О сушке древесины

На сегодняшний день существует множество производителей и видов сушильных камер. Как же выбрать ту камеру, которая необходима Вам? Чтобы разобраться в этом вопросе давайте рассмотрим принцип действия сушильных камер и принципы качественной сушки древесины.

Какими же параметрами определяется качество сушильного процесса?

• Разбег по влажности внутри доски.
• Напряжение древесины.
• Срок сушки.

Ниже представлены основные типы сушильных камер и принцип их действия.

Аэродинамическая сушильная камера представляет собой теплоизолированную камеру с вентилятором. Нагрев воздуха происходит за счет трения о лопатки вентилятора. В такой камере сложно регулировать температуру и скорость потока. Затраты на энергию таковы, что экономически выгоднее возить материал на давальческую сушку(600-800 руб./куб.м. только за электричество). Как плюс такой сушилки можно отметить её сравнительно небольшую стоимость, хотя здесь дешевизна как в мышеловке, которая будет медленно захлопываться, когда Вы начнете сушить в этих камерах.

The principle of their work is the same as the one in the microwave oven. High frequency electromagnetic radiation makes the molecules oscillate faster, thus warming up the wood. This type chambers make the time of wood drying much shorter, but these chambers are very costly, you can dry only small amounts of wood in them, they consume a lot of energy, another drawback is that the irradiators of microwaves go out of work pretty quickly. Summing it all up, you can say: "the engineer's idea is very interesting, but it has not found its wide practical application".

This is the most popular type of drying chambers. The transmission of heat goes through the air, that in its turn goes through the heat-transfers, through which go either hot water or overheated steam. The drying agent (air) circulates about the chamber, going through the packages with wood giving the wood its energy. Depending on the technology and drying phases you can change the parameters of the drying agent: moisten it with the help of spray nozzles in the chamber; lower its moisture content by throwing out the agent oversaturated with water and exchanging it for the dry one; change the temperature by just lowering it in the heat-exchanging calorifer; change the velocity and the direction of the agent by adjusting the inverter motors.

They are like the convention drying chambers in their way of heat transmission - the air circulates about the chamber, going first through the calorifers, then through the packages with wood. Then they get into the condensation device which gets the drying agent rid of its moisture, then send it back to the calorifers for heating. Its basic advantage over the convection chambers is its smaller energy consumption ( here we mean heat, not electricity!), and there are disadvantages, too, we will talk about them a little later…. So which of the chambers do you need? To solve this problem, let us get inside the board (wood), and see what is going on there during the drying process in different types of chambers.

Так какая камера нужна Вам? Для решения этого вопроса давайте залезем внутрь доски и посмотрим, что же там происходит во время процесса сушки в различных типах камер.

At the first stage the moisture in the board is distributed almost evenly and that's how it looks: (The crosscut where the dry board is shown in yellow color and the water in blue).

Then the process of drying starts: the air circulates around the board and it gets drier. At the second stage of drying that's how the board looks at the crosscut:

Now if we use a moisture meter, showing the moisture of the upper layer of the board, it will show us the moisture up to which we initially wanted to dry (8-10%), but inside the board is moist, there is a great strain inside it at the expense of the moisture difference and you cannot use this board for joinery work, and if we take a good needle moisture meter, it will show us its real moisture :20-30%, and we need to go on drying.

The process of drying goes on, and the hot air is circulating around the board again at a high velocity, (in aerodynamic chambers it is next to impossible to regulate the flow rate). The upper layer of the board is over dry now, it becomes crisp (2-3% moist), the dry wood contracts and gets thicker, while from the inner layer the water is removed much more slowly, which means that the contraction of the inner layer goes on more slowly, too, and when the outer layer cannot be "put on" the inner layer - the board cracks. That's how the board looks after drying in an aerodynamic chamber:

Conclusion: it is not possible to dry a board with good quality in an aerodynamic drying chamber.

At the first stage of drying the board looks like this:

During the drying process warm air circles around the board, thus removing the moisture from the outer layers of the wood. Then this air gets into the condenser, warms up again and the dry warm air is sent back to the board. At the second stage of drying the board looks like this:

Then the drying process can go in two directions:

If there are inverters on the motors, the air flow rate slows for a smooth removing of the moisture and to avoid cracking of the board. If there are no inverters - you cannot avoid cracks. At the end of the drying process the distribution of the moisture in the board will look like this: Conclusion: to dry a board in the condensation drying chamber without humidification but with inverter fans is possible, but the board will have a lot of strain.

At the second stage of drying, humidification of air is done to remove strain from the outer layer of the board. After humidification is done, the crosscut of the board looks like this:

As the result of osmos, the water has a tendency to flow into the medium with low humidity,that is from the moist air into the outer layers of the board, and from the center to the edges. Thus we remove the moisture from the inside of the board without doing any damage to the upper layers of the board. But because the condensation chamber is designed for relatively low temperatures, all the water inside the board does not have enough time to get warm enough and flow into the...(???)

Напряжение в такой доске есть и для столярного производства она не годится, но для вагонки или заборной доски - вполне.

At the first stage the board looks like this:

Right after loading the board into the chamber the board is heated to a large degree in the moisture saturated medium, with the increased air circulation, so that the water particles always stay warm. After the first water treatment the board looks like this:

But because the water is heated to about 75-80C, the water is easily removed from it. In the drying process, from 1 to 3 thermo-water treatments are done depending on the thickness of the board. The last thermo-water treatment is done shortly before the drying process is over, when the board has acquired the necessary amount of moisture and can totally remove its strain.

That's how the process of removing moisture from the board looks if the board is dried in a convection-type drying chamber:

a) The board is loaded

b) The first water-treatment and the warm-up are done. After this the moist drying agent is removed from the chamber and exchanged for dry air. The temperature inside the chamber drops, due to which the fast drying outer layer is being dried in a moderate regime, while the inner layers do not have time to cool. In this manner of drying the moisture from the inner layers actively goes out on top.

c) The first stage of drying is over. After this the board, depending on its species and thickness, is either kept in the dry agent up to the final stage of the water-treatment, or an intermediate thermo-water-treatment is done.

d) The final thermo-water-treatment.

The final thermo-water treatment is very much like the initial same treatment, as its purpose is to make the board homogeneous as far as the moisture content is concerned, only now it is the final stage of the process. This is how the board looks when it comes to the final stage of thermo-water treatment:

To totally remove the strain the board should be sealed (pressurized) and strongly heated. The upper layer of the moist wood can serve as the sealing agent. That's what is going on inside the board during its final thermo-water treatment:- the upper layer of the board swells quickly, thus creating a sealing layer, and the moisture inside the board is evenly spread throughout the board. After this all you need is to remove the moistening agent out of the chamber and exchange for a dry one, which will make the outer layer of wood dry in no time.

д) e. That's how the spread of moisture looks on the crosscut of the board dried in a convection drying chamber:

Let the board rest quietly for some time (the time of cooling varies depending on the species and thickness of the board - 1-3 days) to totally remove the strain, and you can use the board for any purpose.

CONCLUSION: as a result of experimenting in different drying chambers one can see that the best results are obtained in a convection-type drying chamber, however, if you cannot afford to maintain a water heater ( you are short of wastes to heat the boilers, you have no gas, it is difficult to deliver black oil fuel, etc.), and at the same time the requirements for drying are not that big, ( drying up to the transportation moisture or preliminary construction), in this case you can install condensation chambers - the heat costs for using these chambers are a lot smaller, but the electricity costs are much bigger.

It is you who must decide which chamber to choose, but you will be much better off if you turn to the specialists of the Company "STROINOVATSIYA". They will help you to choose the chamber that suits your requirements best - price, quality, etc. They can also design a chamber for your manufacturing capabilities. As of today "Stroinovatsiya" is probably the only company in Russia where they make experiments on the topics: "Optimal velocity of the air flow in drying chambers for various wood species", "The conditions of thermo-water treatment of wood", " Estimation of the processes of thawing and heating wood in different seasons", "Optimal conditions for removing the strain (pressure)" and others. All these estimations have been applied in the drying chambers "Scandinavia" and "WOODY".
Also "STROINOVATSIYA" can make a special design for your particular manufacturing facility.