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Title:
METHOD AND EQUIPMENT FOR DRYING AN OBJECT INCLUDING WATER
Document Type and Number:
WIPO Patent Application WO/1998/037371
Kind Code:
A1
Abstract:
In the method for drying an object, e.g. timber or reindeer meat, low-pressure gas mixture, which essentially consists of nitrogen and water vapour, is circulated (207) effectively via the space (202) surrounding the object (203). The method includes cycles, in which during a first phase (101a) the gas mixture is heated effectively within the circulation for heating the object, and during a second phase the gas mixture leaving the space (202) is within the circulation first cooled (204) for condensing and removing water therefrom and for removing (205) the water from the circulation and then heated back (206) to a suitable temperature below the temperature of the object before the gas mixture returns back to said space (202). In the equipment accomplishing the method the effective circulation is made possible as well as the effective heating, cooling and drying by means of cooling and heating again. The successive phases and successive cycles formed thereby are controlled and carried out according to certain drying schedules. The drying may be performed rapidly, gently and with a small consumption of energy.

Inventors:
Saarenp��, Keijo (Patotie 8, Utaj�rvi, FIN-91600, FI)
Application Number:
PCT/FI1998/000151
Publication Date:
August 27, 1998
Filing Date:
February 19, 1998
Export Citation:
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Assignee:
Lvi-insin��ritoimisto, Keijo Saarenp�� (Patotie 8, Utaj�rvi, FIN-91600, FI)
Saarenp��, Keijo (Patotie 8, Utaj�rvi, FIN-91600, FI)
International Classes:
F26B5/02; F26B5/04; F26B21/08; F26B21/14; (IPC1-7): F26B21/02; F26B5/04; F26B21/06
Foreign References:
US4246704A
US3939573A
US4058906A
Attorney, Agent or Firm:
Laurinolli, Tapio (Patenttitoimisto Tapio Laurinolli, P.O. Box 258, Oulu, FIN-90101, FI)
Download PDF:
Claims:
Claims
1. Method for drying an object including water, e. g. timber or reindeer meat, in which method: the object (203) is closed to a chamber or equivalent space (201, 202), wherein a partial vacuum is caused in relation to normal air pressure, so that a gas mixture, which includes water vapour extracted from the object, is formed therein; the object (203) is heated to vaporize water therefrom to the space (202) surrounding it; and water vapour is removed from the space (202); characterized in that the gas mixture is circulated (207) effectively via the space (202) surrounding the object (203), so that the gas mixture leaves from the space (202) to the circulation and returns back to the space (202) from the circulation, and that it includes cycles (101, 102, 103), in which: during a first phase (101 a) the gas mixture is heated effectively within the circulation for heating the object to a desired temperature; and during the second phase (102b) the gas mixture leaving the space (202) is within the circulation first cooled (204) for condensing water therefrom and removing (205) the water from the circulation and then heated (206) to a desired temperature below the temperature of the object before the return back to the space (202) surrounding the object (203).
2. Method according to claim 1, characterized in that it further includes cycles (104, 105) which include a freezing phase (104c), in which the gas mixture is cooled within the circulation so that the water vapour is essentially crystallized to form snow and / or ice.
3. Method according to claim 1 or 2, characterized in that the cycles are repeated cyclically according to a schedule prepared for a drying process for achieving a desired drying result.
4. Method according to any preceding claim, characterized in that the gas mixture and the object are heated during the first phase to 50 + 10 CC.
5. Method according to any preceding claim, characterized in that the gas mixture leaving the space (202) surrounding the object (203) is cooled during the second phase an amount of degrees, which is from 5 to 20 "C.
6. Method according to any preceding claim, characterized in that, despite water vapour, the gas mixture essentially consists of nitrogen.
7. Method according to any preceding claim, characterized in that the circulation of the gas mixture is controlled so that the gas mixture returning from the circulation back to the space (202) surrounding the object (203) becomes essentially moisture saturated before leaving said space.
8. Method according to any preceding claim, characterized in that ultrasound is directed to the object during the drying process.
9. Equipment for drying an object including water, e. g. timber or reindeer meat, which equipment includes: a leakproof chamber or equivalent space (1) for the object (3) to be dried and means (8) for causing a partial vacuum in the space in relation to normal air pressure, for forming a lowpressure gas mixture, which includes water vapour extracted from the object, in the space (1); means for heating the object (3) to vaporize water therefrom to the space (1); and means for removing water vapour from the space (1); characterized in that it includes: means (9, 10, 11, 18) for circulating the gas mixture effectively via the space (1) surrounding the object (203), so that the gas mixture leaves from the space (1) to the circulation and returns back to the space (1) from the circulation; means (36, 60, 35) for cooling and / or heating the gas mixture effectively within the circulation; and means (20 34, 38 50, 53, 54) for controlling circulating means (9, 10, 11, 18) and cooling and heating means (36, 60, 35) to: heat the gas mixture for heating the object (3) to a desired temperature; cool the gas mixture to a desired temperature below the temperature of the object; or to dry the gas mixture by cooling the gas mixture leaving the space (1) surrounding the object for condensing water therefrom and for removing the water from the circulation and by then heating the the gas mixture within the circulation before the return thereof to said space (1).
10. Equipment according to claim 9, characterized in that it includes means (32) for controlling control means (20 34, 38 50, 53, 54), circulating means (9, 10, 11, 18), and cooling and heating means (36, 60, 35) to form cycles repeated according to a certain schedule and certain set values, the cycles including successive heating phases and cooling and drying phases.
11. Equipment according to claim 9, characterized in that it includes means (32, 38, 53, 18) for controlling the drying process so that the gas mixture is maintained essentially moisture saturated at the place in which it is leaving the space (1) surrounding the object (3).
12. Equipment according to any claim 9 11, characterized in that it includes means (37, 17) for introducing nitrogen to the chamber (1) and so for achieving a gas mixture, which essentially consists of nitrogen and water vapour.
13. Equipment according to any claim 9 12, characterized in that the circulating means include a channel structure (9, 10), which essentially covers a wall of the chamber (1).
14. Equipment according to claim 13, characterized in that the channel structure is double so that the inner channel (9) serves as an insulating layer in relation to the outer channel (10).
15. Equipment according to claim 13 or 14 , characterized in that the means (36, 60) for cooling the gas mixture and for condensing water vapour therefrom are located in connection with the outest wall of the channel structure.
16. Equipment according to any claim 9 15, characterized in that the cooling and heating means (36, 60, 35) are connected to means (39, 29, 30, 50, 24, 26, 27, 28) for providing cooling solution and to means (20, 21, 34, 45, 46, 48, 22, 23, 24, 25, 27, 33, 59) for providing heating solution for them, and that the equipment includes a heat exchanger (41), to which said means for providing cooling solution and said means for providing heating solution are connected for heat transfer.
17. Equipment according to any claim 9 16, characterized in that it includes a condensing tank (12) for the water condensed in the circulating and cooling means (10, 36, 60) and in connection therewith means (51, 61, 39, 41) for freezing the condensed water and for recovering the energy released with the freezing.
18. Equipment according to any claim 13 17, characterized in that the space (1) is a horizontal cylindershaped space, the cylinder wall (1, 9) of which is essentially covered by the channel structure and under which the condensing tank is located extending essentially over the whole legth of the chamber (1).
Description:
Method and equipment for drying an object including water The invention is related to a method and an equipment for drying an object including water, e.g. timber or reindeer meat. Especially, the invention is related to so-called vacuum drying wherein the object is closed in a chamber or equivalent, in which partial vacuum is maintained during the drying, to bring down the boiling point of water and to make the vaporization of water easier.

A preferred application of the invention is timber drying, with which a lot of problems are involved. The biggest problem, perhaps, is to make the moisture of the inner part of the timber to move rapidly enough to the outside layer of the timber and therethrough to the ambient gas space, without causing cracks or other drying damages in the timber.

Although many improvements are developed in the methods and equipment of drying timber, still today more than one half of the costs of drying large sized timber are caused by the drying damages.

Newly cut timber includes a lot of intercellular water, and for drying the timber to a water content of about 6 %, required for furniture wood, also the most of the water bound by the cells must be removed. Intense shrinkage of timber begins from the so-called fiber saturation point, which may be, e. g., about 28 %, and on which the removal of the intercellular water is ended and the extraction of water bound by the cells starts. If the outside layer of the timber is then dried too rapidly in relation to the inner part of the timber, the surface layer is hardened and tensile stresses and as a result thereof cracks are caused therein.

The water is advantageously removed from the timber by boiling, because then the drying occurs intensively in both the surface layer and the inner part of the timber, by which the moisture differences and the defects caused thereby are reduced. However, for causing and maintaining boiling a temperature of at least 100 OC is needed under normal pressure, and for many kinds of wood such a temperature causes colour defects, e. g.

blackening, or deterioration of strength. An advantage of the vacuum drying is that boiling is caused and maintained at a remarkably low temperature.

The existence of oxygen in the drying ambient causes at higher temperatures, besides the colour changes, also increased melting of resin. At lower temperatures, which are concerned with vacuum drying, the existence of oxygen contributes to growth of mould and blue stain fungi on the surface of the timber.

As may be assumed, the above problems related to the drying of timber have led to many attempts to develop better controlled and faster drying methods and equipment suitable for carrying out these methods.

FI 930081 presents a drying method intended to be used under normal pressure, wherein the timber is dried by means of an air flow directed thereto. The aim is to keep the inside temperature of the timber above the boiling point, and heating and cooling phases are repeated cyclically between an upper temperature limit (100 to 120 OC) and a lower temperature limit, which may not be significantly lower than the boiling point, set for the inside temperature. The intention is that during the cooling moisture is moved from the inner part of the timber towards the surface, from which it is vaporized during the heating.

In the Finnish Nurvac vacuum drier (WOODWORKING PUUNTYÖSTÖ No. 8/95, p. 14) the timber is placed in a leak-proof chamber, and the temperature is first rised therein under normal pressure. When the starting temperature is achieved, a vacuum pump is switched on, whereby the pressure in the chamber is gradually reduced to about 0,1 bar.

Under such a low pressure water is still boiling at least at about 45 "C. There are fans in the chamber to make the conditions uniform. The vaporated moisture is removed from the chamber through a pipe to a heat exchanger, in which it is condensed and then led out therefrom. With this drier the problem is that the removal of the vaporized moisture through the pipe to outside of the chamber takes a lot of time, and so the whole drying process tends to progress slowly.

In the Moldrup/Safimex vacuum drier (WOODWORKING PUUNTYC)STO No.

8/95, p. 15 - 16), to which also the Finnish patent number 87691 is related, air is first removed from the chamber, so that the pressure is about 0,1 bar. Thereafter, during the following heating phase heat is transferred from the thermobatteries placed in the chamber to the timber through the moisture vaporized from the timber. During the drying phase vaporized moisture is removed from the chamber according to a certain drying schedule.

The equipment includes a separate condensing tank, which is kept at a temperature lower than the temperature prevailing in the drying chamber. The drying process is controlled by opening the pressure control valve between the drying chamber and the condensing tank, always when the pressure in the drying chamber exceeds a certain limit. As the valve is opened, vaporated moisture is removed from the drying chamber to the condensing tank having a lower temperature and pressure. The aim of the control is to manage the temperature and the pressure and so the relative humidity of the vapour based on these conditions, and to keep the temperature and the pressure as uniform as possible in the drying chamber.

Other methods known in the art are accomplished by Brunner Hildebrand vacuum drier (WOODWORKING PUUNTYÖSTÖ No. 8/95, p. 17), and Baureihe KVT drier (Neues Vakuum-Trocknungssystem für Schnittholtz hat Erfolg, Sonderdruck aus Holtz- Zentralblatt, Stuttgart, Vol. 116 (1990), No. 30, Friday, March 9, 1990, p. 430+432).

A problem with the vacuum driers is that the vaporizing heat is carried and delivered to the timber lot to be dried ununiformly, which causes too large a divergence of the percentage of moisture in different parts of the timber lot. The disadvantage is the bigger the bigger the reduction of the air pressure in the chamber is, because then the density of the mixture of the air and the vaporized moisture and at the same time the specific heat capacity is reduced. To eliminate this disadvantage some solutions are presented, in which the area of the heating surface and the circulating velocity of the mixture of air and vaporized moisture are increased, and a long enough cooling and settling phase is provided in the end of the drying process.

The drying process is made slower also by the handicaps related to the transfer of the vaporized moisture from the drying chamber to the cooling surfaces. In some solutions the vacuum pump is used also to transfer the mixture of the air and the vaporized moisture.

Vacuum pumps provide large pressure differences with a small volume flow, and because the specific volume of the water vapour increases as the temperature and the pressure decrease, some problems are caused. The total pressure of a closed space decreases also with the reduction of temperature. As a result, the relative humidity is increased to 100 %, and drying of the timber is stopped. Narrow channels and pipes and restricted areas of heating and cooling surfaces set a limit to the increase of the circulation velocity. The vapour is kept saturated on the cooling surfaces, although the temperature and pressure thereof are reduced. The reduction only causes the transfer of heat energy from the drying chamber to the cooling surfaces and the reduction of the temperature and pressure also in the drying chamber, i. e. the total pressure becomes rapidly even. The saturated water vapour and the water condensed therefrom act as "a plug" making the drying process slower in the plate heat exchanger, especially. This leveling of the total pressure is prevented in some equipment by installing a shut-off device between the drying chamber and the cooling surfaces, whereby the drying process is made cyclic. This makes the drying process slower, and additional measurement and control techniques are required.

Problems arise also in such solutions wherein the water vapour is condensed in the drying chamber, and the condensed water is gathered to the bottom of the chamber. The water is not easily maintained as a liquid but tends to vaporize back because of the small differences between the temperature and vapour pressure of, on the other hand the water film of the timber surface, and on the other hand the condensed water.

An object of the invention is to present a method and an equipment for drying an object including water, e.g. timber or reindeer meat, which remove water both rapidly and gently and with a small energy input, and in which many disadvantages considered above

may be avoided. Another object of the invention is to present a solution, by means of which certain qualities desired by the end user may be emphasized in the dried timber.

According to one aspect of the invention the method for drying an object including water, e. g. timber or reindeer meat, in which method: the object is closed to a chamber or equivalent space, wherein a partial vacuum is caused in relation to normal air pressure, so that a gas mixture, which includes water vapour extracted from the object, is formed therein; the object is heated to vaporize water therefrom to the space surrounding it; and water vapour is removed from the space; is characterized in that the gas mixture is circulated effectively via the space surrounding the object, so that the gas mixture leaves from the space to the circulation and returns back to the space from the circulation, and that it includes cycles, in which: during a first phase the gas mixture is heated effectively within the circulation for heating the object to a desired temperature; and during the second phase the gas mixture leaving the space is within the circulation first cooled for condensing water therefrom and removing the water from the circulation and then heated to a desired temperature below the temperature of the object before the return back to the space surrounding the object.

According to another aspect of the invention the equipment for drying an object including water, e. g. timber or reindeer meat, which equipment includes: a leak-proof chamber or equivalent space for the object to be dried and means for causing a partial vacuum in the space in relation to normal air pressure, for forming a low-pressure gas mixture, which includes water vapour extracted from the object, in the space; means for heating the object to vaporize water therefrom to the space; and means for removing water vapour from the space; is characterized in that it includes: means for circulating the gas mixture effectively via the space surrounding the object, so that the gas mixture leaves from the space to the circulation and returns back to the space from the circulation; means for cooling and I or heating the gas mixture effectively within the circulation; and means for controlling circulating means and cooling and heating means to: heat the gas mixture for heating the object to a desired temperature; cool the gas mixture to a desired temperature below the temperature of the object; or to dry the gas mixture by cooling the gas mixture leaving the space surrounding the object for condensing water therefrom and for removing the water from the circulation and by then heating the the gas mixture within the circulation before the return thereof to said space.

The drying method according to the invention is based on powerful circulation of the low-pressure gas mixture, including water vapour extracted from the object to be dried, with effective, in relation to the object properly placed fans, and on cyclic variation of the temperature of the gas mixture within a suitable temperature range. In the drying method

the cycles are repeated, in which the gas mixture is first heated effectively to bring the temperature of the object to be dried to a temperature above the boiling point of water, for example to 60 "C. Thereafter, in the following cooling and drying phase the gas mixture leaving the space surrounding the object is during the circulation first cooled further to condense and remove the water vapour therefrom and then heated again before returning it back to the space surrounding the object.

Advantageously, the gas mixture includes nitrogen instead of air to avoid the problems caused by oxygen. Nitrogen is either normal nitrogen or, if reindeer meat or equivalent object is dried, special nitrogen for foodstuff handling. Advantageously, nitrogen-water vapour mixture is circulated from the drying chamber to a channel structure essentially covering the wall, which channel structure is double, so that the inner channel serves as an insulation channel and the outer channel as a cooling channel. From the cooling channel the gas mixture is circulated back to the drying chamber through fan channels, wherein it may also be cooled or heated.

The gas mixture acts as a heat carrier, moisture discharger and heat insulator in the insulation channel, the insulation capability thereof being based on the continuous, rapid flow of the gas mixture. Insulation channels and the use of the nitrogen-water vapour mixture as a heat insulator render possible large temperature differences between the drying chamber and the cooling channel and condensing tank thus securing good performance of the drying process.

In an advantageous embodiment of the invention the extraction of water from saturated water vapour is based on centrifugal force, density of water, gravitation, and circulation velocity of the nitrogen-water vapour mixture. Water droplets are drifting to the outer walls of the cooling channel, cool to a temperature close to 0 "C, and run down to the condensing tank, in which they are freezed. The use of the centrifugal force intensifies the extraction of water from the saturated water vapour, which speeds up the drying process.

The drying process consisting of two-phase or three-phase cycles provides a lot of alternatives to plan the drying schedule in such a way that total costs (drying time, damages, energy consumption) may be reduced. The cycles and the phases included therein may be varied according to the kind of wood, water content, and thickness of the timber.

In one advantageous embodiment the cooling surface and the heating surface serve both as heating surfaces during the heating phase, and as cooling surfaces during the freezing phase. During the cooling and drying phase they serve separately as a cooling surface and as a heating surface.

Furthermore, the constructional solutions of the invention: wide circulation channels, large areas of the cooling and heating surfaces, simultaneous use of cooling and heating surfaces, and normal air pressure fans together with channels which make possible to melt, preheat and predry the timber, make the invention different from the prior art vacuum driers and render possible the reduction of the total drying costs.

The present invention will now be described more closely with reference to the accompanying drawings, in which: Figure 1 is a flow chart describing generally the method according to the invention; Figure 2 is a schematic presentation, which describes in further detail phases of the method according to the invention; Figure 3 is a diagrammatic elevation view, partly cut away, of an embodiment of the equipment according the invention; Figure 4 presents the same embodiment completely including a sectional view taken on line A-A of Fig. 3 together with a diagrammatic presentation of other parts of the equipment; and Figures 5 and 6 present a plan view and an elevational view, respectively, of another embodiment of the equipment according to the invention.

In the method according to the invention, the low-pressure gas mixture, including air or suitable gas instead of air and water vapour derived from the object, is circulated effectively via a drying chamber, into which the object to be dried is located. As is demonstrated by Figure 1 cycles 101, 102, 103, 104 and 105 are repeated, which include at least two successive phases: a heating phase 101a, 104a, and a cooling and drying phase 101b, 104b. A cycle may include also a freezing phase 104c. The number of the cyclically repeated successive cycles may vary depending on the application. When drying timber, for example, the process may include first a certain number m of cycles 101, 102, 103 having two phases, and then, as the fiber saturation point is reached, a certain number n of cycles 104, 105 having also the freezing phase. In the following the phases of the cycles are considered in further detail with reference to Figure 2.

During the heating phase 101a, 1 04a the gas mixture circulating via the space 202 is heated powerfully, whereby the pressure thereof increases rapidly, and an overpressure is achieved in relation to the object 203 to be dried. The overpressure, and the fact that the gas mixture is saturated, which means that the density and the specific heat capacity thereof are as high as possible, speed up the transfer of the vaporization heat to the object 203 to be dried. In the example of Figure 2 the temperature of both the gas mixture and the object is first 10 OC (above), and when the gas mixture is heated to 60 "C and is

circulated effectively, heat is transferred therefrom to the object 203 (middle), which eventually reaches the temperature of 60 "C (below).

During the cooling and drying phase 101 b, 1 04b the temperature ofthe gas mixture is reduced by a certain amount of degrees, defined in the drying schedule, by means of the cooling surfaces 204 located in the cooling channels. This amount of degrees, so called drying range, is here 7 "C. Before the return of the gas mixture to the space 202 surrounding the object, the gas mixture is heated approximately by the same amount of degrees by means of the heating surfaces 206 located in the heating fan channels. During the heating the relative humidity of the gas mixture is reduced, and the gas mixture is able to receive water vapour vaporized from the water film on the surface of the object 203 in the drying chamber 202. The water film on the surface takes vaporizing heat from inside of the object, and so the temperature of the object is decreased continuously. In the drying chamber, the temperature of the gas mixture is maintained at a suitable value lower than the temperature T of the water included in the object to be dried, by means of effective fans and simple control techniques. The cooling and drying phase is continued, until the temperature of the object is descended to such a low temperature, e. g. 10 "C, that vaporization of the water from the object is practically stopped.

During the freezing phase 1 04c the temperature of the gas mixture is reduced to -5 "C. Water vapour is then crystallized to snow and ice, and the water content of the circulating gas mixture is only 3 grams in a cubic meter. In an advantageous embodiment the operating gas is almost exclusively nitrogen. When the object is timber, the freezing phases are accomplished below the fiber saturation point, in which the essential shrinkage of the wood starts and drying stresses are produced therein during the cooling and drying phase. By freezing the water film on the surface and the surface layer of the timber, vaporization of the water from the timber is prevented. The moisture differencies inside the timber are rapidly leveled, which prevents formation of drying defects. The spores of mould or other fungi possibly attached to the surface of timber are destroyed. The cell structure of the surface of the wood is expanded during freezing, and the expansion cancels tensile stresses, produced in the surface layer of the timber during the cooling and drying phase, and prevents formation of drying defects. In consequence of this a large drying range may be used, which speeds up the process and provides shorter drying time.

In the following an embodiment of the equipment for carrying out the method of the invention is described with reference to Figures 3 and 4. The equipment includes the following parts: drying chamber 1, transfer carrier 2, supporting grid 4, end walls of the drying chamber 5 and 6, door for ice removal 7, vacuum pump 8, insulation channel 9, cooling channel 10, fan channel 11, condensing tank 12, main wall of the equipment 13,

pressure sensor 14, shut-offvalves 15 and 16, nitrogen valve 17, circulation fan for nitrogen-water vapour mixture 18, normal air pressure fan 19, tube system for outside heat source 20, heat exchanger 21, heating solution pump 22, valves 23, 24, 25, 26, 27 and 28, cooling solution pumps 29 and 30, heating solution pump 31, control center and computer 32, temperature sensor 33, valve 34, heating battery 35, cooling solution channel 36, nitrogen container 37, main temperature sensor 38, cooling solution container 39, diaphragm expansion containers 40, solution cooling unit 41, temperature sensor 42, cooling solution pump 43, temperature sensor 44, heating solution pump 45, air condenser 46, air condenser fan 47, heating solution container 48, temperature sensors 49 and 50, cooling solution channel for cendenser tank 51, damping material for volume change of ice 52, optical density sensor for water vapour 53, measurement sensor for relative humidity 54, protective agent container 55, protective agent 56, valve 57, spray nozzle for protective agent 58, valves 59, cooling / heating rib 60, thermal insulation of condensing tank 61, and normal air pressure channel 62.

Drying chamber 1 is advantageously a cylindrical space free from operating units. The most important operative units are located and the most of the functions of the drying process occur in the circulation channels 9, 10 and 11 for the gas mixture. In relation to the volume of the drying chamber there is room for a large amount of goods 3 to be dried, because there are no operational units causing harm to material transfer, circulation of the gas mixture, and delivering of the mist spray amending the properties of the goods. The basic structure of the equipment is generally symmetrical, and, as is shown in Figure 3, in the longitudal direction of the drying chamber 1 the equipment may be constructed of modules depending on the need of drying capacity. The main wall 13 is made of pressure- tight material with good resistance against mechanical, physical and chemical stresses, for example steel.

In the equipment according to the invention, the insulation channel 9, cooling channel 10, cooling solution channel 36, and normal air pressure channel 62 are located one upon the other in the direction of the radius of the round drying chamber 1, so that two successive channels have one common wall. Advantageously, the channels are essentially as long as the drying chamber. The walls of the circulation channels 9, 10, and 11, and the cooling / heating ribs 60 act as reinforcing elements for the main wall 13, and so the main wall may be made thinner. The circulation channels may be constructed to have large sectional areas, which renders possible the circulation of a big quantity of gas with a small circulation resistance. Respectively, the large cooling and heating surfaces are obtained, whereby big energies may be transferred rapidly. Also the simultaneous use of the surfaces speeds up the drying process. The cooling surfaces are located in the cooling channel 10,

in which the circulated saturated water vapour is condensed to water. The surfaces are formed by the main wall 13 and cooling ribs 60.

The cooling solution channel 36 forms two cooling surfaces, one working under the vacuum, and another working under the normal pressure, which another one at the same time is the inner wall of the normal air pressure channel 62. The normal air pressure channel is used for melting, preheating, and predrying of timber.

The function of the fan channel 11 is to transfer the gas mixture from the cooling channel 10 back to the drying chamber 1. The circulation fans 18 and the heating surfaces 35 are located in the fan channel. The fans are located at both sides of the drying chamber on two levels, so that they never are opposite each other. The motors of the fans act as heating surfaces, too. The location of the fans 18 and the powerful flow of the gas mixture improve delivering and distribution of heat energy to and within the goods 3 to be dried.

The condensing tank 12 is designed in such a way that there is room for the whole water amount of the drying process. Condensed water is advantageously freezed, whereby the heat energy released with the freezing may be utilized in the drying process. The inner wall of the condensing tank is provided with a flexible water-repellent damping material 52 having good thermal conductivity, which material accepts the volume change occurring with the freezing ofthe condensed water. One end wall ofthe condensing tank 12 is pivoted to provide a pressure-proof door 7, through which the ice is removed. When the drying process is in the end and the pressure is settled, heating solution is during a short time conducted to the cooling solution channels of the cendenser tank, whereby a thin water film is formed between the ice and the damping material and serves as a lubricant when the ice is drawn out. By freezing the condensed water it is secured that this water is kept separated from other phases and operations of the drying process.

The cooling solution container 39 serves as a store for cooling solution, and the heating solution container 48, respectively, as a store for heating solution. Both of them are also used for leveling the peaks of energy consumption. The cooling solution pumps 29, 30 and 43, and the heating solution pumps 22, 31 and 45 circulate same water-glycol solution, having the same pressure but, respectively, different temperatures, in different parts of the equipment.

The sensor 54 for measuring relative humidity is located in the group of main measurement sensors. From the nozzles 58, which are located in front of the fans 18, it is possible to spray agent, for example, which reduces moisture related dimensional changes of wood, to deliver it to the surface of the timber. This is necessary, especially when the timber is dried to a very low percentage of moisture. The fans 18 are utilized for delivering

and distributing the spray. When drying reindeer meat, for example, salt solution may me sprayed through the nozzles.

In the following an example is described concerning the application of the equipment of Figures 3 and 4 and the method according the invention to the drying of pine timber.

The required final percentage of moisture is 6 % and the allowed deviation is + 1 %. In the end of the drying process agent, which reduces moisture related dimensional changes of wood, is sprayed to the timber. The application concerned provides also a special drying schedule.

The lot of pine timber 3, stacked with sticks on the carrier 2, is transferred to the drying chamber 1. The pine timber lot 3 is supported by the grid 4. The end walls 5 and 6, pivoted to the main wall 13 of the drying chamber 1, and the ice removal door 7 are closed pressure-tightly.

The vacuum pump 8 is started by the control center 32. The air in the drying chamber 1, cooling channel 10, fan channel 11, and condensing tank 12 is rarefied, and a partial vacuum is formed therein. The main wall 13 of the equipment serves as a vacuum responsive wall. A message from the pressure sensor 14 stops the vacuum pump, when the total pressure in the vacuum space has reached 40 mbar. The valve 15 is closed, and also the valve 16 still remains closed. The vacuum pump 8 is not started again during the drying process. The shielding nitrogen valve 17 is opened, and nitrogen is introduced to the vacuum space from container 37, until the total pressure reaches 100 mbar. The message from the pressure sensor 14 causes the closing of the valve 17. The control center starts the first heating phase. The fans 18 are activated. They are drived under frequency converter control with a high constant speed of rotation. The fans 19 in the normal air pressure channel 62 are not activated during this drying process, because there is no timber to be predried.

Heat energy from outside is taken to the drying process during the first heating phase via the tube system 20. This heat energy is transferred to the water-glycol solution by means of the heat exchanger 21. The pump 22 is run with a high constant speed of rotation. The valves 24, 59 and 25 are open and valves 23, 26, 27 and 28 are closed. The pumps 29, 30, 31, 43 and 45 are not active. The solution cooling unit 41 is not active. The heating ribs 60 intensify the transfer of heat energy to the nitrogen-water vapour mixture.

The control center 32 controls the valve 34 according to the value measured by the temperature sensor 33, so that the temperature of water-glycol solution at the place of measurement is constantly kept at + 60 "C. The solution is circulated via the heating battery 35 and the cooling solution channels 36. The heating phase lasts, until the main temperature sensor indicates that the temperature of circulating nitrogen-water vapour

mixture is + 60 OC, and the message of the sensor stops the heating phase. The pump 22 is then stopped and the valves 25 and 34 closed. In the beginning of the heating phase the vacuum space is rapidly filled by saturated nitrogen-water vapour mixture, and the total pressure rises to about 200 mbar during the heating phase. During the heating phase heat enrgy is stored in the timber to be used later for vaporizing water included in the timber.

The control center 32 starts the first cooling and drying phase. Solution cooling unit 41 and pumps 31 and 43 are activated. The valves 24 and 23 and 28 are opened. The pumps 30 and 22 are started. Also the valve 26 is opened and the pump 29 with frequency converter control is started. The control center 32 controls the frequency converter controlled pump 43 according to the value measured by the temperature sensor 49, so that the temperature of the water-glycol mixture at the place of measurement is kept at a desired value, - 10 "C. In the same way the control center 32 controls the speed of rotation of the frequency converter controlled pump 31 according to the value measured by the temperature sensor 44, so that the temperature of the water-glycol mixture at the place of measurement is kept at a desired value, + 40 "C. The diaphragm expansion containers 40 level the pressure variations in the tube system caused by temperature variations of the water-glycol mixture.

The control center 32 controls the speed of rotation of the frequency converter controlled pump 29 according to the value measured by the temperature sensor 42, so that the temperature of the water-glycol mixture at the place of measurement is descending and is at the place of measurement 7 OC lower than at the place of the main temperature sensor 38. According to the value measured by the temperature sensor 38 the control center 32 controls the frequency converter controlled pump 22, so that the condensing heat energy bound by the water-glycol solution the temperature of 40 "C is conducted via the heat solution container 48 to the heating batteries 35, so that the temperature of the circulating water-glycol solution is rised approximately by the same amount of degrees, as it was reduced by the cooling. According to the values measured by the optical sensor of vapour density 53 and temperature sensor 38 the control center 32 controls the frequency converter controlled fans 18, so that the circulated water-glycol solution is at the place of measurement kept as essentially saturated. When the temperature goes down in the vacuum space, the vapour pressure of the water-glycol mixture and the total pressure are reduced, and the water film on the surface of the timber starts to vaporize by boiling.

The control center 32 controls the frequency converter controlled pump 30 according to the value measured by the temperature sensor 50, so that the temperature of the water- glycol mixture circulating in the cooling solution channels of the condensing tank is at the place of measurement kept at a desired value, - 3 "C. In the condensing tank 12 the water

is freezed, and the damping material 52 receives the growth of the volume. Heat insulating material 61 covers outside the surface of the cooling solution channels 51 of the condensing tank 12.

When the water from the film on the surface of the timber is vaporized by boiling, energy stored in the timber 3 is consumed to release the water molecules from the water film, and the temperature of the lot of the pine timber is reduced continuously. When the production of the condensing heat energy in the solution cooling unit 41 is more than the consumption in the heating batteries 35, the temperature of the water-glycol solution is rising at the place of the temperature sensor 44. Then the control center 32 starts the pump 45 and the fan 47 of the air condenser 46 according to the value measured by the temperature sensor 44, and the excess heat energy is transferred, for example, to the predrying of timber. In view of calculations, and because of melting heat energy of the ice and the fact, that the electric energy of the compressor of the solution cooling unit is changed to heat energy, the drying process produces more heat energy than it consumes.

The cooling and drying phase is continued, until the vaporizing energy stored in the timber has been utilized. At that time the temperature of the timber 3 is reduced to the temperature of+ 10 "C, and the vaporization of the water film on the surface of timber is ceased. Vaporization is continued through evaporation, and the temperature of the timber lot is still going down. The ending of the vaporization by boiling is observed as a considerable reduction of the speed of rotation of the fans 18, and another heating phase is started.

The second and the following heating phases differ from the first heating phase of the drying process so that the valves 28 are open, and that the pump 30, the control thereof, and the solution cooling unit 41 are active. Condensing water is freezed and the melting heat of ice is utilized. The lot of pine timber is heated up to only + 40 CC. The control center 32 drives the frequency converter controlled pump 22 according to the value of measurement of the main temperature sensor 38, until the temperature of the nitrogen- water vapour mixture reaches + 40 "C at the place of measurement. In this drying schedule the heating phase and cooling and drying phase are now carried out successively seven more times.

When the control center 32 indicates the end of the seventh cooling and drying phase, the first phase is started, in which the final percentage of moisture (equilibrium moisture content) is determined. The fans 18 run with a high constant speed of rotation. The solution cooling unit 41 is active. The melting heat of the ice of the condensing tank is utilized. The valves 25, 26 and 27 are closed and the valves 23, 24, 28 and 59 are open.

The pumps 29 and 45 are stopped and the pumps 22, 30, 31 and 43 are active. The control

center 32 controls the speed of rotation of the frequency converter controlled pump 22 according to the value of measurement of the main temperature sensor 38, so that the temperature of the circulating nitrogen-water vapour mixture at the place of measurement is kept at + 10 "C. The relative humidity sensor 54 measures the relative humidity of the circulating nitrogen-water vapour mixture. When the relative humidity is maintained at an essentially constant value during several minutes, the control center determines the final percentage of moisture.

The control center 32 indicates that the constant value of relative humidity is 80 % and determines the value 16.3 % of the final percentage of moisture. Because the required value is 6 i 1 %, the drying process is continued. The control center 32 performs the eighth heating phase and cooling and drying phase.

After the eighth heating phase and cooling and drying phase the first freezing phase is carried out. The valves 26 and 24 are open and the control center 32 drives the pump 29 with a high constant speed of rotation under the control of the main temperature sensor 38, and circulates water-glycol solution, having the temperature of - 10 "C, from the container 39 via the heating batteries 35 and cooling solution channels 36. The water- glycol solution of -10 "C is circulated, until the temperature ofthe circulating nitrogen- water vapour mixture is reduced to -5 CC, and thereafter the circulation is continued in this drying schedule for about 15 minutes.

The control center 32 performs the ninth heating phase and cooling and drying phase and the second freezing phase. In the same way the tenth and eleventh heating phase and cooling and drying phase and, respectively, the third and fourth freezing phase are performed, and thereafter the second determination of the final percentage of moisture is carried out. The control center 32 indicates that the constant value of relative humidity is 30 % and determines the value 6.2 % of the final percentage of moisture, which is in accordance with the requirements. The drying process is ended: the solution cooling unit 41 and all the pumps are stopped and all the valves are closed.

Under control of the control center 32, agent, which reduces moisture related dimensional changes of wood, is next sprayed to the lot of pine timber. The fans 18 run with a high constant speed of rotation. The open container 55 includes protective agent 56 in liquid form. The valve 57 is opened for a time of 15 seconds. The normal air pressure pushes liquid to the nozzles 58, from which it is sprayed to the circulating nitrogen-water vapour mixture and fluctuates further on the surface of the timber and is absorbed to the surface layer. The fans 18 are stopped after 30 seconds from the opening ofthe valve 57.

The valve 16 is opened and normal air pressure is let to the vacuum space, which at the same time speeds up absorption of the protective agent. When the pressure is leveled, the

end walls 5 and 6 and the ice removal door 7 are opened. The valves 23 and 27 are opened and the pump 22 is started to run with a high constant speed of rotation. The control center 32 drives the pump for about 15 minutes. The warm water-glycol solution is circulated in the cooling solution channels 51 of the condensing tank 12, whereby a thin film of water is formed between the ice and the damping material 52 and helps with removal of the ice from the condensing tank 12. The ice is conveyed to outdoor air for melting, and the lot of pine timber 3 is transferred out from the drying chamber 1.

Alternatively, the vaporization heat of water and melting heat of ice released during the timber drying process may be utilized for melting freezed timber or for preheating or predrying the timber. Then the melting, preheating and predrying may be carried out simultaneously with the drying process applying the principle of one drying chamber within another one, whereby the inner drying chamber is according to the invention, and the normal air pressure channels 62 and fans 19 thereof are utilized. The transfer of heat energy happens from the inner drying chamber by means of the outer pump 45, air condenser 46 and fan 47. The air condenser may be replaced by a liquid condenser.

The removal of the ice from the condensing tank 12 may, alternatively, be carried out as a continuous process. Then an ice removal device removes ice from the condensing tank to the normal air pressure in proportion to the formation of ice. The ice serves as a pressure-tight wall or plug and the water as a lubricant and sealing material between the ice and the wall of the condensing tank. In this case the condensing tank may be quite small.

In the following a calculatory eample is presented concerning the effectiveness required for the circulation of the gas mixture in the method and equipment according to the invention. Let us assume that the volume of the drying chamber is 150 cubic meters and a lot of 30 cubic meters of pine timber, having a water content of about 50 %, is to be dried therein. According to calculations with certain values, 260 000 cubic meters of gas mixture must be circulated to remove the required amount, 6000 kilograms, of water. If there are 16 fans in the equipment and the drying time is 10 hours, then the circulation per each fan must be about 1600 cubic meters per hour, i. e. about 2.7 cubic meters per minute. When the sectional area of the channels of Figure 4 (insulation channel and cooling channel together) is assumed to be 3.2 square meters (the distance of 10 cm between the walls), the calculations give a flow rate of about 2.5 m/s for the gas mixture in the channels.

Figures 5 and 6 present a very simple equipment accomplishing the method of the invention. An object 3, e. g. timber, to be dried is located in a first part of a ring-shaped tube. Nitrogen-water vapour mixture is caused to move rotatively along a main wall 71 of

a cooling channel 70 by means of a fan 73 and a spiral-shaped heat exchanger 80, which is located in a second part of the tube. During the cooling and drying phase (Figure 5) the first half 81 of the heat exchanger serves as a cooling surface and the second half 82 as a heating surface. During the freezing phase the heat exchanger 80 serves as a whole as a cooling surface, and during the heating phase as a whole as a heating surface. The centrifugal force helps to extract water droplets from the saturated water vapour. Under the heat exchanger part there is a condensing tank 12.

In the following the principles of the control of the drying process, and embodiments thereof in connection with the equipment described above, are considered more closely.

The control of the drying process is based on utilization of the condition of saturation, the drying range, and the vapour pressure range, and on monitoring the current frequency changes of the frequency converter controlled motors of the circulating fans. The drying range is the magnitude of the temperature difference, amount of degrees, which the gas mixture is during the cooling and drying phase first cooled and then, respectively, heated.

The vapour pressure range is the difference between the pressure of the gas mixture and the water vapour pressure in the material to be dried, which difference is temperature related. When the temperature of the gas mixture (total pressure) is higher than the temperature of the water film on the surface of the material to be dried (water vapour pressure), the vaporization of the water by boiling is stopped. When the temperatures (pressures) are equal, the boiling starts, and when the temperature of the gas mixture (total pressure) is lower than the temperature of the water film (water vapour pressure), the boiling continues until the pressures are leveled. The condition of saturation prevents the drying of the object, i. e. then no difference in moisture content between the surface and inner parts of the object may be produced. The larger is the drying range or the vapour pressure range the shorter is the drying time.

This kind of control is clear and simple and renders the drying process reliable, which contributes directly to the drying velocity, low level of drying defects, and savings of heating energy. The main measurement sensors of the drying process, the main temperature sensor 38 and the optical density measurement sensor 53, are located immediately after the object 3 to be dried in the direction of gas mixture flow. According to the measurement values of these sensors the frequency controlled fans 18 are adjusted steplessly during the cooling and drying phase, so that the drying process is kept in essentially saturated condition at the place of measurement. The stepless control and adjustment of the drying process result in that the water amount condensed from the saturated water vapour is essentially the same as the amount of water which may be

vaporized from the object in the drying chamber, i. e. the drying time is the shortest possible as the drying defects are taken into account.

During the cooling and drying phase the drying process is maintained in a saturated condition between the drying chamber 1 and the heating surfaces 35, and close to the saturated condition in the space after the heating surfaces and in the drying chamber. The larger the drying range is the larger is the distance to the saturated condition and the higher is the probability of drying defects. The use of the vacuum pump to rarefy air only in the beginning of the drying process makes the control of the drying process easier. Then the variations of the total pressure and temperature, which follow the use of the pump, are avoided, and as the amount of variables is reduced the control of the drying process is simplified.

The stepless control of the drying process means that no shut-offdevice, e. g. a pressure control valve, is needed between the drying chamber 1 and the cooling surface 60.

No sensors need to be attached to the object 3 to be dried for the purpose of controlling and adjusting the drying process, and it is not necessary to know the percentage of moisture or amount of water of the object when starting the process. In the beginning of the drying process the final percentage of moisture and the identification of the drying schedule, which is based on the kind of wood and the thickness to be dried, are input to the control center 32.

By means of experimental dryings the values for drying range and water vapour pressure range are determined and the drying schedules for different kinds of woods are prepared, by means of which the shortest possible drying time is achieved taking into account the thickness to be dried and the minimum amount of drying defects.

Additionally, special drying schedules are prepared so that by means of them the properties like colour, strength, stiffness, or dimensional stability in relation to moisture, appreciated by the final user, e. g. a carpenter, may be emphasized.

During the cooling and drying phase the cooling of the water vapour, the cooling efficiency of the cooling surfaces 60, is controlled by the temperature sensor 42, which is located between the cooling surface and the heating surface. According to the value measured by this temperature sensor the control center 32 controls the speed of rotation of the frequency converter controlled pump 29 for cooling solution, maintaining the temperature of the nitrogen-water vapour mixture at the desired value at the place of measurement. The value is the amount of the drying range between the main temperature sensor 38 and this so-called cooling temperature sensor 42. The heating efficiency ofthe heating surfaces 35 is controlled according to the value measured by the main temperature sensor 38, so that the control center 32 adjusts the speed of rotation of the frequency

converter controlled pump 22 for heating solution in such a way that the temperature of the gas mixture at the place of measurement is maintained at a certain value in accordance with the drying schedule.

During the heating phase and the freezing phase the drying process is controlled by the main temperature sensor 38 in accordance with the drying schedule, and the speed of rotation of the fans 18 is limited to a high value corresponding to a certain frequency.

When the water content of the timber is reduced, the duration of the cooling and drying phase become longer. The duration depends also on the kind of wood and thickness of the timber. According to the lengthening of the duration the control center determines approximately the percentage of moisture of the timber, and determines also the final percentage of moisture (equilibrium moisture content) by means of function point method, for example.

A further possibility to amend the method and equipment of the invention may be to direct ultrasound periodically towards the object to be dried during the drying process, especially when timber is dried. It is thought that ultrasound could help leveling the moisture in the object and removal of moisture therefrom. Besides that, it could release the stresses produced in the timber and affect advantageously the cell structure of the timber.

The invention is not limited to the above embodiments, but may be varied within the scope of the accompanying claims.