Filter cartridges and bags are both widely used technologies for achieving optimal filtration performance and cost-effectiveness in liquid filtration. While both methods have advantages, technological advancements in filter bags have led to applications previously dominated by cartridge filters to consider using bag filtration instead. This blog post compares both methods and provides the information you need to make a well informed decision based on performance, cost, and practicality.
Micron ratings
Filter bags are available rated from 1 micron absolute to 1550 micron, whereas filter cartridges are available from 0.1 micron to 200 micron. Whilst both techniques are versatile, filter bags have a wider scope of applications , although filter cartridges tend to be the method of choice in applications requiring fine, sub-micron filtration.
Surface Area and Differential Pressure
Among the most significant differences between filter cartridges and filter bags is the surface area of the filtration media.
- A standard size 2 filter bag offers a surface area of approximately 0.5m². In contrast, a standard 10-inch filter cartridge such as a wound filter or a melt blown filter only provides about 0.055m².
- To match the filtration surface area of one filter bag, you would need about nine 10-inch cartridges. Pleated cartridges however can have filter areas in the region of 0.5m2 per 10 inch length, but their unit cost is significantly higher than a comparable sized filter bag.
Larger filtration surface areas equate to low initial differential pressure across the filter system. This ultimately means that the filters will have an extended service life due to slower clogging and, therefore, provide a practical advantage in applications to have fewer changeouts.
Flow Rates
Filter bags outperform cartridges when it comes to handling higher flow rates. For instance:
- Depending on the filter series, a size 2 filter bag can achieve flow rates up to 25m³/hr.
- A single 10-inch filter cartridge typically only manages around 1m³/hr per unit.
To match the flow rate of a single size 2 filter bag, you would need up to 25 x 10 inch equivalent cartridges, depending on the type of bag used. The larger 7 inch opening diameter versus cartridges which have a 1 inch core reduces flow restriction, leading to higher fluid flow rates.
Cost Efficiency
Filter bags are generally more cost-effective than filter cartridges. A bag filtration system to achieve the same purpose as a cartridge filter requires fewer filter elements, saving you considerable money.
Performance efficiency
Filter bags vary in particle vary in particle capture efficiency between approximately 60% and >99.9% depending on type, whereas filter cartridges tend to have a particle capture efficiency between 75% and 99.89% (Beta 5000), even at very fine, sub-micron ratings.
For applications requiring filtration between 1 micron and 200 micron, bags and cartridges can therefore be competing technologies and can be selected based on cost, performance and operational benefits. For applications requiring filtration greater than 200 micron, filter bags are the go to technology, whereas for sub-micron filtration, cartridge filtration would be the method of choice.
Storage, Disposal and Carbon Footprint
Filter bags are also more effective when issues of storage and disposal are considered:
- One box containing fifty size 2 filter bags requires around 0.1m³ storage and disposal volume. Providing comparable filtration performance could require up to 250 cartridges, which would require a pallet of around 2m3 volume, 20 times that of the filter bags.
- Additionally, fewer bags means reduced shipment costs and less landfill waste at disposal, hence lower disposal costs.
- The equivalent filter bags would have a far lower polymer compared to cartridges, considerably reducing the carbon footprint of your filter system.
Downtime, Ease of Changeout and Solid Collection
When changing filters, the time spent directly impacts overall productivity. Filter bags are changed quicker and easier than filter cartridges, particularly in larger systems. For instance, replacing a size 2 filter bag is equivalent to changing seven 40-inch cartridges, reducing significant downtime.
- Cartridge systems also take longer because more care is required to ensure correct seating at multiple sealing points, and in the case of double open ended cartridges, installing hold down cup and spring devices further increases changeout time.
A fundamental difference between filter bags and cartridges is that solids collect on the interior surface, minimising the risk of contamination upon removal. In contrast, cartridge filters collect solids on the outer surface, potentially leaving residues in the housing and causing contamination. Filter bags are therefore generally preferred by operators, and filter cartridge vessels require more careful cleaning before the process can restart. This difference in contamination risk instils a sense of security and confidence in using filter bags.
Contamination Risk and Sealing Points
Filter bags exhibit a distinct advantage over other systems concerning points of sealing. One size 2 filter bag requires only one sealing point between the housing and the filter bag. On the other hand, the same flow rate can be treated with seven 40 inch cartridges in a filter cartridge system, each having two sealing points, thereby resulting in fourteen sealing points.
A greater the number of sealing points increases the risk of bypass and unfiltered fluid passage, End capped cartridges do have O-ring seals at one end, but these take additional time to install and remove from a vessel.
Conclusion
While there are advantages to both the filter cartridge and the filter bag systems, the scale is tipping more and more toward the filter bag solution due to practical operations, performance and environmental considerations, as well as being more cost-effective and equally efficient. Filter bags perform to higher flow rates, have a long service life, and lower operational costs. They reduce changeout time, decrease storage and disposal volumes, and have a lower carbon footprint, whilst minimising contamination risks with fewer sealing points. Though cartridge filters are the go to technology for sub-micron filtration, one can continue to expect filter bags to increasingly be the solution of choice in a large range of applications.