RECYCLING POLLUTED AIR
Excerpt of the practical Guide de ventilation - 9.1
Liquid product application spraying booth ( Ch. 6)

Three different filtration systems are used: open front industrial booths, dry filtration system, and water curtain system. These devices are not efficient for the elimination of solvent vapors, which should be evacuated in compliance with the regulations concerning classified sites.

Dry filtration system
Can be used in open or closed booths. The dry filters can be composed of either a quilt on non-woven fabrics belonging to the reaction to fire category M1 (according to the norm NF P 92-507) or fireproof pleated cardboard (impact filters).
A device should be installed to check the crushing resistance of the filters to detect pressure drops to prompt the replacement of filters in respect with the required minimal ventilation conditions in the booth, defined above. It could be a differential pressure indicator.

Fiber mat
The aerosol is sucked through a filter panel composed of several layers of fiberglass or synthetic resin.
These materials can be relatively efficient when they are "new" but they clog up progressively. They then lose their efficiency and can become fire hazards. It's not possible to clean them, they have to be replaced regularly.
It is not recommended to use dry filtration systems in booths where either nitrocellulose lacquer or paint are used due to spontaneous ignition hazard of the clogged up filters.

Impact Filters
In this case the filtration panel is made of two-pleated, fireproof and perforated cardboard, with a series of staggered holes giving the filters high capturing efficiency. The paint over spray is carried through the filter several times before it is evacuated, - the different change in direction make the particles adhere to the surface of the filter.
This type of filter offers little resistance to air, the big diameter of the holes maintains a high efficiency with very small variations whatever the saturation of the filter. The airflow rate on this type of filter is limited to 1 m/s.

Ceramic materials
The products are hollow materials in ceramics (rings, cylinders or other articles with various shapes, convex or concave type Raschig rings) placed in baskets under the floor grating of the booth and in the extraction ventilator casing. The polluted air passes through a layer of material and the non-volatile particles are deposited on the surface.
The filtration is not efficient if the ceramic layer is too thin. When the material is clogged up, it must be cleaned which can be hazardous (caustic solutions or toxic solvents have to be used).

Water washing system
The devices using water washing systems avoid clogging up without eliminating periodical cleaning. The use of flocculants or in certain cases of electrical fields promotes the sedimentation of non-volatile matter, which should be evacuated regularly. These systems are not really purification systems - part of the over spray could be dragged through the ventilation ducts.

Water curtains
It can be used both in open or closed horizontal ventilation booths.
A water sheet flows along a vertical or slightly inclined plane. A pump or the push of exhaust air insures water progression and recycling. Several water sheets are needed to obtain a homogeneous water flow. Behind the water curtain, there can be a scrubber where the water is atomized to rid the polluted air of non-volatile matter, which will be decanted in one or more tanks situated in the lower part.

Floor grating washing system
This system can be used both in open or closed horizontal ventilation booths.
A quantity of water flows constantly under the floor grating in spots where paint usually accumulates. The adopted device should ensure a homogeneous airflow to avoid deposits in the pit and to minimize the potential release of solvents, in particular when work is interrupted.

Open front industrial systems
Openwork systems are found in cheap open ventilated booths. They are constructed from assembly sheet metal on which the over spray is projected . Nevertheless, part of it is not arrested and is deposited nearby in the extraction duct on the blade fan and is evacuated into the atmosphere contributing to environmental pollution. Clogging up of the double bottom sheet metal and the ducts is very rapid. The initial efficiency of the extraction decreases rapidly if cleaning is not frequently undertaken. Furthermore, the accumulation of old layers of dried paint is a huge fire risk in clogged up ducts.
The openwork treatment system for polluted air is not recommended and should be used only for very specific operations.