illustrates how in an endeavour to overcome this failing by using a raised ridge sheet, the effective outlet is often reduced to a fraction of the throat width.
2. Louvred Openings
These are a common form of haphazard ventilation (Fig. 3) and suffer from similar disadvant- ages to the Jack roof. Maximum exhaust occurs when the wind
blows parallel to the roof face in which they are fitted, but as the wind direction changes to blow into them, air flow reverses and with the wind at 90° to the roof face, louvred openings are more efficient inlets than they are ex- tractors under the first position. Whilst such a performance may go unobserved in a sparsely occupied stores building, it can be imagined that where a back draught results in heat and fumes being forced down to working level, conditions may well give rise to a complaint, and the mere of the louvres, should they be of the operating type, will scarcely overcome the trouble, for ventilation will cease.
For people working in a very hot condition, a clear view of the sky through the roof may be psychologi- cally beneficial. If the sky is visible the building feels cool and louvred ventilators covering a large area of a roof can be opened mechanically or pneumatically at the touch of a switch. However exhaust through this type of ventilator cannot be guaranteed.
Louvred openings are also used as fire ventilators, being fitted with fusible links and remaining with the blades in the closed position unless there is a fire below the louvre, when the fusible link melts and the blades automatically open to allow the heat and the smoke to escape from the building, so localising the fire.
Slope Mounted Ventilators
A unit type of roof slope ventilator designed with an eye on economy of installation to take the place of a roof sheet or a pane of patent glazing is shown in Fig. 4. This provides
a weatherhole in the roof, but its performance is variable, dependent upon the angle of roof slope in which it is mounted. In steep slopes (north- light blazing) facing the wind, it is liable to act as an inlet, because a pressure is built up, by the wind over the roof surface.
It may be argued that this dis- advantage can be offset by mounting ventilators in both slopes, only half
on the windward side, but this overcomes the reduction of exhaust due to cooler air entering at a high level and means that wind strength and direction exercise an undue in- fluence on the overall exhaust.
Fig.
Fig. 7
THE HONG KONG & FAR EAST BUILDER-VOLUME 19, NUMBER
A
B.
A ventilator of this type however will provide nominal air movement within a building where there is no heat problem.
Ridge Mounted Extractors
By mounting a natural ventilator at the ridge, greatest advantage is taken of effective height and tempera- ture difference and the wind blows directly on it from any direction.
An open circular pipe extending two feet or more above the apex of the roof is a good extractor and several of the better known ventilators of this type consist basically of a stack pipe with a superstructure which in some cases is designed merely to keep out the weather and in others with the twofold object of weathering and increasing wind effect. In some, the weathering louvres permit the wind to blow into the ventilator or are so spaced as to restrict the natural out- flow.
exhaust Many which
satis- factorily when mounted on a flat roof are caused to back draught by the wind sweeping up a sloping roof.
However, from a ridge mounted unit by correct design, the highest exhaust and most consistent perfor- mance of any type of natural extrac- tor can be achieved, mainly because of the all round positive response to wind action. Thus the varying fac- tors of wind direction, the main cause of uncertain performance of other types of natural ventilators. is re- moved.
Desirable characteristics may be listed as:-
1. Generally cylindrical in shape, mounted with a vertical axis, so presenting the same aspect to a wind from any direction.
2. Ample outlet area with main
openings horizontal.
3. Prevention of wind blowing in- to the stack outlet, even when deflected upwards by an inclined roof slope.
A natural extractor, fulfilling these requirements is shown in Fig. 5. It is cylindrical, its suction band “A” preventing a horizontal wind from blowing into the stack and wind baffle "B" preventing the entry similarly of upwardly inclined currents.
If performance is guaranteed and the published capacity tables follow closely the formula: Q = A (10.8V H x T + 30.8V) where Q = volume extracted in c.f.m., A throat area in square feet, Heffective height of ventilator above fresh air inlet in feet, T = mean temperature dif-
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