Mechanically induced air movement is carried out by fans or air propellers, and has the advantage of being "positive" in its action, which no other method of obtaining air movement truly is. It is independent of weather, wind, or temperature; the air can be warmed more precisely, and the whole is capable of very exact control. In many cases exhaust steam is used for the heating, so that this detail costs comparatively nothing, and the power required to propel the air is of small cost also.

There has been a divergence of opinion as to how the air should be dealt with, whether the vitiated air should be extracted, leaving the new air to flow in, as, of course, it freely will through the openings provided; or, whether the new air should be driven in under sufficient pressure (very slight) to force the air in the rooms - the vitiated air - out through the openings provided. The latter is the working principle of the Plenum or pressure system, and one of its advantages is that, the air of the rooms being under a light pressure, every opening (other than the inlets provided), whether around doors or windows or elsewhere, has air passing outwards through it, and no draughts or undesirable air movements can occur. Ventilation, by a fan used exhaustively, requires the air propellor to be placed at the highest part of the building, as a rule, or at a distant point from the inlets, with the ducts conveying vitiated air leading to it. With this method, therefore, every opening in a room, whatever its size, or wherever it leads from, will probably act as an air inlet, in consequence of which many unexpected results transpire.

With the Plenum System the air propeller is fixed so as to come between the outer air and the fresh-air inlets of the room, and, communicating directly with both, the air is driven into the room. There can be no doubt about the superiority of the Plenum over the Exhaust method, and, except to meet some special requirement, it may be said that the latter has fallen into disuse, while the former is employed exclusively when mechanical ventilation is decided on. As previously stated, the action of the Plenum System is positive in causing the circulation of air, besides which it admits of heating the air by a single heater.

In Fig. 74 are given the particulars of a Plenum installation in an Infants' School, previously illustrated and described, to afford warmth and ventilation in winter, and ventilation only in summer.

Mechanical Ventilation 106

Fig. 74.

In the basement plan will be seen the fresh - air conduit, this having its inlet just above ground level (shown on the ground plan). At a point between the inlet and the air propeller or fan is a screen for filtering the air. This, in suburban or rural districts, is a frame carrying wire gauze, its purpose being to arrest insects and floating debris. It is important that access be given to this, for, as stated, it arrests many things, a number of which remain on or in its mesh. The surface must be periodically brushed, or it will retard the passage of air to a very noticeable extent. When the air carries impurities in a fine state, as in manufacturing districts, recourse has commonly been had to coarse muslin, or cheese cloth, as the filtering material. This is an excellent filtering medium, but it requires very frequent cleaning or renewal. There is now most favour shown to a coke screen, this being about a 4-inch thickness of small coke held between two surfaces of small wire netting or coarse metallic gauze. Where convenient, arrangement is made for flushing this screen with water to cleanse it.

Mechanical Ventilation 107

Fig. 75.

After passing the screen, the clean air is next carried through the fan and enters the main horizontal ducts - sometimes spoken of as culverts, when built of brickwork. Leading from these are the many vertical ducts, marked B, leading to the rooms. The delivery openings from the ducts B are situated on side walls, facing outer walls as recommended, and deliver the air about 11 feet above floor line. This high delivery, combined with low outlets for the vitiated air, will be found discussed later on. The outlets for vitiated air are also on side walls - the same walls as the inlets - situated at a low point just above skirting level. These shafts or flues proceed up and join a main shaft, which is shown in the roof space. The used air can be discharged directly into the open air, if desired ; but the roof space quite commonly receives it, and thence it escapes into the outer air as best it may ; though special openings must be provided if the roof be well constructed, say with boarding or felt under the slates, and the beam-filling properly done.

The heating of the air is not accomplished by a single heater in this case. Instead of this, each vertical duct has its own heater, in the form of a radiator, at its base. This will be seen in the section on A A. It is an arrangement that admits of independent regulation of the warmth of the rooms without varying the volume of air.

Mechanical Ventilation 108

Fig. 76.

Fig. 75l shows, in isometric projective, another method of treating a school building (or any similar structure) that extends to three floors. In this case, instead of brick culverts for the main ducts, these are of sheet metal and of rectangular form, and this illustration presents a good idea of the easy curves that should be made when branching or dividing ducts of any kind.

1 This and the two succeeding-illustrations represent works carried out with the Sturtevant Company's fans and heaters.

The fan in this case is of the blow-through kind, the air being driven through - not drawn through - the heating stack. The heater is enclosed in the space shown on the delivery side of the fan. In this installation was provided a novel detail, not shown in the illustration, this being a "tempering coil." It consisted of a coil to temper the air which came to the fan and heater proper; and as this coil was heated by the exhaust steam from the engine employed to drive the fan, the heat units afforded were obtained free. Another detail not appearing is a space beneath the heater stack, so arranged that, by operating a damper, the passage of air can be regulated so as to go through the heater or beneath it, or part each way, so that its temperature may be varied according to the weather.