Wood drying is an essential but energy-intensive step in the wood production process. Whether carried out in traditional kilns or continuous belt dryers, this process often results in significant energy losses, particularly through the discharge of hot, moisture-laden air. At a time when efficiency and sustainability are no longer optional but expected, recovering this lost heat has become not only technically feasible but also economically compelling.

Air-to-air heat exchangers - especially those designed to withstand the harsh conditions of industrial drying - offer an intelligent solution. By recovering thermal energy from exhaust air and reintroducing it into the drying process, they reduce fuel consumption, lower emissions and contribute to a faster return on investment.

Drying technologies and their recovery potential

In wood processing plants, two main technologies dominate the drying stage: kilns and belt dryers. Although they differ in design and operation, both can benefit significantly from heat recovery.

Kilns are widely used because of their simplicity and batch operation. However, they emit large volumes of hot, moist air at regular intervals. This exhaust contains considerable thermal energy, energy that, without recovery systems, would simply be lost to the atmosphere. By integrating plate heat exchangers into these systems, the hot exhaust air can be used to preheat the incoming fresh air, making the whole process more energy efficient.

Belt dryers, on the other hand, operate continuously and are typically used for high moisture materials such as wood chips or sawdust. Their constant airflow and exhaust conditions make them particularly suitable for consistent heat recovery. In fact, some belt dryer installations with heat exchangers have achieved energy savings of 30%, with even greater savings in optimised systems.

High-efficiency dryer with heat recovery and adiabatic humidification

In both the kiln and belt dryer systems, energy efficiency is greatly improved by intelligent airflow design and the integration of heat recovery. In the kiln, ambient air is drawn in and preheated by the warm, humid air flowing out via a plate heat exchanger. Once inside the kiln, the now warmer and drier air is further heated as required and circulated horizontally over the stacked wood. As it absorbs moisture, it becomes saturated and is finally exhausted - after passing back through the heat exchanger to transfer its residual heat to the incoming stream. This loop allows continuous drying with significantly reduced energy consumption.

The belt dryer works on a similar principle but is adapted to dry smaller wood particles such as chips or sawdust. Pre-heated air is introduced above a moving belt and passed downwards through the loose material. As it passes through, it picks up moisture and exits as humid air. This exhaust air also passes through a heat exchanger where it releases its heat to the fresh incoming air. In this case, the heat recovery structure often consists of several recovery units installed in parallel and in series to optimise airflow distribution. The airflow is carefully controlled to ensure uniform drying, while the recovery of thermal energy minimises fuel consumption. Despite the different configurations, both systems benefit from the same core principle: maximising energy recovery to reduce operating costs and environmental impact.

Belt dryer with multiple heat recovery units in series and parallel

Designing for efficiency: technical insights

For heat recovery to be effective in wood drying applications, the heat exchangers must be matched to the process environment. This means selecting corrosion resistant materials - such as full aluminium or epoxy coated aluminium - to cope with the aggressive, acidic nature of the exhaust air, which can contain resins, tannins and wood particles.

Plate spacing is another critical consideration. Narrow passages increase thermal transfer efficiency, but in the wood industry they also risk clogging by airborne fibres. Most systems designed for this sector use wider plate spacing – starting from 8 millimetres - to balance efficiency with durability and ease of maintenance.

In terms of airflow management, well-designed ductwork ensures stable pressure drops and even distribution across the exchanger, minimising the risk of performance degradation over time. The trade-off between heat transfer and pressure drop must always be evaluated during system design.

Plate Heat Exchanger with epoxy coated aluminium plate and full aluminium frame

Aluminium plate epoxy coated with a specific geometry to maximize efficiency, minimizing pressure drop

Economic impact and energy savings

The economic case for heat recovery in lumber drying is compelling, but it must be based on realistic assumptions. While energy consumption varies considerably depending on the species, moisture content and kiln design, a conventional kiln without recovery typically consumes between 500 and 700 kilowatt hours (kWh)¹ of thermal energy per cubic metre of dried lumber. For a mill processing approximately 5,000 cubic metres of wood per year - a medium sized industrial operation - this translates into an annual thermal energy demand of approximately 3,000,000 kWh.

A well-designed air-to-air heat recovery system can recover between 25% and 40%² of this energy, depending on factors such as exhaust air temperature, exchanger design and airflow management. Using an estimate of 30% recovery, such a kiln could save approximately 900,000 kWh of thermal energy per year.

When monetised, this saving becomes even more tangible. Assuming a thermal energy cost between €0.02 and €0.04 per kWh³ - whether from natural gas, biomass or district heating - the annual cost reduction could be between €18,000 and €36,000. These figures are particularly relevant in today's energy market, where price volatility and carbon considerations are driving companies to seek greater efficiency and cost predictability.

The initial investment required for a corrosion-resistant heat exchanger suitable for the wood sector is not insignificant. However, due to the significant energy savings, payback periods are often less than two years, especially in continuous or high throughput operations. Even smaller kilns, drying around 1,000 cubic metres per year, could see significant returns, with savings of €3,600-7,200 per year and a relatively quick payback on the equipment.

In short, heat recovery is not only a sustainable measure - it is a financially sound upgrade. For many operators, the return on investment is measured in months, not decades.

Conclusion

As energy costs rise and environmental regulations tighten, investing in heat recovery is no longer a niche solution - it is a strategic move. With robust exchanger designs now available to meet the unique challenges of the wood sector, drying processes can be optimised without compromising throughput or product quality.

More importantly, the combination of technical simplicity, cost effectiveness and sustainability make air-to-air heat recovery systems an accessible upgrade for both large industrial plants and smaller operations.

Heat that was once wasted is now a valuable resource. The question is no longer whether we can recover it, but whether we can afford not to.

Heat recovery, a duty of our time!

References:

¹. Bilal Lamrani et al., Energy analysis and economic feasibility of wood dryers integrated with heat recovery unit and solar air heaters in cold and hot climates, Energy Volume 228, 1 August 2021, 120598
². Range widely recognised in the market by various kiln and belt dryer manufacturers
³. 0.035€/kWh as refence value for the calculation