Meciianical (or artificial) ventilation

Mechanical ventilation or artificial ventilation involve the use of some mechanical equipment for effective air circulation. It is provided in those circumstances where satisfactory standard of ventilation in respect of air quantity, quality or controlability cannot be obtained by natural means. This system is costly, but it results in considerable increase in the efficiency of persons under the command of the system. There are following systems of artificial ventilation :

(i) Extraction system.

(ii) Plenum system.

(iii) Extraction-Plenum system.

(iv) Air-conditioning.

1. Extraction system (or exhaust system)

This system is based on creation of vacuum in the room by exhausting the vitiated inside air by means of propeller type fans (exhaust fans). Air inlets are formed at a height of 1.2 to 1.8 m through Tobin tubes, and the outlet is arranged within a quarter of a metre of the ceiling on the opposite side of the room from which air enters. The extraction of air from the room permits the fresh air to flow from outside to inside either through Tobin tubes or even through a window. This system is more useful in removing smoke, dust, odours, etc. from kitchen, laterines, industrial plants etc.

2. Plenum system (or supply system)

In this system, fresh air is forced into the room and the vitiated air is allowed to leave through ventilators. The air inlet is selected on that side of the building where purest air is available. The incoming air which is mechanically forced into the room is passed through a fine gauge screen or filter. A constant stream of water is kept flowing down the screen giving a fine mist of water through which the air is drawn by means of blower fan. Thus, all the mechanical impurities arc removed from the air. In summer, this also results in cooling of air. At this point air may be further disinfected by the introduction of ozone from an ozonizing apparatus. In winter, the air may be forced through a battery of hot water tubes and be heated before being forced into the room. In the case of big hall or factories etc., the distribution of this air is done through properly formed sheet iron ducts with properly dimensioned branches. This ventilation system is costly, but is used for factories, conference halls, theatres, big offices, etc.

The ventilation by plenum process may be either downward or upward. In the downward ventilation, the incoming air is allowed to enter at the ceiling height and while mixing with the vitiated air during its downward journey, it is taken out through outlets situated at the floor level. In the upward system, fresh air enters at the door level and moves out at the ceiling level.

3. Extraction-Plenum system

This is an extension of plenum system in which extraction fans are used for the exit of the vitiated air from the room. This system is adopted where the delivery of fresh air is either sluggish or where it is desired to discharge vitiated air containing obnoxious fumes as from kitchens, laterines, or various manufacturing processes, in specially isolated areas.