Coconuts, paper and butterfly wings - the way to safer working environment

You might not think that chaos theory and coconut husks have much to do with your fume cupboard, but Simon Kear of Labcaire systems has other ideas

It‘s interesting to think that the flapping of a butterfly’s wings in Bangkok could have been the cause of the recent Tornado in Birmingham.  And, whilst this article will deal with neither the intricacies of chaos theory nor the mechanics of insect flight, we are concerned with the power invested in the movement of air.  For laboratory specimens, especially in the clinical environment, their integrity depends on their contamination-free status.  For many laboratory workers, it’s their safety that could be compromised by contamination.  Both depend on the absolute control of the air that surrounds them.  That, and old coconut shells and paper.  Let me explain.

Many common laboratory procedures involve a moderate to high degree of risk.  These might be created by the nature of the biological or chemical reagents involved, or the specimens themselves, or a mixture of both.  For instance, reagent preparation, acid handling and standard extraction techniques can result in the generation of hazardous fumes that need to be efficiently and safely contained.  Clinical samples might contain infectious agents that must be isolated from other samples and all workers. Many samples, innocuous in themselves, must also be kept clean and contamination-free – just think PCR.

For many years now, ductless fume cupboards and biological safety cabinets have been the favoured methods employed in these circumstances to achieve a safer working environment. It is the meticulous design of the airflow within these units that safeguard workers and samples alike.

Butterfly Wings
The typical airflow within a recirculating fume cabinet is illustrated in Figure 1.  Air is drawn inwards and upwards, around and away from the operator and any objects on the worksurface, through the filter unit or units by the integrated fan or fans before being ejected from the cabinet.  The activated carbon filter neutralises pollutants and eliminates harmful discharge to the environment and is usually protected from gross dust pollution by removable prefilters.  Replace the carbon filter with a HEPA (high efficiency particulate air) filter and you have a cabinet with the airflow characteristics of a Class I biological safety cabinet.

figure 1: Airflow inside a typical bench mounted, recirculatory fume cupboard


In both cases, the airflow into the front opening of the unit is designed to protect the operator from the potentially harmful sample whilst the filter acts to remove these potentially harmful discharges from circulation.


In order to protect the operator, the environment, and the sample, a more sophisticated airflow design is required (Figure 2).  In this case, barrier air is still drawn inward around and away from the operator but, rather than being allowed to contaminate the sample, is drawn down through the front edge of the worksurface.  It is pulled up the rear plenum, through the prefilter (1), and through the exhaust HEPA filters (3 & 4) to be recirculated to the atmosphere.  However, a proportion of the air is pushed back down through the main HEPA filter (2) into the working chamber and over the sample before joining the incoming barrier air to be drawn up the rear plenum.

The airflow may be sophisticated but the key to the effectiveness of both the recirculatory fume cupboard and microbiological safety cabinet is the choice and efficiency of the filters. 

   Figure 2: Airflow of class II microbiological safety cabinet                                                                                                 

Paper
In a world where tiny hard discs are skimmed by pin-sized magnetic heads to give us handheld MP3 players capable of holding 200 CDs worth of songs, ensuring dust-free conditions in the assembly and manufacture of optical assemblies and electronic components is vital.  The air filtration industry has responded with Ultra Low Penetration filters of staggering efficiencies, such as the 99.99996% arrestance of 0.12µm particles by the Class 1 ULPA filter.

This same 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 ‘sterile air’ in safety cabinets.  Advanced we may have become but the basic technology of the HEPA hasn’t changed significantly for forty years!
 
The filtration media is paper, composed mainly of fine diameter sub-micron glass fibres (Figure 3), typically of 0.3-4µm, folded into pleats and held in a frame.  Different characteristics may be induced through alteration of the number and size of fibres in the paper but a HEPA filter doesn’t sieve the air.  If it did so, its resistance to airflow would be unacceptably high and it would clog very rapidly. 

In fact, 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.

So what has changed? 

Like many other production processes, the robots took over about 15 years ago and gave us machine pleating instead of hand-folding.  This pulls the pleats much closer and, since HEPAs work on the total area of paper through which air is filtered, the depth of the whole filter has been reduced from 150-200mm to today’s 66mm ‘MiniPleat’.  The eradication of bulk has been carried over to more compact LAF designs.  The MiniPleat also offers less resistance to airflow, requiring less energy to filter a given volume of air.  This has led to smaller fans and quieter, more energy efficient units that are more comfortable to work at for long periods.

The other major area for change is the introduction of one-piece sealing gaskets.  Although this may not appear significant, by specifying one-piece seals a manufacturer can eradicate one potential area for leakage and potential contamination.  This is vital in microbiological safety cabinets but the ingress of dirty air to a LAF unit used for PCR work or media preparation can have devastating cross-contamination consequences.  Therefore, it’s precautionary to always choose a supplier that designs in ‘negative pressure sealing’ on every HEPA filter.

and Coconuts
Bringing a touch of the Caribbean to the laboratory scene, the activated carbon that fills the filters in a Labcaire recirculatory fume cupboard is derived from coconut shells.

High temperature steam activation leads to a slow and controlled destruction of the solid charcoal mass to produce millions of pores (Fig 4).  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 bench-mounted fume cupboard, such as the one metre wide Aura 550, is equivalent to 5000 football pitches!

Physical adsorption involves a process in which the atoms of carbon comprising the extensive surface area of activated 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.

In chemisorption, the surrounding molecules of gas are attached to the adsorbent filter surface by one or more of the following: a) converting the gas into another species which is either more readily adsorbed or non-hazardous; b) chemically combining with the gas; c) 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 like the HEPA, 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.

Art and Design
Charcoal and Paper, the everyday stuff of the artist, form the cornerstones of the recirculatory approach.  However, it is the skill and experience of the engineer in matching these to the precise movement of air that produces a safe working environment in our laboratories.

by Simon Kear, Labcaire Systems Ltd, Clevedon, Avon