US2281580A - Fluid heat exchange apparatus - Google Patents

Description

May 5 1942. J, H B 2,281,580

FLUID HEAT EXCHANGE APPARATUS Filed Oct; '28, 1938 5 Sheets-Sheet l 1N VENT OR.

Hobbs M y' 5, 1942. J. c. HOBBS 228L580 FLUID HEAT EXCHANGE APPARATUS Filed Oct. 28, 1938 Fig: 2

5 Sheets-Sheet 2 INVENTOR.

3 James Cflobbs I May 5, 1942. J, HOBBS 2,281,580

FLUID HEAT EXCHANGE APPARATUS Filed Oct. 28, 1938 5 sheets -sheet 4 INVENTOR. v

f N James C Hobbs ATTORNE y 1942- J. c. HOBBS 2,281,580

mm) HEAT EXCHANGE APPARATUS Filed om. 28, 1938 5 Sheets-Sheet 5 Fig 6- INVIENTOR. James C Hobbs Patented May 5, 1942 UNITE S'E'A'I'Efi PATENT OFFICE 2 Claims.

The present invention relates to fluid heat exchange apparatus, and it may be considered as exemplified in a steam superheater.

Present day steam boiler installations are provided with superheaters, and, in the more effi cient plants operating at high pressures, the temperature of the steam is high enough to involve dimculties in avoiding damage to the superheater metal. The superheaters usually consist of a multiplicity of similar tubes through which steam flows from an inlet to an outlet header, the tubes being disposed in a gas pass through which gases of combustion flow and from which the heat of superheat is derived by transfer through the tube walls.

The steam flowing through the tubes in parallel should absorb heat from the gases in such amounts that the steam temperature will be the same at the outlet ends of all of the tubes. Also, if the individual superheater tubes are supplied with the same amount of steam by uniform division of the whole flow between them, then each tube should absorb the same amount of heat from the gases. For this to be possible there must be equalization of gas flow rates past each tube and equality of temperature in the gases approaching the tubes. This is equivalent to the requirements that the gas flow shall be equally distributed over the cross section of the gas stream and that the gases shall be at the same temperature over that cross section.

To fulfill these requirements of uniform gas flow and temperature over a cross section of the gas stream at the entrance to the superheater zone is extremely difiicult, and, in many cases, it is quite impossible. This invention, accordingly, seeks to accomplish the desired results in spite of these conditions.

If the steam flows from a plurality of tube outlets vary as to temperature, yet when mixed together in an outlet header a desired average temperature is attained, there are differences in the effectiveness of the heat absorption of the various tubes. Some of the tubes absorb heat at lower rates than others, while the latter may even be attaining or approaching a steam or tube metal temperature which is hazardous. It is the aim of this invention to so co-ordinate positions of the various tubes with reference to the heating medium that equalization of heat absorption and consequently equalization of steam temperature is more nearly approached.

There are two main causes of inequality ,of gas flow and temperature over a cross section of the gas stream approaching the superheater. One of these originates in combustion conditions in the furnace, and the other in the form of the gas flow path within the enclosing walls between the furnace exit and the entrance to the superheater zone, especially as to changes of direction.

: p r a ow rsroup If the gases leave the furnace horizontally and then flow vertically to the superheater zone there is a tendency for the flow to concentrate at the outside of the, turn. There is also ,a tendency to set up eddy currents, and even a reverse flow may exist in one part of the gas duct with a steady direct flow in another. In addition, when water cooled gas boundary surfaces are present inequality of gas flow will result in inequality of heat transfer rates and gas temperatures at the entrance of the superheater zone even if the velocity and temperature of the gases are equal over the cross section of the stream leaving the furnace.

If the combustion conditions across the furnace are unequal as to rate of combustion per unit of width of gas exit, or are unequal as to excess air, or both, the gases may vary in temperature and velocity over the area of the exit, and this inequality will continue up to the entrance of the superheater zone, subject to further disturbances due to flow conditions over the gas path.

Another cause of inequality of gas temperature and velocity at the furnace exit, which may persist up to the superheater zone, arises from the location of the burners, their adjustment, and. other operating conditions. If the, burners are concentrated near the center of a wide furnace, the gases leaving the furnace exit near the side walls may have velocities or temperatures lower than the gases leaving the central part of the furnace. This would especially be the case if the exit were characterized by large area for low draft loss. If, on the other hand, a plurality of burners are arranged in a row extending all the way across a wide furnace, then, differences in the adjustments of the burners, or diiferences in their capacities may cause inequalities of gas velocity or temperature over the cross section of the furnace, particularly along a line parallel to the line of the burners.

My invention solves the problem of equalizing the temperature of steam leaving the several superheater tubes in spite of such inequalities of velocity and/or temperature of the gases approaching the superheater.

Illustrative embodiments of my inventionv are disclosed in the accompanying drawings, in which:

Fig. l is a vertical section of a. steam boiler which includes the illustrative superheater;

Fig. 2 is a vertical section of part of the-Fig. 1 boiler with certain parts broken away t0 Show the superheater on an enlarged scale;

Fig. 3 is a vertical section takenlon the line 3-3 of Fig. 2;

Fig. 4 is a horizontal section takenhon @the line 4-4 of Fig. 2- and showing thev cross-over connections for the superheater coils of the up- Fig. 5 is a diagrammatic view illustrating another modification of the invention; and

Fig. 6 is a multiple horizontal section on the broken section line 6-6 of Fig. 1.

The installation indicated in Fig. 1 of the drawings includes the furnace I0, the walls of which are defined by steam generating tubes connected into the boiler circulation. The furnace is fired by a plurality of burners I2, I4, and

I6 which are adapted to burn pulverized fuel.

The gases of combustion leave the furnace through the exit I8 and pass upwardly over the tubes of the superheater positioned in the gas pass 22. The superheater consists-of spaced tubes extending across the gas pass, these tubes being bent so as to form fiat coils. The coils are assembled in the gas pass 22 side by side, and are disposed in spaced arrangement over the whole width of the pass between opposite walls, each individual coil extending across the gas pass between the remaining opposite walls. The preferred relationship between the superheater tubes and the gas fiow is such as to cause the gases to fiow transversely of the tubes and in a direction parallel to the planes of the fiat coils.

The superheater coils are arranged in two groups, an upper group 24 and a lower group 26, the groups presenting tube banks over which the furnace gases fiow in succession. The flat coils of one group are connected to those of the other so that the steam flowing through a flat coil of the upper group flows through a flat coil of the lower group which is disposed in a different plane longitudinall of the gas pass. The relative po sitions of the two planes occupied by the two flat coils successively traversed by the steam and the gases are determined with reference to the mean velocities or temperatures of the parts of the gas stream in which the planes are disposed, so as to substantially equalize the amounts of heat absorbed by the steam passing through a plurality of fiow paths formed by such pairs of connected flat coils. Considering one of these flow paths, if the fiat coil of the first group 24 is disposed in a plane where the gas flowing past it has a Velocity or temperature lower than in other planes, the second group fiat coil forming a part of the same fiow path would be disposed in another plane where the gases flowing past it are higher in velocity or temperature. In this way the variations of gas temperature and velocity across the gas stream and along the row 1 of flat coils are not reflected in temperature differences in the steam leaving a plurality of such flow paths, but on the contrary, the effect is to equalize such temperatures.

The desired relative positions of the planes of the connected first and second group flat coils may be predetermined from a knowledge of the distribution of gas as to velocity and temperature over the stream section, or it may be determined from observations of steam temperatures at the outlets of the individual flat coils, which, if found to be at undesirable values, may be corrected by changing the cross-over connections between sections of the first and second group coils.

In the preferred embodiment of my invention a space is left between the first and second groups of superheater coils and in this space are located the cross-over tube connections for the pairs of fiat coils. This space also has the advantage of providing access to the cross-over connections for changing the relative positions of the planes of the flat coils. This space is indicated at in Fig. 2 of the drawings, and the manner in which the cross-over tubes are disposed in that space is indicated in Figs. 2, 3, and 4.

Steam flowing from the drum 32 through the supply tubes 34 and 36 to the coils 38, 40 and 42 of the first group 24 passes through the downwardly extending parts 44, 46, and 48 (see Fig. 3) of the cross-over tubes at a position near the side of the superheater gas pass adjacent the superheater outlet header 50. From this position it passes transversely of the gas pass to the coils 52, 54, and 5B of the lower group 26. The paths for this steam flow are afforded by the transverse sections 58, 60, and 62 of the cross-over tubes, these transverse sections communicating at their opposite ends with the downwardly extending parts 64, 60, and 68 of the cross-over tubes.

Considering Figs. 3 and 4 of the drawings, the coils above indicated at 38, 40, and 42 are the first, third, and fifth coils of the upper group 24. Coils I0, 12, and I4 are, respectively, the second, fourth, and sixth coils from the left-hand wall I5 of the gas pass I8. The latter are separately connected to the second, fourth, and sixth coils 80, 82, and 84 of the lower group 26 by cross-over connections which include the transverse tube 5 80 and two companion tubes directly below it.

by the downwardly extending connections 86, 88,

The coils I0, I2, and I4 are connected to the transverse tube 90 and its companion tubes (in the same horizontal planes as the tubes 58, 80, and 02, and therefore not appearing in Fig. 3)

and I00, and it will be noted that the transverse tube 90 and its companion tubes leading from these downwardly extending sections, have their axes in a vertical plane adjacent the gas pass 1 wall defined by the water tubes I02 and the blocks I04 closing the spaces between those tubes.

At the right-hand ends of the transverse tube 80 and its companion tubes there are the downwardly extending sections I06, I08, and I I0 which directly communicate with the tube sections H2, H4, and H6 of the coils 88, 82, and B4 of the lower group of superheater coils 26.

Similarly, steam flowing through the coils I20, I22, and I24 of the upper group of superheater coils passes through cross-over tube connections w gas pass.

which include a corresponding number of transverse tube sections I26 to the coils I28, I38, and I32, and this system of cross-over connections is continued in each of the superheater gas passes I8 and I40 so that, for instance, the coils of the upper group 24 at one side of the gas pass are individually and directly connected to the coils of the other group 26 at the opposite side of the Thus, approximately half of the coils of the upper group adjacent the gas pass wall the gas pass wall I44.

I42 of the gas pass I40 are individually and directly connected to the coils of the lower group 26 at the opposite side of the gas pass and adjacent The remaining coils of the first group 24, at the side of the gas pass I40 and adjacent the wall I42.

upper group 24 in zones of lower gas velocities and temperatures. In this way the individual steam flows will have their temperatures substantially equalized at their outlets.

In the embodiment of the invention illustrated in Figs. 1, 2, 3, 4, and 6 of the drawings the burners 12, I4, and I6 are grouped centrally of the furnace and the furnace gases passing from the exit I8 and from the gas pass 22 may be at higher temperatures and higher velocities at the central part of the installation. In this arrangement, higher gas temperatures and higher gas velocities would exist adjacent the gas pass walls 16 and I42 of superheater gas passes 18 and I40 a above indicated, and the cross-over connections for the coils of the upper and lower superheater groups are arranged so as to compensate for such inequalities. In this particular installation, there is an additional gas pass between the walls 16 and I42, but this is occupied by the economizer sections I50, I52, and I 52. However, if the illustrative installation were modified so as to have a superheater extending entirely across the width of the installation in a single gas pass, it is within the scope of the invention that the superheater coils be arranged and connected as indicated in Fig. of the drawings. Here, the central coils a and a of the upper group I60 are disposed in a zone of higher gas velocities or higher temperatures and are therefore joined by the cross-over connections I63 and I35 in the space I62, with coils a" and 11 adjacent the gas pass wall I64 and I86 respectively, and in zones of lower gas temperatures or velocities. Conversely, the coils d and d in the lower group I68 and in zones of higher gas temperatures and higher gas velocities are directly joined by the connections I61 and I68 with the coils d" and d' of the upper group I 60 located in zones of lower gas temperatures and lower gas velocities. Of the remaining coils, the coil 0 of the upper group is joined by the connection I to the coil 0' of the lower group on the opposite side of the gas pass. The coil b is similarly joined by the connection I12 to the coil b of the lower group I68. On the opposite side of the upper gas pass the coils c" and b" are joined by the connections I16 and I18 to the coils b' and c' of the lower group I68 and on the opposite side of the gas pass I80.

In connection with the illustrative embodiment it will be noted that the coils of the upper group 24 form a counter-flow superheater section while most of the coils of the lower group 26 constitute a parallel flow section, the steam passing upwardly from the straight tube sections 200, 202, and 204 through successive return bend coils to the outlet header 50. These tube sections 200, 202, and 204 receive steam directly from the uppermost return bend coils 206 of the lower group 25. Preferably, the coils of both groups are supported by lugs 2| 0-2I5 inclusive, secured to the wall cooling tubes I02 and 216. Complementary brackets 2I1-222 inclusive, welded to the return bends of the superheater coils co-operate with the lugs to maintain the superheater coils in their operative positions, the successive tube sections of the superheater coils being connected by the hangers such as those indicated at 223-220 inclusive, for suspending the lower superheater coils from the upper coils which have the brackets welded thereto.

The furnace gases after passing over the coils of the superheater groups 24 and 26 pass over the tubes of an economizer 250 and then past the damper 252. The gases are then discharged through a flue, preferably by means of an induced draft fan. Secondary air is heated by the furnace gases and it then passes through a duct 210 to the chamber 212 enclosing the burners.

The circulatory system of the installation indicated in Fig. 1 of the drawings includes the furnace roof tubes 300, the floor tubes 302, the wall tubes 302, and the headers 303, 308, and 3I0. The furnace side walls also are defined by tubes 3| 2 connected into the boiler circulation through the intermediacy of the upper headers 3I4 and the lower header 3I6 and appropriate connections with the drum 32, the spaces between the furnace wall cooling tubes being preferably closed by refractory blocks similar to the blocks I 04 previously mentioned.

The wall tubes I02 are connected at their upper ends to the water space of the drum 32 and at their lower ends to a header 320 to which the wall tubes 216 of the superheater gas pass are also connected. The latter are joined at their upper ends to the header 322 which, in turn, is connected to the drum 32 by the tubes 324 and 323. Appropriate connections may be provided for the headers 3I0, 3I6, and 320 at the base of the installation.

Whereas, I have described my invention with reference to the particular embodiments shown in the drawings, it is to be understood that the invention is not to be considered as limited to all of the details thereof. It is to be considered of such scope that many of these details may be modified, so long as the essence of the invention is retained. In general, the invention is of a scope commensurate with the scope of the subjoined claims.

I claim:

1. In fluid heat exchange apparatus, means forming a gas pass for furnace gas flow having non-uniform heat exchange values over its cross section, separate and spaced apart tube banks arranged transversely of the gas pass with each bank consisting of substantially uniform fiat coils having their axes disposed in planes extending parallel to the general direction of gas flow, and laterally extending tubular connections each establishing communication between the outlet of a coil in one bank and the inlet of a coil of the other bank, the latter coil being disposed at a different distance from the same side of the gas pass, each of said tubular connections with the coils directly connected therewith forming a separate flow path having parts arranged in zones of different and compensatory heat exchange values, the separate flow paths being arranged in parallel.

2. In fluid heat exchange apparatus, means forming a pass for a furnace gas flow of nonuniform heat exchange values over its cross section, a plurality of spaced apart tube banks extending across the gas pass, said banks of tubes being spaced apart in the direction of gas flow, each bank of tubes consisting of flat return bend coils with their axes in planes substantially parallel to the walls of the gas pass, and tubular means in the space between the banks of tubes, each of said means forming a separate fluid path by the connection of the outlet of a coil of the first bank of tubes to the inlet of a coil of the second bank disposed in a different longitudinal zone of gas flow, the coils joined by each of said means being disposed in zones of different and compensating heat exchange values.

JAMES C. HOBBS.

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