Problem 15.76 At a steam power plant, steam engines work in pairs, the heat outp
ID: 1486001 • Letter: P
Question
Problem 15.76
At a steam power plant, steam engines work in pairs, the heat output of the first one being the approximate heat input of the second. The operating temperatures of the first are 710 C and 460 C, and of the second 435 C and 290 C.
Part A
If the heat of combustion of coal is 2.8×107J/kg, at what rate must coal be burned if the plant is to put out 860 MW of power? Assume the efficiency of the engines is 65% of the ideal (Carnot) efficiency.
Express your answer using two significant figures in kg/s.
Part B
Water is used to cool the power plant. If the water temperature is allowed to increase by no more than 4.5C, estimate how much water must pass through the plant per hour.
Express your answer using two significant figures in gal/h
Problem 15.76
At a steam power plant, steam engines work in pairs, the heat output of the first one being the approximate heat input of the second. The operating temperatures of the first are 710 C and 460 C, and of the second 435 C and 290 C.
Part A
If the heat of combustion of coal is 2.8×107J/kg, at what rate must coal be burned if the plant is to put out 860 MW of power? Assume the efficiency of the engines is 65% of the ideal (Carnot) efficiency.
Express your answer using two significant figures in kg/s.
Part B
Water is used to cool the power plant. If the water temperature is allowed to increase by no more than 4.5C, estimate how much water must pass through the plant per hour.
Express your answer using two significant figures in gal/h
Explanation / Answer
a. heat input = heat output / efficiency = 860 x 10^6 /0.65 = 1323.07 x 10^6 J
coal burning rate = 1323.07 x 10^6 /2.8 x 10^7= 47.25 kg/s
b.heat rejected per hour = (heat input - power output) x 3600 = 463.07 x 10^6 x3600 = 1.66 x 10^12 j
mass of water = 1.66x 10^12 /( 4186 x 4.5) = 8.8 x 10^7 kg or 8.8 x 10^7 ltre / hr