Bag filters for dust separation : design, materials, filter size calculation

Bag filters are the oldest and mot common filter technology used to capture dust in a stream of gas, typically air. Although an old technology, a well sized baghouse with modern filtration media can reach high dedusting performances. This page is helping you to understand how are made bag filters and how to size a baghouse.

1. Bag filters : design

What is a bag filter ?

Bag filters are made of soft flexible filtration media that are mounted on a cage (typically metal such as stainless steel) that give them mechanical strength. Filter bags are either cylindrical or rectangular and assembled in a baghouse which allows to position them easily with the required distance in between each bags for a good filtration.

What is the difference between bag filter and cartridge filter ?

Bag filters have the following pros and cons compared to cartridge filters.

Pros of bag filters vs cartridge filters

• Lower unitary cost
• No risks of clogging in between pleates as it is the case for cartridges.
• Can handle higher filtration velocities

Cons of bags filters vs cartridge filters

• Lower filtration area within the same volume
• More difficult to assemble, takes longer to maintain
• For most common materials, lower filtration efficiency

It is critical to have a good anchoring of the bag to its supporting cage, and of the supporting cage to the supporting plate. This indeed ensures that no bag filter can slip or fall down which would have the consequence to create a dust leak and also potential foreign bodies on the product. The contact in between the bag filter and the supporting plate is also critical to ensure electrical continuity and thus manage the risks of dust explosion that may be triggered by a bag filter getting charged and suddenly generating sparks when the tension is high enough.

2. Bag filters materials

How many types of bag filters are there ?

There are 2 major technologies for the manufacturer of bag filters :

• Woven media filters
• Felt media filters

Woven media filters are the oldest design and are now less widespread. They are mainly used in baghouses that are using reverse air or shaken filters as cleaning mechanisms. The materials used in woven media is typically fiberglass or polyester. Woven media have typically lower efficiencies than needlefelt media due to their very well arranged, woven, structure.

Felt media filters are non woven and are made by short fibers needled together so that they form complex network of fibers. Felt media are sometimes supported by woven layer called scrim. Needlefelt bag filters are usually made of one of the following materials : polyester felt (the most common), aramid felt (higher resistance to temperature vs polyester), PTFE felt (provides superior resistance to chemicals and temperature). Felt filters are the most efficient media type for use in pulse jet cleaning baghouses.

A certain number of enhancement of the filter media can be done thanks to specific treatments or by using a blend of fibers. The most common improvements to bag filter media are listed below. It concerns felt filter media mainly :

• Without treatment : needlefelt filters are sensitive to dust penetration in the filter media. Fine dust will 1st get inside the filter, get stuck in between the fibers then once this 1st phase is done, dust will start accumulating as a layer on the surface of the filter, forming what is called a dust cake. The dust cake actually acts as a filter itself and thus improves the filtration efficiency. However, dust penetration still continues over time and leads to an increase in the pressure drop of the filter and with time leads to its clogging. Those filters are good choices if the dust is not too fine and if the air flow is not too high
• Coating : to coat a filter, a acrylic or PTFE is sprayed on the surface of the filter. The coating is not continuous and thus do not prevent deep dust penetration in the filtration media. However, especially for large particles, it helps the dust cake release and thus offers a better cleanability.
• Membrane : a membrane of PTFE is applied to the surface of a needlefelt polyester media. The membrane is creating a continuous very fine layer which is then preventing even small particles to penetrate in the polyester matrix. This allows to get a better filtration, less pressure drop and a better dust case release and thus more efficient cleaning when the filter is pulsed.

Special filter media offer solutions to some of the common problems experienced by industrialists. [Scoble] is giving some suggestions on which filter to select to answer to specific issues :

Poor dust cake release (clogging of the filter due to a thick layer of material)	Glazed felt filters Use a PTFE membrane
Dust leakage	Use felt filters made of ultra fine fibers (micro-denier) Have a specific fiber blend Use a PTFE membrane
Filter wear	Use coated filter with a silicone solution
Filtration of fibrous dusts	Glazed felt filters Use a PTFE membrane

3. Bag filters : required filtration area calculation, filter sizing

Bag filter sizing : how to size a bag filter system ?

When designing a dust collector using bag filters, the following design sizing criteria must be studied : air to cloth ratio (filtration velocity), intersticial velocity and can velocity.

3.1 Air to Cloth ratio for filter cartridges

The air to cloth ratio is actually similar to the air filtration velocity calculated by dividing the volumetric air flow at the inlet of the dust collector, by the total filtration area installed.

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)

In US, the same calculation is done but with cfm and ft² which gives an air to cloth ratio in ft/min, it is important to know in which units the ratio is expressed as the values

For bag filters, literature references mention filtration velocities up to 0.06-0.07 m/s but more precise values can be obtained from manufacturers' data.

Example : the following data are given by a cartridges manufacturer

The manufacturer gives a permeability of 65 l/dm2.min, which means a filtration velocity or air to cloth ratio of 0.108 m/s which appears slightly higher than the value given above.

The air to media ratio is the most common design criteria discussed for filters. However, for pulse jet systems where the bags are vertically mounted and the inlet of dust laden air below the cartridges, the air velocity below, and in between the filters, should not be neglected as. If it is too high, it will prevent the dust unclogged from the filters during a compressed air pulse, to fall down. The air going upwards will indeed immediately re-entrain the powder which will make the pulse jet cleaning ineffective.

3.2 Can velocity

The can velocity is defined as the volumetric air flow divided by the cross sectional area of the filtration chamber.

Can_Velocity = Q_air/S_chamber

With :

Q_air = total air flow at the inlet of the baghouse (m³/s)
S_chamber = cross sectional area of the filter chamber (m²)

3.3 Intersticial velocity

The intersticial velocity is the velocity in between the filter, which means that it is defined by the volumetric air flow divided by (the cross sectional area of the chamber - the sum of the cross sectional areas of the filters)

Interstitial_Velocity = Q_air/(S_chamber-S_{section_filters})

With :

Q_air = total air flow at the inlet of the baghouse (m³/s)
S_chamber = cross sectional area of the filter chamber (m²)
Ssection_filters = sum of the cross sectional areas of the filters

4. Filter size Excel calculator

Different characteristics related to filter sizing can be estimated thanks to this free Excel calculator : Calculation Tool - Filter sizing Excel calculator (click here)

Warning : this calculator is provided to illustrate the concepts mentioned in this webpage, it is not intended for detail design. It is not a commercial product, no guarantee is given on the results. Please consult a reputable designer for all detail design you may need.

Source

[Scoble] Specialty bag filter media for better performance in tough applications, Scoble, PBE