Technical area

This invention concerns a method for determining the moisture content of wood that is being dried, an arrangement that makes possible the determination of the moisture content in wood that is being dried, a drying chamber and a drying plant.

Background to the invention

When drying various materials, it is of interest to measure and control the moisture content of the material such that the material does not dry too rapidly and become damaged. This is particularly true when drying wood. The detection of the moisture content of the material, the wood, with the aid of various arrangements that are attached to the material that is to be dried, to one or more planks, is previously known. The detection can take place in various ways depending on what the wood is, what condition it is in, and the duration of the drying process.

One problem with this is that the arrangement must be in contact with the wood, and that a limited part of the wood is in this way measured. Several arrangements must be mounted in order to give several measurement points, and this in turn extends the duration of the drying process.

Description of the invention

One purpose of this invention is to offer a method, an arrangement, a drying chamber and a drying plant that make it possible to measure and determine, without contact, the moisture content of wood that is being dried.

This purpose achieved with a method having the characteristics that are specified in claim 1 , an arrangement having the characteristics that are specified in claim 5, a drying chamber having the characteristics that are specified in claim 12 and a drying plant having the characteristics that are specified in claim 13.

The invention will be described below with reference to Figures 1 and 2.

Description of drawings Figure 1 shows a drying plant according to the invention comprising a batch dryer. Figure 2 shows a drying plant according to the invention comprising a continuous dryer.

Description of embodiments of the invention

A method and an arrangement 1 to determine the moisture content of wood 2 that is being dried in the arrangement 1 will be first described, and then a chamber 3 and a drying plant 8.

A method according to this invention has been developed to determine the moisture content of wood 2 that is being dried. The determination of moisture content takes place during the actual drying process.

The method comprises introduction of the wood 2 into a chamber 3 and its placement there, in a compartment in which the wood 2 is to be stored during the drying process. The wood 2 is normally stacked before transport into the chamber 3, or while in the chamber 3. The wood 2 is stacked in layers, with supporting units between the layers, normally laths, and with a certain distance between each plank in a layer.

In order for drying of the wood 2 to take place, warm flowing air A is introduced into the chamber and onwards around and past the wood 2, through the stack of wood, the wood package. When the warm air A warms the wood evaporation of the water in the wood, emission of the water from the wood, into the surrounding air takes place.

Transport of the air, the moist and somewhat cooler air B, and thus also transport of the water that has been emitted from the wood, takes place continuously out from the chamber 3. The air B, which is full of water, is dried, warmed and reintroduced into the chamber 3.

In order to be able to determine the moisture content, the method comprises a transmission, a sending operation, of a pulse through the wood 2 from one side of the wood 2 to the other. Reception of the pulse when it has passed through the wood 2 takes place on the other side of the wood 2.

The method comprises further measurement and recording of the time of transmission of the pulse from its sending to its reception, the time of transmission of the pulse through the stack of wood. Thereafter, calculation of the moisture content of the wood relative to the time of transmission takes place, where a long time of transmission for the puise is related to a high moisture content of the wood and a short time of transmission of the pulse is related to low moisture content in the wood.

The method comprises further repeated transmission and reception of a new pulse following a pre-determined period, and the measurement of the time of transmission of the pulse for each transmitted pulse.

The pre-determined period is selected such that the preceding pulse has passed through the wood and been received before the next pulse is transmitted. Electrical energy is used in the pulse. The pulse comprises a number of wavelengths that travel at different speeds. If a new pulse is sent at too short an interval after the preceding pulse, the reception, the measurement and the recording of the later pulse will comprise noise from the preceding pulse. In order not to transmit a pulse too soon, such that certain frequencies overlap, there must be a pre-determined time delay between transmissions of consecutive pulses. The determination of the time delay requires that the frequency spread, the frequency content, of the pulse is known, or that initial measurements of the time required for pulse decay are carried out.

The initiation of the transmission of a new pulse is thus related to the preceding received pulse. When the preceding received pulse reaches a certain level, a threshold value, the transmission of a new pulse is permitted.

The method comprises also measurement of duration changes between different pulse transmission times and calculation of the change in moisture content of the material relative to the change of duration. A large change of duration indicates a large change in the moisture content, and thus also rapid drying, a large emission of water from the wood. A small change of the duration indicates a small change in the moisture content, and thus also slow drying, a small emission of water from the wood.

The invention concerns also an arrangement 1 that makes it possible to determine the moisture content of wood 2 that is being dried. The method according to the invention can be used when using this arrangement 1.

The arrangement 1 is placed into a chamber 3 into which warm air is caused with the aid of a fan system 4 to flow and to be introduced into the chamber 3 through an inlet system 4.1 and onwards around and past the wood 2, through the stack of wood. The wood 2 is dried by the warm air that passes through the wood 2, where the water in the wood is evaporated, and where the circulating air carries with it water out from the chamber 3. The moist air, and thus also the water that has been emitted by the wood 2, is transported out from the chamber 3 through an outlet system 4.2. The chamber 3 may be a batch dryer chamber or a continuous dryer chamber.

The arrangement 1 comprises a transmitter 1.1 that transmits a pulse through the wood, from one side of the wood 2 to the other side. The arrangement 1 comprises also a receiver 1.2 that receives the pulse that has passed through the wood 2, and an arrangement 1.3 that measures and records the time of transmission of the pulse from the transmitter to the receiver, and also an arrangement 1.4 that calculates and records the moisture content based on the time of transmission measured.

In cases in which a continuous dryer is used, the arrangement 1 should comprise several transmitters 1.1 and receivers 1.2 arranged along the inner surfaces of the continuous dryer on each side of the wood 2 such that pulses can be transmitted through the stack of wood at several locations inside the chamber and can follow the stack of wood during its complete time in the chamber 3. One stack of wood is fed into the chamber in a continuous dryer and then it is displaced within the chamber from the entrance to the exit. When the first stack has been displaced through a certain distance, a further stack of wood can be introduced into the chamber since the wood in a continuous dryer is displaced through the chamber, which has extended dimensions. Several stacks of wood are thus dried at the same time and continuously inside one chamber.

The arrangement comprises also an arrangement 1.5 that initiates repeated transmission of new pulses after a predetermined time.

The arrangement comprises further an arrangement 1.6 that measures and records the change of time of transmission between two transmitted pulses, and an arrangement 1.7 that calculates the change of moisture content based on the change in the time of transmission.

The arrangement 1 comprises both a transmitter and a receiver arranged, located, on two opposite sides of the wood, on each side of the wood, of the stack of wood. The transmitter 1.1 and the receiver 1.2 are arranged in one unit 5. Each transmitter-receiver unit 5 on each side of the stack of wood is connected to an aerial 6 and is in communication with a reporting system 7. The use of an aerial gives the effect that it becomes possible to avoid the necessity of using a coaxial cable that previously has been used to carry information about the drying process and the dryness, from the measurement point to the reporting system.

The arrangement comprises, in cases in which a continuous dryer is used, an arrangement 1.8 that coordinates the times of displacement of the stacks of wood and the transmission of a pulse, it manages the transmission of the sending of a pulse and the reception of a pulse, and it collates the information that has been collected to the correct stack of timber, for the different stacks of timber.

It is appropriate that at least one of the transmitter-receiver unit 5 and the reporting system 7 comprise the arrangements 1.3-1.8 that have been described above, although it is possible that these be independent units in any suitable manner.

The invention concerns also a drying chamber 3 in which it is possible to determine the moisture content of wood 2 that is being dried, which drying chamber comprises an arrangement 1 as specified above. The chamber 3 may be, as has been described above, of batch dryer type or continuous dryer type. The drying chamber 3 may comprise several arrangements 1, transmitters 1.1 and receivers 1.2, transmitter-receiver units 5, arranged on each side of the wood 2, in order to provide information from desired areas or sections of the wood 2 in the chamber 3.

The invention concerns also a drying plant 8, a drying system, a drying arrangement, in which it is possible to determine the moisture content of wood 2 that is being dried, comprising a drying chamber 3 according to the invention which in turn comprises an arrangement 1 according to the invention. The term "drying plant 8" is used to denote the complete facility that has been created for the drying of wood in which the method according to the invention can be used and in which an arrangement according to the invention is located comprised inside a chamber.

One important parameter for this invention is that it is the moisture content of the wood and the change in the moisture content of the wood as time passes that are interesting during the drying process, during the period in which the drying process is carried out