Flexible modular kiln kit can broaden sawn timber sales
Wood-Mizer has launched its own modular kiln system to enable operators of its band sawmills to take a key step and add value to their sawn timber by curing it themselves. For simplicity’s sake, the company has developed the kit for kiln capacities from 4m3 to 20m3. The package is designed to include everything needed to continually and successfully dry loads of quality timber.
Modular components can be installed in standard 20-foot insulated chambers or a chamber can be added by the purchaser to deal with individually required lengths. Actual drying is by a widely used heat/vent system. The company has avoided complex computer controlled systems which often impose un-called for drying regimes. The aim is a flexible response to individuals’ varying requirements.
Wood-Mizer says that it recognises that each log and every board have different characteristics.
The role of its kiln is to respond to each different charge of timber with easily understandable operating schedules that take about five minutes daily to control.
In a business climate where the potential for sales of green boards is becoming limited, the drying of a sawyer’s own boards adds value to them and a new group of customers is reached. The stage after this would be to provide the local market with finished wooden elements like flooring, trim and mouldings produced on the company’s secondary timber processing equipment.
The WMDK modular kiln system’s pre-assembled, electrical control box contains thermostat, thermocouple and a wet/dry bulb thermocouple. The modular fan assembly, in standard 1,200mm sections, is preformed, preassembled and wired. It includes two 3ph 400V 50Hz AC circulation fans, heating elements with safety cut-out switches, cable tray, plenum baffle and wiring. The exhaust assembly includes a 220V AC exhaust fan, fan mounting module and exhaust control valve. Each track assembly includes one modular loading cart and the elements needed for a 1,200mm track section.
Wood-Mizer Drying Solution: Things To Remember
The drying schedules in the manual are to be used when drying timber in the Wood-Mizer Kiln.The schedules outlined are a general guide. However, many kiln operators use more severe drying conditions than those in the schedules while others dry at a more conservative rate.
For faster drying and best results - the species, thicknesses, and moisture content of the timber which take the same schedule should be segregated into (1) kiln whenever possible.
If it is necessary to dry various species and thicknesses of timber in the same kiln, always operate the kiln according to the greenest, thickest, and most difficult to dry species.
The kiln should always be full, or near full, when using schedules specified for species and thickness being dried. This will maximize your kiln efficiency and will help to ensure even and consistent drying.
Pieces which are square, or nearly so, or in dowel form in cross section, will dry more quickly than wider pieces of the same thickness.
Wide timber, ie. timber exceeding 300mm in width, is more apt to caseharden and surface check than narrower boards. For this reason, we suggest that when a large portion of the load is wide boards, that it be dried at a slower rate than narrow stock.
Quarter sawn timber requires a longer time for drying than plain-sawn timber of the same thickness and species.
If mould occurs on the timber, open the exhaust valve to reduce humidity. Mould thrives on near 100% humidity and temperatures between 18C and 38C.
Do not open the door unnecessarily. When the doors are open, you are upsetting the drying conditions and are also wasting heat. In the conditioning stage, it is especially necessary for the door to remain closed to contain the moisture vapor necessary for equalization.
Keep the kiln neat in appearance and in proper working order. Do preventative maintenance every time the kilns are unloaded and prior to re-loading. The life expectancy of the kiln will be affected if regular maintenance is not carried out.
Dimension stock of all sizes when dried as a dimension should be end-coated immediately after it is cut from the green slab or bolt. We especially recommend this for green oak, walnut, hickory, sycamore, and large hard maple dimension stock.
If punctures or holes occur in the skin of the kiln unit as a result of accidents or carelessness, please consult the manufacturer before attempts are made to repair. Some methods and materials would be better than others in the repair of such damage.
Wood-Mizer takes no responsibility for operator error.
Preparing timber to be placed in the kiln is one of the most important phases of the kiln drying process. If the following procedures are taken, the first step toward high quality, kiln dried timber will have been taken.
Timber must be sawn or planed to a uniform thickness. A variation in thickness causes:
- Timber to warp because thin boards between twothicker boards are not held flat;
- Thin boards to over-dry, while thicker boards may still have high moisture, unless the kiln charge is equalized for a longer period of time Unnecessary and valuable kiln space to be used as opposed to more volume of properly sized timber to be dried at the same cost and time;
- Stickers to be broken or cracked rendering them unusable Packages that are stacked high to lean or fall over.
Divider sticks (stickers)must be sawn or planed to a uniform thickness in order for even pressure to be applied to each stick which allows the timber to dry straight. Sticks should be placed directly over each other so that the weight transfer is straight down.
A GREEN BOARD IS VERY SUSCEPTIBLE TO BOWING UNDER WEIGHT WHEN A STICK IS JUST 20mm OUT OF LINE, AND WILL RETAIN THAT BOW IF IT IS DRIED IN THAT POSITION.
Sticks should be placed as close to the end of the board as possible. Since wood gives up moisture as much as (10) times faster from the end as it does from the face and edges, the stickers will prevent excessive air flow around the end of boards, which will slow the drying and help prevent end-splitting.
Coating the end of the boards will also help prevent end-splitting. Stickers should be a minimum of10mm thick to allow enough air to pass through a 1100mm wide package.
If packages are dried side by side, an additional 4mm in thickness should be used for each 1220mm in width, up to a maximum of about 30mm.
More than 10mm in a 1100mm package takes up space in the kiln. There should be a sticker placed at least every 600mm, or just above each bearer in the kiln. It is a good aid to build a stacking frame that will ensure that all your stacks are built to exactly the same dimensions and that your stickers and bearers are always in the same location and will be directly above and below each other at all times.
Hold-Down Bars or Weights
After the timber has been properly stickered, hold-down bars or weights should be placed directly above each row of stickers or across the stickers.These hold-down bars or weights, will hold the top layers of timber flat as well as letting a little air flow between the top layer of timber and the air baffle which lays over the timber. If using straps, they must be stretched enough to maintain downward pressure even after the timber dries and shrinks.
Loading the Kiln
After the package is properly prepared, it is ready to rolled or loaded into the kiln.It should be rolled in as close to the centre as possible to prevent damage to the door seals and to ensure equal airflow on each side of the package. The rows of stickers should all line up. If there are any voids on the ends of the package, they should be baffled in some manner to force all circulating air through the timber for even drying.
Turning Kiln On And Operating
It depends on the species, thickness, and the moisture content of the timber being dried as to which schedule will be used.
The Wood-Mizer manual will help you to determine the correct settings for your material. After using the kiln for a while, you will soon get the feel as to what setting should be used, depending on the species of timber and the thickness being dried.
Most soft woods give up moisture very easily, while some hardwoods - such as Oak, require low initial temperatures and a lot of time to dry properly. The three key elements in drying any specie of wood are; Temperature Speed with which the moisture is extracted from the kiln chamber Air circulation.
The third factor - air circulation - is needed to ensure uniform drying throughout the kiln.
Wood-Mizer kilns have fully reversible and speed controllable fans to ensure that right air-speed is used for your application.
Wood-Mizer Kilns are designed to be very simple and easy to operate, while maintaining very positive control of the three basic elements needed for the highest quality drying that it is possible to achieve. Schedules for the Wood-Mizer kilns have been formulated to achieve proper drying , from the very difficult 50mm oak to the very easy to dry thin softwoods. All species and thicknesses are placed in a schedule that is slower than the maximum that it can tolerate.
This not only improves the quality of the timber, but also increases the efficiency of the drying process. Wood-Mizer has listed most domestic species and thicknesses under the schedule we feel will give the best quality drying; however, after using the kilns for a while, you may wish to alter the schedule depending on the quality of the drying desired.
The difference in moisture content that exists between the centre of a board and the outside of the board. When the difference in moisture content between the inside and outside is too great, surface checking and casehardening will result.Therefore, the moisture gradient is one of the things to watch when drying timber. The lower the gradient is at the finish, the better the timber will work.
SURFACE CHECKS Caused by the moisture gradient becoming too great. This usually occurs when the timber is drying from the green stage down to 20% moisture content. However, it cannot always be detected in air-dried timber until the timber is placed in the kiln and the drying begins. Almost all air-dried oak, especially that stock cut in hot, dry weather, will contain surface checks. It also frequently occurs in the heartwood of other species.
Caused by a steep moisture gradient, but frequently cannot be detected until the centre of the board begins to dry. The outside of the board becomes dry in an expanded or stretched condition - due to being dried too rapidly - while the centre of the board is of much higher moisture content. It can be relieved by a higher humidity treatment which softens the surface and allows it to take its normal shrinkage.
HONEYCOMB OR HOLLOWHORNING
Follows severe casehardening which has not been relieved before the centre of the board becomes fully dry.
WARP AND TWIST
Sometimes caused by improper sticking and in some cases by drying the timber at too low of a humidity.
EMC or EQUILIBRIUM MOISTURE CONTENT - Also known as UWG.
The moisture content that the timber will come to and will remain indefinitely if left under any constant temperature and humidity. This is important for sheds or rooms where dry timber is to be stored for a long period of time.
RELATIVE HUMIDITY The amount of moisture that is in the air in relation to what it will hold at any given temperature. It is expressed as a percentage.
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