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Paper 2 –Set A Answer Key Regn. No: ____ _____________ Name: ___________________ (To be written by the candidates) 5th NATIONAL CERTIFICATION EXAMINATION FOR ENERGY MANAGERS & ENERGY AUDITORS– November, 2007 PAPER – 2: Energy Efficiency in Thermal Utilities Date: 17-11-2007 Timings: 1400-1700 HRS Duration: 3 HRS Max. Marks: 150 General instructions: o Please check that this question paper contains 7 printed pages o Please check that this question paper contains 64 questions o The question paper is divided into three sections o All questions in all three sections are compulsory o All parts of a question should be answered at one place Section – I: OBJECTIVE TYPE Marks: 50 x 1 = 50 (i) Answer all 50 questions (ii) Each question carries one mark (iii) Please darken the appropriate oval in the OMR answer sheet with Black HB Pencil or Black Colour Ball Pen, as per instructions 1. The amount of theoretical air required for complete combustion of one ton of coal as compared to that required for one ton of natural gas is: a) higher b) lower c) equal d) none of the above 2. Furnace oil contains 84% carbon by weight. How many kg of CO 2 does complete combustion of 1 kg of furnace oil generate? a) 2.4 b) 3.1 c) 3.6 d) 4.0 3. The highest % of sulphur is present in a) LDO b) FO c) LSHS d) Kerosene 4. LPG is predominantly a mixture of a) methane & propane b) ethane & butane c) propane & butane d) butane & isopropane 5. In flue gas, the theoretical CO2 is 15.0% and the measured CO2 is 12% by volume. The % excess air is: a) 50% b) 150% c) 25% d) 20% 6. The unit of specific heat is o 3 a) kCal /kg b) kCal/kg C c) kCal /m d) kCal 7. Pour point of LSHS is o o o o a) 72 C b) 25 C c) 50 C d) 100 C _________________________ 1 Bureau of Energy Efficiency Paper 2 –Set A Answer Key 8. For stoichiometric combustion of 1 kg of carbon, the required amount of air will be about a) 31 kg b) 21 kg c) 11.6 kg d) 2.66 kg 9. For coal fired system, the flame length is dictated by a) moisture b) volatile matter c) ash content d) fixed Carbon 10. One kg of wood contains 20% moisture and 5% hydrogen by weight. How much water is evaporated during complete combustion of one kg of wood? a) 0.2 kg b) 250 gram c) 0.65 kg d) none of the above 11. The largest heat loss in the heat balance of a coal fired boiler is due to: a) dry flue gas loss b) loss due to hydrogen in the fuel c) radiation losses d) moisture in the air 12. The maximum possible evaporation ratio of a boiler (From & At 100ºC basis) fired with coal having calorific value of 5400 kCal/kg and operating at 90% efficiency will be a) 5 b) 9 c) 15 d) 20 13. The presence of calcium and magnesium bicarbonates in boiler feed water would form: a) acidic solution b) alkaline solution c) neutral solution d) none of the above 14. Good opportunity for energy savings from continuous blow down water of boiler is by a) reusing the hot water so formed as make-up water b) using the blow down steam to run steam turbine c) utilization of flash steam in deaerator d) none of the above 15. Which data is not required to calculate the efficiency of boiler by indirect method a) flue gas temperature b) ambient temperature c) calorific value of fuel d) blow down rate 16. Water treatment for steam boilers is generally required to: a) remove hydrogen b) prevent formation of scales c) help improve combustion efficiency d) reduce stack temperature 17. Maximum Continuous Rating (MCR) of a boiler is the maximum evaporation rate that can be sustained for a) 24 hours b) 8 hours c) 16 hours d) none of the above 18. A rise in conductivity of boiler feed water indicates a) drop in the contamination of feed water b) greater purity of feed water c) rise in the TDS of feed water d) rise in oxygen level 19. The TDS level in boiler water for boiler blow down, is measured through a) alkalinity of water b) thermal conductivity of water c) electrical conductivity of water d) turbidity of water 20. Heat transfer rate for indirect heating application will be less if we heat with a) saturated steam b) dry steam c) superheated steam d) high pressure steam _________________________ 2 Bureau of Energy Efficiency Paper 2 –Set A Answer Key 21. For flash steam calculation, flash steam quantity available depends upon a) condensate pressure and flash steam pressure b) steam pressure c) steam enthalpy at atmospheric pressure d) total heat of flash steam o 22. The heat which is required to change the phase from water at 100 C to saturated steam at o 100 C at atmospheric pressure is a) sensible heat b) latent heat c) super heat d) specific heat 23. For transporting the steam to the long distance, the best quality of steam is a) dry saturated steam b) slightly superheated steam c) mildly wet steam d) very high pressure steam 24. Velocity of steam in a steam pipe, is directly proportional to a) number of bends in pipe b) specific volume of steam c) length of pipe d) none of the above 25. Latent heat at the critical point of a steam phase diagram is a) 540 kCal/kg b) zero c) infinite d) none of the above 26. Mechanical steam traps work on the principle of a) difference in density between steam and condensate b) difference in thermodynamic properties between steam and condensate c) difference in temperature between steam and condensate d) none of the above 27. The inverted bucket trap operates on the principle of _______ difference between water and steam a) pressure b) density c) temperature d) velocity 28. The hearth pressure in the heating zone of furnace should be a) slightly negative pressure b) slightly positive pressure c) high negative pressure d) high positive pressure 29. For the same size and rating, efficiency of continuous furnaces will be more than batch type of furnaces mainly because of a) reduced opening losses in continuous furnace b) reduced flue gas losses in continuous furnace c) increased flue gas in batch furnace d) loss of energy stored in walls of batch furnace 30. Which loss is the highest in a typical re-heating furnace operating at furnace temperature of 0 1250 C? a) flue gas loss b) wall loss c) necessary opening loss d)cooling water loss 31. A re-heating furnace is operating at 10 TPH and consuming FO of 460 kg/hour for O O reheating the material from 40 C to 1100 C. Considering specific heat of material as 0.13 o kCal/kg C and GCV of FO as 10500 kCal/kg, the efficiency of the re-heating furnace will be a) 25% b) 29% c) 35% d) 40% 32. Furnace wall heat loss does not depend on a) temperatures of external wall surfaces b) velocity of air around the furnace c) thermal conductivity of wall brick d) material of stock to be heated. _________________________ 3 Bureau of Energy Efficiency Paper 2 –Set A Answer Key 33. The insulation material suitable for low temperature application to prevent heat gain is a) mineral fiber b) fiber glass c) silica d) polyurethane 34. The coefficient of thermal expansion of refractory material should be a) low b) high c) medium d) very high 35. Example for basic type of refractory is a) chrome b) chrome magnesite c) alumina d) all the above 36. The unit for coefficient of heat transfer in SI system is a) kCal /m-hr °C b) kCal / m²-hr °C c) kCal /m² °C d) none of the above 37. The use of ceramic fiber results in fuel economy due to a) chemical resistance b) thermal shock resistance c) lower maintenance d) lower heat storage 38. The emissivity of ceramic coatings used in furnace: a) decreases with increase in temperature b) remains constant c) increases with increase in temperature d) decreases with increase in furnace pressure 39. The equipment having the highest efficiency in case of thermal power plant is a) boiler b) turbine c) generator d) cooling tower 40. NOx formation in FBC boilers is eliminated because of a) higher velocity of flue gas in combustion chamber b) higher pressure of the air supplied c) lower temperatures in the bed and combustion chamber d) higher contact of solid particles in the flue gas 41. The difference between mean velocity of solid and gas in a FBC boiler is called a) fluidization factor b) slip velocity c) settling velocity d) none of the above 42. The material used to control SO x in the FBC boiler is a) limestone b) alumina c) silica d) lime 43. For efficient sulphur retention in the bed, temperature of a fluidized bed boiler should be a) 700°C b) 1100°C c) 850°C d) 1000°C 44. In an industry, exhaust gas from the furnace is used for power generation by installing waste heat recovery steam boiler and a steam turbine. This type of co-generation will be called as a) gas turbine b) diesel generator c) topping cycle d) bottoming cycle 45. Which type of steam turbines has high heat-to-power ratio? a) back pressure turbine b) gas turbine c) extraction condensing turbine d) reciprocating engine _________________________ 4 Bureau of Energy Efficiency Paper 2 –Set A Answer Key 46. A waste heat recovery system (Shell and Tube heat exchanger) receives hot fluid at 200°C and leaves at 70°C, cold medium enters at 30°C and leaves at 90°C, the type of flow involved in this is a) cross-flow b) co- current flow c) counter-current flow d) none of the above 47. Among which of the following flue gas temperature options, heat recovery equipment will be the least expensive (assuming same material of construction and heat recovered) o o o o a) 300 C b) 350 C c) 400 C d) 500 C 48. In a heat pump a) work is added to improve quality of waste heat b) heat is added to improve quality of waste heat c) steam is added to improve quality of waste heat d) air is added to improve quality of waste heat 49. A heat pipe can transfer up to ------times more thermal energy than copper a) 10 b) 50 c) 70 d) 100 50. Wicks in the heat pipe are provided for a) forward movement of vapours b) forward movement of hot liquid c) return of hot vapours d) return of condensed liquid -------- End of Section - I --------- Section - II: SHORT DESCRIPTIVE QUESTIONS Marks: 8 x 5 = 40 (i) Answer all Eight questions (ii) Each question carries Five marks S-1 (a) State the stoichiometric combustion equation for Propane (C3H8) (b) How many kg of Carbon Dioxide will be generated by complete combustion of 1 kg of Propane Ans i) C3H8 +5O2 =3CO2 +4H2O 44 + 160 = 132 + 72 ii) 44 kg of propane on combustion produces 132 kg of CO2. Hence 1Kg of propane will generate 3 Kg of Carbon dioxide S-2 In a plant, a boiler is generating saturated steam of 2 TPH at a pressure of 7 kg/cm2 g. The feed water temperature is 60°C and furnace oil consumption is 150 kg/hr. What is the efficiency of the boiler by using direct method of efficiency evaluation? (Calorific value of FO is 10,000 kCal/kg, enthalpy of steam is 660 _________________________ 5 Bureau of Energy Efficiency Paper 2 –Set A Answer Key kCal/kg at 7 kg/cm2g) Ans = 2000 x (660-60) x 100 = 80% 150 x 10000 Note : Deduct 3 marks if 60 is not subtracted from 660 S-3 A reheating furnace output is 10 TPH. The billets are heated from 50OC to 1200OC. The oil consumption rate is 700 liters /hr. Specific gravity of FO is 0.95; Net Calorific value of FO is 9650 kCal/kg, and the specific heat of the billet is 0.12 kCal/kg OC. Find out the thermal efficiency of the furnace on NCV basis. Ans Heat input into the furnace : 700x0.95x9650 = 6.42x106 kCal/hr Heat output of the furnace : 10,000x0.12x(1200-50) =1.38x106 K.Cal/hr. Thermal efficiency of the furnace = (1.38/6.42) x 100 = 21.5 % S-4 Name any five parameters required to evaluate economic thickness of insulation Ans For determination of economic thickness following parameters are required: i) Cost of fuel ii) Annual hours of operation iii) Heat content of fuel iv) Operating surface temperature v) Pipe diameter / thickness of surface vi) Estimated cost of insulation vii) Average ambient air temperature S-5 What are the advantages of fluidized bed combustion over fixed grate boiler? Ans The major advantages are as under: Burn low-grade fuels. Reduces NOx and SOx No clinker formation.Faster response to changing demand. Suited for fluctuating fuel quality Higher combustion efficiency Any other relevant point S-6 For a 5 TPH capacity boiler, estimate the rise in temperature of feed water in an economizer, when the flue gas temperature decreases from 310ºC to 180ºC. Air to fuel ratio and evaporation ratio of the boiler are 22 and 10 respectively. Assume condensate recovery and boiler blow down is nil. Specific heat of flue gas: 0.23 kCal/kg oC _________________________ 6 Bureau of Energy Efficiency Paper 2 –Set A Answer Key Ans For 1 kg of fuel steam generated is = 10 kg For 1 kg of fuel makeup water is = 10 kg For 1 kg of fuel required combustion air is = 22 kg For 1 kg of fuel flue gas generated is = 22 +1= 23 kg In economizer heat given by flue gas = heat received by makeup water 23 x 0.23 x (310-180) = 10 x 1 x T T = 68.77 ºC S-7 Sketch the schematic of “Back Pressure Turbine” and “Extraction Condensing Turbine” Cogeneration systems Ans _________________________ 7 Bureau of Energy Efficiency Paper 2 –Set A Answer Key S-8 Explain briefly with a neat sketch the operation of a Regenerator for waste heat recovery. The heat from flue gases is stored in the refractory bricks alternately between time intervals Air for combustion gets preheated by moving over one set of hot refractory bricks After some time the air moves over another set of bricks Thus preheated air is supplied for combustion -------- End of Section – II --------- _________________________ 8 Bureau of Energy Efficiency Paper 2 –Set A Answer Key Section - III: LONG DESCRIPTIVE QUESTIONS Marks: 6 x 10 = 60 (i) Answer all Six questions (ii) Each question carries Ten marks L-1 A boiler utilizes bio-mass as fuel and the ultimate analysis (by weight %) of the fuel is given below: COMPONENT ANALYSIS % Carbon 45 Hydrogen 6 Nitrogen 0.5 Oxygen 46.5 Sulfur 0.5 Ash 1.5 The component analysis is provided on dry basis and the CO2 content of the flue gas measured is 13%. Using the basic combustion equations estimate the following on dry fuel basis for 100 kg of dry fuel fired: (a) Theoretical amount of air required for combustion (b) Theoretical CO2 content in flue gas. (c) Excess air supplied Ans Basic equations: C+O2 = CO2 2H2+O2 = 2H2O S+O2 = SO2 a) Theoretical amount of air for combustion 12 kg carbon requires 32 kg of oxygen forming 44 kg of CO2. 1 kg carbon requires 32/12=2.67 kg O2. (45)C+(2.67x45)O2= (165.15) CO2 4 kg of Hydrogen requires 32 kg of oxygen.1 kg of Hydrogen requires 8 kg of oxygen. (6) H2+(6x8)O2 =(54)H2O 32 kg Sulfur requires 32 kg oxygen to form 64 kg of SO2. 1 kg Sulfur requires 1 kg Oxygen. ( 0.5)S+(0.5)O2 = (1)SO2 _________________________ 9 Bureau of Energy Efficiency Paper 2 –Set A Answer Key Total oxygen required = (120+48+0.5) = 168.5 kg Oxygen present in fuel = 46.5 kg Oxygen to be supplied = 168.5-46.5 = 122 kg Amount of air to be supplied=122 x100/23= 530 kg b) Theoretical CO2 content in flue gas Nitrogen in flue gas : (530-122)+0.5 =408.5 kg Moles of CO2 in flue gas= 165.15/44= 3.75 Moles of N2 in flue gas = 408.5/28 = 14.6 Moles of SO2 in flue gas= 1/64 = 0.016 Total moles =18.37 Theoretical CO2 by Volume = (Moles of CO2X100)/ Total moles( Dry) =(3.75x100)/18.37 = 20.41% c) Calculation of excess air Excess air= (Theoretical CO2/Actual CO2)-1 X100 = (20.41/13)-1 X100 = 57 % L-2 List any 10 energy conservation opportunities in a steam distribution and utilization system Ans 1) Fix steam leaks and condensate leaks 2) Ensure process temperatures are correctly controlled. 3) Maintain lowest acceptable process steam pressures 4) Remove or blank off all redundant steam piping 5) Ensure condensate is returned or re-used in the process. 6) Recover boiler blowdown. 7) Check operation of steam traps. 8) Remove air from indirect steam using equipment 9) Reduce the work done by steam 10) Insulate pipelines Any other relevant point _________________________ 10 Bureau of Energy Efficiency Paper 2 –Set A Answer Key L-3 A plant proposes to install an air pre-heater to pre-heat combustion air from 30º C to 160º C in the existing boiler. The other performance parameters are as given below. Fuel used : Bio-mass Bio-mass firing rate : 28TPH Sulfur in fuel : 0.1% GCV of fuel : 2300 kCal/kg Boiler efficiency : 70% Flue gas temperature : 300 °C Air requirement : 4.5 kg/kg of fuel Specific heat of combustion air : 0.24 kCal/kg oC Specific heat of flue gas : 0.23 kCal/kg oC Estimate the following: (a) Quantity of flue gas assuming negligible ash content in the fuel. (b) Heat transferred to the combustion air. (c) Exit flue gas temperature and whether it is acceptable. (d) Annual fuel savings using GCV for an operating period of 8600 hours/ year Ans a. Quantity of flue gas Quantity of fuel fired: 28000 kg/hour. Combustion air supplied = 28000 x 4.5 = 1,26,000 kg / hour. Total quantity of flue gas = 28000 + 1,26,000= 1,54,000 kg / hour b. Heat transferred to combustion air Heat transferred = 1,26,000 x 0.24 x (160-30) = 39,31,200 kCal / hour c. Exit flue gas temperature Heat drop for the flue gas = 39,31,200 / (1,54,000 x 0.23) = 111ºC. Therefore the exit gas temperature = 300 – 111 = 189º C. The fuel contains very little sulfur and hence the sulfur dew point is very low. Hence the temperature of 189 º C. is acceptable. d. Annual fuel saving Quantity of heat saved / hour = 39,31,200 kCal/hour. Equivalent bio-mass savings = 39,31,200 / (2300 x 0.7) = 2442 kg / hour Annual bio-mass savings = 2442 x 8600 / 1000 = 21001 MT / year. _________________________ 11 Bureau of Energy Efficiency Paper 2 –Set A Answer Key L-4 (a) Explain why dry saturated steam is preferred over wet or superheated steam for industrial process heating (b) Complete the enthalpy equation hg =…. +……., for wet steam and name the variables (c) Why should one use dry saturated steam at the lowest possible pressure for indirect steam heating? Ans i) Dry saturated steam is the preferred choice because: Wet steam has a lower heat content than dry steam. Superheated steam gives up heat at a slower rate than saturated steam. Dry steam alone condenses quickly, thereby providing a higher heat transfer rate. ii) hg =hf +X*hfg Where hf = Enthalpy of saturated water at a given pressure. hfg =Enthalpy of evaporation hg =Enthalpy of saturated steam. X=Dryness fraction of steam. iii) The latent heat of steam increases with reduction of steam pressure and it is only the latent heat that is transferred during indirect heating applications. L-5 In an industry, an electrical furnace consuming 50 kWh/batch is to be converted into furnace oil fired furnace. Estimate the annual savings with furnace oil replacement considering the following: i) Efficiency of electrical furnace : 75% ii) Efficiency of FO fired furnace (NCV Basis) : 52 % iii) Number of batches : 5000 batches/ year iv) Cost of Electricity : Rs. 4 per kWh v) Cost of FO : Rs. 20 per kg vi) Net Calorific Value of FO : 9650 kCal /kg Ans Energy consumption per batch by furnace : 50 kWh Efficiency of electrical furnace : 75% Useful heat generated : 50 x 5000 x 860 x 0.75 = 161250000 kCal/year Conversion of Electrical Furnace into Oil Fired Furnace For meeting useful heat, requirement of FO : 161250000 /9650 = 16710 kg/year Efficiency of FO fired furnace : 52 % _________________________ 12 Bureau of Energy Efficiency Paper 2 –Set A Answer Key Net FO required to meet useful heat : 16710 /0.52 = 32134 kg/year Cost required to run the electrical furnace = 50 x 5000 x 4 = Rs. 1000000 Cost required to run the furnace by (FO) = 32134 x 20 = Rs. 642680 Cost savings = Rs. 357320 L-6 (a) Calculate the blow down rate for a boiler with an evaporation rate of 5 tons/hr, if the maximum permissible TDS in boiler water is 3500 ppm and with 17 % make up water addition. The feed water TDS is around 350 ppm. (b) Steam at a pressure of 15 kg/cm2g is flashed to a lower pressure of 2 kg/cm2g. If the steam flow rate is 1000 kg/hr. Calculate the % of flash steam and flow rate of flash steam? Given are the data from steam tables : Sensible heat of steam at 15 kg/cm2g : 200 kCal/kg Sensible heat of steam at 2 kg/cm2g : 120 kCal/kg Latent heat of steam at 2 kg/cm2g : 666 kCal/kg Answer Feed water TDS x % Make up a) Blow down (%) = Permissible TDS in Boiler Percentage blow down = 350 x 17/3500 = 1.7 % If boiler evaporation rate is 5000 kg/hr then required blow down rate is: = 5000 x 1.7/100 = 85 kg /hr b) The flash steam quantity can be calculated from the following formula: % Flash steam =S1 –S2 --------------- L2 Where S1 is the sensible heat of high pressure condensate. S2 is the sensible heat of steam at lower pressure L2 is the latent heat of flash steam at the lower pressure. Substituting values= (200-120)/666 =80/666 =12% Flow rate of high-pressure steam: 1000 kg/hr. Flash steam quantity: 1000*(12/100) = 120 kg/hr. -------- End of Section - III --------- _________________________ 13 Bureau of Energy Efficiency