It will be seen that the maximum loss of illuminating power in 400 hours was 6.4 per cent., the average loss being 6.0 per cent.
TYPICAL INCANDESCENT BURNERS.--Of the many burners for lighting by the use of incandescent mantles which have been devised, a few of the more widely used types may be briefly referred to. There is no doubt that finality in the design of these burners has not yet been reached, and that improvements in the direction of simplification of construction and in efficiency and durability will continue to be made.
Among the early incandescent burners, one made by the Allgemeine Carbid und Acetylen Gesellschaft of Berlin in 1900 depended on the narrowness of the mixing tube and the proportioning of the gas nipple and air inlets to prevent lighting-back. There was a wider concentric tube round the upper part of the mixing tube, and the lower part of the mantle fitted round this. The mouth of the mixing tube of this 10-litres-per-hour burner was 0.11 inch in diameter, and the external diameter of the middle cylindrical part of the mixing tube was 0.28 inch. There was no gauze diaphragm or stuffing, and firing-back did not occur until the pressure was reduced to about 1.5 inches. The same company later introduced a burner differing in several important particulars from the one just described. The comparatively narrow stem of the mixing tube and the proportions of the gas nipple and air inlets were retained, but the mixing tube was surmounted by a wide chamber or burner head, in which naturally there was a considerable reduction in the rate of flow of the gas. Consequently it was found necessary to introduce a gauze screen into the burner head to prevent firing back. The alterations have resulted in the lighting duty of the burner being considerably improved. Among other burners designed about 1900 may be mentioned the Ackermann, the head of which consisted of a series of tubes from each of which a jet of flame was produced, the Fouché, the Weber, and the Trendel. Subsequently a tubular-headed burner known as the Sirius has been produced for the consumption of acetylene at high pressure (20 inches and upwards).
The more recent burners which have been somewhat extensively used include the "Schimek," made by W. Güntner of Vienna, which is shown in Fig. 19. It consists of a tapering narrow injecting nozzle within a conical chamber C which is open below, and is surmounted by the mixing tube over which telescopes a tube which carries the enlarged burner head G, and the chimney gallery D. There are two diaphragms of gauze in the burner head to prevent firing back, and one in the nozzle portion of the burner. The conical chamber has a perforated base-plate below which is a circular plate B which rotates on a screw cut on the lower part of the nozzle portion A of the burner. This plate serves as a damper to control the amount of air admitted through the base of the conical chamber to the mixing tube. There are six small notches in the lower edge of the conical chamber to prevent the inflow of air being cut of entirely by the damper. The mixing tube in both the 10-litre and the 15-litre burner is about 0.24 inch in internal diameter but the burner head is nearly 0.42 inch in the 10-litre and 0.48 inch in the 15-litre burner. The opening in the head of the burner through which the mixture of gas and air escapes to the flame is 0.15 and 0.17 inch in diameter in these two sizes respectively. The results of some testings made with Schimek burners have been already given.
The "Knappich" burner, made by the firm of Keller and Knappich of Augsburg, somewhat resembles the later pattern of the Allgemeine Carbid und Acetylen Gesellschaft. It has a narrow mixing tube, viz., 0.2 inch in internal diameter, and a wide burner head, viz., 0.63 inch in internal diameter for the 25-litre size. The only gauze diaphragm is in the upper part of the burner head. The opening in the cap of the burner head, at which the gas burns, is 0.22 inch in diameter. The gas nipple extends into a domed chamber at the base of the mixing tube, and the internal air is supplied through four holes in the base-plate of that chamber. No means of regulating the effective area of the air inlet holes are provided.
The "Zenith" burner, made by the firm of Gebrüder Jacob of Zwickau, more closely resembles the Schimek, but the air inlets are in the side of the lower widened portion of the mixing tube, and are more or less closed by means of an outside loose collar which may be screwed up and down on a thread on a collar fixed to the mixing tube. The mixing tube is 0.24 inch, and the burner head 0.475 inch in internal diameter. The opening in the cap of the burner is 0.16 inch in diameter. There is a diaphragm of double gauze in the cap, and this is the only gauze used in the burner.
All the incandescent burners hitherto mentioned ordinarily have the gas nipple made in brass or other metal, which is liable to corrosion, and the orifice to distortion by heat or if it becomes necessary to remove any obstruction from it. The orifice in the nipple is extremely small-- usually less than 0.015 inch--and any slight obstruction or distortion would alter to a serious extent the rate of flow of gas through it, and so affect the working of the burner. In order to overcome this defect, inherent to metal nipples, burners are now constructed for acetylene in which the nipple is of hard incorrodible material. One of these burners has been made on behalf of the Office Central de l'Acétylène of Paris, and is commonly known as the "O.C.A." burner. In it the nipple is of steatite. On the inner mixing tube of this burner is mounted an elongated cone of wire wound spirally, which serves both to ensure proper admixture of the gas and air, and to prevent firing-back. There is no gauze in this burner, and the parts are readily detachable for cleaning when required. Another burner, in which metal is abolished for the nipple, is made by Geo. Bray and Co., Ltd., of Leeds, and is shown in Fig. 20. In this burner the injecting nipple is of porcelain.
ACETYLENE FOR HEATING AND COOKING.--Since the problem of constructing a trustworthy atmospheric burner has been solved, acetylene is not only available for use in incandescent lighting, but it can also be employed for heating or cooking purposes, because all boiling, most warming, and some roasting stoves are simply arrangements for utilising the heat of a non-luminous flame in one particular way. With suitable alterations in the dimensions of the burners, apparatus for consuming coal-gas may be imitated and made fit to burn acetylene; and as a matter of fact several firms are now constructing such appliances, which leave little or nothing to be desired. It may perhaps be well to insist upon the elementary point which is so frequently ignored in practice, viz., that no stove, except perhaps a small portable boiling ring, ought ever to be used in an occupied room unless it is connected with a chimney, free from down- draughts, for the products of combustion to escape into the outer air; and also that no chimney, however tall, can cause an up-draught in all states of the weather unless there is free admission of fresh air into the room at the base of the chimney. Still, at the prices for coal, paraffin oil, and calcium carbide which exist in Great Britain, acetylene is not an economical means of providing artificial heat. If a 0.7 cubic foot luminous acetylene burner gives a light of 27 candles, and if ordinary country coal-gas gives light of 12 to 13 candles in a 5-foot burner, one volume of acetylene is equally valuable with 15 or 16 volumes of coal-gas when both are consumed in self-luminous jets; and if, with the mantle, acetylene develops 99 candles per cubic foot, while coal-gas gives in common practice 15 to 20 candles, one volume of acetylene is equally valuable with 5 to 6-1/2 volumes of coal-gas when both are consumed on the incandescent system; whereas, if the acetylene is burnt in a flat flame, and the coal-gas under the mantle, 1 volume of the former is equally efficient with 2 volumes of coal-gas as an artificial illuminant. This last method of comparison being manifestly unfair, acetylene may be said to be at least five times as efficient per unit of volume as coal-gas for the production of light. But from the table given on a later page it appears that as a source of artificial heat, acetylene is only equal to about 2-3 times its volume of ordinary coal-gas. Nevertheless, the domestic advantages of gas firing are very marked; and when a properly constructed stove is properly installed, the hygienic advantages of gas-firing are alone equally conspicuous--for the disfavor with which gas-firing is regarded by many physicians is due to experience gained with apparatus warming principally by convection [Footnote: Radiant heat is high-temperature heat, like the heat emitted by a mass of red-hot coke; convected heat is low-temperature heat, invisible to the eye. Radiant heat heats objects first, and leaves them to warm the air; convected heat is heat applied directly to air, and leaves the air to warm objects afterwards. On all hygienic grounds radiant heat is better than convected heat, but the latter is more economical. By an absurd and confusing custom, that particular warming apparatus (gas, steam, or hot water) which yields practically no radiant heat, and does all its work by convection, is known to the trade as a "radiator.">[ instead of radiation; or to acquaintance with intrinsically better stoves either not connected to any flues or connected to one deficient in exhausting power. In these circumstances, whenever an installation of acetylene has been laid down for the illumination of a house or district, the merit of convenience may outweigh the defect of extravagance, and the gas may be judiciously employed in a boiling ring, or for warming a bedroom; while, if pecuniary considerations are not paramount, the acetylene may be used for every purpose to which the townsman would apply his cheaper coal-gas.