSi₂). Temperature controlled by power to the elements. Charge heated by radiation and convection from hot elements and walls. Concept / Approach: In a resistance furnace, current passes through a high-resistivity element. Electrical power P is converted to heat as P = I^2 * R or P = V^2 / R. The resulting element temperature provides radiant and convective heat to the load. There is no arc between electrodes and charge (that would be an arc furnace), nor are eddy currents induced in the charge (that would be induction). Step-by-Step Solution: Identify furnace type: resistance.Map to heating law: ohmic (Joule) heating in elements.Exclude arc and induction mechanisms. Verification / Alternative check: Manufacturer datasheets specify element resistance, allowable current, and power density—hallmarks of resistance heating. Why Other Options Are Wrong: Arc: uses plasma arc; not applicable.Induction: requires magnetic fields and conductive charges; not applicable.None of these: incorrect because the correct mechanism is clearly ohmic heating of elements. Common Pitfalls: Confusing the heat delivery (radiation to the load) with the generation mechanism (Joule heating in elements). Final Answer: flow of current through a heating element.

Question 20

Furnace firing arrangement nomenclature: If the flame is generated beneath the hearth and then sweeps upward into the heating chamber, what is this furnace configuration called?

Accepted Answer

Introduction / Context: Furnaces are often described by how the flame and hot gases are introduced and flow relative to the hearth and charge. This naming helps communicate burner placement, flame direction, and expected heat transfer patterns. Given Data / Assumptions: Flame originates beneath the hearth structure. Combustion products sweep upward into the main heating space. Standard terminology used in furnace practice. Concept / Approach: When primary combustion occurs below the hearth and gases rise into the chamber, the arrangement is termed “underfired.” Sidefired furnaces have burners entering laterally; “covered” is not a standard flow designation; “recirculating” describes flow behaviour but not burner placement. Step-by-Step Solution: Match description (flame under hearth, then up) → underfired.Eliminate sidefired (lateral), covered (nonstandard), and recirculating (descriptive, not positional). Verification / Alternative check: Classic reheating and soaking pit nomenclature aligns with this definition in training materials and OEM docs. Why Other Options Are Wrong: Sidefired: inconsistent with “under the hearth.”Covered: does not define gas entry path.Recirculating: could occur in many layouts; not a burner placement term. Common Pitfalls: Using operational flow descriptors (like recirculation) as if they were standard furnace configuration names. Final Answer: underfired

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