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foul air should be removed through the central outlet. The larger the volume of air coming through the floor the less would be the tension in the building; and when a door was opened the cold air would not shoot across the audience. If this provision had been made there would have been no need for the ventilators in the windows to be opened at all during the meeting referred to, and, with a powerful fan, even in summer time, the atmosphere would be good although the windows were shut.
What has been said about the window openings will show how difficult it is to use them to the best effect even in moderately warm weather when mechanical power is employed. In the first place it should be made impossible for the audience to interfere with the ventilators in any public building. One man has fresh air on the brain, and wants every ventilator open, even if the air coming in is frigid. Another, suffering from neuralgia, and having had the sad experience perhaps of sitting near cold air inlets in the school where he was educated, wants all the openings closed. Only the caretaker should be able to get at these ventilators, and they should be adjusted to the best of his knowledge and experience before the meeting commences. In all cases where mechanical aid is at command, the great point is to ensure that air shall be distributed all over the building near the floor level. Openings 14 inches by 8 inches through which air can come into a hall having a fan in the roof are much too large. Every caretaker who has a fan to superintend should be provided with an instrument which would show the difference between the pressure inside the building and that on the outside, in order that he may not at any time increase the internal pressure too much, and cause intermittent air currents, because too much inlet air space has been afforded by the over liberal opening of the window ventilators.
There is no doubt whatever that air inlets brought through and raised just above the floor of the building are the best. It is folly to object to such on account of supposed dust coming through these floor openings. Buildings kept clean and provided with proper inlets fixed above the floor level are free from dust, and it is only possible to distribute air to an audience in sufficient volume and in comfort in this manner.
The fan referred to was fixed in a turret in the roof, but it is not best in all cases to ventilate by the exhaust method. Where a building is ventilated from below, and the height and area of the mixing chamber are satisfactory, the plenum method, using the fan for forcing air into the building, is sometimes desirable and better suited than an exhaust fan in the roof of a building. In either case, the inlets should be through the floor, of great number and of small area. If the plenum method is used, and the fan is fixed in the basement, it is still necessary to examine the outlet spaces in the roof, as in the case of natural ventilation. If volumes of cold air fall from one end of a structure it may be regarded as certain that a corresponding body of air which has been warmed and sent into the building through the floor is at once expelled upwards and outwards through the roof at the other end before it has done much service in the lungs of the audience. It should not be forgotten that it is not only possible but certain that down draughts and intermittent air currents can be formed even where warmed air is sent by the plenum method through the floor of the building. Whether the plenum or the exhaust method is adopted, no satisfactory result will be obtained until the inlets and outlets are properly adjusted according to the friction and tension incidental to their area and to the part of the building in which they are fixed.
When a building has sufficient inlets through the floor, and full provision made for enough air to enter, the fan will do its work best, and be most efficient for summer ventilation, when it is fixed in the centre of the roof, if the building is high enough, or two fans may be employed if the roof is somewhat low. The reason of this is that the fan will have a vast body of elastic air to pull upon in the building, and there will be a steady and continuous draw upon the air inlets through the floor; and, although the aspirating influence of the wind will affect both the inlet and outlet air at times very appreciably, the large elastic buffer afforded by the inside area of a building will neutralise the tendency for vast volumes of air to be drawn through the floor inlets at the moment of the greatest air pressure outside, and small volumes at a high temperature to come through when the aspirating power of the wind is most exercised upon the inlet air as it comes from the outside.
The chief value of mechanical aid is that the air can be drawn into a building or forced into it under appreciable friction, and therefore through small apertures. Let us suppose there is an unceiled space under a public hall 1o feet high. Into this space, air is forced by a fan to be heated over hot water pipes, and then sent through inlets in the floor into the hall. If there are six inlets only, of area equal to 3 feet each, these will be very sensitive to every swirl of the wind, especially if the inlet gratings in the outside walls are of large area. It is impossible to prevent wind suction upon air inlets feeding a fan, in many cases, simply because the building is so situated that every forward rush of the wind means a greatly reduced pressure around the building. During the time a strong gust is blowing, the inlet air is checked, and where the inlets through the floor above the heating chamber are of considerable area, the aspiration of the wind greatly influences the main body of air inside through these large channels. The atmosphere in the assembly hall of the building becomes attenuated, and its pressure reduced, with the unfortunate result that such a small volume of air passes over the hot water pipes in the basement, that what warmed air does pass through the gratings into the hall is too hot, e audience experiences the effects of the hot room of
a Turkish bath during every strong gust of wind, and a cold air current during a lull in the storm.
The effect of the wind upon the ventilators on the roof may remedy this state of things somewhat, or may even aggravate them according to the position in which the building is situated.
By a judicious use of the fan to force air through many small apertures into a building, and then out through the roof under considerable velocity, the air in the hall or church can be kept from being much influenced by the vagaries of the wind. If the fan is used to exhaust the air from a building, the main point after supplying enough inlet air through the floor is to make certain that no air leaks through the roof to feed the fan, and so render its action inoperative. Where a fan is fixed in the roof, every crack and crevice ought to be stopped. This is not so imperative if the fan is used for plenum ventilation, but the top outlets must be under command, and should not be more than one half the area of the inlets in that case. By such an adjustment of the top outlets it will be possible to use the whole of the natural ventilating power of the building to assist the fan, a power which may be equivalent in cold weather to a velocity of 15 feet per second in the outlets.
For summer ventilation, the fan is most useful, as well as during mild weather in the spring and autumn. By providing extra and larger inlets through the floor in the aisles, etc., for use in the summer in addition to the inlets in each pew, it will be possible to be independent altogether of window openings; and, whilst the fan is working satisfactorily, a far better atmosphere will be maintained with closed than with open windows, because the law is that air2 like all other bodies, will travel along the lines of the least resistance; and if the fan can get its air above the heads of the audience in sufficient quantity because the upper windows are opened, it will do so, leaving the atmosphere in the body of the