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ventilators on the apex of the roof, because they would exceed 1oo feet in length when they touched each other. A ventilating engineer would not recommend more than three ventilators for such a building, hence it will be seen howabsurd and impracticable the above figures are. Let any one try to imagine a continuous Tobin shaft 10o feet long and 5 inches wide belching forth cold air at 32o into the side of a building, and another shaft of similar length doing the same thing on the other side. And this is the practical ventilation of the twentieth century.
If air is to be introduced into a building in its raw, cold condition, it should fall, if possible, through 3o feet of warm air, and not more than 5 square inches per person should be attempted; and even the 5 square inches ought to be greatly subdivided. It will be seen that the fixing of the cold air inlets as high as possible is done so that the cold air shall be warmed before it gets to the audience, and all modern writers on ventilation suggest that air inlets should be raised higher than formerly. But such attempts only deal with the fringe of the matter. If the cold air is warmed in its fall, it implies that the pure air has been largely mixed with the hot products of the breath of the audience, and really rendered so vitiated that it is not fit to be inhaled.
It has been shown already that inlets of the Tobin type are most unsuited to be fixed to buildings where seats abut the outer walls, and in nine cases out of ten they are kept closed, being unfit for winter use, and useless for summer, as window openings are more effective. Where cotton wool is used in such shafts, the volume of air going in is reduced to one-twentieth perhaps, and the inconvenience and draughts are not so bad; but during very cold weather persons sitting near cannot endure them open; and felt and such like material is used to prevent air leaking around the valve when it is closed. Air inlets of the Tobin type or any large cold air inlets are condemned for other reasons than the intolerable floods of cold air which they admit when the temperature is low outside. It is shown in Chapter II. that the wind effects upon Tobin shafts and similar inlets for cold air are very considerable. If the wind blows at right angles to the opening in the outer wall, the extra pressure forces a deluge of cold air inwards. If the wind aspirates on either side of the building, or causes reduced pressure at the back, Tobin shafts may act as outlets at every gust of wind, and as inlets during every lull in the storm.
The wind effects upon outlet ventilators are equally erratic, and generally at cross purposes with the effects upon the inlets. If the wind blows at right angles to the ridge of the roof, an upward twist caused by the upper wind currents, accelerated by the slant of the roof, will cause a powerful upward suction upon the ventilators above the ridge. On the other hand, if the roof of the church forms a line running east and west, and the wind blows from either quarter, the effect upon ventilators above the roof will be normal; whilst upon the full length of the building on both sides the Tobin inlets would be under a powerful aspirating influence, unless some careful provision was made to overcome the aspirating action—a provision which it is hardly necessary to say has not been made because the action of the wind was not studied. If the ventilators are fixed in the side of the roof itself, and flush with the tiles, they will be aspirated powerfully on both sides when an east or west wind is blowing. If the wind blows from the north, from the south, or south-west, the increased pressure of air upon the one side, and the reduced pressure on the other side of the roof, will tend to cause an up current in one half of the ventilator and a down current in the other half, especially if the building is inclined to be top heavy in outlet space. Ventilators in the sides of the roof do not admit of the full ventilating power of the building being used, and are not to be recommended owing to the very variable action of the wind at the outlets.
The position of a church or building, moreover, may be such, and often is, that a strong wind causes a considerably reduced pressure upon the whole building owing to its sheltered position, and the consequence will be that all the inlets and outlets will be aspirated with a force varying according to the gusty nature of the wind. It will not be difficult to realise that if there is a large inlet, or if the outlets are of greater area in the aggregate than the inlets, as they usually are where large outlet tubes are employed, the atmosphere in a building may be turned topsy turvey at every severe gust of wind, especially if its duration is for one minute or more. It is well to remember that a building containing from six to ten thousand cubic feet of air in one hall, offers a very large body' of elastic matter to be expanded by the aspirating influence of the wind. If a string of elastic one foot long is pulled, it soon stretches to its utmost limit; but if that string is six thousand feet long what a distance it will stretch before the limit is reached! The thousands of cubic feet of air in a large building represent such a long thread of elastic, and the aspirating effects of a gust, of wind, the drawing out or expanding the air in a building, like a long elastic thread is elongated by pulling at one end. But immediately the gust of wind ceases, and a lull occurs, the air contracts and snaps back like an elastic thread which has been strained and suddenly released. It is very difficult, therefore, to ventilate buildings when both their inlets and outlets are aspirated and subjected to reduced pressure at the same time, and at every gust of wind.
On the other hand, the reader will see it is possible for some buildings to be sheltered at the top, and subjected to considerable pressure at the bottom whenever the prevailing winds blow; and that under these conditions the ventilation of a building may be much assisted.
It is very evident, however, that the questions of air inlets and outlets are much more complex than a simple calculation of figures, and it is not surprising that the ventilation of large buildings is generally in a most unsatisfactory condition.
The question whether the foul air should be drawn from the top of the building or near the floor level is still in dispute. During the past century, especially about its latter half, it had become recognised that owing to the heat of the breath and the presence of so much water vapour in it, it must rise upward quickly by virtue of its lesser density. The heat of the bodies of the audience is also a factor to be reckoned with, and some have thought that this alone was sufficient to cause an upward movement equal to one inch per second. When the temperature is very low within the building, say at 53°, and that of the air outside is 300, the velocity of the air caused by the warmth of the bodies of a closely packed audience is nearer two inches per second. In the face of the fact that lighting by gas gives rise to much heat, and that even the electric light emits appreciable warmth, it ought to be self-evident to all that the only way to get rid of impurities and a foul atmosphere, is to drive it out at the highest point of a building. The larger number, perhaps, of ventilating engineers concluded that this was the only alternative, but it was thought by not a few that by the aid of powerful fans air could be impelled downward, and so prevent dust and other impurities being drawn in at the floor level, as when upward ventilation is employed. The main contention is, however, that the impurities in the atmosphere of a building are found in great quantity at the floor level, and this is quite erroneous. It has been proved again and again that the most vitiated part of a room, a church or a public hall is near the ceiling, if there is one, but in all cases in the upper part of the building, unless the area of the outlets is greater than that of the inlets near the ground level; in which case the upper layer of the atmosphere will be diluted by cold air which forms down draughts and intermittent air currents. It has been found to be absolutely impossible to remove the heated products downward, and where there are galleries, it could not be done partially without seriously inconveniencing those in the body of the hall or church. During the past century downward ventilation has had a fair trial, for among other places it was tried in the French Chamber of Deputies, under circumstances where monetary considerations did not handicap the system. Wherever it has been tried on a large scale, that failure which common sense, with a knowledge of the physical behaviour of gases, predicts, has invariably been experienced. And yet, even during the opening of the present century, in the pages of the leading engineering and sanitary journals, down
ward ventilation has been advocated, hence it is not so surprising that the science has made such little progress in the past.
As the problem of admitting air into a building is surrounded with many difficulties it can only be dealt with by considering all the present arrangements employed. During recent years the upper portions of the windows of some churches, halls and other buildings have been fitted with framed glass panels which when open and shut form a V, Fig. Io. It was thought that this arrangement would shoot the air some distance into the building, but they are generally closed in cold weather, and are open to the objection already mentioned that if the air descends to the audience it is so