An underfeed stoker introduces solid fuel beneath the burning bed, commonly through a retort by a ram or screw, while combustion air enters through tuyeres. This calculator represents only a simplified screw-volume and fixed-property heat comparison.
Under-feed Heating Furnace Interactive Calculator
Enter an assumed conveyor diameter, pitch, speed, fill fraction and assumed useful-heat fraction. The tool uses fixed bulk density 850 kg/m³ and fixed heating value 30 MJ/kg; it is not a stoker, furnace, combustion or emissions design model.
Equation Used
- Use the assumed effective screw-conveyor diameter for the swept-volume calculation, not the retort diameter.
- The screw has constant pitch and the filled fraction is uniform.
- Bulk density is fixed at 850 kg/m³ and higher heating value at 30 MJ/kg.
- The entered efficiency is an assumed useful-heat fraction applied directly to fuel input.
- Shaft volume, flight thickness, clearance, slip, compression, variable pitch, fuel grading, caking, air flow, combustion, bed behavior, ash, emissions, heat transfer, controls, loads, power, strength and safety are omitted.
Underfeed Retort and Screw Feed
EPA describes underfeed stokers as feeding coal into the firing zone from below by mechanical rams or screw conveyors. U.S. Patent 2,378,805 shows coal introduced by a horizontal screw into the bottom of a retort and then moved upward toward an active combustion zone.
The calculator does not reproduce that complete machine. It uses the ideal volume swept by an assumed full-diameter, constant-pitch screw and scales it by an entered fill fraction.
What This Calculator Represents
The volume product is converted to mass using a fixed 850 kg/m³ bulk-density assumption. Fuel input then uses a fixed 30 MJ/kg higher heating value, and the final heat value is merely the entered efficiency multiplier.
These outputs compare the entered assumptions. They do not rate or predict a particular fuel, screw, retort or furnace.
Ideal Swept-volume and Heat Calculation
fixed-density mass flow = volume rate × 850 kg/m³
fixed-HHV fuel input = mass flow × 30 MJ/kg / 3.6
entered-fraction heat product = fuel input × entered fraction
diameter circumference speed = pi D × rpm
Worked Arithmetic Example
For D=125 mm, pitch=75 mm, 2 rpm and 20% fill, ideal swept volume is about 0.0221 m³/h. With the fixed 850 kg/m³ assumption, the mass-flow product is about 18.8 kg/h. Fixed 30 MJ/kg gives about 156.5 kW fuel input; an entered 80% fraction gives about 125.2 kW.
Model Boundary
- Calculated: ideal swept volume, fixed-density mass product, fixed-HHV input, entered-fraction heat product and circumference speed.
- Illustrated: a schematic horizontal screw, retort, rising fuel and air tuyeres.
- Not calculated: actual screw capacity, shaft or flight volume, slip, compression, caking, fuel properties, air flow, combustion, bed motion, ash, emissions, furnace heat transfer, efficiency, controls, motor load, strength or safe operation.
Frequently Asked Questions
Is the entered diameter a retort diameter?
No. Enter an assumed effective screw-conveyor diameter. The formula uses that diameter to calculate an ideal swept volume.
Are coal density and heating value entered?
No. They are fixed assumptions of 850 kg/m³ and 30 MJ/kg.
Does pitch times rpm equal actual conveying speed?
Only as an ideal no-slip advance used in the swept-volume product.
Is entered efficiency predicted?
No. It is simply a multiplier.
Does the tool calculate combustion air or emissions?
No.
Can it size or operate a stoker furnace?
No.
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