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How to Calculate a Filter Resistance Coefficient

Filters are devices that capture contaminants from a flowing substance, such as air or water. Filters can be constructed from a variety of materials, including cloth, fabric and polymeric blends. Air filters, for instance, have an airstream that passes through the filter media. Particles are trapped in the fibers of the media. Resistance to this flow is often referred to as pressure drop (inlet pressure minus outlet pressure). The resistance coefficient, K, takes into account the fibrous void volume, flow constant and filter surface area.

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Instructions

- 1
  Determine the porosity or fibrous void volume (e) of a filter media. This is a dimensionless (unitless) value that is determined empirically from filter manufacturers. For instance, a polyester filter cloth has a porosity of 0.0032. This means that the ratio of open space between the fibers is 0.0032.
- 2
  Determine the flow constant for the filter system. The flow constant (c) is the ratio of inlet flow to outlet flow and is usually considered to be close to 1.0 for clean filter media. Assume the flow constant is 0.99.
- 3
  Determine the specific surface area (S) per unit volume of media. This is a measure of the media fiber̵7;s surface area that can be found in a specific volume of media. For instance, assume a one-eighth-inch thick polyester filter cloth has a specific surface area of 1,020.
- 4
  Determine the resistance coefficient, K, using the formula K = e^3 x c x S^2. In this example, 0.0032^3 x 0.99 x 1,020^2 = 0.0337.

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