Ventilation, artificial renewal of the air within a confined space, such as a mine or the interior of a building or vessel. This is required when the air is subject to contamination, as by products of respiration or by admixture with other gases. In a mine it is necessary to renew the air in order to carry off the products of respiration of men and horses, the products of combustion of lamps, and, in coal-mines, the inflammable gas which oozes from coal or rushes from 'blowers,' and causes risk of explosions. In general there are two leading methods of ventilation: (a) causing air to go out at the outlet, and allowing air to find its way in by any inlet (vacuum method); (b) forcing air in and allowing it to find its way out (plenum method). The advantage of the second method is that it is known whence the air comes which is forced in, and the access of air from other sources tends to be prevented by the excess of pressure within the confined space. In most cases, however, the vacuum method is much more easily applied.
In mines there are two main methods of applying the vacuum method: (a) by a furnace at the bottom of the upcast shaft; (b) by a vacuum fan at the top of that shaft. In both cases the actual quantity of air which is caused to travel up the upcast shaft must be sufficient to sweep away and sufficiently dilute all the mischievous admixtures in the workings; and the 'draught' must be sufficient to overcome the very considerable friction encountered by the air as it passes through the air-ways. This friction varies at each spot as the square of the velocity there; and therefore it is necessary to arrange the air-ways so as, with a given flow, to have the least velocity possible upon discharge into the outer air. When a furnace is used, air is heated by it and becomes light; it therefore tends to ascend in the shaft; other air takes its place from the workings; and a continuous current is thus set up, the fresh air entering by the downcast shaft. A furnace is, however, somewhat dangerous, for the air from the workings may, upon reaching it, be unsafely charged with gas and coal-dust; and during recent years powerful exhaust fans, like centrifugal pumps (see PUMPS), have been largely employed. The velocity for a given flow is lowest when the air-ways are wide, and the velocity at any point of the workings is less the more numerous the air-ways. The flow through the workings is regulated by opening and closing air-doors or putting up barriers. If the flow through the workings is too much obstructed, the actual outflow of air being maintained by driving the engine hard, the pressure within the air-ways may be so far diminished that gas is exhausted from the coal-seam. In mines water-ganges are employed in order to test the local pressures in the workings; one limb of a U-tube containing a little water is connected with the 'dumb-drift,' and thereby with the upcast shaft; the other limb is open to the air-way; the movement of the water in the U-tube, taken along with the readings of the instruments in the upcast-shaft, indicates the suctional variations of pressure in the workings. The velocity is measured by anemometers, which are often made to signal their readings by electric transmission. Barometers and thermometers are also used in order to supply data for the requisite corrections and adjustment of quantity of air to be set in motion. Besides furnaces and fans, steam and air-blasts have been used in order to force air along; but these waste much power. In ore-mining the fresh and chilled air liberated from compressed-air motors at the working face is often very useful as a means of ventilation by the plenum method at the very point where air is most needed.
In regard to buildings and dwellings the greater care now taken in making doors and windows airtight makes it more necessary than it formerly was to provide proper means of removing vitiated air and supplying fresh. In the old House of Commons Sir Christopher Wren made the chamber communicate by means of tubes with an upper room into which the heated air went while the cold air from that room flowed down; this produced draughts. In 1723 Dr Desaguliers lit fires in the upper room; these fires heated the ventilating tubes from the chamber below, and caused air to ascend in them. He afterwards used a centrifugal blowing wheel, turned by a man, to effect the same purpose. Before 1744 Dr Stephen Hales proposed to use bellows for driving air into a room or a ship, or for expelling it; these being sometimes worked by a roof windmill. In 1749 Mr Samuel Sutton proposed the simple use of an open fire, the air-supply of which is wholly drawn from the interior of a building or the hold of a ship. Count Rumford proposed to take the air-supply of a building down from the roof by means of an air-shaft; a plan which implied that the fire must be kept burning, and extraneous air excluded. In 1811, at the House of Lords, Sir Humphry Davy admitted air, heated if necessary, by numerous apertures in the floor, and used Dr Desaguliers' mode of extraction, but his exit-pipes were too small, and the plan failed. About 1818 the Marquis de Chabannes wrote upon the ventilation of rooms. He proposed the admission of air by tubes heated by the fire, open to the outer air at one end and to the room at the other; and he proposed a chimney ventilator to remove the vitiated air, and also ventilating lamps and ventilating gas-fittings to carry off not only the products of their own combustion, but also an additional quantity of air, through special channels. In the House of Commons he applied steam-heat near the ceiling to warm the air and produce ascending currents. In 1834 Dr Reid of Edinburgh introduced, at the temporary House of Commons built after the fire, the principle of a much larger exit area than had previously been thought necessary. The vitiated air was drawn through a furnace and up a high chimney. There were very numerous apertures of admission for the air, which was filtered from dust, and warmed or cooled, dried or moistened, as might be required. The temperature was regulated by mixing highly heated air with an adjustable quantity of cold air in a mixing and equalising chamber before allowing the mixture to enter the House. Mr Gunney, in the new buildings, made the windows capable of being opened, introduced steam-heating, and took in the air at a low level instead of from the top of a tower as Dr Reid had done at first.
The products of combustion and respiration, comprising carbonic acid, water-vapour, and a small quantity of anthrotoxin (which is a virulent poison when concentrated), are rapidly diffused throughout the air of a room; and more air ought to enter a room, in order to dilute these, than is actually necessary in order to supply the requisite oxygen. The quantity of air required per head per minute has been very variously stated; the figure now usually stated is between 20 and 30 cubic feet per head per minute. The inflowing air should never be allowed to make sharp draughts across a room; for this reason it is frequently much safer to open a window widely than it is to open it a little; but the inflowing stream may be broken up so as to diminish its velocity as far as is consistent with adequate flow. For this purpose perforated zinc, perforated glass, wire gauze, sliding valves, air-bricks, and similar devices may be used; or the velocity may be practically diminished by directing the incoming stream of air towards the ceiling as by louvre ventilators in the window, or by Tobin's tubes. In these, when there is any withdrawal of air from the room, its place is taken by air directed in an upward stream through tubes whose upper end is open at a height of about 6 feet within the room, while the other end is open to the external air-supply at a lower level. In Sir Douglas Galton's grate there is an air-chamber behind the fire, which is continued into an air-flue above. The air-chamber is connected with the outer air; air is heated in the chamber and flue, and leaves the flue, passing into the room near the ceiling; fresh air enters the chamber from outside, and a continuous supply is thus kept up; warm air enters the room, and cold draughts are prevented. In Pierce's stove air from outside flows round the fire within an open stove; it enters the room through a perforated grating at the top, ascends to the ceiling and circulates there. There is still a good deal of divergence of opinion as to whether air should be admitted at the top or the bottom of an apartment. Neither method is entirely free from objection. Cold air admitted at a low level never allows the lower part of the room to become warm; this might, however, be an advantage in sultry weather. Cold air admitted at a high level brings the products of combustion down to be breathed over again. Mr Dye argues in favour of bringing in fresh air, if cold, at an intermediate height, just above people's heads.
For extraction of the vitiated air the open fireplace is to some extent effective even in summer time, for the chimney long remains slightly warm; but the current of vitiated air is better directed, not past the persons in the room, but towards the ceiling, and taken off by a grating opening into the chimney. When this is done down-draughts must be checked by a valve, the fireplace must not be too wide or open, and the chimney itself must not be too narrow. The fault of an open fireplace is in general that when the fire is lit it extracts more air than it is necessary to extract for ventilation purposes, and the air-supply must come in partly through the house or down some other chimney, or may fail to find its way in at all, in which case the chimney smokes. During recent years a great deal of attention has been paid to utilising the motive power of gas-flames as a means of ventilation. In the Paris Gas Company's cooking demonstration rooms in the Rue du Quatre-Septembre, 28, ventilation by this means is said to have been most successfully carried out. In every case a sufficiently large exit-channel ought to be provided, and this is a point apt to be overlooked.
The reader may consult Dr Reid's Illustrations of the Theory and Practice of Ventilation (Longman, 1844); Ritchie's Treatise on Ventilation; Tomlinson's Rudimentary Treatise on Warming and Ventilation; the chapter on Ventilation in Dye's Hood on Warming Buildings (Spon, 1891); Edwards on Ventilation and Heat (Longmans, 1881), a work to which we are largely indebted for our historical retrospect of the subject, and which contains numerous useful illustrations; and also Alan Bagot's Colliery Ventilation (Kegan Paul, 1882).