When To Use Media Filtration Instead of Electrostatic Precipitators

June 26th, 2024 | Posted in Air Cleaners

When mitigating CNC (or manual) machining coolant mist, the technology of choice is nearly always electrostatic precipitators. There are, however, exceptions. Let’s discuss some situations where media filtration is the preferred solution. But first, a quick review of these two technologies is in order.

Electrostatic Precipitators or Electronic Air Cleaners

Coolant mist removal starts with an impinger section. The larger spray and mist droplets impact the impinger and adhere to the plates or mesh. The accumulated coolant flows by gravity to the bottom of the air cleaner and back into the machine tool enclosure. Collection of the fine coolant mist employs two sections, a charging section and a collection section. In the charging section ionizer wires impart a positive charge to the mist particles in the air. The air with charged mist particles then passes through a secondary electric field where they are collected on a series of metal plates called “collector plates.”.

As the coolant mist gathers on the plates, it will form droplets that run back into the machine tool. Some of the particles that are in the coolant may adhere to the collector plates forming a collection of particles on the plates. The debris could be micro sized metal that was in the mist or dust in the air that was caught in the mist.

With time, the collector plates become coated with the particles captured by the air cleaner and the layers of collected mist will act as an insulator and the air cleaner will no longer be effective. In that event, it is necessary to clean the electronic cells. Vertical airflow air cleaners are largely self-cleaning when used for mineral oil coolant as the collected oil mist drips off the collector plates back into the machine tool. In these applications, cleaning may not be necessary for a month or two, or even longer.

It should be borne in mind that not all coolants are amenable to ionization. Consequently, those mists are most often collected with fabric filters, or “Media filtration.” Materials like cast iron create a ultra micro particle that will not work in electrostatic air cleaners where a media filter will have better performance.

Media Filtration Mist Collectors

Media filtration mist collectors also start with an impinger section, typically consisting of either a series of chevon-shaped plates or a coarse metal mesh. This is followed by media filters that may be constructed of any porous material, either natural or man-made. Polyester fabric is a common choice for filter media.

Media filtration collectors remove mist particles by straining, impingement, interception, diffusion and electrostatic charge. Because there is no need for a power supply, or for electronic cells, as is the case with electronic coolant mist collectors, media type coolant mist collectors tend to have a lower initial purchase price, but higher costs over time due to the need to purchase replacement filters.

Media filtration may be preferred in the following circumstances:

• Dry machining cast iron. Cast iron has a higher carbon content than carbon steel and the carbon actually acts as lubricant during the machining of cast iron, obviating the need for coolant in many cases. Like all metals, iron is electrically conductive. Dry metal dust can short out the electrostatic precipitator cells and result in ineffective contaminant capture. Media is preferred for this application.

• Machining carbon dies for electro discharge machining (EDM). EDM dies are commonly made of carbon (graphite). Carbon is also electrically conductive, and media is preferred for this application to avoid the shorting out that would occur with ESP.

• Non-ionizing coolants. There is a small group of coolants that do not ionize in the charging section of an ESP air cleaner, In the absence of ionization, the ESP simply will not work. Media filtration is used for these coolants.

• Corporate culture or preference. Regardless of air cleaning technology selected for a given application, one must deal with the accumulated mist/dust. Depending on the preferences of management and/or the maintenance group, it may be easier to just replace a filter than it is to clean an ESP air cleaner cell.

• Since media air cleaners do not have a power supply and associated electrical components, media air cleaners may offer a slightly more economical price point at the time of purchase.

In comparison with electronic coolant mist collectors, media type filtration has a few drawbacks:

• Media coolant collectors generally require reconditioning by being allowed to drain periodically, usually for a few hours during off-shift times. This limits their use for continuous operations.
• Because of the need for a larger filter volume to allow for coolant accumulation between reconditioning periods, media type mist collectors tend to be bulkier than electronic mist collectors.
• Overcoming the static pressure restriction of a media type filter may require a larger and noisier blower.

The industry trend is toward electronic coolant mist collectors due to their high efficiency in dealing with fine mists and their low maintenance requirements. When purchase price is the driving factor, media types predominate, although filter replacement costs cannot be ignored. Whichever air cleaning technology is selected, Air Quality Engineering has a complete line of both Electrostatic Precipitators and Media Filter Air Cleaners for coolant mist and other commercial, industrial, and kitchen exhaust applications. Contact Air Quality Engineering for guidance regarding optimal coolant mist capture technology for your application.