L aunched at Ligna 2007, Pallmann’s latest-generation refiner, designated the ECO Advanced, went into operation at the Kastamonu City plant of Turkish MDF maker Kastamonu Entegre in March this year. A further seven models have also been sold by Pallmann.
The design of this completely new refiner was developed using computer-aided design and computer simulations. The first was fluid dynamic modelling and then, once the geometry of the machine was determined, an FEM (Finite Element Method) stress analysis was carried out to confirm the geometric postulations.
The most noticeable feature of the new refiner is that it has a tangential outfeed rather than the radial one which has been in use for the last 30 years. And that is patented.
“We needed to design the housing for as even stress distribution as possible and the geometry must be precisely maintained even under the extremes of temperature and pressure during the refining process,” said Franz-Josef Wilbois of Pallmann. “If the casing is too weak it will distort and affect the grinding gap accuracy, and thus the fibre quality, so we designed very rigid structures to achieve a constant, even, grinding gap between the refiner plates. This is in the range of approximately 0.3-0.5mm, with adjustment in steps of 0.01mm.”
In order accurately to assess the fluid dynamics and stress, a virtual net of 2.5 million knots was used in the simulation. Fluid dynamics modelling was used to assess the movement of steam and fibre within the grinding chamber. “The computer time needed for this modelling was one week for one computer model – this gives some idea of how detailed was the analysis,” said Mr Wilbois.
“The fluid modelling shows the velocity profile in the grinding housing with different colours, in a similar appearance to thermal imaging,” he said. “The highest speed occurs at the outer edge of the rotor and decreases somewhat towards the blow valve at the outlet, where pressure drops by four to five bar and is converted to an increase in velocity.
“We looked at speed as well as dynamic and static pressure distribution to get everything right. Low speed means the risk of fibre depositing and so we have almost completely eliminated low speed zones in the refiner and at the outlet valve.”
The simulation enabled the designers to highlight a single particle and follow its path, partly in order to get the dwell-time and thus the fibre dimensions, in the refiner optimised.
“Getting the fibre flow optimised saves energy, hence the ECO part of the name,” said Mr Wilbois.
“We found that our tangential outfeed casing approximately halved the dissipation loss caused by the fluid in the casing compared with a radial outfeed.”
Pallmann has also recently developed a high-efficiency plug screw with, according to Mr Wilbois, the highest compression rate in the industry.
“This saves on dryer energy by removing extra water from the fibre stream, as well as giving a balanced moisture content entering the dryer,” he said. “This is one of the most important parameters in an efficient MDF process and fibre moisture control.
“We are a refiner maker who supplies tailor-made solutions, not off-the-shelf. There are various options on each different refiner type we sell, according to the customer’s specific requirements.”
