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Usage of filters in bulk solids handling : design and operation of filters

Industrial air-solids separation, dedusting

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Section summary
1. Introduction
2. Design
3. Performances
4. Important design considerations
5. Common problems with air filters

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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. They will allow to :

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

• 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, and for economical reasons as keeping materials in the process is reducing losses of salable material.

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

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 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 dyring process, such industrial dedusting filters 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

Figure 1 : example of types and location of air filters in a bulk handling process

2. Dedusting filter Design

Filters are produced commercially under the shape of bag filters (baghouse filters), the material looks like fabric, or as cartridges, the material here is stiffer.

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.

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 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 filter fall back in the receiver, which will decrease the pressure drop generated by the filter. Other designs to decrease the size of the product cake are shaken filters and reverse air flow filters ; however, those processes can 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). A bottle of air is installed very close to the filter and a nozzle equipped with an electro valve is installed for each bacg. 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.

2 different designs for industrial air filtration can be found in powder handling industries. The system can be centralized and deported, with a single baghouse equipped with a fan that will suck air and dust from different equipment through a network of pipes and bring it to the industrial dust collectors (this can be preferred in industries like chemicals, wood, cements, plastics), or localized with a small filters integrated directly to the vents of receivers and other hoppers (this will be preferred in food, dairy, pharmaceuticals industries - see figure 1 above).

3. Dedusting filter Performances

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 + the pressure drop through the cake of product that has deposited over the filter.

This pressure drop is actually a function of the filtration 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

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 dust 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 on the dust collector in order to assess the state of the filter.

Dust filter systems can be a source of contamination in case the filter is not a standalone unit, deported, but integrated to the process as a vent filter for example. 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. Common problems with Air filters

Air filters are usually reliable equipment, however a certain number of issues may require some corrections :

Table 1 : common problems with air filtration

Issue	Root cause and action
Overpressure during pneumatic conveying	The filter installed in the receiver may be too small - control the filter size The filter installed at the reception of the pneumatic conveying line may be dirty and blocked - check the cleaning sequence, air pressure and frequency
Powder leakage through the filter	The filter may be damaged or not properly installed - check the filter and repair The filter may not be of a correct type in terms of filtration
Too much air consumption for pulse jet filters	Incorrect pulse jet settings - check the air pressure, the frequency and if there are no leaks at solenoid valves on the filter flushing system