Paper Mill Heat Recovery

Paper machines dry stock by blowing large volumes of hot air across the moving web, then exhaust the moisture-laden stream to atmosphere. That exhaust is hot and wet – an obvious heat-recovery target – but the moisture means any recovery device must be designed for condensation, corrosion and cleanability from the first sketch.

What the exhaust actually looks like

Hood exhaust commonly leaves at 70 to 95°C with high relative humidity. Sensible plate recovery can pre-heat incoming make-up air or boiler combustion air. Because the air is moist, the cold side of the recuperator will fall below dewpoint and condense; the unit must drain and be washable. Ignoring this is the most common reason paper-mill recovery projects underperform, because standing condensate breeds corrosion and blocks the plate channels.

Design choices

• Sensible plate when you only want temperature recovery and can manage condensate drainage and periodic wash.
• Enthalpy wheel when you also want to recover the latent load to condition process make-up air; budget for a wash section.
• Run-around coil when the hood and intake are remote and a direct path is impractical.

Condensate and corrosion

Condensate from a paper-machine hood is mildly acidic and can carry fines. Use stainless or coated surfaces on the cold side, slope the core for drainage, and provide access for wash-down. A pre-treatment or bypass during start-up avoids standing water on a cold core. Plan the wash cycle into the maintenance calendar rather than waiting for a visible performance drop.

Numbers that matter

A machine hood venting 20,000 m³/h at 85°C to 25°C ambient, at 70% effectiveness, recovers about 20,000 ÷ 3,600 × 1.1 × 60 × 0.70 ≈ 256 kW. Even at part load this is a major fuel saving on the make-up or combustion air. See enthalpy vs sensible, the industrial overview, efficiency and materials for the right condensation-resistant choices.

Control integration and verification

Recovery should be sequenced with the hood damper and the dryer's own heat-input control rather than run as a standalone box beside the machine. When the hood calls for less exhaust, the recuperator should scale with it so you never recover heat that the process is about to discard anyway. Measure the condensate volume during commissioning; a sudden drop often signals a blocked drain or a core that has frozen, and catching it early prevents corrosion. Link the recovered-energy signal to the steam-boiler meter so the benefit shows up as reduced gas consumption, not just as a number on the exchanger display. Over a full production season, compare the metered recovery against the design figure and adjust the wash frequency if performance quietly drifts.

Specification checklist

1. Treat the exhaust as wet from day one; design drainage and wash access up front.
2. Use stainless or coated cold-side surfaces to resist acidic condensate.
3. Slope the core and verify drains are clear during commissioning.
4. Prefer an enthalpy wheel only if latent recovery is genuinely useful to the process.
5. Add a start-up bypass so a cold core never sits in saturated air.
6. Schedule wash cycles before performance complaints appear.

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