69.A 23.5 kg of steam per second at 5 MPa and 400°C is produced by a steam generator. The

feedwater enters the economizer at 145°C and leaves at 205°C. The steam leaves the boiler

drum with a quality of 98%. The unit consumes 3 kg of coal per second as received having value

of 25,102 kJ/kg. What would be the overall efficiency of the unit in percent?

Steam properties:

@ 5 MPa and 400°C: h = 3195.7 kJ/kg

@ 5 MPa: hf = 1154.23 kJ/kg hfg = 1640.1 kJ/kg

@ 205°C: hf = 875.04 kJ/kg

@ At 145°C: hf = 610.63 kJ/kg

A. 65.72

B. 80.67

C. 88.28

D. 78.82