# How much fuel does a recuperator save on a reheating or forging furnace?

Canonical: https://senseible.earth/answers/recuperator-combustion-air-preheat-fuel-saving

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](https://senseible.earth/answers/reheating-furnace-insulation-fuel-savings)
- [Forging footprint per tonne](https://senseible.earth/answers/forging-carbon-footprint-per-tonne)
- [Furnace oil emission factor](https://senseible.earth/emission-factors/furnace-oil)
- [Emissions saved by waste heat recovery](https://senseible.earth/answers/emissions-saved-by-waste-heat-recovery)

Sources: [BEE Energy Auditor Guide Book 2, Energy Efficiency in Thermal Utilities, furnaces and waste heat recovery chapters](https://beeindia.gov.in), [2006 IPCC Guidelines, Volume 2 Energy (default net calorific values and CO2 factors)](https://www.ipcc-nggip.iges.or.jp/public/2006gl/vol2.html)

Last reviewed 2026-10-18.