Mechanical Engineering 2024 Paper II 50 marks Compulsory Explain

Q5

(a) Explain the functions of three types of superheaters used in power boilers. Sketch the heat addition process in them on a T-s chart. Also mention the function of desuperheater (or attemperator). 10 marks (b) Explain the principle of operation of cogeneration plants using a schematic diagram. 10 marks (c) A refrigerator in a laboratory uses R-134a as the working substance. The high pressure is 1200 kPa, the low pressure is 101·3 kPa and the compressor is reversible. It should remove 500 W from a specimen currently at – 20°C (not equal to T_L in the cycle), that is inside the refrigerated space. Find the cycle COP and the electrical power required. The enthalpy of superheated R-134a at 1200 kPa may be taken as 430 kJ/kg at compressor outlet. Use the R-134a property table attached. The refrigerant enters the compressor as saturated vapour. 10 marks (d) SI or CI, which engine emits higher unburnt HC emissions? What are the causes for UHC emissions from IC engines? Explain in brief. 10 marks (e) Draw the schematic arrangement diagram of an air washer. Describe the various air-conditioning processes it can perform. 10 marks

हिंदी में प्रश्न पढ़ें

(a) शक्ति बॉयलरों में उपयोग होने वाले तीन प्रकार के अतितापकों के कार्यों को समझाइए। एक T-s आरेख चार्ट पर उनमें ऊष्मा योग प्रक्रम का खाका बनाइए। साथ ही विअतितापक (तापमान संयामक) का कार्य भी बताइए। 10 अंक (b) एक योजनाबद्ध आरेख का प्रयोग करते हुए, सह-उत्पादन संयंत्रों के कार्य-सिद्धांत को समझाइए। 10 अंक (c) एक प्रशीतक एक प्रयोगशाला में R-134a को कार्यकारी वस्तु के रूप में प्रयोग करता है। उच्च दाब 1200 kPa, निम्न दाब 101·3 kPa तथा संपीडक प्रतिवर्ती है। उसे एक नमूने से 500 W निकालना है जो कि अभी – 20°C पर (चक्र में T_L के बराबर नहीं है) अर्थात् जो कि एक प्रशीतित स्थान पर है। चक्र की COP तथा आवश्यक विद्युत शक्ति ज्ञात कीजिए। संपीडक निर्गम पर अतितापित R-134a की 1200 kPa पर एन्थैल्पी 430 kJ/kg ली जा सकती है। संलग्न R-134a की गुण तालिका, जो कि अंत में दी गई है, का प्रयोग कीजिए। संपीडक में प्रशीतक संतृप्त वाष्प के रूप में प्रवेश करता है। 10 अंक (d) SI अथवा CI, कौन-सा इंजन उच्चतर अदग्ध HC उत्सर्जन निकालता है? IC इंजनों से UHC उत्सर्जनों के क्या कारण हैं? संक्षेप में समझाइए। 10 अंक (e) एक वायु धावक का योजनाबद्ध व्यवस्थात्मक आरेख खींचिए। विभिन्न वातानुकूलन प्रक्रमों का वर्णन कीजिए जिनको यह सम्पन्न कर सकता है। 10 अंक

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Directive word: Explain

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How this answer will be evaluated

Approach

Explain the theoretical concepts for parts (a), (b), (d), and (e) with clear diagrams, while solving part (c) with rigorous thermodynamic calculations. Allocate approximately 20% time each to (a), (b), (d), and (e) as they carry equal marks; dedicate 20% to (c) for the numerical solution. Structure as: (a) superheater types with T-s sketches, (b) cogeneration schematic with energy flow, (c) stepwise COP and power calculation, (d) SI/CI HC emission comparison with causes, (e) air washer diagram with process descriptions.

Key points expected

  • (a) Three superheater types: radiant (wall-mounted, high temp), convective (flue gas, temp increases with load), combined; T-s diagram showing heat addition lines; desuperheater function for temperature control and turbine protection
  • (b) Cogeneration principle: simultaneous electricity and useful heat production; schematic showing boiler → turbine → generator with extraction/condenser and process heat utilization; energy cascade and overall efficiency improvement
  • (c) COP calculation using h_f and h_g from R-134a tables at given pressures; compressor work = h_2 - h_1; refrigeration effect = h_1 - h_4; electrical power = Q_L / COP; note: -20°C is specimen temp, not evaporator temp
  • (d) SI engines emit higher UHC due to flame quenching in crevices, incomplete combustion during cold start, and fuel-rich mixtures; CI engines have lean operation but may have HC from over-mixing; causes: wall quenching, crevice volumes, oil film absorption, misfire, poor atomization
  • (e) Air washer schematic: chamber with spray nozzles, eliminators, pump, recirculated water; processes: cooling and dehumidification, cooling and humidification, adiabatic saturation, heating and humidification

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness20%10For (a) correctly distinguishes radiant vs convective superheater behavior with load; for (b) explains topping vs bottoming cogeneration cycles; for (c) uses correct refrigerant properties and distinguishes between specimen temperature and cycle temperatures; for (d) accurately identifies SI higher HC and explains wall quenching mechanism; for (e) correctly identifies all four psychrometric processes possible in air washerCovers most concepts but confuses superheater types or misses distinction between topping/bottoming cycles; in (c) uses wrong enthalpy values or confuses temperatures; in (d) states SI higher HC but explanation of causes superficial; in (e) misses 1-2 air washer processesFundamental errors: treats all superheaters similarly, describes cogeneration as simple combined cycle, uses wrong refrigerant tables or ignores saturated liquid assumption, claims CI higher HC without justification, describes air washer as simple humidifier only
Numerical accuracy20%10Part (c): correct h_1 from saturated vapor at 101.3 kPa (≈234 kJ/kg), h_2 = 430 kJ/kg given, h_3 = h_4 from saturated liquid at 1200 kPa; COP = (h_1 - h_4)/(h_2 - h_1) ≈ 2.8-3.2; power = 500/COP ≈ 160-180 W; all values from standard R-134a tables with proper interpolationCorrect method but minor interpolation errors or uses approximate h_f values; COP within 10% of correct value; power calculation follows from COPMajor errors: uses h_g instead of h_f for h_4, ignores throttling process, calculates COP as T_L/(T_H-T_L) using wrong temperatures, or arithmetic errors yielding COP > 5 or < 1
Diagram quality20%10(a) T-s diagram with saturation dome, three distinct superheating curves (radiant, convective, combined) showing different slopes; (b) clear cogeneration schematic with boiler, turbine, condenser, process heat extraction, and electrical generator; (e) detailed air washer with spray arrangement, baffle eliminators, water sump, pump, and air flow arrows; all diagrams labelled with state pointsDiagrams present but missing key labels or showing generic schematics; T-s diagram shows only one superheat curve; cogeneration diagram lacks process heat utilization detail; air washer missing eliminators or pumpNo T-s diagram or wrong coordinates (P-v instead); cogeneration drawn as simple Rankine cycle without heat extraction; air washer confused with cooling tower; diagrams unlabelled or technically incorrect
Step-by-step derivation20%10(c) Explicitly states given data, extracts h_1 from saturation table at P_low, identifies h_2 = 430 kJ/kg, finds h_3 = h_4 from saturated liquid at P_high, calculates refrigeration effect q_L = h_1 - h_4, compressor work w_c = h_2 - h_1, COP = q_L/w_c, then power = Q_L/COP; units tracked throughout; (a)(b)(d)(e) show logical flow from principle to applicationShows most steps but skips table lookup demonstration or combines steps; minor unit inconsistencies; descriptive parts have adequate but not rigorous structureNo derivation shown—only final answers stated; or incorrect thermodynamic relations (e.g., COP = T_H/(T_H-T_L) for vapor compression cycle); jumps from given data to final numbers
Practical interpretation20%10(a) Explains why desuperheaters are critical for turbine blade protection and load following; (b) Cites Indian examples: NTPC cogeneration plants, sugar mill bagasse-based cogeneration under PAT scheme; (c) Notes actual power higher due to irreversibilities, discusses effect of specimen temp approaching evaporator temp; (d) Links HC emissions to Bharat Stage VI norms and catalytic converter requirements; (e) Discusses air washer applications in textile mills (Ahmedabad, Coimbatore) for humidification controlMentions practical relevance superficially; generic statements about energy efficiency or pollution control without specific Indian context or industrial applicationsNo practical interpretation; treats all parts as purely academic exercises; no mention of power plant operations, emission norms, or industrial refrigeration applications

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