Title:
Method of and apparatus for superheating
United States Patent 2000906


Abstract:
This invention relates to steam superheater constructions, and has for one of its objects provision of an improved form of superheater highly resistant to burning out, sagging or "creeping", even though subjected over long periods to the impingement thereagainst of radiant heat at high temperature,...



Inventors:
Turner, Charles S.
Application Number:
US51267531A
Publication Date:
05/14/1935
Filing Date:
01/31/1931
Assignee:
Turner, Charles S.
Primary Class:
Other Classes:
29/890.03, 122/481, 138/111, 165/154, 165/169
International Classes:
F22G7/14
View Patent Images:



Description:

This invention relates to steam superheater constructions, and has for one of its objects provision of an improved form of superheater highly resistant to burning out, sagging or "creeping", even though subjected over long periods to the impingement thereagainst of radiant heat at high temperature, and even though the supply of saturated steam to the superheater tubes may be cut off or greatly diminished.

Another object of this Invention is the provision of a method and improved means whereby the temperature of the superheated steam emanating from the superheater tubes may be controlled, automatically or otherwise, within close limits. As commonly constructed, superheater tubes are kept at a temperature sufficiently low to prevent their burning out or "creeping" by the saturated steam which is supplied to them from the main boiler. If the supply of saturated steam is cut off or greatly diminished, however, the heat impinging against the tubes may not be conducted away from them fast enough, in which event sagging or "creeping", or burning out of the tubes is the common result. My invention contemplates the integration of cooling means with the exposed walls of the superheater tubes in a way which will prevent the attainment of dangerous temperatures by such walls when steam in normally operative quantities is absent, in other words, cooling means appurtenant the superheater tubes in such heat-conducting relation to the exposed walls thereof that by supplying a reasonable amount of water, steam, or other cooler fluid, at a minimum difference of temperature, the su;- perheater tubes may be kept safely below their fusing temperature and prevented from "creeping" or burning out.

Another object of this invention is the provision of an improved form of superheater incorporat.0 ing the aforementioned safeguard against dangerously high temperatures, but which nevertheless is more effective in its superheating action than such construction as they are now commonly constructed.

This application is a continuation in part of my previously filed application Ser. No. 496,868, filed Nov. 20, 1930.

Other objects and advantages will be apparent from the following description, where reference - is made to the accompanying drawings illustrating preferred embodiments of my invention and wherein similar reference numerals designate simi!ar parts throughout the several views.

In the drawings: c,. Figure 1 is a sectional view showing somewhat conventionally a typical water tube boiler, and superheater coils arranged therein for radiant heating; serving to illustrate a typical application of one embodiment of my invention.

Figure 2 is a cross sectional view of a superheater tube, showing one method of applying the principles of my invention thereto.

Figure 3 is a cross sectional view of a somewhat modified form of superheater tube embodying my3 invention. Figure 4 is a similar sectional view showing another modification of my improved superheater tube construction.

Figure 5 is a cross sectional view showing still another modification of my improved superheater tube construction.

Figure 6 is a similar cross sectional view of another modification of my invention.

Figure 7 is a similar cross sectional view of another modification of my invention. Figure 8 is a similar cross sectional view of another modification of my invention.

Figure 9 is a similar cross sectional view of another modification of my invention.

Figure 10 is a similar cross sectional view of another modification of my invention.

Figure 11 is a perspective view showing a typical method of installing my construction shown in cross section in Figure 10.

Figure 12 is a longitudinal sectional view through a take-off manifold, which I may use with the tube construction shown in Figure 5.

Figure 13 is a view partly in elevation and partly in section of a header construction and method of leading tubes thereto suitable for use with some forms of my invention.

Referring now to the drawings: Reference character 10 designates the tubes of a water tube steam boiler. Any desired form of boiler or steam generating equipment may be used with my invention, however. Numeral II indicates steam drums from which saturated steam is led off through pipe 12 in the usual or any desired manner. The superheater tubes to which the steam is conducted by the pipe 12 are generally designated 13 in Figure 1, and may take any of the forms indicated in Figures 2 to 10, inclusive, or any of many other modifications thereof which will occur to persons understanding the principles underlying my invention. In the form illustrated in Figure 2, the steam to be superheated is conducted through a plurality of tubes 14, which are nested cylindrically and secured together, as by welding, (welds designated 15) so that an inner passage 16 is formed through which cooling water may be conducted.

If desired, however, the procedure might be reversed, and the cooling water conducted through the tubes 14 and the steam led through the central passage. It will be noted that cooling water passing through the central passage 16 has direct and very effective metallic heat-conductive connection with the outer portions of the tubes 14 which are exposed to the radiant heat within the furnace. Thus if the steam supply normally passing through the tubes 14 is cut off or greatly diminished, the passage of a sufficient quantity of cooling water and/or saturated steam through the central passage SG will effectively maintain the outer portions of the tubes 14 at a temperature low enough to prevent damage. Experience has shown that a direct metallic connection of adequate total area distributed throughout the entire extent of the tube construction is necessary to enable a sufficient heat transfer to take place between the exposed portions of the tubes 14 and the contained cooling water within the space 16 to enable the water to effectively cool the tubes.

In the alternative method of construction shown cross-sectionally in Figure 3, the tubes for the cooling water are formed integrally with the superheater tube by crenelating the same in the manner shown; thus forming a plurality of quite closely spaced cooling chambers 22. The meeting edges of the bent back portions so formed may be sealed by welding as indicated at 23. The steam is conducted through the portion 24, and will be seen to have a greater area of contact with metal heated by conduction from the exposed outer portions of the tube than is the case with an ordinary cylindrical pipe, as each of the tubes 22 may also conduct heat into the interior and give it up to the steam, at least when or where the tubes 22 are empty of cooling water. Here again the distance from the most remote point of the outer wall to water-contactible metal of the cooling tube will be seen to be short, and the provision of a direct metallic connection of adequate area, distributed at frequent. intervals throughout the entire peripheral extent of the exposed walls of the tube construction enables sufficient heat conduction to take place between the exposed portions of the chambers 24 and the contained cooling water within the spaces 22 and to let the water cool the tubes at all points when desired.

If the tubes are formed in the manner shown in Figure 4, the cooling water, when used, may flow through the tubes 25, which, when and where empty of water, increase the conductive metallic surface and aid in heating the steam, as in the other embodiments. The steam is of course conducted through the portion 26.

Still another method of, building up an improved superheater embodying my invention is indicated in Figure 5. It will be seen that this construction is in all essential features similar to Figure 3, but is assembled from pipes 27, and curved plate sections 28, the welds 20 serving to render the structure unitary. The steam may be conducted through the section 23 and the cooling water through the tubes 27.

It will be seen that if the cooling water is taken from the boiler, certain of the welds in the constructions shown in Figures 2, 3, and 5 (also 10) will be subjected to boiler pressure. This is by some considered objectionable, and may be avoided either by constructing the tubes as shown in Figure 4, or by using one of the arrangements shown in Figures 6, 7, 8, and 9. In these the water conducting or cooling tubes are designated 45 with different exponents, and the steam conducting or superheating tubes are indicated by the numeral 46 with different exponents. The welds are designated 41, and it will be observed that in no instance are they called upon to withstand boiler pressure.

The embodiment illustrated in Figure 6 incorporates another important advantage in that the tubes 45' being welded in the depressions in the main tube 46' only along their sides, an effective heat bridge between the tubes is only present along the welded areas. These areas being at and along the surface of the assembly, the water tubes 45' absorb heat from the exposed outer surface most effectively, but the ever-present air gap which exists between metal surfaces in metal-to-metal contact only, has a relatively high insulating value, and so heat exchange between the inner surfaces of the interfitting tubes 45' and the depressions in the large tube 46' is materially limited, thus restricting the ability of the water tubes 45' to absorb heat from the steam, while their efficiency in absorbing heat from the outer walls of the assembly is very great. Of course the actual dimensions of the gap between the interfitting parts are immaterial and may vary while the assembly is in use. If merely made to interflt in the ordinary manner of making such parts, a gap of the order of .002-.005 inch would be present even where the surfaces would ordinarily be described as "contacting." For this reason the tubes 45' may be merely laid in the depressions and welded in the manner shown. In the embodiment shown in Figure 7 also, the water tubes 452 (by reason of their positioning) absorb heat most effectively from the surface of the steam tube 482.

As shown in Figures 9, 10, and 11, the principles of my invention may, if desired, be incorporated in a superheater which is built up in wall form, with alternate cooling and superheating tubes.

In the form shown in Figure 10, the cooling tubes are, excepting for the end ones, which may be separate pipes (48) completely enclosed, and formed by configurations of the superheater tubes, which therefore present an unbroken heat absorbing surface as great as is possible by any construction, while the cooling tubes are in such effective direct metallic connection with them that they provide very effiient cooling action when the cooling fluid is flowing. The superheater tubes in Figure 10 are designated 465. A construction exceeding this in efficiency in some particulars, and in which no welds are subjected to boiler pressure, is shown in Figure 9. In this construction the fully effective metallic connection between the cooling and superheating tubes is, as in the tube form of Figure 6, only present at or near the surfaces, which results in very efficient heat transfer between them only thereralong.

This enables efficient control of the temperature of the wall without undue absorption of heat from the steam in tubes 464. When the cooling fluid is not flowing and the superheating function is in progress, the cooling tubes (454) effectively assist in transferring heat to the superheater tubes (46$).

Figure 11 shows a wall type superheater construction incorporating the tube design shown in Figure 10. The numeral 45 designates a furnace wall, although the tubes may be placed in any desired position.

It wll be seen that in all my improved superheater tube constructions the relative area of heat-conductive metal with which the steam to be superheated is in contact is much greater than is the case in ordinary superheater tube constructions. Thus when or where cooling water is not present; much more efficient superheating action may be obtained than is possible from ordinary tubes.

If desired the cooling water may be taken from a lower part of the boiler and returned to an upper part, as by conducting it from the lower or "sludge" drum 35 through the pipe 21 and the valves 40--40 to the cooling passages of the superheater, which it may enter from below, as through the manifold 9I, (Figure 1), and leave at the top, as through the manifold 20, whence it is returned to one of the upper drums if through the pipe 38, usually in the form of vapor. This method of supplying the cooling water is of course optional, however, and an outside scurce and disposal thereof might be provided if desired. It is only because the unusually efficient construction and arrangement of my superheater tubes provides remarkably efficient heat absorption from the exposed portions of the tubes when the cooling water is flowing that it is possible to use the water from the boiler for this purpose, which water may itself be at a fairly high temperature. Though the difference of temperature may be less than is usually considered necessary, however, it is sufficient to provide efficient cooling, by reason of the effectiveness of heat absorption from the outer walls provided by the cooling tubes in their disclosed arrangement. A certain amount of steam is generated in the cooling coils, especially when the boiler is being started and no steam is passing through the superheater tubes. The steam thus generated is delivered to the steam drums of the boiler to augment its output.

The steam may enter the superheater tubes from above, as through the header IT of Figure 1, and may leave through the header 18 at the bottom. As will presently be explained, the cooling water stands at variant levels in the cooling tubes during normal operation in order to control the temperature of the delivered superheated steam, but no material desuperheating occurs as a result of the downflow of steam through the lower, water-containing portions of the assembly, by reason of the fact that the cooling water absorbs heat much more readily from the outer walls than from the steam. This tendency, which is of course accentuated in the structures shown in Figures 6 and 9 results in control being effected by preventing the outside walls and so the steam from ever reaching a temperature higher than desired, rather than by desuperheating the steam after elevation of its temperature above that finally desired (and consequent excessive heating of the walls). Any desired manifold and header construction may be used for conducting the steam and water to and from the tubes. One form of such manifold suitable for use with the tube construction shown in Figure 5 is indicated in Figure 12, the tubes 21 being led into registering apertures in the manifold 31, and the water flowing to or from them through the pipe 32. The arcuate plates 28 may be cut away within the manifold, as shown in the drawing, to permit the steam to flow into or from the section 30 through the pipe 33. The arrows in Figure 6 indicate a flow of steam into the superheater at this point, and the flow of water as being out, although obviously identical manifolds may be used at each end of the tubes if desired, and the details of their structure will vary to conform to the tube design.

If a wall type superheater construction is used, the headers may.be arranged as shown in Figure 13, in which 50 indicates a steam header, which may run the length of the wall, as may also the water header, designated 51. The tube formation shown in Figure 13 is that shown cross sectionally in Figure 9.

While water is contained in the cooling tubes, that portion of the superheating tubes adjacent. the water-containing portion of the cooling tubes ceases to function as a superheater, by reason of the rapid heat absorption from its walls by the water. The effective superheating area is therefore dependent upon and controllable by variation of the level of the cooling water in the cooling tubes. In the arrangement shown in Figure 1, the higher the level of the water in the cooling tubes, the less effective area has the superheating portion, for only those portions of the superheating tubes above the water function as superheaters. When the boiler is operating, the inflow of water to the cooling tubes may be so adjusted relatively to the rate of vaporization of the cooling water into steam, that the level of the cooling water may be maintained substantially constant at any desired level, and by thus controlling the superheating area, the temperature of the emanating steam may be maintained at any desired point. Likewise, by flooding the superheater tubes when the boiler is being started, the danger of injuring the superheater tubes during the period before saturated steam begins to pass through them may be eliminated, and the presence of sufficient water in the cooling tubes whenever the passage of steam through the superheater tubes falls off or ceases, protects them from harm by the heat which otherwise might not be carried away with sufficient rapidity.

In starting, the operator would ordinarily turn on the cooling water by hand, for which purpose the by-pass valve 40 is provided, (Figure 1). As soon as the steam started to flow normally, however, the operator would shut off the by pass valve, and the superheating action would commence as the water passed out of the cooling tubes. The flow of cooling water thereafter is preferably controlled by an automatic valve 40', across which the hand valve 40 is by-passed. The automatic valve 40' may be thermostatically controlled; a thermostat, which may be of any desired construction, being indicated at 38 in Figure 1. The valve 40' may thereby be governed in relation tq the temperature of the emanating steam, and arranged to open sufficiently to allow the entry of a desired quantity of water when the temperature of the superheated steam reaches a predetermined point, and to open and close proportionately as the temperature of the steam rises above or falls below the desired point. The temperature of the superheated steam emanating from the header may thus be controlled within close limits, as the thermostat may easily be adjusted to maintain the valve 40' open a sufficient amount, in relation to the temperature of the outgoing steam, to maintain the water in the cooling tubes constantly at the proper level to provide the proper superheating area necessary to produce superheated steam of the desired temperature.

What I claim is: 1. In combination with a furnace, a steam boiler, and a source of cooling water, a plurally chambered conduit structure for superheater installations comprising a steam chamber arranged at an angle to the horizontal within the furnace alternatedly spaced portions of whose outer wall are directly exposed to external radiant heating influence, and a plurality of longitudinally extending chambers for the conducting of a cooling medium arranged within the line of the outer surface of the conduit structure as a whole, with a portion of the walls of each directly superheated may be regulated.

2. A conduit assembly for water-cooled superheaters and the like comprising a relatively thinwalled steam conduit having longitudinally extending flutes therein, a cooling conduit having independent walls and projecting into each of said flutes to interflt with the steam conduit, and metallic heat bridging portions integral with each of said cooling conduits and with the steam conduit and arranged substantially at the surface of the latter.c 3. A conduit assembly for water-cooled superheaters and the like comprising a relatively thinwalled steam conduit having exposed surface portions and longitudinally extending flutes therein, a cooling conduit having independent walls projecting into each of said flutes to interflt with the steam conduit, and metallic heat bridging portions integral with each of said cooling conduits and with the steam conduit and arranged substantially at the exposed surface portions of the latter, the other interfitting portions of said steam and watei conduits being arranged adjacent each other but having distinct surfaces.

4. A tubular superheater construction comprising a steam conduit and abutting temperature control conduits spacedly arranged around and having wall portions separate from but interfitting with those of the steam conduit, and metallic heat bridging portions integral with the walls of the steam conduit and temperature control conduits and connecting said walls only near the exposed surfaces of the steam conduit but throughout substantially the entire active superheating length, inner parts of said interfitting wall por.tions being left unconnected.

5. A tubular superheater construction comprising a steam conduit, a plurality of separate temperature control conduits lying against spaced portions of the steam conduit, said temperature control conduits having walls abutting the steam conduit over a substantial area, and metallic Aeat bridging portions integral with the walls of both the temperature control and steam conduits throughout their entire effective length and directly connecting the former to the latter along substantially the entire length, said heat bridging portions being located at exposed areas directly subjectable to radiant heat.

6. A tubular superheater construction comprising an individually complete steam conduit, a plurality of an individually complete separate 05 temperature control conduits abutting the steam conduit, and metallic heat bridging portions integral with the wall portions of both the temperature control and steam conduits throughout substantially their entire effective length and directly connecting the former to the latter only .t exposed surface portions of said steam conduit directly subjectable to radiant heat.

7. In combination with a boiler and a source of heat, a superheater comprising a water . and steam tube assembly including a plurality of alternate steam and water conduit portions arranged around a container area and having alternated exposed outer faces, metallic heat-bridging means integral with all of said alternating exposed faces and completing a continuous outer wall lying outside said contained area, said alternated steam and water conduit portions also having abutting face portions lying within the area bounded by and unconnected by said heat-bridging means, whereby the schism between said abutting unconnected portions resists direct heat transfer between them while the bridging means assists direct heat absorption from the outer wall by both the steam and water conduit portions. 8. In combination with a furnace, a steam space, and a cooling water supply source, a tubular superheater construction arranged within the furnace and comprising a steam conduit, a plurality of separate temperature control conduits abutting the steam conduit, certain of said abutting portions being unconnected, and metallic heat bridging portions integral with the wall portions of both the temperature control and steam conduits and directly connecting the former to the latter only at the exposed surface of said steam conduit, whereby heat interchange between one conduit and the exposed wall portions of the abutting conduit is facilitated, while the schism between the unconnected abutting wall portions between the edges thereof restricts heat interchange therebetween.

9. In combination with 'a furnace, a steam space, and a cooling water supply source, a superheater comprising a water and steam tube 35r assembly including a plurality of conduit portions arranged around a contained area, said conduit portions having individual walls and each enclosing a longitudinal channel, and metallic bridging means integral with all of said conduit portions throughout substantially their entire effective length and cooperating to form a portion of a continuous outer wall as well as to assist in the conduction of heat from the outer wall to the contained area, said contained area being distinct from said longitudinal channels contained within said conduit portions and forming an additional channel.

10. In a fluid-cooled superheater construction, in combination with a furnace, a conduit assembly arranged at an angle to the horizontal within the furnace and comprising exposed wall portions defining a passage for steam to be superheated and separate wall portions defining a passage for fluid for temperature control, metallic heat bridging portions connecting and integral with both such wall portions and arranged substantially at said wall portions exposed to direct radiant heat impingement, means for supplying cooling fluid to the passage for temperature control fluid, and means responsive to the condition of the superheated steam and arranged to regulate the admission of such fluid to said passage to hold the same at a level within the limits of said exposed wall portion, whereby the amount of heat which may thereby be conducted away from the exposed wall portions defining the passage for steam to be superheated may be regulated.

11. In combination with a furnace, a boiler, a steam space and a source of cooling water supply, a superheater comprising a water and steam tube assembly including alternate steam and water conduit portions, the latter being connected to said cooling water supply source at the lower end and to said steam space at the upper end, said portions being arranged around a contained area and having alternated abutting faces exposed to radiant heat within the furnace, and metallic heat bridging means integral with both and connecting the alternated heat-exposed face portions of the steam and water conduit portions throughout substantially their entire effective length.

12. In combination with a steam space and a water supply source, a superheater comprising a water and steam tube assembly, the water tube portions of which are connected at the lower end to said water supply source and at the other end to said steam space, said assembly including a plurality of conduit portions arranged in abutting relation around a contained area, said conduit portions having individual walls and each enclosing a longitudinal channel, and the contained area defining another channel, and metallic bridging means integral with all of said conduit portions throughout substantially their entire effective length and cooperating to form a portion of a continuous outer wall as well as to assist in the conduction of heat from the outer wall to the contained area, said contained area being distinct from said longitudinal channels contained within said conduit portions and forming an additional channel.

13. In combination with a steam space and a water supply source, a superheater comprising a water and steam tube assembly the water tube portions of which are connected at the lower end to said water supply source and at the other end to said steam space, said assembly including a plurality of alternated conduit portions arranged around a contained area, certain of said conduit portions having individual wall sections enclosing isolated longitudinal channels, heat bridging weld portions integral with each of said conduit portions and an adjoining conduit portion and completing an unbroken heat bridge between such adjoining conduit portions throughout substantially their entire effective length.

14. In combination with a steam space and a water supply source, a superheater comprising a water and steam tube assembly the water tube portions of which are connected at the lower end to said water supply source and at the other end to said steam space, said assembly including a plurality of alternated conduit portions arranged around a contained area, certain of said conduit portions having individual wall sections enclosing isolated longitudinal channels, heat bridging weld portions integral with each of said conduit portions and an adjoining conduit portion and completing an unbroken heat bridge between such adjoining conduit portions throughout substantially their entire effective length and between the outside of the assembly and the contained area.

15. In combination with a furnace including a heating chamber, superheating and superheater controlling means including a unitary plurally sectioned metallic superheater tube assembly comprising independent steam and cooling fluid passages, said assembly having an outer wall exposed to direct radiation within the furnace, and said cooling fluid passages being arranged upon said outer wall exposed to direct radiation, said steam and cooling fluid passages being in direct metallic heat-conductive communication with each other and with the outer wall throughout substantially their entire effective length, and all having active portions within the heating chamber, said assembly being arranged at an angle to the horizontal and having separate inlets for steam and cooling fluid, a source of cooling fluid supply connected to the cooling fluid inlet of said assembly, and regulatable means responsive to the temperature of the superheated steam and arranged to hold the cooling fluid in the assembly at a level within the. zone of heating, comprising a throttle valve arranged between said cooling fluid passages and said inlet thereto for controlling the cooling fluid supply to said inlet. 16. In combination with a furnace and a steam boiler, a water regulated tubular superheater assembly exposed to direct radiation within the furnace and comprising a plurality of longitudinal generally parallel cooling fluid conduits arranged around a similarly extending contained area and having component wall portions exposed to radiation within the furnace, and a steam conduit portion lying at least partially within such contained area and metallically integral with said cooling conduits throughout substantially their entire effective heat-absorbing length.

17. In combination with a furnace and a steam boiler, a tubular water-cooled superheater assembly extending through a portion of the furnace at an angle to the horizontal and spaced from the furnace walls, said superheater assembly comprising a metallically continuous outer wall exposed to direct radiation within the furnace and a plurality of wall portions therewithin defining conduits, at least one of said conduits being for steam while a plurality are for cooling water, said cooling fluid conduits being arranged at said outer wall exposed to direct radiation, and a metallic heat bridging portion integral with said exposed outer wall portion and integral with wall portions defining both the steam and water conduits throughout substantially their entire effective length.

18. The method of controlling superheating action in a superheater structure consisting of separate metallic superheater and cooling water tubes arranged at an angle to the horizontal within a zone of heating and in heat interchanging relation to each other throughout substantially their entire effective heat-absorbing length, which comprises supplying steam and water to the respective tubes, throttling the water supply below the evaporative capacity of the tubes whereby the water will assume a level within the tubes, and raising or lowering such level in accordance with increase or decrease respectively in the temperature of the superheated steam.

19. The method of controlling superheater action in a superheater structure consisting of separate metallic superheating and cooling water tubes arranged at an angle to the horizontal within a zone of heating and in heat interchanging relation to each other throughout substantially their entire effective heat-absorbing length, which comprises supplying steam and water to the respective tubes, throttling the wateri supply below the evaporative capacity of the tubes sufficiently to cause the water to assume a level within the tubes, and controllingly varying such throttling of the water supply relatively to the rate of heat impingment to maintain the water in the cooling water tubes at a level conforming to and adapted to delimit the degree of superheating of steam within the superheater tubes. 20. In combination with a furnace, a steam boiler, and a source of cooling water, a superheater element comprising a structurally unitary tube arranged at an angle to the horizontal and having an outer wall exposed to direct radiation within the furnace, including a plurality of longitudinally extending cooling chambers connected to said source of cooling water, and an independent steam passage having spacedly separated portions of its wall exposed to external heat influence, between which exposed wall portions the longitudinally extending cooling chambers are peripherally arranged, the wall portions of said cooling chambers being arranged at and directly connected to said external wall exposed to direct radiation within the furnace for direct heatingabsorption therefrom, said wall portions of the cooling chambers cooperating with the spaced wall portions of the steam passage in constituting 16 the outer conduit wall.

21. The method of controlling superheater action in a superheater structure consisting of separate metallic superheater and cooling water tubes arranged at an angle to the horizontal within a zone of heating and in heat interchanging relation to each other throughout substantially their entire effective heat absorbing length, which comprises supplying steam and water to the respective tubes, directly throttling the inlet of the water supply below the evaporative capacity of the tubes sufciently to cause the water to assume a gravity induced level within the tubes, and raising or lowering such level to vary the effective superheating area and so control the temperature of the superheated steam.