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Industrial Air Filters

Bag filters and cartridges for air dedusting during pneumatic transport

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Section summary
1. Introduction
2. Dedusting filter design
3. Dedusting filter performances
4. Important design considerations
5. Safety of filters, dust explosion prevention
6. Bag filters manufacturers

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1. Introduction

Dedusting filters need to be positioned in powder handling systems at each place where an exchange of air must be performed in between the process and the environment. Industrial air filters will allow to :

• Avoid pollution of the product with dust or any other foreign material when the air is sucked from the environment

• Avoid rejection of dusts in the environment when air needs to be vented. It is necessary for legal reasons but also just to allow a proper and safe work environment without dust.

Filters can be passive (no automatic cleaning system) or active (with automatic cleaning system), depending on their place in the process as well as the service they are used for.

Figure 1 : positions of filters in a typical pneumatic conveying line

In bulk solids handling processes, the typical applications are the following :

• Pneumatic conveying filters located at the beginning of the conveying line (to avoid pollution to go in the process) and at the end, in cyclone filters
• Venting filters in order to perform air dedusting of the air displaced when filling hoppers for examples
• In dust collectors, for example at a spray drying process, such industrial dedusting filter bags will avoid high levels of emissions in atmosphere and will improve the economics of the process by recycling the powder captured
• In air handling units

2. Dedusting filter Design

Filters are produced commercially under the shape of bag filters, the material looks like fabric, or as cartridges, the material here is stiffer. Very small filters can have only a single bag filter or cartridge but most of the time there are many filters supported by a housing (bag filter housing).

Filters are usually achieving a high degree of separation by using the product itself as filtering media. Indeed, the filter will retain the powder which will constitute a fine layer over the filter. This fine layer allows to reach high degree of filtration.

Figure 2 : principle of operation of a pulse jet filter

However, letting this layer of product grow too much will lead to a blockage of the air and the increase of the pressure drop of the filter. It is therefore necessary to clean the filter at regular interval. Nowadays, such a cleaning is performed thanks to the injection, counter current, of a stream of compressed air (or another gas if air is not allowed). These filters are called pulse jet filters. The puff of gas will make the product accumulated on the bag filter or cartridge fall back in the receiver, which will decrease the pressure drop generated by the filter. Other bag filter systems designs to decrease the size of the product cake are shaken filters and reverse air flow filters ; however, those processes cannot be used in continuous, it is necessary to stop the process, clean the filter and restart, that is why pulse jet filters have become very popular.

In order to perform such a backflush, it is necessary to have a strong (to be able to make the material fall), but short, flow of air (not to disturb the conveying process) through the baghouse. A bottle of air is installed very close to the filter and a nozzle equipped with an electro valve is installed for each baghouse filter. When the filter needs to be cleaned, the electrovalves open and the bottle discharges the air. Usual pressure of the bottle is in the range 3-4 barg.

3. Dedusting filter Performances

How to calculate the size of filters ?

The key performance parameter to estimate in order to size properly a filter is the pressure drop that will occur through the filter. It must be noted that the pressure drop is actually composed of the pressure drop through the fabric of the filter (which is depending on the efficiency of the filter) + the pressure drop through the cake of product that has deposited over the filter.

This pressure drop is actually a function of the filration velocity which is often expressed in m/s. If the pressure drop is too high, it is necessary to add bag filters, since the surface is increasing, the filtration velocity is decreasing as well and reduces the pressure drop. Typical filtration velocities for filters equipped with backflush (pulse jet) is 0.01 to 0.075 m/s.

This order of magnitude allows to calculate the filtration area. For example, if you run a 500 m³/h conveying line with a receiver equipped with pulse jet filter, you can estimate the need of filtration area as A=500(m³/h)/(144(m/h))=3.5m²

In addition to the filtration velocity (also called air : cloth ratio), 2 other velocities must be calculated : the interstitial velocity and the can velocity.

Interstitial velocity of filters : it is the upward gas velocity in between the bag filters. It is calculated by dividing the air flow in m³/h by (the cross sectional area of the filter box minus - the bag area = open area in between bags). It should be typically below 1.5 m/s (or 300 fpm). It is important the interstitial velocity is not too high otherwise the dust that falls down from the bags when a cleaning cycle is done will not fall down to the hopper but be carried back straight away to the filter. To be noted that sometimes interstitial velocity is confused with can velocity.

Can velocity of filters : it is the upward gas velocity just before the bag filters. It is thus lower than interstitial velocity. It is calculated by dividing the air flow in m³/h by the cross sectional area of the filter box. Can velocity, as interstitial velocity, should not be too high to prevent that dust cleaned from the bag is immediately pick up by the air going upwards.

It's in practice necessary to ask a company specializing in filtration or pneumatic transport to size the filter, but the simple orders of magnitudes above are useful to check a design existing or a proposal.

4. Important Design Considerations

The backflush of the filter can be considered either based on the pressure drop or on a timing. Even if based on timing, it is wise to consider a pressure differential gauge in order to assess the state of the filter.

Filters can be a source of contamination since some product will fall back in the product stream during the cleaning process : for sensitive products, different set of bags must be used. In this case, filter boxes design allowing a good access and quick change of bag filters is necessary.

5. Safety of filters : dust explosion prevention

Filters have been in the past the source of several dust explosions, especially when they are used in pneumatic conveying. Filters can indeed accumulate static electricity due to the flow of air and material, if they have not been properly grounded, this static electricity will discharge from time to time, from the filter to another piece in the equipment, typically the chassis on which is assembled the filtering media. A spark is created during the discharge and if a dust cloud within its explosive concentration limits is present, an explosion can happen.

In order to prevent such a risk, the filter and its support must be certified for use in area where dust is present (ATEX zone in the European regulation). The chassis supporting the filtration media must be grounded and the filter bag or cartridge must be either conductive, in which case a grounding wire will have to be installed, or with low breakdown voltage and guaranteed for usage in dust explosive atmosphere. Electrical continuity must be ensured.

It is a safety requirement to assess the dust explosion zoning in which the filter will operate, select an equipment certified for this zone, and to ensure by a strict maintenance that the system stays over time as per the manufacturer requirements (especially all grounding cables are connected, the filter media is the same as designed).

Please refer to the Process Safety Handbook for more information about dust explosion prevention and mitigation.

6. Bag filter manufacturers

Filters, or cartridges, now play a key role to ensure the compliance of a factory operator in terms of dust emissions, product containment and dust explosion safety. The choice of a reputable manufacturer is therefore critical. Buying cheap filters from an unknown company can have very expensive consequences in case the filter media breaks, in case the filtration efficiency is not what is expected or if the pressure drop is higher than design (impact on performance).

One should always contact suppliers with several years of experience, interview them, ask questions using the infos of this page, and contract the manufacturer which will offer the best knowledge / price ratio.