Product category:
Separation and Membrane Systems
News Release from: AxFlow | Subject: Self-cleaning disc filters
Edited by the Processingtalk Editorial
Team on 12 September 2005
The advantages of self-cleaning disc
filters
With shrinking budgets and labour forces plus increased production, mechanically cleaned filtration such as the Ronningen-Petter MCF and DCF filters offer a tremendous advantage over replaceable media
a review by Richard Charlton of AxFlow UK Limited When mechanical disc cleaning filter technology emerged during the 1990s, the expectation was that permanent wedge wire media would provide more consistent solids removal than bags, cartridges and clean in-place filters
This article was originally published on Processingtalk on 2 Nov 2004 at 8.00am (UK)
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Available from Axflow Limited, the MCF is a unique filter that employs magnetically coupled drive technology to move the self-cleaning disc, eliminating the need for shaft or external drive seals
Furthermore, this technology would eliminate media replacement and reduce both disposal and labour costs.
A decade later, disc cleaning filters have become well established within the chemicals, paints and coatings industries and are now making inroads into other market sectors.
This is because they have demonstrated that they can improve production throughput, reduce maintenance and ensure a better quality product.
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The key to the design of self cleaning disc filters is the constant low differential pressure that gives a consistently high quality product, low purge quantity (only the waste is removed) and high throughput.
Disc cleaning filters use a special permanent media to filter process material and they clean in-place.
These units consist of a cylindrical filter screen inside a pressure-rated housing.
Unfiltered liquid is pumped into the top of the filter housing and down into the permanent filter element.
Such is the machining quality of the filter, unwanted matter cannot become trapped or wedged in the screen.
Suspended solids are deposited on the inside of the screen, with the clean fluid exiting through the outer side of the screen housing.
The solids collected on the inner surface of the screen are removed by the action of a single spring loaded circular cleaning disc that moves up and down, scraping off solids as it moves.
The disc edge wipes the screen surface clean and all collected debris is moved to a collection chamber at the bottom of the filter.
The debris is then purged by a discharge valve without there being any break in the process flow.
Paints and Coatings.
The paints and colorants industries present particular challenges for traditional open bag and vibrating screen filtration systems due to the nature of these similar products.
DCF filters designed by RPA Process Technologies have proved to be highly effective for two US companies in the paints and colorants industry.
The Color Corporation of America faced several problems when using open bag filters to remove undispersed pigments and contaminants from its products.
Fed by gravity through a bag filter at the base of a blending vessel (let-down tank), colorant was deposited into an accumulating vessel.
From there a pump transferred the product to the designated filling line.
Both the bag and the accumulating vessel were open to the atmosphere and although the release of Volatile Organic Compounds (VOCs) from the bag and vessel were within predefined tolerance levels, the company wanted to achieve zero emission.
There was also a problem resulting from the viscous nature of the product.
When faced with increases in viscosity, blinding of the filter bag speeded up and there was an increase in the differential pressure across the bag.
On occasions, this led to the bag bursting, resulting in product contamination and lost production.
The solution was to replace the existing system with a Ronningen-Petter mechanically cleaned DCF-800 filter and an air-operated double diaphragm pump unit mounted on a mobile cart.
The installation has the pump suction side attached to the base of the let-down tank and the discharge side, incorporating a pulsation dampener, directed to the filter inlet.
The filter outlet is connected to a filling machine and the entire colorant filling process is completely enclosed, thereby eliminating the possibility of VOCs being released to the atmosphere.
Control of the mobile pump and filter unit is via the filling machine.
When in operation, the pump transfers the colorant through the DCF800 filter and filling machine head.
When the system recognises that a container is full, it automatically stops the compressed air supply to the mobile unit.
The DCF filter performs its self-cleaning cycle when instructed by the filling machine.
As a result of the introduction of the DCF filter, higher flow rates have been achieved and the problems of burst filter bags and the time associated with changing bags have been eliminated.
In contrast, latex and stain manufacturer PPG Architectural Products employed a vibrating screen, accumulating vessel and diaphragm pump in its filling operation.
Not only was this system overly complex, it was noisy, required constant operator attention and produced unacceptable levels of product loss.
The system employed an air-operated diaphragm pump at the base of the blend tank to transfer product on to a large vibrating screen that was open to the atmosphere.
Product exiting the screen was fed into an accumulating vessel where another pump transferred it through a polishing filter and into a rotary drum filling machine.
Here, the manufacturer wanted to reduce product waste, reduce operator involvement and simplify the whole process.
Introducing the DCF800 filter achieved these objectives by replacing the vibrating screen, the accumulating vessel and its associated pipework, the second diaphragm pump and its compressed air components, and the required controls.
In addition, operator involvement has been virtually eliminated.
With the DCF800 filter, paint enters the top inlet of the filter housing and passes through the screen.
The screen holds undispersed pigments, paint skins and debris, while the cleaned paint exits the bottom outlet of the filter.
The self-cleaning disc removes debris from the screen into the holding chamber from which it is purged as and when necessary.
Removing the second pump and vibrating screen has reduced noise levels.
Also, significant reductions in product waste have been recorded due to the DCF filter top-to-bottom flow design and debris purging technology.
An additional benefit to have come out of the change to disc filter technology has been that the filter purges residual paint and debris into the plant recycling system, where the solids are separated and the paint re-used.
Magnetic Coupled Filters.
The success of self-cleaning disc filter technology has encouraged Ronningen-Petter to examine the drive mechanism of its filters; the result being that is has introduced a magnetically coupled drive mechanism to move the self-cleaning disc.
This has eliminated the need for shaft or external drive seals, and without the motors and actuators mounted on the lid of the filter unit, the filter chamber can be readily accessed for the rapid removal of the screen.
This is highly desirable in batch processing, as it optimises production uptime.
The take up of the new Magnetically Coupled Filter (MCF) has been swift.
For one major international automotive manufacturer, discarding the OEM-equipped replaceable media filters and installing the MCF has generated over $60,000 of annual savings and a Return on Investment (ROI) in less than three months on their filter purchase.
The process in question was a group of three machining centres employed for machining aluminium transmission parts.
The machine tool coolant to be filtered lubricates, cools and flushes contaminants away from the part and tool.
The fluid is collected under the machining operation where it is routed through a set of weirs to separate out the large particulate.
A pump then sends the fluid to a housing containing three large 75-micron cartridge filters.
Historically, the filters needed to be replaced three times per week, generating a tremendous amount of annual costs to the automotive company.
An in-house filtration audit highlighted the recurring high costs of the cartridges, cost of disposal, labour for change out and machining down time.
Other costs and inconveniences included tool life concerns, the inability to separate harmful chips from the fluid, all resulting in borderline product quality and safety concerns.
In their search for a solution for the costly and recurring problem, the customer selected the MCF, featuring a permanent self-cleaning slotted filter media that effectively removes the harmful aluminium chips and fines from the coolant.
The automotive company chose the MCF filter for its spring-loaded cleaning disc design that continuously regenerates the open area of the filter media.
The engineers responsible for the machining centres requested tighter filtration levels, specifying a 50 micron screen upon installation as a potential opportunity to increase tool life.
The new MCF filter was installed seamlessly into the existing space of the removed cartridge filter.
Start up of the MCF permanent self-cleaning filtration equipment and its ability to more efficiently manage the full coolant flow and chip loading, had an immediate positive impact on the machining centres.
The more efficient cleaning cycle and purge frequency were adjusted to meet customer needs.
Extending the cycle times greatly increased the life of the moving parts and allowed for a greater concentration of chips to be discharged.
The purge, or concentrated chip stream, was re-routed back to a large paper band filter three weeks into the installation for recycling purposes.
The quality of the filtrate under the MCF filter was evaluated every shift for the first few weeks to ensure the quality of the machined parts.
Full evaluation of the economic impact of the MCF installation was completed following three months of continuous operation.
The final review determined that the MCF paid for itself within that three-month evaluation period.
Magnetic drive.
Operating in the same manner as its DCF counterpart, the MCF differs in that it contains a hollow shaft at the centre of the system in which is located a piston with powerful rare earth magnets.
These internal magnets are coupled to external magnets housed in the carrier connected to the cleaning disc.
Two air connections are located at the bottom of the housing where the actuator shaft is locked and sealed.
One connection drives air under the piston with the rare magnets inside the actuator shaft.
The second connection drives air through a small tube at the top of the actuator shaft, directing it over and driving the piston down.
This pneumatic action moves the internal magnets up and down the shaft, with the cleaning disc following behind.
The result is a powerful actuation that does not require a physical linkage passing through the vessel.
The cleaning disc is secured to the carrier assembly and is moved up and down the element by the air and the magnets.
A further enhancement that has been introduced with the development of this new filter is that the cleaning disc is a four-piece component.
This has the benefit of putting equal tension on each piece to give better contact with the filter element.
It also eliminates dead spots, resulting in improved efficiencies.
Conclusion.
With shrinking budgets, increased production and reduction of labour forces, mechanically cleaned filtration such as the Ronningen-Petter MCF and DCF filters offer a tremendous advantage over replaceable media. Request a free brochure from AxFlow ...
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