Air to cloth ratio

The air-to-cloth ratio is the volumetric flow rate of air (m³/minute) flowing through a dust collector's inlet duct divided by the total cloth area (m²) in the filters. The result is expressed in units of velocity

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The air-to-cloth ratio is typically between 1.5 and 3.5 metres per minute, mainly depending on the concentration of dust loading.

For details on how to calculate air-to-cloth ratio, there is a detailed formula and example at http://www.usairfiltration.com/dust_collectors/dust-collector-terminology.htm