Drying plant

The invention relates to a drying plant of the kind that is seen in the preamble of the appended independent claims.

It is well known that it is possible to dry a particulate material such as sawdust by first forming a falling curtain of particles, and by leading a flow of relatively dry air through the curtain.

However, in practice it is difficult, by a compact plant and a constant low air flow, to achieve a high drying capacity, and furthermore it is difficult to in that connection determine the drying time in order to achieve a predetermined moisture content of the material, also for cases with variations of the moisture content of the material being introduced into the drying plant .

The object of the invention is to provide a drying plant by means of which one or more of the mentioned objectives are attained or one or more of the outlined drawbacks are entirely or partly obviated.

The invention is defined in the appended independent claim.

Embodiments of the invention are defined in the appended dependent claims.

The invention will in the following be described in the form of examples, reference being made to the appended drawings.

Figure 1 schematically shows a sectioned side view of a drying plant excluding gas heater.

Figure 2 schematically shows a view taken along line II-II in figure 1.

Figure 3 shows a flow chart for a drying plant according to the invention.

Figure 4 shows in a Mollier diagram, the operation of a drying plant according to figure 3.

In figure 1, three drying devices are shown, which in principle are mutually equal. These drying apparatuses/drying cells have each the form of a housing 10 having a trough- shaped bottom 11. The housing contains a rotationally mounted rotor 20, which has a generally circular-cylindrical envelope, which is coaxial with the rotation axis thereof, which is arranged essentially horizontal. The lower circumference portion of the housing is adjacent to the envelope of the lower part of the rotor 20 over an angle area amounting to approximately +/- 45° from a vertical plane containing the rotor shaft. Outside this angle area, the lower wall parts of the housing extend tangentially to the rotor such as is shown. The upper part of the housing 10 has an upper wall part 14 (which upward delimits the gas openings 30, 31) . A sluice valve 15 through which particle material can be introduced into the housing 10 connects to the wall 14. The sluice valve 15 can extend along the entire length of the rotor and of the housing and can thereby in it self form the wall 14, such as is shown in figure 1. The sluice valve can connect between a lowermost part of the bottom area 11 and the area of a closest subjacent device 10 in the area between the upper parts of the openings 30, 31 thereof. In figure 1, it can furthermore be seen that each housing 10 in two opposite upward converging wall areas have an inlet opening 30 and an outlet opening 31, respectively, for a drying flow of gas. A drying cell is driven batchwise, and the particle material, for instance sawdust, is filled in the housing 10 up to a level corresponding to the clearance of the bottom part/bottom trough from the envelope of the rotor. In figure 1 it can be seen that the periphery of the rotor 20 has wmg-like

elements, which upon the rotation of the rotor drive the particle material in the rotation of the rotor i.e., clockwise in the lowermost drying cell 10, particulate material being thrown upward on the gas outlet side of the drying cell, having a motion component countercurrent to the flow of gas rushing in through the inlet opening 30, so that generally seen, a curtain of particles is thrown upward on the gas outlet side of the drying cell 10 up to in the vicinity of the ceiling of the housing, and then falls downward on the gas inlet side of the housing. The gas openings 30, 31 are shown to lie in planes which upward converge toward each other, so that the gas inlet flow coming in through the opening 30 and flowing in the normal direction to the extension plane of the opening 30, is generally directed toward the area of the rotor shaft, and is deflected obliquely upward to a direction being parallel to a normal of the outlet plane of the outlet opening 31. It will be appreciated that the air current through the openings 30, 31 in principle passes two curtains of particle material moving along a throw parabola, which is somewhat deformed by the flow of gas. The housing 2 of the drying cells 10 has substantially parallel end walls 24, which are lying in the plane of the drawing figure 1 and which are adjacent to a respective end surface of the rotor 20.

The air that is introduced through the cell is preheated to a predetermined temperature in order to efficiently be able to absorb and drive moisture from the particle material/sawdust in the cell.

In figure 1, it is seen that three mutually equal drying cells 10 are arranged vertically above each other, the vertically intermediate cell 10 being rotated 180° in the horizontal plane in relation to the other drying cells, in order to enable a flow of gas from the lowermost drying cell 10 to simply be possible to be led through the intermediate drying

cell, and further from the intermediate cell through the uppermost drying cell, in a meander-shaped path as viewed in figure 1. From figure 1, it can further be understood that the drying cell in the upper end wall thereof has a sluice valve through which the particle material batchwise can be introduced into the respective drying cell. Furthermore, each drying cell 10 has a sluice valve 15 situated at the bottom of the trough through which material that has dried in the cell can be emptied gravimetrically, for instance to a closest sub- ]acent drying cell 10, or to means for transportation away of ready-dried particle material.

From figure 3, it can be understood that outdoor air is led through a first air heater LBl, which advantageously may be supplied with energy from a condenser, which condenses relatively warm water from water steam from a flow of gas or flow of waste gas, for instance from a fuel boiler or from a dry gas flow in the overall plant in which the drying plant is included.

In figure 3, it can be understood that the sawdust material batchwise is led through the drying apparatuses TCl, TC2, TC3, the sawdust from TC3 having a predetermined moisture content, substantially independently of variations of the moisture content of the untreated sawdust that is supplied to TCl.

The fresh air, which is heated to a predetermined temperature in LBl, is led to a substantial share through TC3, and is filtered at the exit of TC3 by means of a filter FTl, after which the exhaust air from TC3 is warmed in a heat exchanger LB2 up to a predetermined temperature, and is subsequently mixed with a shunted part flow of the fresh air, which has been warmed in LBl. The total heated flow of gas is then led through TC2, and is filtered for separation of particles from TC2, after which the flow of gas is additionally warmed in LB3 before it is led

through TCl and possibly out in the environment. Possibly, the heat content of the air released from TCl may be taken care of in some useful way. In figure 4, an operation example is illustrated, wherein the fresh air, i.e., the flow of drying gas is heated from O ⁰ C and X=2 (X=g water/kg dry air) to a temperature level of 70 ⁰ C, before it is led through the sawdust in TC3, where it can be seen that the temperature of the drying gas falls, in the example to T=25, wherein X=20,4 and i=77,l, after which the drying gas leaving TC3 afresh is heated to 70° before it is led through TCl and there, in the example, gets the values T=41, X=52,4, i=176,2. It will be appreciated that by temperature rise in steps of the drying gas before the respective drying cell, favourable and efficient drying conditions are achieved. At, for instance, a temperature of 70 ⁰ C, great quantities of energy are usually at disposal in a sawmill, in which great quantities of sawdust are produced, wherein wood material of different forms, for instance needles, can be combusted, wherein the combustion flue gas can be utilized as a source of energy for preheating of the flow of drying gas to a chosen temperature upstream the respective drying cell. With the drying plant according to the invention, several advantages are attained. The plant and the drying devices/drying cells thereof can be made very compact since the flow of air through the drying cells is constant and low, and by the fact that the obtained high drying capacity is established by the fact that the capacity of the gas to absorb water from the sawdust is provided by the fact that the drying gas several times is imparted an elevated temperature before the passage of the respective cell, and in the flow path between adjacent cells. The drying effect is determined by

M _water =qδx where M is absorbed water, q is the dry air and δx is the alteration of the dry air of the water content. The physical size of the respective drying cell is directly proportional to the flow of air, which results in a drying cell having good surface efficiency.

A drying cell of the kind that is included in the drying plant may have a total width of 2-3 m and a length in the axial direction of the rotor of 3-4 m, and a height of approximately 3 m.

The drying cells which are connected in series in respect of flow of sawdust as well as flow of drying air, operate according to the batch method, i.e., after a drying period of approximately 20 min, dried sawdust is emptied from the drying cell TC3, the sawdust is transferred from TC2 to TC3, and from TCl to TC2, and untreated dry sawdust is introduced to TCl via the sluice valves in question. During the transfer of sawdust between the drying cells and out of the drying cells, the rotor may run, preferably however by a lower speed than what is utilized for establishment of the particle curtains in order to guarantee output through the outlet valves.

In one embodiment, between TC2 and the last drier TC3, the temperature of the particle material may be measured (=t wet for outgoing temperature from TC2) . If the detected temperature does not correspond to the dimensioning temperature for the drying period for a normal set of material in TC3, the drying period of the material is adjusted in TC3. If the detected temperature is higher than the dimensioning temperature, this is an indication of the material, coming in to TC3 being drier than dimensioned value, and therefore the dwell time for the material in TC3 is reduced. In the opposite case, the drying time in TC3 is increased for the material from TC2, in accordance with an empirical value. Such a control system affords an excellent possibility of, in a simple manner, being able to establish a constant moisture content going out of the material from TC3, independently of variations of the moisture content of particle material coming in to TC3.

The filters FTl, FT2, may consist of a straining-cloth, for instance in the form of one or more layers of a wire gauze of the kind that is often used in machines for paper production. Such a straining-cloth may, for instance, be arranged over the outlet opening of the drying cell, or in a separate particle separator. On the side of the straining-cloth situated downstream, an elongate nozzle may be movably arranged transverse to the longitudinal direction thereof, parallel to the surface of the straining-cloth and next to the same, the nozzle having an air outlet gap, which is directed in an upstream direction and toward the straining-cloth, the lamellar air flow from the nozzle bringing particles deposited on the straining-cloth to let go instantaneously so that they can move downward under the impact of the gravity, and eventually be removed from the lower part of the filter cloth.

The flow of gas/the flow of air

In TCl, the flow of gas meets a very humid particle material, which by itself efficiently works as a filter and separates small particles of water and has in practice therefore no possibility of being driven by the flow of gas out of the particle mass in TCl.

Upon the heating, the flow of gas may again be heated by some low temperature source of heat to a relatively low temperature level before the passage of the respective drying cell, whereby the gas becomes moist saturated and cooled, and then, after reheating again it can absorb an additional great quantity of moist, such as is illustrated in figure 4.