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Kiln Airflow Design

Adequate airflow inside a kiln is critical for effective timber drying.  Due to the friction between the rough surfaces of the timber and the flowing air, a boundary layer [with a higher vapour pressure] is formed on the timber’s surface which reduces evaporation effectiveness.  Consequently, the importance of an effective air circulation system inside a kiln should not be underestimated.  As standard, all TF Design Kilns are equipped with high-efficiency, fully reversible fan units from Germany that result in high-quality drying while reducing overall power consumption.

The drying process of timber is essentially a combined diffusion and mass transfer process. Moisture needs to be removed from the surface of the timber boards in a controlled manner. The airflow design inside the kiln has a major impact on the final drying quality.

This can be contributed to the following three facts:

  • The moisture removal rate from the surface of the timber is strongly influenced by the velocity of the air stream. A low air velocity, therefore, results in longer drying times.
  • Diffusion of moisture from the core of the board to its surface is controlled by the temperature of the timber.  The hot air that flows across the timber transfer heat into the boards and elevates the temperature of the boards.
  • Uneven airflow distribution through the timber stacks in the kiln result in uneven drying and quality degrade.

 

Determination of the required air velocity through the stacks:

The target drying rate determines the minimum required air velocity between adjacent layers of timber within the stack.  Too low air velocities will result in uneven moisture distribution through the stack and increased drying times.  Velocities that are too high can again negatively affect the quality of the dried timber.  Studies and experimental data have shown that slow drying rates are required for typical hardwood species, and air velocities up to 2 m/s can be tolerated. Softwood timbers, such as Pine, with associated faster-drying rates, require an airflow between 3 and 6 m/s.

During evaluation of the airflow, it is important to calculate the cross-sectional air flow area of the voids between the layers in the timber stacks. This cross-sectional area is then multiplied by the required air velocity resulting in the target volumetric air flow rate.  A 10% leakage factor is added to account for losses due to the less than perfect stacking of the timber. [The leakage factor can be adjusted according to the prevailing stacking practices of each sawmill.]

Ensuring an equal velocity distribution:

A uniform air velocity distribution through the stack openings is essential for ensuring an even final moisture content distribution throughout the timber stack after completion of the drying process.

The following design parameters play an important role in the air velocity distribution:

  • Symmetry of kiln's design.
  • Selection of the correct fan units.
  • Width of plenum chambers.
  • Baffling around the timber stack.
  • Quality of stacking and equal sized stickers.

 

All the parameters above are interconnected and influence one another. Finding an optimum solution entails the simultaneous solving of a matrix of equations.  During development, TF Design used computational fluid dynamics (CFD) to solve the complicated flow calculations in order to optimize air flow distribution & velocities.

Traditional Fan Impellers:

The traditional locally manufactured impeller blades are designed to operate only in a single air flow direction for a specific rotational direction of the impeller. Reversible flow is achieved by turning every second blade through 180°.  The result is that only half of the blades perform according to their design conditions while the other half are in reverse mode and highly inefficient.

Modern Fully Reversible Fan Impellers:

Fully reversible fan impellers (e.g the imported DLK range of fully reversible impellers) are designed to solve this specific problem. The fan blade uses a typical NACA symmetric profile that has been developed to ensure the same maximum efficiency in both the forward and reverse directions. The implication is that the electrical power consumption for these fans is considerably less than the single direction impellers with every second blade facing in the opposite direction.

The fan cowling is manufactured from Aluminium with a rounded fan inlet. This reduces inlet losses and increases the efficiency of the fan unit. The fan impellers are accurately balanced ensuring a small blade tip clearance. The overall improvement in fan performance, compared to traditionally mounted units, can be as high as 25%.

Fan Motors:

TF Design Kilns incorporate direct mount fan motors inside the kiln as oppose to mounting the motors outside and connecting to the fan using a connecting shaft. 

This provides the following benefits:

  • Reduced vibrations. [Fans & motors can be balanced perfectly and installed as a unit.]
  • Improved shaft alignment [increased bearing reliability].
  • Improved efficiency with little not no losses through the connecting shaft.

High humidity levels and temperatures prevailing inside a kiln create a corrosive and hostile environment for any electrical motor. Consequently, reliability is a major concern considering this highly corrosive operating environment.  TF Design kilns are equipped with high-quality, fully sealed, Siemens electrical motors that are rated for 120°C and 95% relative humidity.

Main factors influencing fan motor reliability:

  • Effective temperature dissipation [Dissipation of heat is of utmost importance and these fans rely on the large fins on the outside of the motors to regulate temperatures.]
  • Bearing reliability [Correct shaft alignment, vibration minimization, and adequate lubrication improves the reliability of the bearing units.  Shaft alignment & vibration minimization is addressed by installing the fan motor as close to the fan as possible. Adequate lubrication is addressed by having re-greasing devices on all fan motors and ensuring that only hight quality Esso Unirex N3 or Klüber Barrierta L55/1 grease is used.

The selection of quality, high-efficiency fan units ensures that TF Design kilns produce high quality dried timber at an even moisture content distribution, without the high electrical power demands normally associated with modern kilns.

TF Design remains committed to continuously develop South African drying technology to provide tailored, cost-effective solutions to its clients.