DRYING WOOD

Traditional Method vs. Heat Pump

Francesc Pérez

June 21, 2001

Drying wood

INDEX

INTRODUCTION........................................................................................................................................3

WHY DRY WOOD?.................................................................................................................................... 4

PROBLEMS IN DRYING WOOD.............................................................................................................5

DRYING WOOD......................................................................................................................................... 5

THE PRODUCT................................................................................................................................................5

THE TRADITIONAL METHOD..............................................................................................................................6

DRYING BY HEAT PUMP................................................................................................................................... 6

STAGES OF DRYING WOOD................................................................................................................................9

ADVANTAGE OF THE HEAT PUMP FOR THE DRYING WOOD...................................................10

APLICATION IN SPAIN.......................................................................................................................... 10

BIBLIOGRAPHY...................................................................................................................................... 12

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Drying wood

INTRODUCTION

The drying of products, understanding as such the dehydration of the

same ones, it is a very old technique used for the conservation of the foods and

other applications like it is the wood. This drying was carried out with the use of

the solar energy and with the wind. This technique still comes using, although

most of the processes require of this basic operation in a more industrial way.

The certain thing is that, in spite of being an old technology, it is not highly

developed and until few years ago these industrial processes were quite

imperfect and they gave insatisfactorials products as a result.

A high control of the variables that to be part of the drying process:

temperature and relative humidity of the air, and speed of circulation of the air,

they are indispensable to obtain a product of quality and homogeneous. Today

in day the regulation systems and control, they offer the possibility of very fine

regulations that it will allow to be able to offer very sure and versatile systems.

The heat pump is presented like one of the most appropriate

technologies for the drying of products thanks to that not only allows the control

of the drying temperature, but also of the relative humidity of the camera, with

independence of the external conditions.

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Drying wood

WHY DRY WOOD?

Some important reasons to dry wood include:

1. Better usability. Wood shrinks as it loses moisture and swells as it gains

moisture. It should be dried to the % MC it will have during use.

2. Reduced shipping costs. Dry wood weighs less (drying may reduce its

weight by one-half or more). It is more profitable to transport wood than

water.

3. Less likelihood of stain or decay during transit, storage, and use.

4. Reduced susceptibility to insect damage.

5. Increased strength. As wood dries below 30% MC, most strength properties

increase.

6. Better “hold”. Nails, screws, and glue hold better in seasoned wood.

7. Better finishing. Paints and finishes adhere better to seasoned wood.

8. Better heat insulation. Dry wood is a better thermal insulator than wet wood.

9. Better preservation. Dry wood must be used when treating with most wood

preservatives.

10.Added value. Drying the wood products before shipment adds value to the

product.

Wood

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Drying wood

PROBLEMS IN DRYING WOOD

There are some negative aspects to drying wood, including:

1. The great amount of energy that must be expended to drive the water out of

wood. As much as 80% of the total energy requirement for a sawmill can be

used in the drying operations.

2. The possibility of drying defects. As wood dries, it shrinks in several

dimensions. If wood is not correctly dried, the dimensional changes will

cause drying defects, including checks, splits, warp, casehardening, and

honeycomb.

Some explanation of these two items is warranted because of their

importance in the wood drying process.

DRYING WOOD

The product

The wood is a fabric, alive and no inert whose balance is bound to the

vital functions of the tree. Therefore, their degradation begins with the pruning

of the tree. From this moment it becomes necessary a new balance with the

means, so that one can obtain an useful and durable product. This way, the

quality, duration and properties of the products elaborated with wood, they will

be in direct relationship to the drying process that has suffered the same one.

The two conditions that they should be completed for the drying of these

products they are:

1. to get an appropriate humidity, in function of the application.

2. to distribute the humidity evenly, don't take place tense.

The drying in closed circuit of air, with control on the temperature and

humidity of the drying camera, one of the improvement technologies supposes

for the drying of the wood. This is gotten by means of the industrial application

of the heat pump.

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Drying wood

The traditional method.

The air, impelled by the fans, crosses the heating system, where it warms

until the fixed temperature; later on it goes by a humidification system that

allows that it becomes moist if it is too dry. Once acclimatized, the air goes to

the piles, so that it crosses the free spaces that are among the heaped checks.

As the air crosses the piles, it leaves loading of humidity and diminishing its

temperature.

Once the air, cold and humid it is to the other side of the wood load the

fans, to complete the circuit and repeat the previous process again absorb it. In

the return circuit, a part of the air with humidity is loaded to the exterior, for

ventilation peepholes, sometimes helped by extraction fans. At the same time, it

enters to the external air, it makes lower the temperature and the relative

humidity of the interior. It fits to point out that although the external air has a

very high relative humidity, the thermal elevation of the system usually makes

that it lowers its humidity strongly, for what the relative humidity of the dry

descends. In occasions, to minimize the losses of heat in a dry, an exchanger is

located between the entrance air and that of exit.

Drying by heat pump.

The application of the technology of heat pump to the drying of the wood

allows the use of a closed circuit of air in the process, since when taking

advantage the possibility of condensation of water on the part of the heat pump,

so that it is not necessary the entrance of external air to maintain the relative

humidity of the camera. The circuit will be the following one: the air that has

passed through the wood (cold and loaded with humidity) it is made it passed

through of a evaporator (cold focus) for it passes the refrigerant (R-134a) in

liquid state to low pressure. The air-cools down until it arrives to the point of it

dewed and it condenses the water that has separated the wood.

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Drying wood

The heat given by the water when passing from state vapour to liquid

state is picked up by the refrigerant that passes to vapour to low to pressure.

This refrigerant in gas state is made pass through a compressor, so that we

have the refrigerant in gas state and high pressure, and therefore high

temperature that takes advantage to heat the same drying air and to close the

cycle. This way we have hot and dry air that makes pass through the wood

again. The great importance of this cycle is due to that when not making that big

quantities of external air enter, don't break the balance achieved by the wood,

and tensions don't take place, so that a drying of high quality is achieved.

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Drying wood

Installation of Heat Pump in drying wood

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Drying wood

Stages of drying wood.

The drying of the wood is carried out in closed cameras, for lots, making

go the camera by different stages:

1. Preheating stage: It has an approximate duration of 20 h. In this stage it

warms the wood. This initial process of heating slow improvement the quality

of the final drying and the beginning of the operation of the heat pump, since

are not connected until the temperature of the camera has reached a pre-set

temperature, with an appropriate level of humidity.

2. Dehumidification stage: The heat pump enters in performance, to eliminate

the water coming from the wood, with temperatures among 55-65ºC: For

each type and size of the wood a curved of drying settle down, so that they

leave advancing stages, every time with bigger temperature and smaller

relative humidity in the atmosphere, conform to it goes advancing in the

drying of the wood. This way, the wood doesn't suffer excessively high

gradients. The circulation of the air is invested each 12 h, so that the biggest

possible uniformity is obtained.

3. Stabilization stage: The cooling of the camera takes place in a controlled

way, so that the wood doesn't suffer abrupt crashes that can cause twists in

the fibbers or fissures.

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Drying wood

ADVANTAGE OF THE HEAT PUMP FOR THE DRYING WOOD

1. Better drying quality, when controlling in a reliable and rigorous way the

humidity of the camera.

2. Smaller cost of operation, saving until 60% of the necessary thermal energy.

3. Bigger security in the operation.

4. Null environmental impact, without emissions to the atmosphere.

APLICATION IN SPAIN

The heat pump in the drying of the wood this developed by STC

(Sistemas de Transferencia de Calor) located in Avda. Siblings Bou, s/n, Box.

1157 - 12080 CASTELLÓN (Spain).

STC, S.A. is a Spanish company been founded with the objective of

investigating, to develop and to manufacture drying teams, for very diverse

products, applying technologies that allow a high drying quality, combined with

measures of energy saving and I respect to the environment.

The drying operation is a very delicate process in which it should be

conjugated all the factors that intervene in the same one appropriately. STC is

specialized in the industrial application of heat pump, system qualified by the

European Union as technology of energy saving. Also, the teams STC can use

other sources of residual heat, for requirements of the facilities.

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Drying wood

Equipment of STC

The quality and duration of the products manufactured in wood will be in

very direct relationship with their drying. A good drying must complete two

conditions fundamentally: to get the appropriate humidity, depending of the

application, and to make that their allotment is uniform, not generating fissures

neither tense in the planks. STC manufactures teams of wooden drying

designed especially to complete the most demanding drying requirements. The

drying is carried out to low temperature, between 50 and 65ºC. These teams

are constituted by system of heat pump, manufactured with materials of first

quality, according to the normative one more demanding.

Reversible ventilation with units of high yield and under consumption.

Square of integrated control, sent by PLC that executes the drying orders with

continuous measures of the camera conditions, with control possibility at

distance and/or via modem. The cameras STC are the best complement for the

drying team. Manufactured with panels of polyurethane isolation and interior-

external foils of aluminium. Profile of aluminium with break of thermal bridge of

own design they frame the panel. The door, folding horizontally, he/she opens

up in vertical by means of automatic mechanism, giving load access in the

whole front of the camera (between 6,0 and 12,5 m).

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Drying wood

When working in closed circuit a great energy saving it is obtained, since

it is not necessary to heat the external air. At the same time, we can control the

humidity of the camera and to maintain it high during great part of the drying

process. Under these conditions the surface of the wood is not dried up and the

chart gives the water with more easiness, getting this way a quick drying and of

great quality.

BIBLIOGRAPHY

James E. Reeb, Wood Products and Utilization Specialist

Website of STC, S.A. http://www.stcsa.es

Technical days in industrial applications of the heat pump.

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