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Pure-air
or raw-air system -- which of the two is suitable for the
requirements in your company? What are the differences between
these two extraction techniques? The following article provides
answers to these questions.
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In
raw-air systems, the blower is situated between
the woodworking machines and the dust extraction or filter unit.
Via the blower, the woodworking machine takes in the air with
its load of dust and chippings (raw air). The raw air is then
pressed into the dust extraction or filter system. During this
process, an overpressure is imposed on the dust extraction or
filter system. This is why raw-air systems are also called overpressure
systems. |
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Raw-air
systems can be designed in a very basic manner. The most common
type is the so-called filtering bag at the top / chippings
bag at the bottom design. The raw air inflates the filter
and the chippings-collection bag. A drawback of this principle
is that the overpressure inside the filters presses the dust
outwards, so that in cases involving minimum leakage the dust
escapes from the dust-extraction or filter system and is distributed
again in the room.
Since
dust and chippings pass constantly through the blower, the
impeller must be designed in a very open manner. Larger particles
must be transported via the impeller without becoming wedged
in the blower. Due to the constant impact of chippings and
dust, the impeller is exposed to a high degree of wear. Its
efficiency is thus clearly lower than that of a pure-air blower.
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Raw-air
dust-extractor systems, however, have the advantage that they
are relatively cost effective due to their simple design.
In
the woodworking trade, raw-air systems may not be installed
in the workshop area. Neither may the air of open systems
erected in special filter rooms or outdoors be returned to
the workshop. As a result, the heat contained in the extracted
air is lost and the workshop cools down.
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In
pure-air systems, the blower is situated behind the filter.
The air with its load of dust and chippings is taken in through
the filter and cleaned before it passes the blower. A negative
pressure is imposed on the casing of the dust-extraction or
filter system. For this reason, pure-air systems are also referred
to as vacuum systems. |
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The
filtering principle of the vacuum system only works when the
filter is fully encased. This more complex design entails
higher production costs. In cases involving leaks, the ambient
air is sucked into the filter chamber for pressure compensation,
so that no dust can escape.
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Since
the blower only comes into contact with air, the impeller
can be designed as a high-performance impeller, i.e. the blower
casing can be provided with very small air slots which, in
conjunction with the closed impeller, help the pure-air sytem
to achieve a very much higher efficiency than the raw-air
blower.
Provided
the series of products has been type approved, pure-air
systems up to an extraction capacity of 6.000m³/h may be installed
directly in the workroom area, so that the warm air is
retained in the workshop. A closed filter design can be used
both on the pure-air and the raw-air side.
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