A laboratory hood is a device that allows the extraction of toxic vapors from products used during handling. Its primary function is to protect the manipulator. The vapors are extracted from the working volume and then either treated by filtration (with charcoal and / or particles), or discharged to the outside.
There are two types of devices to differentiate:
- the hood: a funnel-shaped exhaust causing the evacuation of air by natural depression. Today, it is still equipped with a fan allowing regular evacuation and for greater efficiency, often with side walls. It is used above devices (ovens, distillation devices, etc.) or workstations requiring space (difficult handling such as welding, etc.);
- the fume cupboard: whose name comes directly from the university of the same name, is a Fume hood provided with side walls, but also with a front wall (almost always with guillotine) and a bottom register allowing the high aspiration and low vapors.
There are two types of laboratory hoods:
- the Fumehood on activated carbon (standard ETRAF) without connection to the outside, so permanent recycling in the work area. These work on the principle of chemical adsorption. Their limits: the specificity of the filters and their saturation;
- the hoods and fume extraction outwardly .
The choice is made according to the average exposure value (VME) of the most toxic product used:
- VME higher than 400 ppm: extractor hood, without register or front glass;
- VME between 1 and 400 ppm: laboratory fume cupboard;
- VME lower than 1 ppm: closed and waterproof glove box type system.
The VME of chemicals can be found on the INRS website.
A laboratory fume hood or Low hood cupboard (except with an AC filter) must be fitted with an extractor (fan) which extracts the vapors to discharge them outside. The European standard NF EN 14 175 and its French extension XPX 15 206 impose very specific constraints on the manufacture of fume cupboards. Since 2006, they no longer contain any indication of minimum air speed on the front of fume cupboards. The only quantified constraint included in the French standard concerns the containment measure.
Other types of hoods
Laminar flow hood
In biology, we find Lowhood. A laminar flow hood generates purified air in particles (for example, a hood equipped with an absolute HEPA type filter stops 99.99% of particles larger than 0.3 μm) and above all, without aeraulic disturbances. Here, a flow is said to be “laminar “, if at each point in the flow section, the speed does not differ from the average speed of the flow by +/- 20%. A laminar flow is therefore unidirectional and homogeneous. There are two main categories of laminar flow hoods:
- Horizontal laminar flows :the air treated on the filter (s) is blown towards the manipulator. This technique protects the product but not the handler or the environment from contaminants;
- Vertical laminar flows: the air is blown from the "ceiling" of the hood by a high efficiency filter (HEPA). It comes out either through the facade (same as the horizontal flow), or it is taken up by perforations on the side or rear walls, preventing it from coming out towards the manipulator and the environment.