How much fuel does a recuperator save on a reheating or forging furnace?
It depends on how hot the flue gas leaves and how hot the recuperator makes the combustion air. In the worked heat balance below, preheating air to 300 C on an oil-fired furnace with flue gas at 900 C saves about 17% of fuel. Work out your own figure, then confirm it with fuel per tonne before and after.
Why preheated air saves fuel
In a reheating or forging furnace the flue gas leaves hot, often around 900 C or more. A recuperator passes that gas through a heat exchanger and warms the combustion air. Every kJ the air brings in is a kJ the burner does not have to supply, and the hotter the flue gas, the bigger the gain.
The formula
Fuel saving share = Q air / (NCV minus Q flue plus Q air), all per kg of fuel.
Q air is air mass per kg fuel x specific heat x (preheat temperature minus ambient). Q flue is flue gas mass per kg fuel x specific heat x (flue temperature leaving the furnace minus ambient). NCV is the fuel's net calorific value. The formula assumes the excess air and furnace output stay the same.
What to measure before and after
- Fuel per tonne of stock, over at least a few weeks of similar product mix and furnace hours.
- Flue gas temperature at the furnace exit and after the recuperator.
- Combustion air temperature at the burner, and flue O2, since excess air often creeps up after a retrofit.
- Extra fan power if the recuperator needs a bigger blower, which you subtract as kWh x 0.710.
Practical limits
- Metallic recuperators usually limit air preheat to a few hundred degrees; ceramic recuperators and regenerative burners go higher.
- Burners must be rated for hot air, and hot air raises flame temperature and NOx.
- Scale, dust and leaks in the recuperator cut the saving over time, so log air temperature every shift.
Illustrative example: furnace oil fired reheating furnace
- Furnace oil NCV: 40,400 kJ per kg (IPCC 2006 default for residual fuel oil)
- Flue gas per kg oil (about 20% excess air, assumed): 17.8 kg; leaving at 900 C, ambient 30 C, specific heat 1.15 kJ per kg K: Q flue = 17.8 x 1.15 x 870 = 17,809 kJ
- Combustion air per kg oil: 16.8 kg, heated to 300 C, specific heat 1.02: Q air = 16.8 x 1.02 x 270 = 4,627 kJ
- Saving share = 4,627 / (40,400 minus 17,809 plus 4,627) = 4,627 / 27,218 = 17.0%
- On 300,000 litres a year: 51,000 litres saved; CO2: 51,000 x 2.97 = 151,470 kg, about 151 t a year
Air and flue masses, temperatures and specific heats are assumptions for illustration; measured fuel per tonne before and after is the figure to report.
What Senseible does here
Senseible reads your furnace oil, PNG or coal invoices and shows monthly fuel and CO2, so the change after a recuperator retrofit shows against the months before. Fuel per tonne needs your production figure alongside.
- Reheating furnace insulation savings
- Forging footprint per tonne
- Furnace oil emission factor
- Emissions saved by waste heat recovery
Sources: BEE Energy Auditor Guide Book 2, Energy Efficiency in Thermal Utilities, furnaces and waste heat recovery chapters, 2006 IPCC Guidelines, Volume 2 Energy (default net calorific values and CO2 factors)
Last reviewed 2026-10-18.