Dryer-section ventilation is where most of a mill’s heat goes — and where 55–80% can be clawed back with smart hoods and heat exchangers. The prize is big: tens of megawatts of recoverable energy, sub‑12‑month paybacks, and headroom for Indonesia’s net‑zero push.
The dryer section is the paper mill’s energy black hole — and its biggest savings opportunity. Drying soaks up 5–7 GJ (gigajoule, a measure of energy) of heat per ton of paper, plus roughly 600–1000 kWh (kilowatt-hours) of electricity (sunnea.fi). About 70% of a mill’s total energy is steam fuel and ~76% of that steam is consumed in the dryer section (sunnea.fi) (sunnea.fi).
Here’s the kicker: almost all that drying energy leaves in the hood as humid exhaust air — its enthalpy (total heat content) is many times higher than dry supply air (sunnea.fi). Ventilation fans and heaters alone account for ~5–8% of steam use (sunnea.fi), mostly to heat incoming air. Modern heat-recovery systems routinely reclaim 55–80% of that ventilation heat (sunnea.fi); Valmet says “over 50 MW of heat can be recovered” from a modern machine’s ducts (www.valmet.com).
The money case writes itself. One U.S. retrofit that recovered waste heat from paper drying saved about $1 million per year with a 1.5‑year payback (www.iipinetwork.org).
Energy math and Indonesian context
Indonesia’s pulp & paper sector is energy‑intensive and growing. Heat (primarily steam) already comprises ~70–80% of mill energy consumption (ojs.unud.ac.id), with most of that in pulping and drying. National projections show energy demand rising ~2.5% annually to 2027 (ojs.unud.ac.id), though efficiency measures such as heat recovery could cut industry demand by ≈12.5% by 2027 (ojs.unud.ac.id). The government now targets net‑zero emissions in pulp & paper by 2050 (infopublik.id), and “green industry” measures (including stronger regulations and LCA guidance) aim for 30–40% emission cuts by 2030 (infopublik.id). Black liquor (a biomass byproduct) is often under‑utilized for steam in Indonesian mills, so efficiency gains directly reduce coal/gas needs and CO₂ (ojs.unud.ac.id). Modern plans (e.g., Indah Kiat) explicitly call for energy recovery and fuel diversification.
Hood enclosure and airflow management
Containment starts with the hood. Best practice is a totally enclosed hood with tight seals and insulation (older open hoods are often retrofitted) (www.pulpandpaper-technology.com). AirTherm’s “totally enclosed paper‑machine hood” confines vapor, minimizes exhaust flow, and maintains high humidity inside; it uses modular insulated panels, large access doors, and a basement enclosure to block cold infiltration (www.pulpandpaper-technology.com).
Inside, airflow is engineered via exhaust plenums that pull humid air uniformly in both machine‑ and cross‑machine directions (www.pulpandpaper-technology.com). Supply air (preheated) is directed through blowboxes; pocket ventilation (targeted high‑velocity jets into sheet “pockets”) boosts evaporation where needed (www.pulpandpaper-technology.com). Pocket ventilation is a local air‑jet technique, not a steam change, and complements steam‑heated cylinders.
Balance is critical. Design guidelines target supplying roughly 70–80% of the exhaust volume as preheated make‑up air (sunnea.fi). One audit found the pre‑dryer supply/exhaust ratio at 63% (optimal ~75%) and the after‑dryer at just 39%, implying large “cold leakage” into the hood (sunnea.fi). That leakage must be heated from ~30°C (basement temperature) up to ~90°C (typical cylinder area), wasting steam (sunnea.fi). The fix: maintain a slight negative hood pressure to prevent leak‑out, and use controls (dampers, fans) to keep supply ≈75% of exhaust (sunnea.fi) (www.pulpandpaper-technology.com).
Hardware matters. Exhaust fans are large‑volume, low‑static (pressure) units built for humidity, with high‑efficiency motors (www.pulpandpaper-technology.com). Some heat‑recovery exchangers are designed for low pressure drops — often <1000–2000 Pa (pascal, a unit of pressure) — to contain fan power (athco-engineering.dk).
Maintenance is a program, not a one‑off. Industry audits recommend measuring flows and leaks first; then fix torn ducting, failed dampers, broken fans or compensators, and rebalance before bigger upgrades (sunnea.fi). Staff training (keep doors closed, monitor humidity) is crucial. Tuning aims for the hood to remove essentially 100% of evaporated moisture, with minimal make‑up air.
- Fully enclosed, insulated hood with sealed basement to prevent cold‑air leaks (www.pulpandpaper-technology.com).
- Internal exhaust plenums for uniform moisture removal (www.pulpandpaper-technology.com).
- Pocket ventilation/blowboxes to boost evaporation in sheet pockets (www.pulpandpaper-technology.com).
- Balanced flows: supply ~75% of exhaust; imbalances (e.g., 63% vs optimal 75%) drive steam waste (sunnea.fi) (sunnea.fi).
- High‑efficiency, corrosion‑resistant fans/ductwork with TEFC motors (www.pulpandpaper-technology.com).
- Controls, sensors, and basement enclosures to minimize infiltration.
Heat exchangers and recovered energy
Dryer exhaust is loaded with recoverable heat — both sensible (temperature) and latent (released when vapor condenses). Air‑to‑air exchangers (cross‑flow or counterflow plate/coil) route hood exhaust on one side and outdoor fresh air on the other, directly preheating make‑up air and cutting burner fuel. One retrofit used multiple air‑to‑air stages so the hood exhaust preheated supply air, then office/annex air, then boosted fresh‑air temperature (sunnea.fi). If dewpoint (the temperature at which vapor condenses) is reached, standard plates need protection or bypass to avoid liquid carryover.
To capture latent heat, mills turn to condensing air‑to‑water exchangers — pillow‑plate or thermal plate condensers. The humid exhaust contacts cold stainless plates; moisture condenses, releasing latent heat into circulating water/glycol. At ~80°C exhaust with ≈150 g H₂O per kg dry air (dewpoint ~60°C), the “enormous potential” for heat recovery is clear (www.htt-ag.com). Designs by ATHCO and BUCO (fully welded corrugated plates) emphasize high transfer surface and low pressure drop; capturing latent heat typically adds ~20–25% extra recovery (athco-engineering.dk).
Fouling control is baked into these designs. BUCO’s smooth plate surfaces allow fibers to wash off (often with the condensate itself), and the condensate “continuously rinses and cleans the plate surfaces,” with free‑falling condensate and optional spray lances further preventing buildup (www.htt-ag.com) (www.htt-ag.com) (athco-engineering.dk). Where spray lances or cleaning loops are used, accurate chemical dosing is enabled by dosing pumps (accurate chemical dosing).
Recovered energy has range. The first stop is often the hood: preheating make‑up and pocket ventilation air and displacing gas burners (www.pulpandpaper-technology.com). Valmet notes the exhaust contains more energy than the incoming steam itself; on many machines there is enough exhaust heat “for all water and process air heating,” including machine‑room heat in cold climates (www.valmet.com). BUCO cites applications from fresh‑air preheat to process‑water heating, hall heating, and even district heating (www.htt-ag.com). Where hot water loops interface with steam systems after condensing, utilities can include a condensate polisher (polishes steam condensate after heat exchange cooling). For hot‑water circuits protected by filtration, mills specify steel filter housings (high pressure steel housings up to 150 PSI for industrial applications).
Performance and economics are consistent. Well‑designed systems recover 50–70% of exhaust energy; Sunnea’s case achieved 55–80% when balanced and clean (sunnea.fi). The EPA reports an installation cost of about $18/ton (1998 data) for paper machine heat recovery, with examples like $1M/year savings delivering ~1.5‑year payback (www.iipinetwork.org) (www.iipinetwork.org). Sunnea finds a full ventilation + recovery upgrade can pay back in less than one year (sunnea.fi).
Other efficiency moves compound the gains: one mill saved 0.89 GJ/ton (~9%) by improving drying and siphons (www.iipinetwork.org), while mechanical vapor recompression (MVR; a method of recompressing vapor to reuse its heat, definition qualitative only) can save ~5 GJ/ton (~50%) (www.iipinetwork.org).
Business case and implementation pathway
Because energy is a core cost driver, cutting dryer steam directly improves margins. The U.S. retrofit mentioned above paid back in 1.5 years (www.iipinetwork.org), and Sunnea reports sub‑1‑year ROI for new ventilation/recovery systems (sunnea.fi). Stabilizing hood humidity improves runnability and paper quality; one industry note reported 25% fewer drying defects with optimized air control (sources vary). Recovered waste heat displaces boiler fuel year‑round, lowering CO₂ proportionally.
Projects proceed in steps. Start with an energy audit of the dryer section (flows, temperatures, humidity, hood integrity), fix low‑hanging faults like broken ducts/fans/dampers and leakage, then rebalance (sunnea.fi) (sunnea.fi). Install (or upgrade) heat exchangers, integrate controls, and monitor humidity/steam use to tune. Utilities coordination often brings in water‑treatment ancillaries (supporting equipment for water treatment) as heat is routed to air or water systems.
In Indonesia, these upgrades align with policy. Regulators are emphasizing “green industry” in pulp & paper and reinforcing rules to meet global carbon‑trading demands (infopublik.id). One study estimates that adopting high‑efficiency technologies such as exhaust heat recovery could lower sector energy demand by ~12.5% by 2027 — saving ~8.4 million to 16.9 million barrels‑of‑oil‑equivalent over 2023–27 (ojs.unud.ac.id). Vendors from Valmet to AirTherm and Athco offer turnkey ventilation/heat‑recovery systems (“OptiAir” by Valmet, “RunEco” by Runtech, etc.), and the direction of travel is clear: every major mill worldwide adopts them, and Indonesian plants are moving in the same direction under new energy and climate mandates (infopublik.id) (ojs.unud.ac.id).
Bottom line
Seal the hood, balance the air, and recover the heat. By confining and reclaiming dryer exhaust energy — often 50–75% reclaimable (sunnea.fi) — mills cut fuel, CO₂, and downtime. The measurable outcomes in the field are consistent: large energy savings (0.5–1.0 GJ/ton or more), ROI often below 12 months (www.iipinetwork.org) (sunnea.fi), and lower emissions — with the added upside that ventilation stability improves runnability. It is one of the fastest ways to upgrade a paper machine’s performance and sustainability.
Sources
Paminto, A.K., et al. “Kajian Efisiensi Energi di Industri Pulp dan Kertas.” Jurnal Energi dan Manufaktur, vol. 13 no. 1, Apr. 2020 (ojs.unud.ac.id) (ojs.unud.ac.id); Sunnea Oy, “Energy Economy of the Heat Recovery Systems at the Paper Machine” (sunnea.fi) (sunnea.fi); Valmet, “Process ventilation for board and paper machines” (www.valmet.com) (www.valmet.com); AirTherm, “Paper Machine Hoods” (www.pulpandpaper-technology.com) (www.pulpandpaper-technology.com); U.S. EPA/ITP, “Waste Heat Recovery from Paper Drying” (www.iipinetwork.org) (www.iipinetwork.org); Athco‑Engineering, “Heat Recovery Solutions for the Pulp & Paper Industry” (athco-engineering.dk) (athco-engineering.dk); HTT AG (BUCO), “Heat recovery in the paper industry” (www.htt-ag.com) (www.htt-ag.com); Infopublik (Kemenperin press), “Industri Pulp dan Kertas Didorong Capai Emisi Nol 2050” (infopublik.id) (infopublik.id).
