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THE ACTION OF WIND UPON VENTILATION.
The importance of wind action in causing the movement of air has been long recognised and appreciated, but the value of the aspirating powers of the wind in inducing ventilation in public buildings is much overrated. This overrating is due to a variety of causes.
(1) As to the average velocity of the wind. Where observations of wind pressure are taken, it is usual to fix the anemometer in the most open space where the wind has free play, and also to set the revolving vanes as high as possible. Observations of this kind are comparative, and of value for making tables, but they throw no light whatever upon the manner in which the wind acts at the top of a building surrounded by other houses, or the velocity with which wind travels over such a building. Recent experiments with kites fitted with recording instruments, have shown that the velocity of the wind is generally twice as great at the altitude of 1,ooo feet as it is in the open country near the ground level. Again, the friction caused by the wind passing over buildings is so great that it is scarcely possible to demonstrate it accurately. Those who stand in an open square or park near houses over which the wind blows, will soon learn how greatly the velocity of the wind is accelerated the moment it leaves the roofs of the houses.
If the average velocity of the wind at the recording station
is eight miles per hour, it will be safe to assume that in London and large towns it does not exceed four miles per hour as it sweeps over the roofs and housetops. Furthermore, it must be noted that the average velocity of the wind is largely increased by the equinoctial gales, and these winds are usually very choppy and gusty in the extreme. Nothing tends so much to interfere with steady ventilation and to overturn the mechanical and other arrangements of the inlets and outlets as do these winds. After, therefore, deducting the pressure due to winds which are injurious, the average aspirating power during the periods of the year when it would be most serviceable is a small and very uncertain quantity.
(2) The second reason why the value of wind aspiration by the aid of pneumatic cowls and ventilators has been overrated is because air is one vast ocean, and, in all cases of ventilation, no matter how small or how large the building is, the air is one continuous body or mass. The connection between the air in a building may be so minute that it is just the chinks around the doors and windows, and if the atmosphere in a building is compared to a promontory, then it is united to the mainland by the narrowest isthmus, but there is such an isthmus, and all the physical effects which occur as the result of wind action upon the main body of the air outside are more or less communicated to the air inside the building. The larger the openings—the inlets and outlets—the more the wind effects will be felt inside the building, but the chief point to notice is that the aspiration of the wind may be, an'' often is, greater upon the inlets near the floor level than it upon the ventilators above the roof. In the case of a ch' upon a hillside with buildings above it on the side c hill immediately facing the wind, the wind effects mouth of the ventilators on the roof will be reversed.: 1-of aspiration there will be increased pressure, which forie air outside down into the building. The e '-ch from which, in this case, air is taken to suf
inside, adjoins a street crossing the brow of the hill, but there is also a free space adjoining the church itself. The wind sweeping along this street aspirates powerfully on the inlets at the ground floor level, the result being that if the ventilators on the roof are open they act as inlets, sending volumes of air into the building at every gust of wind, causing such violent down draughts that the ventilators on the roof must be kept closed. In this case, and there are plenty of churches so situated, the wind effects are baneful, and the so-called pneumatic, or air-pump ventilator, is a self-acting down draught producer.
(3) The third reason why the value of the aspirating powers of the wind in inducing ventilation has been so greatly overrated is because the physical laws bearing upon the atmosphere are not sufficiently understood. The law which governs moving bodies not only applies to wind, but the effects are greatly complicated by the elastic properties which air possesses.1 The wind blowing over the tops of houses across a street, not only draws air out of the street, but reduces the pressure upon the air in the street by expanding it with a pull equivalent to the force which gives rise to the velocity of the wind. The air in the street (which must have no outlet in this case) may be compared to the fixed end of an elastic thread, and the effects of the wind blowing over the buildings across the street, to a person's fingers pulling at the free end. As long as the velocity of the wind continues, the air in the street is expanded and drawn out like the elastic. When a gust of wind ceases, the expanding effects cease also, and the air contracts to its original bulk like the elastic does when it is released.
It is this elasticity, this sudden expansion and contraction, which requires to be understood. It is either unknown to the ventilating engineer, or else it is ignored by him, but, all the
'The substance of what follows was communicated to the British Association at the Bradford Meeting in 19oo.
same, it is the most potent factor which increases the difficulty of ventilating buildings, and which renders the aspiration of the wind not only valueless under existing conditions, but absolutely unmanageable. It is no secret to the architect, the ventilating engineer, or even to the public, that what is wanted for churches and halls of assembly is fresh warmed air introduced at the floor level, and that buildings are most sadly deficient in this particular. It is equally well known that cold air inlets cannot be endured when the temperature is low, the consequence being that there is much less pressure inside the building than there is outside. Let it be assumed that the building having the self-acting pneumatic ventilators on its roof is situated in the best position possible for the wind to act upon them—the hall is upon the top of a hillock, the roof is much higher than those adjoining, and the wind sweeps over the open country with great aspirating force. The consequence is that the air inside the hall is drawn out— expanded, like the india-rubber thread mentioned above, and, by reason of the large outlet space on the top, and the small inlet space at the bottom, the air in the building is greatly expanded during the time a gust of wind is blowing; but the moment any lull occurs in the pressure of the wind, or the gust ceases, the air in the hall, like the elastic thread referred to, contracts, and as very little air gets in at the floor level, and that little under much friction, the air in contracting draws a supply, to fill the partial vacuum, along the lines of the least resistance, which in this case is down the ventilator. It will be seen, therefore, that if a building is situated in the best position for ventilators which are designed to aspirate by wind action, and they do aspirate, the effects are as inimical to good ventilation as it is possible to be. The effects to which I have referred were proved by experiments and demonstrated with a large model, and there is no doubt that the ingenuity of the ventilating engineer should be directed to shielding the mouth of a ventilator from wind
action, rather than trying to devise methods to utilise the action of the wind so as to aid the ventilation of churches and public buildings.
It is claimed, sometimes, that certain roof ventilators have a peculiar property of inducing air currents through them, as it can be shown by an anemometer, or any delicate means which demonstrates moving air, that even in weather so calm that no wind can be detected, there is air travelling through the tubes of such ventilators. There is no reason to deny this statement that air ascends ventilators when no wind is blowing, but it should be understood that such will be the case with all ventilators, however little or great their pretensions, whilst the open tube will still be found the most effectual in this particular. It is easy to explain why a current of air generally, fitfully or intermittently, ascends the ventilators in empty buildings when no audience is present. In winter the building is warmed at intervals and the walls retain their heat for some time, so inducing a current to circulate. The inside of the building is sheltered, and warmed by other buildings somewhat. The building whether in a hollow, on level ground, or on a hill top, is influenced by the flowing wind, more especially as the roof outlets greatly exceed the ground inlets in area. In summer the roof is heated by the sun, and a current of air is caused to ascend the ventilators in consequence.
Ventilators, which are called self-acting, do not cause these induced currents—it is the physical effects due to heat and the general aspiration of the wind, which form them—and they are formed whatever kind of ventilator or open tube is used upon the roof
When a current of air is blown right across the outlet plates of the models of pneumatic, self-acting, or pumping ventilators, as is usually done by those who sell them, they exhibit considerable aspirating powers, but the pressure of the breath employed represents the blowing of a violent gale.