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20th National Certification Exam Energy Managers & Auditors SEPTEMBER 2019 Paper 4

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Paper 4 Code: Pink 20th NATIONAL CERTIFICATION EXAMINATION FOR ENERGY MANAGERS & ENERGY AUDITORS - September, 2019 PAPER 4: ENERGY PERFORMANCE ASSESSMENT FOR EQUIPMENT AND UTILITY SYSTEMS Section - I: BRIEF QUESTIONS Marks: 10 x 1 = 10 (i) Answer all Ten questions (ii) Each question carries ONE mark 1. The unit of Specific humidity is kg moisture / kg dry air. True 2. In a water Lithium bromide refrigeration system, the concentration of the lithium bromide is increased, in the evaporator. False 3. For the same no of poles and kW rating, the rpm of an energy efficient motor is higher that of a standard motor. True 4. The atmospheric pressure of 1 kg/cm2 (a) is 76 mm of mercury column. False 5. The capacity of diesel generator designed for sea level condition decreases at high altitude. True 6. Two pumps can be operated in parallel provided their closed valve heads are not the same. False 7. Installing a VFD and operating a screw compressor at 50 Hz will increase the power consumption. True 8. Building energy performance index (kWh/yr/m2) will not include captive power used in the building. False 9. The COP and EER (w/w) in a refrigeration system will be numerically different. False 10. The gross heat rate of the power plant does not include auxiliary consumption. False . End of Section - I . Section - II: SHORT NUMERICAL QUESTIONS Marks: 2 x 5 = 10 (i) Answer all the Two questions (ii) Each question carries FIVE marks L1 A food processing industry has been importing 3 tonnes/hr of steam, at 8 kg/cm2(g), with an enthalpy of 661 kcal/kg, at a price of Rs 3,200 per tonne, from a neighboring industry. The steam metering is done at the point of delivery. However, the seller is demanding for higher price, as the steam has to be transported over a distance of 1 km, through a 100 mm (internal diameter) pipe line, from the boiler house. The thickness of the pipe is 4 mm and it is insulated with 50 mm of insulation. The measured outside surface temperature of insulation is 45 OC, whereas the ambient temperature is 30 OC. Estimate the following: 1. Heat loss in piping distribution in kg equivalent of steam /tonne of steam . 2. Final price of steam per tonne after accounting for distribution losses. (2.5 Marks) (2.5 Marks) Ans : Outer dia. of insulated pipe = (100+4+4+50+50) = 208 mm = 0.208 m = DL = 3.14 x 0.208 x 1000 = 653.12 m2 Surface heat loss per unit area = [10+ (Ts Ta)/20] x (Ts Ta) kcal/m2.hr Total heat loss = [10+ (45 30)/20] x (45 30) x 653.12 = 1,05,315.6 kcal/hr Equivalent steam loss (for surface heat loss in the piping distribution), occurring while transporting 3 tonnes/hr of steam = (1,05,315.6) / (661) = 159.327 kg steam/hr Equivalent steam loss in kg of steam per tonne of steam = (159.327 / 3) = 53.109 kg/tonne steam Additional price to compensate for heat losses per tonne of steam = (53.109 x (3200/1000)) = Rs 169.95/ tonne steam Final price of steam = (Rs.3200+ Rs.169.95)/ tonne = Rs. 3369.95 / tonne steam Surface area of the 1 km pipe L2 A highly viscous oil which requires rapid initial heating, with a flow rate of 20 tonnes/hr, has to be heated in a shell and tube heat exchanger, from 55 OC to 155 OC, using saturated steam at 175 OC. The specific heat of oil is 0.5 kcal/kg OC. Only latent heat of steam at 485 kcal/kg is used for heating. 1. What type of heat exchanger is recommended? 2. Draw a schematic of fluid flows with directions and temperatures. 3. Find the LMTD for this application and also the steam requirement. (1 Mark) (2 Marks) (2 Marks) Ans : 1. Parallel flow Heat Exchanger The appropriate choice is parallel flow heat exchanger, mainly to cater for providing rapid initial heating of the viscous fluid. 2. Schematic Diagram Schematic of Parallel flow Heat Exchanger 3. LMTD and Steam Requirement: T1 at feed end T2 at discharge end = = = = 175 55 120 OC 175 155 20 OC LMTD (parallel) = T1 T 2 ---------------Ln T1 / T2 LMTD (parallel) = 120 20 --------------Ln 120 / 20 Steam requirement = = (20000 x 0.5 x (155 55)) / 485 2061.85 kg/hr = 55.81 OC . End of Section - II . Section - III: LONG NUMERICAL QUESTIONS Marks: 4 x 20 = 80 (i) Answer all the Four questions (ii) Each question carries TWENTY marks N1 The operating details and particulars of a natural gas-fired, smoke tube boiler, are given below : Steam flow Steam Pressure Feed water temperature % O2 in dry flue gas Exit flue gas temperature G.C.V. of natural gas Density of natural gas Cost of natural gas Enthalpy of steam at 10.0 Kg./.cm2.(g) Inlet feed water temperature Loss due to Hydrogen Radiation losses in the N.G. boiler Specific heat of flue gases Ambient temperature Density of air Daily hours of operation Yearly operation = 8 tonnes/hr steam = 10 kg/cm2g. = 80 oC. = 4% = 215 oC. = 13,500 kcal/kg = 0.7 kg/m3 = Rs 27/m3 = 666 kcal/kg. = 80 oC = 9.92% = 1.52% = 0.29 kcal/kg oC = 30 oC = 1.125 kg/m3 = 24 hours = 330 days Composition of natural gas (per kg) Carbon = 0.74 kg /kg Hydrogen = Nitrogen = 0.03 kg /kg Oxygen = Ignore Sulphur & Moisture 0.22 kg /kg 0.01 kg /kg Find out the following a. Steam to fuel ratio, in the existing case, in kg/kg ( 8 Marks) b. Total combustion air required in m3/min ( 4 Marks) c. % improvement in the steam to fuel ratio, when the feed water temperature is raised to 95OC due to improved condensate recovery ( 2 Marks) d. Savings in gas consumption in m3/hr ( 4 Marks) e. Yearly monetary savings ( 2 Marks) Ans : Theoretical air required Excess Air, % Actual Air Supplied (AAS) Mass of dry flue gas (mdfg) L1 L2 Radiation losses L3 Efficiency of natural gas boiler on G.C.V. = = = = = = = = = 11.6 C + [34.8 (H2 O2/8)] + 4.35 S] kg air / kg gas 11.6 x 0.74 + [34.8 (0.22 0.01/8)] 16.2 kg air / kg gas (% O2) / (21 % O2) x 100 (4 ) / (21 4) x 100 23.5% (1+0.235) x 16.2 20.0 kg air / kg gas (Mass of combustion gases due to presence C,N,S) + (Mass of N2 in the fuel) + (Mass of N2 in air supplied) + (Mass of excess O2 in flue gas) = (0.74*44/12) + 0.03 + (20*0.77) + (20 16.2) x 0.23 = 19.02 kg (dfg )/ kg gas = % heat loss due to dry flue gases = Mdfg x Cp x (Tq Ta) -------------------------G.C.V. of fuel = 19.02 X 0.29 X (215 30) ----------------------------------- x 100 13500 = 7.56% = 9.92% (Given) = 1.52% (Given) = 100 [7.56 + 9.92 + 1.52] = 81% X 100 . (0.81 x 13500) / (666 80) 18.7 Steam to fuel ratio = = Amount of gas required for steam load of 8000 kg/hr = (8000 / 18.7) = 427.81 kg/hr = 427.81 x 20 Total Combustion air required = 8556.2 kg/hr = 8556.2 /(1.125x60) m3/min Steam to fuel ratio with feed water temp of 95OC % Improvement in steam to fuel ratio Gas consumption with feed water temp at 95OC Gas savings due to increase in feed water temp Yearly monetary savings = 126.76 m3/min = (0.81 x 13500) / (666 95) = 19.15 kg/kg = ((19.15 -18.7) x 100) / (18.7) = 2.41 % = = = = = = = = = 8000 / 19.15 417.75 kg/hr 427.81 417.75 10.06 kg/hr 10.06/ 0.7 14.4 m3/hr 14.4 x 24 x 330 x 27 Rs. 30,79,296 Rs. 33.793 lakhs N2 Pressurized hot water circulation system is employed for heating in a process industry. Hot water at 140 oC is supplied to a process, through a steel piping of 100 mm internal diameter and equivalent length of 2000 meters by an oil-fired hot water boiler of 6,00,000 kcal/hr output capacity. After each weekend holiday, at the beginning of the first shift during startup, while raising the water temperature from 50 oC to 140 oC, the entire piping system carrying water also gets heated from 50 oC to 140 oC. 1. Find out the start-up heating time if the boiler operates at 90% capacity, during this period. (12 Marks) 2. Also, find out the % reduction in start-up heating load and fuel savings, with each start up, if the initial temperature at the start up is increased to 60 oC due to improved housekeeping and insulation. (8 Marks) Make use of the following data and information: Efficiency of the hot water boiler GCV of fuel oil Specific heat of water Density of water Specific heat of steel Density of steel Outer diameter of the pipe = = = = = = = 80% 10,000 kcal/kg 1 kcal/kg oC 1000 kg/m3 0.12 kcal/kg oC 8000 kg./m3 108 mm Ignore the heat loss from the surface of the insulated pipe during start up, in the calculations. Ans : Outer diameter of pipe Inner diameter Equivalent length of pipe network Hold up volume of water Mass of water Volume of steel pipe Mass of steel pipe = = = = = = = = = = = = = = 108 mm 0.108 m 100 mm 0.1 m 2 km 2000 m /4 x (0.1)2 x 2000 15.7 m3 (15.7 X 1000) 15,700 Kg /4 x [(0.108)2 (0.1)2] X 2000 2.612 m3 2.612 X 8000 20,896 Kg Startup heating load = = = = = Heat required to heat water and steel from 50 oC to 140 oC (Mass x Specific heat x Temperature difference) [15700 x 1 x (140 50)] + [20896 x 0.12 x (140 50)] [14,13,000 + 2,25,677] 16,38,677 kcals Time taken for start-up heating = = 16,38,677 / (600000 x 0.90) 3.035 hrs Temperature differential for heating in the existing case = = Temp. differential when Initial temp is increased to 60oC = = 140 50 90oC 140 60 80oC % reduction in start-up heating load = ((90 80) x 100)/ (90) = 11.11% Savings in fuel due for each start up = (0.1111 X 1638677) / (10000 X 0.8) = 22.757 kg per start up (or) Startup heating Load N3 = Heat required to heat water and steel from 60oC to 140oC = ( mass x Specific heat x temp. difference) = [15700 x 1 x (140 60)] + [20896 x 0.12 x (140 60)] = [12,56,000 + 2,00,602] = 14,56,602 kcal % reduction in start-up heating load = ((16,38,677 14,56,602) x 100)/ (16,38,677 ) = 11.11% Savings in fuel due for each start up = (16,38,677 14,56,602) / (10000 X 0.8) = 22.757 kg per start up The management of a process industry is planning to switch over from the existing 300 TR directly-gasfired double effect absorption water chiller to a 300 TR centrifugal water chiller, as a cost saving measure. The double effect absorption chiller is rejecting its heat in to a cooling tower. The proposed centrifugal chiller will be rejecting its heat to the same cooling tower. The management is also planning to connect the heat load of a water-cooled process heat exchanger to the same cooling tower. The cooling water entering the heat exchanger will cool the hot oil from 110 OC to 50 OC. The hot oil flow rate in the heat exchanger is 20,000 kg/hr. Make use of the following data: C.O.P. of double effect absorption chiller Electrical energy input to centrifugal chiller motor GCV of Natural Gas Cost of Gas Efficiency of gas firing Electrical energy cost Specific heat of oil to be cooled by water Motor efficiency Annual operating hours = = = = = = = = = 1.2 0.8 kW/TR 9450 kcal/m3 Rs.27/m3 80% Rs.8.5 / kWh 0.5 kcal/kg OC 87.5 % 7920 hrs. Find out the following -: a) The yearly monetary savings in operating centrifugal chiller in place of the double effect absorption chiller. (8 Marks) b) C.O.P. of the centrifugal chiller. (2 Marks) c) Whether the capacity of the cooling tower is sufficient to take the additional heat load of the process heat exchanger, in addition to that of centrifugal chiller. (10 Marks) Ans : C.O.P. of double effect chiller 1TR (Ton refrigeration) Heat input to double effect chiller (Generator) = = = = Overall heat input considering gas firing efficiency = = Operating cost of double effect chiller = = Electrical input power in centrifugal chiller = Operating cost of centrifugal chiller = = Saving in cost = = Yearly monitory saving = = = 1.2 3024 kcal/hr (3024/1.2) kcal/hr 2520 kcal/hr (2520 kcal/hr / 0.80 Effy of gas firing) 3150 kcal/hr ((3150 x 27) / 9450) Rs.9 /TR 0.8 KW/TR (0.8 X 8.5) Rs.6.8 / TR Rs.9.0 Rs.6.8 Rs.2.2 / TR (2.2 x 300 x 7920) Rs.52,27,200/Rs.52.27 Lakhs Heat rejection load from double effect chiller for 1 TR = (Chilling load at evaporator + Heat input to generator) = (3024 kcal/hr + 2520 kcal/hr) = C.O.P. of centrifugal chiller (1 TR) = = Heat rejection load for 300 TR double effect chiller = = Capacity of the cooling tower should be = 5544 kcal/hr (3024) / (0.8 x 0.875 x 860) 5.02 (5544 X 300) 16,63,200 kcal/hr 16,63,200 kcal/hr. Heat rejection load to cooling tower in the case of of 300 TR Elec l Centrifugal chiller power for 1 TR = (Electrical Input x Motor eff.) In case of centrifugal chiller, heat rejection / TR Heat rejection load of 300 TR centrifugal chiller = = = = = = (0.8 kW/TR X 0.875) 0.7 kW / TR ((3024) + (0.7 x 860)) 3626 kcal/TR (3626 x 300) 10,87,800 kcal/hr Heat load on the cooling tower due to process heat exchanger oil cooling = 20,000 X 0.5 X (110 50) = 6,00,000 kcal/hr Total heat rejection load on the cooling tower = 10,87,800 + 6,00,000 = 16,87,800 kcal/hr Cooling tower capacity is NOT adequate to take the heat load of process heat exchanger in addition to heat rejection load of the centrifugal chiller N4 A Answer any ONE of the following An integrated cement plant is having vertical roller mill (VRM) of 200 TPH capacity for cement grinding, drawing hot air (Temperature: 380 oC and Sp.heat Cp : 0.246 kcal/kg oC) from clinker cooler exhaust. The operational data while grinding PPC, is as under : Particulars Unit Value Particulars Unit Value VRM output (dry basis) oC TPH 200 Mill exit temperature Average feed temperature oC 52 Total gas mass flow rate of circuit at process fan inlet kg/hr 487490 Avg. feed moisture % 3 Stack exhaust gas mass flow rate kg/hr 128124 Ambient temperature oC 30 False air into the circuit (% of total fan flow) VRM motor efficiency % 95 Sp. heat (Cp) of material Mill water spray Coal NCV TPH kcal/kg 3.5 7000 Sp. heat of mill false air Sp. heat of mill exit air Coal cost Rs./M T 9000 VRM motor operating power kcal/kg 540 Latent heat of evaporation of water % kcal/kg oC 90 15 0.21 oC kcal/kg kcal/kg oC 0.238 0.239 kW 4000 A recent energy audit of the plant recommended to install Waste Heat Recovery System (WHRS) for power generation, that may be sold out at Rs. 5.0 per kWh and cement mill heat requirement can be fulfilled by installing a separate Hot Air Generator (HAG) with 90% thermal efficiency. Feed Moisture VRM Cement Mill False Air Bag House Recirculation air Cooler hot air HAG Proposed Heating System Schematic Diagram of Cement Mill (VRM) Circuit Exhaust Stack Estimate the following: a) Calculate the heat and mass balance of input and output components of the VRM (cement mill), considering radiation and convection heat loss to be negligible and also estimate the heat requirement (kcal/hr) of VRM. (6 Marks) b) Determine the amount of hot air being drawn from the clinker cooler. (4 Marks) c) The power generation potential in the cooler hot air, which is presently used for VRM (cement mill) heating, at 28% overall efficiency of WHRS. (6 Marks) d) Hourly coal requirement in HAG. (2 Marks) e) Hourly monetary saving of WHRS power generation using HAG, for cement mill heating. (2 Marks) N4A Sol Water Balance: Dry Feed = 200 TPH Feed moisture = 3% Wet Feed = 200 / (1 - 0.03) = 206.186 TPH Moisture = (206.186 200) = 6.186 TPH Total moisture (including water spray) = 6.186 + 3.5 = 9.686 TPH Air Balance: False air = Total process fan mass flow rate x 15/100 = 487490 x 0.15 = 73123 kg/hr Recirculation air = Total mass flow rate at Process fan Inlet mass flow rate exhaust from stack Recirculation air = 487490 128124 = 359366 kg/hr Input Heat Components: S.No. Description Sensible heat in dry feed 1. (Hfi) Sensible heat input in feed 2. moisture (Hwi) Sensible heat input in mill water spray Sensible heat in false air 3. (Hfa) 4. 5. 6. Calculation Hfi = mfi x Cp x T = 200x1000x0.21x(52-30) Hwi = mwi x Cpx Ta = 6186 x 1 x (52-30) = 3500x 1 x (30-30) Value (kcal/hr) 924000 136092 0 kcal/hr Hfa = mfa x Cp x T =73123.5 x 0.238 x (30-30) 0 kcal/hr Sensible heat in hot air (Hha) Hha= mha x Cp x T Hha is unknown Sensible heat in recirculation air (Hrec) Heat input equivalent of electrical energy (HElect) Hrec= mrec x Cp x T = 359366 x 0.239 x (90-30) HElect= P x 860 x motor eff. = 4000 x 860 x 0.95 Total Heat Input Output Heat Components: Sensible heat in product 1. (cement) (HProd) Sensible heat in mill exit gas 2. (HEG) Heat of evaporation of 3. moisture in feed (HEvep) Heat of evaporation of Water (mill spray) Total Heat Output Heat supplied from clinker cooler air VRM heat requirement Amount of hot air drawn from cooler Power generation potential in cooler hot air Hfi + Hwi +Hfa+Hha+ Hrec+ HElect Hprod= mp x Cp x T = 200000 x 0.21 x (90-52) HEG = mg x Cp x T = 487490x 0.239x (90-30) HEvp= mw x [540+ Texit] = 6186 x [540+(90-52)] = 3500x1x[540+(90-30)] = Hprod + HEG + HEvp (Hha) = 14262114.6 9481400.4 = Hha+ 9481400.4 = 4780714.2 + 9481400.4 mha = 4780714.2 /(0.246x (380-30)) P = 4780714.2 x (0.28/860) 5153308.4 3268000 Hha+9481400.4 1596000 6990606.6 3575508 2100000 14262114.6 4780714.2 14262114.6 55525 kg/hr 1556.5 kW Hourly coal requirement in HAG Revenue from WHR power (Rs. per hour) Cost of coal consumption in HAG (Rs./hr) Monetary Saving mcoal = 4780714.2 /(7000 x 0.9) R = 1556.5 x 5 = 758.8 x 9 S = 7782.5 6829.2 758.8 kg/hr Rs. 7782.5 per hour Rs. 6829.2 per hour Rs. 953.3 per hour OR N-4 B In a textile process house, a stenter is running at a speed of 75 meters/min where, the dried finished cloth is leaving at 6% moisture and 75 oC, whereas the wet cloth is entering at a temperature of 25 oC. The hot air for drying in the stenter is heated by circulating thermic fluid, which in turn is heated in a dedicated furnace oil-fired thermic fluid heater, having an efficiency of 84%. The furnace oil consumption in the thermic fluid heater is 85 kg/hr. The unit takes measures to reduce the inlet moisture. The inlet moisture is now found to be 55%, at the same temperature of 25 oC. The outlet conditions remain the same. The stenter operates 24 hours a day and 30 days a month. The other data is given below -: Stenter dryer efficiency G.C.V. of furnace oil Weight of 10 meters of dried cloth at the outlet = = = 50% 10000 kcal/kg 1 kg Find out : a) Feed rate in kgs/hr b) Percentage reduction in stenter drying load with the change in inlet moisture. c) Furnace oil savings in Tonnes/month. N-4 B Sol (12 Marks) (6 Marks) (2 Marks) Ans: Moisture % at stenter inlet Temperature at stenter inlet, Tin Moisture % at stenter outlet Temperature at stenter outlet, Tout Stenter speed Dried cloth output = % moisture (unknown) = 25 oC = 6% moisture, = 75 oC = 75 meters / min = (75 x 60 x (1/10)) = 450 kg/hr Wt. of bone-dry cloth at outlet per hr (W) = 450 x (1 - 0.06) = 423 kg/hr Hence, Wt. of outlet moisture per kg. of bone dry cloth (mo) = (450 x 0.06) / 423 = 0.0638 kg/kg bone dry cloth Heat supplied by stenter for drying = (Fuel consumption x GCV x Heater eff x Dryer eff) = (85 x 10,000) x 0.84 x 0.50 = 3,57,000 kcal/hr Heat load on the dryer (Heat Consumed) = W x (mi mo) x [(Tout Tin) + 540] kcal/hr = 423 x (mi 0.0638) x [(75 25) + 540] Heat supplied by stenter for drying = Heat load on the dryer (Heat Consumed) 3,57,000 kcal/hr = 423 x (mi 0.0638) x [(75 25) + 540] Inlet moisture per kg of bone dry cloth, mi = 1.494 kg moisture / 1 kg bone dry cloth Total weight of inlet cloth = (1 + 1.494)] Inlet moisture %, wet cloth = (Inlet moisture per kg of bone dry cloth)________ (bone dry cloth + Inlet moisture per kg of bone dry cloth) = (1.494 x 100)/(1+ 1.494) = 60 % Reduction in moisture inlet, the moisture will be 55% Hence, feed rate = 423 x (100/(100-55)) = 940 kg/hr mi = (940 x 0.55)/( 423) = 1.222 kg/ kg bone dry cloth Stenter dryer load with 55% inlet moisture = 423 x (1.222 0.0638) x [(75 25)+540] = 2,89051.974 kcal/hr = say 2,89,052 kcal/ hr Reduction in stenter drying load = 3,57,000 - 2,89,052 = 67,948 kcal/hr % Reduction in stenter drying load = (67,948 x100) / (3,57,000) = 19% Monthly furnace oil savings N-4 C = 0.19 x 85 x 24 x 30 = 11,628 kgs / month = 11.63 tonnes/month In a particular biomass power plant, 33.6 TPH of steam at 63 kg/cm2g, 450 OC is expanding to 0.1 kg/cm2(a), and temperature of 45 OC. The boiler and the turbine are designed for superheat temperature of 475 0C. The following data has been given. Enthalpy of steam at turbine inlet with 450 OC Actual enthalpy at turbine outlet at 0.1kg/cm2(a) Combined efficiency of gearbox and generator Enthalpy of steam at turbine inlet with temp of 475 OC Enthalpy at turbine outlet under isentropic condition (with 475 OC at inlet, exhaust pressure, 0.1 kg/cm2(a) Isentropic efficiency of the turbine with turbine inlet at 475 0C Biomass Boiler Efficiency Calorific value of biomass fuel Cost of biomass fuel Electricity price for power sold Yearly hours of operation Auxiliary consumption = = = = 787.9 kcal/kg 564.78 kcal/kg 92% 802.4 kcal/kg = = = = = = = = 511.77 kcal/kg 79% 72% 3450 kcal/kg Rs 3.3 /kg Rs 6/ kWh 8000 hrs. Remains Same Calculate the following: ( Each 4 Marks) a) Power generated in kW with turbine inlet temperature of 450 OC. b) Steam rate in kg/kWh with improved turbine inlet temperature of 475 OC. c) Additional power generated in kW with improved turbine inlet temperature of 475 OC, assuming steam flow rate remains the same. d) Increase in fuel consumption kg/hr with improved turbine inlet temperature of 475 OC, assuming steam flow rate remains the same. e) Yearly benefit by operating the turbine at inlet temperature of 475 OC. N-4 C- Ans: a) Power generated in kW with turbine inlet temperature of 450 OC Turbine power output with inlet temp 450OC = m (h1 h2)/ (860) x Comb eff ( gg). Where; m = 33,600 kg/hr; h1 = 787.9 kcal/kg; h2 = 564.78 kcal/kg Power output = (33600 (787.9 - 564.78) x (0.92))/ (860) = 8019.86 = say 8020 kW b) Steam rate in kg/kWh with improved turbine inlet temperature of 475 oC Steam rate with improved turbine inlet temperature of 475OC = where s = isentropic turbine efficiency = 860 / [(h1 h2s) x s x gg ] 79% = 0.79 gg = combined gear box and generator efficiency = Steam rate at inlet of 475 OC = 860 / [(802.4- 511.77) x 0.79 x 0.92)] 92% = 0.92 = 4.071 kg/kWh (Or) Steam rate with improved turbine inlet temperature of 475OC: Turbine isentropic efficiency, 79% = (Actual enthalpy drop / Isentropic enthalpy drop) x 100 0.79 = (802.4 H2) / (802.4 511.77) Actual enthalpy at the turbine exhaust, H2 = 572.8 kcal/kg Power generated in kW with turbine inlet temperature of 475 OC = (33600 x (802.4 572.8) x (0.92))/ (860) = 8252.8 = say 8253 kW Steam rate with improved turbine inlet temperature of 475OC = 33600/8253 = 4.071 kg/kWh c) Additional power generated in kW with improved turbine inlet temperature of 475 assuming steam flow rate remains the same Power output with inlet temp of 475OC = 33600 /4.071 = 8253.5 kW = Say 8254 kW Additional power generated = 8254 8020 = 234 kW Additional revenue through power sold = 234 x 6 = Rs.1404/hr O d) Increase in fuel consumption kg/hr with improved turbine inlet temperature of 475 assuming steam flow rate remains the same C, O C, = 33600 (802.4 787.9)/(0.72 x 3450) = 196.135 kg/hr Increase in fuel consumption e) Yearly benefit by operating the turbine at inlet temperature of 475 OC Increase in fuel cost = 196.135 x 3.3 = Rs 647.25/hr = (1404 647.25) x 8000 = Rs. 60,54,000 /- (or) = Rs 60. 54 lakhs Yearly benefit, net increase in revenue OR N-4 D A Multispecialty hospital has conducted energy audit of all their utilities. In the existing system, an electrical chiller is operated and the operating cost is Rs. 11.25 / TR. Steam from the boiler, is used for hot water generation by indirect heating. Latent heat of steam is 500 kcal/kg and steam cost is Rs 2.85 / kg. Other data s for existing system: Electrical Load of the Hospital Cost of Grid Electricity : 625 kW : Rs 9.25 / kWh The audit has proposed to install trigeneration system with a gas engine of 700 kW. The gas engine is operating at 28 % efficiency. Chilled water will be produced through a single effect Vapour Absorption Chiller Machine (VAM) in the trigeneration system, using the entire heat rejected to the jacket cooling water. Hot water requirement will be met using heat recovered from the engine exhaust. The data pertaining to tri-generation system is given below: Cost of Gas GCV of Gas Heat Rejected by the engine to the Jacket cooling water COP of VAM Heat utilized from engine exhaust for hot water generation for hospital purpose Temperature of inlet water for hot water system Temperature of outlet water from hot water system = Rs 45/ sm3 = 9000 kcal/sm3 = 29% of the engine heat input = 1.65 = 20% of total engine exhaust heat = 30o C = 60o C Calculate the following: 1. 2. 3. 4. Hourly Gas Consumption in sm3/hr (2 Marks) TR delivered by VAM (6 Marks) Quantity of hot water generated from exhaust heat for hospital purpose in kg/hr (4 Marks) Annual cost savings in Rs. lakhs/yr on account of Trigeneration system, for 7500 hours of operation. (8 Marks) Ans: 1. Hourly Gas Consumption Power Generation = 625 kW Gas Engine Efficiency = 28 % Heat rate = 860 / 0.28 = 3071.43 kcal/ kWh Hourly Gas Consumption = (625 x 3071.43) / 9000 = 213.29 sm3/hr 2. TR delivered by VAM = 213.29 sm3/hr x 9000 kcal/sm3 Input heat = 1919610 kcal/hr Heat used for power generation = 1919610 x 0.28 = 537491 kcal/hour Balance heat available after power generation = 1919610 - 537491 = 1382119 kcal/hr Heat Utilized for VAM through jacket cooling water = 1919610 kcal/hr x 0.29 = 556687 kcal/hr COP of VAM = 1.65 COP,1.65 = (TR X 3024 kcal/hr) / (Input Heat, 556695 kcal/hr) TR delivered by VAM = (1.65 x 556687) / 3024 = 303.8 TR 3. Quantity of hot water generated from exhaust heat for hospital purpose in kg/hr Heat available for hot water generation = 1919610 x ((100 29 28 ) /100) = 825432 kcal/hr (Or) Engine Exhaust Heat = Heat Input Heat output for power Heat for VAM thro jacket cooling water = 1919610 537491 556687 = 825432 kcal/hr 20 % of the heat in the exhaust is used for hot water generation from 30oC to 60 oC for hospital purpose = 825432 x 0.20 kcal/hr = 165086.4 kcal/hr Equivalent Qty of hot water generated from 30oC to 60 oC for hospital purpose = 165086.4 kcal/hr / (60 -30) = 5503 kg/hr 4. Annual cost savings due to Tri-generation for 7500 hours of operation. Cost of Existing System: Cost of grid power per hour = (625 kW x 9.25 Rs./kWh) = Rs. 5781.25 / hr Cost of chiller operation per hour = (303.8 TR x 11.25 Rs/TR) = Rs. 3417.75 / hr Cost of hot water generation from boiler = [(5503 kg/hr x 1 x (60-30)) / (500 kcal/kg stm)] = 330.18 kg steam/hr x 2.85 Rs./kg steam = Rs 941 / hr Total Operating cost of existing system = Rs (5781.25 + 3417.75 + 941) /hr = Rs 10140 /hr Cost of operation with tri-generation = Gas consumption x Cost of gas = 213.29 sm3/hr x Rs.45 / sm3 = Rs. 9598.1 /hr = (Existing Cost / hr Trigeneration Cost / hr ) Hourly savings = Rs.10140 / hr - Rs. 9598.1 /hr = Rs 541.9 / hr Annual savings for 7500 hrs operation = Rs 541.9 / hr x 7500 hrs / yr = Rs 40.64 lakhs /yr . End of Section - III

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