GB2178149A - Method and apparatus for drying timber - Google Patents

Abstract

A method of seasoning timber involves exposing timber to a vacuum whilst at or at less than substantially ambient temperature, the residual pressure of the vacuum preferably being less than or equal to the saturation vapour pressure of water at the ambient temperature. Water evaporates/boils off from the timber without the need for the input of large quantities of heat, although, to prevent excessive cooling of the timber and slowing of the rate of seasoning, an apparatus for carrying out the method may be provided with a heat reservoir/exchanger (27) for recovering thermal energy from gases extracted from a process vessel (21) containing the timber by condensing the water vapour in the exhaust gases. Then, if exposure to the vacuum is temporarily interrupted by allowing atmospheric air to enter the process vessel (21), the air may first be heated by being passed through the heat reservoir/exchanger (27) before contacting the timber and returning the timber to substantially ambient temperature. <IMAGE>

Description

SPECIFICATION Method and apparatus for drying timber Green, i.e. wet, wood is normally seasoned, i.e. dried, in order to make it lighter, stronger, better able to retain nails, less prone to insect and fungal attack, and easier to machine, paint and glue. More importantly, the removal of the water enables the timber to accept preservatives, and, if just sufficient water is removed for the timber to be in equilibrium with its eventual surroundings, it will not, in use, warp. The timber processing industry mainly uses two methods of seasoning timber: airdrying and kiln-drying.

For air-drying, the timber is placed in stacks and the ambient temperature is used to evaporate water within the wood prior to the evaporated water being carried away by air currents moving past the timber. This method suffers from the disadvantage of having a drying rate which is dependent upon environmental factors such as the relative humidity, temperature and wind movement, and, even under ideal circumstances, timber will take several months to dry to the correct moisture content (i.e. the percentage of the weight of water contained in the timber to the dry timber weight). As a result of the long seasoning time, a large quantity of timber will be seasoning at any one time, thereby tying up a large amount of capital, and a large area of land will be needed on which to conduct the seasoning operation, thereby tying up a further large amount of capital.In summer in the U.K., timber can only be dried to a moisture content of about 15%, and this is insufficient for timber which is going to be used in heated buildings where a moisture content of 9-12% is to be expected. For this reason, and in order to overcome the unpredictable variability of the drying rate, timber is frequently seasoned in kilns instead of being seasoned by air-drying. In kiln-drying, the timber is placed in a chamber inside which the temperature, humidity and air flow are controllable. Whilst this method does require the provision of a kiln and the consumption of a considerable amount of fuel, it does have the advantage over air-drying of allowing the timber to be seasoned to any desired moisture content in a shorter time.

Kiln-drying, therefore, reduces the uncertainty inherent in the seasoning process and produces a uniformly better quality product than that achievable by air-drying. The only reason that air-drying is still used is that kilndrying, even with all of its advantages of a better quality product and a shorter seasoning time, requires the use of large quantities of fuel to produce the conditions of elevated temperature needed to cause the water to evaporate from inside the wood. We have now appreciated that using hot air to season timber is not the only way to effect seasoning in a controlled manner.

The idea of subjecting timber to a vacuum in order to remove water is known. For example, in the so-called Boulton process, timber is impregnated with creasote by being immersed in a bath of heated creasote which is maintained at a temperature between 85 and 95"C and subjected to a vacuum in order to lower the boiling point of the water in the wood so that the water may evaporate more quickly and the space then left within the wood filled with creasote. It is also known to subject timber, whilst being immersed in a bath of water-borne preservative, to pulses of reduced pressure followed by pulses of elevated pressure in order that, when the pressure is reduced, air and water are removed from the wood to create a space which, when the pressure is elevated, is filled with the water-borne preservative.However, in this impregnation method the residual pressure of the vacuum (i.e. the pressure of the remaining gas relative to a perfect vacuum) is in the order of 125mm of mercury.

In accordance with the present invention a method of seasoning timber comprises the step of exposing the timber to a vacuum whilst at or at less than substantially ambient temperature.

This method of seasoning timber does not require the application of heat, other than perhaps a small quantity to compensate for any reduction in temperature, due to loss of latent heat of vaporization, which lowers the vapour pressure of the remaining water and requires an ever higher vacuum, and consequently it does not incur the large fuel costs associated with kiln-drying. This saving is achieved without losing the benefits associated with kilndrying since the seasoning still occurs within a controlled environment, thereby enabling the timber to be seasoned at a predetermined rate and to a predetermined moisture content.

The compensatory small quantity of heat may be supplied to the timber by interrupting its exposure to the vacuum by exposing it to atmospheric air having the ambient temperature and pressure. Once sufficient heat has flowed from the atmospheric air to the timber, the timber is re-exposed to the vacuum. This compensatory cycle may be carried out as often as is necessary during the seasoning process.

A pump used to establish the vacuum will normally also be used during the seasoning operation itself to remove water vapour from around the timber and thereby encourage more water to evaporate/boil off. Otherwise, a static equilibrium is reached once a certain amount of water has evaporated/boiled out of the timber. The output gases from the pump will usually be hotter than the ambient temperature and saturated with water vapour. If these gases are cooled in a heat reservoir/ex changer, some, but preferably all, of the water vapour can be condensed and the thermal energy liberated upon its condensation may be stored, i.e. when the heat reservoir/exchanger acts in its heat reservoir mode.Some of this stored energy will pass, by conduction (assuming the heat reservoir/exchanger is in thermal contact with a process vessel in which the timber is being seasoned), back to the process vessel and then, primarily by radiation, from the walls of the process vessel back to the timber.

However, most of the energy liberated upon condensation of the water vapour will remain stored in the heat reservoir/exchanger. This will occur as the timber is being exposed to the vacuum. Then, assuming the timber is being treated by being subjected to one or more cycles of (after exposure to the vacuum for a period of time) exposure to air at atmospheric pressure and then re-exposure to the vacuum, the air at atmospheric pressure may be passed through the heat reservoir/exchanger and heated, i.e. when the heat reservoir/exchanger acts in its heat exchanger mode, before being allowed to bathe the timber. Thus, the compensatory heat may be provided by thermal energy originally extracted from the timber, i.e. the majority of the thermal energy removed from the timber as water evaporated/boiled off during previous exposures to the vacuum is recycled back to the timber.

The switch from exposure to the vacuum to exposure to atmospheric air may conveniently be triggered by the temperature of the heat exchanger/reservoir rising above a given threshold (thereby making it possible to prevent thermal damage to the equipment), and the switch back to exposure to the vacuum may be triggered by the temperature dropping below the same or a lower threshold.

The residual pressure of the vacuum is preferably lower than or equal to the vapour pressure associated with the water within the timber in order to allow this water to boil off into the vacuum. In other words, the residual pressure of the vacuum should preferably be less than or equal to the saturation vapour pressure of water at the temperature of the timber, which will normally be equal to or less than the saturation vapour pressure of water at the ambient temperature, since the timber will usually be at or at less than substantially ambient temperature. Therefore, an operator could work on the assumption that the residual pressure would need to be less than or equal to the saturation vapour pressure of water at the ambient temperature.For example, if the ambient temperature is 10 C, the residual pressure would have to be lower than or equal to 9.2mm of mercury. Other values are 4.5mm, 17.5mm, 31.8mm and 55.3mm of mercury at 0 C, 20"C, 30 C and 40"C, respectively. Thus, at 20"C, the residual pressure would only have to be lower than or equal to 17.5mm of mercury. In general, in the U.K., it is believed that a vacuum of lower than 30mm, preferably lower than 10mm and possibly even below 5mm, of mercury is desirable.As 40"C is the maximum ambient temperature that could reasonably be expected anywhere in the world, the residual pressure could be expected to be always less than 55.3mm of mercury (unless the seasoning apparatus is in a hot part of a factory, for example).

As the residual pressure is reduced to the saturation vapour pressure of the water, the water will evaporate at an ever increasing rate until it eventually is boiling out of the timber.

If the pump is capable of removing water vapour at a rate faster than it can boil off out of the timber, the residual pressure of the vacuum may be reduced to below the saturation vapour pressure, thereby achieving even faster seasoning. However, boiling off the water too vigorously may lead to there being a moisture content gradient from the surface to the centre of the timber which is steep enough to cause warping after subsequent machining, i.e.

the problem known as case-hardening. For this reason, the timber may be allowed to achieve a more uniform moisture content by being exposed to a vacuum from which water vapour is not being removed (i.e. if a pump is being used, the pump is turned off and the timber is left to "soak" in the vacuum).

Experiments have revealed that this method, even when applied for only a first period of 8 hours followed by a second period of 8 hours to samples of very wet softwood post (i.e.

the moisture content was initially greater than 30%, which corresponds to full scale deflection on an electrical resistance moisture meter), enables the moisture content to be reduced to an average value of 24%. It is envisaged that seasoning to a moisture content of 9-12% will usually take up to 48 hours, when a residual pressure of, for example, 5mm of mercury is being used. It might take even longer if the factors (such as timber species, timber dimensions, initial moisture content and ambient temperature) affecting the seasoning time are particularly unfavourable.

The invention will now be desribed by way of example with reference to the accompanying drawings, in which: Figure 1 is a section through a vacuum chamber and side view of the associated ancillary equipment of an experimental apparatus for carrying out the method in accordance with the present invention; and, Figure 2 is a diagrammatic side view of a production plant for carrying out the method in accordance with the present invention.

Referring to Figure 1, a pressure chamber 1 comprises a bowl shaped lower portion 2 and a lid 3 mounted on a seal 4 to enable the pressure chamber 1 to be made air-tight. The lid 3 has viewing ports 5 for allowing visual monitoring of the condition of the timber 5 positioned within the pressure chamber 1. A pump 7 is used, via an orifice 8 in a side wail of the lower part 2, to reduce and maintain the pressure within the pressure chamber 1 at a value lower than or equal to the saturation vapour pressure of the water in the timber 5 at the ambient temperature. A gauge 9 is used to monitor the residual pressure within the pressure chamber 1 and a vent valve 10 is provided to allow pressure equalization on completion of the seasoning process and to allow entry of air for the previously described compensatory cycles.

The seasoning method in accordance with the present invention is carried out by inserting the timber 5 in the lower part 2, placing the lid 3 on top of the lower part 2 to seal the pressure chamber 1 against unwanted entry of air. A motor 11 is used to power the pump 7 which evacuates the pressure chamber 1 by extracting the air through the orifice 8. Once the correct residual pressure has been achieved, this being determined by viewing the pressure gauge 9, the pump 7 is used to maintain this pressure. One or more times during the seasoning of the timber 5, the pump 7 may be turned off and the vent valve 10 opened to allow ambient air to enter the chamber 1 to transfer heat to the timber 5.

The vent valve 10 is then closed and the pump 7 restarted to reestablish the residual pressure. Then, when the timber 5 has been dried to substantially the correct moisture content, the pump 7 is turned off and a valve 12 shut to prevent air or lubricating oil from the pump 7 from entering the chamber 1 via the orifice 8. The timber 5 then proceeds towards reaching an equilibrium with the water vapour remaining in the chamber 1, and this results in it obtaining a more uniform moisture content distribution. Finally, once the desired amount of seasoning has been effected, the vacuum is neutralized by allowing entry of atmospheric air through the relief valve 10 and then the lid 3 is removed and the seasoned timber 5 is available for further processing or for sale.

This experimental apparatus is only capable of seasoning small quantities of timber and, for this reason, a production plant has one or more separate vacuum chambers, each having door(s) which open to allow the chamber to be charged with a large quantity of timber prior to being shut to seal the chamber and each being of a size compatible with the desired overall throughput of timber per unit time. Thus, if the seasoning operation takes 48 hours and 75m3 of seasoned timber is required in that period, one chamber with a capacity of 75m3 or three chambers each with a capacity of 25m3 could be used.

Such a plant, but showing only a single vacuum chamber for the sake of clarity, is shown in Figure 2. The vacuum chamber of the plant is a cylindrical process vessel 21, which may be of the type commonly used in timber impregnation processes. The cylindrical shape provides the process vessel with a large surface area to volume ratio, thereby facilitating heat flow into the vessel. The process vessel 21 has a door 22 at one end, through which timber-conveniently stacked on trolleys arranged to move on rails (not shown)-may be inserted into and removed from the process vessel. The door 22 closes against a seal to ensure that the process vessel is air-tight.A pump 23 (rated down to 0.003mm Hg), driven by a motor 24, extracts gases from the process vessel through a duct 25 and discharges those gases to the atmosphere via an oil trap 25, a heat reservoir/exchanger 27 and an exhaust pipe 28. The exhaust gases are passed through the oil trap 25 in order to remove any lubricating oil which has escaped from the pump. This recovered oil may be drained through a drain cock 29 and then reused.

The heat reservoir/exchanger 27 contains a number of parallel connected tubes 30 which enable atmospheric air to flow through the heat reservoir/exchanger 27 whilst in thermal contact with, but physically separate from, the exhaust gases passing through the heat reservoir/exchanger 27 from the oil trap 25 to the exhaust pipe 28. The atmospheric air enters the heat reservoir/exchanger 27 through an inlet 31, passes through the tubes 30 whilst in thermal contact with the exhaust gases, along an inlet pipe 32 and, if an air valve 33 is open, through an inlet 34 and into the process vessel 21.

In operation, the door 22 of the process vessel 21 is opened and a load of green timber (e.g. a 35m3 load if the process vessel 21 has a length of 25m and diameter of 2m) is loaded into the process vessel, possibly in stacks on trolleys running on rails. The door 22 is then shut. The plant includes a control unit (not shown) which receives inputs from a temperature sensor 35 mounted on the exhaust pipe 28 adjacent to the heat reservoir/exchanger 27 or in the heat reservoir/exchanger 27 itself and from one or more electrical resistance moisture meters which have been inserted into the timber now in the process vessel (a number of moisture meters being desirable in order to obtain, by averaging, a more reliable value), and the control unit is able to open and ciose the air valve 33 and switch on and off the motor 24.

Thus, when it is desired to commence the seasoning operation, the control unit closes the air valve 33 and energizes the pump motor 24 to cause the pump 23 to start to reduce the pressure inside the process vessel down to the appropriate residual pressure, e.g. the saturation vapour pressure of water at the temperature of the timber. This temperature can be measured, and hence the corre sponding residual pressure of the vacuum determined, by having temperature sensor(s) mounted inside the process vessel 21, the temperature sensor(s) feeding into the control unit, which determines the desired value of residual pressure.

As the residual pressure inside the process vessel 21 drops down towards the desired residual pressure, an equilibrium is reached when the actual residual pressure is equal to the saturation vapour pressure of the water in the timber, with water boiling out of the tim ber at the same rate as water vapour is extracted from the process vessel 21 by the pump 23. As water boils out of the timber, the latent heat removed from the timber by that water results in the timber cooling, the saturation vapour pressure of the water remaining in the timber dropping and the pump 23 being able to lower the residual pressure until a new equilibrium is established. After a while, the rate of heat flow into the process vessel 21 is equal to the rate of loss of latent heat from the timber, and a final equilibrium, and hence a final residual pressure, are reached.Even when the final equilibrium has been reached, the pump 23 is kept running in order to remove water vapour and thereby allow more water to boil out of the wood. The actual residual pressure can be sensed and used as an additional input to the control unit.

The exhaust gases from the vacuum pump 23 pass through the oil trap 26, where they deposit any lubricating oil, and then into the heat reservoir/exchanger 27 in which the water vapour present in the exhaust gases is encouraged to condense. As it condenses, it liberates its latent heat and this thermal energy is transferred to the heat reservoir/exchanger 27 (which is made of a material having a relatively large specific heat capacity and of such a bulk as to have a large overall heat capacity). The condensed water is drained off through a drain cock 35 and the remaining exhaust gases pass out to the atmosphere through the exhaust pipe 28.

The length of the duct 25 to the pump should be made as short as possible in order to enable the pump 23 to scavenge gases from inside the process vessel 21 as efficiently as possible at the low residual pressures which are achieved. The duct running into the pump will be generally cool, but anything in the exhaust gas flow path downstream of that point (e.g. the pump 23 and the heat reservoir/exchanger 27) will generally be hotter than ambient temperature. In order to allow heat from these hot objects to flow back to the process vessel 21 and then to the timber, even though admittedly during the vacuum part of the seasoning process relatively little heat can be transferred by conduction or convection back to the timber, these parts of the plant are placed adjacent to, so as to be in thermal contact with, the process vessel 21.

A thermally insulative housing 37, preferably lined on its inside with a reflective surface so as to reflect radiation, is positioned over these hot parts in order to encourage heat, other than that flowing by conduction, to flow to the process vessel 21. Heat flow back to the timber, especially during the vacuum part of the seasoning process, is further assisted by painting the inside and outside of the process vessel with matt black paint.

The duct 25 is kept outside the housing 37 because it is a comparatively cold part. The size of the surface area of the process vessel 21 covered by the insulative housing 37 should be kept as small as possible in order to maximise the size of the thermal path between the process vessel 21 and its surroundings.

In order to aid heat conduction from the heat reservoir/exchanger 27 to the process vessel 21, the former may be positioned, as may other components, under the latter.

It may prove necessary to lag the parts of the exhaust gas flow path between the pump 23 and the heat reservoir/exchanger 27 to ensure that water vapour does not condense before it reaches the heat reservoir/exchanger 27, condensation in the oil trap 26, where condensed water would mix with condensed lubricating oil, being something to be avoided.

The heat reservoir/exchanger 27 is arranged to ensure that as much of the water vapour as possible condenses out of the exhaust gases before they are discharged by the exhaust pipe 28 into the atmosphere.

Once the timber has been exposed to the vacuum for a period of time (e.g. 1 hour), the temperature detected by the temperature sensor 35 rises above a predetermined threshold value and the control unit then opens the air valve 33, thereby causing atmospheric air to enter the inlet 31, pass through the tubes 30, inlet pipe 32 and inlet 34 and into the process vessel 21 to neutralize the vacuum. As the atmospheric air passes through the tubes 30 in the heat reservoir/exchanger 27, the air will be heated, i.e. receive some of the thermal energy from the heat reservoir/exchanger.

When the heated air passes into the process vessel 21 and contacts the timber, the timber receives back by conduction from the heated air some of the thermal energy previously lost when water evaporated/boiled off during the earlier vacuum phase of the seasoning operation. Also, now that there are sufficient gas molecules in the process vessel 21, heat may pass by convection from the walls of the vessel to the timber. During the exposure of the timber to atmospheric air, the pump 23 is left running in order to draw air through the process vessel 21. This ensures that as much as possible of the thermal energy stored in the heat reservoir/exchanger 27 is returned to the timber.

As this plant has more than one process vessel (vacuum chamber), the heat reservoir/exchanger of one vessel may be used to heat the atmospheric air being passed through another vessel if the two process vessels are operated out of phase, i.e. the one is exposing its timber to a vacuum whilst the other is exposing its timber to atmospheric air, then vice versa.

After a predetermined period of time (e.g. 1 hour), or once the temperature detected by the temperature sensor 35 drops below a second threshold value, the control unit closes the air valve 33 so as to allow the pump 23 to re-establish the vacuum in the process vessel and thereby re-expose the timber to the vacuum. The control unit allows this cycle of exposing the timber to the vacuum, then to atmospheric pressure and then again to the vacuum to continue for as long as is necessary to achieve the desired moisture content in the timber. The electrical moisture resistance meters embedded in the timber allow the control unit to monitor the moisture content and, once the desired moisture content has been reached, to terminate the seasoning operation by de-energizing the pump motor 24 to allow the timber to "soak" in the vacuum and then opening the air valve 33 to neutralize the vacuum.

In order to prevent, during the parts of the seasoning operation in which the timber is exposed to atmospheric air, the atmospheric air sucked in through the inlet 31 from being so humid as to result in the timber being covered in condensed water when the atmospheric air contacts the timber, the atmospheric air may have its relative humidity decreased before being allowed to enter the heat reservoir/exchanger 27. For example, the air could be dried chemically or chilled to below its dew point (thereby allowing water to condense out and be drained away) before being allowed to return to atmospheric temperature.

With the above type of production plant, it should be possible to season wood from a 100% moisture content, i.e. green, state to a 25% moisture content in a period of 48 hours whilst using during the vacuum parts of the seasoning process a residual pressure of about 5mm of mercury.

By recycling thermal energy from the timber to the heat reservoir/exchanger 27 and then back to the timber, it is possible to prevent the temperature of the timber from always being so low as to severely reduce the rate of seasoning and/or require the constant use of very low residual pressures for the vacuum.

However, if it is desired to enable the atmospheric air sucked into the process vessel 21 to carry a still larger quantity of compensatory thermal energy to the timber, a heat pump may be used to pump heat from the surroundings and/or the exhaust pipe 28 to raise still further the temperature of the air entering the pressure vessel.

Claims (13)

1. A method of seasoning timber comprising the step of exposing the timber to a vacuum whilst at or at less than substantially ambient temperature.

2. A method according to claim 1, in which the residual pressure of the vacuum is less than or equal to the saturation vapour pressure of water at substantially ambient temperature.

3. A method according to claim 1, in which the residual pressure of the vacuum is less than or equal to the saturation vapour pressure of the water within the timber.

4. A method according to claim 1, in which the residual pressure of the vacuum is less than 55.3mm of mercury.

5. A method according to claim 4, in which the residual pressure of the vacuum is less than 30mm of mercury.

6. A method according to claim 5, in which the residual pressure of the vacuum is less than lOmm of mercury.

7. A method according claim 5, in which the residual pressure of the vacuum is less than 5mm of mercury.

8. A method according to any of the preceding claims, further comprising the steps of exposing the timber to air at ambient pressure and at least ambient temperature and then reexposing the timber to the vacuum.

9. A method according to claim 8, in which the air at ambient pressure is above ambient temperature, having been heated by thermal energy liberated from gases removed to create and maintain the vacuum used in the preceding step of the method.

10. A method according to claim 9, in which the thermal energy liberated from the gases results mainly from the latent heat released upon the condensation of water vapour present in the gases.

11. A method of seasoning timber substantially as described with reference to the accompanying drawings.

12. Timber seasoned by a method in accordance with any of the preceding claims.

13. Apparatus for carrying out a method in accordance with any of claims 1 to 11.