The invention also relates to a system for performing the method as defined in the introductory portion of claim 3.

When wooden objects are to be treated, these are stacked in twos with the right side toward each other, separated by one or more spacers in the form of so-called joists. The finished stack is placed on a carriage or the like with a device for turning the entire stack (or pack) so that the wooden objects face upwards with the narrow long side. This ensures that the oil to be ab- sorbed in the wooden objects does not accumulate as a small puddle on the top side of the wood. This results in an even absorption of the oil and a nice application of colour.

The wooden objects are introduced into an autoclave which is surrounded by a storage tank. When the autoclave is closed, it is evacuated together with the storage tank to a suitable negative pressure or vacuum. The ad- vantage of evacuating both the storage tank and the autoclave is that filling may then take place without any form of pressure change in the wood, and thereby no risk that the wood absorbs oil during this phase.

If a small absorption of oil is desired, the prevacuum is omitted so that the air in the cells of the wood will only be drawn out during the drying phase.

Filling of the autoclave takes place in that the oil is pumped from the stor- age tank to the autoclave by a pump. This takes place without any change

in pressure, since the autoclave and the storage tank are openly connected with a plurality of pipes at the top of the autoclave.

When the liquid level has reached above the pack of wood, the filling is stopped so as to form at the top of the autoclave an air gap between the oil and the top of the autoclave.

When the heating is terminated, a vacuum is created in the autoclave and the storage tank to reduce the boiling point of the water. To load the wood as little as possible, this vacuum is preferably created in three steps.

Pressure Temperature Boiling point Equilibrium moisture Step 1 0.30 bar 75 °C 69 °C 1.10% Step 2 0.20 bar 75 °C 60 °C 6.50% Step 3 0.15 bar 75 °C 54 °C 4.50% It is important that the pressure always increases during this part of the process since, otherwise, the wood will begin to absorb oil.

In the drying method, the wood is dried to a predetermined temperature, and then the pressure in the autoclave is reduced so as to bring the boiling point of the water in the wood at any time below the temperature in the pre- heated wood. This ensures that the water in the wood passes into vapour form and creates a positive pressure in the cells of the wood which acceler- ates the drying rate from taking days to taking hours. The reason is the great transfer of heat from the oil to the wood instead of the traditional heat transfer from air to wood.

When the vapour leaves the wood, it bubbles up through the oil and passes up through the pipes that lead out to the storage tank, where the entrained oil is separated and the vapour is sucked out to a cooling device, e. g. a

plate cooler, where the vapour condenses.

The condensed water is pumped from a measuring tank to a condensate tank by a pump, with measuring of the amount of water pumped out. This amount pumped out is used for determining when the drying process is to be stopped.

If a great absorption of oil in the wood is desired, the autoclave may be pressure-equalized to a desired pressure according to how large an amount of oil is to be absorbed in the wood. In the pressure equalization, the oil will penetrate into the wood and replace part of the water which has been drawn out of the wood in the drying process.

Further, the same pressure may be used during the emptying of the auto- clave as is used at the end of the drying process, which will give a minimum absorption of oil in the wood.

The oil is returned from the autoclave to the surrounding storage tank in such a manner that the pump starts and stops at intervals so that a specific temperature is maintained in the storage tank.

After the oil has been emptied from the autoclave, a vacuum is established so that any excess oil is pressed out of the wood. The time that elapses while this vacuum is being established and optionally maintained, gives the oil time to drip off the wood.

When this aftervacuum has been allowed to act, the pressure is equalized to atmospheric pressure so that the door may be opened, but at the same time the oil present on/in the surface of the wood is pressed into the cells of the wood, which means that the wood leaves the system drip-dry.

Before the system may be opened, the oil which has come out during the aftervacuum phase is returned to the storage tank by a pump.

After treatment, the wood is left to dry (hardening of the oil) in a form of covered, preferably heated construction for a period of time, e. g. 3-5 days, before it is delivered to the customer.

The advantage of using this process and this system is that the great cir- culation and heating with a heat exchanger instead of using heating pipes at the bottom of the autoclave, give a considerably better heat transfer to the wood, which in turn gives the high drying rate relative to known sys- tems.

By having the same pressure in both the storage tank and the autoclave it is ensured that it is possible to control the absorption of oil during the proc- ess, in that filling and emptying may be carried out independently of the pressure in the autoclave.

By withdrawing the vapours via the storage tank total separation of oil and vapour is ensured, which means that the condensate from the process may readily be used for new mixing of impregnation liquid.

The arrangement of the storage tank around the autoclave gives a good heat transfer from the storage tank to the autoclave and thus a good ther- mal economy as well as a minimum of heat loss during filling. Also achieved is a great drying effect of the surface of the wood during the after- vacuum phase, as the temperature in the autoclave is kept at a relatively high level for drying.

Further, the separation of oil from vapour is facilitated by mounting a plural- ity of pipes to the top of the storage tank, said pipes preferably extending

from the autoclave and in an upward direction.

In case of impregnated wood, the oil treatment substantially only gives col- our to the wood, and in case of non-impregnated wood a protection treat- ment of the wood is ensured as well as colouring of the surface.

Optionally, treatment with oil without noticeable colour may be carried out.

In the following, reference will be made to the drawing, in which fig. 1 shows a schematic representation of the system with an auto- clave filled with wooden objects, a storage tank arranged around the autoclave, said storage tank being filled with oil, and a condensation container for receiving separated conden- sate, fig. 2 shows the system of fig. 1, where the oil has been conveyed into the autoclave and vapour is conveyed into the condensate container, fig. 3 shows the system of fig. 1, where the oil has been pumped back into the storage tank and residual vapour is conveyed to the condensate container, fig. 4 shows a system seen from the side, fig. 5 shows a system seen from the front, fig. 6 shows an apparatus for stacking and turning wooden objects for introduction into the system, with the apparatus seen in the direction of introduction,

fig. 7 shows the apparatus of fig. 8 with the wooden objects before they are turned for introduction into the system, and fig. 8 shows the introduction apparatus with objects placed in the autoclave.

The method according to the invention and the system for working the in- vention will now be described in preferred embodiments.

When wooden objects 1 are to be treated, these are stacked in twos with the right face facing toward each other, separated by one or more spacers in the form of so-called joists. The finished stack 2 is placed on a carriage 3 or the like by a device 4 for turning the entire stack 2 so that the wooden objects 1 face upwards with their narrow long side. This ensures that oil 5 to be absorbed in the wooden objects 1 does not accumulate as a small puddle on the top side of the wood 1. This results in an even absorption of the oil 5 and a nice application of colour.

The wooden objects 1 are introduced into an autoclave 6, said autoclave 6 being surrounded by a storage tank 7. When the autoclave 6 is closed, it is evacuated together with the storage tank 7 to a suitable negative pressure or vacuum. The advantage of evacuating both the storage tank 7 and the autoclave 6 is that filling may then take place without any form of pressure change in the wood 1, and thereby no risk that the wood 1 absorbs oil 5 during this phase.

If a small absorption of oil is desired, the prevacuum is omitted so that the air in the cells of the wood 1 will only be drawn out during a drying phase.

Filling of the autoclave 6 takes place in that the oil 5 is pumped from the storage tank 7 to the autoclave 6 by a pump. This takes place without any

change in pressure, since the autoclave 6 and the storage tank 7 are openly connected with one or more pipes 8 at the top of the autoclave 6.

When the liquid level has reached above the stack of wood 1, the filling is stopped so as to form at the top of the autoclave 6 an air gap 9 between the oil 5 and the top of the autoclave.

When the heating is terminated, a vacuum is created in the autoclave 6 and the storage tank 7 to reduce the boiling point of the water which is naturally contained in the wood 1. To load the wood 1 as little as possible, this vac- uum is preferably created in three steps.

Pressure Temperature Boiling point Equilibrium moisture Step 1 0.30 bar 75 °C 69 °C 1.10% Step 2 0.20 bar 75 °C 60 °C 6.50% Step 3 0.15 bar 75 °C 54 °C 4.50% It is important that the pressure always increases during this part of the process since, otherwise, the wood 1 will begin to absorb oil 5.

In the drying method, the wood 1 is heated to a predetermined temperature, and then the pressure in the autoclave 6 is reduced so as to bring the boil- ing point of the water in the wood 1 at any time below the temperature of the preheated wood 1. This ensures that the water in the wood 1 passes into vapour form and creates a positive pressure in the cells of the wood which accelerates the drying rate from taking days to taking hours. The reason is the great transfer of heat from the oil 5 to the wood 1 instead of the traditional heat transfer from air to wood.

When the vapour leaves the wood 1, it bubbles up through the oil 5 and passes up through the pipes 8 that lead out to the storage tank 7, where the

entrained oil 5 is separated and vapour is sucked out to a cooling device (not shown), e. g. a plate cooler, where the vapour condenses.

The condensed water is pumped e. g. from a measuring tank to a conden- sate tank 10 by a pump, with measuring of the amount of water pumped out. This amount pumped out is used for determining when the drying proc- ess is to be stopped.

If a great absorption of oil 5 in the wood 1 is desired, the autoclave 6 may be pressure-equalized to a desired pressure according to how large an amount of oil 5 is to be absorbed in the wood 1. In the pressure equaliza- tion the oil 5 will penetrate into the wood 1 and replace part of the water which has been drawn out of the wood 1 in the drying process.

Oil 5 is returned from the autoclave 6 to the surrounding storage tank 7 in such a manner that the pump starts and stops at intervals, so that a specific temperature is maintained in the storage tank 7.

After the oil 5 has been emptied from the autoclave 6, a vacuum is estab- lished so that any excess oil 5 is pressed out of the wood 1. The time that elapses while this vacuum is being established and optionally maintained, gives the oil 5 time to drip off the wood 1.

When this aftervacuum has been allowed to act, the pressure is equalized to atmospheric pressure so that the door 11 of the autoclave 6 may be opened, but at the same time the oil 5 present on/in the surface of the wood is pressed into the cells of the wood, which means that the wood 1 leaves the system drip-dry.

Before the system may be opened, the oil 5 which has come out during the aftervacuum phase is returned to the storage tank 7 e. g. by a pump. After treatment the wood 1 is left to dry (hardening of the oil) in a form of covered, preferably heated construction for a period of time, e. g. 3-5 days, before it is ready for delivery.