Kiln treatment for sterilization of insects-infested timber Technical description of the certification of sawmill kilns


Full text


Boris Haj ek

Kiln treatment for sterilization

of insects-infested timber

Technical description of the

certification of sawmill kilns

Paper presented at the European COST Action E15

Workshop, Helsinki, Finland, June 11-13,2001



Boris Haj ek

KILN TREATMENT FOR STERILIZATION OF INSECTS-INFESTED TIMBER Technical description of the certification of sawmill kilns

Trätek, Rapport 10112033 ISSN 1102-1071 ISRN TRÄTEK - R — 01/033 — SE Nyckelord insects kiln drying sterilization wood Stockholm december 2001


Rapporter från Trätek - Institutet för träteknisk forsk-ning - är kompletta sammanställforsk-ningar av forskforsk-nings- forsknings-resultat eller översikter, utvecklingar och studier. Pu-blicerade rapporter betecknas med I eller P och num-reras tillsammans med alla utgåvor från Trätek i lö-pande följd.

Citat tillätes om källan anges.

Reports issued by the Swedish Institute for Wood Technology Research comprise complete accounts for research results, or summaries, surveys and studies. Published reports bear the designation I or P and are numbered in consecutive order together with all the other publications from the Institute.

Extracts from the text may be reproduced provided the source is acknowledges.

Trätek - Institutet för träteknisk forskning - betjänar sågverk, trämanufaktur (snickeri-, trähus-, möbel- och övrig träförädlande industri), skivtillverkare och bygg-industri.

Institutet är ett icke vinstdrivande bolag med indust-riella och institutionella kunder. FoU-projekt genom-förs både som konfidentiella uppdrag för enskilda företagskunder och som gemensamma projekt för grupper av företag eller för den gemensamma bran-schen. Arbetet utförs med egna, samverkande och ex-terna resurser. Trätek har forskningsenheter i Stock-holm, Växjö och Skellefteå.

The Swedish Institute for Wood Technology Research serves sawmills, manufacturing (joinery, wooden houses, furniture and other woodworking plants), board manufacturers and building industry. The institute is a non-profit company with industrial and institutional customers. R&D projekcts are performed as contract work for individual

indust-rial customers as well as joint ventures on an industrial branch level. The Institute utilises its own resources as well as those of its collaborators and outside bodies. Our research units are located in Stockholm, Växjö and Skellefteå.



Skadeinsekter i levande skog kan spridas genom torkat virke. Dessa insekter kan avdödas vid torkning av virke och därmed förhindra spridningen.

Nödvändiga torkningstemperaturer för avdödning av insekter Hgger inom temperatur-områdena för vanliga torkningsscheman i Sverige.

Det har konstaterats att uppvärmning av trämaterialet i hela tvärsnittet till 56° C i 30 minuter vid alla luftfuktigheter räcker för att avdöda dessa skadeinsekter.

Statens Jordbruksverk, Växtinspektionen i samarbete med Trätek certifierar sågverk som kan nå denna temperatur i virket under torkningen.

Torkningsscheman för olika typer av torkar (kammar- respektive kanaltorkar) måste god-kännas.




Boris Håjek AB Trätek Box 5609, S-114 86 Stockholm Sweden ABSTRACT

Some wood-destroying insects attack fresh or green wood. Fortunately all insects can be killed by exposure to temperatures which are within the range of most kiln-drying schedules.

Generally it has bin found that exposure to 56°C in the centre of the boards at the relative humidity appropriate to any drying schedules for 30 minutes will kill wood-destroying insects.

The Swedish Board of Agriculture in collaboration with Trätek certificate sawmills with regards to possibilities for sawmills to reach the approved temperature in boards.

Drying schedules for different types of kilns (batch and progressive) must approve.


Many countries outside the EC and some countries within the EC (protected zones) demand that imported lumber is free from insects.

One way to get the lumber free from insects is by heat treatment in kilns during the drying in "high" temperature.


According to the Swedish University of Agricultural Sciences, Prof. Åke Lindelöw, the temperature at which death occurs in insects depends on:


duration of exposure

interaction with other factors, especially humidity.

Large insects may withstand higher temperature by cooling through evaporation. Some die at +38°C at high humidity, but can survive up to +48°C i f the air is dry.

At exposure during a longer period, the effect of humidity is the opposite. At exposure to a dry environment, the insects die due to desiccation.

For small insects, the humidity does not affect the temperature when they die.

For most insects, the short-term exposure leading to death, is within the range 40-50°C.

In accordance with the above, the Swedish Board of Agriculture decided on a voluntary approval of kilns for heat treatment of wood. At the moment, timber for export is ocularly inspected in the harbour of export or at the exporting sawmill.

Conditions of approval

The following conditions for approval shall apply:

1. All sawn wood in the kiln shall be heated up to at least 56°C during 30 minutes for the whole basal area.

2. Control of kiln shall have been carried out to establish the fact that the requirements for a batch kiln or channels are available to reach a dry bulb temperature of at least 56°C during two hours at moisture content of 18-20%. In this connection calibration of temperature through the kiln temperature sensor shall have been made as well as a review with the personnel in charge.

3. A running documentation shall be carried out by the enterprise of all drying and marking that takes place in accordance with these statutory provisions. Calibration of temperature sensors in the kiln shall be carried out and documented by the enterprise every six months. 4. The enterprise shall be able to present a system, which guarantees that each package, or

the like has gone through drying process in accordance with these statutory provisions.


Only wood, which has been dried in an approved kiln, shall be marked.

Example of marking



2. T E C H N I C A L D E S C R I P T I O N O F T H E C E R T I F I C A T I O N O F S A W M I L L K I L N S 2.1 Type of kilns

2.1.1 Compartment (Batch) kilns

The compartment kiln is the original type of kiln. It offers full possibility of varying the temperature, the humidity and the flow velocity of the kiln air during drying.

At the passage of the kiln air through the timber pile, the temperature will fall on the exhaust side. The size of the temperature fall will depend on the moisture content of the wood. When the circulation air is reversed, the lowest temperature will be in the middle of the timber pile.

60 58 56 54 52 o 50 48 s U 46 a o. a 44 H 42 40 38 MC = 18% 27% 120 h STOP 90 h 0 - 3 0 h S T A R T

= 85 - 58% WET buib temperature


-1-1.5 3.0

Length, m 45


2.1.2 Progressive kilns

The progressive kiln is shown below.

—1 1 H -47 u 45 o A3 2? mt 41 3 •** SS 39 u V 37 Q. S 35 33 D R Y temperature W E T bulb temperature 15 Length, m 30

hi the kiln, the timber moves in the opposite direction of the circulation air. hi the progressive kiln, you cannot choose freely between drying schedules, but the choice is determined by the evaporation along the kiln.

The highest temperature, which is kept constant for each timber dimension, will be in the outtake end of the kiln.

2.1.3 2-step progressive kilns

The progressive kiln, with a temperature fall from the middle to both the intake and outtake end, is shown below.

DRY temperature

WET bulb temperature

N.B. A new type of kiln, the OTC kihi, has the same design as the kihi above but the circulation air blows in the opposite direction, that is from the middle towards the door.


2.2 Wood temperature

At the start of the drying of green wood with moisture content above the fibre saturation point, the wood temperature is equal to the wet bulb temperature of the circulation air.

The equipment for measuring the dry and wet temperature, that is the air humidity, is called psychrometer, see below.


With decreasing moisture content, the wood temperature will rise towards the dry temperature of the circulation air. The difference between the temperature of the air and that of the wood will thus depend on the final moisture content of the wood.

In order to achieve the 56 °C wood temperature across the whole cross section during

"normal" drying processes, a temperature of 2 °C higher, that is 58 °C, is required according to Trätek's measurings and judgement, see below.

Temperature of the air and in the middle off 22 mm

boards towards the end of the drying

tair tward 1 board 2 board 3 42 43 44 45 Drvina time, h 46 47

With regard to temperature falls in compartment kilns of different blow depths, the air temperature registered by the control unit equipment must be higher than 58°C, according to Trätek higher by 2°C, that is 60°C.

With regard to the inaccuracy of the temperature gauges (approx. ± 1-2 °C), the air

temperature should reach yet another 2°C higher, that is 62°C, in order to guarantee the 56°C level with a verv great certaintv over the whole cross section of the wood.


Finally, it was decided that the control measuring of the wood temperature will not have to be made on kilns with a drying temperature towards the end of the drying, at least 62°C for at east 2 hours.

At drying temperatures of between 56 and 62°C, the measuring of the drying temperature is performed at the same time as that of the wood temperature.

Of the measurings performed so far, all kilns in which the air temperature towards the end of the drying rose to at least 58°C were approved of.

With regard to this, the earlier approved lowest air temperature of 62°C (for approval without measuring of the wood temperature) could be lowered to 60°C with a maintained security that the wood temperature will rise to 56°C.

2.2.1 Measuring of the wood temperature

When the temperature of the drying air reaches 50-60°C, the measuring of the air temperature and the temperature of the wood should be performed.

AB Trätek, the Swedish Institute for Wood Technology Research, uses a mini data logger ( 4 x 7 cm, see below) containing a built-in thermistor for measuring of the air temperature. An extemal gauge could be connected to this logger. This gauge should be placed in a hole bored in the wood ( 0 = 8 mm). As a rule, the temperature will be registered in the air and in the wood during the whole drying process.

After finished drying, the logger should be sent to Trätek by post for printing of the registered temperatures.


2.3 Graplis showing the trends

Drying temperatures (dry bulb and wet bulb) are registered continuously during the whole drying process by a data control unit. These temperature courses are called trend graphs. The trend graphs should be at the approved level for the sawmill and the kiln or, as indicated earlier, the dry bulb temperature should reach 60°C or higher towards the end of the drying process.

Below is an example of a trend graph for a progressive kiln (constant maintaining of the temperature) and for a compartment kiln.

Progressive Kiln U o 1i tm 3 2 *i a E DRY temperature • i| ^yu ^1 i r ^ ^ j 1 1 • U 11:< Batch Kiln 90 U 75 o 2 a 30 !5 D R Y temperature * V W E T bulb temperature 120 Boris Håjek


2.4 Pitfalls

As earlier mentioned, the wood temperature is equal to the lower wet temperature when the moisture content of the wood is above the fibre saturation point.

In a 2-step progressive kiln, the highest temperature is in the middle of the kiln after one third or one half of the drying time. At this stage, the wood is still so humid that its temperature cannot rise towards the level of the dry temperature. For an example, see below.

fSBasnrFiÄSi assrm B ^ T u w m u 1 mnssam

B5S5ETB5 1

.•::'..:,T-is:a.'.it:.»v k - ^ / r r - r - i r X i c a m u A y : •c. l/iU.w'^' til I» 12 caVMa —

DRY temperature

Wood temperature

C» ( I M l u too MIB.M I2.(D;00 afr4>:00

With regard to this, the temperature should always be measured for approval of these types of kilns for drying of boards.

2.5 Calibration of the temperature gauges

Calibration of the gauges should be done in order to secixre that the temperature measurings are correct. This calibration of the gauges could be done in several ways:

1. Calibration with an electric thermometer 2. Calibration in boiling water

3. Calibration in ice-water

4. Comparison of all the dry and wet gauges in between themselves (after removal of the wet wick).

5. Built-in calibration with the help of computer


Detta digitala dokument skapades med anslag fran

Stiftelsen Nils och Dorthi Troédssons forskningsfond


• J S T I T U T E T F Ö R T R Ä T E K N I S K F O R S K N I N

Box 5609,114 86 STOCKHOLM Besöksadress: Drottning Kristinas väg 67 Telefon: 08-762 18 00 Telefax: 08-762 18 01 Vidéum, 351 96 VÄXJÖ Besöksadress: Universitetsplatsen 4 Telefon: 0470-72 33 45 Telefax: 0470-72 33 46 Hemsida: http://wwv^ Skena 2, 931 77 SKELLEFTEÅ Besöksadress: Laboratorgränd 2 Telefon: 0910-58 52 00 Telefax: 0910-58 52 65





Relaterade ämnen :