Drying of Wood

I. Read and learn the following words and word combinations:

remaining – той, що залишається

excess – надмірний, зайвий

precise dimensions – точні розміри

to machine – піддавати механічній обробці

to maintain – утримувати в хорошому стані

fire-retardant chemicals – хімічні речовини, що надають вогнетривкості

relative humidity – відносна вологість

stain – забарвлення (вада деревини)

framing in construction – конструкційний матеріал

cabinets – корпусні меблі

millwork – столярні вироби

accelerated air drying – прискорене атмосферне (повітряне) сушіння

fan – вентилятор

suction fan – всмоктувальний вентилятор

exhaust fan – витяжний вентилятор

kiln – сушарка

lumber stack – стіс пиломатеріалів

pre-drying – попереднє сушіння, підсушування

drying rate – швидкість сушіння

chamber – камера

progressive kiln – сушарка безперервної дії

progressively – поступово

batch kiln – сушарка періодичної дії

wide range – широкий діапазон

wet-and-dry-bulb thermometer instrument – психрометр

forced air circulation – примусова циркуляція повітря

spacer – прокладка

truck – вагонетка

track-type kiln – рейкова сушильна камера

package-loaded kiln – пакетно-завантажувальна сушарка

fork-loader – навантажувач вилкового типу

instead of venting – замість випускання (повітря)

dehumidifier coils – змійовик-осушувач (вологого повітря)

steam – пара

predetermined set – зазделегідь визначений набір (температур і вологості)

II. Match the English words and word combinations with their Ukrainian

equivalents.

1. excess water a) прискорене атмосферне сушіння

2. relative humidity b) рейкова сушильна камера

3. drying rate c) змійовик-осушувач

4. predrying d) стіс пиломатеріалів

5. accelerated air drying e) режим сушіння

6. chamber f) вагонетка

7. progressive kiln g) швидкість сушіння

8. batch kiln h) відносна вологість

9. kiln drying schedule i) випускання вологого повітря

10. forced air circulation j) камера

11. stack of lumber k) сушарка періодичної дії

12. track-type kiln l) попереднє сушіння

13. truck m) сушарка безперервної дії

14. venting humid air n) зайва (надлишкова) вода

15. dehumidification coils o) примусова циркуляція повітря

III. Read and translate the following text. Drying of Wood

In the living tree, wood contains large quantities of water. As green wood dries,

most of water is removed. But the remaining moisture content is usually rather high if wood is not specially dried. Drying is required for wood to be used in most products. Properly dried wood has many advantages over green lumber. First, removal of excess water reduces wood’s weight. Second, proper drying reduces shrinkage and swelling of wood in use. Third, as wood dries, most of its strength, electrical, and thermal insulating properties increase. Properly dried lumber can be cut to precise dimensions and machined more easily and effectively; also, paint, varnish and other finishes are more effectively applied and maintained. Wood must be relatively dry before it can be glued or treated with decay preventing and fire-retardant chemicals.

The key to successful and efficient drying is control of the drying process. The use of optimum temperature, relative humidity and air circulation is very important. The usual strategy is to dry as fast as possible. Slower drying can be uneconomical, also it can result in stain.

Softwood lumber which is to be used for framing in construction is usually dried to an average moisture content of 15%. Softwood lumber for many other uses is dried to a lower moisture content of 10% to 12%, and 7% to 9% for furniture. Hardwood lumber for furniture, cabinets and millwork is usually dried from 6% to 8% moisture content.

Lumber drying is usually done by some combination of air drying, accelerated air drying (or pre-drying) and kiln drying. Accelerated air drying involves the use of fans to force air through lumber stacks in a shed. Pre-drying reduces energy costs in kiln drying. All these methods reduce shrinkage, warping and checking, but kiln drying kills fungi and insects in wood.

In kiln drying, higher temperatures and faster air circulation are used to increase drying rate greatly. Most dry kilns are thermally insulated chambers. There are two classes of dry kilns: a progressive kiln is one in which the lumber moves progressively from one end to the other, becoming drier as it moves along. Today the progressive kiln is used mostly for drying thin size materials such as veneer. The other is a batch kiln in which lumber is completely loaded in one drying operation and remains stationary during the entire drying cycle. Temperature and relative humidity are kept as uniform as possible throughout the kiln and can be controlled (regulated) over a wide range. But, as the wood dries, temperature and relative humidity are changed according to a schedule of drying. The temperature and humidity in the kiln are usually measured by a wet-and-dry-bulb thermometer instrument.

All dry kilns use some type of forced air circulation, with air moving through lumber perpendicular to the length of the lumber and parallel to the spacers that separate each layer of lumber in a stack. The forced air circulation is produced by using the system of various fans. Fresh air is taken in by suction fan and spent air is exhausted by exhaust fan.

Batch kilns are generally divided into three types. One is the track-type kiln where lumber is loaded on kiln trucks which are rolled in and out of the kiln on tracks. Another type is the package-loaded kiln, where individual stacks of lumber are put into place in the kiln by a fork-loader. The third common type of kiln is the dehumidification kiln. Instead of venting humid air to remove water, as the other two types do, water is removed by condensation on cold dehumidifier coils.

These kilns are most commonly heated by steam.

Kiln schedules. A kiln schedule is a predetermined set of temperatures and relative humidities for various stage of drying. In most schedules, the temperature is gradually increased and the relative humidity decreased. The schedule must be developed so that the drying stresses are not higher than the strength of the wood at any given temperature and moisture content. Otherwise, it can lead to drying defects such as cracks, checks, shrinkage or discoloration. Kiln drying schedules vary by species, thickness, grade and end use of lumber.