Re-Circulating Fume Cupboards

Many schools in recent years have opted for re-circulating fume cupboards.
This design of fume cupboard has the advantage of not needing extract ducting, minimising disruption during installation and reducing the initial costs. However the limitations of the filters and need for regular filter replacement restrict the use of the fume cupboard and increase the ongoing costs.

Re-circulating schools fume cupboards usually feature clear sides for ease of demonstration and are mobile, allowing them to be moved between classrooms.

We test and maintain fume cupboards across a large number of schools, academies and colleges and have seen an increase in the popularity of these type of fume cupboards (especially in newly built schools over the past 10 years). Rather than expelling the fumes to atmosphere above the building (where dispersal eliminates any risk) a re-circulating fume cupboard draws the fumes across a filter before expelling clean air back into the classroom.
We increasingly find this air is not as clean as expected and fumes are being released back into the classroom, due to poorly fitted or saturated filters.

As most school fume cupboards use carbon filtration a face velocity test will give very little indication of its efficiency or effectiveness. A filter saturation & seal test is the only way of truly proving the filter. However CLEAPSS G9 suggests filter testing is an unnecessary expense where the filters are changed regularly (although a filter seal test must always be performed after a filter change).

Filter change frequency can vary massively between manufacturers. with some recommending very frequent changes to safeguard themselves (and increase sales!?!), while CLEAPSS G9 / BB:88 suggests the following:

• Filters in fume cupboards with normal use, used in teaching laboratories, are changed every 4 years.
• Filters in fume cupboards with heavy use, used in teaching laboratories, are changed every 3 years.
• Filters in fume cupboards used in prep. rooms are changed every 2 years

POSITIVES OF RE-CIRCULATING FUME CUPBOARDS IN SCHOOLS

• Lower initial purchase cost
• No building adaptions needed
• Can serve multiple classrooms (mobility)

NEGATIVES OF RE-CIRCULATING FUME CUPBOARDS IN SCHOOLS

• Higher on-going running costs (Regular filter changes)
• Higher risk to users if regular filter changes are not carried out
• Filters limit the types of chemicals that can be used
• Filters can become instantly saturated in the event of a spill

Where possible we would always recommend a ducted fume cupboard over a re-circulating one, especially in prep rooms where the frequency, concentrations and volumes of chemicals used are much greater.