Mist collectors and fume collectors reduce the amount of mist, aerosols and fumes in the workplace atmosphere. They can be used for treating media streams involving combinations of liquid/vapor, or solid/vapor. Mist collectors and fume collectors are commonly used alongside machining equipment and other devices which generate oil mists or plumes of smoke or debris during operation. Typically, they are designed either as source collectors to capture material directly or as ambient collectors to treat air as it circulates through an environment.

Smoke and fumes are sub-micron particulate created by processes such as welding, rubber and plastic processing, high speed machining with coolants, tempering, and quenching.

Mist Collector diagram. Image Credit: Emifree GmbH

Types of Mist Collectors

Media Filter Collectors

Media filter mist eliminators, also called cartridge mist collectors, use HEPA type filters to remove industrial mist and smoke oil. In operation, air containing fumes or mist is drawn into the cartridge media by a fan or blower. It is then forced through the filter where the droplets, mist, and particles are trapped or filtered out by the media material.

Media filter mist collector diagram. Image Credit: Sentry Air Systems, Inc.

The efficiency of media filter collectors varies depending on the filter type used. Pressure drops for these systems are usually higher than other types of mist collectors, so power requirements are often higher. Current cartridge mist collectors are typically modular and include options for pre-filters, HEPA, activated carbon, as well as high-powered fans that can reach as high as 18,000 cfm. Pre-filter options include:

• Metal screens are used for heavier dust/grit from wet grinding.
• Metal mesh is used for most water-soluble mist.
• Polypropylene mesh is used for most straight oil mist.
• High efficiency 1st stage is used for oily smoke.

Cartridge or media filter mist collectors have been used for all types of wet grinding, cast iron machining, and die casting. Other applications the cartridge style of mist removal include rubber and plastics, food processing, and occasionally welding fume.

Centrifugal Collectors

Centrifugal mist collectors incorporate rotating filter elements which form trapped mist back into larger droplets. The rotating centrifugal force then causes the enlarged droplets to be propelled free of the filter drum towards the inside wall of the casing, where they are subsequently removed.

Centrifugal mist collector diagram. Image Credit: BPA Air Quality Solution LLC

An advantage of centrifugal mist cooling systems is that they do not depend on gravity for the smoke oil or mist to drain, allowing the compressor to run continuously. However, they are only about 97% efficient for one micron sized particles, and typically require the addition of final stage filters to collect smoke and fine mists. They generate noise and vibration while operating, and have considerably higher maintenance requirements and costs than other types of collectors. Maintenance includes periodic drum cleaning, regular inspections, and a complete rebuild every 3 to 5 years due to wear on moving parts.

Electrostatic Collector

Electrostatic precipitator (ESP) mist collectors use high voltage to charge and gather the smaller mist particles and droplets. Mist or smoke streams enter the collector and are passed through a region containing one or more high voltage electrodes. The electrode(s) apply a charge to the particles in the stream, causing them to stick to oppositely charged plates or surfaces in the system. The collection surfaces are routinely cleaned either manually or automatically.

Image Credit: Ceramic Technic Co., Ltd.

ESP mist collectors typically provide the most efficient particle collection of sub-micron smoke and fumes at constant flow and low pressure drop. They also eliminate the need to use and replace expensive HEPA filters. However, ESP collectors may cost more initially than other types. Also, manually cleaned ESP collectors are not suitable for heavy fumes with larger particles because the limited surface area would require excessive cleaning maintenance.

Well engineered electrostatic mist collectors can collect particles as small as 0.003 microns in diameter, and are the most commonly used systems for filtering mist and oil smoke in industrial applications.

Collection Categories

Particulate collectors can also be categorized based on what types of media they can treat their mode of operation.

Media Types

Mist and fume collector units are often suited best for certain types of media with different particle sizes.

• Water mists have particle sizes from 2 to 20 microns.
• Oil mists have particle sizes from 0.8 to 5 microns.
• Smoke has particles ranging from 0.07 to 1.2 microns.

Collection Method

Mist and smoke eliminators units are design for ambient operation or source collection.

• Ambient units are designed to continuously filter airborne particles as air circulates through an environment. They are used in applications where there is no specific source of the mist or fumes or where the use of ducts and source-capture arms is limited.
• Source collectors are designed with ducts and arms to capture and filter air at the source. They can be either stationary or portable units.

Performance Specifications

Performance specifications are the ratings assigned by a manufacturer that describe a collector's performance. These are the specifications most important for industrial buyer to consider when selecting a suitable mist or fume collector for their application.

Maximum air flow or gas flow rate is the maximum amount of air or gas flow that the collector can accept, measured in cubic feet per minute (cfm). This effectively defines the cleaning capacity of the mist or fume collector.

Minimum particle size filtered indicates the minimum diameter of aerosol or smoke particles that the collector is capable of capturing, measured in micrometers (µm). This rating defines the range of capability of the collector.

Efficiency, collection efficiency, or capture rate defines the percentage of mist or smoke pollutants removed by the collector. Certain types of filters have rated efficiencies for particulate collection.

High efficiency particulate air (HEPA) filters are replaceable extended-media dry-type filters in a rigid frame having a minimum particle collective efficiency of 99.97 percent for 0.3 micron particles or larger, and a maximum clean filter pressure drop of 2.54 cm (1") water gauge when tested at rated air flow capacity.

Ultra low penetration air (ULPA) filters are extended media dry filters in a rigid frame that have a minimum particle collection efficiency of 99.999 percent for 0.12 micron particles or larger.

American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) filters are designed up to efficiencies of 95 percent for 0.5 micron particles or larger.

Activated carbon and specialty blended filters are used to treat gas streams with specific types of pollutants.

Additional Specifications

Mist collectors and fume collectors are rated using a number of additional specifications which may be important to consider.

• Connections - the electrical connections for the collector unit.
• Motor/Power - the specifications of the collector's motor, or the power requirements for the collector, measured in horsepower (hp).
• Size - the dimensions (length, width, depth) of the collector unit, given in feet or inches.
• Sound level - the noise produced by the collector during operation, measured in dBA.
• Storage capacity - the amount of storage space for captured pollutants. This determines how frequently the filter or collector must be cleaned or replaced.
• Weight - the weight of the collector unit, given in pounds (lbs).

References

BPA Air Quality Solutions LLC - Technologies for Reducing Oil and Mist in the Work Environment

Donaldson Company, Inc. - WSO Mist Collectors

Sentry Air Systems, Inc. - Mist Collection and Mist Elimination