Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/178

Rh 160 VENTILATION Ventila tion by window charge outside the products of combustion) ; or the tube may draw its supply, not directly from the outer atmosphere, but from a hot- air flue or from a room or corridor where the air has been already warmed. The opening should always be about the level of a man s head, but the tube need not extend down to the floor : all that is essential is that it should have sufficient length to let the air issue in a smooth vertical current without eddies (fig. 3). These inlets are at once so simple and efl ective that no hesitation need be felt in introducing them freely in the rooms of dwelling-houses. When no special and door, provision is made for them in the walls, the advantage of a current entering vertically may still be in some FIG. 3. Short degree secured by help of certain makeshift contriv- 5 ances. One of these, suggested by Dr Hinkes Bird, is to open one sash of the window a few inches and fill up the opening by a board ; air then enters in a zig-zag course through the space between the sashes. Another plan is to have a permanent vertical slot between the sashes by making the top of the lower sash stand out a little from the bottom of the upper one. Still another plan is to have a light frame of wood or metal or glass made to fit in front of the lower sash when the window is opened, forming virtually a Tobin s tube in front of the window (see fig. 4, where a portion of the frame is broken away to show the position of the window sash). This last contrivance allows the fresh air as ready access as may be wished ; Fig. 4. FIG. 4. Ventilating inlet fitted to window. FIG. 5. Ventilating inlet at foot of door. The arrows indicate the direction of the current of air when the door is closed and, unlike the others, it is still effective when the blind is drawn down. A Tobin s tube, however, is better placed against a dead wall than below a window, for the ascending current is liable to be broken by the window recess and by the down-draught which a window causes by its cooling action on the air of the room. The principle of giving entering currents an upward direction is turned to useful account in a simple contrivance (see fig. 5) for preventing the disagreeable cold draught which comes along the floor of a room from the chink beneath the door. The clearance under the door is made a little greater than usual, and a thin piece of wood is set on the inner side as close as possible to the floor and at a dis tance of half an inch or so from the surface of the door ; the air then enters in a vertical stream. Arrange- As an example of the systematic ventilation of dwelling-rooms ments in on a large scale, the following particulars may be quoted of arrange- barracks ; ments that have been successfully used in English barracks for more than twenty years. One or more outlet-shafts of wood are carried from the highest part of the room, discharging some feet above the roof under a louvre ; the number and size of these shafts are such as to give about 12 square inches of sectional area per head, and the chimney gives about 6 square inches more per head. About half the air enters cold through air-bricks or Sheringham valves at a height of about 9 feet from the floor, and the other half is warmed by passing through flues behind the grate. The inlets taken to gether give an area of about 11 square inches per head. A fairly regular circulation of some 1200 cubic feet per head per hour is found to take place, and the proportion of carbonic acid ranges from 7 to 10 parts in lO^OO. 1 In public In the natural ventilation of churches, halls, and other large buildings, rooms we often find air admitted by gratings in the floor or near it, an offensive plan, since it fouls the air, besides causing objection able draughts, unless the temperature is very carefully regulated. The inlets should consist, like Tobin s tubes, of upright flues rising to a height of about 6 feet above the floor, from which the air proceeds in vertical streams. If the air is to be warmed before it enters, the supply may be drawn from a chamber warmed by hot-water or steam pipes or by a stove, and the temperature of the room may be regulated by allowing part of the air to come from a hot chamber and part from outside, the two currents mixing in the shaft from which the inlets to the room draw their supply. If a basement or 1 Ue Chaumont, in Parkes s Hyrjienc, Oth ed., &amp;gt;. 171. story below the room to be ventilated is available, a good plan is to carry the inlet tubes vertically down through it and warm the air in them, so that the height of the warm column assists the flow. Outlets usually consist of gratings or plain openings at or near the ceiling, preferably at a considerable distance from points vertically above the inlet tubes. One of the chief difficulties in natural ventilation is to guard them against down-draught through the action of the wind. Numberless forms of cowl have been de vised with this object, and often with the further intention of turning the wind to useful account by making it assist the up-current of foul air. Some of these exhaust cowls are of the revolving class : Exhaus- a hood or trumpet-shaped mouth, opening horizontally and sup- cowls. ported about a vertical axis so as to be free to turn, is kept facing away from the wind by means of a large vane. To make the wind help the up-current, a horizontal conical tube is fixed within the cowl, pointing towards the wind and discharging through the trumpet-mouth of the cowl, where it exhausts by suction, on the } rin ci pie of the jet-pump. Revolving cowls are liable to fail _by sticking, aiid, apart from that, when the wind blows in shifting gusts they cannot respond quickly enough to its changes of direc tion to prevent it from occasionally blowing down. Fixed cowls are to be preferred ; they are designed in many forms, of which Mr Buchan s may be cited as a good example. Fig. 6 shows this ventilator in horizontal section : aa is the ver tical exhaust flue through which the foul air rises ; near the top this expands into a polygonal chamber, Ibbb, with vertical sides, consisting partly of perforated sheet -metal plates ; outside of these are fixed vertical curved guide-plates, c,c,c,c ; the wind, blowing between these and the polygonal chamber, sucks FIG. C. Sectional plan of Buchan s air from the centre through the per forated sides. Perhaps no form of cowl is entirely free from liability to down-draughts. The most complete safeguard is to place in the exhaust flue a set _ j i | of flap- valves open- &quot; ing only outwards. Fig. 7 shows the ar rangement of exit- valves employed by Mr Buchan ; &quot; the valves a, a, a are flaps of oiled silk , working on a wood en grid bb, which is Flf; - 7 - ~ Exit val ves in exhaust flue, inclined enough to let them hang free of it when no current is passing ; beyond them is a door closing-valve c, worked by hand by the cord d ; and the whole is enclosed in a box, with glass sides ee, through which the action of the valves may be seen. When the outlets arc guarded by valves of this type they may discharge through a plain box with louvred sides ; an exhaust cowl, however, may still be used with advantage to assist the ventilation under favourable conditions of the wind. 2 The two things that supply motive force in automatic ventila tion the difference of temperature between inner and outer air and the wind are so variable that even the best arrangements of inlets and outlets give a somewhat uncertain result. To secure a strictly uniform delivery of air, unaffected by changes of season or of weather, the influence of these irregular motive forces must be as far as possible minimized, and recourse must be had to an artificial method of driving the air. Artificial or Mechanical Ventilation. This finds application on Artific the largest scale in the ventilation of collieries, by methods which vcntila are fully described under COAL (vol. vi. pp. 70-71) and MINING (vol. tion. xvi. p. 460). Motive force is supplied to the up-cast shaft either by a furnace at the base, which heats the rising column of air, or (in more modern practice) by a centrifugal fan, such as Guibal s, exhausting air from the top. The long galleries and workings through which the air has to be driven oppose so much resistance that the pressure required to move a sufficient volume of air is im mensely greater than is ever necessary or desirable in the artificial ventilation of buildings. A broad distinction may be drawn between what are sometimes Vacuu called vacuum and plenum methods of artificial ventilation. In and the former, as in colliery ventilation, the motive force is applied at plenur the outlets : air is drawn from the rooms, and the pressure of their metho atmosphere is less than the pressure outside. In the latter the motive force is applied at the inlets : air is pushed in, and the pres sure within the room is greater than outside. The plenum method has distinct advantages : it makes the air escape instead of coming 2 For an account of tests of various forms of ventilatinj Ilellyer, The Plumber and Sanitary nouses, 4th ed., 1SS7. cowls, see S. S.