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Main Advantages of the Microwave Heating

  • Speeding up: minutes instead of hours!!
  • Volume Energy transmission - cold vessel
  • ENERGY Savings, Room Savings
  • Efficiency 70%
  • Automatization
  • Safety *
  • Environment *

*  Comparing to the processes using the combustion


REFERENCES MW

Drying, glueing and forming of wood

Moisture and drying

Drying in general is the water transfer from material to the surrounding medium – most often below the boiling temperature. Free water is contained in cellular hollows while the fixed water is also bounded in cellular walls. Moisture 25-30 % (depends on wood sort) matches the fibre saturation it means the water is removed from cavities while it is still contained in walls. When drying the wood below this point of fibre saturation results in volume decreasing, shrinkage and shape changes.

Conventional heating

Classical heat sources are increasing the surface temperature of the dried material and saturated steam boost on the surface. Pressure gradient enables vapour to release to the space. At the beginning the water content is decreased in upper layers that release the capilar pressure transferring the moisture from the inner part to the surface layer. Thermal gradient and moisture gradient have the opposite orientation in conventional drying.

Dielectric heating

Microwaves solve again this disadvantage and avoid overdrying of the surface layer that can act then as the thermal insulator.   High frequency energy influence to the depth of the material. The thermal gradient occurs helping to push out the moisture and creates water pump mechanism that does not depend on the thermal conductivity (which is usually low for wood). Drying is processed at relatively low temperatures. Although drying can be accelerated by increasing the temperature, maximum drying speed is limited by the material properties.

Dielectric heating operates both in microwave range and in radio frequency band. Drying is relatively flexible and assures good homogenity. The process must be slow enough to prevent inner tension and deteriorating the product quality. Dielectric method is predetermined again namely for high-end and special application of higher value. The drying time is cut down to minutes from previously needed days or weeks? Specific example is drying of special golf driver or treating the special tanoak wood cutting down the process from 2 years to 4 hour microwave operation and 2 weeks of subsequent final drying (without MW).

Radio frequencies

27.12 MHz enables the depth of impact in order of meters. Deployed microwave power is from tens of kW up to 1500 kW.  Radio frequency heating is on the levels above 50 kW far more economic then microwave band. This determines using radiofrequencies for treating the saw-timber. It is widely applied in plywood production where the electrodes are simultaneously the press plates. Applying the radio-frequency to plywood erase the humid spots and equalizes moisture preventing occurrence of steam bubbles and preventing disruptions in process of lamination. RF energy is also applied to selectively to glue in plywood and woodfibres products (HDF,MDF) Plastification (softening) the wood at 18-24 % on equalized humidity within full section can be accelerated to couple of minutes comparing with conventional steaming process.  
 

Glueing

Microwaves and RF have potential advantage also for jointing and linking both for wooden plates, plywood and layers. Firm and consistent joint is achieved much faster by selective heating of the susception layer while maintaining the rest of the wood at low temperature. It can bring the effect both for mass production and for energy savings.
 

Wood forming and modification

Microwave nad radiofrequency heating for forming, curving and modifying of the wooden part is most often used in combination with the steam.