Mist Collection Technologies

January 11th, 2023 | Posted in Air Cleaners

There are three common types of mist collectors:

• Electrostatic Precipitators
• Media Filtration
• Centrifugal Collectors

Let’s look at how they work and their relative strengths and weaknesses.

Electrostatic Precipitators or Electronic Air Cleaners

Coolant mist removal starts with an impinger section. This typically consists of either a series of chevron-shaped plates or a coarse metal mesh. 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. In order to filter the fine coolant mist, the mist collectors employ 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.” The cleaned air is then recirculated into the workplace.

Depending on the air flow rate and the number of cells in the mist collectors, these units have a collection efficiency of 90% to 99%.

I think it is not the mist collected that gives the problem but small particles and contaminates in the mist or air that can that create the insulator.

Eventually the collector plates become coated with contaminate particles that have been captured by the air cleaner and the built-up layers of collected particles then act as an insulator, which reduces the effectiveness of the air cleaner. In that event, it becomes necessary to clean the electronic cells. Frequency of cell cleaning will vary with the type of coolant used and the condition of the workplace atmosphere. 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.

For water-soluble coolant applications, a mixture of coolant and tramp oil (e.g., from way lube) may accumulate on the collector section. This condition may require cleaning every few weeks or months. Electronic air cleaners have several advantages:

• No expense due to periodic filter replacement.
• Since there is little static pressure restriction as the cleaned air passes through the air cleaner, a quieter fan can used, reducing the level of noise in the shop.
• Due to the favorable ratio of surface area to volume exhibited by fine mists particles, electrostatic air cleaners are well-suited to the very fine mist generated by high pressure coolant systems.
• One manufacturer (Air Quality Engineering) offers an electronic mist collector with a switch that allows for collecting either mineral oil based or water-based coolants.
• Because they are largely self-cleaning, these electronic mist collectors are suitable for continuous operation.

One disadvantage of electronic mist collectors is their slightly higher initial purchase price compared to media filters.

It should be borne in mind that not all coolants are amenable to ionization. Consequently, those mists are most often collected with media filters.

Media Filtration Mist Collectors

As with electronic air cleaners, media filtration mist collectors start with an impinger section. Again, this typically consists of either a series of chevron-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 media 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.

The ability of the filter medium to pass air is stated as “permeability” and is defined as the cubic feet of air that is passed through one square foot of media per minute at a pressure drop of 0.5 in. w.g. Obviously, the filter media must be permeable, or no air would pass through it. Just as obviously, the more efficient a filter is, the less permeable it will be. A balance must be struck between air flow rate (permeability) and how clean the air is coming out of the mist collector (efficiency). A highly efficient media filter that cannot be effectively cleaned or reconditioned represents an excessive resistance to airflow and is not an economical engineering solution.

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.

Centrifugal Collectors

Centrifugal collectors enjoy the engineering elegance of having the impinger section and the air mover combined into one unit, the centrifugal blower. The blower both moves the air and slings the mist particles against the blower housing. The mist droplets adhere to the housing and drip back into the machine tool enclosure. These units are effective for heavy coolant spray.

However, “…high efficiency centrifuges are not as efficient on small particles as electrostatic (and) fabric…” collectorsⁱ. Recognizing this characteristic, centrifugal units require a “post-filter.” In actuality, the “post-filter” is the primary filter for capturing fine mist as the centrifugal units are not effective at these particle sizes.

Since the blower is on the “dirty” side of the airflow, the impeller may accumulate coolant, dust and possibly fine chips or filings of metal, or other material produced by the machining operation, resulting in an out-of-balance condition. If the unit is hard mounted to the machine tool cabinet, the resultant vibration may affect product quality.

In the end, “There is no such things as a free lunch.” 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. Centrifuges are an old technology best suited to heavy spray rather than fine mist.

i “Industrial Ventilation: A Manual of recommended Practice, 24th Edition.” ACGIH, 2001 pg. 4-22.