US2148025A - Locomotive superheater - Google Patents

Description

Feb. 21, 1939. K, KOEHLER 2,148,025

LOC OMOT I VE SUPERHEATER Filed Jan. 21, 1957 INVENTOR.

ma/k011i ATTORNEY Patented Feb. 21, 1939 UNITED STATES LOCOMO-TIVE SUPERHEATER Karl Koehler,

Kassel, Germany Application January 21, 1937, Serial No. 121,611

2 Claims.

The invention relates to superheaterelements, each located in a flue of a flue boiler, particularlysueh as used in locomotives, each element having two annular channels, one surrounding 4 the other, the steam being delivered into the inner channel and taken off from the outer channel.

The object of the invention is either to raise the superheat temperature, keeping the superheating surface of the usual size (roughly of the evaporatingsurface in the case of locomotives), or to obtain the superheating temperatures (roughly 400 C.) customary in locomotive operation today, but to do this with a substantially smaller superheating surface, a surface that may for example be smaller by A; than the customary one. In doing this the pressure drop of the steam occasioned by the flow of the steam through the superheater elements and the draft loss of. the gases flowing through the flues are not to be increased, the pressure drop of the steam, on the contrary, being lower than heretofore. In addition, the heating gases coming from the flues are to have as nearly as possible the same temperature as the gases leaving the tubes. To accomplish this the invention, in the first place, gives to the central tube of the annular element, through which heating gases flow, the same or approximately the same diameter as that of the tubes of the boiler in question, this diameter being computed by means of Wagners factor, which is the ratio of the free gas area of a tube or flue .to the total surface contacted by the heating gases in such tube or flue. The most favorable empiric value of this ratio for locomotives is from 1:410 to 1:420 and this is the value which it is desirable to obtain. By doing this it is possible in the simplest way to divide the gas stream in the flues into an outer annular current and a central portion flowing through the central tube of the element, both correct according to Wagners factor. steam area with relatively large diameter of the ordinary serpentine superheater element is replaced by a particularly thin annular cross section with relatively large diameter and at the same time the condition can be met that the steam both enters and leaves the superheater element at the velocity of about 12 mm./sec. which is today regarded as the most favorable rate of flow from the standpoint of heat transfer. As a result of this and of giving the steam path the technically correct form with only one reversal of direction, the pressure drop is smaller than was possible in all other former superheater elements. At the same time the heating surface of the portion of the superheater element in which the steam flows in the same direction as the gases, i. e. the outer heating surface is increased as compared with that of the portion in In addition, the usual circular Germany January 24, 1936 which there is counterflow of the steam and gases, so that as far as therate of heat transfer is concerned, the portion in which the flow is in the-same direction works under at least as favorable conditions as that in which the flow is in opposite directions.

Further characteristics of the invention consist in the particular shape given to the end of the element at which the flow of the steam is reversed and in the form of the piece serving toadmit andtake off the steam to and from the element.

In the drawing Figs: 1 to 3 show an illustrative example of. how the invention may be carried out. Fig. 1 is a vertical central longitudinal section of a locomotive boiler equipped with superheater elements in accordance with the invention; Fig. 2 shows a superheater element as viewed from the smoke box and Fig. 3 shows a lateral view of a superheater element, parts being in sectionand other parts being broken away.

The flues of the boiler, in which superheater elements are located, are designated by the reference numeral I andthe tubes, which have no superheater elements, are shown at 2. Each of the superheater elements in the flues I consists essentially of three concentrically or co-axially arranged tubes, namely, the innermost one 3, constituting a gas passage and of approximately the same diameter as one of the tubes 2, the outer tube 4 and an intermediatetube 51 At the smoke box end the tubes 3, 4 and 5 are connected into a special connecting piece 6 having two portions 1 and 8 which are'bent oif laterally away from the axis of the element. The outer tube 4 is connected directly to the bent piece or elbow I and the intermediate tube 5 is. directly connected to the elbow 8, whereas the innermost tube 3 opens into the smoke chamber l9 through the elbow 3. The elbow l is penetrated in a manner which will be understood readily from an inspection of the drawing by intermediate tube 5 within which lies the innermost tube 3, and the elbow 8 is penetrated only by the innermost tube or flue 3. The elbows are bent away from the axis of the flueboth in a vertical direction as Well as laterally so that between the elbows "I and 8 there remains a free space, as clearly shown in Fig. 2, through which the flues are readily accessible for cleaning, etc. To the ends of the elbows 'l and 8 there are connected pieces 9 and it which serve to. connect the superheater element to the superheated steam chamber H and saturated steam chamber I2 respectively of the header. The gases coming from the fire box 13 flow through the flues I in the direction indicated by the arrows; part of them flowing through the innermost tube Band part of them through the fax annular channel l4 between the tube 4 and the flue I.

The steam flows from the saturated steam chamber l2 by means of the connecting length l and the elbow 8 of an element into the annular channel between the innermost tube 3 and the intermediate tube 5, the flow in the annular channel being counter to the flow of the heating gases. At the return bend l'l the steam flow is reversed and then flows through the channel 18 in the same direction as the gases, being delivered to the elbow and so to the connecting piece and superheated steam chamber ll.

Annular superheater elements located in boiler flues have been proposed heretofore, but the required relation between the quantities: steam cross section, superheater surface and heating gas section necessary for a solution of the problem was not present. They did not therefore accomplish the desired purpose and were not used to any extent, being moreover inadequate structurally for continued service. In contrast to this the newly proposed superheater elements have the advantages mentioned at the beginning of the specification. The decrease of the superheating surface presents far-reaching advantages. as far as weight and cost are concerned because the number of flues can be reduced, their evaporating surface being replaced by tubes of considerably smaller diameter. As a result, the diameter of the boiler can be reduced, this resulting in further saving in weight and cost.

An advantage which is inherent in this type of annular superheater element is that a higher degree of superheat can be given to the steam by the central gas stream, since these gases give up heat only for superheating and under counterfiow conditions and their temperature therefore is not lowered by their giving up heat to water cooled surfaces, so that the heat gradient is very favorable.

It will of course be advantageous for high superheating if the gases flowing through the central circular passage could be delivered to the smoke tube at a higher temperature than the temperature of the gases leaving the tubes and the outer annular space of the flues. This, however, would impair the boiler effici-ency. If this boiler efficiency is to be kept at the usual figure, the diameter of the central tube of the superheater element would become so small that the important advantage of having the steam fiow in a very thin stream would be lost and moreover this central passage of the unit would plug up.

It is essential for the invention that:

1. The gases flow through the central circular space in a direction counter to the steam which they heat;

2. That the inner one of the two annular steam passages extends as far as possible into the smoke box so that the gases flowing through the central passage may give up additional heat to the steam;

3. That the central tube be designed in accordance with Wagners factor so that the gases flowing through it reach the smoke box at a temperature which makes the operation economical.

If one were to make the attempt to obtain higher superheats solely by increasing the superheating surface or by reducing the cross-sectional area of the gas passages in order to have a more rapid gas flow and resultant more favorable rate of heat transfer, then Wagners factor would become considerably smaller for the flues than for the tubes. If the draft loss in two groups of tubes is the same, then the gas velocity in each group will depend directly upon the resistance in such group to the flow, and the relative resistances are represented by Wagners factor for the two, and as a result more of the heating gases will, in the supposed case where the gas area is reduced to enhance superheating, flow through the unoccupied tubes and the desired improvement in superheating will either not result at all or only to a very small extent. In addition, if Wagners factor is made materially different for the flues than for the tubes, then the temperatures of the heating gases leaving the two will vary widely and as a result the economy of the boiler will of course be lowered. By using superheater elements in accordance with the invention the efficiency of the superheater is improved and therefore the overall boiler efliciency increased. (The term superheater efiiciency is to be understood as meaning the product of the ratio of the superheating surface to evaporating surface and the ratio of flues to tubes.) The improvement in this superheater efficiency is brought about by increasing the number of the flues relatively to that of the tubes and at the same time by increasing the amount of superheating surface as compared with the evaporating surface. By using a tube having the correct Wagners factor for a given boiler as a basis for constructing the entire superheater element, the circular cross section of the steam. current present in the ordinary serpentine unit in use today is replaced by a very thin annular stream of steam. As a result the heat from the surfaces heated by the gases can be delivered more effectively to the whole volume of steam. The entire thickness of the steam current becomes practically a turbulent layer, such turbulence being caused by the friction against the walls and the socalled cold center of the circular section which is present in the ordinary superheater is entirely absent. This cold center of the ordinary unit of circular section is brought into contact with the hot walls of the unit only at the return bends and here at the expense of a considerable pressure drop. In an element in accordance with the present invention, however, each particle of steam is sufficiently often and sufficiently long in contact with the hot wall. As a measure of this heat penetration the ratio can be used: heated circumference to depth of stream of steam. For every circular cross section, in which of course the thickness of the stream is approximately equal to half the diameter, this ratio with usual tube diameter and usual wall thickness is about 7.52 to 7.96. That this heat penetration figure represents actual conditions is illustrated in an actual case where the outer diameter of the superheater tube of a flue superheater was increased from 36 to 38 mm., the inner diameter being in each case 30 mm. The heat penetration for-the old tube was that for the new one is or 2.531r

This small difference clearly reflects the advantage which it is possible to obtain by increasing the wall thickness, namely that the same superheat temperature can be obtained more quickly than before. The thickness of the steam stream in the proposed new superheater element will, depending upon the size of the locomotive and the corresponding size of the flues, lie between 6 and 10 mm. Inasmuch as the diameter of usual tube is from 30 to 65 mm. the heat penetration value for a superheater in accordance with the invention will be about 12 for the inner and about 20 for the outer annular stream of steam, that is about 16 for the entire element. This fact and the further fact that the central stream of heating gases does not have its temperature lowered by giving upheat for Water evaporation but gives up its heat only for superheating and this with a favorable heat gradient and under counterflow conditions and that therefore the steam is heated to a higher degree explain the higher efficiency of the new superheater element.

By superheating steam of pressures such as are usual today to temperatures such as are usual in locomotive operation, the volume of the steam in flowing through the superheater is increased to about 1.5 of its entering volume. If the cross section for the steam remains constant as is the case in the usual superheater element of serpentine type, this means an increase in the steam velocity of about 50%. This means a decreased heat transfer rate, and this disadvantage is even more serious from the fact that this higher velocity occurs mainly in the portion of the superheater in which the steam flows in the same direction as the gases. By replacing the ordinary circular cross section of the usual superheater element into two very thin annular streams, one surrounding the other, the inner path being used for the entering steam, these deficiencies are automatically removed if the annular cross section of the outer path is made 1.5 times as large as the area of the inner annulus. The steam will therefore enter and leave with about the same velocity and the element further provides a larger heating surface in the unifiow part of the path to make up for less efficient action of this surface. The steam velocity which has been found by experience most advantageous for heat transfer is about 12 mm./sec. and this velocity has been used in designing the superheater element in accordance with the present invention.

Finally, the essential advantage of the superheater element according to the present invention should be mentioned that the pressure drop as the steam flows through the element is only slight.

I am aware that attempts have been made heretofore to obtain the advantages resulting from subdividing the relatively thick stream of steam of the ordinary element into several smaller circular or flat streams. Furthermore it has been proposed to arrange in sequence in a single element tubes of increasing diameter, this diameter being given the correct value for the hydraulic radius of the entire element so that the portions with the smallest tube diameter and the greatest length lie in the hottest zone and vice versa. Naturally these arrangements resulted in more favorable superheating conditions but this advantage was offset by the fact that the steam flow through the superheater resulted in increased and undesirable pressure drop.

The element according to the present invention results in a substantially decreased pressure drop. This is due to the fact:

1. By directing the steam flow at the inlet and outlet in a technically correct and favorable manner and doing this also at the return bend. In the latter the only consideration is to bring about the reversal of the steam without causing any eddying. If this were done in return bends of the ordinary superheater, the superheating would suffer because it is at these very return bends that the powerful eddying, which of course means pressure drop, mixes the cold core of the steam with the superheated exterior portion and brings the core in contact with the hot walls so it may be heated. This is not required in an element according to the present invention because of the small depth which permits each particle of steam to come into contact with the hot wall sumciently often and for sufficiently long a time.

2. By keeping the steam flow at a velocity most favorable for heat transfer and doing this also in the second zone, without repeated decrease or acceleration of such velocity such as occurs for example in serpentine units of increasing tube diameter.

3. By using a single return bend causing a prompt reversal of the steam without eddying (see under 1).

The inner diameter of the flue is computed in accordance with Wagners factor for tubes. This results in an equal heating gas velocity in the tubes and in those portions of the flues in which the evaporation occurs. This meets the requirement that equal velocities of heating gases are most favorable for a uniform evaporation.

The adjusting of the amount of superheating surface to the steam delivery of the boiler can be made in the simplest manner by placing the required number of units in parallel, just as in the case of the superheater elements of usual form.

What I claim is:

1. In a locomotive having fire tubes, and flues for superheater elements, the combination with each flue of a superheater element in the flue, the superheater element comprising three coaxially arranged tubes which together with the flue define a central gas path circular in crosssection, two annular steam paths, and an annular gas path between the flue and the element, means to cause steam flow through the inner annular steam path in a direction counter to that of the gas flow and through the outer annular steam path in direction with the gas flow, the ratio of the cross-section of the free central gas area to the total inside area of the central tube and the ratio of the cross-section of the annular outer gas path to the sum of the outer surface of the outermost of the three tubes plus the inner surface of the flue each being substantially equal to the ratio of the free cross-sectional area of one of the fire tubes to its total internal surface.

2. In a locomotive having fiues for superheater elements, a smoke-box into which said flues open, a superheater header in the smoke box above the flues, a superheater element in each line and each comprising three co-axially arranged tubes, an elbow into which the outermost of the three tubes opens and through which the intermediate of the three tubes extends, a second elbow into which the intermediate tube opens and through whose curved outer surface the innermost of the three tubes opens into the smoke box, the intermediate tube extending forward from the first elbow a comparatively long distance.

KARL KOEI-ILER.

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