Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/177

Rh VENTILATIO N 159 [ochani il and it u nil entila- on. Dhimney fraught. Other )Utlets. enumerate such obvious requirements were it not that, in providing appliances which are intended to act as venti lators, one or other of the three essentials is not unfre- quently overlooked. In systems which are distinguished by the general name of mechanical or artificial ventilation special provision is made for driving the air, by fans, or by furnaces, or by other contrivances to be described more fully below. In what is called natural ventilation no special appliance is used to give motive force, but the forces are made use of which are supplied by (1) the wind, (2) the elevated temperature of the room s atmosphere, and (3) the draught of fires used for heating. A careful distinction should be drawn between cases in which these motive forces are skilfully taken advantage of, by the use of proper inlets and outlets, to give the best result attain able without special appliances for driving the air, and other cases, unfortunately too common in practice, where the ventilation is left to take care of itself. In a fair comparison of a mechanical with the natural system we should exclude examples in which the ventilation is hap hazard, or in which as near an approach to no ventilation is reached as the conditions of modern architecture will permit. Domestic Ventilation. The chief agent in domestic ventilation is the chimney ; when a bright fire is burning in an open grate, it rarely happens that any other outlet for foul air from a room need be provided. The column of hot air and burnt gases in the chimney is less heavy, because of its high temperature, than an equal column of air outside ; the pressure at the base is therefore less than the pressure at the same level outside. This supplies a motive force compelling air to enter at the bottom through the grate and through the opening over the grate, and causing a current to ascend. The motive force which the chimney supplies has not only to do work on the column of air within the chimney, in setting it in motion and in overcoming frictional resistance to its flow : it has also to set the air entering the room in motion and to overcome frictional resist ance at the inlets. In many cases the latter part of the chimney s work is the more considerable of the two. From want of proper inlets air has to be dragged in at a high velocity and against much resistance, under the doors, between the window sashes, and through a hundred other chinks and crevices for which we have to thank imperfect carpentry and half-seasoned timber. Under these con ditions the air enters in small streams or narrow sheets, ill-distri buted and moving so fast as to form disagreeable draughts, the pressure in the room is kept so low that an opened door or window lets in a deluge of cold air, and the current up the chimney is much reduced. If the attempt is made to stop draughts by applying sand-bags and listing to the crevices at which air streams in, matters only become worse in other respects ; the true remedy of course lies in providing proper inlets. 1 The discharge of air by an ordinary open fire and chimney varies widely, depending on the rate of com bustion, the height and section and form of the chimney, and the freedom with which air is entering the room. About 10,000 cubic feet per hour is probably a fair average, about enough to keep the air fresh for half-a-dozen persons. 2 Even when no fire is burning the chimney plays an important part in ventilation : the air within an inhabited room being generally warmer than the air outside, it is only necessary that an up-current should be started in order that the chimney should maintain it, and it will usually be found that a current is, in fact, passing up. 3 When a room is occupied for any considerable length of time by more than about half-a-dozen persons, the chimney outlet should be supplemented by others, which usually take the form of gratings in the ceiling or cornices in communication with flues leading to the open air. Frequently these openings are protected from down- draught by light flap valves of oiled silk or sheet mica, opening outwards. To increase the efficiency of the ventilating action of thechimney, Dr Arnott advocated (in 1849) that an opening should 1 The absence of proper inlets for air in a house where several fires are burning involves a danger that is much more serious than other effects of bad ventilation. When the air which is required to take the place of that discharged by the chimneys can only struggle in through small openings, the pressure within the house falls considerably below that of the outer air, the water traps under basins and closets are liable to be forced, and foul air is drawn in from every leak in soil-pipe or drain. The writer has found a house drawing what seemed to be its main supply of &quot; fresh &quot; air from the public sewer, through a defective joint between the soil-pipe and the (untrapped) house-drain. 2 .Report of the Barracks Commissioners, 1861. 3 See observations by De Chaumont, in Parkes s Hiiqiene, 6th ed. . p. 173. be made near the ceiling, into the chimney, guarded by a flap valve of this type, with the object of providing a direct exit for the foul warm air that gathers in the upper region of a room, especially when gas is burnt. To make Arnott s valve of much service it should be larger than the size usually supplied ; even then it has the drawback that, notwithstanding the protection given by the valve, enough back-flow may occur to blacken the wall and ceil ing with soot near the opening. If a valve near the ceiling be pro vided, it is probably better in most cases to lead its outlet shaft direct to the open air than to lead it into the chimney. With regard to inlets, a first care must be to avoid such currents Inlets, of cold air as will give the disagreeable and dangerous sensation of draught. At ordinary temperatures a current of outer air to which the body is exposed will be felt as a draught if its velocity exceeds 2, or at most 3, feet per second. The current entering a room may, however, be allowed to move with a speed much greater than this without causing discomfort, provided its direction keeps it from striking directly on the persons of the inmates. To secure this, it should enter, not horizontally nor through gratings on the floor, but vertically through openings high enough to carry the entering stream into the upper atmosphere of the room, where it will mix as completely as possible with warm air before its presence can be felt. A favourite form of inlet is Sheringham s (see fig. 1). When opened it forms &amp;lt;^ a wedge-shaped projection into the room, and admits K- air in an upward stream *~&quot; through the open top. It FlG - ! Shtringhani air inlet, is usually placed near the ceiling ; but a lower place in the wall would be better. Other inlets are made by using hollow perforated blocks of earthenware, called air-bricks, built into the wall ; these are often shaped on the inner side like an inverted louvre-board or Venetian blind, with slots that slope so as to give an upward in clination to the entering stream. In another and most valuable form of ventilator, the introduction Tobin of which is due to Mr Tobin of Leeds, the fresh air enters verti- tube, cally upwards. The usual arrangement of Tobin s tube (shown in front elevation and section in Jig. 2) is a short vertical shaft of metal plate or wood which leads up the wall from the floor level to a height of 5 or 6 feet. Its lower end communicates with the outer air through an air brick or built opening in the wall ; from its upper end, which is freely open, the current of fresh air rises in a smooth stream, clinging, as it were, to the wall, and scarcely chang ing its direction until it has passed far above the level of the opening. Various forms of section may be given to the tube : if placed in a corner it will be triangular or segmental; against a flat wall a shallow rect- angular form is most usual ; a lining of wood forming a dado may even be made to serve as a Tobin tube by setting it out a little way from the wall. The tube is often furnished with a regulating valve ; but this is a doubtful advantage, as it tempts the inmates to stop the ventilation for no better reason than that the room is cold ; in exceptional circumstances, such as the presence of an invalid, the opening may be stopped or reduced by laying a board over it. Contrivances are occasionally added for cleansing the enter ing air. A muslin or canvas bag hung in the tube, or a screen stretched diagonally across it, may be used to filter out dust ; the same object is served in some degree by forcing the air, as it enters the tube at the bottom, to pass in close contact with the surface of water in a tray, by means of a deflecting plate. These complications have a double drawback : they require frequent attention to keep them in order and by putting resistance in the way of the stream they are apt to reduce the efficiency of the ventilation. 4 The air entering by a Tobin tube may be warmed by a coil of hot pipes within the tube or by a small gas-stove (provided of course with a flue to dis- Fio. 2. Tobin tube. 4 When the air is not filtered, and when it has been warmed before entering, the vertical direction of the stream is readily traced by dust which is deposited on the wall in a nearly upright column, spreading slightly fan-wise as it rises. With cold air the deposit of dust is com paratively slight. The difference is due to the fact noticed and ex plained by Mr John Aitken, that air quickly deposits any suspended particles when it is brought into contact with a surface colder than itself, but retains them in suspension if the surface be warmer than the air (Trans. Roy. Soc. Edin., vol. xxxiii., 1884, p. 239). Another domestic illustration of the same fact is given by the greater dustiness of walls and furniture in a stove-heated room than in a room heated by an open fire.