Filterworld Limited is BS-EN-ISO9001:2000 accredited. Our scope of supply covers the Design & Manufacture of Filters and Filter Systems.Filtration - Technical Page
Compressed air filtration is simply the removal of air borne particles from a moving pressurised air stream. To this end, millions of pounds have been ploughed into research on filtration media and as much again on the investigation of particle dynamics.
First of all, filtration media doesn't possess any intelligence. It doesn't matter if the particle is inert organic, viable organic or inert inorganic. If the media can capture a particle, it will.
So what is a particle?
The smallest particles are a bit esoteric; these sub-atomic 'things' have wonderful names like Quarks, Neutrinos and the like. High-Energy physicists can play with these, they don't affect the types of filters that we are talking about.
The next group of particle comprises of whole atoms, then whole molecules. Even these are almost infinitely smaller than the tiniest particle that our filters can deal with. A small anomaly exists when defining the size of a particle. Engineers tend to talk about 'diameters'. Particle physicists talk about particle radii. In truth it doesn't really matter one way or the other, because particles are very rarely round, and who is going to argue about the exact size and shape of a bit of material which is only 0.01 micron in diameter, or radius.
However -For those of us that do want to argue, the following definitions of size are available.
Feret's Diameter. This is depicted as dimension 'A', it is the overall length from 'tip-to-tail' of the particle.
Martin's Diameter. This is depicted as dimension 'B', it is the length of a theoretical horizontal line, which passes through the centre of gravity of the particle, to touch the outer boundary walls of the particle.
Projected Area Diameter. This is depicted as dimension 'C' and is the diameter of a theoretical circle, which would contain the same projected area as the irregular particle.
Equivalent Diameter. This is the diameter of a sphere, which would contain the same volume as the irregular particle.
Aerodynamic Diameter. This is the diameter of a spherical particle that exhibits the same settling velocity as the irregular particle.
Some Particle Definitions.
Grit. This is defined in the Clean Air Regulations 1971, as 'particles of solid matter, which are retained in a sieve of 75 micron nominal aperture in conformity with BS410:(updated 1986)'.
Dust. This is defined in BS3405 as 'particles of solid matter, which will pass through a sieve of 75 micron nominal aperture in conformity with BS410:(updated 1986)'.
Fume. This is regarded as solid particulate matter, less than 1 micron in diameter.
Mist. This is particulate matter formed by condensation of a vapour, with a range in diameter from 0.5 micron up to 5 micron.
Spray. This is larger particulate formed by atomisation and break up of bulk liquids.
Filterworld Elements.
The coalescing filters made by Filterworld are not like a sieve. They don't have filter elements in them resembling a minuscule matrix of equal size holes. Holes through which gas passes and particulate matter gets trapped.
Filterworld filters rely upon various principles of physics, which date back for centuries. We'll start at the small end and see how we can catch little particles, then work up towards the bigger particles.
Brownian Motion:
This is named after an English Botanist called Robert Brown who first noticed the effect in 1827. Einstein later used the effect in 1905 to prove his Kinetic-Molecular Theory, all a bit highbrow really.
Brown noticed that small dead pollen samples moved about erratically when suspended in a liquid. Because they were dead, something else must have been moving them. He concluded that they were being bombarded by something so small that they were invisible to the naked eye. Molecules and atoms.
Brownian motion is the random movement suffered by a small particle due to the ongoing bombardment from molecules. Typically a particle must be equal to or smaller than 0.1 micron diameter, (0.05 micron radius) to travel under Brownian motion. The actual distance that such a small particle can travel in a straight line before it gets bounced off-course is called its Mean Free Path. Brownian motion also helps to explain why heat conductivity takes place.
This means that particles smaller than 0.1 micron will zip around all over the place, regardless of the direction of compressed air flow.
The filtration media is comparatively vast and deep compared to these sub-micron particles. This makes it almost inevitable for a particle travelling with Brownian motion to collide with a fibre contained within the filtration media bed.
Phoretic Forces.
This sounds quite grand. What it means is that the tiny particles actually cover so much ground in getting from A to B, that their chances of bumping into a similar sized particle, or into a larger particle is quite high. Smaller particles then either combine to become a bigger particle (agglomeration), or a smaller particle sticks to the back of a huge particle (accretion). Either way, the tiny particle no longer moves randomly and now follows the direction of air flow.
Thermophoresis.
Temperature gradients cause the effects of Brownian motion to increase. Compressed air is rarely cold, this means that the tiny particles are bouncing around with an even greater energy. This increases the capture rate.
Diffusion.
A global terms which describes the combined effects of Brownian motion and Thermophoresis. If you run a hot bath and let it stand for a while, you can put in some pink (if you like) bath oil. Even though the bath water is perfectly still, the pink bath oil will slowly spread through the water until it is fully dispersed. This is called diffusion. In compressed air, particles smaller than 0.1 micron move through a filter element in a state of diffusion.
Inertial Impaction and Interception.
The filter media fibres catch particles upwards of 5 microns, because they are too big to get out of the way in time. Inertial impaction is the result of a particle whizzing through thousands of fibres at such a rate that eventually it leaves the air stream that is carrying it, to collide with a fibre. It's like taking a motorbike down a winding country lane just a bit too quickly. Interception occurs when a particle is too heavy to be affected by the changes in direction of air currents. It simply carries on in a straight line, into the fibres.
Greenfield Gap.
In between 0.1 micron and 5 micron we have something called the Greenfield Gap. This particle band-width is the most difficult to capture and is the basis of much on-going research.
However, capture these particles we do. This can be demonstrated using laser particle counters and stack impactors. Although the theory seems vague, the practical results are conclusive. High efficiency filter elements can easily and cheaply remove all particles down to 0.01 micron. It's the theory that is sadly lacking.
The most common view is that although no specific mechanism can be held up as the reason for the capturing this particle band-width, it is probably a mixture of known mechanisms such as agglomeration, impaction and interception.
Coalescing Filtration.
The particles will eventually bump into a filter fibre for all of the reasons shown above. Having done so, they stick on to the fibres for several reasons.
Electrostatic Adhesion.
All particles carry a small electrical charge, this is caused by the frictional forces of the particle passing through air space. The particle will stick to a filter fibre for the same reason that you can make a balloon stick to a wall.Friction.
Hard matter particles are ragged around the edges. They get caught on the filter fibres and cannot be dislodged. This is one of the reasons that it is almost impossible to clean a deep bed coalescing filter cartridge once it has been used.Accretion.
We've used this word once already, in this case we mean the particle sticks to the surface of the fibres in the same way that they try to stick to each other.
The oil and water particles will ultimately agglomerate on to the filtration media fibres to form droplets of liquid. These droplets drain through the filter bed until they are of such a size that they can drop off the bottom of the filter element into the filter bowl. The hard particles will either remain trapped inside the media, or some of it will wash through, to be contained within the much larger liquid droplets. This process is called coalescing.
Finally, the next trick is to ensure that the flow of compressed air is kept well below the point at which pneumatic conveying would take place. It would be a pretty pointless exercise going to the expense of all this technology, only for the liquid droplets to be picked up by a fast moving air-stream and taken out of the filter housing to be conveyed down line.