1. THE REQUIRED EXTRACTION CAPACITY
Woodworking machines must be designed so that at a minimum air speed of 20 metres per second, exposure to wood dust remains below the MAC value of 2 mg/m3. (The Maximum Acceptable Concentration is the legal limit for exposure to harmful substances).
In practice, an air speed of 25 m/s is used for standard machines. Multi-axis machines for solid woodworking, such as four-sided planing machines and tenoning machines often require 28 m/s, and larger machines for sheet metal working (CNCs, dividing saws and edgebanders) usually require 30 to 35 m/s of air speed.
The air velocities mentioned are the values measured in the diameter of the suction opening, not those in the opening of any hood present on the machine. In that, speeds are usually lower.
The documentation of the relevant machine manufacturer should always be checked for correct specifications.
However, the determination of extraction capacity is not so much about the airspeed (all Riedex extraction units more than achieve that), but to achieve the total required air volume. For this, the diameters of the intake openings on the machines should be included in the calculation.
The table below gives the air volumes (flow rates) for different diameters of inlets for applicable air velocities. The unit used is m3/hour. For flow rates in m3/minute, the values should be divided by a factor of 60.
To determine the exact capacity required, one therefore first looks at the diameter of the suction nozzle(s) on the machine and then chooses the air speed according to the machine specifications, or according to the above-mentioned purpose of use. If a machine has several suction nozzles, it is important to determine whether they should be extracted simultaneously or separately.
Example: a circular saw has 2 extraction nozzles, 120 mm under the table and 80 mm on the protective hood. Both are extracted simultaneously. It is a standard machine, so the air speed should be 25 m/s. The table shows that the required extraction capacity is then 1018 + 452 = 1470 m3/hour.
Adding the capacities of these machines gives the minimum extraction capacity that an extraction unit must have. (So do not simply add up the capacities of all connected machines, as each machine must be fitted with a shut-off valve, which must be closed when not in use).
2. THE FREQUENCY OF USE
It will be obvious that an extraction unit located in the sawing department of a DIY store will not have to endure as much as the same unit deployed in the factory sawing of plasterboard. The more dust the unit extracts, the greater the air resistance of the filters will be and the more the extraction capacity will be limited. The actual extraction capacity moves between two limits: maximum and minimum volume (see our extractors' product brochures). Not only the volume of dust, but also the switch-on time plays a role in this. During operation, the dust settles as a layer with a thickness of up to several millimetres on the filter elements. This dust layer creates air resistance; the filter material itself has hardly any resistance. When expanding, the air pressure drops and the filter elements relax. The dust layer then breaks off, assisted by the filter cleaning mechanism. Thus, the more often a unit is switched on and off, the better the air passage of the filters.In practice, when selecting an extraction unit, one assumes maximum volume for intermittent use and minimum volume for long-term, intensive use.For extremely intensive situations (especially when extracting fire-resistant board material), one can opt for continuous filter cleaning during operation ("online" cleaning option).
3. THE MATERIALS TO BE EXTRACTED
Whereas previously only "normal" wood, chipboard and later some MDF were processed, nowadays the number of different materials has become enormously diverse and, consequently, there has also come a lot of variation in the properties of released chips and dust.Here, when choosing the right extraction unit, similar guidelines apply as in the previous section. If the waste being extracted is mainly coarse in nature, then the maximum volume is assumed. If, on the other hand, it consists mainly of fine dust, then the minimum volume.
If the waste contains aluminium particles, other filter material should be used (i.e. ultrafibre) and if the dust is silica-containing (Promatect, Fermacell etc.), Teflon-coated filter material is required. Feel free to ask us for further advice on your specific application.
Sticky material such as uncured glue or resin cannot be extracted, nor can smoke and welding fumes. If the extracted air contains volatile substances harmful to health but not harmful to the extraction unit, extraction is possible, provided the filtered air is discharged to the outside.
4. COLLECTION AND DISPOSAL OF WASTE
For those with little waste, collection in plastic bags is the most obvious solution. The plastic bags are placed in the rolling containers under the unit and changed when they are full. The full plastic bags are taped shut and can thus be disposed of. The collection volume of the roll containers is limited to 160 litres each due to health and safety regulations. As long as the bags do not have to be changed more often than once a day, this works fine.
If more waste is released, the possibility of collecting it in some form of container should be considered. Here, the extraction unit can, for example, be placed on an elevation (RV version), with a small container driven under it. This does raise the question of how to empty the container when it is full. This is because many waste management companies do not have the option of coming to collect the loose waste (dust!) at home. There are also specialised companies, dedicated to processing and recycling wood dust. These companies work with special moth containers (capacity 30 cubic metres), into which the waste has to be blown. The Riedex extraction units in the RV version can be equipped with a blower unit for this purpose.
However, working with such mothballed containers does incur costs and it is therefore only interesting at waste quantities of at least about two cubic metres per week.
Recently, briquette presses have been booming. A briquette press can often be placed under the extraction unit. It compresses the waste into small lumps, which require little storage space and want to burn well. This also gets rid of a lot of junk around the moth storage area and reduces the risk of fire. A very elegant solution therefore, with a price tag incidentally, but increasingly popular.