Civil Engineering Reference
In-Depth Information
Fig. 6.6
Example of radiation losses chart (each curve corresponds to a boiler rated power value)
Heating Value is assumed as heat input in the fuel), the humidity of the
combustion air, and the water contained in the fuel. Industrial boilers not
equipped with waste heat recover equipment such as air preheaters or
economizers have very large flue gas losses because of the high stack-gas
temperature (473-573 K; 200-300
C; 392-572
F);
• Losses due to incomplete combustion. These consist mainly of combustible
material in the flue gas such as carbon monoxide, hydrogen, and hydrocarbons.
Additionally,
refuse may contain unburned solid fuels and other solid
combustibles;
• Radiation losses through the exterior surfaces of the boiler. Approximate evalu-
ation of these losses by charts developed by the ABMA (American Boiler
Manufacturers Association) or similar ones is shown in Fig.
6.6
;
• Additional losses.
The indirect method is based on the determination of the above-mentioned
losses, by first evaluating singly the heat losses per unit of fuel and then converting
these values to a percentage loss by means of the heating (Higher or Lower)
Heating Value.
The waste heat going up the stack is evaluated as the sum of three components:
1. Heat loss (sensible heat) due to dry gas is equal to the kilograms of dry gas per
kilogram of fuel multiplied by the specific heat of the combustion gases (roughly
1 kJ/kg
F) multiplied by the temperature difference
between the stack exit gas (
t
s
) and the inlet air for combustion (
t
a
) The stack
exit temperature
t
s
is closely related to the recovery equipment temperature such
as economizers and air preheaters;
K or 0.24 Btu/lb
