GB2190179A

GB2190179A - Predrying kiln for timber

Info

Publication number: GB2190179A
Application number: GB08710547A
Authority: GB
Inventors: Kiski Matti Kippo; Boudewijn Victor Albe Dommelen
Original assignee: WHITTAKERS Ltd
Current assignee: WHITTAKERS Ltd
Priority date: 1986-05-02
Filing date: 1987-05-05
Publication date: 1987-11-11
Also published as: DE3714519A1; IT1204547B; IT8720331A0; GB8710547D0

Description

SPECIFICATION Predrying kiln for timber The present invention relates to a predrying kiln for timber. It is known to mount sawn timber in a predrying kiln so as to reduce the moisture content thereof prior to the timber being placed in a final dryer in which the drying process is completed. In the predryer air at a relatively low temperature of the order of 40[deg]C is passed over the timber whereas in the final dryer air of a much higher temperature is used. The predrying treatemnt removes water which can be fairly readily driven off whilst the final drying treatment proper removes chemically bound water. In the predrying kiln the sawn timber is typically placed in stacks and air is blown from one end to the other of the kiln to subject the timber to the predrying treatment. Also, finned heat exchanger tubes are placed between the stacks of timber to increase the heat input and obtain a satisfactory degree of predrying at a reasonable rate. However, it is found that as the air passes from the inlet end of the kiln to the outlet end of the kiln, the moisture content of the air increases. This reduces the tendency of the air to remove water from the timber and this reduces the efficiency of the process. Thus, the timber adjacent the inlet end is predried to a greater extent than the timber adjacent the outlet end. The present invention provides a pre-drying kiln in which the build up in the degree of saturation of the air is alleviated so as to maintain better the efficiency of the predrying treatment and to ensure that all of the timber is subjected to substantially the same amount of predrying. In accordance with one aspect of the present invention there is provided a predrying kiln for timber which comprises a chamber having a first, predrying section arranged to receive sawn timber in stacks, and a second air circulation section, a partition separating the sections of the chamber, an air inlet to the first section from the second section, an air outlet from the first section to the second section and fan means for establishing an air circulation through the second section, the air inlet, the first section and the outlet, wherein there is also provided a separate air circuit, means for withdrawing a portion of the air from the main air circulation into the separate air circuit, means in the separate circuit for adjusting both the temperature and moisture content of the withdrawn air,and means for injecting the so conditioned air back into the main air circulation between stacks of timber. The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic plan view of a predrying kiln of the present invention with a roof removed and with part of a partition broken away; Figure 2 is a sectional view along the line A-A of Figure 1; and Figure 3 is a sectional view along the line BB of Figure 1. In the drawings, there is shown a predrying kiln 10 comprising a chamber 12 defined by a floor 14, upstanding walls 16 extending around the periphery of the floor and a roof 18 mounted on the walls 16. The chamber 12 is divided into an upper section 20 and a lower, predrying section 22 by means of a partition 24 which is horizontally disposed and is mounted adjacent the upper end of the chamber 12. The predrying section 22 contains an air inlet 26 at one end of the partition 24 and an air outlet 28 at another end of the partition 24. The inlet 26 and the outlet 28 are located adjacent opposite ends of the chamber 12. An air handling housing 30 is also placed outside the chamber 12. The housing 30 comprises an air withdrawal duct 32 containing an inlet port, and a fan housing 34 containing a fan for drawing in a portion of returned air through the duct 32. The housing 30 also contains a heat exchanger 38 through which a heating medium is circulated to heat air that is being fed into the chamber 12. Further, the apparatus 10 also contains an outlet stack 40 in communication with the chamber 12 and an inlet stack 42 in communication with the duct 32 both of which stacks contain air dampers. The stack 40 is used to vent a quantity of returned air from the upper section 20 to atmosphere. The stack 42 is used to draw in a quantity of fresh air from the atmosphere into the duct 32. In this way the overall water content of the air in the chamber 12 can be controlled to be within desired limits. At the air entry end or inlet port of the ducts 32 an air flow resistance such as a filter pad or orifice plate is placed to facilitate the airflow into the duct 32. From the heat exchanger 38 an air duct 52 leads back into the chamber 12. The upper section 20 contains an upstanding dividing wall 48 mounted on the partition 24 and extending transversely across the chamber 22 to divide the upper section 20 into separate parts. A pair of fans 50 are mounted in the wall 48 and are arranged to circulate air from the outlet 28 in a closed circuit to the inlet 26, of the lower predrying section 22. The air duct 52 is a variable air supply duct which extends laterally and then longitudinally of the chamber 12 parallel to the general air stream. A plurality of branch, variable air supply ducts 54 extend downwardly from the longitudinal section of the duct 52. The ducts 54 are arranged to extend to a plurality of outlets between stacks of sawn timber contained in the predrying section 22. A pair of short branches 55 facing the fans 50 extend horizontally above the partition 24 from the lateral section of the duct 52. The branches 55 supply conditioned air to timber adjacent the inlet 26 (see Figure 2). The timber is typically stacked in known manner on hobbs and rails 56 or any other known manner of stacking the timber. In use, sawn timber to be predried is mounted on the hobbs and rails 56 in the predrying section 22 of the kiln 10. The ducts 54 extend between the stacks. When the kiln 10 is loaded it is closed off. The fans 50 are caused to operate so that air is circulated along the upper section 20 towards the inlet 26. The air then passes through the stacks of timber and travels through the predrying section 22 to the outlet 28. At the outlet 28 the air re-enters the upper section 20 and is drawn back into fans 50. Simultaneously, a portion of the return air is drawn by means of the fan 34 into the duct 32 through the stack 42 which is connected directly to the duct 32. The presence of a resistance at the air inlet port of the duct 32 reduces the pressure therein to below atmospheric pressure. Thus, air is drawn into the housing 30 via the stack 42. Simultaneously the chamber 12 acquires a pressure above atmospheric pressure and this enables air to be released readily through the stack 40. The mixed air enters the heat exchanger 38 in which it is heated to, for example, about 60[deg]C before entering the duct 52. The duct 52 directs air to the branch ducts 54. Air flows in the duct 52 and each of the branch ducts 54 are controlled by a damper. As described above the air tends to collect water and approach saturation as it approaches the outlet 28. It is desired to maintain the atmosphere inside the predrying section 22 at a level at which the dry bulb temperature is of the order of, for example, 40[deg]C whilst the wet bulb temperature is of the order of, for example, 37[deg]C. The kiln 10 also contains a plurality of spaced points monitoring means by which the humidity and temperature of the atmosphere in the kiln 10 can be ascertained at any time. In order to avoid the dry and wet bulb temperatures becoming too close at any point in the kiln conditioned air is fed along the ducts 52 and selected ducts 54 to inject relatively dry heated air into the predrying section 22 at desired points. The amount of air fed along a particular duct 54 is controlled by an air damper means to maintain the desired wet and dry bulb temperature differential. Further, means is provided for sensing the overall moisture content of the air adjacent the outlet 28. Depending on the overall moisture content of the air adjacent the outlet 28, a quantity is automatically vented to atmosphere through the stack 40 and a corresponding amount of fresh air is automatically fed in through the stack 42. This is controlled by the dampers in the stacks 40 and 42. The overall dry bulb temperature of the predrying kiln 12 is controlled by the temperature of the injected air heated by the heat exchanger 38. Modifications and variations such as would be apparent to a skilled addressee are deemed within the scope of the present invention. For example, discrete air circuits could be provided for separately adjusting the temperature of withdrawn air and the humidity of withdrawn air. Further, a number of kilns could be placed longitudinally side by side with a common division between adjacent kilns. The kilns would preferably be separately controllable in terms of temperature and humidity. In a further modification, the stacks of timber could be charged at a side of the kiln and removed at the opposed side whilst the main air circulation flows transversely to the direction of increment of the timber. In this embodiment, the timber would preferably be in continuous motion across the kiln during the predrying operation. Also, steam injection points may be provided in walls of the chamber 12 and in the duct 52 to enable steam to be injected into the chamber 12 to steam wood contained therein and induce stress free relaxation in timber in the chamber 12. This treatment of the timber is separate from the predrying treatment described above.

Claims

CLAIMS The claims defining the invention are as follows:

1. A predrying kiln for timber which comprises a chamber having a first, predrying section arranged to receive sawn timber in stacks, and a second, air circulation section, a partition separating the sections of the chamber, an air inlet to the first section from the second section, an air outlet from the first section to the second section and fan means for establishing an air circulation through the second section, the air inlet, the first section and the outlet, wherein there is also provided a separate air circuit, means for withdrawing a portion of the air from the main air circulation into the separate air circuit, means in the separate circuit for adjusting both the temperature and moisture content of the withdrawn air, and means for injecting the so conditioned air back into the main air circulation between stacks of timber.

2. A predrying kiln according to claim 1, in which an air handling housing is provided for conditioning withdrawn air, and an air withdrawal duct containing an inlet port in the chamber is arranged to withdraw air from the main air circulation and direct the withdrawn air into the air handling housing.

3. A predrying kiln according to claim 2, in which the air handling housing is located externally of the chamber.

4. A predrying kiln according to claim 2 or 3, in which the air handling housing contains a heat exchanger through which withdrawn air is passed to heat the withdrawn air prior to injection into the main air circulation.

5. A predrying kiln according to any one of claims 2 to 4, in which the air handling housing contains an air outlet stack and an air inlet stack, both of said stacks containing means for regulating the amount of air flowing through them at any one time.

6. A predrying kiln according to any one of the preceding claims, in which the fan means for establishing the main air circulation is located in the second section.

7. A predrying kiln according to any one of the preceding claims, in which means is provided for sensing the temperature and/or humidity between adjacent stacks of timber and means is also provided for controlling the amount of conditioned air which is injected between adjacent stacks of timber dependent on the sensed temperature and/or humidity.

8. A predrying kiln substantially as hereinbefore described with reference to the accompanying drawings.