This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The different kinds of extract-cowl are very numerous indeed, but they fall into two distinct classes: - (1) Cowls with movable parts; and (2) Cowls without movable parts.
(1) Cowls with movable parts are made both for inlets and outlets. The "lobster-back" rotary cowl, shown in Fig. 588, is an example in point. If arranged so as to expose the open side to the wind, it will act as an inlet, if there reverse as an outlet; consequently its power in either din tion is in proportion to the external force of the wind, provided no other power is exercised from within. Should the latter take place, one may either counterbalance the other, or, what is more probable, the current will vary in proportion as to whether that within or that without is the stronger.
Rotary ventilators, such as those shown in figs. 589 and 590, are often attractive because they give ocular demonstration that air is moving, but without the accompaniment of mechanical force they are most delusive. Fig. 589 is a form at one time employed, fixed in windows or doors, but now rarely seen. The revolving of the blades doubtless has the effect of diffusing the air, but such can be accomplished with better results, as previously indicated, without any movable parts.
Fig . 500.-Howorth's Archime-dean Screw Revolving Ventillator, with part removd to abow the Screw.
When rotary ventilators , such as Fig. 590, are used as outlets, they impede the flow instead of assisting it, unless wind is blowing with sufficient force to revolve the outer louvred cap; consequently the discharge is in varying proportion according to the power of the wind outside, and the cowl would probably be equally effective without the revolving parts. Some people suppose that down-draughts are prevented by this form of ventilator, but much depends upon the direction in which force is exercised, if from within, (say) resulting from an open fire in an apartment unprovided with other sufficient inlet, the tendency will be to suck air in past the screw, when the external air is comparatively calm; and, if wind be blowing, antagonistic forces may be brought into play, one being the suctional force exerted in conseqnence of the fire, which will endeavour to draw air in, the other being the force of wind acting on the louvred cap by which the screw will be revolved and made to draw the air out, the result of which might be to reverse the action in the flue, and cause smokiness within.
Fig . 501 -Donald & Slme 's Extract - cowl.
Fig 592 - Kite's Extract-cowl in Wood Turret.
Fig. 593. - Bedford's Exhaust ventilator.
Fig 594 - Donald & Stme's Extract-cowl.
When turned by mechanical power, however, rotary ventilators may be usefully employed to extract air from apartments io which there are suitable inlets and no open fires.
(2) Cowls without movable parts are preferable, where mechanical power is not available. Speaking broadly, we may include in this category not only those circular, octagonal, or square cowls, upon which winds from every quarter take effect, but also the rectangular dove-cot ventilators with blank ends, and what are known as "concealed roof ventilators ".
The most common form of extract-cowl is the circular cowl, with vertical plates and openings so arranged that the wind, passing over and between the plates, sucks air out through the openings, and so through the central shaft, with which these are connected. The arrangement of the several parts by different makers differs considerably, but the general principle remains the same. Fig. 591 illustrates a good cowl of this sort, and Fig. 592 a somewhat different one, designed for inserting into a wood turret, as shown.
Cowls with the air-slits arranged horizontally instead of vertically, are also successful. Fig. 593 shows a good example, which is fitted with condensation-channel* inside and outlet-holes, and with internal valves for opening and closing. A simpler form is shown in Fig. 594, and another in Fig. 595, the Utter having a rather curious appearance.
Fig. 596 shows a cowl, in which there are both horizontal and vertical openings, but experiment alone can indicate whether this modification increases the extracting power. See also Fig. 587.
Fig 505 - Renton Gibbs's Extract-cowl.
Fig. 596. - Walker's Extract-cowl.
A different arrangement is shown in Fig. 597; in this, the wind passing horizontally through the upper part draws the air up through the holes in the perforated plate, while back-draught is prevented by the light mica disc, which is hinged on one side In another cowl, made by the same firm, the mica disc is arranged to move up and down a central rod, and this arrangement appears preferable. In either ease, the weight of the disc must be overcome before extraction can take place.
The dove-cot type of extract-ventilator is shown in Fig. 598, and a modified form in Fig. 599. These are not as universally successful as the cowls previously described, but in some situations their appearance may be preferred The concealed roof ventilator" was designed to meet the wishes of those to whom all projections of ventilators above the roof are eyesores. A good ventilator of this kind, having outlets for moisture, is shown in Fig. 600. These ventilators are not as efficient as good cowls, but they have their uses Extract-venticulor they must be placed in pairs, the wind blowing right through both ventilators "Water-spray ventilators" have been used with considerable success. In.
Flue-outlets, as shown in Fig. 601, may assist as outlets when there are no fires lighted, but when the fires are used they will almost invariably become inlets, and consequently at such times should be closed. For double chimney-stacks these ventilators the force exercised by a spray of water within a suitably-constructed tube is employed for inducing a current of air either into or from an apartment Where water at sufficient pressure is available, these ventilators may be usefully employed. Their application, however, must necessarily be very limited. because of the cost involved in many localities, compared with the result effected, which in most instances can be secured l»y far simpler means. In cold weather than is also the liability to freezing of the water. Fig. 602 illustrates an extract-ventilator of this kind, in which the water-spray from the nozzle a draws the air vertically downwards. Other appliances are made in which the Spray acts vertically upwards, and others again in which it acts horizontally.
Fig. 597 - Sugg's Extract-cowl.
Fig 598.-Kite's Extract ventilator of the Dove-cot Type.
Fig 599 -Baird. Thompson. & Co's
Fig. 600. - Donald and Stme's Concealed Roof Ventilator
Fig. 601. kite's Chimeny attack Ventilator.
Fig. 602. - Section of Kite's Water-spray Vertical Exhaust, driving downwards.