Forced Draft Fan Sizing For Fired Heaters / Boilers

Fired Heaters / Steam Boilers require combustion air for combustion of the fuel to generate heat for heat transfer to the fluid being heated which in case of steam boiler would be water. Control of combustion air is an absolute must to control the combustion process to provide optimum heat transfer in any fired heater / boiler.

A Forced Draft (FD) fan is commonly employed in many fired heater / boiler applications for supplying combustion air to the combustion box of the fired heater. Normally the FD fan is part of the fired heater package supplied by the package vendor. However, during the design phase of any plant / unit a electrical load list may be required during the FEED stage of engineering. It is quite possible that at that stage the vendor data related to the fired heater may not be available and the design engineer would require to provide an estimate of the power requirement for the FD fan. In such a case it becomes essential that the design engineer does the preliminary FD fan sizing calculations. Also if the fired heater is debottlenecked for increased capacity it may require checking the adequacy of the existing FD fan for the increased capacity

The purpose of this blog entry is to guide a design engineer in estimating the power requirement of a FD fan for fired heater / Boiler application.

The estimation methodology is described in steps considering the adequacy check of an existing FD fan which can be also extended for initial estimation for a new fired heater. Readers can build an excel sheet based on the steps.

1. Estimate the theoretical air requirements by combustion calculations for the fuel being combusted. A combustion calculations spreadsheet is available in the "Free File Repository" of "Cheresources" which would help in determining the theoretical air requirement as kg of air / kg of fuel.

2. Estimate or obtain (from vendor) the maximum fuel consumption for the maximum or design firing rate of the fired heater as kg of fuel / h.

3. The above two steps will provide the theoretical air requirement in kg / h.

4. Find out the maximum average ambient air temperature which the FD fan can see at the site ambient conditions. This will be required for calculating the air density.

5. Consider excess air requirement for the combustion process based on the fuel type. Some guidelines are provided below:

a. Eastern bituminous coal: 25%
b. Western subbituminous coal: 20%
c. Lignite Coal: 20%
d. Oil (#2 & #6): 10-20%
e. Natural Gas: 5-10%
f. Refinery Gas: 8-15%
g. Blast-furnace gas: 15-25%
h. Coke-oven gas: 5-10%

6. Consider air leakage through the combustion air preheater as a percent of theoretical air. Typically a value of 2% of theoretical air is quite adequate for preliminary sizing.

7. Consider air leakage through the furnace or heater shell as a percent of theoretical air. Typically a value of 7.5% of theoretical air is quite adequate for preliminary sizing.

8. Sum up the theoretical air including the leakage losses to get the final theoretical air in kg/h.

9 Add up the excess air required based on the fuel type and as per guidelines given in step 5 to the theoretical air as calculated in step 8.

10. What you have now is the calculated air flow requirement in kg/h.

11. It is recommended that an additional safety margin of 5-10% on the calculated air mass flow be provided in order to take care of any unknowns during operation.

12. Consider the various pressure drops in the FD fan discharge system for preliminary sizing. Some recommended values are as follows:
a. Air Preheater Pressure Drop: 40 mmWC
b. Duct Pressure Drop: 20 mmWC
c. Heater Wind box Pressure Drop: 45 mmWC
d. Burner Pressure Drop: 30 mmWC

The above are recommended values for the case where data is not available. Readers may use actual data in case it is available.

13. A safety margin of 20%is recommended on the sum of the pressure drops calculated as per step 12 to account for unknowns.

14. You have now the pressure drop available or the differential pressure for the FD fan to be sized based on steps 12 & 13.

15. Calculate the air density for the given average maximum ambient temperature using:
rho = P*M / R*T*Z
where:
rho = air density, kg/m³
P = atmospheric pressure at the heater location, bar(a)
M = MW of air = 28.96
R = Gas constant =0.0831447
T = absolute temperature, K
Z = compressibility factor (normally =1 at the ambient conditions)

16. Divide the calculated air flow rate as calculated in step 11 by the density to get the volumetric flow rate of air in m³/h.

17. Use the volumetric flow rate calculated in step 16 and the differential pressure calculated as in step 14 to determine the shaft power of the FD fan using the equation:

Shaft Power, kW = Volumetric Flow (m³/h) / 3600*Differential Pr (kPa)

18. Consider a fan efficiency of 75% and motor efficiency of 90% and divide the shaft power obtained as per step 17 with these efficiencies. This will provide you the motor power.

19. Consider a standard commercial motor size closest to the calculated motor power as per step 18.

The exercise of sizing the FD fan motor is complete.

Comments and questions are most welcome.

Regards,
Ankur.