Principles of Liquid Filtration

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.

Micron

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

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

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

12 Particle Size Ranges

Example	Lower Limit	Upper Limit	Typical Contaminant
	(Micron)	(Micron)
1	0.3	0.4	Smoke, Paint Pigments
2	0.4	0.55	Bacteria
3	0.55	0.7	Lung Damaging Paint
4	0.7	1	Atmospheric Dust
5	1	1.3	Molds
6	1.6	2.2	Flour Mill Dust
7	3	4	Cement Dust
8	4	5.5	Pulverized Coal
9	5.5	7	Commercial Dust
10	7	10	Pollen
11	10	75	Silt
12	75	1000	Sand

4. The micron unit of measurement is used not only to measure the size of a contaminate particle, it is also used 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.

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 micron would have a window opening of 410 micron, whereas a 50 mesh fabric with a yarn diameter of 200 micron would have a window opening of 310 micron. It's main value, now, is in the determination of a percentage of open area in a structure, which 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 exact particle size retention for a non-woven media.