View A Video on Mist Collectors - A Quick Introduction
Mist collectors are air pollution control devices designed specifically for the removal of mists and vapors from process air streams. The use of these filtration devices provides many benefits. Product purity is increased significantly as is throughput, while operating costs, air pollution and downstream corrosion are reduced. In addition to the clean gas that exits the collector, the liquids may also be recovered and even recycled in some applications.
Mist collection systems can be used for abrasives, coolant, smoke, oil, water and many other materials if properly engineered. Metal finishing and forming industries as well as chemical processing, brine desalination, marine, food processing, paper and pulp, agriculture and many other industries take advantage of mist collectors in daily operations. They are especially popular in applications where vapor quality is of utmost importance or where space and weight savings are integral. Installed in line, mist collectors have minimal impact on the gas flow of an operating system, another point of interest for many users. It is, however, important that mist collectors be accessible to technicians as regular maintenance is essential to optimal performance as particulates often adhere to the vapor and become trapped in the collection screen. To accommodate, most demisters are cleanable and relatively easy to replace.
Also known as mist eliminators or demisters, the design of these collection tools can vary significantly, but operate off of the same general principals. The most important feature of a mist collector is the filter. Often several such obstacles are used in industrial collectors to capture droplets contained within a vapor stream. Different structures are used in to aggregate the mist into droplets that are heavy enough to drop out or separate from the air stream. Mesh type coalescers, vane packs, or pads made of fabric-like materials are among the most popular filters implemented in mist collectors. The filter or obstacle must be permeable with very close mesh-like openings through which vapors may pass. Often the smaller the openings the more effective the device is at removing mist from the process stream. No matter the specific type or size, the filters cause a drop in pressure which draws moisture to the collection area. As aforementioned, mist collectors enable the capture and reuse of the liquids they remove. This necessitates a collection chamber. Often the base of the collector serves as the chamber. Droplets are filtered out through a valve in the bottom while vapors free of mist and debris rise and exit from the top of the compartment. It is important to consider the end goals, process stream and materials when selecting a mist collector for industrial use.
Mist Collectors - Air-Clear, LLC
Mist Collectors - Air-Clear, LLC
Mist Collector — Function and Maintenance
Mist collector is an air filtration system utilized for eliminating metalworking fluid (MWF) aerosol from exhaust airstreams, before being discharged into the ambient air. Like other air pollution control systems, such as wet scrubbers, oxidizers, and dust collector, many different types of commercial mist collection systems are available. However, generally, most of the commercial collectors are multiple-staged that are configured in series. The mist collectors primarily focus on swarf—fine chips of metal or other material—at the first stage. The first stage removes most of the swarf to reduce the mist loading that reaches the final stage. The final stage collector is usually consists of high efficiency particulate air (HEPA) filter. For extensive applications, a three-stage collector is used with a 95% HEPA filter; such designs are applicable for MWF operations.
- First stage filtering
The first stage filters, or pre-filters, are typically a metal-mesh woven together to capture particles, swarf, with greater dimensions. It actively gets rids of swarf to reduce the load for the later stages depending on the design; a standard design is a two second-stage, and specialized designs have third stage filters.
- Second-stage filtering
The second stage air filters are more efficient than the first in capturing particles; it utilizes cartridge filters or pockets to catch particulates.
- Final-stage filtering
At the last, 95% HEPA filters arrest particulates out of the exhaust stream. A HEPA filter, certified by US government standards, must remove 99.97% of particles (up to size of 0.3 µm) from the air that passes through it. They provide excellent efficiency when they are new. However, their lifetime is determined by the amount of particulates they are exposed to; the cumulative mist load is a big factor that defines life. And replacement is often expensive, thus, first and second stage filters must be effective, so that HEPA do not receive extra mist load.
Maintenance of Filters
Most of the filters perform well only when they are new, and eventually lose their effectiveness, over time, being loaded with liquid. If you want them to perform properly for a longer time, it is critical that you get someone to inspect and clean air cleaner filters, periodically. Replacement should be the last option, as it often is expensive. As mentioned earlier, the easiest way to expand their life is to have well-performing primary metal-mesh and cartridge filters, get them replaced first, if they get loaded, since they are least expensive filter in the mist collector.
Poor maintenance of mist collectors can significantly increase the loading of mist in the discharged air, rendering the system useless. Moreover, the aerosol caught in the mist collector over time turn into rancid, if they are left in the collector unchecked, adversely affecting the performance. Evidently, it is crucial to have proper maintenance.
For proper working, it is advised that the aerosol collected must be removed at the end of shift. If the collected aerosol, somehow, drains back into the fluid system, it can contaminate the fluid. Any microbial contamination poses serious health problems and can reduce the fluid life.
Different Designs of Mist Collectors
Occupational Safety and Health Administration (OSHA) mandates giving workers safe and healthy work environment in a range of industries. To comply with the air-pollution control regulations, facilities are fitted with a cache of air filtration systems, with different goals in line, such as wet scrubbers to filter harmful gas out of exhaust stream or mist collectors to clean aerosol from the air to provide a better working conditions for floor employees.
Indoor air quality standards advocate use of mist collectors in all metalwork facilities, as it cuts down the harmful effects of metalworking fluids, which if remain unchecked can adversely harm employees. Moreover, mist collectors improve part quality, by capturing the particulates in the air, thus, keeping the work surfaces clean, which also directly reduces housekeeping and maintenance costs.
Mist collectors, generally, unite mist droplets into drainable liquid, with the help of filters. However, there are different ways to remove droplets from airstream. Therefore, it is necessary that to consider the distinctions to determine what design will suit a given application.
Mist Collection through Electrostatic Precipitation
Mist collectors that employ electrostatic precipitation to remove aerosol are fitted with ionizer that draws mist-filled air, and endows charge to each droplet, either positive or negative. Then the charged droplets are accumulated at the grounded plate in the collection cells, which are run by high-voltage. The droplets then are drained out.
Advantages of electrostatic precipitation units:
- No air filters to replace, which often is expensive
- Low consumption of electricity
- High efficiency if maintained well
However, their frequent maintenance needs have put them out of favor, as the operational cost piles on with time.
Mist Collection through Inertial Separation
Some designs rely on inertial separation for removing tiny aerosol droplets. The airstream is directed around a collection surface, and the droplets in the airstream that have momentum on the impact coalesce, which is drained out of the system. This type of collectors offers one advantage, as they do not need primary air filters. They, however, require periodical maintenance to clean out the components that is contaminated. Moreover, they have sub-par efficiency, if the droplets are smaller than 2 microns in diameter are involved, as the momentum is directly proportional to weight.
Mist Collection through Fibrous Filter
In this type of collectors, for removing the droplets a fibrous filter is utilized, just like dust collector but with media. The droplets in the air stream stick to the fibers in the filter media, and when droplets gets large enough, the gravity drains them out. High efficiency can be achieved with this system. However, efficiency is determined by the size of the fibers. That is where problem starts, as when smaller fiber are used—which require resin to hold them together—it prevents effective draining after being plugged. Conversely, if the larger fibers are used, efficiency to capture small droplets goes down.
To mitigate this problem, pre filters with larger mesh or screens are fitted to capture large droplets, while a second layer with smaller fibers catches the remaining droplets. Sometimes, an optional third layer with HEPA filters are added, based on the need.