An underfeed boiler stoker introduces solid fuel beneath the burning bed. The illustrated American stoker uses a rotating screw to move coal from a hopper into the retort; other underfeed arrangements use a reciprocating ram. Air enters through tuyere openings near the bed. The calculator compares entered coal and air rates and opening area, without predicting combustion quality, steam output or equipment capacity.
American Boiler Stoker Interactive Calculator
Compare air mass flow, volume flow at an entered density, tuyere opening area and coal rate relative to an illustrative reference.
Equation Used
Coal firing rate multiplied by the selected air-to-coal ratio gives combustion air mass flow. Dividing by 60 and by air density converts that mass flow to fan volume in CFM. Tuyere area is the selected open percentage of grate area.
- Steady entered coal rate; the calculation does not solve fuel distribution or burn rate from feeder motion.
- Air density is an input. The 0.075 lb/ft³ example defines the volume-flow conversion rather than predicting inlet conditions.
- Reference load divides coal rate by an illustrative 2,000 lb/hr denominator; it is not rated utilization.
- The open-percentage slider spans 8–12% for comparison; those endpoints are not a design recommendation.
The visual depicts a screw-fed underfeed variant; ram-fed versions also exist. The 2,000 lb/hr denominator is an example reference, not a verified stoker rating. Airflow results do not establish fan pressure or combustion performance.
Fuel enters below the firebed
The rotating screw carries coal from the hopper into a retort beneath the bed. Incoming material advances upward toward the burning zone. Tuyere openings admit air near the fuel bed; the illustration also shows the surrounding air plenum and rising gases.
Ram-fed underfeed stokers move fuel intermittently instead of using the illustrated screw. Fuel-feeding geometry and control arrangements vary between machines. Underfeed describes where fuel enters the bed, not one universal feeder or rated output.
Parts in the cutaway
- Hopper: supplies coal to the feeder.
- Screw and retort: advance fuel beneath the bed.
- Tuyeres: provide air openings around the firing zone.
- Air plenum: distributes supplied air toward those openings.
- Firebed and furnace: contain the burning fuel and gas path.
Moving particles and flames illustrate flow direction. They do not calculate fuel residence time, mixing, ash behavior, emissions or flame temperature. Actual combustion depends on fuel properties and the complete furnace and air system.
Mechanical fuel feeding
Underfeed stokers have been used in heating and industrial boilers. Mechanical feeding reduces the need to add each charge by hand, but it does not automatically establish low emissions, complete burnout or a particular steam output.
A specific installation needs compatible fuel properties, feeder and grate ratings, air-system performance, ash handling and boiler controls. A screw-fed arrangement, a ram-fed retort, a spreader stoker and a pulverized-fuel burner are distinct systems.
This page explains the illustrated fuel path and arithmetic relationships. It is not a recommissioning procedure, a named museum installation report or a hydraulic-drive conversion specification.
Air mass, air volume and opening area
Let C be the entered coal rate in lb/hr and AF the assumed air-to-coal mass ratio in lb of air per lb of coal. Air Mass = C × AF. At air density ρ in lb/ft³, Fan Flow = C × AF/(60ρ), in ft³/min.
This is volume flow at the density entered. It is not automatically a standardized CFM value, and it does not determine fan pressure, power or a point on a fan curve. The appropriate air-to-fuel ratio requires fuel composition and combustion information; heating value alone does not specify it.
Tuyere Area = Grate Area × Tuyere Open/100. This is a geometric area, independent of the air-flow arithmetic. Changing the opening percentage therefore does not change Fan Flow in this model, even though actual pressure loss and distribution would depend on the air passages.
Reference load = 100C/2000 percent. The fixed 2000 lb/hr denominator is only a comparison reference. A result above 100% does not establish overload, and one below 100% does not establish safe operation. Slider ranges are not ratings or recommended firing settings.
Worked example: the entered mass-flow balance
Enter Coal Rate 2000 lb/hr, Air per Coal 9 lb/lb, Air Density 0.075 lb/ft³, Grate Area 50 ft² and Tuyere Open 10%. These are illustrative assumptions, not a boiler operating prescription.
Air Mass is 2000 × 9 = 18,000 lb/hr. Fan Flow is 18,000/(60 × 0.075) = 4000 CFM at the entered density. Tuyere Area is 50 × 0.10 = 5.0 ft², and Reference load is 100%.
Halving Coal Rate to 1000 lb/hr gives 9000 lb/hr of air, 2000 CFM and a 50% reference comparison; opening area stays 5.0 ft². At the original mass flow, changing density to 0.060 lb/ft³ increases the calculated volume flow to 5000 CFM.
Changing only Tuyere Open to 8% gives 4.0 ft² of openings and leaves the calculated air flow unchanged. None of these outputs predicts steam pressure, fan pressure or acceptable combustion.
Different solid-fuel arrangements
| Arrangement | Fuel path | Distinction |
|---|---|---|
| Screw-fed underfeed | A rotating screw advances fuel beneath the bed. | The illustrated feeder type. |
| Ram-fed underfeed | A reciprocating pusher advances charges into a retort. | Feed rate needs actual displacement and fuel-delivery data. |
| Spreader stoker | Fuel is distributed over a grate. | Different fuel introduction and combustion arrangement. |
| Pulverized-fuel burner | Prepared fine fuel enters a gas stream and furnace. | Not represented by the retort illustration. |
Efficiency, emissions, fuel sizing, maintenance and turndown are installation-specific. The air-mass calculation cannot assign universal values to these systems.
Underfeed-stoker calculation questions
No. It shows a screw-fed underfeed variant. Ram-fed stokers also exist, but this illustration does not calculate ram stroke or delivery per stroke.
Opening area and air flow are separate calculations here. Area is grate area times open fraction; air flow is entered coal rate times air ratio divided by density. Pressure loss through openings is not solved.
No. It compares the entered rate with a fixed illustrative 2000 lb/hr denominator. There is no equipment capacity model.
No. It is an input assumption. Fuel composition, moisture, mixing and operating requirements matter; this calculator does not determine an appropriate excess-air setting.
No. It gives volume flow at the entered density. Selecting a fan also needs pressure losses, operating conditions and the fan’s performance data.
No. Fuel energy, combustion and boiler heat transfer, losses, feedwater and steam conditions are absent.
No. It has no ash-fusion, bed, combustion-control or fault model. Use the installation’s operating and service procedures rather than adjusting pressure or fuel settings from this arithmetic.
No. It has no cylinder, force, structural or control inputs. Matching an assumed stroke or supply pressure does not establish a suitable conversion.
References and further reading
U.S. EPA: Emission Factor Documentation for AP-42 Section 1.2, Anthracite Coal Combustion describes underfeed arrangements with ram or screw feeding, retorts and tuyere air. Its discussion of combustion systems provides context; it does not validate the illustrative input values as settings for a particular boiler.
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