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The Coshh regulations were up-dated again last year for the third time since they were introduced in 1988. Health and safety at work, as enshrined in the 1974 Act of Parliament of that name, is taken ever more seriously and the issues are ignored by companies at their peril. Dust-filled workshops are not acceptable to the Health & Safety Executive (HSE) and there are legally enforceable limits of exposure to certain dusts, significantly silica and crystalline quartz (maximum 0.3mg per cubic metre of air).
While the HSE would rather persuade companies to improve conditions than prosecute them, if a case does go to court the fines can be unlimited and directors and owners can go to prison if convicted. Making workshops healthier and safer can, in any case reduce insurance premiums.
For masonry companies dry cutting or carving stone, the HSE-acceptable solution to dust is commonly what they call LEV (localised exhaust ventilation) and what the rest of us would know as a dust extraction unit. But there are some points to consider about dust extraction systems. Andy Quail, an engineer with a science Masters degree who is chief executive of extraction unit manufacturers J B Thorne, explains some of them.
Adding dust extraction to a working area, both to improve the environment of the person working on a particular machine and to reduce the dust levels in the workshop as a whole, is not always as successful as might be expected. Systems can turn out to be less efficient at removing dust and more expensive to run than forecast.
On some machines, the whole dust extraction system is compressed into free space within the machine-stand without consideration of the implications. The result is poor (sometimes even non-existent) dust capture, poor dust removal, lack of protection for the operator and environment, and wasted energy.
In order to capture dust generated by specific operations, the choice of air volume to be moved and the velocity at which it can be moved is critical. This is a function of the operation, machine design, the amount and the size of particles produced by the operation, and access and closeness of the dust extraction hood to the dust source. These factors determine the size and configuration of the dust extraction system.
Such a system will consist of fixed element - capture hood, straight duct, bends, flexible ducting and filters, etc - activated by a fan. For successful dust extraction system, the correct amount of air should be extracted from the dust-generating source, capturing as much as possible of the dust generated.
The air and dust is drawn into a ductwork system to carry the dust to a filter. The capture hood, pipework, filter and fan will all affect the performance of the whole plant.
If the hood, pipework and filter are incorrectly configured and sized, the fan will not achieve the performance required to remove the dust from the operation. As a result, the system will not perform effectively, efficiently or economically, making the investment in the system a waste of money and exposing operators to potentially harmful dust.
Recent tests carried out on a simple extraction system demonstrated that a fan capable of moving 600m2 of air an hour was only achieving 150m2 through the dust extraction system. Further examination showed that the fan, when tested in ideal conditions, would indeed achieve the required performance of 600m2 an hour, but it was failing to do as part of a dust extraction system because of the configuration and design of the fixed parts of the system.
In this example, the hood was a simple rectangular box with a small-bore pipe welded in place to connect to the rest of the pipework. It was the small-bore pipe which was causing the problem. By increasing the pipe to the full width of the hood an improvement was achieved in capture rates through increased air columns and reduced pressure losses.
Pipe fittings to convey the dust filter were also small in diameter and there was further loss of performance caused by abrupt changes in direction in the piping together with a right-angle connection into the fan assembly.
Compounding the problem still further, part of the duct was made of flexible pipe with an extremely rough internal surface.
Losses from the rough duct surfaces and the right-angle bends can result in coefficients of resistance in the order of 1.3, compared with the results in the order of 0.25 with smooth duct surfaces and large-radius bends. All these factors combined to increase the energy required to move air and dust through the system, reducing the overall performance.
To move a given quantity of air and dust through a fixed system, a pressure equal to the system resistance must be generated. The flow rate, or velocity, of any given system will be determined generally by the properties of the dust being extracted and the energy needed to convey it to the filter. The system pressure is a function of the velocity squared.
A filter sock with a small filter area relative to the amount of air being moved will soon become blocked with dust, increasing the system resistance still further and, as a result, reducing the suction available at the hood.
The location of the fan and the way the air is introduced to, and dischargede from, it will influence the performance of the whole system. In many small systems, if the air is drawn into the fan from a right-angle bend the loss of performance can be as much as 50%.
Not only is such a system failing to achieve the performance required, it is taking energy to run without achieving the correct level of extraction, making the investment expensive in energy terms and failing the Coshh requirements and, as a result, CE regulations.
A correctly proportioned extraction system fitted together with the base of the unit can provide an effective, efficient and economic solution to dust problems.
The use of small, compact filters with the correct amount of surface area will cut running costs by reducing the filter resistance. It will also give longer filter life, thus reducing running costs and giving better long-term protection to the operator.
With the correctly proportioned ductwork and capture hood, energy can be saved in the long term by reducing losses within the system and focusing the maximum amount of extraction force where it is needed to protect the machine operator.
The design of the collection unit can incorporate special features to regain pressure lost through changes to duct shape, as well as reduced velocities and turbulence.
With the assistance of experienced dust extraction design engineers, many of the systems in use today could be made more effective, efficient and economical to achieve a greater degree of protection for the machine operator and the workshop environment in general and save money on the running costs of the system.
For more information about health and safety issues, including copies of the health and safety execution (HSE) booklet Five steps to risk assessment call HSE infoline on Tel: 0541 545500. To discuss dust extraction systems call J B Thorne Ltd on Tel: 0116 260 5757
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