Bag Filters / Filter Media Overview

Felts
Operating on the principle of depth filtration, needled felt filter bags are effective at removing both solid and gelatinous particles. 100% polyester fibers are needled to create a tortuous path in a three-dimensional medium. Materials available are Polypropylene, polyester, nylon, PTFE and Nomex.

Felt Advantages

• High dirt holding capacity
• Capability to remove gelatinous particles
• Cost-effective filtration due to long service life
  
  

Monofilament Mesh
Also operating on the principle of surface filtration, monofilament mesh filter bags are manufactured from mesh materials in which each strand is a single filament. The non-deformable filaments are woven in a specific, consistent pattern and fusion welded for extra strength. Monofilament meshes are structurally very strong and are available in both nylon and polypropylene with micron ratings from 5 to 800.

Monofilament Mesh Advantages Include:

• Absolute rated medium
• High mechanical strength

Oil Adsorption

Manufactured from a combination of polypropylene felt, meltblown polypropylene and a protective polyester mesh cover, the oil adsorption filter bag series is designed to remove oils and hydrocarbon liquids from aqueous fluids, while providing the benefits of particle retention.

Oil Adsorption Advantages Include:

• Unparalleled adsorption characteristics for oils and hydrocarbons
• Higher flow rates
• Superior levels of adsorption efficiency via controlled flow velocity

Liquid Filtration
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.

1. A micron is a metric unit of measurement where one micron is equivalent to one-thousandth of a millimeter [1 micron (1µ) = 1/1000 mm] or 1 micron (micrometer) = m1/1,000,000 of a meter.

2. Visualizing a micron

• a human red blood cell is 5 microns
• an average human hair has the diameter of 100 microns
• most humans cannot see anything smaller than 40 microns with the unaided eye

3. The following chart relates to the size of some common particles:

Fiber Content
Filtration media, whether they are woven or non-woven, are constructed from either natural or synthetic fibers. Today, the only natural fibers still used in limited applications are wool and cotton. Their primary benefit is in their ability to withstand higher temperatures.

Development of synthetic fibers, such as polyester, polypropylene, nylon, aramid, rayon, viscose and polyethylene has all but eliminated the use natural fiber media in liquid filtration. When we select a media for a specific application, its fiber content can be critical due to the fiber's ability to withstand specific chemical and thermal environments.

Surface Media
1. Surface media are filtration structures which remove contaminate particles on the surface of the structure. They are generally two dimensional woven structures and are only as deep as the diameter of the yarn from which they are woven. They will only trap particles that are larger than the window opening of the structure. Their advantage is that they can be woven with great precision, therefore offering exact window openings, providing entrapment of particles of a specific size, Their disadvantage, however, is that they do not offer high 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.

2. Not all surface media is woven. There are surface structures that are constructed mainly from matted fibers which are bonded together with heat or binding agents. They are commonly known as spun bonded or point bonded structures and are primarily used in multi-layer applications as covers or bypass and transfer layers due to their low inherent strength characteristics.

3. The most widely used surface media are woven structures using either multifilament or monofilament yarn made from polyester, polypropylene and nylon.

Multifilament Media are woven from strands or yarns, which consist of many smaller strands or fibers that have been spun or twisted together. They have the advantages of being the most inexpensive woven media, and therefore produce a very low cost disposable filter bag. An individual yarn may vary 50% or more in diameter and generally produce a woven structure that has a rectangle rather than a square window opening. Contaminate particles tend to get trapped within the fibers of the yarn making this media very difficult to clean and re-use. Despite these shortcomings, they are well suited to a great many filtration applications where high precision is not required and low cost is key.

Monofilament Media are woven from strands or yarns which have a consistent diameter and a very smooth surface. The disadvantage of this type of yarn is that they weave a very precise filtration structure with consistent square window openings. The yarns smooth surface enables it to be cleaned and re-used in many applications. Monofilaments offer precision rated media with high mechanical strength.

Depth Media
Depth media are filtration structures which remove contaminant particles both on the surface and within the depth of the media. They are typically of needled felt or meltblown construction in a three dimensional structure. This creates a tortuous path for particles to follow, often resulting in particles of a size smaller than the structure.

The advantages are: high dirt holding capacity, higher void volume or pore volume, the capability of removing gelatinous particles and particles smaller than the mean pore openings, and most importantly, a long service life due to the three dimensional structure