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DO NOT LET the dust settle
Much metal treatment equipment today comes with dust extraction which is sometimes ineffective, and below required recommended standards. Yet such machinery must now be fitted with dust extraction equipment, following the implementation of CE Mark and COSHH regulations.
Many machinery designers incorporate fans, filters and interconnecting pipework based on the space available around the machine, and do not always consider good dust collection practice. Adding extraction as a means of meeting customer specification without consideration of effective performance leads to inefficient and uneconomic operation.
Quite often the whole system is compressed into a free space within the machine stand without consideration of the implications. The result is poor or non-existent dust capture, poor dust removal, lack of protection for the operator and environment, and wasted energy.
SYSTEM THEORY
To capture the dust generated by specific operations, the choice of air volume and velocity is critical to the success of the plant. This is a function of the operation, machine design, the amount and size of particle produced, access and closeness to the dust source. These factors determine the size and configuration of the dust extraction system.
Such a system consists of a number of fixed fittings such as capture hoods, straight duct, bends, flexible ducting and filters activated by a fan. For a 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.
This is drawn into a ductwork system to convey the dust to a filter and fan. The capture hood, pipework, filter and fan are a fixed system and once installed will 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, efficiently or economically, making the investment in the system a waste of money.
SYSTEM PERFORMANCE
Recent test carried out on a simple extraction system fitted within a machine stand demonstrated that a fan capable of 600m3/hr was only achieving 150m3/hr through a dust extraction system. The specification claimed 600m3/hr. Further examination showed that the fan, when tested in ideal conditions, would achieve the required performance. Failure occurred due to fixed parts of the system design.
In this example the hood was a simple rectangular box with a simple small bore pipe welded in place to connect to the rest of the pipework. By increasing the pipe to the full width of the hood, and by enclosing the dust source, an improvement was achieved in capture, through increased air volume and reduced pressure losses.
Pipe fittings to convey the dust to the filter were small in diameter, with abrupt changes in direction together with a right angle connection into the fan assembly. Part of the duct was made of flexible pipe with a very rough internal surface. Losses from rough duct surfaces and right angle bends can result in coefficients of resistance in the order of 1.3, compared with results in the order of 0.25 with smooth duct surfaces and large radius bends. All these factors combined to increase the energy to move air and dust through the system, reducing the overall system 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 will be determined generally by the dust being moved and the need to convey it to the filter. The system pressure is a function of the velocity squared.
The filter sock with a small filter area relative to the amount of air recommended will soon become blocked with dust, increasing the system resistance still further and as a result reducing the suction available at the hood. This is exacerbated because the filter is being used to hold the dust until the operation is finished, when the filter can be emptied.
The location of the fan and the way the air is introduced and discharged for the fan will influence the performance of the whole system. In many small machine tools, if the air is drawn onto the fan from a right angle bend the loss in performance can be as much as 50%, with similar affects occurring on the discharge.
Not only is such a system failing to achieve performance, it is taking energy to run without achieving the correct level of extraction, making the investment uneconomic in energy, wasting vital investment capital and failing the COSHH requirement 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 reduce the energy used by reducing filter resistance, and will also give longer filter life, thus reducing running costs and giving better long term protection for 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 focussing the energy where it is required to produce the protection for the operator.
The design of the collection unit can incorporate special feature to regain pressure lost through changes to duct shape, as well as reduced velocities and turbulence.
With assistance of experienced dust extraction design engineers, much fettling and grinding equipment in use today could be improved to achieve the required protection for the operator and environment in an efficient and economical way, ensuring best returns from capital invested.
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