Background of the invention In the MDF process, vapor is used for blowing the fibers from the defibrator to a pneu- matic dryer where the fibers are dried by means of flue gas or hot air. In most cases, glue that joins the fibers together in the fiberboard manufacturing process is dispensed into the blow line. The method is simple and reliable. A high vapor speed in the blow line ensures that the fibers and the drying gas mix well together and prevents the fibers from sticking together, forming harmful fiber bundles.

The weakness of the method is the transfer of the vapor to the dryer and the mixing thereof with the drying gas because, in this case, the volatile organic compounds (VOCs) con- tained in the vapor end up in the open air together with the drying air. Simultaneously, the possibility of utilizing the heat content of the vapor at a higher temperature level is lost.

The increase in wet temperature can be compensated for by raising the flow temperature of the drying air, but on the other hand, this increase causes the glue to harden too early, in- creasing glue consumption and possibly decreasing the quality of the fiberboard.

These disadvantages are eliminated in the method according to International Published Application WO 99/01606 in which the vapor is separated from the fiber before the dryer and used for preheating the drying air. The volatile organic compounds are conducted to a combustion or recovery plant in a concentrated form. The problems of this method are the separation of the fibers from the carrier vapor and the mixing of them with the drying air without forming harmful fiber bundles and without causing the fibers coated with glue emulsion to stick to the separator or to the walls of the dryer.

Description of the invention General Description The method according to Claim 1 makes it possible to separate the blow vapor from the fibers, to transfer the fibers to a pneumatic dryer and to mix the fibers with the drying-air flow in such a way that no harmful fiber bundles are formed and the fibers do not stick to the walls of the separator. The method according to the invention can be carried out by means of a device according to Claim 5.

The fibers, which are carried by blow vapor and coated with glue emulsion, are tangen- tially introduced into a separator consisting of a multi-vane rotor resembling the vane wheel of a fan, of a cylindrical housing and of straight end walls. In the rotor, the vapor flows from the outer periphery towards the shaft and is removed from the rotor in the di- rection of the shaft, via an annular zone in the proximity of the shaft.

Centrifugal force causes separation of the fibers from the vapor between the vanes, and the fibers are removed tangentially from the periphery of the rotor before a revolution has been completed, i. e. when the fibers have traveled less than 360° on the rotor periphery. An auxiliary blower supplies a partial flow of drying air to the fiber outlet from another direc- tion, to transfer the fibers thus separated to the dryer and mix them with the main stream of drying air.

In other words, centrifugal force is used for changing the fiber carrying gas from vapor to air, substantially avoiding mixing of vapor and air.

The separation capacity of the apparatus can be influenced by adjusting the rotation rate of the rotor. The power consumption due to rotation of the rotor can be decreased by using serrated rotor vanes, whereby buildup of scale on the inner surface of the housing is still prevented by offsetting the teeth of adjacent vanes.

Further, the fibers can be removed from the separator by means of a pressure-tight dis- charger, such as a screw plug or a shutter feeder. In this case, no auxiliary blower is re- quired, and the fibers may be radially removed. It is preferable for the operation of the separator to give the feed and discharge openings a width equal to the width of the rotor

vanes and the shape of a relatively narrow rectangle. The vapor inlet is preferably a cone rectangular in cross-section and having a sufficiently small cone angle to keep the flow in continuous contact with the walls.

In comparison with a cyclone, the method according to the invention ensures a shorter residence time, maintains the fiber speed better and requires less space.

Detailed description The invention is apparent from the accompanying drawing, showing a MDF raw material production plant drying section according to the invention.

Figure 1 shows a device according to the invention applied to the process of drying fiber for use as MDF raw material. A split device 2 can direct the vapor/fiber flow arriving from a defibrator through pipeline 1, either to a cyclone 3 or to a dryer 4. Before the dryer 4, a dispenser 5 doses the glue substances necessary for the fiberboard manufacture into the vapor/fiber flow, into pipeline 6, as an aqueous emulsion. After the dispenser 5 there is a split device 7 that can direct the vapor/fiber flow directly to the dryer 4 through pipeline 8 or to a separator 10 according to the invention through pipeline 9. A blower 12 makes the transfer of the fibers from the separator 10 to the dryer 4 and the mixing of them with the drying-air flow more effective. Preferably, the blower 12 takes its air from the channel of the dryer 4, before the fiber inlet. The vapor from the separator 10 is transferred, through a pipeline 13, to a heat recovery and VOC gas treatment, for example in accordance with International Published Application WO 99/01606.

Figure 2 shows a cross-section of separator 10. The separator comprises a cylindrical housing 15 to which are tangentially connected a vapor/fiber flow inlet 16 and a fiber outlet 17, which are substantially of the same width as the housing, and a vane wheel 18 rotating concentrically in housing 15.

The vapor is removed from housing 15 through outlets 20 connected to one end or each end 19 thereof.

Figure 3a is a cross-sectional view of the separator of Figure 2 taken along line A-A. The vapor is removed through an outlet 20 provided at only one end of the housing 15. In this embodiment, the shaft 21 of vane wheel 18 does not extend through housing, 15 but is

supported and mounted in bearings on only one side of the housing. Such a structure is preferable in small devices.

Figure 3b shows a structure suitable for larger devices. The shaft 21 of vane wheel 18 ex- tends through housing 15 and is mounted in bearings at both ends. Correspondingly, there are vapor outlets 20 at both ends 19 of housing 18.

The vapor/fiber flow enters the separator 10 through an inlet 16, extending over the entire width of vane wheel 18. Vane wheel 18 rotates in the direction of the vapor flow at such a high rate that its peripheral speed exceeds the vapor flow rate at inlet 16. The wall curva- ture of housing 15 and the high speed of the vane wheel result in a great centrifugal force that throws the fibers, which are heavier than the vapor, against the wall of housing 15 and then out of housing 15 through outlet 17. The vapor flows radially through the passages 22 between the vanes of wheel 18 to the inner periphery of vane wheel 28 and then out of separator 10 through an outlet or outlets 20.

In outlet 17, the fibers thus separated are immediately mixed with a hot-air flow blown into the channel through an outlet 23 (Figure 2) and carrying the fibers to the dryer 4 (Figure 1).

The separation efficiency of the separator 10 can be influenced By changing the rotational speed (the peripheral velocity) of vane wheel 18.

By changing the angle a between the vapor/fiber inlet 16 and the fiber outlet 17, see Figure 2. The larger a is, the better the separation efficiency. Figure 1 shows an embodi- ment in which angle a is 180°; in Figure 2, the angle is smaller than 90°. The choice of an- gle is also influenced by the sticking tendency of the fibers and by the location of the de- vice.

By polishing or coating the surfaces that come into contact with the fibers, especially the inner surface of housing 15, to prevent the fibers from sticking to the surfaces. A suit- able coating material is, for example, Teflon (PTFE) because it stays clean, is wear resis- tant and has a low friction coefficient.

By properly adjusting the pressure of the vapor passing from the separator 10 to a heat re- covery/VOC treatment 14 through vapor outlet 20 and pipeline 13, the vapor is separated from the fibers in the most efficient way. In the optimum situation, a small amount of dry- ing air flows from outlet 17 against the fiber flow and is mixed with the outgoing vapor flow.