A METHOD FOR THE CONTROL OF DRYING OF VENEER

Abstract: A method for the during of veneer, in which the drying takes place in a kiln (1) not unknown in itself, in which the drying-air circulates in the drying compartment from blowers (6) to heat exchangers and passes the veneer absorbing the water evaporated from the veneer. For the improvement and automation of the control, a major characteristic of the present invention is that, through observing the moisture content of the veneer produced (10) and through measuring the parameters affecting the moisture of the product (10) in the kiln, sufficient information is gained for the automatic regulation of the drying process, in which the adjustable factors consist of the speed of the air circulating in the dryer, the outflow of air, and the speed of the veneer going through the kiln, to obtain desired moisture content in the dried veneer (10).

Description: A METHOD FOR THE CONTROL OP DRYING OP VENEER The object of this invention is a method for the control of the drying of veneer. In this method the drying takes place in a kiln, not unknown in itself, in which the drying-air circulates in the drying compartment from blowers to heat exchangers and passes the veneer, absorbing the water evaporating from the veneer. In this method, also the moistur of the veneer produced, is controlled. In the plywood industry the veneer is lathed from roundwood i a veneer lathe, after which the veneer is dried in a kiln to attain moisture content appropriate for further processing. A veneer dryer is normally a fairly large kiln with tracks ca ing the veneer, and with blowers blowing heated air into the drying compartment. The drying power can be regulated through adjusting the speed and the humidity of the air circulating in the kiln and the speed of the veneer passing the dryer. The regulation of these parameters is necessaj to provide constant quality of the dried veneer or to change the drying conditions deliberately. The control of the quality of the veneer is traditionally carried out manually by sorters, who, by feeling the veneer wi their fingers, establish its quality. As veneer that is too moist is rejected, the drying parameters are so adjusted that the dryness of the veneer is assured. The parameters are adjus manually by trial. The above method has certain disadvantages. The quality of the veneer suffers from over-drying; it shakes and blisters. Over drying also wastes energy. Manual adjustment is difficult, because the exact loading of the dryer is not known. The loadi of the kiln is also a factor affecting the drying-power needed and it varies. Further disadvantages of manual adjustment are the long transitional shut-off times and the large reject batc as drying conditions change, e.g. in moving over to another woo grade, when the parameters are readjusted by trial, and it take time before the conditions of a large kiln stabilize to produce reliable evidence of the results of the change. Prom the Finnish patent publication 58021 a method is known for regulating the outflow of air, through measuring the pressure o the air in the kiln, by a valve, to keep the humidity of the ai within certain limits. The humidity of the air in the kiln is only one of the many factors affecting the moisture of the end product. Moreover, th adjusting of-the valve has to be done by -iand; automation and remote control are difficult to carry out, and the gain remains doubtful. The object of the present invention is to avoid the disadvantag described above. It is characteristic of the invention that, through measuring the parameters affecting the moisture of the product in the kiln, sufficient information is gained for the automatic regulation of the drying process, in which the adjust able factors consis :- * of the speed of the air circulating in the kiln, the outflow of air, and the speed of the veneer going through the kiln, in a way not unknown in itself, to obtain desired moisture content in the dried veneer. In this method, the kiln automatically reacts to changes in the drying conditio thus eliminating waste of energy. All the essential parameters are considered. By regulating the speed of the veneer in the ki also the loading can be affected, and by drawing up drying para meters for each wood grade or production type, unnecessary loss are eliminated in connection with -.changes in production. An essential feature of this invention is also the fact that th parameters observed in the kiln are the moisture content of the incoming veneer, the loading of the kiln, the speed of the veneer in the kiln, the speed, the humidity and the temperature of the air flowing in the kiln. The knowledge of the state of OMPI ^ NATI these parameters produces an adequate picture of the drying process. Essential of this invention is also the fact that the humidity of the air is measured through compilation of values given by various temperature sensors. . This is an accurate -. ' method of moisture measurement, suitable for automatic control. Another characteristic of the present invention is that, as nee for adjustment arises, the adjustable factors are exploited ful one by one, till the desired result is achieved. Thus the whole range of •. ithe adjustment • can be exploited when necessary, to maintain constant moisture content of the veneer. A further characteristic of the invention is the order in which the.adjustments are made: the speed of the infeed air blowers, the speed of the outflow air blowers,and the speed of the wires carrying the veneer sheet. This order is important for the econo of the drying process, because with the use of circulating air blowers primarily, heat loss is eliminated, and the speed of production is maintained. The invention is described in further detail in an example with figure 1• and 2. Fig. 1. presents schematically a device employing the present invention, as seen from the side. Fig. 2. presents the same device as seen from the head. Undried veneer (2) coming from the lathe is fed into the kiln (1 in (3). The veneer sheet is conveyed on a track consisting of wires (4) and folding wheels (5). The kiln (1) is divided into compartments with own infeed air blowers (6). The blowers blow air through heat exchangers (7) heated with steam, after which the air is led through nozzles onto the veneer. The air circulat in the kiln (1) as shown in figure 2. The kiln has hygrometers OMPI for incoming and outgoing veneer (14 and 15) required by the regulation system, a device measuring the humidity of the air (16) a thermometer (17) and a pulse sensor (18) for the measuring of the spee of the veneer. Through the regulation of the speed of the circulation of air by means of an inverter regulator (8) to meet the parameter value in the controlling computer, the moisture content of the end prod can be affected. If the circulation of the air is sped up, the drying of the veneer becomes more powerful and vice versa. By means of the outflow air blower (11) and its regulator (12) the humidity of the air in the kiln is adjusted. The more humid the air is, i.e. the less of it is allowed to flow out, the more efficient is the transfer of heat into the veneer, and the evaporation of water from the veneer is sped up. This produces energy savings. Through regulating the rotation speed of the engine (13) the speed of the veneer sheet in the kiln, and thereby also the loading of the kiln,- can be affected. Ωiiε is abased on the fact that the speed of the infeed of the wet veneer (2) is not related to the operating speed of the kiln (1). During the operation of the kiln (1), each of the above parameters can be adjusted independent of each other within the regulation system, or a part or all of the adjustments can be left to the computer (9). In a fully-automate mode, which is the normal form of operation, the regulation system first of all changes the speed of the circulation of air with blowers (6) as need for adjustment arises. Only if this is found insufficient, the regulation system moves over to adjust the following parameter: the speed of the out air blower. If desired results are still not achieved, the last possibility is the regulation of the speed of the wires (4) with the engine (13). An expert appreciates that the applications of the present inven¬ tion are not limited merely to the above example, but they can OMPI vary within the limits of the claims put orward below. Therefo the structure of the dryer does not need to be the one describe above; e.g. a roll dryer is possible. There are also various possibilities for the control system. In this case it is " • computerized, but it does not have to be.

Claims: CLAIMS 1. A method for the control of drying of veneer, in which the drying takes place in a kiln (1), not unknown in itself, in whic the drying-air circulates in the drying compartment from blowers to heat exchangers (7), and passes the veneer absorbing the wate evaporated from the veneer, and in which the moisture content of the veneer produced (10) is controlled, c h a r a c t e r i by that, through measuring the parameters affecting the moisture of the product (10) in the kiln, sufficient information is gaine for the automatic regulation of the drying process, in which the adjustable^factors consist of the speed of the air circulati in the kiln, the outflow of air, and the speed of the veneer going through the kiln, in a way not unknown in itself, to obtai desired moisture content in the dried veneer (10). 2. A method complying -with claim 1, c h a r a c t e r i z e d by that the parameters observed in the kiln (1) are the moisture content of the incoming veneer, the loading of the kiln, the speed of the veneer sheet in the kiln, and the speed, the humidity and the temperature of the air circulating in the kiln. 3. A method complying with claims 1 and 2, c h a r a c t e r i z by that the humidity of the air is measured through compilation of values given by various temperature sensors. 4. A method complying with claims 1, 2 and 3 ι c h a r a c t e r i z e by that, as need for adjustment arises, the adjustable factors are exploited fully, one by one, till the desired result is achieved. o A method complying with claim 4, c h a r a c t e r i z e d by that the order in which the adjustments are made, is: the speed of the infeed air blowers (8), the speed of the outflo air blowers (11) and the speed of the wires ( 4) carrying the veneer.

Inventors: KOVALA KEIJO, KUHNO PAULI, MAHLBERG OLLI, KOVALA, KEIJO, KUHNO, PAULI, MAHLBERG, OLLI
Applicants: SCHAUMAN WILH OY, OY WILH. SCHAUMAN AB
Application year: 1984
Main classification: F26B21
IPC codes: F26B13, F26B25