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more or less effectual. Pictures, skirting-boards, cornices, mirrors may all be utilised, both to conceal openings in walls and to give the air a proper direction.

Arnott's Valve is a valve which is used as an exit ventilator. It consists of a light metal flap, swinging inside a metal framework in such a way that it only opens towards the chimney flue; back pressure from the flue causes it instantly to shut.

Boyle's Exit Ventilator is on the same principle, but instead of metal there are a number of light talc flaps.

Openings in Doors.-These are not much made use of; but sometimes a panel is made to open, either pivoted or otherwise. Currall's ventilator for doors is precisely the same as the one for windows just described.

(44) Special Shafts or Tubes.

Tobin's Tubes.-One of the best known and most efficient forms of ventilation is by means of what is called a Tobin's tube. It is a right-angled tube, the one limb leading in a horizontal direction into the outer air; the other, vertical, being in the room. The vertical pipe should not much exceed six feet. Some Tobin's tubes are provided with valves, in order to throw them out of use when not required. The air streams in, and is directed up towards the ceiling, and then falls like water from a fountain, in fine streams, becoming partly warmed during the descent. These tubes may be placed in the corners of the room. They are susceptible of ornamentation, and are nearly always efficient. If a tray of water, or a piece of canvas kept constantly moist, be placed in the horizontal limb, and the air deflected upon the wet surface by metal plates, most of the dust and soot, which otherwise in smoky towns effects an entrance, is deposited.

With the exception of Tobin's tubes, most of the shafts or tubes act in extracting and not in delivering air.

MacKinnel's tube does however combine the two systems and acts both as a delivery and an exit tube. It consists of two cylinders, one inside the other, passing from the ceiling into the outer air. The inner tube is longer than the outer one and projects above it outside, while at its lower end it also projects into the room and carries a circular rim, the rim being parallel

with the ceiling. A gas burner may be placed in the inner tube to cause a greater extraction; the vitiated air passes up the inner tube and escapes, the colder external air flows down through the annular space between the outer and inner tube, but is prevented from spraying like a douche on the heads of the inmates by the circular rim which compels the air to pass downwards in curved streams, this system has answered well in the case of square or round one storied buildings such as stables.

(45) Cowls and Extraction Shafts Generally.

A chimney is of course an excellent extraction shaft when a fire is burning; the amount of air which passes up is enormous, and if the mean temperature of the shaft is known can be calculated from the formula given at page 69. The efficacy of cowls has probably been overrated. In the numerous experiments carried out by the cowl committee, consisting of Mr. Rogers Field, Mr. Peggs, and Mr. Eassie, the result was, speaking generally, that the much advertised cowls had little advantage over a simple tube or shaft. The motive power in the absence of wind is the difference of temperature between the external and internal air, but when wind is present the wind passing over the top of a shaft will aspirate the air and cause movement independent of temperature.

Cowls are under certain circumstances advantageous, for instance, the common way of ventilating ships and steamers is by means of a cowl turned towards the wind, the air is in this way forced into the lower and enclosed parts of the vessel, a windsail again acts in the same way and may be considered a canvas cowl. Cowls whether aspirating or the reverse, fixed or revolving, owe any efficacy they possess to wind, and they are of course of no more use than ordinary tubes in foggy still conditions of the atmosphere just when ventilation is most urgently needed.

If more than one extraction tube is applied for the purpose of ventilating a room, the principle already enunciated that the separate shafts must be of equal height should be borne in mind.

CHAPTER VI.

SYSTEMS OF VENTILATION COMBINED WITH WARMING OR
DEPENDENT ON MECHANICAL AGENCIES.

(46) Gas as an Aspirating Agent.

GAS and lights generally may be made to purify the air of a room, instead of contaminating it, by placing some simple tube arrangement with a trumpet mouth over each light, and leading the tube out into the open air. The aspirating force of gas may be utilized for the purpose of ventilating small spaces placed in difficult positions. For example, a water closet in the centre of a building may be kept fairly sweet by carrying a tube direct from the outer air in a horizontal position to the floor of the closet, and a shaft led vertically upwards to the roof, and placing in the shaft a tiny jet of gas; in this way the direction of the air is controlled, and the shaft at all times will be warmer than the external air. Special lights for rooms, such as the Wenham, have, when properly fitted up, arrangements by which the products of combustion are carried away by a flue, while the gas itself is supplied with the upper layer of air near the ceiling already somewhat impure.

(47) Pritchett's Miniature Hot Water Apparatus.

This is a combined system, by means of which a room is at once warmed and ventilated. It consists of a small boiler, which can be heated by a spirit lamp, or placed on a fire. Attached to the boiler are two cylinders, each made double; the water circulates in the double casing of the cylinders, the effect being to make two shafts, one for the admission of air, the other for its extraction. The cylinders are placed vertically, the outer cylinders being the

hottest and used for the extraction of foul air, the inner cylinders are cooler, but are yet sufficiently hot to warm the entering air.

(48) Ventilation Depending on Water Power.

There are several systems which owe their efficacy to the mechanical force of water, for instance, the Eolus water spray ventilator consists essentially of a fly-wheel fitted with fans. A jet of water directed against this causes it to revolve. Such a system can of course be made either to propel air in or to extract air from a room. This simple form of mechanical ventilation has much to recommend it. By regulating the water supply the rate at which the fans revolve can be adjusted to a nicety.

(49) The Ventilation of Underground Passages such as Mines. This section of the subject may be illustrated by an abstract of Mr. Morrison's short account of the ventilation of underground passages, such as mines and tunnels by mechanical means.

The Furnace. The simplest plan is the furnace, which at one time was the only system employed, and it is still used in many collieries. There is a good deal to be said in favour of its simplicity, but it is only in the case of very great depths that this system can compete for economy with mechanical methods, and even then there are various drawbacks. The useful effect rarely exceeds 5 per cent. of the actual energy given out by the coals, and although, at a colliery, this is often, but wrongly, considered a matter of minor importance, in tunnel ventilation it is of the first consequence.

Air Pumps.-One of the first mechanical ventilators was invented by Mr. Struvé, of Swansea. It consists of a piston, somewhat resembling a gasometer, working in a brick chamber, in an annular space filled with water. The air is admitted by, and expelled through, flap-valves. It is well suited for extracting large quantities of air from collieries at pressures of 5 inches or 6 inches of water; but the large amount of clearance renders it unsuitable where the pressure is sufficient to cause a practical difference in the density of the air. The effective duty is stated

1 Vide Inst. Mech. Eng. Proceedings, 1869, p. 133; and Trans. Inst. of Eng. in Scotland, vol. xiv. p. 72.

* Vide Minutes of Proceedings Inst. C.E., vol. x. p. 38.

to vary from 40 per cent. of the actual power put into the pump to 40 per cent. of the gross boiler power.

There is a pair of ventilators of this description at Cwm Avon,1 with pistons 18 feet in diameter and of 7 feet stroke, working 8 strokes per minute.. This machine exhausts from 40,000 to 56,000 cubic feet per minute, with a water gauge of 3 inches. A slightly smaller ventilator at Risca Colliery1 exhausts 43,800 cubic feet per minute, with a water gauge of 23 inches.

Exhausters on a somewhat similar principle have been erected at the St. Gothard Tunnel works.2 These are cylinders hung one at each end of a rocking beam, which alternately dip into annular tanks of water. The space to be ventilated is connected with these air-cylinders by pipes with inlet valves, and the tops of the air-cylinders are furnished with outlet valves. Each time the cylinder rises it fills with air from the pit or tunnel, which in falling it expels through the valves on the top. These exhausters are therefore single-action pumps, while Struvé's are double action. They are intended to work with a water gauge of 6 inches, and their general design renders them suitable for much higher water gauges than Struvé's.

Lemielle's Ventilator.-Lemielle's ventilator consists of a vertical drum, with movable leaves or vanes, placed eccentrically in a casing, so that the leaves lie close against the drum on one side of the casing, but expand as they pass the other, and thus sweep out a certain amount of air at each revolution. There is a ventilator of this description at Page Bank Colliery, Durham, 23 feet in diameter and 32 feet high. It usually works up to about 60,000 cubic feet per minute with a 26 inch water gauge, and but occasionally to 97,000 cubic feet per minute with a 66 inch water gauge. The useful effect is 36 per cent. of the gross boiler

power.

The Steam Jet.-In some collieries the steam jet has been tried with success, as at Lower Moor Colliery,5 Oldham.

The

1 Vide South Wales Inst. of Engineers, vols. v. and vi., passim. Paper by C. Cope Pearce.

2 Engineering, 7th May, 1875.

3 Vide North of England Institute of Mining Eng. Trans., vols. xviii. p. 63 et seq., and xix., p. 2. Steavenson on Mechanical Ventilation. Inst. Mech. Eng.

Proceedings, 1858, p. 63.

4 Vide Inst. Mech. Eng. Proceedings, 1869, p. 147.

5 Engineer, 19th Jan., 1872.

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