EP0746736B1

EP0746736B1 - A method and plant for drying wood

Info

Publication number: EP0746736B1
Application number: EP95911542A
Authority: EP
Inventors: Eric BJÖRKMAN; Stefan Ericsson
Original assignee: PROPAC AB
Current assignee: PROPAC AB
Priority date: 1994-03-03
Filing date: 1995-03-03
Publication date: 1999-06-02
Anticipated expiration: 2015-03-03
Also published as: ATE180887T1; EP0746736A1; SE501508C2; AU1907295A; DE69510042D1; WO1995023946A1; DE69510042T2; SE9400745L; SE9400745D0

Abstract

The invention relates to a method and to a plant for drying with circulating drying air wooden elements that are stacked in mutually spaced relationship in a chamber (2) which is essentially sealed from the outside. The heat content and the moisture content of the drying air can be controlled, wherein the moisture content of the drying air circulating in the chamber (2) is controlled by ventilation and the drying air is cooled by and condenses on one or more cooling surfaces (11) disposed in the chamber as the air passes in contact with said surfaces, these surfaces being cooled by replacement air which enters the chamber (2) from outside the chamber.

Description

The present invention relates to a method and plant for drying wooden elements with the aid of circulating drying air, said elements being stacked in mutually spaced relationship.

Wood driers are normally divided into two categories, depending upon their construction, namely circulating air driers and condensation driers. With regard to their construction and working methods, such plants are relatively simple since they need only be provided with means for controlling air speed, air temperature and air humidity in order to achieve a drying function.

When wooden elements are to be dried, the elements are stacked in closed spaces, so-called drying chambers, into which warm air and/or steam is delivered from a heating unit with the aid of adjustable fans, the temperature of said spaces being raised to 50-100°C. The warm air is forced to circulate, or flow, through the stacked wooden elements and in so doing takes up, absorbs, moisture from the wood. In this regard, the warm air is given a dryness and wet temperature status which is suitable to the quality of the wood concerned, by controlling the heat delivered to the air and also the moisture ventilated therefrom. When the air becomes saturated with water vapour and is unable to absorb further moisture from the wood, it is necessary to reduce the moisture content of the drying air in order for the drying process to continue. This is either effected by ventilation, i.e. by forced introduction of dry, cold ambient air and the release of moisture-saturated drying air or by cooling and condensing the moisture-saturated air in the chamber.

However, both of these methods require large energy inputs in order to function effectively, since in the case of ventilation, it is necessary to heat the cold ambient air to the relevant drying temperature before introducing the air to the wood located in the drying chamber, and also because energy must be supplied in order to cool and condense the moisture-saturated air in said chamber.

In drying plants which work in accordance with the air circulating principle, it is necessary to use large capacity fans in order to obtain an intensive throughflow of air and an effective drying effect. Consequently, large volumes of air are heated and pass through the plant per unit of time. It is known that when drying wood in accordance with the air circulating principle, the consumption of heat can be reduced when ventilation is effected so that the ventilated or exiting exhaust air (of higher temperature) is allowed to give part of its enthalpy, i.e. heat content, to the incoming supply air (of lower temperature) in a heat exchanger.

Theoretically, this heat recovery in the drying process can be continued until the exhaust air has been cooled to the lowest supply air temperature, wherein most of the enthalpy of the exhaust air is returned to the drying process. In existing wood-drying plants, this principle is often applied to the economic limit of the heat exchanger with regard to its size, since a large amount of energy is required to dry the wood is large and often corresponds to about half of the total drying costs. The drawback with air circulation drying plants is that they incur high installation costs, requiring well-insulated chambers, and also high operating costs unless provided with advanced heat exchange systems. Such systems are relatively expensive, however.

As before mentioned, drying plants which operate in accordance with the condensation principle include a condenser for cooling the moisture-saturated drying air and a heating battery for heating the demoisturized air. The cooling effect of the condenser is normally achieved by passing a coolant through the condenser from a compressor cooling unit or by passing a forced flow of cold ambient air through the condenser in a circuit which is closed from the drying chamber. One method of providing an energy lean system for the condensation of moisture-saturated drying air and for heating the demoisturized air is to provide the drying plant with a heat pump whose coolant absorbs heat from the moisture-saturated air (which is therewith cooled) via the condenser, wherein the heat absorbed by the coolant is delivered to the heating battery.

Of the aforesaid methods for demoisturizing and heating the drying air, only drying plants that are provided with the heat pump system fulfil the low energy consumption requirements placed on wood-drying plants today. However, these systems are much too complex and incur much too high installation and maintenance costs to provide a viable solution to the problem.

From DE 26 54 835 it is known to regulate the moisture content of circulation air by cooling and condensing the same as it passes one or more condenser surfaces disposed in the chamber. A condenser body is forming part of one of the walls of the drying chamber and is being selected from a material having high heat conductivity characteristics. The condenser surface is cooled from outside the chamber by aid of the heat exchange taking place between the air circulating in the chamber and the relatively cold ambient air surrounding the chamber. In contrast to the present invention, the drying plant according to said document is not constructed for ventilation of the chamber.

From US 3 262 216 is known a drying plant whereby the moisture content of the drying air circulating in the chamber is regulated by passing a condenser surface whereby said surface is arranged for taking advantage of the relatively cold earth on which the drying plant is situated. Further in contrast to the present invention, the drying chamber of said document is not ventilated. Drying air is directed from the drying chamber and passed through a closed system for heating and removing moisture from the air.

The present invention is based on the aim to provide a wood-drying plant of the type defined in the introduction which has a low energy consumption and which will condense the drying air rotating in the chamber in the absence of mechanical auxiliaries, such as cooling compressors. Such a plant is constructed on the principle of generating different pressures in the chamber and by effecting exchanges of air in the chamber by causing the warm moisture-saturated drying air that is formed successively in the chamber by being heated to exit from that part of the chamber which exhibits the relatively high pressure, i.e. that part of the chamber which, in accordance with the invention, is located in the region between the stacked wood and the pressure side of respective fans. This is achieved with the aid of a passageway provided in this part of the chamber, preferably at the top of the chamber, and including an adjustable throttle valve. The relative decrease in pressure that occurs in the chamber enables replacement air to be taken into the chamber from the ambient surroundings. According to the present invention, this replacement air, i.e. cold, dry ambient air, is caused to flow into that part of the chamber which is under the relatively low pressure, through the medium of a heat exchanger which is mounted in the chamber and which forms a wall on which the warm, moist drying air circulating in the chamber condenses. This results in an exchange of heat between the circulating chamber air and the inflowing cold ambient air.

The use of this replacement air in the chamber to condense the water vapour present in the air circulating in the drying chamber reduces the need for air circulation and therewith also the amount of energy required to heat the replacement air taken into the chamber during the drying process.

It can be expected that the cost of energy will increase in the future and therewith elevate the requirement for drying plants that have low energy consumptions. Furthermore, there is always found a need for simple and inexpensive plant constructions which in combination with low energy consumption result in a low total cost.

The aim of the present invention is realized with a method and plant that have the characteristic features set forth in the following Claims.

The invention will now be described in more detail with reference to a practical, but not limiting, example of the invention and also with reference to the accompanying drawings, in which

Fig. 1 and Fig. 2 are respectively schematic vertical sectional and horizontal sectional views of an inventive wood-drying plant; and Fig. 3 and Fig. 4 are respectively schematic vertical sectional and horizontal part-sectional views of a ventilation system intended for use in plants constructed in accordance with the invention.

The Figures illustrate an inventive drying plant which comprises typically a housing 1 which includes a drying chamber 2 in which wooden elements (not shown) are stacked in mutually spaced relationship. Although not shown, the drying chamber includes an input port which, when closed, essentially seals the chamber 2 from the outside. The chamber has mounted therein a wall 3 which extends from the floor to the ceiling of the chamber 2 and therewith delimits a corridor from that part of the chamber in which wood to be dried is placed. Placed transversely in the corridor is a fan stand 5 which supports a plurality of stationary fans 4. The fans 4 may conveniently be axial fans which are driven by a variable-speed reversible electric motor 6 through transmission means in the form of power transmission couplings. The motor is placed outside the chamber, so as not to be overheated by the high temperatures that prevail in the chamber. Mounted adjacent the fans is a heating battery 7 which is supplied with hot water and which functions to heat the air circulating in the drying chamber. The air is either pressed or sucked through the heating battery 7, depending on the direction of rotation of the fans 4. It will be understood that a plurality of water nozzles may be arranged in a known manner adjacent the fan-heating battery arrangement for bringing the wooden elements to a state for further treatment.

The wood is dried with external air, which is drawn by suction in through one of the air intakes 8 located adjacent the heat exchangers 9 mounted at the corners of the corridor, depending on the direction of rotation of the fans 4. Each heat exchanger includes a heat-transferring, double-walled elongated rectangular body which has provided at its upper end a passageway or air intake 8 for the introduction of dry, cold external air and an air outlet 10 at its bottom end. The air drawn in through the intake 8 constitutes the heat-absorbing medium and flows through the heat exchanger and out through the outlet. Check valves are provided in the air intake 8 or in the air outlet 10, to prevent heated dry air from flowing back through the heat exchanger 9. In addition to preheating the incoming ventilation air and condensing the water vapour bound there in, the heat exchangers 9 are also positioned to deflect the air flows at the corners of the chambers. To this end, each heat exchanger 9 includes at least one air-cooling surface 11 which faces in towards the chamber and which forms a wall on which the bypassing drying air condenses. In order to achieve effective condensation, one elongated side of the heat exchangers 9 is placed adjacent the chamber wall. In this regard, the heat exchangers are preferably disposed with the other longitudinally extending edge of the heat exchangers 9 adjacent the chamber wall so as to form therebetween a gap through which the incoming ambient, replacement air is able to pass. The volume of air drawn in through the air intake 8 by suction is determined by the throughflow area of an adjustable control throttle valve 12 placed in a passageway 13 at the top of the chamber is open.

In order to be able to guide the air into the passageway 13 and the valve 12 irrespective of the direction in which the air flows into the drying chamber, the fan stand 5 is provided at its upper end with a flap 14 which can be swung or flipped between two end positions. The flap 14 and the fan stand 5 divide the corridor into two halves, of which only one half is able to communicate via the fans 4. In operation, the area between the stacked wooden elements and the adjacent chamber walls is sealed in a known manner, wherewith the resultant overpressure thus prevailing on the pressure side of the fans 4 will cause the flap 14 to flip so as to provide a free passage from this half of the chamber to the passageway 13. In order to achieve uniform drying of the wooden elements stacked in the chamber, it is necessary to reverse the direction of air flow at regular intervals. This can be achieved simply by reversing the direction of rotation of the fans 4, such that what was the suction side of the fans is now the pressure side. This change in pressure occurring across the flap 14 will cause the flap to flip.

The fans 4 are started up after having placed wooden stacks in the chamber 2 and having closed the intake port. In normal operation, the drying air is heated in the chamber by the heating battery 7, while air is caused to circulate in the chamber 2 by the fans 4, in the arrowed directions. The heated air is caused, in a known manner, to circulate or flow through the stacked wooden elements and continuously absorbs moisture from the wood until the air is fully saturated. It is necessary to demoisturize the saturated air flow in order for the air to again absorb moisture from the wood, which can be achieved by adjusting the extent to which the valve 12 is open.

When the valve 12 is fully or partially open, warm air which is fully or partially saturated with moisture will flow out through the passageway 13, preferably by virtue of the overpressure prevailing on the pressure side of the fans and also by virtue of a chimney effect. The outflowing air will be replaced with dry, cold ambient air which flows in through the air intake 8 of the heat exchanger 9 located on the suction side of the fans 4, therewith cooling said heat exchanger. While the valve 12 is open and air circulates in the chamber, part of the water vapour in the air flow circulating around the drying chamber will condense as it passes the cooled heat exchanger, by virtue of the heat transfer that takes place between the air flow circulating around the drying chamber and the cold, dry ambient air passing through said heat exchanger. It will be understood that the volume of replacement air taken from outside the drying chamber can vary momentarily even though the air inlet area of the valve 12 has not been changed. For instance, such momentary variations may be due to a change in the pressure conditions in the drying chamber, for instance as a result of an increase or decrease in fan speed.

The invention affords a number of advantages over known techniques. For instance, the drying air can be condensed without requiring the provision of separate ventilation fans or cooling compressors that are required in hitherto known drying plants. It will be understood, however, that the inventive plant may also be provided with auxiliary fans for supporting ventilation.

Tests carried out on a pilot plant have shown that a significant part of the water vapour normally removed by ventilation readily condensates. It has also been found that the drier consumes only a small amount of energy, since the valve opening is very small during operation of the plant. Instead of comprising a double-wall elongated body, the heat exchanger 9 may comprise a single-wall, elongated sheet metal element placed in the corners of the chamber 2.

Claims

A method of drying with circulating drying air wooden elements stacked in mutually spaced relationship in a chamber (2) which is sealed essentially from the outside and in which the heat content and moisture content of the drying air can be regulated, characterized in that the moisture content of the drying air circulating in the chamber (2) is regulated by ventilation in which a controlled volume of drying air is released from the chamber (2); and the thus departing air volume is replaced continuously with fresh supply air from outside the chamber, said air entering the chamber (2) being caused to pass one or more surfaces (11) which are disposed in the chamber and are common to the drying air circulating in the chamber (2) and the fresh ambient air entering said chamber and which surfaces function to exchange heat between the drying air circulating in the chamber (2) and the ambient replacement air entering said chamber thereby causing the circulating drying air to be cooled and condensed as it passes said surfaces (11).
A method according to Claim 1, characterized in that the controlled release of drying air from the chamber (2) and the intake of replacement air to the chamber (2) from outside the chamber is effected through pressure differences in the chamber.
A method according to Claims 1-2, characterized in that each air cooling surface (11) is placed in a corner of the chamber (2) and functions to deflect the drying air circulating in the chamber.
A method according to Claims 1-3, characterized in that the controlled release of drying air from the chamber (2) and/or the intake of replacement air entering the chamber from outside said chamber is/are supported by a fan.
A plant for drying with circulating drying air wooden elements that are stacked in mutually spaced relationship, wherein the plant includes a chamber (2) in which the wooden elements are stacked and which is sealed essentially from outside, and further includes means for regulating the heat content and the moisture content of the drying air circulating in the chamber (2), characterized in that the plant is constructed for ventilation of the chamber by controlled discharge of drying air from the chamber (2) and replacing the departing air volume with fresh supply air from outside the chamber; and in that arranged in the chamber (2) are one or more surfaces (11) intended to be cooled by the replacement air entering the chamber from outside the chamber and which surfaces is/are common to the drying air circulating in the 5 chamber and to the fresh air entering said chamber and which function to exchange heat between the drying air circulating in the chamber (2) and the replacement air entering the chamber from outside said chamber, whereby the drying air circulating in the chamber is cooled and condensed as it passes by said surfaces (11).
A plant according to Claim 5, characterized in that the plant is effective in releasing drying air from the chamber (2) and taking replacement air into the chamber from outside said chamber by means of pressure differences in said chamber.
A plant according to Claims 5-6, characterized in that each cooling surface (11) is placed in a respective corner of the chamber (2) and is effective in deflecting the drying air circulating in the chamber (2).
A plant according to Claims 5-7, characterized in that the plant includes a fan for supporting the release of drying air from the chamber (2) and for taking replacement air into the chamber (2) from outside said chamber.