Electrostatic precipitators

Overview:
Electrostatic filters are highly versatile in their application, and are widely used owing to their low pressure losses and to the ease of cleaning the electrostatic filter cells. At this point we would like to briefly point out the three most important application areas for electrostatic filters (also see here the VDI Directive 3678, Part 2: “Electrostatic Filters and Filtration of Process and Room Air” – VDI-Richtlinie 3678 Teil 2, Elektrofilter-, Prozessluft- und Raumluftreinigung).

Oil-mist filtration:
Judging from the number of systems applied, the most important use for electrostatic filters is without doubt the separation of oil mist: a pollutant that occurs in metal-cutting and machining processes. There are two different solutions available for removal of oil mist:

  1. Central exhaust systems, which serve several emission sources via a piping system
  2. Decentral exhaust systems, which are installed directly at each work place.

Both of these types of exhaust systems have their own advantages, depending on the particular operating conditions encountered. As a result, it is not possible to provide generally valid recommendations as to which type is fundamentally better. For oil-mist separation, metal and metal-mixture filters are primarily used, depending on the concentration of untreated gas, and on the size of the droplets constituting the mist. These filters are applied as pre-filters, in combination with electric filters. The purpose of these filters, however, it not preliminary separation, but uniform directional guidance of the stream of air.

The following factors are crucial in determining the degree of filtration possible:

  • The velocity of air at the ionization electrode (for decentral systems, it is somewhat lower than for central systems)
  • The thickness of the ionization electrodes (ideally, as small as possible)
  • The uniformity of the air flow.

As a rule, the high voltage is positive. If toxic substances are in the process air, the use of filter paper with a high filter class (greater than E10 as per EN 1822), and with hydrophobic and oleophobic characteristics, is advisable: with installation of the filter behind the electrostatic separator.

HVAC electrostatic filters:
One possibility of application that is steadily growing in importance is the use of HVAC electrostatic filters. These filters, in combination with filtering separators that serve for pre-filtration, are integrated into HVAC ducts and function there as supply-air and recirculated-air filters.

The installation of electrostatic filters at such points offers the following advantages:

  • Extensive elimination of micro-organisms (a key factor in the sick-building syndrome, SBS)
  • No filter cake through which air must flow (which eliminates one of the chief causes of odours)
  • Very low pressure differentials (which lowers energy costs)
  • Practically no production of waste to be disposed.

The high-voltage supply is positively charged, which assures levels of ozone concentration considerably below any applicable limit values.

A mobile scrubber unit fully automatically cleans the system.

Systems that must separate organic hydrocarbons (for example, in the kerosene at airports) employ activated carbon as a post-filter stage.

The structural design of a HVAC electrostatic filter is modular – as for all large systems built according to Penney’s Principle. This means that a series of small electrostatic-filter cells is installed, for example, in a stack or battery system, in vertical or horizontal configurations.

Separation of solvents:
The separation of solvent mist is one of the few filtration applications that applies negative high voltage. Treatment of the incoming air takes place by employment of wet scrubbers or coolers. Solvents occur especially in the production of textiles and carpets.

Additional applications of electrostatic filters include the separation of metal oxides and superfine dust.