Fume Cupboard Operation Now Affected by the Latest Health and Safety Regulations as well as Air Pollution Regulations!

By Karlheinz Stasch

If you currently operate a laboratory that uses fume extraction and control equipment you need to look closely at your obligations under the latest regulations.

Your managers could be leaving themselves open to legal action and stiff fines from two parties. The first is the possibility of action brought by operators of equipment that no longer meets either Australian Standards or current occupational health and safety regulations. Those working in surrounding buildings may also be affected by effluent sucked into their air conditioning systems from nearby exhaust stacks that do not perform adequately!

The second area of likely trouble could be incorrect control of emissions at the exhaust stack, causing higher than acceptable concentrations of the toxic product that was exhausted from the laboratory in the first place. Here, the pollution control people as well as the occupational health people have a public duty to ensure that concentrations of noxious fumes are safe.

So what do you need to consider when reviewing your own equipment or when specifying new equipment?

First and foremost, attention is drawn to the Australian Standard AS/NZS 2243.8. In this standard will be found the latest safety requirements for this type of equipment. In the area of emission control, Paragraph 3.2.7 of this document is very specific in its advice on the subject of what is an acceptable exhaust velocity and an acceptable concentration level of contaminants at the stack exit for all sash opening positions.

It is quite obvious that any attempt to produce the required face velocities at all sash positions, as called for in under the standard, (0.5m/s) can be achieved by merely altering the speed of the exhaust extract fan based on sash position. Lowering the sash from fully open to its lowest position will affect speed of the fan. It will only be running at a fifth of its full speed. Thus the exhaust velocity will be only a fifth of its maximum and desirable output but at the top of the stack the concentration level of pollutants will be about five times the original level. This approach compromises safety of the environment for energy saving reasons.

Smoke tests to check the real situation under varying wind conditions should be conducted at the stack by any laboratory which may have installed this type of control equipment.

Particular attention must be paid to any air inlets that may be in nearby buildings to see just what is being picked up by such inlets and then circulated throughout those buildings.

In a letter recently received from Mr Lyall Munslow-Davies, Senior Physicist, Radiation Protection Office, Health Department of Western Australia, Sir Charles Gairdner Hospital and University of Western Australia:

"It is also obvious that with variable speed extract fans that the concentration of contaminants in the discharged air will be higher with the sash closed than with the sash fully open or with a by-pass system in operation. Your ("ATT Pty Ltd) calculated factor of five contaminant remains constant.

However, the significance of this higher concentration depends upon the clearance and dilution of the exhaust as it leaves the discharge flue. This needs to be considered for two scenarios:

People working on the roof or in close proximity to the discharge need to breath clean air to comply with occupational health requirements. The best way of achieving this is to shut down fume cupboard operations while the work is carried out.

During routine operations, adequate dilution of the exhausted air needs to be achieved before it enters any air intakes to buildings. Our wind tunnel and modelling studies show that dilution factors of hundreds to thousands are often necessary to prevent complaints of odour from building occupants.

Only where the dilution from exhaust to intake is marginally low would the factor of five achieved from a by-pass system make a significant difference. This could also be important if the fume cupboard operations could not be stopped and people had to work near the discharges".

Computer Engineering Models

For those interested in the engineering computer models of the likely effects of varying the fan speed their attention is drawn to Fig 1.

So what is the most effective solution to this vexing problem?

Examination of this set of graphs will clearly show that the by-pass damper method (in the graphs shown as "Green Line") of providing makeup air from outside the laboratories' own air supply clearly meets the Australian Standard requirements across the full range of sash openings.

Even the earlier single and two speed fans produce far safer results than the technique of slowing down the exhaust extract fan, the variable speed fans method (shown in the graphs as "Red Line") to achieve satisfactory sash face velocities. 

Now look at Fig 2. Here we see the models showing a comparison of the more popular systems as a prediction of the percentage of contaminant concentration at the exit point. Our assumed starting point is that the discharged air contains 75% of the maximum contaminant concentration permitted at a maximum air discharge rate, normally achieved when the sash is fully open and all fans are running at their full speed. We assume that contaminant concentration monitors with switch off facilities are installed at the exhaust of each extraction method. The fume cupboard with the variable speed fan (Red Line) would be switched off before the sash is lowered to 1/3 of its full movement to ensure that there was no health risk

In Fig 3 the models predict the actual fume extraction rates in litres per second by each of the common systems for a fume cupboard in which sash opening is 130 cm wide.

Fig 4 shows the comparisons of likely face velocities for each of the systems at common sash opening positions.

This data can now be applied scientifically to any laboratory situation and a prediction made of the likely performance of any installation at the site.

Designers have not only been pondering over the best way to meet standards, protect the health of the operators and meet clean air requirements. There is also the never ending search for ways to reduce operating costs and save energy.

Fig 5 clearly shows the benefits to be gained by using an electronic by-pass damper system in the fume cupboard. It draws 368 l of outside air into the exhaust system for sufficient dilution of the fumes. Competing systems rarely merge additional outside air into the exhaust system for a safer dilution.

Those responsible for laboratory effluent now face a much tighter set of controls both in the laboratory as well as in surrounding areas. The public, the authorities and nearby workers will no longer accept conditions that existed in the past. (Note)

The conclusion of this study suggest two considerations. The variable speed fan option is the last option to be chosen from the five variations in fans exhausting a fume cupboard for health hazard reasons alone. Second, an operator who uses a variable speed fan in which an electronic control converts a single phase into three phases will have chosen the poorest form of motor control. No energy saving will be achieved by slowing down the speed of such a small motor. This type of electronic system is also well known as a producer of electronic noise. Electronic laboratory instruments and devices can be affected in their use and accuracy by this unwanted side effect.

The author of this article has developed and is marketing an electronic by-pass damper system plus full computer control system with an Intel based microprocessor for fume cupboards as shown as "A" in the graphs. Recently he was shown a video about variable speed fan systems in connection with fume cupboards in research laboratory. The health professionals in this company, concerned about the health of their employees proved, by using coloured fumes, that the nearby air conditioning system was sucking these fumes inwards and distributing them into the offices when the sashes of the fume cupboards were lowered!

A plan to alter this health hazard is under way by this research laboratory.

Note:

Readers' attention is drawn to the appropriate sections of the 1991 publication... Exposure Standards for Atmospheric Containments in the The Occupational Environment, published by the National Occupational Health and Safety Commission, as well as the appropriate Australian Standards and the NSW State Pollution Control Commission Quarterly Air Monitoring Report No 2, 1990 page 5 section 4. Air Quality Criteria.

Note this article was published in the Aug/Sept 1993 issue of What's New In Scientific & Laboratory Technology. For further information please contact Advanced Technology Transfer Pty Ltd, Ph: (61 - 2) 9488 8924, Fax: (61 - 2) 9449 9924, e-mail: info@att.com.au