Controlling the MDF Dryer

The MDF dryer appears to be a relatively straight-forward operation. The fibre from the refiner is discharged into a hot air stream, which provides the energy to evaporate the water from the fibre.
In reality, the operation is quite complex because the endpoint is not the lowest moisture content which can be achieved, and it is necessary to stop the drying process at a point where the fibre moisture content is still changing rapidly.

The fibre moisture content after the dryer is required to be that which is required at the press, usually between 9 and 12 %, requiring that the drying process be stopped by control of the process rather than by coming up against an endpoint determined by the fibre characteristics.

Temperature measurements along the dryer tube show that most of the temperature drop between the inlet and outlet of the dryer occurs within 10 m or so of the fibre entry point. This shows that most of the energy exchange between the gas stream and the fibre has occurred by this point, and that most of the drying has been done. The remainder of the dryer acts to achieve equilibrium between the fibre moisture and the humidity and temperature of the air, particularly in the cyclone.

It is this equilibrium relationship at the end of the dryer which makes control so difficult. In fact it is possible under some conditions for a conventional control system to act in the wrong direction. The result of this is that moisture control is difficult, and slow to respond. In many cases the dryer remains in manual control, or is only placed in automatic control when stability has been achieved.

It is possible to change the dryer characteristics so that controllability is improved significantly. The change is simple, and cheap to install. It can be turned off at any time. It allows any of the existing moisture control schemes to be more effective, reducing moisture variation in fibre from the dryer.

The benefits of reduced moisture content are well established including:

• Density profiles are more consistent.
• Higher face densities.
• Increased fibre moisture content allows faster press speeds.
  

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