CHAPTER V.

REGENERATIVE BURNERS.

[Sidenote: Temperature of a gas flame.]

As was remarked in the introduction to this treatise, recent years have witnessed a very considerable advance in the construction of gas-burners, and in the amount of light capable of being developed from each cubic foot of gas consumed. Undoubtedly the most noticeable feature of this advance is the successful application of the regenerative, or, as it would be more appropriately designated, recuperative system. Briefly stated, this consists in utilizing the heat of the products of combustion from the gas flame (which otherwise would be dissipated into the atmosphere) to raise the temperature of the gas before it is ignited; and, likewise, of the air necessary for combustion. The temperature of an illuminating gas flame is usually estimated to be between 2000° and 2400° Fahr.; and as the products of combustion must leave the flame at a temperature little, if at all, inferior to the former figure, it must be evident that there is an ample margin of heat for employment in this direction. A considerable proportion of the large amount of heat conveyed by those products of combustion which, under ordinary circumstances, is imparted to the surrounding atmosphere--often elevating its temperature to an unnecessary and prejudicial extent--is, by this method, returned to the flame; intensifying the process of combustion, and augmenting, in a remarkable degree, the illuminating power developed from the gas consumed. Thus the ultimate effect of the operation is to produce a concentration of heat in the flame, and the conversion of superfluous heat into beneficial light. Within a comparatively recent period, the utility of this process was strongly disputed; and it was stoutly maintained, by many persons, that as the immediate effect of ignition was to cause a temperature of more than 2000° Fahr. to be attained, the heating of the gas and air prior to their combustion could produce little or no beneficial effect upon the illuminating power of the flame. However, the falsity of this view of the case is conclusively demonstrated by practical experiment; the remarkably high results yielded by burners that have been constructed upon the regenerative system sufficiently attesting the correctness of the principles upon which they are founded.

Although, in general, both the gas and air supplies are heated, it is chiefly due to the latter that the beneficial effect noticed is produced; and this for two reasons. First, because the quantity of air is so much greater than the gas it is required to consume; being, at the nearest approach to theoretical perfection, fully six times its volume. Second, because four-fifths in volume of the air consists of inert nitrogen, which does not contribute anything to the heat of the flame, but, when applied in its normal, cold condition, abstracts no inconsiderable proportion of heat from it. Yet the heating of the gas itself is not without very appreciable influence. In an ordinary gas flame there is always an area of non-illumination around, and extending to a variable distance from the burner head. This is caused partly by the conduction of heat from the flame by the burner; but, in a greater degree, by the cooling action of the issuing stream of cold gas, as is shown by its extending farther from the burner in proportion to the pressure or velocity with which the gas issues. The prejudicial effect due to the former is obviated to a great extent by constructing the burner of steatite, or other non-conducting material. To remedy the latter, nothing will avail but the heating of the gas supply.

[Sidenote: Effects of heating the gas and air.]

The effect of heating the gas is to enlarge the area of the illuminating portion of the flame, and, in a minor degree, to enhance the intensity of incandescence to which the carbonaceous particles are raised. When the gas issues from the burner at a temperature little inferior to the temperature of ignition, the hydrocarbons it contains are immediately decomposed; the liberated particles of carbon are raised to the temperature of incandescence; and the illuminating area of the flame is extended downwards, even to the surface of the burner. The heating of the air operates chiefly to produce and maintain a more elevated temperature of the flame; and, in this manner, contributes to the development of a higher illuminating power from the same area of flame. In the case of ordinary gas flames, the cold atmosphere by which they are surrounded, by abstracting heat from the flame, prevents the most favourable conditions for the development of light from being attained. When, however, the air immediately surrounding the flame has been previously heated, the particles of carbon (the incandescence of which furnishes the desired illuminating power) attain to a much more exalted temperature; and, consequently, give out a greater degree of light.

But there is yet another direction in which the prior heating of the air supply contributes to the development of improved illuminating power. Being heated, its density is lowered; so that in any given volume of air there is less weight of oxygen than when cold. The consequence is that as less oxygen is presented to a given surface area of flame, the separated particles of carbon remain for a longer period of time in the incandescent condition before being entirely consumed. Thus there are three distinct results produced by heating the gas and air before combustion--namely, first, the particles of carbon are liberated earlier in the flame; second, they are raised to a more exalted temperature; and, third, they remain for a longer time in the incandescent condition. The combined effect of all three is the improved illuminating power developed from the gas consumed.

[Sidenote: Bowditch's regenerative burner.]

So far back as the year 1854, the principle of heating the air supply to an Argand burner, by means of waste heat from the flame, was partially applied, with some success, by the Rev. W. R. Bowditch, M.A., of Wakefield. Mr. Bowditch's burner, which is shown in the accompanying diagram, contained, in addition to the ordinary chimney, an outer glass chimney, which extended for some distance below the inner one, and was closed at the bottom; so that all the air needed to support the combustion of the gas was required to pass down the annular space between the chimneys, and in its passage became intensely heated by contact with the hot surface of the inner chimney, as well as by radiation from the flame itself. This burner contained many defects. Amongst others, the inner chimney could not long withstand the intense heat to which it was subjected, and, in consequence, had to be frequently renewed; the heating of the air was not effected solely by the products of combustion, but, perhaps in a greater degree, by the abstraction of heat from the flame itself; while, at best, this heating was but partial. Yet, these defects notwithstanding, the burner showed very clearly the beneficial results attending even a partial application of the principle; as, in the illuminating power it developed from the gas consumed, a clear gain of 67 per cent. over the ordinary Argand burner was obtained. Although the drawbacks connected with the construction of Mr. Bowditch's burner prevented its ever receiving general, or even extensive adoption, its simplicity has gained for it the distinction of being freely copied by so-called inventors of a later day.

[Sidenote: Invention of the Siemens regenerative burner.]

It was left to Herr Friedrich Siemens, of Dresden, to produce a burner which, while applying the principle of regenerative heating in the most scientific and complete manner, should also be adapted to the ordinary conditions of gas lighting. After much experimenting on the subject, a burner embodying the essential features of the regenerative system was invented by this gentleman in 1879; and so great was the advance which its performances manifested over anything previously attained, so wide the prospect of further achievements which was opened out, that it may fairly be said to have inaugurated a new era in gas illumination. In this burner the products of combustion were made to give up a considerable portion of their heat to the gas and air, as the latter passed to the point of ignition; the flame itself not being called upon to contribute in any degree to this result. Although, as was but natural, the first attempts towards the construction of such a burner were very crude, and but partially successful in their results, the inventor persevered in his endeavours to work out his ideas into practical and thoroughly satisfactory shape. It was not until after it had gone through many modifications that the burner acquired the peculiar form which now distinguishes it, and attained to its present stage of perfection. Before proceeding to describe an example of the burner as now constructed, it is necessary to state that the principles embodied in Herr Siemens's invention are equally well adapted--and, indeed, are applied with equal success--to the construction of flat-flame and Argand burners; but as the distinctive features of the invention are common to both classes of burners, it will be quite sufficient to describe in detail one of the latter type.

A prominent feature in the appearance of the Siemens burner, as will be seen from the annexed illustration, is a large metal chimney, for creating a draught to carry away the products of combustion. The entrance to this chimney is situated a little above the apex of the flame; but there is a branch flue connecting the main chimney with the interior of the burner. The body of the burner is of metal, and its interior is divided into three concentric chambers. Of these, the innermost is open at the top, and is surmounted by a porcelain cylinder, which, when the gas is lighted, is surrounded by the flame. This chamber is closed at the bottom, but communicates at the side with the before-mentioned branch tube, or flue, leading to the main chimney. The intermediate chamber communicates, at its lower extremity, with the gas supply; and terminates, a short distance from the top of the burner, in a number of small metal tubes, which convey the gas to the point of ignition. The outer chamber is open both at top and bottom, and is for conveying air to support the combustion of the gas. In order to promote greater intensity of combustion, there is a notched deflector at the summit of the latter chamber, and another on the lower part of the porcelain cylinder, which cause the air to impinge more directly upon both sides of the flame. There is also an arrangement for introducing air between the outer casing of the air chamber and the glass chimney which encloses the flame; its object being to keep the chimney cool.

[Sidenote: Action of the Siemens burner.]

The action of the burner is as follows:--When the gas is ignited at the ring of tubes, the heated air and products of combustion, which rise from the flame, create a draught in the main chimney. Through the communication established by means of the lateral flue, a partial vacuum, or area of low pressure, is induced in the innermost chamber of the burner, and within the porcelain cylinder which surmounts it. As the flame terminates close to the mouth of the latter, the greater portion of the products of combustion, instead of going into the main chimney, are sucked into the porcelain cylinder; and thus a current is set up through the interior of the burner, and by the lateral flue, to the main chimney. The heat carried away by the products of combustion is communicated, through the walls of the chambers, to the entering gas and air; and by this means the latter are heated to a very high temperature before they issue from the burner and are consumed. The consequence is that a much greater intensity of combustion is maintained; the carbon particles are separated earlier in the flame, and are raised to a more exalted temperature; and the ultimate effect is a higher yield in illuminating power per cubic foot of gas consumed. Independent tests by various experienced photometrists have conclusively shown that a light equivalent to that from 5 to 6 candles is obtained per cubic foot, from gas which, in the standard "London" Argand, yields a light of only from 3 to 3-1/2 candles.

[Sidenote: Defects of the Siemens burner.]

While the advantages of the Siemens burner are many and obvious, it is not without its disadvantages. These partly arise from causes connected with the very observance of the conditions necessary to secure the efficiency of the burner. With every advance in the more efficient operation of gas-burners, increased care and attention are demanded in their employment, in order to obtain the benefits they are calculated to yield. Indeed, it would almost appear that the nearer the approach to perfection which is made in the construction of a burner, the greater must be the drawbacks to its general adoption. Thus, in the burner under notice, if the gas supply is allowed to become in excess, the tail of the flame enters the porcelain cylinder, and soot is deposited in the interior of the burner; obstructing the passages, and impairing the burner's action. Then, to cause the burner to yield its highest results, it is necessary that the air supply be accurately adjusted to the quantity of gas being consumed. To this end the entrance to the air chamber, at the bottom of the burner, is covered by a perforated semi-circular cup, by turning which the quantity of air entering the burner can be increased or diminished as required. Moreover, the bulky construction of the burner, with its accompaniment of chimney and flue, and its complicated arrangement of tubes and chambers, imparts to it a somewhat clumsy and inelegant appearance, which is calculated to impair the favour with which its remarkable performances cause it to be regarded. But these drawbacks are far outweighed by the undoubted advantages conferred by the burner--in improved illumination combined with economy of combustion, and the facilities it affords for securing perfect ventilation.

Encouraged by the success of Herr Siemens, other inventors have followed in his footsteps; with the result that there are now a variety of burners before the public, embodying the same principles, but differing in the details of their construction and in the measure of their efficiency. Of these may be mentioned Grimston's, Thorp's, and Clark's; and without describing in detail the construction of the several burners (of which further particulars will be found in the "Register of Patents" in the _Journal of Gas Lighting_[14]), it must suffice to refer to the salient points and distinctive features of each.

[14] See Vol. XL., pp. 786, 950; and Vol. XLII, p. 836.

[Sidenote: Grimston's regenerative burner.]

Grimston's burner (shown on the next page) consists, in effect, of an Argand burner turned upside down; the gas issuing from the bottom ends of a number of small tubes placed in a circle. The jets of flame--first directed downwards from the mouths of these tubes--by a conoidal deflector in the centre of the ring, are caused to spread outwards, and assume a horizontal direction; and by their amalgamation with each other a continuous sheet or ring of flame is produced. The horizontal direction of the flame is maintained by its passing underneath a metal flange, faced with white porcelain, or other refractory material; the supply of gas being adjusted so that the flame just terminates at the outer edge of this flange. Before entering the chimney, the products of combustion are caused to flow through a number of vertical tubes contained in a cylinder, which is concentric to an inner cylinder containing the gas-supply tubes. The outer cylinder is traversed by the air needed for the support of combustion, which is to become heated before reaching the point of ignition; and in order the more completely to enable the products of combustion to impart their heat to the entering air, the cylinder is further intersected by strips of wire gauze, which pass around and between the tubes (see fig. 22, on next page). By these means the air is intensely heated; and, passing among the narrow burner tubes through which the gas is conveyed, gives up a portion of its heat to the latter before the point of ignition is reached. Thus, in a very simple manner, both air and gas are raised to a considerable temperature before combustion takes place.

With regard to the efficiency of the burner, at the exhibition of gas appliances held at Stockport in 1882 (where a gold medal was awarded to it, as well as to Thorp's burner, to be referred to hereafter), with a consumption per hour of 9·84 cubic feet of 17·5 candle gas, an illuminating power of 60·67 candles was obtained (equal to 6·16 candles per cubic foot); while, on another occasion, when the burner was consuming 8·94 cubic feet per hour, an illuminating power of 51·5 candles (equal to 5·76 candles per cubic foot) was obtained from gas of the same quality. It is claimed for this burner that equally good results are obtained with small sizes as with large; and this, if borne out in actual practice, should go far towards ensuring the success and extensive adoption of the burner.

[Sidenote: Thorp's regenerative burner.]

Thorp's burner produces a cylindrical flame, like that of the Argand, but without the aid of a glass chimney which is a necessary adjunct to the latter burner. By means of a deflector on the inner side of the flame, the latter is made to curve outwards and assume a somewhat convex form, so as to obviate the shadow which otherwise would be cast by the gas chamber at the bottom of the burner. Above the flame is a cylindrical chimney, divided by a vertical partition into two concentric chambers, which are intersected by a series of metal gills, or projections, continued through both chambers. The outer chamber is for conveying away the products of combustion; the inner one for the passage of air to feed the flame; while down the centre of the inner chamber there passes a tube conveying the gas to the point of ignition. The hot products of combustion pass up from the flame through the outer chamber, and give up the greater portion of their heat to the projections; by which it is conducted into the inner chamber, and transferred to the incoming air. A common imperfection of regenerative burners is that, in consequence of the diminished rate at which the gas flows through the burner when expanded by heat, when starting the burner the gas must be only partially turned on, and the quantity gradually increased as the burner becomes heated; thus necessitating considerable attention. To prevent the need for this attention, there is in Thorp's burner an ingenious contrivance for automatically regulating the quantity of gas admitted to the flame. The central gas-tube, which is referred to above, contains a brass rod, fixed at one end, and at the other connected to a valve controlling the quantity of gas that enters the tube. At first, when the gas is lighted, this valve is almost closed; but as the rod becomes heated it elongates, gradually opening the valve until the full quantity of gas is admitted which the burner is intended to consume. At the Stockport exhibition, Thorp's burner was tested with the following results, as recorded in the Judges' report. After it had burned about two hours, "it gave an illuminating power of 183 standard candles, while burning 27 cubic feet of gas per hour (equal to 6·77 standard candles per cubic foot), with gas of 3·5 candles per cubic foot.... In another experiment with the same quality of gas, after burning half an hour it yielded, under similar conditions, 154 candles with a consumption of 25·29 cubic feet per hour, which gave an illuminating power of 6·02 candles per cubic foot."

[Sidenote: Clark's regenerative burner.]

There is nothing in Clark's burner that calls for special notice. In its main features it appears to be constructed upon similar lines to Grimston's burner, although the coincidence is doubtless only accidental.[15] It must, however, be added that in the details of its construction it is much simpler than the latter burner; and certainly it appears to lose very little in efficiency from its greater simplicity, as the following extract from a report by Mr. F. W. Hartley, the well-known photometrist, will show:--"With a consumption rate of 5·3 cubic feet of gas per hour, the amount of light yielded horizontally was equal to 29·79 times that of a standard candle. The light yielded per cubic foot of gas burned per hour was therefore equal to 5·62 times that of a standard candle." And the amount of light delivered immediately downwards is said to be "very sensibly greater than the amount of light delivered horizontally." Like the Grimston burner, it is of the inverted Argand form; the gas issuing from a chamber at the bottom of a tube which descends through the centre of the burner. The products of combustion escape through a chimney; and in so doing give up a portion of their heat to the entering air, which is conveyed to the point of ignition through horizontal tubes that intersect the chimney. The burner is enclosed in a suitable lantern, the lower half of which consists of a semi-globular glass; a similar arrangement being adopted in connection with the Grimston and Thorp burners.

[15] In justice to Mr. Clark it should be mentioned that, since the above appeared in the _Journal of Gas Lighting_, the attention of the writer has been called to the fact (which had been overlooked by him) that Clark's patent was taken out some months before that of either Grimston or Thorp.

The three burners last mentioned have not been before the public sufficiently long to enable a reliable opinion to be formed as to their value in actual and prolonged use. Although there is no reason for supposing that such will occur in the present instance, it so often happens that the results indicated by apparatus in the experimental stage, or while still under the control of the inventor, are not borne out in practice, that it would be unwise to express any decided opinion as to their ultimate worth from existing information. It is, however, to be earnestly hoped that the marked favour with which they have been received will not be impaired on improved acquaintance; but that further experience will justify the anticipations that have been excited by the excellent performances of the burners hitherto, and demonstrate at once their durability and real usefulness.

Since writing the above, considerable activity has been shown by inventors in producing new burners upon the regenerative principle, or in improving upon existing models. Of course, as yet it is too early to arrive at a satisfactory estimate of their actual value or relative worth; but it may be hoped that, from the increased attention being devoted to the subject, some real and practical results will flow, by which the gas-consuming public will be the gainers. So far, the most promising of this class of burners that has been brought into actual use, since the introduction of the Siemens burner, is the one represented below.

It is a modification, in the direction of greater simplicity, of Thorp's former burner, illustrated and described on p. 69 of this treatise; and as its construction is based upon the same lines as that burner, further description is not required.