Guide to dry filtration systems
Air pollution control is important to keep employees and workspaces safe, as well as ensuring businesses comply with any environmental permit conditions requiring them to reduce their emissions to the atmosphere. Businesses which operate in environments with contaminated air may require a dry filtration system to help clean the air of unwanted particulates.
In this guide we are focusing on how different types of dry filtration systems can reduce air pollution, how they work, and their strengths and weaknesses. In most scenarios, a dry filtration solution is preferable to a wet scrubber as they are cost effective, simple to maintain, and don’t require potentially complicated effluent discharge systems.
We’ll be focusing on the following types of dry scrubbers:
- Bag Filter
- Shaker Filter (a type of bag filter)
- Cartridge Filter
What are dry filtration systems?
Dry filtration systems are a type of equipment used in industrial settings to clean the air of unwanted particulates. They are often mislabelled “scrubbers”, however dry scrubbers are a separate type of equipment used for a different purpose.
Like wet scrubbers they remove unwanted particulates from an air stream. However, dry filtration systems rely on a media such as a cartridge or a bag to filter the dust from the air, rather than adding a liquid. In the case of cyclone filters, no media is added and instead centrifugal force flings the particulate out of the air stream.
As with any air pollution system, it’s important to have an in-depth understanding of the application it is being used for to ensure the solution is appropriate.
Dry filtration systems are suitable for a huge range of applications, including:
- Dry food production (i.e. cereals, flours)
- Grinding, metalwork and machining
- Wood industry
- Materials handling
Types of dry filtration systems
Cyclones (or cyclonic separators) rely on a centrifugal effect to remove particles from the air. They are primarily used as a low-cost pre-filtration method as they work well for large particulates, allowing for finer filtration of smaller particles later in the treatment process with a reduced burden. This type of system can also be used to remove sparks which could cause fire or damage to a further filtration system.
Cyclones operate on the law of inertia, which is the idea that a particle will continue to move at a constant speed unless it is acted on by a force. Dirty air enters the scrubber, where the chamber creates a spiral vortex (similar to a tornado). The finer particles have less inertia so will travel up the vortex, while the large particles are flung towards the walls and drop down into a hopper to be collected and removed.
Cyclones are typically best for dealing with larger particulates and as such are usually not suitable for food applications where the particulates are too fine. They are popular as a pre-filter but care needs to be taken as a build up of too much fine particulate can mean the resulting dust cake is not permeable enough and cause it to suffer a higher differential pressure, making it less efficient.
Cyclones are also a good option for furnace and grinding applications as a spark arresting method.
- Allows for continuous disposal of solid particles
- Low maintenance option due to the lack of moving parts
- Can provide an alternative to a spark arrester
- Not suitable for removing fine dust
- Generally not efficient as a filtration system in its own right
What do we mean by air pollution?
In a factory environment, air pollution refers to any unwanted substances contaminating the air. This can include gases, fumes, dust and even odour.
Employers have a legal obligation to protect their employees from exposure to substances that could be harmful, as well as fulfilling Environment Agency rules around the amount of pollution they exhaust to the atmosphere.
When we approach an air pollution project, detailed analysis, including environmental surveys and LEV surveys, give us the insight we need to understand the specific requirements. Dry filtration systems are just one potential solution to solving air pollution problems.
The bag filter is a good option for environments which require continuous operation as it can be left to run without interruption and requires minimal intervention for cleaning.
This type of dry filtration system is generally more tolerant of difficult or arduous applications and working conditions than cartridge filters.
Bags of up to 5m in length are typically made of materials such as polyester needle-felt with an internal metal cage to support them and prevent them from collapsing under suction. The contaminated dust enters the filter and settles on the outside of the filter bags (the easiest way to think of this is the opposite to a vacuum cleaner where the dust is inside the bag).
Air is blasted from the compressed air manifold above and puffs out the bags. This blasted air uses reverse pulse jet flows at pre-programmed intervals, either timed or using differential pressure sensors to shake off the dust cake that has formed. The particulates removed from the air then fall to a hopper below.
The most common bag material is polyester needle-felt as it provides an economical and durable solution that is suitable for a wide range of applications. However, different materials can be used for different applications such as high temperature of chemical resistance. Bags can receive different surface finishes such as hydrophobic to improve performance in the presence of high moisture levels, and is also possible to seed the bags with limestone or bicarbonate of soda to protect them from excess oil.
- Can operate 24/7 until bags need cleaning
- Continuous operation through the pulse jet cleaning
- Good for aggressive environments where there is a need for low emissions
- Can be coated to different media to suit a variety of environments, e.g. high temperatures
- Adjustable pulse intervals and duration
- Standard filtration efficiency on straight forward applications is not as high as cartridge filters
- Higher differential pressure than a cartridge filter resulting in higher fan power requirements and increased running costs
- Not as compact as a cartridge filter for straight forward applications
Shaker filters are a type of bag filter. In this type of dry filtration system, dust collects on the outside of the filter bags in the same was a standard bag filter. Instead of air blasts, a belt at the bottom is used to shake off the dust gathered on the bags. The heavier dust falls directly into the hopper below for disposal while filtered air passes through the bag filter and is discharged via a fan.
The filter typically runs for four hours before cleaning is required; however the cleaning is usually hassle free and relatively straightforward. Dust is dislodged by taking the equipment offline and shaking the media. The filter needs to be taken offline to clean as the shaking isn’t powerful enough to dislodge the dust with the extraction on. The shaker filter is good for situations where there is a lighter dust burden and no requirement for continuous operation.
- Typically lower cost than pulse jet cleaned bag filters
- A good option where there isn’t a compressed air supply available
- Good for low burden environments
- Simple design
- Need to stop the filtration to shake the bags
- Unsuitable for high dust burden applications
- Unsuitable for applications producing dusts that are difficult to dislodge from the media
Cartridge filters are a cost-effective method of dry filtration system for many applications because they pack in a lot of media compared to bag filters which results in smaller filter casing sizes. They also have a lower differential pressure than bag filters requiring smaller fans and motors and reducing running costs, making them a popular choice within many sectors, including the food industry.
Cartridges are typically made from cellulose or polyester materials and coatings can be applied to improve their effectiveness, depending on the application.
Many manufacturers arrange the cartridges horizontally a number of rows high, which allows more filter area in a small footprint. However it’s important to consider access arrangements to these filters as they can be quite high. In some cases the dust, especially when dense, can build up on top of the cartridge and flatten the pleat, reducing the effective filtration area. The lower rows are more susceptible to this as dust can migrate from the upper to lower cartridges.
Vertical set-ups allow the dust to naturally drop down but are limited in height which may result in a bigger footprint. This set-up tends to be a good option for internal locations where headroom is an issue.
This type of dry filtration system requires pulse jet cleaning. Cartridge pleats are glued into place so are unsuitable for high temperature applications where the glue is at risk of melting. It’s also important to note that care needs to be taken when setting the pulse air for cleaning as this could burst the glue or media.
Cartridge filters are typically more efficient than standard bag filters when dealing with dry powders such as food ingredients and fumes from applications like welding. Care must be taken when considering applications where there may be the presence of oil on dusts or high humidity, hygroscopic dusts that can readily absorb moisture, sticky dusts or dusts that have hook or needle-like fibres, as these have a tendency to pack into the pleats of the cartridge. They cannot be removed as easily as on a bag filter due to the less aggressive nature of the pulse jet cleaning.
Special care must also be taken with abrasive dusts that tend also to be heavy – these can easily flatten the pleats and cause abrasion damage.
- Cheaper option making it good for cost-driven environments
- Effective for coping with fine dust in food applications (e.g. flour and cocoa power)
- Can be a good option if space is restricted – particularly if horizontal cartridges are used
- Usually a more efficient method than a bag filter, especially if specific emissions targets or workplace air recirculation is required
- Often a less flexible option due to their inability to cope with higher burdens
- Less suitable for humid environments and abrasive dust than standard bag filters without the addition of mitigating measures
- Pulse jet cleaning is less aggressive than a bag filter making them less suitable for hostile or arduous environments/applications
Our air pollution services
We know that product quality, environmental concerns and the health and wellbeing of employees is paramount. As well as dry filtration systems, we specialise in solving odour control, ventilation and dust extraction problems – all via our unique and independent approach to environmental technologies.
We’re experts in designing and building effective dry filtration systems across a range of sectors and industries.