US2423997A - Ramified tubular gas heater - Google Patents

Description

July 15, 1947. RUEGG 2,423,997

RAMIFIED TUBULAR GAS HEATER Filed Feb. 26, 1945 Fig.3 9

' Atcorneqs Patented July 15, 1947 RAMIFIED TUBULAR GAS HEATER Rudolf Ruegg, Zurich, Switzerland, assignor to Aktiengesellschait Fuer 'Iechnische Studien, Zurich, Switzerland, a corporation of Switzerland Application February 26, 1945 Serial No. 579,847

4 Claims.

This invention relates to a gas heater with tubular elements arranged in a combustion chem-- ber. In particular it relates to a gas heater of this kind which is suitable especially for thermal power plants, in which a. gaseous working medium, preferably air, after having been raised to a higher pressure in at least one compressor, is brought to a higher temperature in a ,heater by an indirect supply of heat from an external source and thereafter expanded in at least one turbine whilst giving up energy to at least one consumer of useful output.

In the heaterof such thermal power plants a given quantity of heat has to be supplied from an external source to the circulating working medium in order to heat the latter to a certain final temperature. With given conditions in the combustion chamber the quantity of. heat that can be supplied to the working medium from an external source,.is proportional to the product of length and. external diameter'of the heater tubes arranged in the combustion chamber. For a given degree of heating the quantity of working medium flowing through must then be proportional to said product, in order that the necessary quantity of heat shall be taken up and furthermore dangerous accumulations of heat in the tubes avoided. From the equation of continuity it also follows that the quantity of air. flowing through v the tubes is proportional to the product of its sure losses occurring in the heater tubes, to adoptv short tube length, it seems obvious to choose a small velocity of flow and a relatively large internal diameter for the tubes in view of the pressure losses.

However, there are various circumstances which render inadvisable the adoption of too small flow velocities for the working medium that has to be heated. Thus, for example, the eiliciency of thermal power plants of the kind herein referred to can be raised by increasing the temperature to which the working medium is heated. However, since the heat transmission in tubular gas heaters between tube wall and working gas is not particularly intense, the use of high temperatures of the working medium makes it necessary to employ materials capable of resisting high temperatures for those parts of the heater which are Germany March 22, 1944 2 subjected to the highest temperatures. But such materials are expensive and apart from this they are not easy to machine, so that it is important to reduce to a minimum the quantities of such materials that are actually needed. That is the case when a high flow velocity is adopted for the medium to be heated, since the latter can then carry away a great deal of heat from the tubes and thus prevent accumulations of heat therein. However, the extent of the maximum admissible pressure losses usually prohibits the adoption of such a high velocity of the working medium to be heated as would appear desirable from the point of view of preventing accumulations of heat in the heater tubes.

At the same time unduly low flow velocities of the 'working medium can not be chosen, since this would render necessary the adoption of excessively large tubes. On the other hand if the tube diameter is increased, then for adhering to a given, maximum admissible tube wall temperature under otherwise identical conditions-i. e., the same degree of heating of the working medium, the same pressure and the same supply of heatthe velocity at which the working medium to be heated flows through the heater tubes, must, according to the latest lmowledge acquired in the investigation of flow and. heat transmission phenomena, be increased to a degree that is proportional to the cube root of the tube diameter. But, as already mentioned, every increase in the velocity of flow involves higher pressure losses, since in the first approximation these are, as is known, proportional to the square of the velocity and are further proportional to the quotient of the tube length and tube diameter. The pressure losses in the tubes are thereby also proportional to the pressure loss coemcient that depends on the roughness of the tubes and the Reynolds number.

In view of these facts it thus unexpectedly again appears'more advantageous to adopt small internal diameters for the heater tubes. But with small inside tube diameters influences such as can easily be disregarded in the case of larger tube diameters, make themselves apparent to a much greater extent. For example in the'case of tubes having a small internal diameter the wall thickness cannot simply be reduced proportionally to diameter the heat stressing on the internal tube surface (expressed in kcaL/sq. m. see.) is much greater than on the external surface that is di- 3 rectly subjected to the heat supply. Thus, if a certain tube wall temperature shall not be exceeded an endeavour must be made to realize higher heat transmission coeflicients in the internalsurface of the tubes. This can be brought about by imparting a higher velocity to the medium flowing through the tubes. However, this again involves greater pressure losses. Furthermore it must be considered that the pressure loss coeflicient in tubes of small internal diameter and for conditions that are otherwise the same, is much greater than in tubes having a large internal diameter. Thus for these various reasons a considerable increase in the detrimental pressure losses takes place in tubes of small internal diameter. These new aspects, which in other connections are not valid, lead to a gas heater, which comprises tubular elements arranged in a combustion chamber and connected to at least one distributor and at least one collector for the gas to be heated, and of a design complying to a very great extent with the above mentioned relations,

new aspects and requirements arising in connection with the best possible utilization of the radiation heat, this gas heater being furthermore of.

sufllcient reliability in operation and involving relatively low manufacturing costs.

According to the present invention all these advantages are achieved in that each tubular element of the gas heater comprises, when considered with reference to the direction of flow of the gas to be heated, a first tube section connected to the distributor and ramifying by stages in an increasing number of branches which become by stages increasingly smaller in diameter,

and a second tube section constituting -a direct continuation of said first mentionedsection and comprising a number of branches decreasing in number by stages and which become by stages increasingly larger in diameter, said second tube section being connected to the collector. In a heater of this kind the most suitable adaptation of the tube diameter to the temperatures of the combustion chamber can be brought about, since tubes of smallest diameters can be provided only just at those points where the greatest supply of heat takes place and where, as a consequence, small tube diameters and high velocities of the working medium are particularly desired. A heater according to this invention further offers the advantage that intermediate headers can be eliminated at those points where a change in the tube diameter is provided for. This is of importance as such intermediate headers increase the manufacturing costs, since they involve the use of a considerable amount of material that cannot be utilized for the transmissionof heat. If such intermediate headers are installed outside the combustion chamber and only the tubes of small diameter subjected to the radiation from the flames, then the further drawback is involved that sections of the tubes have to be passed through the brickwork of the combustion chamber. said tube sections being consequently lost as far as the transmission of heat is concerned. This has a detrimental influence with regard to the pressure losses. If on the other hand the intermediate headers are arranged in the combustion chamber then they must be protected by an insulating layer against excessive heat radiation, which make necessary the use of even more material. Contrary hereto in a gas heater according to the present invention almost the whole pressure drop which, from the point of view of efficiency of the plant can still be accepted, is utiii ed the heater tubes.

exclusively for producing a sufllciently great velocity for the working medium passing through With regard to the heating surface proper there is practically no section where pressure losses occur, which would not simultaneously contribute to an-improvement in the heat transmission.

In the accompanying drawing a preferred embodiment of the invention is illustrated by way of example. In this drawing:

Fig. 1 shows in a simplified mode of representation a vertical section through a heater,

Fig. 2 shows parts of a tubular element of this heater on a larger scale, developed in the plane of the drawing, and

Fig. 3 shows a section on the line III-III of Fig. 1.

In Fig. 1 the numeral i denotes the combustion chamber of a gas heater arranged for burning pulverized fuel, and i denotes a burner to which the pulverized fuel is supplied. The combustion chamber l is surrounded by walls 2 of refractory material. Above the top wall 2 of the combustion chamber l are arranged a distributing header 3 and a collecting header 4 for the gas to be heated, which flows to the header 3 through a tube 5 and leaves header 4 through a tube 6. The letter E denotes tubular elements of the heater. each of which is connected through a tube section I to the distributing header 3 and through a tube section 8 to the collecting header 4. Each tube section I ramifles by stages into an ever increasing number of branches, the diameter of which, considered in the direction of flow Z of the gas to be heated, becomes by stages increasingly smaller. The various groups of branch tubes of the tube section I, the diameters of which become by stages increasingly smaller, are denoted by the numerals I 1 I and 1 To the group of branch tubes 1 are connected, seen always in the direction of flow Z of thegas to be heated, a plurality of branch tubes which form a direct continuation of the section I. These branches which constitute a second tube section 8, decrease. in number by stages and become by stages increasingly larger in diameter. The tube section 8 is connected to the collecting header 4; the relative groups of branch tubes, the diameter of which becomes by stages increasingly larger, are denoted by the numerals 8 8 8 As shown, the group I with the greatest number of branches is situated within the range of the highest combustion chamber temperatures. Thereby the tube elements l l arranged nearest to the wall 9 of the combustion chamber, 1. e., to the wall with an opening for the flue gas discharge into a chan- .nel It, can conveniently be installed in such a manner that those of their branches which are situated in the proximity of the inlet end of said channel Ill are, when considered with reference to the direction of flow of the flue gases R and as shown in Fig. 3, arranged in one row, so that a relatively large, unobstructed flow cross section between the various elements ll results here .for the flue gases. The foremost branch of each row of these tubular elements I I, which as a" result of the radiation from the flames, the gas radiation and the transmission by contact (cross current flow) is subjected to an increased supply of heat, is preceded by a relatively short protecting tube I! of relatively small diameter through which an intense exchange of heat takes place and which consequently assumes a relatively low tube wall temperature. By means of such protecting tubes l2 the tubular elements II are, at

least partly, protected against excessive'heating. The principle of the invention can be applied to heaters having combustion chambers of different forms and no necessary limitation to the typical embodiment chosen for illustration in the present application is implied.

It is not necessary for all heater elements of to be heated, each of said tubular elements consisting, considered in the direction of fiow of the gas to be heated, of a first section connectedto said distributor and ramifying by stages in an increasing number of branches which become by stages increasingly smaller in diameter, and of a second section constituting a direct continuation of said first mentioned section and having a plurality of branches decreasing in number by stages and which become by stages increasingly larger in diameter, said second section of the tubular elements being connected to said collector.

2. A gas heater comprising in combination, means forming a combustion chamber; a burner operating in said chamber and creating therein a maximum-temperature zone; tubular elements arranged in saidcombustion chamber and overlying the side walls thereof; at least one distributor and at least one collector for the gas to be heated, each of said tubular elements consisting, considered in the direction of flow of the gas to be heated, of a first section connected to said distributor and ramifying by stages in an increasing number of branches which become by stages increasingly smaller in diameter, and of a second section constituting a direct continuation of said first mentioned section and having a plurality of branches decreasing in number by stages and which become by stages increasingly larger in diameter, said second section of the tubular elements being connected to said collector, and-the greatest ramifications of said two sections of each tubular element being situated in said maximumtemperature zone.

3. A gas heater comprising in combination, means forming a combustion chamber and a channel leading from said combustion chamber {or the discharge of the products 01' combustion: tubular elements arranged in said combustion chamber and overlying for the most part the side walls thereof; at least one distributor and at least one collector for the gas to be heated, each of said tubular elements consisting, considered in the direction of fiow of the gas to be heated, of a first section connected to said distributor and ramiiying by stages in an increasing number of branches which become by stages increasingly smaller in diameter, and of a second section constituting a direct continuation of said first mentioned section and having a plurality of branches decreasing in number by stages and which become by stages increasingly larger in diameter, said second section of the tubular elements being connected to said collector, and those of said tubular elements that are located in the proximity of the wall of the combustion chamber where said channel for the discharge of products of combustion is situated, being arranged in such a manner that the branches of each such element lie in the neighbourhood of said channel, considered in the direction of flow of the products of combustion, in a row, an unobstructed flow cross section for the products of combustion remaining between the various tubular elements.

4. A gas heater comprising in combination, means forming a combustion chamber and a channel leading from said combustion chamber for the discharge of the products of combustion; tubular elements arranged in said combustion chamber and overlying for the most part-the side walls thereof; at least one distributor and at least one collector for the gas to be heated, each of said tubular elements consisting, considered in the direction of fiow of the gas to be heated, of a first section connected to said distributor and ramifying by stages in-an increasing number of branches which become by stages increasingly smaller in diameter, and of a second section constituting a direct continuation of said first mentioned sectionand having a plurality of branches decreasing in number by stages'and which become by stages increasingly larger in diameter, said second section of the tubular elements being connected to said collector, and those of said tubular elementsthat are located in the proximity 01' the wall of the combustion chamber where said channel for the discharge of products of combustion is situated, being arranged in such a manner that the branches of each such element lie in the neighbourhood of said channel, considered in the direction of flow of the products of combustion, in a row, an unobstructed flow cross section for the products of combustion remaining between the various tubular elements; and relatively short protecting tubes 'of relatively small diameter in which an intense exchange of heat takes place and which consequently assumes a relatively low tube wall temperature, the foremost branch of each of said rows of ramifications being preceded by one of said short protecting tubes.

RUDOLF RUEGG.

, REFERENCES CITED The following references are of record in the file of this patent:

FOREIGN PA'I'E'NTS Number Country Date 1,361 Great Britain 1854 39,504 The Netherlands Nov. 16, 1936

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