Electrostatic
Filters
Electrostatic
filters remove dust from the air by electrostatic attraction. Dust laden air
entering the unit passes over an ionizer (below figure). This induces a
positive electrical charge on the dust particles. The airstream then passes
between positively and negatively charged plates. The positive plates repel the
charged dust particles towards the negative plates which are at the same time
attracting the dust particles. The dust collects on the negatively charged
plates.
Electrostatic
filters have a mechanical pre filter to remove the larger particles and a post
filter to collect any large clumps of aggregated dust which may become
dislodged from the unit. Some units have automatic cleaning systems which
periodically wash down the collector plates which become coated in accumulated dust.
In other systems the collector array is removed via a side hatch for cleaning
before re assembly. Electrostatic filters once seen as a low maintenance low
pressure drop option have recently fallen out of favour. This is due to the
increased cost of mechanical parts and the high cost of replacement plates that
become less effective after 'pitting' and accumulation of inground atmospheric
particulate staining.
Activated Carbon
Filters
Activated
carbon filters are used to remove gaseous pollutants and odours from the
airstream which cannot be removed by mechanical or electrostatic filters. The carbonaceous
material is first processed to produce a char and then heated to 800-1000oC to
give it its micropore structure which enables the adsorption of gaseous contaminants
and odours. While most activated carbon filters are made of base carbon, the
carbon can be impregnated to improve its ability to adsorb certain types of
contaminant such as nitrogen dioxide(NO2) and sulphur dioxide(SO2) which are
particularly damaging to documents and works of art. Carbon filters can be constructed
of loose carbon, bonded carbon biscuits, carbon impregnated paper or fiber and
pleated granular mat depending on the application. The filter is commonly
housed in a metal frame and should always be preceeded by a pre-filter. In
certain instances the carbon can be reactivated and reused at the end of its
life. The degree of effectiveness of a carbon filter is generally related to the
amount of time that the air spends within the carbon. This is known as the
dwell time. The greater the dwell time (lower the air speed or greater the
carbon area) the more effective the carbon filter will be at odour or gas
removal. Pressure losses through carbon filters can be high and manufacturers should
be consulted to select a filter to optimize gas removal and minimize pressure
drop.
