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Bible Encyclopedias
Ventilation
1911 Encyclopedia Britannica
or (Lat. ventilare, from ventus, wind), the process and practice of keeping an enclosed place supplied with proper air for breathing; and so, by analogy, a term used for exposing any subject to the winds of public criticism. The air which we breathe consists chiefly of two gases, oxygen and nitrogen, with certain small proportions of other gases, such as carbonic acid (carbon dioxide), ozone and argon. Oxygen, which is the active and important constituent, and on which life and combustion depend, forms about one-fifth of the whole, while nitrogen, which is inert and acts as a diluent, forms nearly four-fifths. Of this mixture each adult person breathes some 2600 gallons or 425 cub. ft. in twenty-four hours. In air that has passed through the lungs the proportion of oxygen is reduced and that of carbon dioxide increased. Of the various impurities that are found in the air of inhabited rooms, carbonic acid gas forms the best practical index of the efficiency of the ventilation. The open air of London and other large inland towns contains about four parts by volume of the gas in io,000 of air. In the country, and in towns near the sea, two to three and a half parts in 10,000 is a more usual proportion. Authorities on ventilation usually take four parts in io,000 as the standard for pure air, and use the excess over that quantity in estimating the adequacy of the air supply. But they differ as to the proportion to which the carbonic acid may be allowed to rise under a good system of ventilation. It is generally admitted that the air in which people dwell and sleep should not under any circumstances be allowed to contain more than ten parts in 10,000. This has been accepted as the permissible proportion by Carnelley, Haldane and Anderson, after an extensive examination of the air of middle and lower class dwellings.
The rate at which an adult expires carbonic acid varies widely with his condition of repose, being least in sleep, greater of in waking rest, and very much greater in violent exercise. As a basis on which to calculate the air necessary for proper ventilation we may take the production of carbonic acid by an adult as o6 cub. ft. per hour. Hence he will produce per hour, in 6000 cub. ft. of air, a pollution amounting to one part of carbonic acid in io,000 of air. If the excess of carbonic acid were to be kept down to this figure (1 in io,000), it would be necessary to supply 6000 cub. ft. of fresh air per hour; if the permissible excess be two parts in io,000 half this supply of fresh air will suffice; and so on. We therefore have the following relation between (I) the quantity of air supplied per person per hour, (2) the excess of carbonic acid which results, and (3) the total quantity of carbonic acid present, on the assumption that the fresh air that is admitted contains four parts by volume in ro,000: - Some investigators have maintained that, in addition to an increased proportion of carbonic acid, air which has passed through the lungs contains a special poison. This view, however, is not accepted by others; J. S. Haldane and Lorrain Smith, for instance, conclude "that the immediate dangers from breathing air highly vitiated by respiration arise entirely from the excess of carbonic acid and deficiency of oxygen" (Journ. Path. and Bad. 1892, I, 175). Carbonic acid, however, is not the only agent that has to be reckoned with in badly ventilated rooms, for the unpleasant effects they produce may also be due to increase of moisture and temperature and to the odours that arise from lack of cleanliness. Again, though there may be no unduly large proportion of carbonic acid present, the air of an apartment may be exceedingly impure when the criterion is the number of micro-organisms it contains. This also may be greatly reduced by efficient ventilation. Comparisons carried out by Carnelley, Haldane and Anderson (Phil. Trans., 1887, 178 B, 61) between schools known to be well ventilated (by mechanical means) and schools ventilated at haphazard or not ventilated at all showed that the average number of micro-organisms was 17 per litre in the former, and in the others 152. Results of great interest were obtained by the experiment of stopping the mechanical ventilators for a few hours or days. Tested by the proportion of carbonic acid, the air of course became very bad; tested by the number of micro-organisms, it remained comparatively pure, the number being, in fact, scarcely greater than when ventilation was going on, and far less than the average in "naturally ventilated" schools. This proves in a striking way the advantage of systematic ventilation.
In the ventilation of buildings four main points have to be considered: (I) the area of floor to be provided for each person; (2) the cubic capacity of the room required tor each occupant; (3) the allowance to be made for the vitiation of the air by gas or oil burners; and (4) the quantity of fresh air which must be brought in and of vitiated air that must be extracted for each individual. The first will depend upon the objects to which the room is devoted, whether a ward of a hospital or a school or a place of public assembly. The purity of the air of a room depends to a great extent on the proportion of its cubic capacity to the number of inmates. The influence of capacity is, however, often overrated. Even when the allowance of space is very liberal, if no fresh air be supplied, the atmosphere of a room quickly falls below the standard of purity specified above; on the other hand, the space per inmate may be almost indefinitely reduced if sufficient means are provided for systematic ventilation. Large rooms are good, chiefly because of their action as reservoirs of air in those cases (too common in practice) where no sufficient provision is made for continuous ventilation, and where the air is changed mainly by intermittent ventilation, such as occurs when doors or windows are opened. With regard to the third point, in buildings lighted by gas or oil the calculations for the supply of fresh and the extraction of foul air must include an allowance for the vitiation of air by the products of combustion. The rate at which this takes place may be roughly estimated in the case of gas by treating each cubic foot of gas burnt per hour as equal to one person. Thus an ordinary burner giving a light of about twenty candles and burning 4 cub. ft. of gas per hour vitiates the air as much as four persons, and an incandescent burner as much as one and a half persons. A small readinglamp burning oil uses the air of four men; a large central table lamp uses as much air as seven men.
As to the fourth point there is great diversity of opinion. To preserve the lowest standard of purity tolerated by sanitarians, ventilation must go on at the rate per person of r000 cub. ft. per hour, and 3000 cub. ft. per hour are required to preserve the higher standard on which some authorities insist. E. A. Parkes advised a supply of 2000 cub. ft. of air per hour for persons in health and 3000 or 4000 cub. ft. for sick persons. In the case of a public assembly hall no great harm will occur to an audience occupying the room for a comparatively short time if 30 cub. ft. of air per minute are provided for each person. The United States book on school architecture gives a practical application to its remarks on this subject as follows: The amount of fresh air which is allowed to hospital patients is about 2500 cub. ft. each per hour, Criminals in French prisons have to content themselves with 1500 cub. ft. per hour. Assuming that we care two-thirds as much for the health of our children as we do for that of our thieves and murderers, we will make them an allowance of moo cub. ft. each per hour, or about 16 cub. ft. per minute. Forty-eight children will then need an hourly supply of 48,000 cub. ft. Definite provision must therefore be made for withdrawing this quantity of foul air. No matter how many inlets there may be, the fresh air will only enter as fast as the foul escapes, and this can only find an outlet through ducts intended for that purpose, porous walls and crevices serving in cool weather only for inward flow. What, then, must be the size of the shaft to exhaust 48,000 ft. per hour? In a shaft 20 ft. high, vertical and smooth inside, with a difference in temperature of 20°, the velocity will be about 22 ft. per second, or 9000 ft. per hour; that is, it will carry off 9000 cub. ft. of air per hour for every square foot of its sectional area. To convey 48,000 cub. ft., it must have a sectional area of 51 sq. ft.
A general idea of the floor area, cubic space and fresh air supply per inmate allowed by law or by custom in certain cases is given in the table below: - 1 In calculating the cubic capacity per person the height should not be measured beyond 12 ft. above the floor.
The supply of fresh air indicated in the table should not be regarded as entirely satisfactory, for the standard of purity suggested is low, and ought to be exceeded, but it might deter many from moving in the matter if a proper and higher standard were to be laid down at first.
One of the most important points is the proper warming of the fresh air introduced into buildings, for unless that be done, when a cold day occurs all the ventilating arrangements will probably be closed. The fact should not be lost sight of that the air in a room may on the one hand be quite cold and yet very foul, and on the other, warm and yet perfectly fresh. To avoid draught the air should enter through a large number of small orifices, so that the currents may be thoroughly diffused. This is done by gratings. The friction of their bars, however, seriously diminishes their capacity for passing air, and careful experiments show conclusively that very ample grating area is required to deliver large volumes. The same remark applies to extracting-flues. Owing to the small size and the roughness of the surface the velocity of the upward current is small, and the quantity of air that passes out is often much less than is requisite.
Means of Ventilation
In order that the atmosphere of a room should be changed by means of air currents, thereby securing proper ventilation, three things are necessary; (i) an inlet or inlets for the fresh air, (2) an outlet or outlets for the vitiated air, and (3) a motive force to produce and maintain the current. 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.
Natural 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 resistance at the inlets. 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 many other chinks and crevices. Under these conditions the air enters in small streams or narrow sheets, illdistributed 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. The discharge of air by an ordinary open fire and chimney varies widely, depending on the rate of combustion, the height and section and form of the chimney, and the freedom with which air is entering the room. About 10,000 cub. ft. per hour is probably a fair average, about enough to keep the air fresh for half a dozen persons. 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.
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 Other of gratings in the ceiling or cornices in communication outlets. with flues leading to the open air. These openings should be protected from down-draught by light flap valves of oiled silk or sheet mica.
With regard to inlets, a first care must be to avoid such currents 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 3, or even 2 ft. 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 the Sheringham (fig. 1). When opened it forms a wedge-shaped project io n into the room, and admits air in an upward stream through the open top. It should be placed at a height of 5 or 6 ft. above the level FIG. 1. - Sheringham Air Inlet. of the floor. Other inlets are made by using hollow perforated blocks of earthenware, called airbricks, 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 inclination to the entering stream.
In another and most valuable form of ventilator, the Tobin tube, the fresh air enters vertically upwards. The usual arrangement of Tobin tube (shown in front elevation and section in fig. 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 lower end communicates with the outer air through an air-grating in the wall; from its upper end, which is freely open, the current of fresh air rises in a smooth stream. 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 rectangular form is most usual, or it may be placed in a channel so as to be flush with the face of the wall; 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, and contrivances may be added for cleansing the entering 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.' 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 discharge outside the products of combustion), or the tube may draw its supply, not directly from the outer atmosphere, 3. - Short but from a hot-air flue. The opening should FIG.
Tobin Tube. 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 effective that no hesitation need be felt in introducing them freely in the rooms of dwellinghouses. When no special provision is made for them in by the walls, the advantage of a current entering vertically may still be in some degree secured by help of certain makeshift contrivances. 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 zigzag course through the space between the meeting rails of the sashes. 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 tube in front of the window.
As an example of the systematic ventilation of dwelling-rooms on a large scale, the following particulars may be quoted of arrangements that have been successfully used in English barracks. One or more outlet-shafts of wood fitted with flap valves to prevent down-draught 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 sq. in. of sectional area per head, and the chimney gives about 6 sq. in. more per head. About half the air enters cold through air-bricks or Sheringham valves at a height of about 9 ft. from the floor, and the other half is warmed by passing through flues behind the grate. The inlets taken together give an area of about II sq. in. per head. A fairly regular circulation of some 120o cub. ft. per head per hour is found to take place, and the proportion of carbonic acid ranges from 7 to 10 parts in Io,000.
' 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 comparatively slight. The difference is due to the fact, noticed and explained 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. 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.
In the natural ventilation of churches, halls and other large rooms we often find air admitted by gratings in the floor or near it; or the inlets may consist, like Tobin tubes, of upright flues rising to a height of about 6 ft. 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. 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 devised with this object, 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, made to various designs and dimensions and put in rotation by the force of the wind. Revolving cowls are liable to fail cowls. by sticking, and, generally speaking, fixed cowls are to be preferred. They are designed in many forms, of which Buchan's may be cited as a good example. Fig. 4 shows this ventilator in horizontal section: as is the vertical exhaust flue through which the foul air rises; near the top this expands into a polygonal chamber, bbbb, 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 air from the centre through the perforated sides. The efficient working of an exhaust cowl, however, depends almost entirely upon the favourable conditions of the wind. 2 The two things that supply motive force in automatic or natural ventilation by means of exhaust cowls and similar appliances - 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. As an example, it is evident that on a hot day with little movement in the air this mode of ventilation would be practically ineffectual. Under other conditions these automatic air-extractors not infrequently become inlets, thus reversing the whole system and pouring cold air on the heads of the inmates of the apartment or hall. To secure a strictly uniform delivery of air, unaffected by changes of season or of weather, it is necessary that the influence of these irregular motive forces be as far as possible minimized, and recourse must consequently be had to some mechanical force as a means of driving the air and securing adequate ventilation of the building.
