Forging, heat-treatment and steel-finishing lines run furnaces that exhaust 300 to 700°C flue gas. The cleanest recovery is on the flue itself with an alloy recuperator, but that equipment is expensive and exposed to the harshest conditions. A robust, lower-cost opportunity sits on the building ventilation side: pre-heating the large make-up air the shop needs for comfort and fume control with warm exhaust from cooling and process areas.
Two recovery layers
- Process exhaust to combustion air: a high-temperature recuperator on furnace flue gas raises combustion-air temperature, cutting fuel use directly and improving flame stability.
- Shop exhaust to make-up air: a sensible plate or run-around coil warms the general ventilation air the shop needs for worker comfort and fume capture.
Why start with the building side
Direct flue recovery needs alloy, refractory-rated hardware and careful thermal shock management. The building-side exchanger works with ordinary materials at 80 to 150°C, so it installs faster and cheaper while still saving real energy. Many plants fund the building-side unit first, then justify flue recuperators from the demonstrated savings, because the payback evidence is already on the meter.
Material and temperature limits
Even at 150°C, shop exhaust can carry oil mist or scale dust. Specify accessible, washable plates and verify the cold-face temperature stays above dewpoint to avoid acidic condensation. Where exhaust is genuinely dirty, a run-around coil keeps the recovery fluid loop separate from the airstream, which also simplifies maintenance for the plant team.
| Stream | Temp | Benefit |
|---|---|---|
| Furnace flue | 300 to 700°C | Fuel saving (alloy unit) |
| Cooling-zone exhaust | 80 to 150°C | Make-up pre-heat |
| Shop relief | 25 to 40°C | Comfort heating |
Example
A forge venting 15,000 m³/h at 120°C recovered at 65% yields about 15,000 ÷ 3,600 × 1.1 × 95 × 0.65 ≈ 283 kW of useful pre-heat. Read industrial recovery, run-around coils, materials and efficiency to scope the right device.
Metering and phasing the investment
Before any capital is committed, put temporary meters on both the furnace flue and the shop make-up air for a representative week; the real duty frequently differs from the spreadsheet because furnaces cycle and shops leak. Use that measured data to phase the work sensibly: install the building-side unit first for a fast, low-risk payback, then build a business case for the alloy flue recuperator backed by numbers the finance team will trust. Route the recovered heat to the make-up air that the fume-extraction system already demands, so you are not creating new conditioned loads to absorb it. Keep the maintenance team in the design loop so wash and access points match how they actually service the shop, which is the difference between a unit that runs for years and one that is quietly bypassed within twelve months.
Specification checklist
- Quantify both flue and building-side streams before committing capital.
- Start with the building-side unit for fast, low-risk payback.
- Reserve alloy flue recuperators for proven, high-utilisation furnaces.
- Use washable plates where oil mist or dust is present.
- Keep the cold face above dewpoint to avoid acidic condensation.
- Meter both streams so the flue project business case is ready.
Specifying a unit for your project?
EN159 builds plate, rotary, heat-pipe, run-around coil and counterflow air-to-air exchangers. Send airflow (m3/h), inlet temperatures and available space and we will size it.
Request a quote