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Exhaust Fume Filtration
Activated carbon, the most common purification filtration
medium, adsorbs chemicals with a molecular weight above 30
and a boiling point above 60°C. It is also effective
with many other chemicals because of their particular molecular
structure.
Adsorption takes place in the active filter zone,
the small cross-section of the filter bed in which the material
to be removed comes in contact with the filtering medium (see
illustration). This active filter zone moves upward as the
filter becomes saturated. When it reaches the filter's top
surface, there is an initial breakthrough by the contaminant
gas; thereafter the percentage of contaminant gas that escapes
filtration increases until total saturation of the filter
is reached.
Enhancing Filter
Performance
To optimize filtration efficiency, any ductless
exhaust fume system must ensure that the exhaust stream is
exposed to the purification medium for an adequate time. Terra
exhaust purification systems satisfy this condition in several
ways.
First, our specially selected impeller
ventilators are designed to ensure the optimal linear velocity
of the airstream— sufficient to maintain a negative pressure
in the work area that prevents backflow, but low enough to
allow adequate adsorption of fumes by the filtering medium.
Our prefilters also extend exposure time by reducing the airflow
through the filter.
Monitoring Filter Effectiveness
A ductless exhaust fume system needs to be monitored in two
areas. The user must detect the period between initial contaminant
breakthrough and the point at which the contaminant gas reaches
the Threshold Limit Value (TLV) for that substance. The velocity
of the airstream must also be monitored to ensure an adequate
negative pressure and to detect ventilator failure.
Through periodic testing of the exhaust airstream, you can
determine the useful filter life in your particular application
and gauge filter effectiveness with an elapsed time meter.
An electronic airflow velocity monitor also helps to ensure
safe, effective operation by providing a continuous velocity
indication and providing an alarm when velocity falls below
a user-defined threshold.
Put Safety First!
To ensure safety, make sure that filter performance is properly
monitored, as explained above. Avoid filter use in applications
involving very toxic substances, very high volumes of contaminants,
and unknown or highly volatile chemical reactions. Always make
sure that filters are promptly changed when the Threshold Limit
Value (TLV) has been reached for any contaminant gases present. |
