There are two primary forms of extraction: Filtered Recirculating Extraction (recirc) and Vented to Atmosphere Extraction.
First let’s cover the basics as Vented to Atmosphere systems can also include many forms of inline filters such as spark traps, dust filters, grease traps/ filters and more.
The key difference is that the Filtered Recirculating Extraction systems have much higher levels and grades of filtration and are designed to return cleaned, filtered air back into the immediate environment, using an extraction unit that has filters built into it. Whereas Vented to Atmosphere Systems do just that – they take the dirty air and vent it out side via an exhaust duct, usually via a fan in the duct.
Different types of Filters.
There are primarily 3 main types of filters available in Recirc Systems, depending on what contaminate you are extracting, Pre-Filters, Dust (or Particulate) Filters and Carbon (or Gas) Filters with several different types of each main filter.
Pre-Filters are often the first filter in Recirc Systems. These are primarily for protecting any subsequent filters. Pre-Filters usually take the form of a foam type pad, a “VEE” Pleated filter or sometimes a bag filter.
Pre-Filters are designed to stop the largest particulates from getting into the main filters. They are usually quite course, low efficiency and designed to be changed fairly regularly.
Dust/ Particulate Filters
Dust filters (also called Particulate Filters) are designed to filter solid particulates such as dust and swarf from the extracted airstream. Dust filters come in a variety of forms and grades of efficiency. The main types are bags filters, panel filters and HEPA Filters.
The different grades will filter out different particle sizes with the most efficient being a High Efficiency Particulate Air filter or HEPA Filter.
HEPA filters are composed from layers of randomly arranged fibres (usually fibreglass) and the fibres vary in size. HEPA filters remove at least 99.97% of contaminates in the airstream down to 0.3 microns.
Bag filters are more akin to the old-fashioned Vacuum cleaner bags but with generally much higher levels of filtration. They can be used as a pre-filter to collect the largest amount of large dust particles or as the main filter after a pre-filter. The grade of dust filter depends on the application it is needed for.
The grade of dust filter needs to be selected based on the type and size of particulate being extracted. For example, a fine HEPA filter is wasted on heavy dust like wood chips as it will block up too fast. Likewise, an F8 dust bag filter is no good for fine dusts as the filtration rate is to low to remove enough of the dust from the air.
Other types of particulate filters often found include Cartridge Filters. These are often in large dust extraction systems and usually have internal cleaning systems - usually via an air blast system - that can be manual or automatic. Some forms of these filters can be treated to carry an electrostatic charge to help aid particulate attraction (makes the dust stick more). Cartridge filters, Bag filters and HEPA filters all work on the same principles of:
- Impact (where larger particules, larger than the weave, meet the surface of the filter),
- Impingement (medium sized particules meet fibres with in the weave) and
- Diffusion (where small particules are attracted towards the fibres)
Image Credit Wikipedia
Filtration efficiency can increase as dust builds up in the filter but this will also lead to an increase in flow resitance (the air can struggle to get through the dust layer). These types of filters are also prone to blocking easily if the dust is greasy or waxy and can wear easily if the material is highyl abrasive.
Particulate filters can also come in the form of Cyclone filters. Cyclone filters consist of a circular chamber, tapered at the bottom. Dirty air is fed into the top of the cyclone and swirled around the chamber. The heavier dust particles are thrown out onto the camber walls by centrifugal force. The heavier dust particulates fall down to the collection bin and the clean air escapes out the top. The down side to Cyclone filters is they are not very effective at small particules - Cyclones are ineffective at filtering particules below 2 microns and are only really good at heavier large particules (like metal swarf) with a particulate size over 8 microns.
Other particulate filter types include ElectroStatic Precipitators that use electrocally charged plates to attract fine dusts. These are unsuitable for heavier dusts, are costly and require specialist cleaning knowledge. Wet scrubbers and self induced spray collectors are systems that use water to "collect" the dust. The water droplets adhere to the dust particules, causing them to fall out of the air stream and into the water. Wet collectors are usually primarily used for highly combustible or explosive metal dusts such as Aluminium or Titanium and are specific to specialist applications.
Wet Scrubber Cyclone Filter ElectroStatic Preceptor Cartridge Filter
Image Credit Health and Safety Executive HSG258: Controlling Airborne Contaminates in the Work Place.
Carbon filters (sometimes called Gas Filters) are made from many, many tiny pieces of activated carbon to remove contaminates from the airstream by chemical adsorption.
Carbon filters are used in applications where there is just a gas or vapour element. A prime example of this is conformal coating – where a layer of a thin polymeric film is applied to electrical components such as PCB’s. The polymeric film gives of vapours that can be harmful to the operative.
Activated Charcoal has a huge surface area in relation to its size, one pound (454g) of activated carbon has a surface area of around 100 acres (1 km/ kg). The large surface area of the charcoal gives it countless bonding sites. When chemical particulates pass through the carbon they are attracted to the surface and become trapped there.
Activated Carbon, also called activated charcoal or activated coal, has been treated to be extremely porous. Standard activated carbon relies on this porousness to be effective but it can be further treated to enhance its ability to adsorb other chemicals more effectively.
Activated Carbon Granules Carbon under a Microscope
Properties of activated carbon include:
- Has a capacity for virtually any vapour contaminant – it will adsorb almost any vapour
- Has a large capacity for organic molecules, especially solvents
- Will adsorb and retain a wide variety of chemicals at the same time
- Has an extremely large capacity to catalytically destroy ozone, a major component of smog
- Works well in a wide range of temperature and humidity conditions
- Adsorbs odours and chemicals preferentially to moisture.
- Can be used as a carrier of one material to attract other
- Is inert and safe to use.
The amount of carbon in the filter matters.
The more time the carbon has to be in contact with a contaminant the more time it has to adsorb that contaminant. We refer to this as the dwell time. In order for the carbon filter to be effective it needs to be as deep as practically possible. The speed of the air through the carbon needs to be considered as well. If the air flow is to fast then the air will pass through the carbon too quickly.
It is also important to ensure that the amount of carbon is appropriate for the amount of fume being produced. A thin amount of carbon adsorbing a high volume of fume can be over saturated in weeks, making it useless and forcing constant changing of filters.
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