Filter elements, the heart of dedusting systems - Where they differentiate and how they operate

In the production of the pharmaceutical and chemical industry, dusts occur time and again. As this is not desirable, measures must be taken to prevent the occurrence of these dusts. One possibility is the employment of filter systems, so-called dedusters. These systems separate the dust in the air and then collect it.

For this purpose, the filter systems are equipped with filter elements. Dust particles are caught in their filter material or filter mats, preventing further transport with the airflow. Various types of filters are available, as well as various materials from which filters can be made.

The principle of filtration is explained below to help understand what is important in the selection of materials.

Criteria that affect filter selection

The selection of number, type and materials of the filter elements - also called the filter cell - depends on the nature of the substance to be filtered as well as the conditions under which filtration takes place. The following conditions are to be observed:

Volume flow rate [m³/h]:
The volume flow rate describes the amount of air to be filtered per hour. As filter elements are limited by their design, the number of filter elements must be increased as the volume flow increases.
Composition of the medium:
Quite often, the applications involve air loaded with particles. However, other gases and dusts may also be contained in the medium. The composition of the medium flowing through the filter determines the selection of the materials of the filter, as the substances must be compatible with each other and may not react, so that further hazards are excluded.
Air temperature [°C]:
The temperature of the air flowing through the filter cells - just as its composition - determines the filter materials. Higher temperatures require the use of correspondingly heat-resistant materials, so that premature wear or even destruction of the filter cell is prevented.
Dust load of the air [mg/m³]: 
The dust loading indicates how much dust or how many particles are in the air to be filtered. This quantity determines the type of filter. A filter element only has a certain service life, i.e. the absorption capacity of dust, until it is clogged. The more particles in the air, the faster the filter becomes clogged - and reaches its operating limit. Depending on the type of filter, it can either be cleaned and is thus ready for use again or - if this is not possible - the filter must be replaced.
Requirements from the process:
 Based on the process, the following questions arise:
- What is to be done with the separated material?
Should it merely be separated or even be reused?
 - How is the filter system to be operated? Are downtimes possible due to service required in the process, or does the filter system have to be
operated continuously?
 - What about regulations that must be observed due to applicational requirements, e.g. ATEX?
- Are there any containment requirements?

The answers are important for selection of filter elements, as they determine the dedusting ability and the number of filters or the type of design.
-  When materials are to be recycled, the filter cell must be dedustable. If not, the substance remains in the filter material and is disposed of together with the filter cell.
- If the filter system is to be operated continuously, additional filters must be planned so that the volume flow can be distributed accordingly during dedusting (during dedusting, filtration is not possible at the filter being dedusted, which is why the volume flow is distributed to the other filters).
- If an explosive atmosphere is present and the risk of explosions exist, the filter cell and its components must be connected to ground to prevent electrostatic effects.
- When there are air purity requirements, the filter groups must be observed with respect to the filter classes.

Design and operating method of a filter cell

To better understand the influence of these criteria, the operating principle of the filter elements must be known.

The dust, respectively the various particles contained in it, have different properties that are decisive for the filtration efficiency.

Apart from these properties of the dust itself - e.g. sticky dust causes the filter material to clog more quickly, while pourable dust is very easy to filter - the structure of the individual particle and the quality of the filter cell must also be taken into account.

Filter cell design

1	Filter material	4 Seal
2	Spacer	5 Metal frame
3	Casting compound

The filter cell consists of a frame that accommodates the V-shaped folded filter material. The air flows into the filter cell. Due to the folding of the filter material, the air must pass through the material in order to escape via the other side of the filter. Thus, the particles contained in the air must also pass through the filter material. On their path, they interact with the fibers of the filter material and adhere to them.

The effects of filtration

To explain the principle of filtration - i.e. the mechanical separation of materials from gases - the view must be directed from the filter cell as a whole to the filter fiber in particular: Here, physical effects are at work on a small scale that are important for the overall result.

The particles flowing through the filter cell follow the airflow. The size and structure of these particles determine what happens inside the filter cell - at the filter fibers.

Inertial effect: A particle with large mass is relatively inert. When the air flows past the filter fiber, the particle cannot follow, is forced against the fiber and adheres to it.
Diffusion: A particle of very small size flows along with the airflow, but does not follow the streamlines and swerves around. In this path of motion, it can be forced against a fiber or other particles, and adhere there. The cause of this diffuse behavior is based on Brownian molecular motion.
Interception: A large sized particle does follow the airflow on the streamlines, but is too large to avoid the filter fibers. On its flow path, it comes into contact with the filter fiber because of its dimensions, and adheres there.
Electrostatic attraction  (Van der Waals force): Motion and the associated particle friction can cause the particles and the filter fibers to become electrostatically charged. If the charge is opposite in each case, the particle is attracted by the filter fiber and adheres there.

These effects ensure that the particles in the airflow are retained in the material of the filter cell. How well this works is indicated by the efficiency, also called separation degree. It indicates how many particles remain in the filter cell or respectively how many particles are still in the exhaust air after flowing through the filter cell. The separation degree is given as a percentage (number of separated particles in relation to the inflowing particles). The influence of the various effects on the separation degree is shown in the following diagram:

1	Diffusion	A	Separation degree [%]
2	Interception	PM	Particle size [µm]
3	Inertial effect	MPPS	Particles 0.1-0.3 µm  –> Minimum of filtration

As particles of various sizes and properties are present in the air, the effects mentioned act in a correspondingly different way. Minute particles are separated due to diffusion. Larger particles remain in the filter material due to the inertial effect or interception. In between are overlapping effects that influence the separation degree of the filter cell.

The exemplary diagram clearly shows that at a certain particle size, a minimum of filtration is given. This low point is known as "MPPS = Most Penetration Particle Size". Its range exists for particle sizes between 0.1 µm to 0.3 µm and includes the filtration performance of the particles that experience has shown to be the most difficult to filter. Filter cells are classified based on the separation degree at this point. Using the classification into different levels or classes based on different standards, the filter cell can be described accordingly so that it can be put into contrast with the containment requirements for the process.

SUMMARY:

Knowledge of the prevailing conditions as well as the properties of the occurring dust play a very important role in the planning and dimensioning of filter systems and especially of filter elements. This results in the requirements that a filter cell inside the filter system must meet in order to produce exhaust air of the desired quality. With this knowledge, the filter cell that best fits the process can be determined from the abundance of available filter cells. A discussion with the manufacturer's engineers is of fundamental importance, as they possess the know-how for dimensioning and selection of the right components.

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