NATURAL VENTILATION OF
INDUSTRIAL BUILDINGS
THE ventilation of industrial build-
ings using systems of natural ven- tilation is widely accepted throughout the world as a means providing satis- factory working conditions, mainten- ance free and without power costs.
Although forces of wind and tem- perature difference are inherently variable. it is possible by correct de- sign and installation of equipment and reasonable appreciation of at- mospheric conditions at times. when ventilation is most needed, to over- come adverse tendencies often quoted in condemnation.
of
Correctly proportioned systems natural outlet and inlet ventilation can deal with, besides general ventila- tion, problems of heat, steam and fume removal, also drying processes, but none of these applications is like- ly to respond to a haphazard ap- proach.
In all parts of the world, natural ventilation offers an economical means of preventing wide fluctuations of temperature in "thin skin" build- ings due to solar heat.
Natural ventilation results from the effect of the wind and temperature difference in the air inside and out- side a building. The wind acts in two distinct ways, either promoting cross ventilation by blowing in the open- ings on the windward side of building, causing an outflow from openings on the lee side, or it exerts an extract effect when blowing across openings in the wall and roof.
the
The temperature difference results from solar heat, process heat losses and occupancy as well as space heating. Because fresh air enters at outside temperature, the warmer air in the building, being less dense, will rise towards the roof space causing an upward air movement or stack action.
In considering these two effects it will be apparent that maximum air flow will occur when the two forces operale in conjunction and because the wind varies both in direction and strength, it is the prime requirement that outlets are designed so that the wind does not come into opposition
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to the natural outflow, due to tem- perature difference. If to this can be added the feature of the wind pro- ducing a suction in the outlet, so much the better.
It is this necessity for harnessing the effects of the wind which has had a profound influence on the develop- ment of natural exhaust equipment and it is now generally accepted that natural schemes must consist of a system of suitably designed inlets at a low level, coupled with outlets on the roof which have only a positive reaction to the wind from any quar- ter.
The functions of a natural exhaust ventilator are:-
1.
2.
To provide an unrestricted out- let to permit a continuous out- let of warm air due to gravity stack action.
To utilise the kinetic energy of the wind to assist convection regardless of wind direction.
3. To weather the outlet.
Any openings high in the wall or roof would meet the first requirement if it were not for the effects of the wind which can easily overcome the air flow due to stack action, if it blows into the outlet. A wind of 10 m.p.h. corresponds to a velocity head of 0.05" w.g., whilst that produced by the quite favourable conditions of temperature difference of 20°F and an effective height of 50 feet, amounts only to 0.008" w.g.
In considering the second require- ment it should be remembered that the wind produces maximum aspira- tory effect when blowing across an opening, a factor which makes it un- desirable to have extractors which have openings in other than horizontal surfaces. Extract devices with open- ings in inclined or vertical faces are dependent upon shutters to keep out the wind when its direction is unfa- vourable, and few industries would tolerate a ventilator which lets in rain.
To examine some of the better known exhaust units and deduce their likely performance under vary- ing condition:
1. The Jack Roof
This type (Figure 1) which has many variations is accepted by some as a means of achieving ventilation naturally, but that its performance is only indifferent is shown by the frequency with which louvred sides are seen boarded up
It is best as an extractor when the wind blows along its length, when both outlet areas are sub- jected to the suction effect of the wind blowing along the louvres. With a cross wind however, the windward side admits air and although the leeward side extracts, it is fed largely by air from the opposite side, and a strong cross wind often results in draughts reaching floor level and it is typical that snow and rain are driven in through the louvres. Inconsistency of design in the Jack roof arises from its apparent lack of weathering, and Fig. 2
Fig. 1
OUTLET
THECA!
Fig. 2
Fig. 3
THE HONG KONG & FAR EAST BUILDER VOLUME 19, NUMBER 3