Wood and the technology of wood drying (Древесина и технологии сушки древесины). Учебно-методическое пособие

to crack; to express; to keep; to dry; to rate; to recover; to cool; to prevent; to move; to follow; to attack; to slow; to damage

7. Compare the following pairs of words and translate them:

permit – permissible; create – creature; known – unknown; permeable – impermeable; danger – dangerous; differ – difference; cold – coldness; doubtdoubtful; aim – aimless; form – reform

TEXT A

WOOD DRYING

Wood drying may be described as the art of ensuring that gross dimensional changes through shrinkage are confined to the drying process. Ideally, wood is dried to that equilibrium moisture content as will later (in service) be attained by the wood. Thus, further dimensional change will be kept to a minimum.

It is probably impossible to completely eliminate dimensional change in wood, but elimination of change in size may be approximated by chemical modification. For example, wood can be treated with chemicals to replace the hydroxyl groups with other hydrophobic functional groups of modifying agents. Among all the existing processes, wood modification with acetic anhydride has been noted for the high anti-shrink or anti-swell efficiency (ASE) attainable without damage to wood. However, acetylation of wood has been slow to be commercialized due to the cost, corrosion and the entrapment of the acetic acid in wood.

Drying timber is one method of adding value to sawn products from the primary wood processing industries. According to the Australian Forest and Wood Products Research and Development Corporation, green sawn hardwood, which is sold at about $350 per cubic meter or less, increases in value to $2,000 per cubic meter or more with drying and processing. However, currently used conventional drying processes often result in significant quality problems from cracks, both externally and internally, reducing the value of the product. For example, in Queensland(Anon, 1997), on

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the assumption that 10% of the dried softwood is devalued by $200 per cubic meter because of drying defects, saw millers are losing about $5 million a year. In Australia, the loss could be $40 million a year for softwood and an equal or higher amount for hardwood. Thus, proper drying under controlled conditions prior to use is of great importance in timber use, in countries where climatic conditions vary considerably at different times of the year .Drying, if carried out promptly after felling of trees, also protects timber against primary decay, fungal stain and attack by certain kinds of insects. Organisms, which cause decay and stain, generally cannot thrive in timber with moisture content below 20%. Several, though not all, insect pests can live only in green timber. Dried wood is less susceptible to decay than green wood as the moisture content in green wood is generally above 20%. In addition to the above advantages of drying timber, the following points are also significant:

1.Dried timber is lighter, and the transportation and handling costs are reduced.

2.Dried timber is stronger than green timber in most strength properties.

3.Timbers for impregnation with preservatives have to be properly dried if proper penetration is to be accomplished, particularly in the case of oil-type preservatives.

4.In the field of chemical modification of wood and wood products, the material should be dried to certain moisture content for the appropriate reactions to occur.

5.Dry wood generally works, machines, finishes and glues better than green timber (although there are exceptions; in many ways green wood is easier to turn than dry wood). Paints and finishes last longer on dry timber.

6.The electrical and thermal insulation properties of wood are improved by drying.

The chief difficulty experienced in the drying of timber is the tendency of its outer layers to dry out more rapidly than the interior ones. If these layers are allowed to dry much below the fibre saturation point while the interior is still saturated, stresses (called

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drying stresses) are set up because the shrinkage of the outer layers is restricted by the wet interior. Rupture in the wood tissues occurs, and consequently splits and cracks occur if these stresses across the grain exceed the strength across the grain.

The successful control of drying defects in a drying process consists in maintaining a balance between the rate of evaporation of moisture from the surface and the rate of outward movement of moisture from the interior of the wood. One of the most successful ways of wood drying or seasoning would be kiln drying, where the wood is placed into a kiln compartment in stacks and dried by steaming, and releasing the steam slowly.

[from http://en.wikipedia.org/wiki/Wood_drying#Wood_drying]

ASSIGNMENTS

1. Translate the following word-groups into English:

пиломатериалы, уксусная кислота, лиственные породы дерева, климатические условия, вырубка деревьев, испарение жидкости, консерванты, волокно, трещина, грибковые пятна, теплоизоляия, ацетилирования древесины, застой влажного воздуха

2. Answer the following questions:

1.What does wood drying mean in its strictest sense?

2.What results can be achieved by chemical modification in drying process?

3.What climate conditions influence timber drying?

4.What are the main elements affecting timber drying process?

5.What is the most successful way of wood drying?

3.Divide text A into logical parts and find the topical sentence of each part.

4.Retell text A in short using topical sentences.

5.Read and translate text B.

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TEXT B

The external drying conditions (temperature, relative humidity and air velocity) control the external boundary conditions for drying, and hence the drying rate, as well as affecting the rate of internal moisture movement. The drying rate is affected by external drying conditions.

Temperature: If the relative humidity is kept constant, the higher the temperature, the higher the drying rate. Temperature influences the drying rate by increasing the moisture holding capacity of the air, as well as by accelerating the diffusion rate of moisture through the wood. The actual temperature in a drying kiln is the drybulb temperature, which is the temperature of a vapor-gas mixture determined by inserting a thermometer with a dry bulb. On the other hand, the wet-bulb temperature is defined as the temperature reached by a small amount of liquid evaporating in a large amount of an unsaturated air-vapor mixture. The temperature sensing element of this thermometer is kept moist with a porous fabric sleeve usually put in a reservoir of clean water. A minimum air flow of 2 m/s is needed to prevent a zone of stagnant damp air formation around the sleeve. Since air passes over the wet sleeve, water is evaporated and cools the wet-bulb thermometer. The difference between the dry-bulb and wet-bulb temperatures, the wet-bulb depression, is used to determine the relative humidity from a standard hygrometric chart. A higher difference between the dry-bulb and wet-bulb temperatures indicates a lower relative humidity. For example, if the dry-bulb temperature is

100 °C and wet-bulb temperature 60 °C, then the relative humidity is read as 17% from a hygrometric chart.

Relative humidity: The relative humidity of air is defined as the partial pressure of water vapor divided by the saturated vapor pressure at the same temperature and total pressure. If the temperature is kept constant, lower relative humidities result in higher drying rates due to the increased moisture gradient in wood, resulting from the reduction of the moisture content in the surface layers when the relative humidity of air is reduced.

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Air circulation rate: Drying time and timber quality depend on the air velocity and its uniform circulation. At a constant temperature and relative humidity, the highest possible drying rate is obtained by rapid circulation of air across the surface of wood, giving rapid removal of moisture evaporating from the wood. However, a higher drying rate is not always desirable, particularly for impermeable hardwoods, because higher drying rates develop greater stresses that may cause the timber to crack or distort. At very low fan speeds, the air flow through the stack is often laminar flow, and the heat transfer between the timber surface and the moving air stream is not particularly effective. The low effectiveness of heat transfer is not necessarily a problem if internal moisture movement is the key limitation to the movement of moisture, as it is for most hardwoods.

[from http://en.wikipedia.org/wiki/Wood_drying#Influence_of_ temperature.2C_relative_humidity_and_rate_of_air_circulation]

6.Suggest a title for text B.

7.Speak about temperature, relative humidity, and air velocity during timber drying.

8.Translate from Russian into English:

1.Древесина имеет весьма сложную структуру, которой трудно дать математическое описание.

2.Вакуумная сушка древесины получила широкое распространение в странах с высокоразвитой деревообрабатывающей промышленностью.

3.Cушка древесины за счет положительного градиента температуры получается не только более быстрой, чем конвективная, но и более качественной.

4.Время высыхания и качество древесины зависят от скорости воздуха и его равномерного движения.

5.Технология вакуумной сушки привлекательна прежде всего тем, что она предоставляет реальную возможность значительно сократить продолжительность сушки при сохранении качества высушенных пиломатериалов, а в ряде случаев повысить его.

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6.Сухая древесина менее подвержена гниению.

7.Натуральная воздушно-сухая древесина находится в состоянии естественного застеклования, возникшего в процессе биосинтеза.

9. Look through the texts again to find two facts which were quite new to you and two facts which were already known to you.

Section 2.

REVISION GRAMMAR EXERCISES

1. Translate the following sentences paying attention to the function of “it”.

1.It is not the sound of the voice that travels over the wire when we talk over a telephone.

2.When a ray of any kind strikes an atom, it may knock an electron out of the atom.

3.It seems unlikely that this experiment will give good results.

4.It is probably impossible to completely eliminate dimensional change in wood, but elimination of change in size may be approximated by chemical modification.

5.It is known that personal experience is necessary for every specialist.

2. State the function of “one”, translate into Russian:

1.One can easily solve a difficult problem using a computer.

2.You sit in that chair and I’ll sit in this one.

3.“Which of the two women is Alice, the younger one or the older one?”

4.This text is easier than the one we translated before.

5.One should take into consideration all the notes given below.

6.One of these problems has been solved with the help of a computer.

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7. One must know that the light year is the distance that the light will travel during one year.

3 Give the comparative degree of the following words:

quick, soon, long, impatient, little, many, far, beautiful, fine, famous

4. Translate into English using “that (of)” where it is possible:

1.Население США больше, чем население Великобритании.

2.Горы Италии выше, чем горы Уэльса.

3.Эти вопросы интереснее, чем вопросы, которые мы обсуждали вчера.

4.Эта шляпа красивее, чем шляпа вашей сестры.

5.В 1896 году Г. Маркони создал приемник, схожий с приемником А.С. Попова.

5. State where “the” is used as an article and where as a conjunction:

1.The earlier we start the sooner we finish.

2.The contribution made by Russian chemists to the world science is widely known.

3.The more difficult the work is the more eagerly he fulfils it.

4.The physical laws that underlie the architecture of the Periodic System are strict.

5.The United States is one of the largest countries in the world.

Section 3.

IDIOMS (set expressions)

In some phrases you cannot get the meaning by adding together the meaning of each of the words within it. They are called idioms or set expressions and function in speech as single words.

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EXERCISES

1. The common expressions on the left should be familiar to you. From the list at the right find the meaning of the expressions:

2. Find sentences which are an idiom.

1.

a)In the long run they were both good fellows more or less on the same side.

b)We saw the two boys running along the street.

2.

a)I saw through Anna very rapidly.

b)You can see TV sets in every house nowadays.

3.

a)Betty went out of her way to be nice to Herb.

b)Can you find your way home?

4.

a)And now you can start with your work. Take your time.

b)He spent much time on this invention.

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5.

a)I’d like to know more about her. That’s girls got something. Just keep your eye on her.

b)Valves at the transmitting station are used to keep the electric circuits oscillating.

3. Fill in the blanks with the following words:

humidity, content, it is probably, one method, timber, wood drying, stronger.

1.A higher difference between the dry-bulb and wet-bulb temperatures indicates a lower relative … .

2.Drying timber is … of adding value to sawn products from the primary wood processing industries.

3.One of the most successful ways of … or seasoning would be kiln drying.

4.Organisms, which cause decay and stain, generally cannot thrive in timber with moisture … below 20%.

5.Dried timber is … than green timber in most strength properties.

6.Paints and finishes last longer on dry … .

7.… impossible to completely eliminate dimensional change in wood.

4. Say in English according to the pattern:

Pattern: чем скорей, тем лучше - the sooner the better

чем дальше … тем интереснее … чем меньше … тем хуже … чем больше … тем скорее … чем дольше … тем меньше …

Section 4.

CHECK YOUR PROGRESS

1. What are the new words which you added to your reading vocabulary?

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2.What is an idiom or a set expression?

3.Did you learn any new information from the texts of this unit?

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