Activated Carbon Filters

High temperature steam activation leads to a slow and controlled destruction of the solid charcoal mass to produce millions of pores.  It is this induction of a highly porous material that guarantees an extremely large internal adsorption surface.  For instance, the surface area of the carbon filter in a standard fume cupboard is equivalent to 5000 football pitches.

Physical adsorption involves a process in which the atoms of carbon present attracting forces outward from the surface.  These physical forces, commonly known as Van der Waal’s forces, attract the molecules of surrounding gases, resulting in the adsorption of molecules at the surface of the activated carbon.  For substances that are not so well adsorbed, specially impregnated carbon filters that entrap fumes and gases through chemisorption may be used.

The process of chemisorption attracts the the surrounding molecules of gas to the adsorbent filter surface by:

• Converting the gas into another species which is either more readily adsorbed or non-hazardous.
• Chemically combining with the gas.
• Promoting catalytic activity whereby the gas/vapour reacts with air or with itself to produce another species which is either more readily adsorbed or non-hazardous.

Consequently, even compounds with low adsorption ratings such as ammonia, hydrogen sulphide, inorganic acids, mercury and formaldehyde, can be effectively trapped onto carbon with specially impregnated carbon filters, and rendered harmless.

Selecting the correct grade of carbon is important but so is the sizing of the fan and filter bed in order to provide containment as well as filter efficiency.

Unlike particulate filters, carbon systems are dynamic rather than absolute filters.  Although a gas molecule may be adsorbed onto the carbon bed, certain forces may cause it to move from site to site within the filter bed.  These forces can be heat, air velocity, molecular energy and displacement by a chemical that is more readily adsorbed.

Therefore, there is always an active zone within the filter where the main adsorption is taking place.  As the carbon becomes loaded so this active zone moves through the bed. If it approaches the outlet, then desorption from the filter will allow the chemical to re-enter the atmosphere and the filter must be replaced.

However, to achieve containment at the working aperture of the fume cupboard, a face velocity of between 0.3 and 0.5m/sec is required.  This volume of air must pass through the carbon filter and, to achieve effective filtration, the minimum dwell time within the filter bed must be long enough to allow adsorption and prevent excessive desorption.  It is for this reason that filter bed depth is a critical design criterion.

High Efficiency Particulate Air (HEPA) Filters

A HEPA filter is constructed in the same manner as an ULPA Filter and must trap 99.97% of air contaminants and particulates 0.3 microns or larger in the complex web of fibres. Depending on the size of the particle, this can happen in four different ways: Inertial Impaction, Diffusion, Interception, or Sieving.

Larger contaminants are trapped via inertial impaction and sieving. The particles either collide with the fibres and become trapped or are trapped while attempting to travel through the fibres. Medium sized particles, as they move through the filter, are grabbed by the fibres via interception. Smaller particles are dissipated as they travel through the filter and eventually collide with a fibre and are trapped.

An advantage of the HEPA Filter is that as they become saturated with contaminant, airflow capacity decreases, and static pressure increases, as a result the filters become more efficient as the filter loads.

As soon as the recommended speed of 0.45 m/s at the filter surface can no longer be met, a replacement of the filter is required. This process is monitored by a differential pressure gauge.

Ultra-Low Penetration Air (ULPA) Filters

ULPA filters guarantee efficiency to 99.99996% arrestance of 0.12µm particles. This filter technology provides surgically clean air for operating theatres, protects foodstuffs and pharmaceuticals during manufacture, provides safe working conditions for workers at nuclear power plants and produces the clean air in safety cabinets.

The filtration media is paper, composed mainly of fine diameter sub-micron glass fibres, typically of 0.3 – 4µm, folded into pleats and held in a frame. The gap between the fibres is large compared to the size of particles to be removed.  Particles are captured through contact with a fibre and retained by surface or electrostatic forces.  Although particles are brought close to the fibre by the airstream, final collision is generally as a result of inertia, Brownian motion or electrostatic attraction.

For more information on how our Market Leading Clean Air Solutions can benefit you, contact Monmouth Scientific’s Technical Sales Experts on;+44(0)1278 458090 or email [email protected].