ference in degrees fahrenheit, V = wind velocity in miles per hour.
Control over the exhaust can
be given by a centrally pivoted damper fitted in the throat.
The manufacturers make a feature of its construction in a protected metal throughout, which is highly. resistant to any corrosive atmosphere, emphasising that ventilators are usually subjected to more corrosive attack than any other part of a struc- ture. Because the maintenance of roof ventilators is notoriously diffi- cult, the choice of a suitable corro- sive resistant material is certainly an important consideration.
Continuous Ridge Ventilators
Perhaps the most prominent de- velopment in natural ventilation is the introduction of a type of extract ventilator termed the Continuous Ridge Ventilator (Fig. 6). As its name implies it is mounted on the ridge of a building where it will give a guaranteed exhaust whatever wind conditions prevail. It is available in a range of throat sizes from six to 18 inches. The larger sizes 36 to 120 inches are necessary to deal with high heat loss problems, but for the exceptional instances where this type of ridge ventilator is unable to provide the exhaust open- ing necessary to remove excessive heat an individually designed monitor ventilator is the only satisfactory solution. Available with throat open- ings between 10 ft. and 20 ft. wide, it is intended for use in solving the steel, glass and similar industries, and is widely used in such hot spots all over the world.
The Basis for Design of Natural Schemes of Ventilation
A natural scheme of general ven- tilation will be judged by its effi- ciency on those days when ventilation is most needed, days when shade temperatures are relatively high and wind speed is low. It follows then that figures for temperature rise and wind speed should be conservative and a reasonable basis now well established in practice is that of 20°F and 4 m.p.h. Higher wind speeds may be used for buildings in the ex- posed positions, whilst a reduction to two m.p.h. may be advisable for buildings in sheltered positions.
Temperature rise should always be considered in relation to summer conditions although an increase to 30°F is often justifiable where the process gives off great quantities of heat.
Natural Ventilation in the Tropics
Thin walled buildings are widely used in the tropics for industrial and domestic dwellings and in many of these, using solar heat as the motive power, air temperature at occupation level is kept near outside air tem- perature.
A most interesting application is one illustrated in Fig. 7, relating mainly to office and domestic dwellings, where the ceiling shown is an essential barrier to radiation from the roof covering.
The roof space is sealed except for extract ventilators at the ridge and openings in the ceiling having a total area in excess of the throat areas of
the ventilators, fresh air being emit- ted only at occupation level.
This arrangement results in air being drawn continuously from out- side to replace heated air from the roof space which rises through the ventilators.
In Calcutta in a building of this type, before the installation of a ceiling, air temperatures at working level often rose 15°F above the already high shade temperature not uncommon in that part of the world. The installation of a ceiling improved matters, but not enough to make conditions bearable for the occupants. After fitting natural roof ventilators calculated to provide an air flow of about 15 air changes per hour, aver- age inside temperatures recorded during a period April to June were within I°F of outside shade.
Typical Installation Errors
Unmerited condemnation of na- tural extractors having its origin in incorrect installation is not unknown and usually arises from a failure to appreciate the natural laws govern- ing their performance.
Listed are some of the reasons for unsatisfactory results based as nearly as possible in the order of prominence as suggested by experience:
1. Too small an outlet area. 2. Insufficient area and poor distri-
bution of fresh-air inlet.
3. The use of outlets which, due to poor design, are uncertain in per- formance
4. Incorrect positioning of otherwise
cfficient extractors.
LU PAN-PATRON SAINT OF CHINESE BUILDERS (From page 77.)
One more humorous legend has it that Lu Pan built himself a large wooden "aeroplane" on which he flew from place to place. He was on a job 1500 li away from his home and as he had been absent for a long time, his parents noticed with con- sternation that his wife was pregnant. Peace came to the household when Lu Pan produced his aeroplane to show that he had been visiting her regularly.
Another relates how Lu Pan once borrowed the palace of the Dragon King of the Eastern Seas to serve as a model he was erecting and nearly got into trouble for keeping it longer than he promised.
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Lu Pan was also said to have in- vented some of the basic tools of carpentry such as the saw and the plane.
He is also alleged to be responsible for the custom of putting gilt studs on gate panels as an emblem of security because
because they represented rock shellfish which shut themselves in tightly at a touch.
Lu Pan's temple in Kennedy Town is comparatively new, having been built in 1928 to replace an eighty year old shrine. It is essentially Taoist in style with the roof ridge decorated with the two dragons and
the lustrous pearl, while the facade over the door is filled in with frescoes illustrating famous poems. Above the lintel of the door is the dedicatory tablet, Lu Pan Hsien Shih Miao "the Temple of Lu Pan the Funda- mental Teacher." The door posts are inscribed: "Lu's architectural precepts hold good forever" and "His method of measuring will last a thousand years."
The pillars supporting the roof are inscribed: "He was so outstanding an architect as to qualify for canoni- sation" and "His tools and methods of construction were so brilliant that they can be used to cultivate peace."
THE HONG KONG & FAR EAST BUILDER-VOLUME 19, NUMBER 3