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Patent Searching and Data

Document Type and Number:
WIPO Patent Application WO/1999/060317
Kind Code:
In a method for the drying of feed pellets in particular, and a drying plant for the implementation of the method, and in which the drying medium is air in a forced flow and with adjusted temperature and relative humidity, it has been aimed at achieving at all times an optimal drying of the drying matter as this is fed forward within the drying plant. This is achieved by means of a two- or multistage drying process, in which each stage receives a separate flow of air, i.e. a flow in which the drying properties (temperature, moisture content, flow rate) of the air is adjusted to the consistency and heat condition (moisture and temperature) of the drying matter within the respective drying stage. In the plant there are at least two (36, 38) of a plurality of drying matter conveyors (36, 38, 40) enclosed in a separate chamber (30, 32, 34) with in- and outlets for drying matter and drying air, respectively, and which are communicating between themselves or with the surroundings only through ducts provided with gates (42, 42', 42'', 42''').

Application Number:
Publication Date:
November 25, 1999
Filing Date:
May 14, 1999
Export Citation:
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International Classes:
F26B17/04; F26B17/14; F26B21/06; (IPC1-7): F26B3/06; F26B17/02; F26B21/06
Domestic Patent References:
Foreign References:
Attorney, Agent or Firm:
Håmsø, Borge (Håmsø Patentbyrå Ans P.O. Box 171 Sandnes, NO)
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1. A method for drying divided/pelletized material, in particular of animal and/or vegetable origin, in the form of a foodstuff or feed material, whereby these foodstuffs and feed pellets, respectively, are dried by means of drying air in a forced flow, while said pellets are being moved within the drying plant, c h a r a c t e r i z e d i n that the drying matter is subjected to a drying in two or more stages by means of two or more separate air flows drying the divided/pelletized material, the air flows being present in two or more different zones passed by the divided/pelletized material as it moves within the drying plant, each of the flows of drying air exhibiting parameters, such as temperature, relative humidity and possibly flow rate, which are mutually independent but are adjusted to the varying properties of the drying matter at all times.
2. A drying plant for the implementation of the method specified in claim 1, comprising for example three drying units (30,36; 32,38; 34,40), between which the product under treatment is displaced during the drying, and comprising conveyors, for example in the form of driven, endless conveyor belts (36,38), which may be arranged one above the other and/or beside each other, and which are preferably formed of perforated or in another way airadmitting material for the flowthrough of drying air while said drying matter is being moved forward by the respective conveyor, c h a r a c t e r i z e d i n that each of said drying units (30,36; 32,38; 34,40) is provided with an outer tight jacket defining a chamber (30,32,34) with an air inlet (82,84) and a used air outlet (44,46,48; 86,88), each air inlet being connected to an air supply device for the supply of air to the respective air inlets, which air is adjusted, with respect to relative humidity and temperature, to the dryness and heatcontaining properties exhibited by the product at the current stage of treatment in the drying, and that each of at least two conveyors (36,38) is enclosed in a separate chamber (30,32) which forms a drying zone each and has an inlet and an outlet for drying matter and an inlet and an outlet for drying air, so that pellets from each chamber (30, 32) may be fed into an adjacent chamber and out of the last chamber (34) which may be constituted of a conventional drying device, and so that separate, mutually independent flows of drying air may be added to the respective chamber (30,32) and leave the same after having subjected the drying matter therein to a partial drying adjusted according to its condition in terms of temperature and relative humidity.
3. A drying plant as claimed in claim 2, in which the conveyors (36,38,40) are arranged one above the other, so that the drying matter may be transferred by the effect of falling from an abovelying conveyor onto an underlying conveyor, and in which the drying matter of the uppermost conveyor (36) is arranged at one axial end thereof, c h a r a c t e r i z e d i n that the chamber (30) of the uppermost conveyor (36) has a downward outlet for drying matter at the other axial end of the conveyor (36), which outlet merges into an upward inlet belonging to a chamber (32) of a directly underlying conveyor (38) and so on.
4. A drying plant as claimed in claim 3, c h a r a c t e r i z e d i n that in the common paths between the different drying stage chambers (30,32,34), i. e. where the product outlet of an abovelying drying chamber merges into the product inlet of an underlying chamber, and in the product inlet of the uppermost chamber (30) and in the product outlet of the lowermost chamber (34) are arranged gates (42,42', 42'', 42''') to seal the respective chamber to the surroundings and other chambers, respectively, which gates permit a transfer of drying matter from the abovelying to the underlying chamber.
5. A drying plant as claimed in claim 2, c h a r a c t e r i z e d i n that the drying zones (30,36; 32,38; 34,40) or two (30,36 and 32,38) or some of them are placed entirely or approximately at the same level and have an intermediate elevator (72) arranged thereto for the transfer of drying matter from one drying zone (30,36) to the next (32,38).
6. A drying plant as claimed in claim 2, c h a r a c t e r i z e d i n that in the first, upper drying zone there is arranged a tight, essentially horizontal chamber (30) with a conveyorbelt (36), while in each of the second, third and possibly further drying zones are arranged counter flow type box dryers (76,78) known in themselves (from drying cabinets), arranged one below the other with a gate (42C) in the transition passage between the counterflow type box dryers (76,78), a gate (42B) in the transition passage between the chamber (30) and the uppermost counterflow type box dryer (76) and a gate (42D) in the product outlet for finished product in the lowermost counterflow type box dryer (78).
7. A drying plant as claimed in claim 2, c h a r a c t e r i z e d i n that the drying plant is essentially formed of three or more counterflow type box dryers (76,78) which are arranged mainly one below the other with a gate (42C) in each of the transition passages there between, a gate (42B) in the product inlet of the uppermost counterflow box dryer, and a gate (42D) in the product outlet for finished product of the lowermost counterflow dryer (78).

The invention relates to a method and a plant for drying material which has been divided, comminuted, pelletized, granulated etc.

More specifically, the invention relates to a method for drying divided/pelletized material, especially of animal and/or vegetable origin, in the form of a foodstuff or a feed material, whereby these food or feed pellets are dried by means of drying air in a forced flow, while said pellets are transported within the drying plant. Likewise, the invention relates to a drying plant for the implementation of the method and is, moreover, of the kind specified in the preamble to claim 2.

In known technique the same air is lead for example through several drying units placed one above the other, which may each comprise a conveyor in the form of a horizontally running conveyor belt, the air flow running from the bottom

upwards. The pellets are added on the uppermost conveyor belt in a warm and moist condition and pass across any intermediate conveyor belts downwards to the lowermost conveyor belt. The direction of transport from the top downwards is reversed from one conveyor belt to the next, so that the uppermost conveyor belt may have the pellets added for example at its left-hand end if the upper section of the conveyor belt has an extent from left to right. At the right- hand end of this upper conveyor belt said pellets leave the belt and fall down on to the right-hand end of the underlying conveyor belt to be moved by this towards the left, and so on. Each conveyor belt may, by identical lengths, be slightly offset in the horizontal direction, so that its receiving end portion extends somewhat beyond the discharge end of the above-lying conveyor belt.

In such a structure the lower conveyor belt receives the air flow as the air temperature and dryness are at their maximums, while the upper conveyor belt receives the air in its coldest and moistest condition, cooled and with condensate added during its passage across the lower and the possible intermediate conveyor belt with drying matter/feed products lying thereon. This involves that feed products, articles of food etc., for example in the form of pellets, present on a lower conveyor belt where the warmest and driest drying air is effective, is"over-dried"on the surface, which reduces the product quality.

When pellets are supplied to an uppermost conveyor belt, the air coming from below will have become more humid and colder on its way up through the drying layers on the underlying conveyor belt (or in drying boxes in drying cabinets).

As mentioned, the pellets will contain water vapour and additionally have an aqueous film which covers each pellet.

When warm pellets, for example at a temperature of about 70 °C, get into contact with even warmer air, which has, for example, a temperature of 120-150 °C, water in the form of water vapour will constantly be extracted from the pellets.

Said animal and/or vegetable products require a gentle drying process in order for the physical properties of the feed and the foodstuff not to be harmed or deteriorated during the drying. Foodstuffs for human beings and feed materials for animals and fish are considered sensitive biological material. A manufactured, not dried nor cooled, product of the kind in question may contain more than 25 % of water and have a temperature of more than 80 °C, when exiting the processing machine.

Such a product needs drying and cooling before it may be used. Forming (extrusion) of pellets normally does not take more than 2-5 minutes, including pre-conditioning, expansion (and pressing) or extrusion. In long conditioning more than thirty minutes may pass.

The biological value in most of the raw material components of the feed material and foodstuffs in question, especially vitamins and colorants (with the exception of carbohydrate material) is reduced whenever they are exposed to heat and moisture above specific values.

The finished formed product, for example in the form of pellets, is transported from the processing stage to the drying plant to be dried to an extent corresponding to a

desired water content. The drying process normally takes a long time and may last for over an hour.

The known drying process depends on several factors: i) particle size (pellet size) ii) the diffusion constant of the product (varies from substance to substance) iii) drying air volume per time unit iv) drying air flow rate v) drying air temperature vi) water vapour absorbing capacity of the drying air The drying process may be divided into two stages of drying rates: a) Stage 1 with a constant drying rate, in which there is a great flow of water from within the product (diffusion) causing the product surface to be moist, and b) Stage 2 with a decreasing drying rate, in which the flow of water from within is reduced and is no longer able continuously to provide the surface of the product with water.

When the drying rate is reduced, the moisture of the product is defined as critical moisture. As mentioned, this is a condition arising when the diffusion forces no longer carry water out to the pellet surface.

By a constant drying rate the water evaporation is so great that the drying medium (for example air) may be relatively hot (>100 °C) without heating and damaging the product which is dried because of the energy of the air being used for the evaporation of water, not for heating the product. At this stage of the drying a certain cooling of the product may in fact be observed. It is important to see to it that the dried product is not over-heated. By decreasing drying rate, after critical moisture has been reached, high temperatures will be rather unfavourable.

An interrupted flow of water to the product surface will result in heating of the product and undesired product changes. Consequently, it is desirable to maintain a continuous flow of water to the product surface.

By decreasing drying rate high air temperatures will have a disturbing and negative effect as to the desire of maintaining a relatively regular flow of an, at the start of the process, unlimited and later in the process limited flow of water through capillary effect out to the product surface.

This may be done by reducing the air temperature, so that the flow of water from the product surface is as large as the rate of diffusion into the product. If too hot air is used in a later phase of the drying process, the result may be over- drying of the product surface and undesired product heating.

In known dryers in the form of drying cabinets comprising one or more drying sections, the product to be dried is subjected to random flows of air of varying parameters such as temperature and relative humidity. This involves that the drying air has varying water absorption capacities at different points in the drying cabinet. Consequently, the

product is dewatered irregularly at said different points.

Thus, in known technique it has not been taken into account that particularly at the end of the drying process the use of high heat is unfavourable, to avoid that the technical quality and the biological nutritional value of the product are reduced. Generally, it can be established that the products in question, including feed materials, are not processed with the correct amount of air at the right point and for the correct period of time in those known drying processes.

More precisely, known drying plants for feed comprises three main components: a drying cabinet of one or more sections; an air heating and circulating system with pertaining channels, fans, cyclones etc.; a control device for the control of said system.

Known and conventional drying cabinets for continuous operation consist of either a box dryer or a multilayer belt dryer of one or more drying sections in one and the same cabinet. Drying plants used currently in the feeds industry work as counter-flow dryers, i. e. the warm air mostly encounters the driest product which is leaving the process.

The more layers of products passed by the drying air, the more humid the air, As previously mentioned, the uppermost product layer contains the most water. When the drying air reaches this upper, moistest product layer, it contains relatively much water which it has absorbed from the underlying layers of product, and its ability to absorb water from the upper product layer is reduced. This may lead to inefficient drying of the upper product layer, particularly at the dryer entrance. In particularly unfavourable

conditions condensation of water from"saturated"drying air may occur in the upper part of the drying cabinet.

In order to have a reduction of the relative humidity of the air, heater batteries are used, in some of these known drying cabinets, to heat the air and reduce its relative humidity.

To increase the efficiency of the drying air and at the same time avoid condensation problems, there is often provided an increase in both the amount and temperature of the drying air on the way to the drying cabinet. In some cases the temperature of internal, recycled air is increased to over 120 °C. High air temperature at the product outlet of the drying cabinet has a negative effect on the physical properties and nutritional value of the dried product.

Drying methods and plants according to known and conventional technique suffer from numerous defects and drawbacks.

The drying air is flowing in the direction against the flow of product, so that the drying capacity of the drying air is reduced on the way to the moistest product. The same drying air, whose drying properties and capacities are gradually changing, flows all the way through the drying cabinet and gets into contact with product present in different phases of the drying, i. e. with a water content of 20-25 % in an initial phase to 4-8 % in the final drying phase. High temperatures of the entering air over-dry the product on its surface in the last drying phase. In addition to the negative effects of heating and over-drying on the physical and nutritional qualities of the product, the drying cost increases at the same time. High moisture in the drying cabinet and the air system (channels, cyclones, fans etc.)

may cause condensation and moreover create serious germs problems, for example by salmonella attacks and growth of fungi in the feed and feed residues. Ready dried product treated in accordance with known technique has not been treated correctly in the different drying phases with air presenting the correct and controllable parameters. Neither has the processing been related to correct periods of time during the drying process, which will likewise be difficult to check and control.

The object of the invention has been to enable stepwise drying of the product, whereby an air flow adjusted according to the product consistency and condition of each step, exhibits the drying properties-temperature, relative humidity, flow rate-which are adjusted to the drying phase/degree of dryness of the respective product. Likewise, the aim has been to provide a drying plant arranged for the convenient implementation of the method.

Said object has been realized according to the invention by means of the method and the plant, respectively, which excel through the features specified in the subsequent claims.

The invention makes it possible to adjust the temperature and air humidity of each separate air flow to the product consistency in question in the respective drying phase.

Thereby the products may be dried in a more gentle and controllable manner. Such a division of the drying process will contribute to achieving a better utilisation of the energy of the drying air, so that the drying air energy is spent for the evaporation of water from the product/product matter, whereby the product/product matter is not unduly

heated. Finally, according to the invention, condensation in the drying cabinet, air channels, cyclones etc is avoided,.

By dividing the drying plant into different zones, for example three different drying zones, the product conveyor of each drying zone may be encased in a housing provided with inlets and outlets for product, for example feed pellets, and each zone receives a separate flow of air through the air inlet of the housing, the air leaving the respective drying zone through the air outlet of the housing. Pipelines, each leading to a separate drying zone housing pass through separate heater batteries and may have a separate set of sensors arranged thereto, comprising for example four sensors, each having functions associated with: a) air flow rate, b) relative humidity of the air, c) air temperature and d) product temperature and e) product moisture. The temperature and relative humidity of each air flow and the amount of air supplied per time unit (air flow rate) may thus in a simple manner be adjusted to the condition/consistency of the product, for example said feed pellets, expressed by moisture content and temperature. The parameters of the drying air may be set and adjusted.

According to the invention at least three drying phases or three drying zones are preferred. Since, of course, the same product/feed material is involved and being treated successively in three drying phases, the partly dried product leaving the first drying zone, must be able to be transferred to a product inlet connected to the second drying zone, and so on. To enable utilization of gravity for the transfer of product/feed pellets from drying zone to drying zone through the effect of falling, it is convenient to arrange the different drying zones above one another.

Such a difference in level between the different drying zones is not critical to the functioning of the invention.

If suggested by the prevailing space conditions, it is fully possible to place two, three or more drying zones at the same, or at approximately the same, height, or two out of three drying zones may be placed in the same horizontal plane, while the third drying zone is placed at a lower or higher plane than the two mentioned. For two adjacent drying zones positioned successively at the same level, an intermediate elevator will be arranged, for example in the form of an encased, slanting, driven cup-elevator or other elevator, whose casing at each end has a gate arranged thereto. Thus, by such a drying zone arrangement there will be, in addition to the elevator, two gates between two adjacent drying zones.

By drying zones offset in height and comprising driven endless conveyor belts, preferably extending horizontally, it may thus be an advantage to displace the individual conveyor belts relatively, so that the product outlet of the housing of the upper conveyor belt is vertically aligned with the product inlet of the housing of the underlying transport belt, and so on, so that the vertical projections of these conveyor belts may essentially overlap one another at adjacent end portions. Alternatively, the conveyor belts may be arranged as specified initially on page 1. To achieve the best possible control with the drying in the different drying zones, the product temperature at the end of each drying zone should be controlled. Likewise, the relative humidity before and after each drying zone should be controlled.

By drying zones offset in height and comprising, in the first drying zone, a chamber with a horizontally extending conveyor belt and two, alternatively three, counter-flow type box dryers arranged at the product discharge end thereof, and arranged one below the other, of the kind included in drying cabinets, or, alternatively, three counter-flow dryers arranged one below the other, each of the counter-flow dryers are encased in a separate chamber (like the conveyor belts), and each counter-flow drying chamber is provided with an air inlet of its own, i. e. an inlet connected to a source of pressurized air, in which the air is conditioned and exhibits properties, such as moisture content, temperature, and possibly has a specific flow rate, all adjusted to the drying condition of the product in the drying zone in question.

Referring to the accompanying drawings, the invention will be explained in the following in connection with multistage drying of feed pellets, only to be considered non-limiting example of a product which could with advantage be dried in accordance with the method in a corresponding drying plant according to the invention, whereby: Fig. 1 illustrates, generally in the form of a flow chart, a simplified multistage drying plant according to the invention; Fig. 2 illustrates, a highly schematic, partial view of an alternative arrangement of the two drying zones with an intermediate elevator; Fig. 3 shows schematically a third embodiment with alternative arrangement, as compared to Figs. 1 and 2, of the last two of three drying zones.

Reference is made first to Fig. 1 and the embodiment visualized there: This figure illustrates a multistage drying plant, in which the drying effect is based on the use of heated, dry air which is brought to flow in a closed circuit, so that heat recovery may take place, and in which there is arranged, upstream of a heat reovery unit 10, a dust separator in the form of a cyclone or a nozzle filter 12.

From the heat recovery unit/plant 10 a pipeline LI leads to a first heater battery 14 which has a pressure fan 16 arranged thereto, which effects the forced flow of the drying air through the closed circuit.

The dustless air heated to a first drying air temperature and which has been, so far, the same for the three drying stages, is distributed after the first heater battery 14 to three branch pipelines L2, L3, L4, each leading to a separate heater battery 18,20,22. In each branch pipeline L2, L3, L4 has been inserted a controlled air valve 24,26,28. The branch pipelines L2, L3, L4 are continued downstream of the heater batteries 18,20,22 by separate air supply pipes L2', L3'and L4', leading to separate drying chambers 30,32,34, whose housings accommodate a conveyor each, in the form of a driven endless conveyor belt 36,38,40 of an embodiment known in itself. Each conveyor belt 36,38,40, together with the pertaining drying chamber/housing 30,32,34 in which it is encased, constitute a drying zone which receives a separate flow of drying air exhibiting an optimal temperature, relative humidity and flow rate for the degree of dryness and the temperature of the respective product in

each individual drying zone. Alternatively, one or more fans may be used for each drying chamber 30,32,34.

The housing of each drying chamber 30,32,34, which by its interior defines a separate drying zone, has, in the exemplary embodiment, an essentially horizontal extent at a different level; and in the vertical projection, one end portion of each of two adjacent drying chambers will overlap each other, while the chambers 30,32,34 are offset relatively in the longitudinal direction of their conveyor belts, so that the product outlet of the uppermost drying chamber 30 is in vertical line with the inlet of an under- lying drying camber 32 for partly dried product, said product outlet coinciding with said product inlet and accommodating a first gate body 42. In a corresponding, combined outlet/inlet between the drying chambers 32 and 34 there is similarly arranged gate bodies 42', 42'', 42'''to permit feeding, transfer, and discharge of product under minimal leakage of air into the upper chamber 30, from chamber to chamber and out of the lowermost chamber 34.

Each chamber 30,32 and 34 has an air outlet 44,46 and 48 at the end opposite the air inlet, and branch pipelines 50,52, 54 from each outlet 44,46,48 lead to the main pipeline Ll for air which downstream of the dust separator 10 and the heat recovery plant 12 ends its closed circuit at the first heater battery 14.

Each air outlet 44,46,48 has a separate set 56,58,60 of sensors arranged thereto, which is extended by cables to a control unit 62. The same applies to the controlled air valves 64,66,68, each mounted into a separate branch pipeline 50,52,54 from the air outlets 44,46,48.

The reference numeral 70 defines a tank or other container for receiving dried product, for example in the form of feed pellets.

The first heater battery conveniently utilizes heat recovered in the recovery plant 12, while the remaining heater batteries, one for each drying zone, may be based on electricity or on heat from vapour/gas. L1'indicates the connection between the heat recovery plant 10 and the first heater battery 14.

The method and the operation, respectively, of the drying plant according to the invention is discussed in the foregoing specification, and therefore, finally, should only be repeated briefly the paths of the product and the drying air through the drying plant.

Moist and warm product of animal and/or vegetable origin enters through the gate body 42"'arranged in the inlet of the uppermost drying chamber 30. The product to be dried, for example in the form of feed pellets, falls from there down onto the conveyor belt 36 which transports said pellets to the receiving end of the conveyor belt 38 of the underlying drying chamber 32. Through a combined outlet/inlet with a gate 42 arranged therein, these partly dried pellets land on the belt 38, where they are influenced as they are moved, by another separate air flow through the pipeline L3', which air flow is adapted according to the fact that said feed pellets have already been subjected to a first drying stage in the drying chamber 30.

Feed pellets in the different drying chambers 30,32,34 which exhibit deviating temperatures and degrees of dryness

(moisture content) at the entrances to the various drying zones, may thus be processed and dried by means of a separate air flow for each drying zone, whereby the parameters (temperature, relative humidity, flow rate) of the air flows for each drying zone may be selected separately and independent of each other, but dependent on the corresponding parameters of the product/feed pellets and at the desired drying rate.

The conveyor belts 36,38,40 are preferably formed with through perforations perpendicular to the belt surfaces, so that drying air may flow from below upwards through the respective conveyor belt during drying of the pellets present on the same.

The respective controlled air valves 24,26,28 and 64,66, 68 are controlled from the control unit 62 on the basis of signals received by the control unit 62 from the sensor sets connected to the air supply pipelines L2', L3', L4'for the drying chambers 30,32,34 and from the sensor sets connected to the branch lines 50,52,54 from the"used"drying air outlets 44,46,48 of the drying chambers 30,34,34. Besides there will be arranged measuring instruments at the outlet of each drying zone/stage to check the moisture and temperature of said feed pellets.

The displacement rate of the conveyor belts 36,38,40 may be individually adjustable for each drying zone 30,32,34.

An alternative positioning of the individual drying zones or of the first two drying zones of a plurality of zones is illustrated schematically in Fig. 2. Parts in Fig. 2 corresponding to identical or similar parts in Fig. 1 are

identified by the same reference numerals. Thus, the two adjacent chambers have also in Fig. 2 been given the identifications 30 and 32, as the conveyor belts are identified by 36 and 38; gate 42'''of the chamber 30 and gate 42 between the chambers 30,32.

The two adjacent drying zones 30,36 and 32,38 shown in Fig. 2 are at the same elevation, at such a horizontal distance from each other that there is room for a pellet transferring, driven, slanted cup elevator 72 which is encased in a separate chamber 74, a gate between the chamber 74 of the elevator 72 and the drying zone 32,38 being identified by 42A. When the space available for the installation of the multistage drying plant has a relatively small area but a great height, it is normally convenient to choose the embodiment according to Fig. 1; in the opposite case the embodiment suggested in Fig. 2 is preferred.

The invention is not limited to the shown and described embodiments. Conveyor belts need not be used. Instead the conveyors may consist of sliding plates pushing feed pellets in each drying zones in front of themselves on a perforated, air admitting, support plate by means of a chain traction or hydraulic/pneumatic piston or telescope cylinders. The conveyors for animal/vegetable products such as feed pellets and the like should preferably be perforated or configured so that the flow of drying air has a convenient access to said feed pellets. Thus, in the conveyors may be included perforated lamellas, S-lamellas, netting or the like. Each drying chamber may comprise several conveyors, both parallel to its length and in its height.

Two or more than three drying zones/stages may be used.

When feed pellets are fed down into the uppermost drying chamber 30, the transfer of them between the drying chambers (from 30 to 32 and from 32 to 34) and the discharge from the lowermost drying chamber 34, the falling effect advantageous in itself is not critical for the invention to function according to its object, as alternatively there may be inserted elevators for the transport of the product into the drying plant, out therefrom and between the individual drying chambers. Air transport of pellets etc. is likewise conceivable, i. a. between drying chambers.

In the embodiment according to Fig. 3 which is a schematic presentation of a partial view of a plant for the implementation of the method according to the invention, the first drying zone is substantially identical to the first drying zone of the plant according to Fig. 2 and comprises a horizontally extending chamber 30 encasing an endless conveyor belt 36, and with an upper product inlet with a gate 42'''at the left-hand end and a product outlet at the right- hand end. Air, whose moisture content and temperature are adjusted according to the drying of the product in the first drying zone, enters from below at the left-hand end of the chamber 30 through the inlet 31, while an outlet 33 for used drying air is arranged closer to the right-hand end, on the top side of the chamber 30.

The underlying outlet at the right-hand end of the chamber 30, together with the inlet of an underlying counter-flow or counter-flow type box dryer 76, form a transition passage in which there is mounted a gate 42B.

Through a corresponding transition passage with a gate 42C, the counter-flow or counter-flow type box dryer 76 is

arranged by its bottom discharge 80 to feed further dried product into an underlying counter-flow or counter-flow type box dryer 78, in whose outlet there is arranged a further gate 42D, from where the finished dried product is fed out.

Each counter-flow dryer or counter-flow type box dryer 76 and 78 is encased in an outer tight chamber, provided itself with a lower air inlet 82 and 84, respectively, and outlet 86 and 88, respectively, and a bottom discharge 80,80'.

In the embodiment according to Fig. 3 the chamber 30 of the first drying zone with the conveyor 36 can be replaced by a counter-flow or counter-flow type box dryer of the same or similar embodiment as 76 and 78.

A drying cabinet working with several drying stages may be used for continuous operation. Filling of the drying cabinet with undried raw material of the relevant kind may take place continuously or by batch, while the bottom discharge of the respective counter-flow box dryer starts at intervals for partly discharging of the product when a predetermined layer thickness/height thereof has accumulated. The dryers work in the way that the air enters below the bottom discharge, passes the product and exits in the upper part of the drying cabinet.