Welcome to Dustbuster

Hi my name is Tony Lawson. For the last 15 years I've been deeply involved in the design and development of filter media technology from the design of the media to the processes and systems used to manufacture and test them. In that time I've gained a lot of experience of the technology that I'd like to share with you. 

One thing I have seen over the last few years is a lack of understanding of how filters work at a very basic level and what test is applied to which market or application. Even when that is understood, there is a lack of comprehension of the basic tenets of the results and what they mean. This blog is therefore not really aimed at the experts who sit on the ISO committees but at the "newbies" or those who want to learn a little more about the technology.  

So I've decided to call this blog "Dustbuster" (as all the other titles related to filtration are probably taken) but the purpose of this is to try and educate you on the issues of filtration as I see them from fundamentals to future developments.

Today I will introduce you to the concept of a filter. Whilst the science of filtration can be mathematically complex the concept of what a filter does and how it functions is relatively straightforward to understand. 

What is a filter?

The role of a filter is to protect something or someone from external contamination. Contamination can be present in the external environment in a range of different forms:

• ·      Solid particulate contamination of air with soot and dust of major concern
• ·      Liquid particulate contamination of air such as water or hydrocarbons
• ·      Molecular separation of contaminant gases from air
• ·      Liquid/liquid separation of water from oil or oil from water

Function of a Filter

Filters work by the simple expedient of blocking the transport of the contaminant through the filter. As the fluid passes through the filter, the filter imposes a resistance to the flow, a pressure drop. The pressure drop of a filter is intimately related to the face velocity or flow rate of the fluid. The illustration below shows a typical HVAC filter element tested to EN779:2012. This, almost linear response can be applied to nearly all filter types in air filtration applications.  

As the media is exposed to the contaminated environment, the performance alters. The build-up of contaminant on the media leads to a decrease in pore size as the pores in the media become blocked. This increases the resistance to fluid flow, increasing the pressure drop of the media but also increasing the filtration efficiency. The illustration below comes from a typical ISO 5011 test on an automotive filter as it is loaded with contaminant. The response is very typical with a classic, and rapid increase, in pressure drop at the termination of the test. 

At a given increase in pressure drop the filter is considered to reach its terminal pressure drop. The level of the terminal pressure drop is determined primarily by the strength of the media. Filter failure or rupture will lead to the object or person being protected being exposed to the contaminant, the terminal pressure drop of the element is well below the absolute failure point of the media.

Mechanisms of Filtration

Widely different mechanisms are employed to separate particles, depending upon the size of the particles and the properties of the fluid. The illustrations below show the main interactions between particles and the filter medium underlying the filtration process.

The continuous fluid phase (solid or liquid) passes the fibre, creating flow lines around it. The particles are intercepted by one of a series of primary mechanisms:

• Interception effect: smaller particles will follow the flow lines around the fibre but if they touch the surface, they can stick to the fibre.
• Inertia effect: When large particles of high mass approach the fibre their inertia causes them to collide with the fibre.
• Diffusion effect: The smallest particles, with diameters less than 0.5µm, move irregularly through the fluid driven by Brownian motion. These particles are less affected by the flow of the fluid and will meander into the fibre in a random process. 

• Sieving: When a particle is too big to pass through the pore

           The process of filtration can be affected by other interactions such as:

•       electrostatic forces generated by surface charges on “electret fibres”

•      surface tension forces such as wetting to create coalescence effects
•      forces induced externally such as centrifugal forces applied prior to the filter.

On striking the fibres the particles are retained by atomic level adhesion forces such as van der Waals forces or electrostatic forces.

Once on the filter, the contaminant undergoes a series of secondary mechanisms that concentrate up the contaminant allowing it to be separated and whose mechanisms are often key to the performance of the element in the final applications. There are many secondary mechanisms but I will illustrate here the three main ones seen. 

• Dust cake formation: Once the contaminant is trapped on the media the particles become a filter for further particles in the fluid stream. These become trapped on the existing contaminant building up a layer of dust on the upstream that causes the pores to become blocked, more efficient but at the same time having a higher pressure drop.

• Coalescence: In liquid contamination e.g. oil mist removal from compressed air, the droplets are attracted to each other forming a larger droplet that coalesces and separates from the filter itself through gravity to a drain.

• Pulse cleaning: In some air filtration market applications, the dust cake can be mechanically removed through the application of a reverse pulse of air passed through the element in a reverse direction

     These principles can be applied to most filters and are the basis of the majority of standard filters used globally ranging from vacuum cleaners to high efficiency clean room filters. In my next blog, I will start to talk about testing and standards used for standard testing globally. 

     If you have any comments or suggestions please feel free to contact me at ajlawson2206@gmail.com or post comments below. 

     Tony