The Cost of a Non-Standard Wound Polypropylene Cartridge Filter

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  • August 21, 2023
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What is a Wound Polypropylene Filter Cartridge?

These are filter cartridges that are used to remove impurities in the form of sediments in a liquid. They are manufactured by winding a continuous strand of polypropylene yarn over a perforated supporting core. This core is also normally made of polypropylene. The criss-cross winding creates a multi diamond shaped pattern across the length of the filter. The closeness of the winding pattern can be varied to produce filters of different porosities. A very close pattern would mean that only very fine particles can pass through it. This would mean that its porosity is low. A more spaced our winding would mean that particles of a larger size can pass through it. For eg a low porosity 1 Micron Rated cartridge would have a close pattern. This filter would only allow particles of size smaller than 1 micron to pass through it. A higher porosity 10 Micron rated cartridge would have a more spaced our pattern. This would allow particles of less than 10 micron to pass through it.
Filter Cartridge
Polypropylene (PP) is the popular choice for these filters because of it is insoluble and inert to a wide range of liquids. In applications involving drinking water, food and beverages, dairy, pharmaceutical – a virgin grade PP must be used. Recycled PP can have detrimental health effects.

They work on the “Depth Filtration Technology”

The sediments are trapped within a bed of depth of fibres by adsorption or absorption. The irregular pores in the bed causes a torturous path to trap fine particles. The filtration happens due to direct interception, inertial impaction, diffusion and electro-kinetic effect. There is a gradual increase in pressure drop across the cartridges until such time that the cartridges are fully choked. In these filters, the trapped particles are more difficult to remove. This can be done by back washing and chemical treatment. These are NOT recommended as such procedures severely effect the micron rating of the filters.
Schematic Drawing Cartridge Filter

International Standard Dimensions of the Wound PP Cartridge Filter

The Wound PP Filter product is a popular choice across the world. There are millions of installations of housings around the world that have these filters fitted inside them. The housings are deployed for the following international standard dimensions of the filters.
Outer Diameter 63mm (2.5”)
Inner Diameter 27mm / 28mm (1.1”)
Lengths 254mm (10”) / 508mm (20”) / 762mm (30”) / 1016mm (40”)
A Wound PP 10” cartridge of 5 Micron Porosity would have a weight of around 165gms

Non-Standard / Low Quality Cartridge in Terms of Dimensions

Many a times, a small change in dimensions of the outer diameter and inner diameter of the cartridge can go undetected to the naked eye. The schematic drawing shows a classic example. It compares a standard cartridge (OD: 63mm / ID: 28mm) vs a non-standard cartridge (OD: 58mm / ID: 32mm). The change is a small one, but when you analyse, the non-standard cartridge has approximately 27% less yarn in terms of volume and 15% less yarn in terms of weight. This would be a significant cost saving for a company which is mass producing such filters.
This small change in dimensions could go undetected by an innocent buyer. They fall prey to a lower cost cartridge while comparing it with the ideal cost of a standard cartridge. However, this lower cost cartridge would significantly change the performance parameters of the cartridge. The “flow rate” through the cartridge and its “dirt holding capacity” would be drastically reduced. The life of the cartridge would be reduced. This would thereby increase the spare cartridge cost and the down-time cost of changing the cartridges in a big way. The cost saving enjoyed while procuring the non-standard cartridges is nullified and reversed in the long run.
Cartridge Standards

Non-Standard / Low Quality Cartridge in Terms of Weight

The PP yarn is the media that provides the filtration. The inner core is just a support to the yarn. Therefore, it is quite logical that it is the weight of the PP yarn which is of importance while ascertaining the quality of a cartridge and pegging it against a price.
Refer to the below table.
Cartridge In Terms Of Weight

The non-standard cartridge has been produced using lesser pp yarn. This is compensated by using a heavier supporting core. The weight of the finished cartridge is thus maintained to manipulate the sale. The non-standard cartridge is offered in the market at a lower price than a standard cartridge whilst confirming that the weight of the cartridge is same as the standard cartridge. As explained earlier the flow rate and the dirt holding capacity of such cartridges will be lower. It would reach its change over pressure drop levels much faster.  

 

Recycled PP is heavier in weight compared to virgin grade PP. Using recycled PP in both the yarn and core of the cartridge can also increase the weight, whilst reducing the product cost. A heavier weight cartridge can create a misconception of being a superior cartridge as it uses more PP. However using the recycled PP does not increase the depth holding capacity in any manner. Moreover it is harmful for drinking water, pharmaceutical, food and beverage industry.

Non-Standard / Low Quality Cartridge in Terms of Micron Rating

A user selects a cartridge of a certain porosity to address the needs of their application – drinking, product processing, machinery requirement, process requirement etc. Ethics would demand that a supplier provides a cartridge, rated to the micron rating as required by the user. For eg, if a user needs a cartridge rated to 1 micron porosity, the supplier needs to give a 1 micron cartridge and not a 5 micron cartridge. Using a higher porosity might affect the process greatly.

 

As explainer earlier, the wound pp cartridge can be made in different porosities by machine settings. The lower porosity cartridge could have the yarn strand closer to each other and they would be “tighter” and slightly “heavier”. For a user, it would be very difficult to identify and confirm the porosity of a cartridge by looking at it. The user would need to trust the word of the supplier, the marking on the label if any on the cartridge or a test certificate given by the supplier.

 

It is quite unfortunate that in many cases, cartridges of the same porosity are labelled with different porosity ratings and made available for sale. The advantages to an unscrupulous manufacturer indulging in such practices would be,

  • Mass production of a particular porosity would mean less machine down time and cost saving
  • Selling a 10 micron cartridge with a label of 0.5 Mic / 1 Mic / 5 Mic would mean cost savings as the 10 micron cartridge would be slightly lighter in weight requiring lower amount of pp yarn.
  • Non availability of accessible and economical testing rigs would mean that the chances of the customer verifying the porosity is very bleak.
Let us take a reverse osmosis plant as an application. The wound pp cartridges are popularly used as a pre-treatment of the input water to reduce the sediments going into the membrane. Normally a 1 Micron or a 5 micron cartridge is selected for this process. Using a higher porosity cartridge like a 10 micron one could allow that much more sediments to pass through it and reach the high pressure pump and membrane. This would severely affect the life of the pump and also choke the membranes faster. The pump and membrane being expensive components, the cost to the user because of a non-standard cartridge could be very high.

Non-Standard / Low Quality Cartridge in material grade of polypropylene

Polypropylene can be recycled. This process would include different additives to bring back the pp in a raw material form suitable for a particular application. For eg recycled PP could be used to manufacture PP yarn. Recycled PP being cheaper, is a raw material option available to manufacture wound pp cartridges. Such cartridges would be available at lower prices and pose a better proposition to the buyer.
Applications like drinking water, food and beverages, pharmaceutical, dialysis etc require cartridges manufactured using virgin grade pp only. Recycled pp could have additives that might cause cancer and other health hazards.

Conclusion

Filtration is an important process in any application. When it involves drinking water, the trade of supplying filters becomes a noble one. It is a moral responsibility of the manufacturer of the filter to provide standard products meeting necessary norms. At the same time, it is important for a customer to be educated and aware of the right product while having options to choose from.

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