Freshly cut wood typically has a moisture content (by weight on a dry weight basis) of about 80%. If such wood were to be used immediately for the making of a timber product such as a piece of furniture, the wood would gradually dry out and in so doing would change shape and thereby distort the timber product. To avoid this problem, wood is dried to somewhere in the region of 20% to 8% moisture content for soft and hard woods respectively, before it is used for making a timber product.

Traditionally, wood has been seasoned by stacking the cut wood and leaving it to dry in the air for several vears. Alternatively wood can be dried in a few weeks usinε a kiln.

Both of the above methods of seasoning are unsatisfactory. Since the production of seasoned timber using air drying or kiln drying takes a considerable time, large stocks of wood must be held in order to ensure a constant supply of seasoned timber. The storage of large stocks of wood over a long period of time is expensive. Furthermore, both processes usually require periodic restacking of the timber and in addition, much of the wood is unusable after seasoning as a result of splits and severe distortion. Beech wood for example typically has a wastage rate of between 30% and 40% using either of the above processes.

It is an object of the present invention to provide an improved apparatus and process for drying timber.

According to the present invention, a timber drying apparatus comprises a housing defining a microwave cavity for containing a quantity of timber, a microwave generator coupled to the housing for introducing microwave energy into the cavity, temperature sensing means arranged to sense the temperature of timber within the cavity, and/or weight sensing means for sensing the weight of timber within the cavity, and control means for varying the power output ofthe microwave generator according to the sensed temperature

and/or weight.

The apparatus may also include ventilation means for moving air through the cavity to adjust the humidity and/or air temperature in the cavity. Preferably the ventilation means includes a humidity sensor at an air inlet and/or outlet ofthe cavity. The humidity sensed by the humidity sensor or sensors may be used as a control input for the ventilation means in order to determine a desired air flow through the cavity.

The apparatus preferably further includes a timber carriage for supporting timber within the cavity. Preferably the carriage is operable to impart an oscillatory motion to timber carried thereon and/or to rotate the timber within the cavity to improve the uniformity of microwave irradiation of the timber. The apparatus may instead or in addition include stirring means for varying the spacial distribution of the microwave energy within the cavity to improve the uniformity of irradiation ofthe timber. Furthermore, the timber may be slowly moved through the cavity to achieve the same effect.

The carriage preferably comprises a plurality of spiders having radially arranged arms emanating from a central hub. The timber to be dried may be mounted at the distal ends of the arms and the hub is preferably rotatably mounted to an axle. By mounting the timber to be dried between two such spiders, the timber may conveniently be rotated within the microwave cavity. One or more further spiders may be mounted along the length of the timber to reduce the possibility of long lengths of timber sagging during drying.

The cavity preferably has a closable opening for the insertion of and removal of timber for drying. The cavity may be arranged to have a plurality of openings and the apparatus may thereby be operable on a continuous basis by regularly introducing new batches of timber into the cavity using, for example, a conveyor belt.

The temperature sensing means preferably includes means for measuring the surface temperature ofthe wood. These preferably include at least one infra-red pyrometer which

are typically arranged to measure the temperature in the middle and one third of the way in from the ends of the wood. Since there is a predetermined correlation (which may be determined using intemal temperature probes in conjunction with surface measurements) between internal and surface temperature for each type of wood, a surface measurement is effective for measuring the internal temperature. The internal temperature itself is important since it is this which largely determines the intemal pressure ofthe timber which builds up as the moisture is heated in the wood pores. The temperature sensing means may instead or in addition include one or more fibre optic temperature sensors. These two types of sensors have been found to be especially advantageous in the present apparatus since they are non-metallic and produce little or no masking of the microwave radiation. Furthermore in the case of the pyrometer, the sensing apparatus may be mounted outside the cavity.

The sensed temperature may be used as a control input to the control means to enable a predetermined temperature/time curve to be maintained during drying. For example, some wood is best dried at a relatively constant temperature. The sensed temperature alone or in combination with the sensed weight can then be used to determine an appropriate microwave generator power output. The temperature/time curve is preferably kept below a predetermined maximum temperature to avoid distortion ofthe timber.

By accurate control ofthe output power ofthe microwave generator, many ofthe problems of uncontrolled microwave irradiation of timber are avoided. Without such control, the moisture in the wood may be removed too rapidly and cause the pore structure of the timber to rupture. This leads to general distortion of the timber in the form of splits, "honeycombing" in which large holes are formed in the internal structure ofthe wood and

"collapse" in which the honeycombing becomes so severe that it causes the wood structure to collapse.

Preferably the microwave energy is in the frequency range greater than 100 MHz and/or less than 300 Ghz. Typically the frequencies (which are government approved frequencies) are 434 MHZ, 896 MHZ, 915 MHZ, 2.45 GHz and 4.75 GHz. Longer

wavelengths provide better penetration ofthe wood structure and are preferred for large pieces of timber. Typically, a frequency of 896 MHZ permits timber up to a cumulative total of 200 mm thick to be dried. At a frequency of 2.45 GHz. wood of a cumulative thickness of at least 100 mm can be dried. Larger pieces also need to be dried more slowly since the moisture tends to escape only at the end of the wood and thus the pressure build¬ up in the centre ofthe wood is greater for larger pieces.

Preferably the control means is operable to derive a measure ofthe rate of change of the weight ofthe timber from the sensed weight thereby to derive a measurement ofthe rate of drying ofthe timber (the drying rate may be derived by assuming that weight loss is due to moisture evaporating from the timber). It has been found that different types of wood have a characteristic maximum drying rate above which significant distortion ofthe timber occurs. Therefore, the control means is preferably arranged to control the power output of the microwave source to ensure that the drying rate of the wood is kept below a predetermined maximum. The rate of weight change may also be used to determine when the wood is substantially dry. For example, the change in weight typically reduces to or very close to zero when the wood is sufficiently dry and the apparatus may be arranged to indicate that the wood is sufficiently dry when the change in weight over a predetermined time is zero or less than a predetermined maximum or the total weight loss during drying is the same or more than a predetermined minimum weight loss.

Preferably the microwave generator is operable to produce several different power outputs. Altematively, the microwave generator may produce a fixed power output and be switched on and off intermittently.

According to a method aspect of the invention, a method of drying timber using microwave energy comprises placing the timber into a microwave cavity, repeatedly sensing the weight and/or temperature ofthe timber and controlling the power output of a microwave generator coupled to the cavity, according to the sensed weight and/or temperature.

The invention will now be described by way of example with reference to the drawings in which

Figure 1 is a side elevation of a timber drying apparatus in accordance with the invention:

Figure 2 is a cross-section along line A-A of Figure 1 ; and

Figure 3 is a plot of temperature and weight against time for an exemplary quantity of timber being dried in the apparatus of Figure 1.

With reference to Figure 1. timber drying apparatus comprises a microwave generator for generating energy in the 896 to 915 MHZ bandwidth. This is coupled to a multi-mode cavity by wave guides. The energy is coupled into the cavity by mode stirrers which operate in a conventional manner.

The cavity has an air inlet and an air outlet. The humidity of the air passing through the inlet and outlet is sensed by humidity sensors on the inlet and outlet respectively. To force the air through the cavity, a fan is mounted in the air inlet.

Infra-red sensors are mounted in the walls ofthe cavities. In the embodiment shown, four sensors are mounted in the side wall. However, the location ofthe sensors may be varied and should be chosen to give consistent and accurate measurement of the surface temperature ofthe timber within the cavity.

Referring also to Figure 2, the side wall of the cavity has an openable door to permit a timber carriage to be inserted and removed from the cavity. The carriage comprises four support arms emanating radially from a central hub. The timber is supported on the arms and is rotated about the central hub by a motor. The hubs are supported on load cells from which the weight ofthe wood and carriage can be sensed, thereby permitting the change in weight of the wood to be monitored during drying. In the embodiment shown, the carriage has in addition to the arms at each end ofthe cavity, a set of arms in the centre of

the cavity to support the timber along its length dunng drying The timber mav instead or in addition to being rotated, be "shuffled" in an oscillatory motion by moving the carriage back and forth in a generally horizontal plane and/or in a generally vertical plane In addition or instead, the timber may be subjected to rotational oscillation bv rotating the timber, preferably about the central hub. alternately in one direction ol rotation and then in the other direction of rotation This helps to achieve a uniform irradiation ofthe timber The microwave energy is distπbuted within the cavity using devices such as mode stirrers. phase change devices and power adjusting devices

The weight sensing means may be used in addition to the humidity sensors as a control input to control means for varying the air flow through the cavity

Figure 3 shows the weight and temperature ot a block of ash approximately 205mm \ 320mm x 30mm The upper plot shows the weight reduction in the block as the moisture is removed In this particular experiment, the temperature peak at 13 minutes corresponded to a popping sound being produced by the wood The sound would have been caused by rupturing of the structure of the block This peak is undesirable and the power should preferably be controlled to achieve a temperature curve similar to that of "Temp 5" on Figure 3. This peak also corresponds to an increased drying rate (as shown by the steeper weight curve between 10 and 15 minutes) By producing such a plot for a particular type of wood, it is possible to determine a safe maximum drying rate which can then be entered into the control means to ensure that the power output of the microwave generator is controlled appropπately for subsequent pieces or blocks of timber

It will be noted that all the temperature curves nse steeply as the wood becomes dry This steep πse may be used to determine when the wood is dry