Power Plant Engineering

(C) Heat Carried Away by Exhaust Gases
Let W
2
= Weight of exhaust gases leaving per minute in kg.
(sum of weight of air and fuel supplied)
T
3
= Temperature of flue gases supplied per minute °C.
T
4
= Temperature °C of exhaust gases.
K
P
= Mean specific heat at constant pressure of exhaust gases

---

256
POWER PLANT ENGINEERING
The heat carried away by exhaust gases
= W
2
× K
P
× (T
4
– T
3
) kg cal.
(D) Heat Unaccounted for (Heat Lost Due to Friction, Radiation etc.). The heat balance
sheet is drawn as follows:
Item
Head units kcal
Percent
Heat in fuel supplied
(a) Heat absorbed by I.H.P.
(b) Heat rejected to cooling water
(c) Heat carried away by exhaust gases
(d) Heat unaccounted for (by difference)
Total
A typical heat balance sheet at full load for Diesel cycle (compression ignition) is as follows:
(1) Useful work = 30%
(2) Heat rejected to cooling water = 30%
(3) Heat carried away by exhaust gases = 26%
(4) Heat unaccounted (Heat lost due to friction, radiation etc.) = 10%.
SOLVED EXAMPLES
Example 1. A diesel engine has a brake thermal efficiency of 30%. If the calorific value of fuel
used in 10000 kcal kg, calculate the brake specific fuel consumption.
Solution. 
η
b
= Brake thermal efficiency = 0.3
I.H.B. hr = 632.5 kcal
η
b
= H.P. hr equivalent/(w × C.V.)
where,
w = Specific fuel consumption per hr.
C.V. = Calorific value of fuel = 10,000 kcal/kg.
0.3 = 
632.5
(
10,000)
w
×
w = 0.21 kg/H.P. hr.