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Module 4Problem: 1

What is the highest cycle efficiency possible for a heat engine operating between 900 C and

30 C ?

Answer : 74.2%

Problem:2

Two reversible heat engines operate in series between a source at 600 C and a sink at 30 C.If the engines have equal efficiencies and the first rejects 400 kJ to the second, calculate:(i) the temperature at which heat is supplied to the second engine;(ii) the heat taken from the source;(iii) the work done by each engine.Assume that each engine operates on the Carnot cycle.

Answers: 241.3 C ; 679.1 kJ ; 279.1 kJ and 164.4 kJ .

Problem:3Estimate the overall cumulative efficiency for domestic electrical and gas water heating, takinginto account energy losses inFuel productionFuel transportationEnergy conversion to electricityTransmission of electricityHeating of waterUse the conversion efficiencies given in the table shown below:Step Efficiency of step(percent)ELECTRIC (coal-fired)Production of coal 96Transportation of coal 97Generation of electricity 33Transmission of electricity 85Water heating efficiency 92GASProduction of natural gas 96Transportation of natural gas 97

Heating efficiency 64Answers: 24.03% and 59.6%

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Specific heat of waste water and fresh water, 4.2 kJ/kg K .Overall heat transfer coefficient, 2500 W/m K.For a single shell-pass, two tube-pass heat exchanger take the followingcharacteristicwhen R = 1.Effectiveness 0 0.46 0.53 0.56 0.57 0.58 0.58 0.58NTU 0 1.0 1.5 2.0 2.5 3.0 3.5 4.0Answers : 73.08 kW ; 10.08 m2 ; 23.7 C .

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