Electrostatic & Activated Carbon Filters


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.