How does our wood fired log kiln actually work?

Wood fired log kilns have been used for centuries to dry timber efficiently and sustainably. Despite their long history, many people remain curious about how these kilns work and why they are still relevant today. This post explores the inner workings of our wood fired log kiln, breaking down the process into clear steps and explaining the key components involved.

Picture of our highly insulated wood fired log kiln taken with a thermal camera on a cold evening.

What is a Wood Fired Log Kiln?

A wood fired log kiln is a type of kiln that uses wood as the primary fuel source to generate heat for drying logs or lumber. Unlike electric or gas kilns, wood fired kilns rely on burning wood to create the necessary temperature and airflow for drying. We use wood because its far more sustainable than any other form of energy.

The main goal of the kiln is to reduce the moisture content of the wood to a level suitable for burning in a way that produces the maximum amount of heat without excessive, tar, smoke or ash.

How Heat is Generated and Controlled

At the heart of the wood fired log kiln is the firebox or combustion chamber. This is where wood logs are burned to produce heat. Our kiln uses a 55kw biomass system that burn logs that have been dried in the kiln, so its completely self sustaining.

The heat generated in the firebox is transferred to a 1000 litre water filled thermal store and then into the drying chamber via a forced air heat exchanger.

Controlling the temperature is crucial. A specialised computer adjusts the airflow and temperature by altering the fan speed and regulating the burner temperature to maintain a steady heat level of 77 degrees centigrade. The water thermal store maintains a constant temperature of 77 Degrees Centigrade, even if the burner runs out of fuel.

Airflow and Moisture Removal

Drying wood requires removing moisture trapped inside the logs. The kiln achieves this by circulating hot air through the stacked wood. The airflow carries moisture away from the wood surface and out of the kiln through automated exhaust fans that remove the moist air from the steaming logs and replaces it with fresh air from outside once per hour.

The kiln’s design ensures even airflow distribution to prevent uneven drying. Logs are stacked with small gaps between them to allow air to pass freely. Fans or natural draft systems help maintain consistent air movement.

Moisture exits the kiln as steam or water vapor. The kiln sometimes looks rather like a steam engine when the exhaust fans are running!

Stages of Drying in a Wood Fired Log Kiln

Drying wood in a kiln is a gradual process that involves several stages:

1. Initial Heating

   The kiln temperature is slowly raised to avoid shocking the wood. This stage helps evaporate surface moisture.

   
   

2. Free Water Removal

   Water inside the wood cells, known as free water, begins to evaporate. This phase is relatively quick and reduces moisture content significantly.

   
   

3. Bound Water Removal

   After free water is gone, the kiln continues to heat the wood to remove bound water, which is chemically attached inside the wood fibres. This stage takes longer and requires careful temperature control.

   
   

4. Conditioning

   Once the desired moisture level is reached, the kiln temperature is lowered, and humidity is increased slightly to relieve internal stresses in the wood.

   
   

5. Cooling and Equalizing

   The wood is cooled gradually to room temperature, and moisture levels are equalized throughout the logs to around 10%

   
   

Materials and Construction of the Kiln

Wood fired log kilns are typically constructed from durable materials that can withstand high heat and moisture.

Our kiln utilises a recycled 28 foot Asda refrigerated lorry body, which is very highly insulated and constructed of fiberglass which is impervious to the hot damp environment inside the kiln when it is operating.

Advantages of Wood Fired Log Kilns

Wood fired log kilns offer several benefits that make them attractive for certain users:

• Cost-effective fuel when wood is readily available

• Sustainable and renewable energy source compared to fossil fuels

• Ability to dry large volumes of wood without relying on electricity or gas

• Traditional method that can be built and maintained with local materials and skills

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Challenges and Considerations

Operating a wood fired log kiln requires skill and attention. Some challenges include:

• Maintaining consistent temperature and airflow to avoid defects

• Managing smoke and emissions to comply with environmental regulations

• Ensuring safety around open flames and hot surfaces

  
  

Despite these challenges, we prefer a wood fired kiln, as using any other source of energy to produce the heat would be environmentally damaging and expensive. All of our wood comes from a woodland management project in Surrey, making it 100% sustainable.

Drying Logs in a Wood Fired Kiln

The operator starts by stacking the logs with around 5cm gaps for airflow. The firebox is loaded with seasoned hardwood to produce steady heat. Over several hours the kiln temperature is gradually increased to about (77°C). The operator monitors vents and adds wood to maintain heat.

After about 4 - 5 days, the moisture content drops from around 60% to about 10%, suitable for burning. The logs are then conditioned and cooled before removal.

Wood fired log kilns combine simple principles with practical design to dry wood effectively. Understanding how heat is generated, how airflow moves moisture, and how drying stages progress helps users operate these kilns safely and efficiently. Wood fired kilns remain a valuable tool for producing quality dried logs.