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has been caused in cold weather that the shafts are rarely opened at all. And even when the Tobin shafts are closed, during strong winds, the down draughts through the gratings, which were intended for outlets in the front wall, render the rooms very draughty and cold in winter. Nor is this all, for the outlets into the passages at the back of the front rooms deliver huge volumes of foul air into them, and cause extra pressure upon the doors of the rooms on the other side, so that when a door is opened, the Tobin shafts and the exits for foul air through the outer walls at the back of the building pour out the foul air from the front rooms. Furthermore, the aspirating effects of the wind give rise to a decreased pressure upon the back part of the building, and the Tobin shafts frequently act as outlets, especially during gusty winds. Again, the Tobin shafts upon each end of the building are aspirated by the wind, and act as outlets during every strong gust or blow, and as inlets during every lull, so that a steady flow of air is impossible.
The block, Fig 8, roughly represents the ground floor of a Board School, showing classrooms, C Ra and C Rb, with central assembly hall. The classrooms are all heated by fires, the flues of which run up in the corners of the rooms, as marked at F. P. By the side of each flue an exit shaft for foul air extraction is carried up, and this is heated by the chimney. No provision has been made to admit fresh air, and if the windows are closed in winter, the air in the classrooms is under much tension. In other words, the pressure of the air outside is appreciably greater than that of the air inside, and the pull upon the air caused by the foul air-shaft as well as that due to the chimney, draws out or expands the air in the classrooms to a very appreciable and sensitive limit. When the wind blows powerfully in the direction of the arrow it aspirates upon the windows in the two ends of the building, as also upon those on the further side, the consequence being that much less air gets into the classrooms, and, as the fires already strain the atmosphere to its utmost limit of tension, every severe gust of wind causes the chimneys to smoke. The side of the building against which the wind blows has extra air supplied by the wind forcing it through the window chinks, and the ventilation is fairly good; but when the wind blows from the opposite direction, the classrooms, C Rb, are affected whilst the chimneys, being rendered too sensitive, always smoke during gusty winds.
With a view to obviate these results the windows are kept more or less open; and during very cold weather, when there is little wind, the fires draw very well; but it is difficult, indeed, to picture the condition of the nerves of the head and the eyes of the boys and girls who have to sit for many hours right under the deluge of cold air flowing over them. It is most unfortunate that such a state of things is allowed to exist. Furthermore, it is found that the aspiration of gusty winds in spring and autumn when they blow in the direction of the arrow, will draw air out of the rooms if the windows are open and overturn both the smoke in the chimney flue and the air in the foul air exit, the consequence being that to avoid the downpour of smoke and soot the windows must be shut, and the condition of the ventilation at such times is deplorable. If the wind blows from the opposite direction, then the classrooms on the other side are similarly affected; and if the wind blows strongly from D to D, the classrooms on both sides are more or less subjected to the evils which result from the aspiration of the wind.
The blockj H L, Fig. 9, is a rough outline of a hospital building, the wards being cut short as shown by the broken lines. XXX are Tobin shafts, and from the action of those in Fig. 7 it will be seen how with one wind they will be inlets, and with an opposite wind, outlets. The heating of this first floor is by hot water pipes chiefly, but at either end there are two coils, and opposite each coil in the oute wall there is an inlet grating. The hospital is on h'
ground, and there is an open space at each end—the consequence being that very powerful suction is exercised upon the gratings at the ends of the building when a strong wind is blowing. If the hand is held near the gratings it will be found that the wind pressure upon the Tobin shafts facing the wind supplies enough air to let the wind aspirate freely through the gratings at either end, and the air heated by the coils as it passes through them is drawn out through the gratings into the open air—a result little expected, doubtless, by those who fixed the coils.
In order to condense the matter of this chapter, let it be assumed that two air inlet gratings, G, were fixed in the outer walls of each classroom in the Board School, the effects produced upon these would be similar to those upon the windows, and they would sometimes act as outlets. Where air gratings are fixed near the ground to supply fresh air to batteries of pipes, or to the flow and return under the aisles of churches and halls, it is very necessary to see that the aspiration of the wind does not prevent air entering the building, especially if the top exit space is sufficiently large to cause intermittent air currents in the - structure. The church, C3, Fig. 6, has two such inlet gratings, I I, and before provision was made to prevent wind suction upon them the volume of fresh air was much intercepted when the wind blew down the street in the direction of the arrow. In this case, as in that of the gratings at each end of the hospital, it is possible to prevent wind aspiration. The results pointed out are, however, rarely ever suspected, and, until the action of the wind is better understood, it is to be feared that little effort will be made to remedy matters.
Example after example can be given of buildings where
natural ventilation, as it is sometimes called, has been
adopted, and where calculations had been most carefully made
f the exact outlet space necessary when so many Tobin shafts
^n area, were furnished, but the position of the building in the town, the probable action of the prevailing winds, and the ground swirl or suction, never received a thought. The consequence is that all these nice calculations are useless, and the system (?) of ventilation has its action reversed every time there is a strong breeze or wind blowing. The larger the apertures in the shaft inlets the more violent will be the wind effects; and before leaving the question of Tobin shafts, it is well to learn definitely that whether these or other form of inlet is provided, the area of each should be small and much subdivided.
The outlets in the form of cowls, roof ventilators, turrets, louvres, etc., are usually fixed upon the top of the roof, and, when these are so situated as to be above the surrounding buildings, the wind effects upon them need not trouble the caretaker much. As already hinted, the turrets should be so made that the wind does not aspirate strongly upon them. In the case of churches, halls, or other buildings upon a steep hill side, and covered in front up to half of their height by other structures, the outlets upon the roof or even the crevices in it, if there are no ventilators, will be under back or increased pressure when the wind blows against the face of the hill. In the case of mission halls, music halls, and small halls of assembly, these are frequently below adjoining buildings, and if one of the sides is exposed to the wind and it blows powerfully against it, the extra pressure at the top of the building hinders ventilation greatly. Where the pressure of the wind acts upon the bottom of a building as well as upon the top, the ^-effects are not so marked—it is when the bottom of a hall protected by a wall or lower structure, and the top is osed to the increased pressure and violence of the wind, the effects are most pronounced; and under these cir\nces it will be found that the roof exits will act best ipen than when open wide.