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.
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Applications of Mist Collectors
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.
How Mist Collectors Work
Also known as mist eliminators or demisters, the design of these collection tools can vary significantly, but they 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 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.
Installed in line, mist collectors have minimal impact on the gas flow of an operating system. It is, however, important that mist collectors be accessible to technicians, as regular maintenance is essential to optimal performance, since particulates often adhere to the vapor and become trapped in the collection screen. To accommodate this, most demisters are cleanable and relatively easy to replace.
Mist Collector Design and Customization
Mist collectors, generally, unite mist droplets into drainable liquid with the help of filters. However, there are different ways to remove droplets from the airstream. Therefore, it is necessary to consider these distinctions to determine what design will suit a given application.
Here are some ways mist collectors can remove droplets from the airstream:
- Mist Collection through Electrostatic Precipitation
- Mist collectors that employ electrostatic precipitation to remove aerosol are fitted with an ionizer that draws mist-filled air and endows a charge to each droplet, either positive or negative. The charged droplets accumulate at the grounded plate in the collection cells, which are run by high-voltage. The droplets are then drained out.
- Here are some advantages of electrostatic precipitation units:
- No air filters to replace, which is cost-effective.
- 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 before being drained out of the system. This type of collector offers one advantage, as it does not need primary air filters. It does, however, require periodical maintenance to clean out the components that are contaminated. Moreover, this collector has 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 collector, a fibrous filter is utilized for removing the droplets, just like a dust collector but with media. The droplets in the airstream stick to the fibers in the filter media, and when the droplets get 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 the problem starts, as when smaller fibers are used, effective draining is prevented.
- Conversely, if the larger fibers are used, the ability 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, if needed.
Proper Maintenance of Mist Collector Filters
Most of the filters in mist collectors perform well only when they are new and eventually lose their effectiveness over time because they are 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 the 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 and get those replaced first, if they get loaded, since they are the least expensive filter within 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 can become rancid if left in the collector unchecked, adversely affecting the overall performance. It is advised that this aerosol be removed at the end of each shift. If the collected aerosol is allowed to drain back into the fluid system, it can contaminate the fluid. Any microbial contamination poses serious health problems and can reduce the fluid life.