furnaces, boilers, kilns, ovens, and dryers, air and fuel are burned to generate heat. Part of that heat is transferred to the heating device and to the particular load. Energy transfer has a practical limit. When that limit is reached, spent combustion gases are passed through a flue or chimney. Those spent gases have significant thermal energy. Some waste-gas heat related losses are inevitable—however, reducing gas exhaust losses can save considerable energy. Some methods to achieve this:
- Minimise exhaust gas temperatures – A cause of excessive temperatures in exhaust gas can be poor heat transfer in the heating system. If transferring the maximum heat possible to the heating system is not achieved, the furnace will be at an unnecessarily higher temperature.
- Minimise exhaust gas volumes – Fuel-air ratios must be carefully controlled. Operating the heating equipment at the best fuel-air ratio for particular processes will also help control fuel consumption. Usually all that is required is correct maintenance of the equipment.
- Use oxygen-enriched combustion air – Ambient air contains approximately 21% oxygen with nitrogen and other inert gases as balance. Exhaust gas volumes can be reduced by increasing the percentage of oxygen in the combustion air, even to the point of using 100% oxygen. There is potential for significant fuel savings. Making decisions on the level of oxygen content will involve balancing fuel cost savings with the cost of using additional oxygen.
- Reduce air-infiltration in heating equipment through minimising gaps, eliminating leaks and using pressure-controlling equipment – A well-designed furnace, kiln, oven or boiler prevents air leakage into a heating system or leakage of flue gases. But with use over time, cracks and small gaps appear around doors and joints. Cold air is naturally drawn through such gaps and into the heated equipment, causing heat loss. By reducing air leakage in a furnace, air infiltration heat losses can be reduced. This can be done in a number of ways, including:
– replace or repair seals or insulation where gaps have appeared
– ensure furnace doors are closed properly
– in the case of a major leak that cannot be sealed, install a pressure control system or a manually adjustable damper at the level of the leakage (with ‘draft gauge’ to monitor the furnace pressure) in the flue or chimney if a pressure control system cannot be used or is not economical.
- Appreciate the benefits of taking an integrated approach – Any reduction in heat losses through wall, conveyor, air filtration or radiation losses will lower the amount of fuel needed to generate heat in the first place, thereby reducing the amount of flue gas needing to be released.