Dust filtration : Filters air-cloth / air-media ratio calculation

The performance of a filter is linked, amount other parameters, to the size of its filtration area compared to the air flow to filter. It is possible to calculate this ratio, called air to cloth or air to filter media, for each filter and to determine if a filter is undersized or oversized compared to the manufacturer recommendations.

1. Air to Cloth ratio / Filtration velocity

For a baghouse, the air to cloth ratio, also called air to media ratio, is determined as the ratio in between the air flow at the inlet of the baghouse, and the actual filtration surface available. It can be calculated with the following expression :

Air_To_Cloth_Ratio = Q_air/S_{filter_effective}

with :

Q_air = total air flow at the inlet of the baghouse (m3/s)
S_{filter_effective} = actual filter surface available (m2)

Performing the dimensional analysis of the air to cloth ratio, one will note that it is actually a velocity (m/s), thus when people refer to air to cloth ratio or filtration velocity they actually point to the same thing. The air to cloth ratio is often expressed as a filtration velocity in m/s or cm/s. It is thus very important to understand the units to which refer the air to cloth ratio being discussed : air to cloth ratio in US, expressed in ft3/min/ft2 are different than when expressed in m3/s/m2 !

To be noted that it is the effective filtration area that is considered, as filters pores are not 100% available. Typical available pores represent 70 to 90% of the total filtration surface.

It is crucial to size the filter so that the air to cloth ratio is within good design standards :

• If the air to cloth ratio is too high, which means that the filtration area is small compared to the air flow, then the pressure drop through the filter will be too high, the filter will clog quicker, it will be necessary to increase the pulse jet cleaning and thus consume more energy, it can even risk early breakage and increase the frequency at which the filters must be changed.
• If the air to cloth ratio is too low, which means the baghouse is oversized, there will be less operational issues however the investment costs will not be optimum

2. Filter Air to media recommendations

The air to cloth ratio applicable for a filter depends on several parameters such as the type of filter (filter bag, cartridge...), the material to filter, the load of dust in the air to filter, the cleaning technology used (pulse jet filter, shaken filter)... thus it is difficult to give definitive values for the best air to cloth ratio, it is recommended to discuss case by case with suppliers.

For pre-design and troubleshooting only, the following references given in the litterature can be useful to the reader, note that they are valid for bag filters :

Filter	Air to Cloth ratio (m3/s/m2)	Source
No cleaning system	0.01 to 0.015	Principles of Powder Technology, Rhodes, Wiley, 1990
Shaking	0.01 to 0.03	https://www.neundorfer.com/
Reverse air	0.005 to 0.02	https://www.neundorfer.com/
Pulse jet	Up to 0.03	Principles of Powder Technology, Rhodes, Wiley, 1990
Pulse jet	0.01 to 0.075	https://www.neundorfer.com/
AHU air conditioning filters	0.25 to 1.5	Principles of Powder Technology, Rhodes, Wiley, 1990

The following data table (Turner J.H. et al JACPA 1987) is referring to specific applications, the values are however consistent with the ranges given above :

Dust	Gas to cloth ratio for felt pulse jet filters
Alumina	4.07
Asbestos	5.08
Cocoa, chocolate	6.10
Cement	4.07
Coal	4.07
Enamel frit	4.57
Feeds, grain	7.11
Fertilizer	4.07
Flour	6.10
Fly ash	2.54
Graphite	2.54
Gypsum	5.08
Iron ore	5.59
Iron oxide	3.56
Iron sulfate	3.05
Leather dust	6.10
Lime	5.08
Limestone	4.07
Paint pigments	3.56
Paper	5.08
Rock dust	4.57
Sand	5.08
Sawdust	6.10
Silica	3.56
Soap, detergents	2.54
Starch	4.07
Sugar	3.56
Talc	5.08
Tobacco	6.61
Zinc oxide	2.54

References for filter catridges are more difficult to find in the litterature, note that [Osborn] mentions that air to cloth ratios for cartridges filters are 3 to 4 times lower that the equivalent for bag filters [Osborn]. If the ratio is lower, it means those cartridges require more air filtration area than bag filters. At the end, cartridges have the advantages of having more area / length of filter thus it can compensate.

3. Other design considerations

Calculating the air to cloth ratio and sizing the baghouse within the range advised above is helpful for getting an idea of the size of a filter, or checking  that an installed filter has the right size, however, the actual sizing of the filter should always be done by a specialized company as the design will change depending on parameters such as : temperature, dust load, dust type...

For example, the reference EPA/452/B-02-001, Chapter 1, Baghouse and filters, is giving some coefficients depending on the application for pulse jet filters :

• Nuisance venting : 1.0
• Product collection : 0.9
• Process gas filtration : 0.8

This reference is also proposing to directly estimate the gas to cloth ratio for pulse jet bag filters :

V = 2.878*A*B*T^-0.2335*L^-0.06021*(0.7471+0.0853*ln(D))

With :

V = gas to cloth ratio (ft/min)
A = material factor (given in a table of EPA/452/B-02-001)
B = application factor (see above)
T = temperature (F) in between 50F and 275F
L = inlet dust loading (g/ft3) in between 0.05 and 100
D = mean particle diameter (microns) in between 3 and 100

Source

Principles of Powder Technology, Rhodes, Wiley, 1990
https://www.neundorfer.com/
EPA/452/B-02-001, Chapter 1, Baghouse and filters
Turner J.H. et al JACPA 1987
[Osborn] The importance of a correct air-to-filter-media ratio for a dust collector, Osborn, PBE, 2018