For a coal or briquette fired furnace or hot air generator, fuel is the biggest running cost, often far bigger than maintenance. The good news is that most fuel waste comes from a handful of causes, and many of them can be fixed without buying new equipment.
1. Measure before you change anything
You cannot reduce what you do not measure. Start a simple daily log and you will see where the losses are.
- Fuel consumed per shift or per hour, against output or dryer load
- Flue gas temperature at the chimney or exit of the heat exchanger
- Oxygen or CO₂ in the flue gas, with a portable analyser
- Shell and duct surface temperatures at fixed points
- Unburnt fuel in the ash
2. Get the combustion air right
Too much air is the most common hidden loss. Every extra kilogram of air is heated and then thrown out through the chimney. Too little air is just as bad, leaving unburnt carbon in the ash and smoke at the stack. Adjust dampers and fan settings so the flue gas oxygen stays within the range recommended for your grate or firing system, and check it regularly as fuel quality changes.
3. Control fuel quality and moisture
Wet coal and briquettes waste heat simply boiling off water before they start to burn. Store fuel under a shed, keep it off the ground, and avoid buying lots with high moisture. Ask suppliers for the gross calorific value (GCV) of each lot, and test it occasionally, since a cheaper fuel with lower GCV can cost more per unit of heat. Keep the fuel size consistent; too many fines either fall through the grate or choke the airflow.
4. Improve firing practice
- Feed fuel in small, regular quantities instead of large charges
- Keep the fuel bed even, without bare patches or heaps
- Clean and de-ash on a fixed routine
- Keep firing and inspection doors closed as much as possible
- Train operators and make one person responsible for each shift
5. Fix the grate
Broken or warped grate bars let unburnt fuel drop into the ash pit, and uneven air distribution creates cold spots. If the ash still contains a lot of black, unburnt fuel, the grate and air distribution need attention.
6. Stop heat escaping through the walls
A worn refractory lining or damaged insulation lets heat leak out through the shell. Hot spots on the casing are a clear sign. Repairing the lining and adding a proper insulation layer, such as insulating bricks or ceramic fibre behind the hot face, keeps that heat inside the chamber where it belongs.
7. Seal air leaks
Cold air entering through cracks, loose doors, peep holes and flue joints cools the gases and forces the fans to work harder. Seal openings, replace door gaskets and repair cracked casings and ducts.
8. Keep heat transfer surfaces clean
In a hot air generator, soot and ash deposits on the heat exchanger act like a blanket. Less heat reaches the air, and more leaves through the chimney, which shows up as a rising flue gas temperature. Clean the tubes and surfaces on a fixed schedule. A commonly used rule of thumb for heaters and boilers is that every 20 °C reduction in flue gas temperature improves efficiency by roughly one percent.
9. Recover waste heat
If the flue gas still leaves hot after the heat exchanger, part of that heat can be recovered, for example by preheating combustion air or the incoming process air. The right option depends on your layout and should be checked by an engineer.
10. Insulate ducts and match the load
Uninsulated hot air ducts lose heat before the air reaches the dryer. Insulate them, and avoid running the unit far above or below its designed load for long periods, as efficiency usually drops at both ends.
Where to start
Begin with the steps that cost little: measurement, air settings, fuel storage, firing practice and sealing leaks. Then plan lining and insulation repairs into your next shutdown. Each step on its own may save a few percent; together they make a real difference to the monthly fuel bill.
JISPL builds and lines hot air generators and furnaces, and repairs linings and insulation on existing units. If you want us to look at your furnace or HAG, send us the details.
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