Ventilation used in a water treatment plant may have one or more of the following purposes:
- controlling odours;
- providing fresh air to personnel working in rooms and doing so in compliance with legislation on the environment in the workplace;
- removing pollution and routing it to odour control facilities;
- preventing atmospheric pollution generated by a treatment from escaping to the outside when a slight vacuum occurs in the rooms (environmental protection);
- controlled atmosphere :
- dehumidifying the air in the rooms to avoid any risk of condensation and the attendant corrosion problems;
- heating the rooms;
- dissipating the heat generated by rotating machines such as air blowers, diesel generators, electric motors, etc;
- smoke extraction: in the event of a fire, especially by partitioning off risky rooms.
the different types of ventilation
The sole purpose of this system is to ensure that pollutants cannot escape to the outside. These are always structures that cannot be inspected exept for exceptional work.
Confinement can signify providing the structure with a sealed cover that has no ventilation. This is designated simple containment. A venting system, with or without protection, must be included to allow for water level fluctuations.
When gas is produced or injected, confinement must include provisions for collecting the gas at a rate that is at least equal to that of its production or injection.
Suction may also be included to enhance confinement efficiency by creating a vacuum compared with the external atmosphere. This is called vacuum containment.
Note: confined structures can be erected inside buildings that are normally visited by personnel. These buildings will then be free of a considerable proportion of the pollution emitted.
Openings are made through the walls in order to renew the air in the rooms.
Although simple and inexpensive, this method still has a number of drawbacks :
- the external environment cannot be safeguarded against noise and pollution;
- flow control is problematic (in particular, it depends on the wind direction);
- problems providing appropriate heating under these conditions.
This type of ventilation is not recommended and can only be considered in special cases of small rooms (transformer cell ventilation, for example).
Air comes into the room either through openings designed for this purpose or in the form of “leaks” that are not usually deliberate.
If S is the total section of the openings (including leaks) and Q the flow drawn up, it is agreed that there will be no backflow of air from inside the rooms towards the outside when V = Q·S–1 is > 2 m·s–1 and in the event of moderate outside disturbance (slight breeze).
A 0.7 mm negative of WC is usually enough to ensure that pollutants are not diffused to the outside of the rooms.
This form of ventilation does not transfer any energy into the premises. Consequently, the atmosphere in the rooms will not be homogenous and this often leads to the presence of "dead spots" where pollution concentration can be significantly higher than the mean value calculated. This aspect results in significantly over-designed drawn up flow rates and, accordingly, over-designed attendant odour control facilities.
However, in local ventilation applications, this is the only method allowing us to capture pollution at source. Therefore, it is used for confined structures or when hoods are fitted round equipment that is the source of major pollution emission.
Dead spots inside the hoods are inevitable. At individual points, pollution concentration can be from 2 to 5 times higher than the average concentration (extracted air). This is responsible for increased risks of corrosion.
When these hoods are installed in a building, the speed of air entering through the holes that have been made deliberately for operating purposes or as the result of leaks can be reduced to 0.5 m·s–1. This is possible when the inside of the building is not affected by outside atmospheric events.
combined blowing and suction
Blowing covers a large area and transfers its kinetic energy to the rooms, thus ensuring that the ambient air is homogenous. Additionally, when fresh air is blown into the circulation areas, it produces a greater of degree of comfort for personnel.
The blown flow rate is established on the basis of the flow rate drawn up for the purpose of maintaining the appropriate negative pressure inside the rooms. Unlike simple suction, the flow rates drawn up do not require major over-dimensioning.
In fact, it is even advisable to limit acceptable leakage flow rates as much as possible as well as areas generating leaks accordingly (better airtightness of buildings).