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

of wood varies with the ambient relative humidity (a function of temperature) significantly, to a lesser degree with the temperature. Siau (1984) reported that the EMC also varies very slightly with species, mechanical stress, drying history of wood, density, extractives content and the direction of sorption in which the moisture change takes place (i.e. adsorption or desorption).

Shrinkage and Swelling

Shrinkage and swelling may occur in wood when the moisture content is changed. Shrinkage occurs as moisture content decreases, while swelling takes place when it increases. Volume change is not equal in all directions. The greatest dimensional change occurs in a direction tangential to the growth rings. Shrinkage from the pith outwards, or radially, is usually considerably less than tangential shrinkage, while longitudinal (along the grain) shrinkage is so slight as to be usually neglected. The longitudinal shrinkage is 0.1% to 0.3%, in contrast to transverse shrinkages, which is 2% to 10%. Tangential shrinkage is often about twice as great as in the radial direction, although in some species it is as much as five times as great. The shrinkage is about 5% to 10% in the tangential direction and about 2% to 6% in the radial direction.

Differential transverse shrinkage of wood is related to:

-the alternation of late wood and early wood increments within the annual ring;

-the influence of wood rays on the radial direction;

-the features of the cell wall structure such as microfibril angle modifications and pits;

-the chemical composition of the middle lamella.

[from http://en.wikipedia.org/wiki/Wood_drying#Wood.E2.80.93 water_relationships]

ASSIGNMENTS

1. State whether the following statements are true or false:

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1.Free water may contain chemicals, altering the drying characteristics of wood.

2.Free water is in the same thermodynamic state as liquid water: energy is required to overcome the capillary forces.

3.If water vapour pressure in the ambient space is lower than vapour pressure within wood, desorption doesn’t take place.

4.Because of its hygroscopicity, wood tends to reach a moisture content that is in equilibrium with the relative humidity and temperature of the surrounding air.

5.The greatest dimensional change occurs in a direction tangential to the growth rings.

2. Answer the following questions:

1.What forms of water does wood contain?

2.How is moisture content of wood calculated?

3.What is equilibrium moisture content in wood?

4.How can wood volume change?

5.What types of wood shrinkage do you know?

3. How would you use the following phrases when discussing the text? Make up sentences based on text A:

-freshly felled logs;

-free water is;

-water in cell lumina in the form of water vapour is;

-the oven-dry wood;

-to give off moisture;

-shrinkage and swelling;

-differential transverse shrinkage of wood is related to.

4. Give the English equivalents of the following:

-недавно срубленные бревна;

-дерево постоянно обменивается влагой с окружающей средой;

-дерево содержит воду в двух состояниях: свободная вода и пар;

-водородные связи;

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-давление пара;

-поперечная усадка древесины.

5.Retell text A: a) in short; b) in details.

6.Read text B. Dictionaries are allowed.

TEXT B

MOISTURE CONTENT DIFFERENCES

The chemical potential is explained here since it is the true driving force for the transport of water in both liquid and vapour phases in wood. The Gibbs free energy per mole of substance is usually expressed as the chemical potential. The chemical potential of unsaturated air or wood below the fibre saturation point influences the drying of wood. Equilibrium will occur at the equilibrium moisture content (as defined earlier) of wood when the chemical potential of the wood becomes equal to that of the surrounding air. The chemical potential of sorbed water is a function of wood moisture content. Therefore, a gradient of wood moisture content (between surface and centre), or more specifically of activity, is accompanied by a gradient of chemical potential under isothermal conditions. Moisture will redistribute itself throughout the wood until the chemical potential is uniform throughout, resulting in a zero potential gradient at equilibrium. The flux of moisture attempting to achieve the equilibrium state is assumed to be proportional to the difference in chemical potential, and inversely proportional to the path length over which the potential difference acts.

The gradient in chemical potential is related to the moisture content gradient as explained in above equations. The diffusion model using moisture content gradient as a driving force was applied successfully by Wu and Doe. Though the agreement between the moisture-content profiles predicted by the diffusion model based on moisture-content gradients is better at lower moisture contents than at higher ones, there is no evidence to suggest that there are

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significantly different moisture-transport mechanisms operating at higher moisture contents for this timber. Their observations are consistent with a transport process that is driven by the total concentration of water. The diffusion model is used for this thesis based on this empirical evidence that the moisture-content gradient is a driving force for drying this type of impermeable timber.

Differences in moisture content between the surface and the centre (gradient, the chemical potential difference between interface and bulk) move the bound water through the small passageways in the cell wall by diffusion. In comparison with capillary movement, diffusion is a slow process. Diffusion is the generally suggested mechanism for the drying of impermeable hardwoods. Furthermore, moisture migrates slowly due to the fact that extractives plug the small cell wall openings in the heartwood. This is why sapwood generally dries faster than heartwood under the same drying conditions.

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

7.Give the main points of text B in three sentences.

8.Translate the following sentences into English:

1.Различают абсолютную и относительную влажность древесины.

2.Абсолютная влажность древесины — это отношение веса содержащейся в древесине влаги к массе абсолютно сухой древесины, выраженное в процентах.

3.Относительная влажность древесины — это отношение веса содержащейся в древесине влаги к весу сырой древесины, выраженное в процентах.

4.Древесину по влажности делят на следующие категории: сырая – 23% и более; полусухая – 18–23%; воздушно-сухая – 12–

8%; сухая – 8–12%.

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5.Разбухание древесины проявляется при нахождении материалов при повышенной влажности воздуха длительное время.

6.Усушка – изменение размеров древесины при потере ею влаги в результате сушки. Усушка происходит естественным образом. Прямым следствием усушки является образование трещин.

7.Чем больше влажность древесины, тем сложнее её использовать в производстве.

Section 2.

REVISION GRAMMAR EXERCISES

1. Write the numbers of the sentences which contain the emphatic construction “it is … that’’.

1.Here is a letter. It is addressed to your brother.

2.It is the violin string that vibrates.

3.It is clear that the device is out of order.

4.It is his method that was used in our experiment.

5.It is a well-known fact that nowadays exchanges are operated automatically.

6.It was in 1869 that Mendeleyev published his Periodic Table.

7.It was the diaphragm that was placed very close to the poles of the magnet.

2. Complete the following sentences and translate them into Russian:

1.It was found that … .

2.It is known that … .

3.It will be shown that … .

4.It has been proved that … .

5.It can be said that … .

6.It must be known that … .

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3. Translate the following sentences paying attention to the – ing forms. Define what part of speech they are:

1.Scientific information is growing very fast.

2.Most people like travelling.

3.He objects to their taking part in this work.

4.Translate the text without using a dictionary.

5.Introduction the new method into practice requires much time.

6.We think of visiting his new experimental laboratory.

7.A barometer is an instrument for measuring the pressure.

8.The reading-room is overcrowded.

9.The students reading a book study in group.

10.The meeting was over.

11.You can improve your knowledge of English by reading more.

12.After finishing his work he helped us.

4. Fill in the blanks with the following prepositions:

in, at, to, from, after, on, past, till, before, by

1.Don’t be late. Come … nine sharp.

2.What time is it now? – It is a quarter … six.

3.Don’t be in a hurry. It is only a quarter … eight.

4.The lectures last … eight … the morning … two … the afternoon.

5…. 1869 Mendeleyev published his Periodic Table.

6.There is a delay … your telegram can reach the person it is addressed to.

7.… week-ends they go in for sports if the weather is fine.

8.The laboratory will have developed and improved telephone set

… the end of the year.

9.… several unsuccessful attempts, in 1866 Britain and America were linked by the underwater cable.

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Section 3.

WORDS HAVE MANY MEANINGS

Exact meaning is very important to scientists and engineers. They prefer to use technical terms when they can, because these terms do not carry old meanings with them. When something new comes along, new terms are invented with only one meaning assigned to them.

But even scientists do not always succeed in pinning down one word to one meaning. And, of course, scientists cannot avoid some words, such as “field”, ‘force”, that have been used a long time in ordinary affairs.

Most of the words in common use have more than one meaning and readers often become confused because they put the wrong meaning on a word. They assume that words are always used with the same familiar meanings.

We have to choose, from the many meanings of a word, the one meaning that fits what we are reading. We cannot assume that any word always has one meanings we are most used to.

EXERCISES

1. Two common ideas are given for each word. Below them the word is used in two sentences. Decide which idea applies to each sentence. Write its letter.

1. number

a) quantity or amount

b) the particular numeral assigned to anything in order to fix its place in a series

He lives in room number 16. The cost of sending a telegram depends on the number of words in it.

2. address

a) make a speech to

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b) send

Mr. Green will now address the meeting. There is a delay before the telegram can reach the person it is addressed to.

3. pass

a)go through

b)give by handing

The letter was passed round to all the members of the family. The vibrations from the violin string pass through the air in waves.

4. find

a) learn by study

b)get back after a search

It was found that electricity and magnetism were connected. Did you ever find that pen you lost?

5. move

a)change position

b)take action

The diaphragm moved inwards towards the magnet and outwards from it. Nobody seems willing to move.

6. place

a)put in a certain place

b)have

They place confidence in a party leader. He placed the diaphragm very close to the poles of the magnet.

7. current

a)stream of water, air, gas

b)flow of electricity

A cold current of air came in when the door was opened. The waves of electric current pass along the line and finally reach the receiver of the telephone.

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2. Read the sentences. Find the appropriate meaning for the words in bold type.

1.The chemical potential of sorbed water is a function of wood moisture content.

2.The bulk of water contained in the cell lumina is only held by capillary forces.

3.Water is a polar liquid.

4.If water vapour pressure in the ambient space is lower than vapour pressure within wood, desorption takes place.

5.Shrinkage and swelling may occur in wood when the moisture content is changed.

6.The chemical potential is explained here since it is the true driving force for the transport of water in both liquid and vapour phases in wood.

7.The gradient in chemical potential is related to the moisture content gradient as explained in above equations.

8.In comparison with capillary movement, diffusion is a slow process.

3. Arrange synonyms in pairs and translate them:

a)vapour, wood, water, material, content, pressure, comparison, period, mechanism, condition, equilibrium, energy;

b)phase, substance, steam, balance, tension, device, power, liquid, lumber, state, capacity, simile.

Section 4.

CHECK YOUR PROGRESS

1.Why cannot we assume that any word always has one meaning?

2.What is the best translation for the English emphatic construction

“it is …that”?

3.Give a summary of the main text of the unit.

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Unit 4

Section 1.

PRE-TEXT EXERCISES 1. Check up if you read these words correctly:

shrinkage [ˈʃrɪŋ.kɪdʒ], moisture [ˈmɔɪs.tʃər], acetic acid [əˌsiː.tɪkˈæs.ɪd], timber[ˈtɪm.bər], layer[ˈleɪ.ər], matter [ˈmæt.ər], unknown [ʌnˈnəʊn], material [məˈtɪə.ri.əl], opponent [əˈpəʊ.nənt], stable [ˈsteɪ.bl], split [splɪt], crack [kræk], hydroxylion [haɪˌdrɒk.sɪlˈaɪən], humidity [hjuːˈmɪd.ɪ.ti], equation [ɪˈkweɪ.ʒən]

2. Give the initial forms of the following words:

dried, lacks, lighter, determined, pests, subsequently, adds, necessarily, owes, kept

3. State to what parts of speech the following words belong:

according, generally, climatic, conditions, handling, rationalizing, higher, to prevent, porous, tissues, promptly, impregnation, importance, susceptible, described

4. Find the nouns in the following word-groups:

large number; under any conditions; the chief difficulty; in the field of chemical modification; dried wood is less susceptible to decay;

5. Translate the following word-groups into Russian:

drying time and timber quality; the dry-bulb and wet-bulb temperatures; timbers for impregnation; the electrical and thermal insulation properties; relative humidity; air circulation rate; at very low fan speeds; the key limitation to the movement of moisture

6. Check if you remember the following verbs:

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