US2202835A

US2202835A - Mercury boiler

Info

Publication number: US2202835A
Application number: US225464A
Authority: US
Inventors: Jr Bevis P Coulson
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1938-08-17
Filing date: 1938-08-17
Publication date: 1940-06-04
Anticipated expiration: 1957-06-04

Description

June 4, 1940. a R N, R 2,202,835

MERCURY BOILER Filed Aug. 17, less 2 Sheets-Sheet 1 I 4/ VARIABLE SPEED MOTOR Inventor": BGViSPCOLLISQnJP,

His Att Carney.

June 4, 1940. B. P. coULsoN, JR

MERCURY BOILER Filed Au 17. 1938 2 sheets-sheet? Fig.2.

Patented June 4, 1940 p UNITED STATES PATENT OFFICE MERCURY BOILER Bevis P. Ooulson, Jr., Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 17, 1938, Serial No. 225,464

I 31 Claims. (Cl. 122-235) The present invention is a continuation in part groups. In the present instance each group conof my application Serial #54275 filed on Decemsists of three twisted tubes bent inwardly, that is,

her 13, 1935, and relates to mercury boilers havtowards the center of the drum IS with the ing containers for mercury liquid to be heated upper ends 25 uniformly spaced apart and conand evaporated. nected to the bottom of the drum. The groups The general object of my invention is to proor ropes of twisted tubes are a convenient arvide an improved construction and arrangement rangement for holding the tubes together and of such boilers whereby the amount of mercury also assure uniform heat delivery to the tubes liquid per kilowatt output necessary to operate of each group. In addition, the arrangement lo the boiler is considerably reduced. also reduced the delivery of heat by radiation For a consideration of other objects of my and convection to these groups due to mutual invention and of What I consider to be novel, shielding- Mutual Shielding and reduced heat attention is directed to the following description delivery to these ropes or groups is important and the claims appended thereto in connection because the tube portions forming said ropes are H with the accompanying drawings. disposed in a most vulnerable position.

In the drawings, Fig. 1 represents a perspective, The lower ends of the tubes 23 project through somewhat simplified View of a mercury boiler in the walls It] and II of the furnace casin and accordance with my invention and Fig. 2 shows a are connected to a plurality of independent headmodification of my invention. ers 25. Mercury liquid is conducted to these in- Qo The arrangement has a casing with front and dependent or lower headers 26 by means includrear walls It! and side walls II. A lower section irlg a co d W HbB Connected o the formed by these walls rests on the foundation. outer end of the drum l9 and having two An upper section is supported by beams l2 and

branches

28 and 29 which in turn are connected l3 and an intermediate section forms sliding to subbranches 30 for conducting mercury liquid 1:; joints l4 and IS with the lower and upper secto the headers 26. To reduce the liquid space tions respectively. The intermediate section is in the drum l9 and the headers 20 and 2|, disyieldably supported by means of supporting placing blocks 3| are provided therein. hangers is held on fixed T beamsll and com- During operation, mercury liquid is conducted prising yieldable spring means H8. The lower from the drum and through the down-tubes 21,

no ends of the supporting hangers l6 are connected 28, 29, to the lower headers 26, whence the 30 to a horizontal drum I9 and front and rear headmercury flows into the tubes 23 which may be ers 20, 2| communicating with the drum. The termed up-tubes. In these up-tubes the mercury ends of the drum l9, as clearly shown in the is heated and partly evaporated, the mixture drawings, project through the walls of the inof mercury liquid and vapor being discharged termediate section and the headers 20, 2| cominto the drum l9 and the headers 20, whence municate with the drum and are disposed at the vapor is conducted in known manner to turleast partly outside the furnace walls. The lower bines or like apparatus, not shown, and the liquid casing section and the lower part of the interis recirculated through the down-tubes. The cirmediate section form a combustion chamber 22 culation in these parts of the boiler takes place which is substantially rectangular in cross secby gravity and is known as thermal or natural 40 tion. Fuel and air are supplied to the combuscirculation. The mercury liquid in the downtion chamber by means, not shown, in order to tubes having greater specific weight than the maintain combustion therein. The walls of the mixture of mercury liquid and vapor in the upcombustion chamber are lined by rows of tubes tubes causes such mixture to flow upward, thereor heating elements 23. The tubes lining the by effecting circulation. front and the rear wall have upper ends 24 The upper part of the boiler forms a convecslightly bent outward and connected to the headtion chamber or space in which the available ers 20, 2!. The tubes lining the side walls have heat energy of the combustion gases discharged upper ends 25 connected to the bottom of the from the combustion chamber is utilized to heat to drum l9. Slag screens are provided in the upper and evaporate mercury conducted to mercury part of the combustion chamber which screens heating elements which receive mercury liquid in accordance with my invention are formed by partly from the drum l9 and partly from an exintermediate portions of the heating tubes linternal source such as a mercury condenser or ing the side walls of the combustion chamber or condenser boiler usually provided in a mercury furnace. To this end these tubes are twisted in power plant. The heating elements provided 55 above the combustion chamber include in the present instance four banks of tubes with regard to each side of the boiler. These tubes are arranged symmetrically with respect to a vertical plane through the center of the drum is and are located at least partly below the cold liquid level in the drum. An upper or first bank of tubes 32 forms a mercury preheater. Mercury liquid is conducted thereto by a conduit 33 from a condenser or like source. The conduit 33 is connected to a header 3 external the side wall of the boiler and having connections 35 to the upper or first bank of tubes. Each tube of t is bank has a number of hairpin bends to form a grid. The other ends of the tubes are connected to a plurality of separate headers 35 external the side walls H. The arrangement may be such that the tubes 32 are alternately connected to headers 35 on one side wall and to corresponding headers on the other side wall. The mercury preheated in the first bank of tubes is conducted from the headers 36 through tubes 3?, 38 into a tube or conduit 39 connected to the inlet of a circulating pump M. The conduit 39 also receives mercury from the drum :9 of the boiler. The pump 1%] is driven by a variable speed motor ii and discharges through a conduit 4! into a header 2-2, which latter extends along the side walls of the boiler and conducts mercury liquid to a second bank of tubes 53 and a third bank of tubes 4 on each side of the boiler. The second bank of tubes 43 is disposed above and adjacent the roped portions of the tubes lining the side wall. Each tube of the second bank has a number of hairpin bends forming a grid with an inlet portion it connected to a header Alli, which latter receives mercury liquid from the conduit 62 through a conduit ll. The third bank of tubes it is similar in design and arranged above the second bank of tubes The individual tubes of the third bank have inlets connected external the side walls to individual headers 48, which latter receive mercury liquid from the conduit 52 through conduits 39.

The second and third banks of tubes, as explained above, have inlets connected in parallel to the conduit 52. The outlets of the second and third banks of tubes are connected to the inlets of a fourth bank of tubes 5%. This fourth bank of tubes comprises a plurality of rows of larger hairpin-bent tubes disposed above the drum and extending across the convection chamber. In the present instance the first tube of the fourth bank has an inlet 5! which is connected to the first and second tube of the second bank Similarly, the first and second tubes of the third bank 44 have outlets connected to the inlet of another tube of the fourth bank. Preferably the inlets of the tubes of the fourth bank are alternately connected to pairs of tubes of the second and third bank on one side of the boiler and to pairs of tubes of the second and third bank on the other side of the boiler. The individual tubes of the fourth bank form grids similar to those of the other banks and the outlets 52 of the tubes 56 of the fourth bank are connected to the drum i9. Secured to these outlets near their connections with the drum if! is a baffle of heat insulating material 53-. The baflle projects downward and prevents short circuiting of the second and third bank of tubes, or from another viewpoint, assures the flow of combustion gases from the combustion chamber through said second and third bank of tubes. As indicated on the clrawingathe tubes of the fourth bankhave a large diameter as compared with that of the second and third bank to provide for sufiicient space for the vapor formed in the tubes of the fourth bank.

It is important to note that the discharge ends of the bank of tubes i3 and 44 are connected to the upper end of the tubes 56 while the lower or discharge ends of the tubes 50 are connected to the drum l9. With this arrangement the flow of mercury through the fourth bank of tubes which may be termed inverted convection heating tubes has a general downward direction opposite to that of the direction of flow of heating gases passing through the spaces formed between said tubes. As the inverted convection surface is installed above the boiler drum, and as this surface is inverted, the mercury there n, when operating, will drain into the drum, under all conditions, when the unit is shut down. Mercury is expelled from the furnace Wall tubes by vapor formation and by expansion, and this expelled mercury is, 3

therefore, available for the inverted convection surface when the boiler is in operation. hus no extra mercury is needed for this heating surface. When the unit is started there will be only a small amount of heat absorbed by the infor the mercury to flow through the inverted convection tubes as fast as possible so that the tubes will hold the least quantity of mercury during the starting cycle.

To cause a rapid liquid flow, the direction of flow should be downwards through the length of g the tubes. The tubes should, therefore, be sloping downward. Moreover, when vapor is formed in the inverted convection surface, the downward flow of the mixture will be increased in velocity.

hen a mixture of liquid and vapor passes through a tube, the liquid has a tendency to remain behind the vapor. In other words, the vapor passes through the tube at a higher velocity than the liquid, to the effect that the tube contains more liquid than would be the case if such cury. The tendency to slip reaches a maximum in case the fluid flow is in a vertical, upward direction. Therefore, in order to reduce the density of the mixture in the inverted convection tubes, mercury is supplied to the top end of these tubes and flows in a downward direction.

Another advantage for the downward flow of the mixture in the inverted convection tubes is the matter of heat transfer from the tube to the mercury. The rate of heat transfer is best when the friction pressure drop of the mixture is high. As the mixture flows downward through the inverted convection tubes, vapor is formed due to release of pressure and also due to the heating gases passing upward through the spaces formed between the tubes. The formation of vapor along the length of the tubes increases the velocity of the mixture and, although the density will thereby be reduced, the friction pressure drop will be much increased. Thus the best heat are provided for supporting the tubes.

transfer is in the region of the hottest flue gases. As the specific heat of mercury vapor is about one-twentieth that of steam, the heat transfer from a tube to dry vapor is very poor." It is important, therefore, to circulate through the inverted convection tubes sufficient liquid with the vapor to always maintain a wetted surface of the steel. The minimum circulation ratio (i. e. the quantity of liquid by weight to that of vapor) shouldinotbe less than 5 to 1. To give a high heat rate it is, therefore, necessary always to circulate through the inverted convection tubes mercury liquid with the vapor.

Another important advantage for the downward flow of the mixture in the inverted convectiontubes as against an upward flow in such a bank of tubes is the matter of pressure drop across such tubes. Inthe case of downward flow the risers supplying mercury to the top of such a bank of tubes can be straight and short, and therefore they can be relatively small in cross section. In such a tube the vapor velocity can be relatively high and, therefore, the pumping of the liquid by the vapor will be possible with less slip of the liquid than would be the case in a large tube with a reduced vapor velocity. In the case of the upward flow in a convection bank of tubes, the path of the fluid in the upward direction would have many'bends and changes in direction, and it would be a relatively long tube. The pressure drop, therefore, would be much greater than in the former case. If, however, to decrease the pressure drop the tubes were made larger, the velocity of the mixture would be less, thus increasing the slip and decreasing the quantity of liquid pumped through the tubes. In the case of downward flow, the tube can be large and this is important in the design of the inverted convection surface.

During operation, mercury is forced by the circulating pump it through the second and third banks of tubes 43 and 44, whence it flows into the fourth bank of tubes 50 in which it is partly evaporated, the mixture being discharged into the drum I 8. Thus, the second, third and fourth bank of tubes, together with the drum, form a circuit in which the parallel connected second and third banks of tubes are connected in series between the drum and the fourth bank of tubes. As stated above, all these tubes are supported on the intermediate section of the casing which in turn is supported through the intermediary of the drum by yieldable supporting holders l5, l8. In addition, a plurality of supporting rods 54 held on a top plate 55 of the casing through the intermediary of springs 56 The connection between the supporting rods 54 and the tubes of the fourth bank is accomplished by a plate 5'! secured to the lower ends of the rods 54 and engaging the hairpin-bent inlet portions of the fourth bank of tubes. In order to protect the rods 54 against the heat of the combustion gases, some of the tubes of the first bank 32 are provided with U-shaped portions 58 adjacent the rods 54. These U-shaped portions, together with the adjacent rods 54 are covered by lagging or heat insulating material 59. In this manner the rods 54 are protected against excessive heat of the combustion gases. The support of the second and third bank by the rods 54 is accomplished indirectly in that the outlets of the tubes of the second and third bank are connected to the inlets of the fourth bank, which inlets are directly held on the plates 51. The upper section of the casing forms a discharge channel 60 for gases discharged from the boiler.

The arrangement of the heating elements in the boiler forms two separate circuits, one circuit including the tubes 23 in the combustion chamber, which tubes form wall tubes, the mercury being conducted through these tubes by natural, gravity, or thermal circulation, and a second circuit formed by the tubes of the second, third and fourth bank through which mercury is forced by the action of the circulating pump 40. Thus, the boiler represents a combination of forced and natural circulation boilers permitting the utilization of the best features of the two kinds of boilers, to the effect that the power necessary for circulation and the amount of mercury required for operation of the boiler may be kept at a minimum. The roped type slag screen provided at the top of the combustion chamber and formed in accordance with my invention by the wall tubes forms a means of collecting slag and causes uniform heat transmission to the tubes, and provides for sufiicient passages connnecting the combustion chamber and the convection chamber.

The grids of the hairpin-bent tubes for the second, third and fourth bank of tubes are simple in design. They may be manufactured at comparatively low cost and thus constitute an inexpensive convection surface for heating mercury forced therethrough. The mercury in its flow through said convection surface is at first passed through the second and third banks of tubes which are nearest the combustion chamber and therefore exposed to the hottest flue gases, the mercury in these tubes is liquid permitting high heat transfer. As'stated above, the vapor is formed in the fourth bank of tubes 50 through which the mercury passes in counterflow. The upper ends of said tubes form the inlets and are disposed in the coolest region of the flue gases.

design the amount of mercury necessary per kilowatt may be reduced by more than 50% as compared with boilers having tubes substantially filled with mercury liquid and embodying either the natural flow or the forced circulation principle. As during operation the wall tubes in the combustion chamber become gradually covered with slag, the relative heat transfer to the convection surfaces formed by the second, third and fourth row of tubes increases. The combustion gases leaving the combustion chamber through the spaces defined between adjacent ropes of tubes are first passed through the second and third banks of small sized tubes containing liquid mercury and then through the fourth bank of larger sized tubes containing both liquid and vapor or fog. The gases leaving the fourth bank of tubes pass through the preheater 32, whence they are discharged through the flue channel 6%. All the mercury supplied to the boiler from a condenser or like source first flows through the preheater 32 and then through the .convection surfaces defined by the second, third and fourth banks of tubes whence the mixture of liquid and vapor flows into the drum l9. The mercury vapor is discharged from the drum l9 and the liquid iii third bank without forming vapor.

of the tubes is about 13:1.

discharged from the-convection tubes-into the drum is recirculated through the wall tubes in the combustion chamber or the convection tubes.

It is known that a tube filled'with mercury liquid may be exposed to more intense heat than one filled with a mixture of mercury liquid and vapor. Therefore, to assure high heat transfer to the second and third banks of tubes, the mercury therein is subjected to high pressure to prevent evaporation thereof. The fluid in the second and third banks of tubes is subjected to the static head'of the fluid in the fourth bank of tubes which is located at a higher level and also to the pressure drop across the fourth bank of iubes. Thus, a large amount of heat can be absorbed by the liquid mercury in the second and The circulating pump ill forces mercury liquid through the second and third bank of tubes against a pressure considerably greater than the drum pressure. This greater pressure, as just pointed out, is due to the static head of the mercury and the pressure drop across the fourth bank of tubes 50. As stated before, the mercury pump 40 receives mercury liquid from both the preheater 32 and the drum iii. In a preferred embodiment, the arrangement is such that during normal operation about six times as much mercury is taken from the drum than is received from the preheater. This means that about seven times the amount of mercury fed to the boiler is circulated through the convection surface tubes. Thus, if the convection surface tubes generate 50% of the total vapor, the ratio of liquid to vapor at the exit end This high ratio permits high heat transfer to the convection surface tubes.

By way of example, in such a system the circulating pump may have a suction pressure of 155 lbs. and a discharge pressure of 510 lbs., the diffe ence being 355 lbs. per square inch. Of this differ encc, 1.59 lbs. may be used to overcome the static head and across the tubes of banks 2 and 3 which are in parallel to give high velocity and make possible reduced tube size with resultant small mercury volume. The rest of the pressure drop. l95'lbs, is used across the fourth bank and provides the pressure being released throughout 1e tube length of the fourth bank, and the pressure to produce high velocity of flow of the mixture of liquid and vapor. Thus, with the pressure of 155 lbs. per square inch. in the drum, the pressure at the entrance of the fourth bank of large tubes in the present example is 195+155 350 lbs. per square inch. At such a pressure the mercury liquid can be heated to about ll F. before vapor formed. If, as pointed out above, the circulating pump forces seven times the feed to the convection surface tubes, the heat absorbed by the liquid is that required to heat this quantity of mercury from 975 F. (which is the temperature of the mercury in the drum at the aforementioned pressure of 155 lbs. in the drum) to ll20 F. The pressure to which the mercury is subjectcd in the fourth bank of tubes is reduced as the mercury passes therethrough, the pressure being at the inlet 350 lbs. and at the outlet 155 lbs. Thus, vapor is formed throughout the length of these tubes due to the decrease in pressure and also due to heat supplied to the tubes.

The ratio of liquid to vapor at the exit of the convection surface tubes, as stated above, is preferably of the order of 13:1, permitting high heat transfer. With regard to the radiation surface tubes, that is, the wall tubes exposed to the radiation in the combustion chamber, said ratio is higher, preferably of the order of :1. The location of the fourth bank of tubes above the drum and the pump also permits a saving of mercury in that the mercruy liquid contained in the tubes of the fourth bank is discharged into the drum l9 as the boiler is shut down and thus may be used to start the boiler. After starting of the boiler the displacement of mercury by vapor in thefurhace wall tubes, due to expansion and evaporation of the mercury liquid therein, is more than enough to supply the mixture of vapor and mercury in the large convection tubes and the additional mercury'in the drum during operation.

The combined natural and forced circulation type boiler according to my invention includes only a single drum. This is possible due to the length of the convection surface tubes which greatly reduces the number of tubes to be connected to the drum. As the tubes are of considerable length, only comparatively few of them are needed. Therefore, only few holes are required in the drum for connection to the con vection surface tubes. By way of comparison. in a boiler of the same capacity based on a different design having complete thermal. circulation, two drums of larger diameter are required due to the greater number of shorter tubes. The provision of only one drum reduces the manufacturing cost because the assembly of a few longv tubes with a drum is less expensive than that of many short tubes with a drum. A single drum also reduces the amount of mercury required for operating the boiler. The reduction in the required amount of mercury is not only due to the fact that two or more drums provide additional space which has to be filled with mercury but also to the fact that the mercury disposed by vapor fromthe wall tubes raises the liquid level to a higher level in the one drum than would be the case if two or more drums were required.

In order to protect the ferrous metal of the mercury heating elements against the dissolving action by the mercury at boiler operating temrperature and also to assure clean surfaces of the heating elements in contact with the mercury, agents may be added to the mercury liquid.

Such agents are disclosed in the patent to A. J.

40 will assure uniformity of the mixture through out the system by effectively mixing the substances added to the mercury.

The circulating pump speed may be controlled in response to changes in load to vary the amount of mercury recirculated through the convection tubes. Advantage may be taken of the variable pump speed to control the circulation of mercury in the. convection tubes so as to maintain the desired pressure distribution in the different banks of tubes at different loads. The pressure created by the circulation pump 49, as stated be fore, is segregated into two pressures, one for forcing the mercury through the second and third banks of tubes 43 and M, and the pressure for forcing the mercury through the fourth bank of tubes 58. This makes it possible to add inexpensive additional tube surface in the cooler region of the flue gases in the form of large tubes requiring little mercury. The surfaces of these tubes forming the fourth bank are well protected from high temperature flue gases because first the flue gases before reaching the fourth bank have been cooled by the small tubes; second, the feeding of each large tube of the fourth bank is assured by connecting their inlet to two small tubes in parallel; third, the velocity of how through the large tubes of the fourth bank is increased by the formation of mercury vapor which takes place substantially uniformly throughout the entire length of said tubes; and finally, the slag screen formed at the top of the combustion chamber and the small tubes cool the flue gases sufiiciently to substantially eliminate the deposit of slag on the large tubes of the fourth bank.

In order to reduce the stresses and particularly the bending stresses set up in. the upper portions of the groups or ropes of twisted tubes, these ropes may be arranged in the form of catenaries, that is, so that each rope is curved downward from a straight line between the points of support. An arrangement of this kind is shown in Fig. 2.

The arrangement of Fig. 2 comprises lower walls 'lI, intermediate walls 12 and upper walls 13 forming combustion and convection chambers. The intermediate wall 12 forms sliding joints M and 15 with the lower and upper walls II and 13 respectively. The support for the boiler includes vertical post 18' and horizontal beams ll, the intermediate wall 12 being flexibly supported on the latter by means including springs 18. A drum or vessel 19 in this arrangement is centrally disposed at an intermediate point of the combustion chamber, thus dividing the latter into a primary combustion space 88 below the drum I9 and a secondary combustion space 8I above the drum I9. The convection chamber is formed by an upper portion of the intermediate wall I2 which is reduced in cross section as compared to the cross section of the combustion chamber. The heating elements in the combustion chamber, more specifically in the primary combustion chamber below the drum It comprises

heating tubes

82 and 83 lining the rear wall of the chamber and having upper ends connected to

headers

84 and 85 respectively, which latter conduct heated mercury discharged from the

tubes

82, 83 to the drum 19. Mercury liquid is conducted to the lower ends of the

tubes

82, 83 by means of downcomers 86 corresponding to the downcomers 27, 29, 30 of Fig. 1. Lower portions of the side walls in the combustion chamber are lined by

heating tubes

81 and 88 which have lower ends connected to the downcomers 88 external the combustion chamber. Intermediate portions of the side well heating tubes form groups, each groups including several tubes twisted to form a rope. The several ropes thus formed are arranged in staggered relation on opposite sides of the combustion chamber, thus forming large spaces to facilitate the flow of combustion gases from the primary to the secondary combustion chamber. The upper or end portions of the twisted tubes 89 are aligned to form protecting walls 98, 9| below the drum 19,

thus defining a housing 92 below the bottom of the drum 19 to protect the latter from radiation in the combustion space. In the present instance the housing 92 includes walls 93 of heatinsulating material below the drum l9 and covering the inner surfaces of the Walls of heating tubes 98, 9!. The upper ends of these heating tubes are connected to the drum 19.

The arrangement so far described comprises two parallel fluid circuits through which circulation of fluid takes place by gravity during operation. Both circuits are supplied at their lower ends by the downcomers 88, and the first circuit including the

tubes

82, 83 lining the rear wall of the furnace (the front wall tubes, not shown, may be similar) and the other circuit including the tubes Ill which have lower portions lining the wall, intermediate portions forming ropes 89 and upper or end portions forming protecting walls 98. The circulation of fluid in other heating elements contained in the combustion and also in the convection chamber is effected by means of

pumps

94, 85 arranged on the same shaft and driven by an electric motor 95. The speed of the motor 96 may be varied in response to changes of operating conditions. In the present instance the electric circuit 97 of the motor includes a resisting element 98 which may be short-circuited by a contact-making member 98 connected to a device I80, which latter is responsive to pressure changes in the header 84. Increasing pressure in the header 84 causes downward movement of the contact-making member 88 whereby the resistance element 98 is inserted in the circuit of the motor and the speed of the latter is reduced, thus effecting reduced circulation of fluid through certain elements of the boiler, as will be described presently. In other cases it may be desirable to provide a pressuretemperature differential responsive device for controlling the operation of the circulating pump in response to changes of a condition in the boiler. Such an arrangement is disclosed in the Patent 2,088,623 to E. S. Thompson issued on August 3, 1937, and assigned to the same assignee as the present application.

The two pumps 94, 95 are connected in series.

The pump 98 has an inlet conduit Illl for receiving fluid from a condenser or other suitable source, not shown. The pump 94 has a discharge conduit i82 connected to the inlet of the pump 95. The conduit I02 in addition receives mercury liquid from the drum 19 through a con duit I83. The pump 95 has a discharge connected by conduits I84 and I85 to headers I188 and I0! respectively, located on opposite sides of the boiler. From the headers I88 and it! the mercury is forced downward into tubes I88 and I89 lining the opposite side walls of the boiler, more particularly the portions of the side walls above the aforementioned tubes 8'1. The lower ends of the tubes I88, I89 are looped and connected to the lower ends of catenary-shaped rows of tubes H8 and III. In the present instance I have shown two ropes III] and III respectively, symmetrically arranged with respect to the center of the combustion chamber. The tubes I II), II I together with the aforementioned roped tubes 89 form slag screens in the combustion chamber. The tubes are arranged in staggered relation to form wide spaces between them so that slag will not bridge the tubes at any load. The spacing of the slag screen thus formed is very important. While the tubes should be sufficiently far apart to prevent slag from bridging the gap, they should also form a shield to prevent overheating of the heating elements in the convection space by radiation in the combustion space. By staggering the tubes they may be spaced far apart and still constitute an effective shield against radiant heat. Slag screens increase greatly the radiant heat-absorbing surfaces, thus generating a large amount of vapor and at the same time they are self-protecting against flame impingement by slag formation.

The upper ends of the roped tube portions I I9, I l I are alined to form two walls of heating tubes H2 and IE3 symmetrically arranged above the drum l9 and somewhat inclined towards the center of the secondary combustion chamber. A housing lid of heat-resistant material is fromed above the drum E9 in the space between the walls of heating tubes H2, H3, the latter thus lining the outer surface or side walls of the housing i M. This housing protects the top of the drum, and vapor discharge conduits I I5 and H6 connected to the drum 7%. The upper ends of the walls of protecting heating tubes H2 and Ill; are connected to the upper ends of banks of inverted convection heating tubes IiI located in the convection chamber of the boiler. Each inverted convection heating tube has a plurality of individual hairpin-shaped portions. The lower ends of some of the inverted convection heating tubes are connected to upper ends of heating tubes i it and i I9 lining the side walls in the secondary combustion chamber and having lower or end portions 526 which are in the form of catenaries and connected to discharge heated mercury into the drum 1.). The lower ends of others of the inverted convection heating tubes are connected to the upper ends of down-tubes $22 and 23 lining the rear wall of the secondary combustion chamber and having lower ends connected to the headers 83 and respectively. Thus the heating elements in the secondary combustion chamber and in the convection chamber comprise several fluid circuits through which mercury is forced from the headers HIS and lfil. I'he circuits discharge mixtures of liquid and vapor into the drum l9 and the headers 8d, 85. One of these circuits includes side wall tubes 5'38 in the primary combustion chamber, roped tubes H 3, protecting wall tubes H2, inverted convection heating tubes H1 in the convection chamber and wall heating down-tubes 22 in the secondary combustion chamber. With the combined pressure of the circulating pump and the static head, it is possible for the mercury flowing downward in the wall tubes I98 of the primary furnace to receive a large amount of radiant heat from the furnace without vaporizing. vaporization takes place in the up-flow tubes II 2 of the secondary furnace due to release of pressure incident to release of static head and also in the bank of inverted convection tubes I IT.

The drum 79 has a bafiie I25 which is concentrically spaced from the walls of the drum to form a channel H26. All of the tubes 9i], 9! and I26 are connected to the drum 79 to discharge heated fluid into said channel I228. The vapor discharge conduits H5, H6 are connected to the space formed inside the bafile I25. During operation the bafile I25 acts as a liquid vapor separating device, the liquid being conducted to the bottom of the drum where a pool is formed and the vapor being discharged to the conduits H5 and H6. The boiler has a bottom I27 with cooling tubes I28.

The ash and dust contained in the gases and not caught by the screen do not receive radiant heat from the primary furnace on their passage through the secondary furnace and through the convection chamber and therefore they are kept at low temperature, that is, at the temperature of the flue gases in the convection chamber. Hence, the ash being cool is dry when entering the convection chamber so that little slag will accumulate on the inverted convection heating tubes. A large portion of ash and dust is caught by the furnace slag screen and either flows 01' drops to the slag pool on the bottom 1'2! of the combustion chamber. Thus the convection surfaces in the convection chamber require less cleaning and less fly-ash leaves the stack. While flowing through a plurality of staggered, horizon tally and vertically spaced rows of furnace slag screens the gases become well mixed and the combastion is completed in the secondary furnace.

Having described the method of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof, I desire to have it understood that the apparatus shown is only illustrative that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A mercury boiler including the combination g of a drum, a combustion chamber below the level of the drum, a plurality of heating elements disposed in the combustion chamber and having up per ends connected to the drum, conduit means for conducting mercury liquid from the drum to the lower ends of the heating elements, the arrangement being such that circulation through the heating elements takes place by gravity tion, other heating elements having e; u. connected to the drum and being exposed only to combustion gases discharged from the combustion chamber, and means including a pump and a heating element located below the drum for forcing mercury into the upper ends of the other heating elements.

2. A mercury boiler including the combination of a casing defining a combustion chamber, a drum disposed above the combustion chamber and projecting through the casing, a plurality of tubes lining the inner surfaces of the combustion chamber and connected to the drum, upper portions of at least some of said tubes being twisted in groups to define ropes so that the twisted tubes of each group mutually support each other and effect uniform heat supply to the different tubes and also provide for sufficient spaces intermediate roped groups for the discharge of gases from the combustion chamber.

3. A mercury boiler including the combination of a casing defining a combustion chamber, drum disposed above the combustion chamber. a plurality of tubes lining the inner surfaces of the chamber and connected to the drum, upper portions of said tubes being twisted in groups to define ropes so that the twisted tubes of each group mutually support each other and effect uniform heat supply to the different tubes and also provide for sufficient spaces intermediate roped groups for the discharge of gases from the combustion chamber,a space formed by the casing above the combustion chamber for receiving conibustion gases from the latter, banks of tubes din posed in said space and connected to t'-:.e drum. and pumping means for effecting circulation of mercury from the drum through said banks into the drum.

l. A mercury boiler including the combination of a casing defining a combustion chamber, a drum above the combustion chamber, heating elements disposed within the combustion chamber and connected to the drum whereby heat is transferred to said elements in the form of radiation, a space defined by the casing above the combustion chamber for receiving combustion gases from the latter, a first bank of tubes disposed in the all) upper portion of said space, means for conducting mercury to said first bank of tubes, a second and third bank of tubes disposed in the lower portion of said space, pumping means for receiving mercury liquid from said first bank of tubes and forcing it through the second and third bank of tubes, and a fourth bank of tubes disposed below the first bank and above the third bank of tubes and connected in series between the third bank of tubes and the drum.

5. A mercury boiler including the combination of a casing forming a combustion chamber, a drum in the casing above the combustion chamber, a plurality of tubes in the combustion chamber and connected to the drum and arranged so that mercury flows therethrough by natural circulation, a space for receiving gases from the combustion chamber, a first bank of tubes in said space defining a preheater for mercury, means for conducting mercury liquid from an external source to the preheater, a plurality of banks of tubes in said space having inlet portions disposed in a region in which the temperature of the combustion gases is a maximum, pumping means for receiving mercury from the preheater and from the drum and forcing it through said plurality of banks of tubes into the drum.

6. A mercury boiler including the combination of a casing substantially rectangular in cross section and defining a combustionchamber, a plurality of heating tubes for mercury lining the walls of the combustion chamber and connected to the drum, upper portions of the tubes on opposite walls being twisted in groups to form ropes to effect uniform heat distribution among the different tubes and to provide intermediate spaces for the discharge of combustion gases, down-tubes and a plurality of independent lower headers connected between the down-tubes and the lower ends of the heating tubes.

7. A mercury boiler including the combination of a drum, a combustion chamber, heating elements in the combustion chamber exposed to radiant heat and connected to the drum, the flow of mercury through said heating elements taking place by natural circulation, a space for receiving combustion gases from the combustion chamber, heating elements disposed in said space and having lower ends connected to the drum, pumping means for forcing mercury from an external source and from the drum through said heating elements, and means including a variable speed motor for driving the pumping means in response to load conditions.

8. A mercury boiler including the combination of walls having lower portions forming a combustion space and upper portions forming a convection space, a drum located at a level near the lower end of the convection space and heating elements comprising first tubes located below the level of the drum and second tubes located above the level of the drum, the first tubes having lower ends connected to the drum to receive mercury therefrom and the second tubes having lower ends connected to the drum to discharge heated mercury into the drum, the upper ends of the first tubes being connected to discharge mercury heated therein into the upper ends, of the second tubes to maintain the density of mercury liquid and vapor contained in the second tubes at a minimum during operation.

9. A mercury boiler including the combination of a casing defining combustion and convection spaces, a drum located in the casing at a level below the convection space, an inverted convection heating element located in the convection space and having a lower end connected to discharge heated mercury into the drum, a heating element located below the level of the drum, and conduit for connecting the. discharge end of the heating element to the upper end of the inverted convection heating element to conduct mercury from the heating elementto the convection heating element.

10. A mercury boiler including the combination of a casing defining combustion and convection spaces, a heating element located below the level of the drum, an inverted convection heating element located in the convection space and having a plurality of hairpin-shaped series-connected portions in which mercury liquid is evaporated, means including a vertical tube located in the convection space and connected to the upper end of the inverted convection heating element to conduct heated mercury from the first named heating element to the convection element, and means connected to receive heated mercury discharged from the lower end of the inverted convection element.

11. A mercury boiler including the combinaton of walls forming a combustion space and a convection space, a drum located at a level near the lower end of the convection space, banks of heating tubes located in said spaces at a level below that of the drum, conduit means for conducting mercury liquid from the drum into the lower ends of said heating tubes, and banks of inverted heating tubes located in the convection space having upper ends connected to the upper ends of said heating tubes to receive heated mercury therefrom and lower ends connected to the drum to discharge the mixture of mercury liquid and vapor formed during operation into the drum.

12. A mercury boiler including the combination of walls having lower portions forming a combus, tion chamber and upper portions forming a con-- vection chamber, a drum located at a level near the lower end of the convection chamber and supported on said walls, banks of inverted heating tubes located in the convection chamber above the drum and having lower ends connected to the drum, and means including a heating element located below the liquid level in the drum for conducting heated mercury liquid to the upper ends of said inverted heating tubes.

13. A mercury boiler including the combina tion of walls having lower portions forming a combustion chamber and upper portions forming a convection chamber, a drum located at a level near the lower end of the convection chamber and supported on the walls, banks of inverted heating tubes located in the convection chamber above the drum and having lower ends connected to the drum, and means for conducting heated mercury to the upper ends of said inverted heating tubes, said means comprising heating tubes located at a level below that 'of the drum and having upper ends connected to the upper ends of the inverted heating tubes and pumping means connected to the lower ends of the last mentioned tubes.

14. A mercury boiler including the combination of walls having lower portions forming a combustion chamber and upper portions forming a convection chamber, a drum located at a level near the lower end of the convection chamber, banks of inverted heating tubes located in the convection chamber above the drum and having lower ends connected to the drum, and means for conducting heated mercury liquid to the upper ends of said inverted heating tubes, said means including banks of pr heating tubes located in the convection space above the inverted heating tubes and having upper ends for connection to a source of mercury and other heating tubes located at a level below that of the drum and connected in series between the ends of the first mentioned preheating tubes and the upper ends of the inverted heating tubes.

15. In a mercury boiler including the combination of walls forming a combustion space, a drum, heating tubes lining the wall of the combustion space and having lower ends arranged to receive mercury from the drum and upper ends connected to the drum to discharge heated mercury into the drum, upper portions of said tubes be ing arranged in groups, each group including several tubes twisted to form spaced ropes ex tending across the upper portion of the combustion space.

16. A mercury boiler including the combination of walls forming a combustion space, a plurality of heating tubes lining the walls, means for conducting mercury liquid to one end of the tubes, portions of the tubes being arranged in groups, each group including a plurality of tubes twisted to form a rope extending across the combustion chamber, said groups forming a slag screen, and drum means for receiving heated mercury discharged irom the tubes.

1'7. A mercury boiler including the combination of walls forming a combustion space, a plurality of heating tubes lining the walls of the space, means for conducting mercury liquid to be heated to the lower ends of the tubes, a drum for receiving heated mercury discharged from the upper ends of the tubes, at least some of the tubes having portions arranged in groups, each group comprising several tubes twisted to form a rope in the form of a catenary.

18. A mercury boiler including the combination of walls having lower portions forming a combustion space and upper portions forming a convection space, heating tubes lining the walls of the combustion space, inverted convection tubes disposed within the convection space, a drum located at a level intermediate said tubes and arranged to receive heated mercury from the lower ends of the inverted convection tubes and the upper ends of the wall tubes, and means for conducting preheated mercury liquid to the lower ends of said heating tubes, at least some of the wall tubes having upper portions arranged in groups, each group including a plurality of tubes twisted to form a rope, each rope being downwardly curved and extending across the combustion space to form a slag screen and to protect the convection tubes from radiant heat.

19. A mercury boiler including the combination of walls forming a combustion space, heating tubes lining the walls of the space and having upper portions arranged in groups, each group including a plurality of tubes twisted to form a rope with the ropes arranged at different levels and in staggered relation to form large spaces for the flow of combustion gases, means for conducting mercury liquid to the lower ends of the tubes, and means for receiving heated mercury from the upper ends thereof.

20. A fluid heating element for use in boilers comprising a plurality of tubes having portions aligned to form a wall, and other portions twisted to form a rope.

21. A fluid heating element for use in boilers comprising a plurality of tubes having portions aligned to form a wall, and other portions twisted to form a centenary-shaped rope.

22. A mercury boiler including the combination of lower, intermediate and upper wall portions forming sliding joints between them and defining combustion and convection spaces, heating elements disposed in said spaces, rigid supporting means for the lower and upper wall portions, and flexible supporting means for the intermediate wall portion to permit expansion of the latter relative to the lower and upper wall portions during operation.

23. A merctuy boiler including the combination of a casing forming combustion and convcction spaces and having an intermediate portion with a sliding fit with the other portions, heating elements located within the casing, a drum projecting through the intermediate portion and connected to the heating elements, and means connected to the drum for flexibly supporting the intermediate casing portion.

24. A mercury boiler including the combination of a casing forming a combustion space, a drum projecting centrally through the space and dividing it into primary and secondary combustion spaces, up-tubes lining the wall of the primary combustion space and having lower ends connected to receive mercury to be heated and upper ends connected to the drum, down-tubes lining the walls in the secondary combustion space and having lower ends connected to discharge heated mercury into the drum, and means including a heating element in the primary combustion space for supplying heated mercury to the upper ends of the down-tubes.

25. A mercury boiler including the combina-- tion of a casing forming a combustion space, a drum projecting centrally through the space and dividing it into primary and secondary combustion spaces, up-tubes lining the wall of the primary combustion space and having lower ends connected to receive mercury to be heated and upper ends connected to the drum, down-tubes lining the walls of the secondary combustion space and having lower ends connected to discharge heated mercury into the drum, and means for conducting heated mercury to the upper ends of the down-tubes, said means comprising groups of twisted heating tubes forming a slag screen in the upper portion of the primary combustion space.

26. A mercury boiler having a casing forming combustion and convection spaces, a drum centrally located in the combustion space and dividing the latter into a primary and a secondary combustion space, up-tubes lining the walls of the primary combustion space and having lower ends for receiving mercury to be heated and upper ends connected to the drum, down-tubes lining the wall of the secondary combustion space and having lower ends connected to discharge heated mercury to the drum, inverted convection heating tubes located in the convection space above the secondary combustion chamber and having lower ends connected to discharge mercury into the down-tubes, and means including a heating element in the primary combustion space for conducting preheated mercury to the upper ends of the inverted convection heating tubes.

27. A mercury boiler having a casing forming combustion and convection spaces, series-connected down-tubes and up-tubes subject to radiant heat in the combustion space, inverted convection heating tubes located in the convection space above the combustion space and having an upper end connected to receive heated mercury from the upper end of the up-tubes, means for supplying mercury to be heated to the upper ends of the down-tubes, and a drum connected to receive heated mercury discharged from the lower end of the inverted convection heating tube.

28. In a boiler a combined slag screen and baffie structure comprising groups of heating tubes, each group including a plurality of tubes having portions twisted to form a rope and other portions aligned to form a wall, the rope portions being spaced to form a slag screen and the aligned portions defining a wall to direct the passage of gases.

29. A mercury boiler having a casing defining a combustion space, a drum centrally located in the combustion space and dividing it into a primary and a secondary combustion space, a slag screen in the primary combustion space including a plurality of groups of heating tubes having portions twisted to form ropes and other portions aligned to form a bafile protecting the bottom portion of the drum from. the radiant heat in the primary combustion space.

30. A mercury'boiler including the combination of a casing forming a combustion space, a drum. centrally located in the space and dividing it intoa primary and a secondary combustion space, a vapor discharge conduit connected to a top portion of the drum and projecting through the casing, and means protecting said conduit and the top portion of the drum from radiant heat comprising means including walls of heating tubes enclosing the conduit and the upper portion of the drum.

31. A mercury boiler including the combination of a casing forming a combustion space, a drum centrally located in the space and dividing it into a primary and a secondary combustion space, a vapor discharge conduit connected to a top portion of the drum and projecting through the casing, means protecting said conduit and the top portion of the drum from radiant heat comprising means including walls of heating tubes enclosing said conduit and upper drum portion, and a slag screen in the primary combustion space comprising groups of heating tubes, each group including a plurality of tubes twisted to form a rope with the discharge ends of each rope connected to the lower ends of said walls of tubes.

BEVIS P. COULSON, JR.