Method for controlling the gas circulation rate in a timber drying oven

Abstract: Procedure for the regulation of gas circulation in a timber treatment or drying oven, said oven comprising a drying space (1) for timber (2), a fan (3) designed to produce gas circulation in the drying space and a heating unit (4) for heating the gas. According to the invention, the change in the condition of gas circulation is measured across a certain distance in the gas circulation and the intensity of gas circulation is adjusted as necessary in view of the measured change.

Description: The present invention relates to a procedure as defined in the preamble of claim 1 for the regulation of gas circulation in a timber treatment oven. Such a procedure is known, for example, from EP-A-0 142 071. In both heat treatment, warm air drying, hot drying and hot air drying of timber, large treatment chambers are used in which the timber to be treated is piled in a way that leaves enough space among the timber to permit circulation of gas, such as air, in the chamber. In the closed chamber, the gas is circulated by means of fans past electric resistors so that the air heated by the resistors causes heating or drying of the timber. When hot gas is effectively circulated in the oven, the heat can be uniformly distributed in the entire timber lot being treated. At present, regulation of the process is mainly effected by measuring the humidity and temperature of the gas at one point or by measuring the degree of drying or heating of the timber. Based on these data, the temperature and humidity of the gas is then adjusted. In addition, the fan can be somewhat adjusted mainly empirically to regulate the intensity of the gas circulation. Generally, however, a constant intensity of gas circulation is maintained throughout the treatment process. It is also known in prior art to use two different sensors to measure temperatures and to control the fan. In EP 0 142 071 the direction of gas circulation is determined with the temperature or humidity measurements. However the speed or intensity of gas circulation is kept constant. In WO 97/08504 drying of wood is devided into five different phases and the flow of circulation air is kept constant except in the last phase, where, in order to save energy, the flow of circulation air can be reduced. In US 4,922,624 a temperature drop in a certain zone is measured and the circulating air flow is reduced in that zone to a minimum to prevent further drying of wood in that zone when certain temperature drop is measured. The main drawback with prior-art technology is energy consumption. The energy needed for the drying and heat treatment and the electric energy required by the fan motor make up a substantial part of the total costs of the treatment, so reducing this energy consumption is of a decisive importance for economic utilisation of the processes in question. As the proportion of electric energy is 20 - 50 % of the total energy requirement and the heating energy is obtained from a cheaper source, such as fuel oil, wood chips etc., the energy consumed by the fan forms a significant part of the total costs. The object of the present invention is to eliminate the drawbacks mentioned above. A specific object of the present invention is to produce a new type of procedure for the regulation of gas circulation that allows the energy consumption of the fan in timber treatment ovens to be significantly reduced. As for the features characteristic of the invention, reference is made to claim 1. In the procedure of the invention, a timber treatment or drying oven is used, which comprises a drying space for the timber to be treated, a fan designed to produce gas circulation through the timber and a heating unit for the heating of the gas to be circulated. Accordion to the invention the change in the condition of gas circulation is measured substantially continuously throughout the treatment process at two points in the gas circulation on different sides of the timber being treated and the intensity of gas circulation is continuously adjusted as necessary in view of the measured change. The changes of condition measured from the gas circulation may be e.g. changes in the humidity, temperature or electric conductivity of the gas or a pressure difference or changes in gas composition. When the measurement of the change in the condition of gas circulation is implemented as continuous on-line measurement, the intensity of the gas circulation can also be adjusted continuously. As compared with prior art, the procedure of the invention has significant advantages. In tests carried out, it has been established that, when the gas circulation is regulated by the method of the invention, the consumption of fan energy can be reduced by amounts varying between 10 - 50 % in different treatment processes. Thus, the energy saving achieved in an oven of standard dimensions may be as large as $ 100.000 per annum as compared with empirical frequency converter regulation. In the following, the invention will be described in detail by referring to the attached drawing, which presents a diagram of a heat treatment oven in which the procedure of the invention can be applied. The heat treatment oven illustrated by the drawing comprises a closed drying space 1, where the timber 2 has been piled in a way that leaves enough space among the timber to permit circulation of gas, such as air. In addition, the heat treatment oven has a fan 3 to effect gas circulation and a heating unit 4, i.e. a suitable heat exchanger for heating the air being circulated. The heat treatment oven also comprises an outlet valve 5 for letting out some of the gas flow from the gas circulation and a replacement valve 6 for the intake of replacing gas into the gas circulation, as well as dampening devices 7 for increasing the humidity of the gas circulation when necessary. To implement the procedure of the invention, the heat treatment oven is provided with temperature or humidity sensors 8 and 9, placed in the gas circulation on either side of the timber 2. The procedure of the invention is applied as follows. When the timber 2 is in the drying space 1, gas is circulated through the timber by means of the fan 3, simultaneously heating the gas by means of the heating unit 4. Thus, the gas flows from the fan to the heating unit, where it is heated rapidly while its relative humidity falls at the same time. From here, the warm gas flows among the timber 2, where it is cooled as it heats the timber and humidity is transferred from the timber into the gas. Thus, while passing through the timber, the gas cools off and becomes more humid. If the gas flow is too intensive, then the temperature difference obtained from the sensors 8 and 9 is too small, in other words, it is unnecessary to use such an intensive air flow to achieve a sufficiently uniform transfer of the required heat energy from the heating unit 4 to the timber. On the other hand, in a corresponding case, the sensors 8 and 9 can be used to measure humidity, in which case it will be discovered that the difference between the humidity values measured at the measuring points is too small, which means that, due to an excessive intensity of the gas flow, too little humidity is transferred from the timber to the gas flow. In this situation, by reducing the speed of rotation of the fan 3, the gas flow as well as the energy consumption of the fan can be reduced. On the other hand, when the gas flow is too small, the differences in the temperature or humidity as measured by the sensors 8 and 9 will be too large, from which it can be inferred that the process is inefficient and needs a more intensive gas flow. In the foregoing, the invention has been described by way of example by the aid of the attached drawing, but different embodiments of the invention are possible within the claims.

Claims: Procedure for the regulation of gas circulation in a timber treatment or drying oven, said oven comprising a drying space (1) for timber (2), a fan (3) designed to effect gas circulation in the drying space and a heating unit (4) for heating the gas, characterised in that the change in the condition of gas circulation is measured substantially continuously throughout the treatment process at two points in the gas circulation on different sides of the timber (2) being treated and the intensity of gas circulation is continuously adjusted as necessary in view of the measured change. Procedure as defined in claim 1, characterised in that the change measured in the condition of gas circulation is a change in the humidity of the gas. Procedure as defined in claim 1, characterised in that the change measured in the condition of gas circulation is a change in the temperature of the gas.

Inventors: LALLUKKA TERO, LALLUKKA, TERO
Applicants: STELLAC OY
Application year: 1998
Main classification: F26B21
IPC codes: F26B21