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FUNDAMENTALS
Filter Bags
Liquid filter bag products are used in a multitude of industrial applications. Whether you’re working on your first liquid filter bag system or refurbishing an existing system, the following pages provide the information necessary for understanding liquid filter bag systems and their operation.
Let Industrial Filter Manufacturing ltd. guide you through the types of filter bag systems, the definition of a micron, types of filter bag media, and bag efficiencies.
Filter Bag Systems
There are two basic types of filter bag systems, Open System and Closed System.
Open System
An Open System is the most economical bag filter system consisting of a filter bag simply tied onto a pipe or secured to an adapter head through which unfiltered liquid passes.
Tie-on Filter Bag System
Adapter Head System
Adaptability is one clear benefit of a tie-on filter bag system. Eliminating the need for specialty hardware, tie-on filter bags offer versatility and simplicity for many industrial applications. Any media, any shape, any size, we can manufacture to your exact specifications.
Strainer Bag System
Perhaps the simplest filter bag is the gravity flow strainer bag. These bags can be fabricated from almost any media but are most commonly constructed from polyester and nylon woven media. Standard sizes include one, two, three, and five-gallon configurations and are usually used in a correspondingly sized container. These strainer bags are produced with an elastic, drawstring, or a raw top. They are the most economical choice for coarse gravity flow filtration.
The adapter head system is a low cost, user-friendly solution for gravity fed or low-pressure filtration. The adapter head is available in either Celcon® or stainless steel with a 1 1/2 “ NPT connection. Filter bags for the adapter head system can be of any combination of media/micron. They are installed by simply sliding the ring over the adapter head and securing the bag, creating a positive seal. Removal of the bag is quick and easy with no clamps to undo, or tools required. For an even easier change-out, our filter bags come complete with integral handles and custom ring size for a perfect fit. With the absence of a seam lump, filtration is bypass-free.
Note: For pressures higher than 10 psi a stainless steel restrainer basket is recommended.
Closed System
A Closed System is a pressurized filtration system. Liquid is delivered through a filter vessel inlet into the top of a filter bag supported by a retainer basket. Fluid travels through the supported filter media where the contaminant is entrapped. The cleaned filtrate exits through an outlet connection. Contaminate particles are retained inside the filter bag for ease of change-out and disposal. The bag filter media is selected based on the particle size retention required.
Advantages:
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Extended service life due to minimal initial pressure loss
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High dirt loading
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Quick and easy bag change-out - labour cost savings and reduced downtime
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Cost effective - tailored to suit your system parameters
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Minimal process fluid loss
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Reliable, consistent performance.
What's a Micron?
Liquid filtration involves the removal of contaminant particles in a fluid system. The grade of filter chosen for a specific application is usually determined by the size of the particle to be removed. Contaminant particles are measured using the "micron" unit of measurement.
Micron
A micron is a metric unit of measurement where one micron is equivalent to one one-thousandth of a millimetre.
[1 micron (1μ) = 1/1000 mm]
or
1 micron (micrometer) = 1/1,000,000 of a metre.
Mesh vs. Micron
The old standard imperial system of gauging a woven filtration media's ability to remove contaminant particles was the mesh system. This system simply counted the number of strands or yarns per inch of woven media. Hence, a 100 mesh media has 100 yarns per inch of media.
This system falls short because the actual window opening of a woven structure can vary as the diameter of the yarn varies. For example, a 50 mesh fabric with a yarn diameter of 100 microns would have a window opening of 410 microns, whereas a 50 mesh fabric with a yarn diameter of 200 microns would have a window opening of 310 microns.
The main value for the mesh system now is in the determination of the percentage of open area in a structure. This value is calculated by using the yarn diameter and the mesh count in order to determine the potential flow rate of a liquid through a woven filtration media.
The micron system, however, attempts to measure an exact window opening for a woven media and an exact particle size retention for a non-woven media.
Visualizing a Micron
A human red blood cell is 8 microns. An average human hair has a diameter of 50 microns. Most humans cannot see anything smaller than 40 microns with the unaided eye.
The following chart relates to the size of some common particles:
Microns (Range) Contaminant
0.3 - 0.4 Smoke, Paint Pigments
0.4 - 0.55 Bacteria
0.55 - 0.7 Lung Damaging Paint
0.7 - 1.0 Atmospheric Dust
1.0 - 1.3 Molds
1.6 - 2.2 Flour Mill Dust
3.0 - 4.0 Cement Dust
4.0 - 5.5 Pulverized Coal
5.5 - 7.0 Commercial Dust
7.0 - 10.0 Pollen
10 - 75.0 Silt
75 - 1000 Sand
The micron unit of measurement is used not only to measure the size of a contaminate particle, but also to measure the size of the openings in filter media, hence, a media's micron rating. This system of measurement is more accurate when gauging woven filtration structures, such as monofilaments, than it is for gauging non-woven structures, such as felts.
Filter Bag Media
Filtration media, whether they are woven or non-woven, are constructed from either natural or man-made fibres.
The development of synthetic fibres such as polyester, polypropylene, nylon, aramid, rayon, viscose, and polyethylene, has all but eliminated the use of natural fibre media in liquid filtration. When we select a media for a specific application its "fibre content" can be critical due to the fibres' ability to withstand specific chemical and thermal environments. For more information on chemical compatibility, visit the Tools and Resources page.
Surface Filtration
Depth Filtration
Surface media removes contaminate particles on their surface. They are generally two-dimensional woven structures and are only as deep as the diameter of the yarn from which they are woven. They only trap particles that are larger than the window opening of their structure. The advantage of surface media is that they can be woven with great precision. Their disadvantage is that they offer lower particle loading or "dirt holding capacity" because they have little depth. As a result, they tend to plug up or "blind off" more quickly than their "Depth Media" counterparts.
Not all surface media are woven. Some are constructed mainly from matted fibres bonded together with heat or binding agents and are known as "spun-bonded" or "point bonded" structures. They are primarily used in multi-layer applications as covers for bypass and transfer layers due to their low inherent strength characteristics.
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Depth media removes contaminant particles both on the surface and within the depth of the structure. They are, typically, of needled felt or melt blown, three-dimensional construction. This structure creates a tortuous path for particles to follow, often resulting in particulates of a size smaller than the actual pore openings being trapped.
Advantages:
• High dirt holding capacity
• Higher void volume or pore volume
• Ability to remove gelatinous particles
• Ability to remove particles smaller than the mean pore size opening
• Long service life
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