WO2015072919A1

WO2015072919A1 - A method and system for drying wood in a drying compartment

Info

Publication number: WO2015072919A1
Application number: PCT/SE2014/051367
Authority: WO
Inventors: Thomas Gustafsson; Eric BJÖRKMAN
Original assignee: Coldbay Ab
Priority date: 2013-11-18
Filing date: 2014-11-18
Publication date: 2015-05-21
Also published as: SE1351363A1; CA2930233A1; EP3071912B1; EP3071912A4; CA2930233C; PL3071912T3; EP3071912A1; SE538180C2

Abstract

A system and a method for drying wood in a drying compartment, comprising providing a flow of air over the wood in the drying compartment, measuring parameters related to temperature and humidity in the flow of air, iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on an estimate of the humidity of the wood and the air flow speed over the wood, comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity, iteratively adjusting the estimated humidity of the wood in the calculation, and providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood.

Description

A METHOD AND SYSTEM FOR DRYING WOOD IN A DRYING COMPARTMENT

Technical field

[0001 ] The present invention relates generally to a method and a system for drying wood in a drying compartment.

Background art

[0002] When drying wood in a drying compartment, such as a batch kiln or a progressive kiln, bundles of sawn timber are arranged in the compartment and a continuous and uniform flow of air is provided throughout the timber bundles.

Systems for wood drying are e.g. disclosed in SE 512988 C2 and SE 1200230 A1 .

[0003] The rate of drying depends on properties of the wood, air flow and other climatic conditions in the drying compartment. Heat is provided to the drying compartment to speed up the process of drying, but this consumes energy and is therefore costly.

[0004] In SE 528467 C2, a method for drying wood is disclosed wherein the wood is subjected to periods of hard drying, during which periods air change is effected, and that between the periods of hard drying the wood is subjected to periods of less hard drying having essentially less air change.

[0005] There is a need to optimize the drying of wood in order to provide sawn and dried timber of good quality while minimizing energy consumption and processing times during drying. In particular there is a need to be able to indicate the humidity in the wood in order to optimize the drying process.

Summary of invention

[0006] An object of the present invention is to provide a method and a system able to indicate the humidity in the wood in order to optimize the drying process.

[0007] Thus the invention relates to a method for drying wood in a drying compartment, comprising providing a flow of air over the wood in the drying compartment, measuring parameters related to temperature and humidity in the flow of air, iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on based on an estimate of the humidity of the wood and the air flow speed over the wood, comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity, iteratively adjusting the estimated humidity of the wood in the calculation, and providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood.

[0008] The invention further relates to a method for indicating the humidity in wood in a drying compartment, comprising providing a flow of air over the wood in the drying compartment, measuring parameters related to temperature and humidity in the flow of air, iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on based on an estimate of the humidity of the wood and the air flow speed over the wood, comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity, iteratively adjusting the estimated humidity of the wood in the calculation, and providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood.

[0009] The parameters related to temperature and humidity in the air may comprise a dry-bulb temperature and a wet-bulb temperature.

[0010] The calculation of the expected dry-bulb temperature and wet-bulb temperature may further be based on an amount of heat provided to the drying compartment.

[001 1 ] The calculation of the expected dry-bulb temperature and wet-bulb temperature may further be based on a flow of fresh air provided into the drying compartment.

[0012] The calculation of the expected dry-bulb temperature and wet-bulb temperature may further be based on an amount of humid air evacuated from the drying compartment. [0013] The dry-bulb temperature and wet-bulb temperature may be measured in the air flow before and after passage over the wood in the drying compartment.

[0014] The flow of air may be a circulating flow of air in the drying compartment.

[0015] The method may comprise changing parameters of temperature, air flow and/or humidity of the air, thereby exciting the climate in the drying compartment. The climate in the drying compartment may be persistently excited, e.g. by means of changing parameters of temperature, air flow and/or humidity of the air.

[0016] The method may further comprise indicating that the drying may be discontinued when the indicated humidity of the wood reaches a predetermined threshold or is within a predetermined interval. Thus the drying may be

discontinued when the humidity of the wood is has been lowered to a certain level (e.g. within the range 15-20 % or 17-19 %, preferably below 18 %) to reduce the risk of over- or underdrying of the wood.

[0017] The method may further comprise controlling heat provided to the drying compartment, the flow speed of air over the wood, a flow of fresh air provided into the drying compartment, a flow of humid air evacuated from the drying

compartment, or combinations thereof, based on the indicated humidity of the wood during drying. Thus the energy consumed during drying and the overall drying time may be minimized by controlling the operational parameters based on the indicated humidity of the wood, during the drying process.

[0018] The drying compartment may be a batch kiln or a progressive kiln.

[0019] The invention further relates to a system for drying wood in a drying compartment, comprising a fan for providing a flow of air over the wood in the drying compartment, a sensing means for measuring parameters related to temperature and humidity in the flow of air, a processing unit configured to iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on based on an estimate of the humidity of the wood and the air flow speed over the wood, comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity, iteratively adjusting the estimated humidity of the wood in the calculation, and providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood.

[0020] The invention further relates to a system for indicating the humidity in wood in a drying compartment, comprising a fan for providing a flow of air over the wood in the drying compartment, a sensing means for measuring parameters related to temperature and humidity in the flow of air, a processing unit configured to iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on based on an estimate of the humidity of the wood and the air flow speed over the wood, comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity, iteratively adjusting the estimated humidity of the wood in the calculation, and providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood.

[0021 ] The invention further relates to a computer program product comprising readable code means, which when run in a processing unit causes the processing unit to perform the method as disclosed herein.

Brief description of drawings

[0022] The invention is now described, by way of example, with reference to the accompanying drawings, in which:

[0023] Fig. 1 shows a batch kiln comprising a drying compartment for drying wood.

[0024] Fig. 2 cross section of a wood bundle in a drying compartment. [0025] Fig. 3 shows a side view of a wood bundle.

[0026] Fig. 4 shows an element i in a single half channel of a wood bundle.

[0027] Fig. 5 shows a progressive kiln comprising a drying compartment for drying wood. Description of embodiments

[0028] Fig. 1 shows a system in the form of a batch kiln 101 comprising a drying compartment 102 for drying wood. Wood is organized in a number of bundles 103 in the drying compartment. The kiln comprises a fan 104 for providing a circulating flow of air 105, 105' over the wood, and heating means 106 for heating the air. An evacuation duct 1 1 1 is provided to be able to evacuate humid air and to provide fresh air into the drying compartment when needed.

[0029] A sensing means in the form of a set of thermometers 107a, 107b, 108a, 108b is arranged in the drying compartment for measuring the dry-bulb

temperature and wet-bulb temperature of the circulating air. The wet-bulb temperature indicates the moisture content in the air. Alternatively any

measurements related to the temperature and moisture content in the air may be made. The thermometers may be arranged to measure the dry-bulb temperature and the wet-bulb temperature in air before (107a, 107b) and after (108a, 108b) passing the wood bundles, but a single point of measurement suffice.

[0030] The system further comprises a control unit 109 connected to the fan 104, the heating means 106 and the thermometers 107a, 107b, 108a, 108b, and comprising a processing unit 1 10. The processing unit is configured to iteratively calculating expected values of the dry-bulb temperature and the wet-bulb temperature in the flow of air based on based on an estimate of the humidity of the wood and the air flow speed over the wood. These calculated expected values of the dry-bulb temperature and the wet-bulb temperature are compared with the the measured dry-bulb temperature and the wet-bulb temperature from the set of thermometers. In the iterative method, the estimated humidity of the wood in the calculation is changed such that to adjust the calculated expected values of the dry-bulb temperature and the wet-bulb temperature to the measured dry-bulb temperature and the wet-bulb temperature. A converging estimated humidity of the wood is thereafter supplied as an indication on the actual humidity in the wood. The iterative method is described in further detail below. [0031 ] In a drying compartment the wood is organized in bundles with spacing sticks. Fig. 2 shows a cross section of a wood bundle 203 with channels made by spacer sticks. A single channel 212 is marked with a dashed rectangle. The wood is assumed to have a uniform thickness of 2 χ d. The distance between two sticks is I and the height of a stick is 2 χ h. The bundles are stacked together to completely fill the cross section of the drying compartment. The spacer sticks allows for the drying air to flow over the surface of the wood through the channels that are made by the spacing sticks 213, see Fig. 2. Fig. 3 shows a side view of a wood bundle 303 with a single channel 312 through the bundle. It is assumed that the flow velocity is uniform over the wood surface and that the average condition for the channels may be represented by one single channel, marked as a dashed rectangle 312 in Fig. 3.

[0032] A single channel 212, 312 consist of the air channel between two wood surfaces and the adjoining wood up to half the thickness of the wood plank or board. The single channel is divided into m elements in the main direction of the air flow. In an element the upper and lower wood plank is divided into n segments from its surface to its center, while the air volume between wood planks is divided into two segments, see Fig. 4, showing an element i in a single half channel 412 in a wood bundle.

[0033] The wood package has length L, width W and height H. The width is measured along the main direction of the air flow. The air volume in an element is V_a = I x h x w, where w = W/m. The wood volume in an element is V_w = I χ d χ w.

[0034] In the following, an embodiment of the method of indicating the humidity in the wood is described. The state in the drying compartment outside the wood bundles is defined by the dry bulb temperature T°₀ and the moisture content X°₀, related to the wet bulb temperature.

[0035] The state in air segment i is defined by the dry bulb temperature TO and the moisture content X'₀. [0036] The state in wood layer j in segment i is defined by the temperature T'_j and the moisture content X'_j.

[0037] The airflow through an element has the speed v and with the cross section area of the air segment as A_cs = I ^x h gives that the mass flow of air through the element is vA_csp_a where p_a is the density of the air.

[0038] The energy rate balance for an air segment is assumed to depend on air transport, evaporation q_eva_P and convection q_COnv heat transfer rates between the drying air and the wood surface (Eq. 1 );

[0039] The heat transfer rate due to evaporation is assumed to depend on the latent heat of vaporization L_water and the specific water vapor mass flow m[dot]_v such that (Eq. 2)

Qev = L_wat< ^, _Tm_vl w

[0040] The specific mass flow rate is depending on temperature and moisture content in the drying air TO, XO and in the wood surface ΤΊ, X

[0041 ] The heat transfer rate due to convection is (Eq. 3)

[0042] Equations (1 )-(3) gives the state space equation (Eq. 4)

^{U J}- 0 i-0 ,- -pi _v £ rp-i γ-% ,.{} ··,

[0043] where f⁰, is a nonlinear function depending on the state variables and the parameters p⁰,. [0044] The energy rate balance for the wood surface segment in element i is depending on evaporation, convection, diffusion and condensation (Eq.5) w*w<^* w^— — — — ((cond f <i ift

(it

[0045] The heat transfer rate due to diffusion is assumed to be (Eq.6)

[0046] The heat transfer rate due to condensation is assumed to be (Eq.7) q_amd ~ ki ':^'/:> - T_j)

[0047] The energy rate balance for the interior wood segment j in element i is depending on condensation and diffusion (Eq.8)

[0048] Equations (Eq.5)-(Eq.8) give the state space equation (Eq.9)

ΡΓ' . . . . . .

— ρ(ητι γ* ητ% Y* _r \

at

[0049] where f^j, is a nonlinear function depending on the state variables and the parameters pV

[0050] The balance equation for mass rate transfer in the air segment in element i is (Eq.10)

[0051 ] The balance equation for mass rate transfer between air and wood in element i is (D being a diffusion coefficient) (Eq.11) d X

/■ * p X^ Χ{)Α_Χ - m w

dt

[0052] The balance equations for mass rate transfer between interior wood segments are (Eq.12)

[0053] The dynamics for the vector valued state variables T and X can be summarized in a vector valued state equation (Eq.13-14)

[0054] where the state vectors T = [T°₀... T^m _n] and X = [X°₀... X^m _n] each has (n + 1 ) x m + 2 elements and P is a vector of unknown parameters.

[0055] The values of the state vectors T and X are unknown except for T°₀ and X°₀that are continuously measured.

[0056] An estimate (^ΛΤ and ^ΛΧ) of the state vectors and unknown parameters ^ΛΡ are calculated as (Eq.15-17) df

/(/ . , P) + is tip; - if: ÷ K₂(x$ - ,

di

- if) + K₄( :$ - .v; i

dP

di ⁱ a o -'-0 / T^" i ¾ .-VQ ^ΛΟ) [0057] The coeffcient vectors K-i , K_2, K_3, K4_, K_5, KQ are selected such that the estimation errors ei = ^ΛΤ - T and e₂ = ^ΛΧ - X converge asymptotically to zero. Thus the converging moisture content X (the humidity) of the wood may be provided as an indication of the actual humidity of the wood, in order to control the drying of the wood.

[0058] In the method, the climate in drying compartment (T°₀ and X°₀) is excited by changing parameters of temperature and/or humidity of the air. In a preferred method the climate is changed such that the system may be considered to be persistently excited.

[0059] One example of providing an excitation of the climate in the drying compartment, such that the climate changes are persistently exciting, is disclosed in SE 528467 C2. In this document a method for drying wood is disclosed wherein the wood is subjected to periods of hard drying, during which periods air change is effected, and that between the periods of hard drying the wood is subjected to periods of less hard drying having essentially less air change. Similar excitation of the climate in a drying compartment during wood drying may be performed in batch type of kiln and a progressive kiln.

[0060] A system in the form of a progressive kiln is shown in Fig. 5, comprising a drying compartment 502 for drying wood. Wood is organized in a number of bundles 503 progressively moving through the drying compartment. The kiln comprises a fan 504 for providing a circulating flow of air 505, 505' over the wood, and heating means 506 for heating the air. An evacuation duct 51 1 is provided to be able to evacuate humid air and to provide fresh air into the drying compartment when needed.

[0061 ] A sensing means in the form of a set of thermometers 507a, 507b, 508a, 508b is arranged in the drying compartment for measuring the dry-bulb

temperature and wet-bulb temperature of the circulating air before (507a, 507b) and after (508a, 508b) each wood bundle. [0062] The system further comprises a control unit as described in relation to Fig. 1 , connected to the fan, the heating means and the thermometers and comprising a processing unit. Thus the climate in the drying compartment may be excited and the humidity in the wood in the bundle may be indicated by the method as disclosed herein.

Claims

1 A method for drying wood in a drying compartment, comprising providing a flow of air over the wood in the drying compartment,

measuring parameters related to temperature and humidity in the flow of air, which parameters comprise a dry-bulb temperature and a wet-bulb temperature, iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on an estimate of the humidity of the wood and the air flow speed over the wood,

comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity,

iteratively adjusting the estimated humidity of the wood in the calculation, providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood, and

the drying is discontinued when the indicated humidity of the wood reaches a predetermined threshold.

2 The method according to claim 1 wherein the calculation of the expected values of the parameters related to temperature and humidity is further based on an amount of heat provided to the drying compartment.

3 The method according to any one of the preceding claims wherein the calculation of the expected values of the parameters related to temperature and humidity is further based on a flow of fresh air provided into the drying

compartment.

4 The method according to any one of the preceding claims wherein the calculation of the expected values of the parameters related to temperature and humidity is further based on an amount of humid air evacuated from the drying compartment. 5 The method according to any one of the preceding claims wherein the values of the parameters related to temperature and humidity are measured in the air flow before and after passage over the wood in the drying compartment.

6 The method according to any one of the preceding claims wherein the flow of air is a circulating flow of air in the drying compartment.

7 The method according to any one of the preceding claims comprising changing parameters of temperature, air flow and/or humidity of the air, thereby exciting the climate in the drying compartment.

8 The method according to claim 7, wherein the climate in the drying compartment is persistently excited.

9 The method according to any one of the preceding claims further comprising controlling heat provided to the drying compartment, the flow speed of air over the wood, a flow of fresh air provided into the drying compartment, a flow of humid air evacuated from the drying compartment, or combinations thereof, based on the indicated humidity of the wood during drying.

10 The method according any one of the preceding claims wherein the drying compartment is a batch kiln.

1 1 The method according to any one of the claims 1 -9 wherein the drying compartment is a progressive kiln.

12 A system for drying wood in a drying compartment, comprising a fan for providing a flow of air over the wood in the drying compartment, a sensing means for measuring parameters related to temperature and humidity in the flow of air, which parameters comprise a dry-bulb temperature and a wet-bulb temperature,

a processing unit configured to iteratively calculating expected values of the parameters related to temperature and humidity in the flow of air based on an estimate of the humidity of the wood and the air flow speed over the wood, comparing the measured parameters related to temperature and humidity with the expected parameters related to temperature and humidity,

iteratively adjusting the estimated humidity of the wood in the calculation, and providing a converging estimated humidity of the wood as an indication on the actual humidity in the wood, and

13 A computer program product comprising readable code means, which when run in a processing unit causes the processing unit to perform the method according to any one of claims 1 -1 1 .