Microwave pre-heating – the option to increase production
Since its invention in the early 1940’s the Microwave has grown to become part of our everyday life at home. This technology is only just now at the edge of becoming a standard piece of equipment in the wood based panel industry.
In the mid 1990’s (approximately 12 years ago) the first application of a Microwave preheating system in combination with a continuous press was installed for LVL production. Since that time six (6) more continuous LVL presses have been built with Microwave preheating systems. This is still the state of the art technology and the only way to produce LVL on a continuous press as other preheating systems, such as steam injection, can not transfer the heat through the veneer billets.
The challenge in the use of a Microwave preheating system in a continuous process is the sealing of the Microwave oven. This is realized with a complex reflection and absorption channel. The opening of this channel has to be adapted to the board thickness and width to prevent leakage from the system.
Two (2) different frequencies are allowed, from the regulating authorities, for industrial applications, e.g. 915 MHz in North America and 2.45 GHz in Europe.
In 2007 Dieffenbacher invested in a 1.2 m (4ft) wide microwave preheating system for its Research and Development Centre in Eppingen, Germany. As can be seen on the thermograph picture the microwave technology heats up the mat very evenly over the width and thickness.
A temperature increase of 20 to 25 K (36 to 45 °F) could be said to be a common design parameter that is proven today. The required space in an existing forming line is only about 10 m (33 ft). As the Microwave system is installed directly before the press in feed there is no risk of precuring of the resin.
However microwave technology is not only applicable for LVL production.
A microwave is the only preheating technology (except high frequency units which are too costly and take up extensive space) in which only the temperature of the mat is increased without influencing other process parameters, such as humidity, resin curing times etc.
For this reason the technology is also highly suitable for applications in MDF and PB production.
So far all this may sound very promising, excellent temperature distribution, no influence on other process parameters, no pre-curing etc.
But questions may arise about when to choose a Microwave preheating system, will it be efficient and are there economic advantages to the system. To answer these questions a detailed review of each situation needs to occur.
Of course there are plants where there is no possibility to elongate the press due to the existing building. There are other cases in which the range of products is so wide (from thin board to thick board of 60 mm or more and from low density to high density) that the throughput of the line varies by up to 40 %. In such cases the degree of utilization of the chip or fiber preparation is dramatically low for part of the production.
This is not economical and the preparation line might even be under loaded so that stable process parameters are no longer possible A typical example, of under loading the preparation line, is the production of HDF and MDF on the same line. HDF can be produced with much higher throughput than high grade MDF. However, the MDF production could be increased to the level of HDF throughput by installing a Microwave preheating system. In doing so the electrical energy consumption of the mill will increase, but the specific consumption per m_ will decrease because about 40 to 50% of the electric energy consumption in a plant is not directly related to the throughput. So for example the consumption of all the pneumatic systems and filters is only slightly dependent on the actual throughput of the press. Thus MDF production with a Microwave preheating system will even reduce the variable production costs. Further benefits are achieved as the total output of the plant is increased. Microwave systems can improve significantly the profitability of plants with a wide spread product mix.
Picture 1: Continuous Press with Microwave
Picture 2: Thermograph laboratory board
Picture 3: HDF production / example
For more technical information, please contact:
Dieffenbacher GmbH + CO. KG
Dr. Gernot von Haas
Head of Research
Heilbronner Straße 20
75031 Eppingen
Germany
Phone: +49 7262 65-445
Fax: +49 7262 65-8445
Email: gernot.haas(at)dieffenbacher.de
Web: www.dieffenbacher.de
Contact person, sales department:
Dieffenbacher GmbH + CO. KG
Maren Jakob
Heilbronner Straße 20
75031 Eppingen
Germany
Phone: +49 7262 65-393
Fax: +49 7262 65-144
Email: maren.jakob(at)dieffenbacher.de
Web: www.dieffenbacher.de