Heat loss
due to dry flue gas in Boilers-Online calculator
What do you mean by dry flue gas loss in
Boilers?
Dry flue gas heat loss refers to the energy carried away
by exhaust gases (excluding moisture content) leaving the boiler stack.
What are the gases
associated with dry flue gas?
Gases mainly consist of nitrogen (N₂), Carbon dioxide (CO₂),
Oxygen (O₂), and traces of other gases.
On what factors dry
flue gas loss depends?
Dry flue gas loss in Boilers is mainly depends on;
Flue gas temperature
Flue gas quantity
Excess air
Moisture in the fuel
Efficiency of Economiser and Air pre-heaters
What is the significance of calculating heat
loss due to dry flue gas in Boilers?
In most industrial boilers, dry
flue gas loss can account for 5% to 15% of total heat loss,
making it the largest single loss component.
Calculating heat loss due to dry flue gas in
Boilers is utmost important because;
Indicates Boiler Efficiency
Dry flue
gas carries a significant amount of heat energy out of the boiler stack. By
calculating this loss, you can determine how much heat is not being utilized for steam
generation.
Helps Reduce Fuel Consumption
When flue gas heat loss is high, it means fuel
energy is being wasted.
Identifying this loss allows operators to:
Optimize combustion
Adjust excess air levels
Improve burner performance
Optimizes Flue Gas Temperature
Stack
temperature directly affects dry flue gas loss.
High
stack temperature = More heat loss
Proper
control ensures:
Maximum
heat transfer in boiler
Minimal
energy escaping through chimney
Improves Combustion Efficiency
Dry flue
gas loss depends on:
Excess
air supplied
Fuel
composition (carbon, hydrogen, etc.)
Flue gas
temperature
By analysing
this loss, you can:
Maintain
ideal air-fuel ratio
Avoid
incomplete combustion
Prevent
energy wastage
Reduces Environmental Impact
Lower
heat loss means:
- Less fuel burned
- Reduced emissions (CO₂, NOx)
Calculating
heat loss due to dry flue gas is crucial because it:
- Directly reflects boiler
efficiency
- Helps save fuel and cost
- Enables better combustion
control
- Supports energy conservation
and environmental compliance
Heat loss
due to dry flue is calculates as below;
Heat
loss% = Mass of flue gas (Mf) X Cp X (Tf-Ta) X 100 / GCV of fuel
Where,
Mf = Mass of dry flue gas (kg/kg of fuel)
Cp = Specific heat of flue gas
(kcal/kg°C)
Tf = Flue gas temperature (°C)
Ta = Ambient temperature (°C)
Dry flue gas
loss increases as the mass of flue gas increases
This mass of flue gas increases due to;
Type of the
fuel
More excess
air
Over
loading of the Boiler
High
moisture content in the fuel: High moisture content fuel drag more air for
combustion
Also dry
flue gas loss increases as the temperature of the flue gas at APH outlet
increases, this is mainly due to poor heat transfer in Economiser and APHs.
Let’s
calculate heat loss due to dry flue gas
in boilers with sample values:
Mf = 9
kg/kg fuel
Cp = 0.24
kcal/kg°C
Tf = 150°C
Ta = 28°C
GCV = 46000
kcal/kg
Heat
loss% = Mass of flue gas (Mf) X Cp X (Tf-Ta) X 100 / GCV of fuel
Heat
loss% = 9 X 0.24 X (150-28) X 100 / 4600
Heat
loss% = 5.2%
A boiler of capacity 150 TPH, consumes coal of GCV
3900 kcal/kg at the rate of 45 TPH, a total combustion air of 350 TPH is being
supplied to ensure complete combustion. Calculate the heat loss due to dry flue
gas. Assume APH outlet flue gas temperature 150 deg C and Ambient air
temperature 30 deg C.
Coal consumption; 45 TPH
Total combustion air = 350 TPH
Total flue gas generated = 350 + 45 = 395 TPH =
395/45 = 8.77 kg/kg of fuel
Note: Neglected ash percentage in coal
Heat
loss% = 8.77 X 0.24 X (150-30) X 100 / 3900
Heat
loss% = 6.47%
