WO1990001664A1

WO1990001664A1 - A method of drying wood

Info

Publication number: WO1990001664A1
Application number: PCT/SE1988/000395
Authority: WO
Inventors: Leif Nilsson
Original assignee: Drytec Ab
Priority date: 1987-02-03
Filing date: 1988-08-03
Publication date: 1990-02-22

Abstract

A method for drying wood, preferably sawn wood, in a wood-predrying chamber, characterized in that the wood, prior to being introduced into the drying chamber (1), is predried in a predrying chamber (8) with air which is taken from the ambient surroundings and which is caused to pass through a heat-recovery battery (15, 13) connected to the exhaust-air ventilating system (7) of the drying chamber, such as to at least raise the temperature of the incoming air and therewith considerably decrease the relative humidity of the incoming air. The thus heated air is blown through the wood in the predrying chamber in a direction such that the wood-batch which is next in turn for introduction into the drying chamber (19) is the first batch to be exposed to the drying influence of the incoming air.

Description

A METHOD OF DRYING WOOD

The present invention relates to a method of drying wood, preferably sawn wood, in wood-drying chambers.

Although the drying of sawn wood in open-air drying timber yards has been found the method requiring the lowest energy input, this energy-lean method has been replaced in the majority of timber mills, both in Sweden and in other countries, with artificial drying methods, for instance such methods which involve the drying of wood in wood dryers in the form of batched dryers or compartment kilns, or progressive kilns. This switch has been made because of the long drying times involved, because of the uneven results obtained due to changes in prevailing weather conditions during the wood-drying period, and because of the large, not-readily handled wood-stacks in the lumber yards entailed by the often air drying method entails.

It can be mentioned by way of example that the number of batched dryers in Sweden is estimated to be about 1,000 dryers distributed between roughly 315 sawmills, each dryer being operative to dry about 4 million cubic metres of wood each year, whereas the number of progres¬ sive kilns (wandering kilns) is estimated to be around 380 kilns distributed between about 100 sawmills and operative to dry approximately 5,100,000 cubic metres of wood each year. About 2,200,000 cubic metres of wood is dried annually in the lumber yards of some 2,000 low- production sawmills. A number of sawmills use both progressive kilns and batched dryers.

Because of the desire to be able to adapt the drying of wood to customer requirements in a better way than has hitherto been possible, and also to achieve lower mois¬ ture ratios than those achieved hitherto, there is a general desire to find ways and means of increasing dryer capacities. Sawn wood is normally dried to "export dryness" , i.e. to a moisture ratio slightly lower than 20-25%. The demand for wood which has been dried to lower moisture ratios, down to 15%, is becoming progres¬ sively higher, for instance wood which shall be used in the manufacture of window frames and like structures.

Artificial wood dryers are highly energy consuming. No direct attention has been paid to this fact for many years, because the crude material used to produce ther¬ mal energy in a sawmill, such as bark, sawdust and wood chips, has been considered to be readily available at no cost. During the 1980s, however, these wood fuels, and other wood fuels, have become highly significant and are in demand as energy-producing raw materials. From the aspect of economy, it is often beneficial for a sawmill to invest in energy-saving measures, and since the amount of energy consumed when drying wood is very high, approximately 300 kilowatt hours/cubic metre of wood, a small percentual saving in energy consumption will result in a high annual saving. When extending a drying plant, it is often more advantageous to the sawmill, from an economic aspect, to invest more in energy-saving measures than in a new boiler system.

Approximately 80-90% of the thermal energy supplied to a wood dryer, in the form of warm water, hot water or steam, and also in the form of electrical power to the fans servicing the dryer, is lost through the ventila¬ ting chimneys in the form of hot steam, so-called waste heat. This waste heat corresponds to about 65% of the total amount of energy generated by the boiler system of a sawmill of average size, the drying plants of which take about 75% of the total amount of energy produced. It is known to install heat-exchangers for the purpose of preheating incoming supply air to the drying chamber (so-called air-exchange dryers) with the aid of exhaust air from the drying chamber, but because the exhaust air has a low temperature, e.g. about 45°C, upstream of the heat exchanger and approximately 36°C downstream there¬ of, and is also highly humid, i.e. about 80% upstream of the heat exchanger and 100% downstream thereof, the air can only be used with difficulty, and consequently the saving in energy amounts to only 10-25% of the total energy consumed by the dryer and more than 2/3rds of the energy content of the exhaust air is lost. It is also known, to this end, to install heat pumps which are intended to raise the temperature of the air or the water in the boiler return pipe. One drawback with this solution, however, is that additional energy is required to drive compressors and like apparatus, and the instal- lation is both complicated and costly to achieve, there¬ with often casting doubt on the benefits afforded by such a solution.

Consequently, an object of the present invention is to provide a method whjich will enable the waste heat from existing drying chambers to be recovered and utilized more effectively than has been possible hitherto, either for the purpose of improving the drying capacity of said chambers or of reducing energy requirements.

This object is achieved by the inventive method, accord¬ ing to which prior to being charged to an existing drying chamber, the wood is predried in a predrying chamber with the aid of external air which is caused to pass through a heat-recovery battery connected to the exhaust-air ventilating system of the drying chamber, so as to at least raise the temperature of said external air, whereafter the predrying air is blown through the wood located in the predrying chamber, in a direction such that the wood-batch which is next in line for introduction into the drying chamber proper is the batch which first meets the air heated in the heat-recovery battery.

This predrying of the wood results in a significant reduction or decrease in the amount of freely-bound moisture in the wood prior to its introduction into the drying chamber proper, without requiring the input of additional energy, and according to calculations it is possible to reduce the water content of the wood to be charged to the drying chamber proper by up to 60%, and that the inventive method will afford an ultimate energy saving of more than 50%.

The invention will now be described in more detail with reference to the accompanying drawing, in which Figure 1 illustrates schematically and in section a plant for carrying out the inventive method; and Figure 2 is a schematic, longitudinal sectional view taken generally on the line II-II in Figure 1.

The reference 1 in the drawing identifies a drying passageway of a progressive kiln. The passageway has located therein a plurality of trollies 2, which are laden with stacks of wood 3. The loaded trollies 2 are moved into the passageway 1 through an entry port 4. Drying air is circulated in the passageway, in the direction of arrows 6, with the aid of a fan 5. Exhaust air exits from the passageway 1 through an exhaust duct 7. In accordance with the invention, there is arranged upstream of the entry port 4 leading to the passageway 1 a predrying chamber 8 of simple and inexpensive con- struction, which need only include a ceiling 9 and walls 10 which adjoin the end wall of the passageway 1 ad¬ jacent thereto. The ceiling 9 and the walls 10 may, for instance, be made of a lightweight material and may be insulated, e.g. with polyurethane, or uninsulated. The length of the predrying chamber 8 is preferably adapted to the capacity of the downstream drying chamber 1 and trollies 2 loaded with wood 3 are disposed in a manner such as to enable the trollies to be moved in turn into the passageway 1, immediately a dried wood-stack is moved out of said passageway.

Mounted in one wall 10 of the predrying chamber, in the close vicinity of the openable and closeable entry port 4 of the passageway 1, are a number of powerful fans 12, preferably axial fans, which are carried on a stand 11. Also mounted on the stand 11 is a heat-recovery battery 13 which has the form of a heat exchanger which is con¬ nected to a heat-recovery battery 15, which also has the form of a heat exchanger, by means of conduits 14 including a circulation pump 15, and which being moun¬ ted in the ventilation duct 7 of the drying chamber, the exhaust air exiting from the drying passageway 1 will pass through the heat-recovery battery and deliver heat to the heat-carrying medium, e.g. a water-glycol mixture, circulating through the two heat-recovery batteries.

The wood located in the chamber 8 is predried with air which is taken from the ambient surroundings and drawn- in through the heat-recovery battery 13 by the fans 12 and heated by said battery, whereafter the thus heated air is blown by the fans 12 into the predrying chamber 8 and through the wood stacks 3 placed therein in the direction shown by the arrows 16, i.e. in a direction such that the wood stack 3 next in line to enter the drying passageway 1 is the first of the wood-stacks present to meet the heated predrying air entering the predrying chamber. This predrying air then flows through the remaining wood stacks 3 in the predrying chamber 8 and exits into the surrounding atmosphere. The flow of predrying air shall be several times larger than the flow of exhaust air from or the flow of supply air 2 the drying chamber.

For the purpose of guiding incoming predrying air through the wood-stack 3 in the predrying chamber 8, flaps 17, which function as seals, may be mounted, in a known manner, at least between the wood-stack 3 located nearest the entry port 4 and the walls 10 and ceiling 9 of the predrying chamber. Alternatively, flaps 17 may also be located between several wood-stacks 3 and the walls and ceiling of the predrying chamber.

Thus, in accordance with the inventive concept, prior to entering the drying passageway 1, the wood is predried in a predrying chamber 8 with air which is taken from the ambient surroundings and caused to pass through a heat-recovery battery connected to the ventilation system of the drying chamber proper. The temperature of the air is raised as it passes through the battery and the thus heated air is blown through the wood located in the drying chamber, in a direction such that the wood- stack 3 which is next in line for introduction into the drying passageway 1 is the first of the stacks to meet the incoming drying air, said air then passing through the remaining wood-stacks in the predrying chamber c and subsequently being blown from said chamber into the surrounding atmosphere.

By using the energy content of the exhaust air in this way, less energy is required for drying the wood and the capacity of existing wood dryers can be increased, by shortening switching times from, e.g. 8-9 hours to 6 hours. This measure results in a capacity increase of approximately 30%, due to the fact that the moisture ratio in the wood has been lowered by 30-45% by predry¬ ing the wood while using the heat content of the exhaust air. This increase in capacity is thus achieved without increasing energy input.

The invention thus provides a method for predrying wood at low temperatures and under controlled conditions which not only result in lower energy consumption or increased capacity, but also provide advantages from the aspect of wood quality. Because the air blown into the predrying chamber has a relatively low moisture content in comparison with the ambient air taken in and flows constantly through layers of wood dispersed in stacks on the trollies, the risk of mould forming in the wooa- layers is minimal. The risk of wood cracking or spur¬ ting due to the effect of the sun in the spring and summer months is also reduced, due to the fact that the wood-stacks are kept covered in the predrying chamber 8.

In the case of normal intermediate storage of wood prior to being dried in a wood-drying plant, and when drying wood in accordance with the open-air drying system, there is a risk that the humidity in the microclimate, i.e. at the surfaces of the wood, will be so high as to result in mould damage. When practicing the method according to the present invention, the water present in the wood surface of the outermost stack or stacks is precipitated (thawing effect) , and consequently the oxygen availability is too low for mould to grow, and on the blowing side of the stack Rh is too low to permit any such growth whatsoever.

The invention is not restricted to the afore-described and illustrated embodiment, and modifications and changes can be made within the scope of the inventive concept defined in the claims. Although the invention has been described and illustrated with respect to a drying passageway in which no heat-exchange system is provided for heating the supply air entering the drying passageway with the aid of the air exiting therefrom, it will be understood that the invention can be applied to all types of wood dryers and thus wood dryers which are equipped with conventional heat-exchange systems for transferring thermal energy from the exhaust air to the drying-chamber supply air.

Claims

1. A method for drying wood, preferably sawn wood, in a wood-predrying chamber, characterized in that the wood, prior to being introduced into the drying chamber (1) , is predried in a predrying chamber (8) with air which is taken from the ambient surroundings and which is caused to pass through a heat-recovery battery (15, 13) con¬ nected to the exhaust-air ventilating system (7) of the drying chamber, such as to at least raise the tempera¬ ture of the incoming air and therewith considerably decrease the relative humidity of said incoming air, and in that the thus heated air is blown through the wood in the predrying chamber in a direction such that the wood- batch which is next in turn for introduction into the drying chamber (1) is the first batch to be subjected to the influence of the incoming predrying air.

2. A method according to Claim 1, characterized in that the airflow in the predrying chamber (8) is several times larger than the flow of exhaust air from the drying chamber or the flow of supply air to said cham¬ ber.

3. A method according to Claim 1 or 2, applied in drying chambers provided with a heat-exchange system for preheating the air supplied to the drying chamber with the aid of the exhaust air exiting therefrom, charac¬ terized in that the exhaust air exiting from the drying chamber is passed through the conventional heat-exchange system prior to its heat exchange with the external or ambient air used for predrying of the wood.