FI97995C - Method and apparatus for drying wood and wood products - Google Patents

Description

97995

METHOD AND EQUIPMENT FOR DRYING WOOD AND WOOD PRODUCTS -FÖRFARANDE OCH ANORDNING FÖR TORKNING AV TRÄ OCH TRÄBASERADE PRODUKTER

The invention relates to a method for drying wood and wood products according to the preamble of claim 1.

The invention also relates to an apparatus according to claim 7.

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One such method and apparatus is known, for example, from EN B 87691. A disadvantage of the known methods and apparatus is e.g. their poor suitability for cold climate countries, especially in terms of heat recovery. According to the publication, the solution according to the publication presents a so-called an open system in which a pressure control valve opening and closing at certain pressure values is placed between the drying chamber and the cooler sealing tank, which is used to control the drying process. This system requires large 20 sealing tanks to operate.

It is an object of the present invention to provide a new type of drying method and apparatus in which the pressure, the temperature and the amount of water to be removed are continuously controlled in a continuously closed cycle in order to obtain optimal drying conditions. In addition, the heat recovered from the dryer is utilized more efficiently in the method and equipment by means of a special condensing heat exchanger.

This object is achieved by a method according to the invention, which is characterized by what is stated in the claims below.

The method and apparatus according to the invention provide fast drying with low energy consumption. In addition, thanks to the heat recovery system, the heat recovered from the dryer can be better utilized by means of a special condensing heat exchanger. In the method according to the invention, the control of the process takes place by adjusting the flow of steam and the condensation of steam 40 and, if necessary, the rate and amount of water removal.

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The solution enables e.g. flexible placement of the heat exchanger in connection with the drying chamber. Low-cost utilization of distillates is also possible.

The following describes in more detail the method and apparatus according to the invention with reference to the drawings, in which

Fig. 1 shows diagrammatically a drying room according to the invention, Fig. 2 shows diagrammatically another drying room according to the invention,

Figure 3 shows a condensing heat exchanger according to the invention.

The process of the invention is commonly referred to as vacuum drying.

The drying chamber 1 of the vacuum dryer receives the initial and additional heat from the boiler plant (not shown in the figure) via pipes 10. 20 Heating can also be done by other possible methods, such as electricity. A drying chamber according to the invention is disclosed in FI B 87691. The publication also explains the basics of vacuum drying. Depending on the stage of the drying process, a significant part of the heat demand 25 of the drying process can be covered by the heat provided by the secondary circuit of the condensing heat exchanger 5. In the figures, the water flow directions are indicated by arrows.

The drying chamber 1 contains wood to be dried, which is preferably stacked in layers in a stack of wood so that open spaces are formed between the layers of wood 30. The drying chamber is preferably a cylindrical space constructed to be pressure resistant. The drying chamber is provided with one or more heating elements 11 such as a fin coil or the like. The dimensions of the drying chamber vary depending on the application. In a preferred case 35, the chamber is about 18 m long and 3.5 m in diameter.

In the initial heating, the temperature of the drying chamber 1 and the batch of wood (not shown in the figure) placed therein is raised to 20 to 90 ° C, preferably 45 to 80 ° C. The vacuum pump is then started

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97995 3 2, which sucks air out of the drying chamber 1. The pressure in the drying chamber is then 0.02 to 0.9 at. In this pressure range, the boiling point of water varies in the temperature range 17.2 to 96.2 ° C, preferably 45 to 80 ° C. It is also possible to first lower the pressure in the drying chamber and then raise the temperature.

The valve TV l.i opens and the boiler circuit circulating water pump P3 starts. In the drying chamber 1, the liquid, such as water, starts to evaporate and the temperature is raised so high that the liquid in the dry wood boils. Convection fans 12 or the like located in the drying chamber circulate this saturated or superheated steam, preferably between the strip-packed timber. By replacing the removed air, preferably with steam, the harmful effect of oxygen in the drying process is prevented. 15 As a result of heating and water evaporating from the wood, the steam pressure in the drying chamber starts to rise. In this case, a vacuum pump 3 is started, which sucks water from the lower part of the heat exchanger 5 (continuous removal), keeping the vacuum in the drying chamber at the desired value as defined by the drying program. The flow of steam removed from the drying chamber 1 can be regulated, for example, by controlling the speed of the pump 3. The water or other liquid sucked by the vacuum pump 3 causes a steam flow from the drying chamber 1 to the heat exchanger 5, where the steam condenses into a liquid. The liquid is discharged to a sewer, for example. 25 The amount of liquid leaving and, if necessary, the temperature are monitored with a measuring device 4.

The heat exchanger 5 secondary flow is directed primarily to the drying chamber 1 to maintain the temperature, for example 30, in the alternative production systems (production facilities heati piping system denoted with the reference numeral 6) for heating, and then cooling the device 7, for example jäähdytyspatte-morale. Most of the waste heat of the cooling device 7 can also be used for heating the production facilities (shown by reference numeral 35 8), whereby only during the warm season is the heat dissipated.

The purpose of this secondary circuit is to cool the temperature of the water circulating in the circuit to such a low level that, when it returns to the heat exchanger 5, it condenses, the temperature of which is below the drying temperature.

4 97 995 The heat exchanger 5 secondary flow is passed along the pipe through the pump P3 into the drying chamber 5 arranged in one or more of the heating element 11, such as a battery. The drying chamber is preferably provided with fans 12 which circulate 5 steam in the drying chamber 1 in order to ensure rapid and even heating of the timber. From time to time, the direction of air flow can be changed, preferably by changing the direction of rotation of the axial fan 12. This enhances the transfer of moisture to the steam and provides a more even drying result. The motors 10 of the fans 12 are preferably located outside the drying chamber. The blades of the fans are shaped so that they give good efficiency when rotating in both directions. The radiators 11 of the secondary loop is guided to the valve TV 2.2. The TC 2.2 controller controls the TV 2.2 valve. The temperature of the flow 15 of the heating network 6 of the production facilities is measured by a sensor TE 2.2, which is connected to

to the controller TC 2.2. When the temperature is within the desired temperature range, water is directed through valves 2.2 and 2.1 to the heating network 6 of the production facilities. If the temperature is higher than TE

2.2 The setpoint is controlled by the water flow from the valve TV 2.2 to the radiator-20 m 7.

The return current of the heating network 6 is led through the valve TV 3.1 to the heat exchanger 5 or if the temperature measured by the measuring element TE 3.1 is too high, the valve TV 3.1 directs it to the flow line of the radiator 25 for cooling.

Figure 2 shows an alternative solution. Central to this is the alternating operation of the vacuum pumps 2, 3, which enables more versatile control of the drying process.

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The deaeration vacuum pump 2 draws in air through the condensing heat exchanger 5. This has the advantage that if, during drying, there is a need to remove more steam than the amount of water removed by the vacuum pump 3, the automation V switches on the vacuum pump 2. The vacuum pump 2 preferably has a higher power than the vacuum pump 3. The power of both vacuum pumps can be adjusted, for example, by increasing or decreasing the speed of their motors according to the drying program pulse so as to achieve a suitable steam flow from the drying chamber to the heat exchanger. The drying conditions are regulated by a control * »system called with open drying formulas. Drying formulas refer to the optimization of drying conditions e.g. regarding the quality and drying time of the 5 wood to be dried.

In the solution according to the examples, the process is controlled by a control system, some components of which are shown in the accompanying figures. The TC 1.1 controller controls the control valve TV 1.1 using the TE 1.1 and TE 1.2 elements. In a preferred case, the drying program keeps the set temperature in the drying chamber constant. The temperature can be changed, for example, from the control center or by means of a drying program.

15 The TC 2.1 control unit controls the control valve of the TV 2.1 sensor TE

2.1 and the sensor TE 2.2 so as to obtain the desired temperature in the production rooms 6. The TC 2.1 controls the control valve TV 2.2 so that when the temperature of the water passing through the TV 2.1 rises above the set value TE 2.2, the valve TV 2.2 directs the water flow 20 to the cooler 7.

Valve 3.1 directs constant temperature water to the return line of the heat exchanger 5. If the water temperature is too high, the valve directs it to the flow line of the radiator 7 for recirculation through the radiator 7.

The heat generated by cooling can be used to heat production facilities, especially during the cold season. Otherwise, the heat generated is removed.

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Figure 3 schematically shows a condensing heat exchanger according to the invention. The heat exchanger consists of a tank 14 into which the steam of the drying chamber 1 is led from the drying chamber 1 through a steam removal pipe 13 and in which the tank has a tube coil 35 15 inside which water circulates and on the outer surface of which the steam condenses into water. The water flows to the bottom of the tank from where it is removed by means of a pump 3 to a sewer or the like. The amount of water accumulating in the lower part of the tank is limited by the electrode controllers Y, E. The controllers give an impulse to the control device V, which controls the alternating operation of the vacuum pumps 2, 3 in the case according to Fig. 2. In this case, the water level remains in the desired range YR, AR. The control can also work so that the power of the pump is increased as the water level approaches the upper limit YR and the power decreases as the water level 5 approaches the lower limit AR. The control of the pumps can be relative or stepped.

It will be clear to a person skilled in the art that the invention is not limited to the above-mentioned application examples, but can be varied within the scope of the following claims.

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Claims (7)

97995 PATENT CLAIM ET A method for removing moisture from solid wood and wood-5-based products or the like, comprising placing wood or wood-based products in a drying chamber (1), then closing and deaerating the drying chamber (1) while supplying heat to the chamber to remove liquid in the wood by evaporation and regulating the pressure and / or temperature in the drying process 10 and removing the steam generated during the drying process from the drying chamber in a heat exchanger (5) connected thereto, characterized in that the pressure in the drying chamber (1) and the steam flow from the drying chamber 5) from the lower part and thus the vacuum is kept according to the program that the temperature in the drying chamber (1) is regulated by introducing heat from the secondary circuit of the heat exchanger (5) and that the flow of liquid leaving the heat exchanger (5) is regulated. Method according to Claim 1, characterized in that the flow of steam and / or effluent is controlled by controlling the rotational speed of one or more pumps (2, 3). Method according to Claim 1 or 2, characterized in that the steam flow is controlled by a throttling control. Method according to Claim 2, characterized in that the pumps (2, 3) are controlled by alternating operation in accordance with the drying formula of the control system. Method according to one of Claims 1 to 4, characterized in that the heat recovered in the heat exchanger (5) is used primarily in the drying process and secondarily for heating production facilities or the like. 8 97995 Method according to one of Claims 1 to 5, characterized in that in the secondary circuit of the heat exchanger (5) water is circulated to one or more heating elements (11) of the drying chamber (1) and / or to the heating network (6) or similar and / or cooling device ( 7) and finally via the return line back to the heat exchanger (5). Apparatus for use in the method of claim 1, the apparatus comprising a drying chamber (1), one or more heating elements (11) for raising and maintaining the temperature in the drying chamber (1), one or more tubes for sucking steam and air from the drying chamber (1), a steam extraction tube (13) connected to a heat exchanger (5) in which the steam condenses into a liquid, as well as control devices for the drying process, characterized in that steam is sucked into the heat exchanger (5) by one or more pumps (2, 3), liquid is removed from the heat exchanger (5), and (5) the secondary circuit is connected to a pipe network in which the flowing water can be directed to pass through the heating chamber heating element (11) and / or one or more heat distribution networks (6) and / or cooling device (7) so that the secondary circuit ends in the heat exchanger (5). Apparatus according to claim 7, characterized in that sensors (Y, E) or the like are arranged in the lower part of the heat exchanger (5), which provide a control pulse to the control unit (V) controlling the alternation of pumps, keeping the liquid level between the desired upper limit (YR) and lower limit. (AR). 30 97995