Ventilation filtration

Source
The dust resulting from the operations in shotblasting installation come from three origins:
- the contaminants removed from the parts to treat, (sand, calamine, rust, coatings, burrs, etc...)
- the abrasive particles made of broken fragments or small flakes,
- the parts particles coming from the shotblasting machine, due to wear or a pulling out.

Aim of the ventilation
The physical and respiratory protection of the operator being ensured by safety helmets fed with new air, thus it is necessary to ventilate and remove dust from the enclosure, it means to harness, carry, separate and evacuate the dust emitted in order to fulfil the 3 following purposes:
1) ensure a good visibility during the shotblasting while limiting the concentration areas,
2) realize as fast as possible the cleaning up of the cabin after the operations being stopped,
3) protect the installation environment and above all the employees who work close to it.
To this aim, the chosen medium is air, the collection will be qualitative and quantitative and a good result will be dependent on the air circulation speed, it means its carrier power.
These rules of the art are governed by the practical guide of ventilation N° 14 ED 768 of INRS (Safety and research national institute) established under the aegis of the CNAM (State health insurance office). The main recommendations are hereafter.

Design and choise

Principle
In order to be efficient, the ventilation has to implicate all the useful volume of the enclosure while avoiding as much as possible the under-ventilated areas. This necessitates to place the devices of dusty air collection in direct opposition with the ones for new air introduction, in order to obtain a vertical or horizontal displacement of air in the projection area.
Therefore the choice is made according 2 kinds:
1) vertical ventilation.
- through air entry divided up in ceiling and through collection under the grating of the recovery floor,
- through air entry divided up in ceiling and through collection thanks to 2 inner longitudinal banquettes,
- through introduction housing in ceiling and through external sensors at the bottom of the partitions.
2) horizontal ventilation.
- through air entry on the surface of a partition or a door and through collection by 2 vertical housings or a surface on the opposite partition,
- through air entry thanks to 2 vertical housings on a partition and through collection by 2 vertical housings or a surface on the opposite partition,
- through air entry thanks to 1 horizontal housing in pediment and through collection by 2 vertical housings or a surface on the opposite partition. (picture 2)

Recommendations:
The vertical ventilation, more favourable to the dust draining, is particularly recommended for the cabins with a lenght superior to 15 m, and becomes imperative for a use with 2 operators.

Features

Principle
Air introduction and extraction surfaces have to be sufficient in order to sweep the work area and avoid discomfort sensations to the operator dued to over important air speeds.
The extraction of dusty air has always to be made mechanically with a fan.
The introduction of new air can be made:
- naturally by the depression created in the cabin by the ventilation group,
- mechanically by a blowing housing fitted with a fan.
In the second case, all the arrengements have to be taken so that the cabin be in light depression.

Introduction and extraction speed
The air extraction surfaces have to be calculated so that the collection speed at their level does not exceed 2m/s, whatever the principle retained is.
The air introduction surfaces have to be calculated so that the air entry speed at their level does not exceed 2m/s for an entry in ceiling and 1m/s for a horizontal entry which is a nuisance for the operator.

Flows of extracted air
It depends on the kind of ventilation and the rate of pollution in the cabin, without never being inferior to:
- 400 m3/h for 1 m2 of the cabin ceiling surface (width x lenght) for a vertical ventilation,
- 1000 m3/h for 1 m2 of the cabin frontal surface (width x lenght)for an horizontal ventilation.
These values have to be increased by 40% for polluting or toxic operations or for cabins with lenghts superior to 15 m for horizontal ventilation.
The renewal rate per hourof the total air volume of the cabin has to be at least 80 in a normal situation and 120 in a polluting or toxic situation or for the big cabins.

Dust removal

Reminder
The dust removal system of the air extracted from the cabin has to be in accordance with the standards of rejection in the environment (order of March, 1st 1993). The separation standards are varied and the choice has to take into account:
- the granulometry of the dusts to collect,
- the dust concentration per m3 of air.

Types of dust collectors
1) mechanical dust collectors, using the simple physical phenomena such as the reduction in pressure chamber or the cyclone.
2) dust collectors with porous elements of filtration which are at the moment the most used and efficient processes with in particular:
- the air bag filters,
- the sleeves filters,
- the cartridges filters,
which work dry and where the air crosses a porous nature in order to deposit the dust particles. A device of periodic, mechanical or pneumatic decolmatage? makes it possible to conserve to the filtering element its stopping power while maintaining a constant flow. (picture on top)
3) wet way dust collectors are machines in which the the dusts are separations through water action. (picture below).
- either by the force of inertia which carries the dust of the air vein in contact with a partition covered with water.
- or by the close contact of the dusts of the air vein which cross a filtering cavity sprayed with water.

Nota bene:
The wet way dust collectors are used above all when the dusts are inflammable. In the case of these dust collectors, the dusts thrown in the water form sediments which are difficult to evacuate, that is why there are some ejection automatic systems.

Evacuation

Outside rejection
In order to protect the operators, this solution will be used in most of the case. It becomes necessary when the dusts filtration system is not very efficient (cyclones or wet way dust collectors) and recommended outside the heating periods. One will prefer a vertical rejection through the roof rather than the horizontal outflow through the front or through gable in order not to be submitted to the winds pressure and to move away from the floor at the maximum the pollutants.

Inside rejection
The air recycling process in the premises can be envisaged only if the air is purified enough and meets the conditions of the order dated May, 8th 1985 and the technical note dated November, 5th 1990. In any case, the equipment has to be fitted with a by-pass selector which will reject the air outside, outside the heating periods and whatever the period is, when the purifying conditions are not respected any more.

Recycling process in cabin
This solution is not recommended in any case because it enriches the cabin atmosphere with fine particles dangerous to the respiratory tract.
It is banned when the fine dusts are likely to be ablazed and to cause a fire and even an explosion.
It is banned also when the air recycled in the cabin is reheated by a air vein burner.