Wood Harvesting Equipment
The most important equipment to harvest woody biomass is undoubtedly the wood chipper, a machine that cuts the wood into small pieces with sharp tools like knives. There are several types of wood chippers:
- disc chippers
- drum chippers
- screw chippers
The market is dominated by the disc and drum chippers. There are very few screw chippersbeing produced.
There are also other machines that break the wood in small pieces: shredders and grinders. These machines do not produce wood chips but a material called hogfuel. Hogfuel is difficult to handle because of the rough surface and uneven length of the particles.
Each machine type has it's advantages and disadvantages and it's best application for the job.
Disc chippers
In a disc chipper, the knives are mounted in slots in a disc. The wood is cut against an anvil and once cut, passes through the disc, where casting plates will throw the chips out of the chute. The casting wings also generate a generous air stream, helping in blowing the chips out of the spout.A drawing the internal workings of a disc chipper can be seen in the link below.
DiscChipper1 (15Kb)
Since the knife slot can be rather large, often stick breakers are inserted in the slot opening. These stick breakers prevent overlong particles from passing though the slot opening.
The quality of the wood chips from a disc chipper is usually good. The particles are rather uniform in length and shape because the angle of attack of the blade towards the wood is more or less the same all the time. Because of the rotary motion of the disc, the wood is always pushed to the outside of the disc. This concentrates the wear on the blade to the outside 1/3 of the knife. The angle of attack is shown in the drawing below. The angle of attack of a disc chipper is typically around 37 degrees. At that angle experience has shown that the energy consumption of the chipper is at its lowest. The more oblique the chipper is cutting towards the fibre direction of the wood, the higher the energy consumption, see picture in link below.
DiscChipper2 (221Kb)
The size of the chips can be adjusted by pulling the knife further out of the disc, while adjusting the anvil at the same time.
Usually the wood is fed into the chipper by a set or sets of feeding rollers, that push the wood into the chipper. At first, when the knives are really sharp, the chipper pulls the wood in by itself, but as time passes, the knives loose their very sharp edge and the wood has to be helped in. The feedspeed has to match the chipping speed, otherwise the feed rollers will press the wood against the disc and then the wood will act as a brake.
Drum chippers
In a drum chipper the knives are mounted on theperifery of a large steel drum. In front of the knife there is typically a pocket, where the chips that have been cut are stored until they can be released behind or below the drum.The internal working of the drum chipper can be seen on the picture in the link below.
DrumChipper1 (290Kb)
A drum chipper does not have fan blades on the drum and therefore an extra fan has to be added to blow the chips out of the spout, or the chips can be dropped onto a conveyor belt.
A drum chipper produces irregular chips because the angle of attack of the blades towards the wood varies with the diameter of the wood. In a thick piece of wood, the angle of attack when the knives first hit the wood is 34 degrees, while at the bottom it might be 79 degrees. The top chips are nice and blocky, while the bottom chips are long and sliver like. This is shown in the next link.
DrumChipper2 (129Kb)
The size of the chips can be adjusted by adjusting the knives and anvil or by inserting a screen with larger or smaller diameter holes in the exhaust opening of the chipper. Particles which are too large to pass through the screen aretaken around once more and cut against the anvil.