Question

pdf 9: Liquid water at a pressure of 8 MPa and temperature of 60c enters a boiler at a rate of 240 kg/sec. The water leaves as steam at a pressure of 8 MPa and a pressure of 420 C. How many tons of coal per hour are needed for this boiler if the heat exchanger efficiency in the combustion chamber is 92%. Coal has a heat content of 26 million B To solve this problem, use Tables A-6 and A-7 with interpolation to find enthalpies After Friday you can do Carnot Heat Engine problems (Section 7.5-7) 10: 7-77 11: 7-78 12: 7-82E (note that the efficiency can be written using the ratio of heat transfers or the ratio of temperatures). 13: 7-83E (Since this heat engine is reversible it can be assumed to have an efficiency equal to the theoretical maximum (Carnot) efficiency) 14: 7-86 (hint: compare the actual and theoretical (Carnot) efficiencies this is an invalid claim for more info, read Section 7-5 on perpetual motion machines)
please answer all of question 9 thank you

Answer 1

Given p1 = 8mpa, T1 = 60degree ms = 240kg/s p2 = 8mpa, T2 = 420degree c Let mc denote the mass of coal needed in tonic per hour eta comb = 92% = 0.92 Heat content of coal, C.V = 26 times 10^6 BTU/ton From stem tables, at p1 = 8Mpa, saturation temperature (T1) sat = 295.01degree c Since T1 < (T1) sat, hence at level of boiler, we have sub cooled liquid. at p1 = 8mpa, T1 = 60degree c, specific enthalpy, h1 = 257.849kJ/kg (from steam tables) since, T2 > (Tsat) 1, hence of outlet of boiler, we have superheated steam. from steam table, h2 = 3194.228 kJ/kg Heat transfer to steam, per heat Qs = ms(h2-h1) times 3600 kJ/hr = 240 times (3194.228-257.894) times 3600 kJ/hr Heat generated by burning of coal, Ec = mc(c.v) eta comb. where mc = mass of coal required in tonic per hour C.v = 26 times 10^6 BTU/toh = 26 times 10^6 times 1.055 kJ/ton (since 1BTU = 1.055kJ) Qc = mc times 26 times 10^6 times 1.055 times 0.92 kJ/hr