Power Plant Engineering

DIESEL POWER PLANT 255

8.17 Heat Balance Sheet

It is a useful method to watch the performance of the diesel power plant. Among all the heat
supplied to an engine only part of it is converted into useful work, the remaining goes as waste. The
distribution of the heat imparted to an engine is called as its heat balance. The heat balance of an engine
depends on a number of factors among which load is primary importance. The heat balance of an inter-
nal combustion engine shows that the cooling water and exhaust gases carry away about 60-70% of heat
produced during combustion of fuel. Heat balance sheet is a useful method to watch the performance of
the plant. In order to draw the heat balance sheet of Diesel engine, the engine is run at constant load and
constant speed and the indicator diagram is drawn with the help of indicator. The following quantities
are noted:

1. The quantity of fuel consumed during a given period.


2. Quantity of cooling water and its outlet and inlet temperatures.


3. Weight of exhaust gases.


4. Temperature of exhausts gases.


5. Temperature of flue gases supplied.
   To calculate the heat in various items proceed as follows.
   Let
   W = Weight of fuel consumed per minute in kg.
   G = Lower calorific value of fuel, kcal per kg.
   Then heat in fuel supplied per minute = WCV kcal.
   The energy supplied to Diesel engine in the form of fuel input is usually broken into following
   items:

(A) Heat Energy Absorbed in I.H.P. The heat energy absorbed in indicated horsepower, I.H.P.
is found by the following expression:

Heat in L.H.P. per minute

(I.H.P. × 4500)/J kcal

(B) Heat Rejected to Colling in Water.

Let

W 1 = Weight of cooling water supplied per minute (kg)

T 1 = Inlet temperature of cooling water in °C

T 2 = Output temperature of cooling water in °C

Then heat rejected to cooling water = W 1 (T 2 – T 1 )

(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.

KP = Mean specific heat at constant pressure of exhaust gases