US2919551A - Turbine with reheat cycle - Google Patents

Description

Jan. 5, 1960 o. F. CAMPBELL TURBINE WITH REHEA'I' CYCLE 2 Shee ts-Sheet Filed March 24, 1955 INVENTOR 0. F. CAMPBELL TURBINE WITH REX-[EAT CYCLE Jan. 5, 1960 2 Sheets-Sheet 2 Filed March 24, 1955 INVENTOR NN muzmzm OLIVER F. CAMPBELL United States This invention relates to steam turbines and in par ticular it is concerned with turbines which employ a reheat cycle, for example, compound or topping turbines.

The use of high pressure steam with low superheat results in certain losses in stream turbine practice known as high moisture loss. A method which has been employed to reduce such loss is re-superheating or reheating. Specifically, exhaust steam from a high pressure turbine is passed into contact with a source of heat whereby heat is added to the exhaust and the resulting drier steam then is introduced into a low pressure turbine, This process is known as reheating. The advantages of reheating, as measured by cycle efliciencies, are small but the practical gains due to the desired decrease of moisture loss of steam turbine engines are considerable. Gains of 4 to 6 percent in turbine cycle heat rate usually result.

Thus, the major justification and advantage in employing reheat is the avoidance of the extremely high superheat temperatures, otherwise required in high pressure turbine plants having a plurality of stages, to keep the expansion substantially out of the wet region. But the use of reheat greatly affects capital costs since additional flow area must be provided in low pressure turbine stages to accommodate the resulting increased specific volume of steam, and additional equipment such as auxiliary superheaters or a boiler of increased capacity are necessary. Ordinarily, increased capital costs involved counterbalance the resulting advantages. Consequently, the design of turbine systems to include reheat cycle generally is restricted to such times as an additional advantage appears in the form of abnormally high fuel costs since the increased efliciency of reheat appears in a reduction in heat rate or a reduction in the heat consumption per unit of output. Even during the times of abnormally high fuel costs, it has been found that the use of a reheat stage will be economical only once or perhaps twice in any turbine or steam engine system.

I have now discovered an apparatus combination including a turbine adapted to improve the operating efliciency of steam turbines by minimizing or eliminating moisture loss. I have also discovered an apparatus combination including a turbine, adapted to employ the reheat cycle, I have also discovered an improved method of operating a steam turbine system which incorporates the reheat cycle. These advantageous results are obtained in accordance with my invention by employing, in conjunction withturbines and turbine systems, a defined .method and means to effect heat transfer.

produced is such that pressure loss on the steam is more limited than would otherwise occur and the exhausting .steam does not evidence the'water content otherwise present and moisture losses are minimized. The disposition of a turbine directly within a heated bed of fluidized solids,

I, "thereby-eifecting the addition of heat to the steam, also avoids employing especially high steam temperatures and pressures of the entering steam.

In conventional turbine practice steam is withdrawn from a high pressure turbine of a compound or topping atent C ice system after partial expansion and then is returned to a re-superheat section of the boiler or, alternatively, is conducted to an auxiliary superheater whereupon heat is added at a higher temperature thereby increasing the mean eifective temperature at which the over-all heat content of the steam is added. The resulting reheated steam is then readmitted to the surbine system, i.e. to a low pressure stage, for expansion to final pressure. In my invention, the exhaust steam from the high pressure turbine is passed in indirect heat exchange relationship with a heated bed of finely divided solids maintained in a state similar to that of a boiling liquid, or fluidized as the handling of the solids in this manner is known. Surprisingly, I have discovered that the resulting efliciency of heat transfer from the fluidized solid to the exhaust steam is so high per unit of extra cost involved that resuperheat or reheat advantageously may be employed at least two times, for example with three or four stages, and conceivably as many as five times. In other words, by the use of my discoveries a topping or compound sysstem of turbines having a plurality of stages, i.e. two or more, and as many as six, ranged in order of decreasing pressure, can be devised. In view of the known fact that heat transfer by convection is more rapid, and hence more efficient, than by conduction, I consider my discovery to be contrary to previously known art.

Thus in an embodiment employing reheat, my invention comprises a combination including a turbine disposed within a bed of fluidized solids maintained within a large vessel, and heat exchange means, communicating with the turbine exhaust, also disposed within the bed. Means are included to facilitate introduction of particle solids and a suitable gaseous material to eifect the fluidization. The steam leaving the heat exchange means is conducted to either a similar combination or merely to a low pressure turbine.

In still another embodiment of my invention employing reheat, the exhaust from a high pressure turbine is conducted to a heat exchange means maintained within a bed of fluidized solids in a vessel separate from the turbine. This embodiment of the invention thus includes reheat and is advantageous in that it can be adapted to existing turbine systems without extensive modification of existing equipment. The invention as thus embodied includes a turbine having its exhaust conduit communicating with a heat exchange means disposed in a bed of heated fluidized solids in a vessel separate from the turbine. Reheated steam is conducted to a second lower pressure turbine. In this embodiment, a plurality of turbines and separate reheat stages can be combined to form a system having at least two turbine stages and advantageously as many as three, four, five or six stages.

The method of heating which is to be employed in accordance with my invention comprises, broadly, the maintenance of finely divided solids in a state resembling a boiling liquid. This is accomplished by passage of a gas known as a fluidizing gas, which may be steam, air, gaseous hydrocarbons, etc., upwardly through a confined space containing finely divided solids. The fluidizing gas is fed to the unit at a rate such that the mass of solids maintained in the unit is suspended in the gas and maintained within the unit as a mass of solids in gas.

The finely divided solids normally used have a range of particle sizes which may be from about 10 to 200 microns. However, it is advantageous that the majority of the particles in a given mixture have diameters in a more narrow range, for example, 40 to 60 microns. The remaining particles would have smaller and larger diameters. A typical mass of solids would analyze with about 5 to 10 percent of fines,that is, particles having less than a diameter of 20 microns, 10 to 15 percent of particles with a diameter of 20 to 40 microns, 40 to percent of particles having diameters of the order of 60 to 80 microns and the balance of larger particles. While the typical analysis of particle sizes described is preferred for use in my invention, any other range or analysis of particle sizes which does not deleteriously affect the economics and feasibility of the process may be employed.

For the present invention, it is desirable that fluidizing gas used be combustible in addition to being inexpensive. Thus, I prefer to employ low boiling hydrocarbon fractions, natural gas, producer gas, marsh gas and the like in conjunction with a suitable quantity of an oxygencontaining gas, for example, air. Liquid and solid fuels also can beemployed as a source of fluidizing medium if desired. Where solid fuels containing ash are employed, the ash also can be introduced into the bed and any excess ash accumulating in the unit can be removed, for example by bleeding a stream from the bed from time to time. These fuels may be passed to the fluidized bed containing unit through a conduit having suitable means to initiate and maintain combustion of the combustible gas and then into the fluidizing unit whereupon heat and fluidizing gas are simultaneously introduced into the unit. The heated gas passes upwardly through the unit supporting the bed of solids in fluidized condition and heats the solids. Gases leaving the fluidized bed normally contain a small portion of fines or other particles entrained. Accordingly they are passed into cyclone separators or similar means positioned within the upper portion of the fluid bed containing unit whereby solid particles are removed and returned to the bed and efiluent gases are permitted to exit.

Heat transfer can be accomplished in accordance with my invention by passing exhaust steam into indirect heat exchange relationship with a bed of fluidized solids as above described. Advantageously, exhaust steam enters the heat exchange area through a conduit connected to a suitable tubular coil supportingly maintained in the fluidized bed. A characteristic of fluidized beds is that each point in the bed is substantially the same temperature as any other point and there are no hot-spots. Hence, the placing of the heat exchange coil or means at any place within the bed, considering heat exchange rates, will be just as eflicient as placing them at any other point. Extremely high heat transfer rates obtainable in my system permit the use of much smaller coils than heretofore and thereby further reduce capital costs involved. Another advantage resides in the fact that the turbulent solids effect a cleansing or scaling action on the coils during contact whereby heat transfer rates are maintained high and maintenance problems to remove scale and the like are substantially non-existent. The simplicity of the heat exchange unit is another factor contributingto decreased capital costs and less maintenance.

My invention will be described further in conjunction with the attached drawings in which Figure l is a vertical elevation of a turbine system incorporating an embodiment of my invention; and

Figure 2 is a flow sheet showing a system employing the principles of my invention.

Referring to Figure 1, the numerals 12, 14, 16, 18 and 20 are large closed vessels, which can have any shape desired though preferably they are cylindrically shaped, capable of maintaining a bed of fluidized solids therein. Disposed within each vessel is a turbine, indicated in the drawing by the numerals 32, 60, 62, 64 and 66. These turbines are connected to one another, mechanically, by their shafts with the turbine 66 being connected by its shaft 68 to a sixth turbine 70 which, as is shown, need not be placed within a vessel. The shaft 72 of turbine 70 is connected to a suitable means for employing the produced mechanical power, for example a generator 74. Since each vessel is substantially identical with the others, in the embodiment shown, only vessel 12 will be described in great detail. Vessel 12 is .provided with a conduit 22 in its lower end to facilitate the introduction of solids and fluidizing gas. Control of flow in the conduit is aided by a valve 24. Within the lower portion of vessel 12 is placed a perforated grid 26 capable of supporting a bed of fluidized solids. In the upper end of vessel 12 a conduit 28 is provided for exiting fluidizing gas. Advantageously, separating means 30, e.g. cyclone separators, are provided in the upper part of vessel 12 to separate fines entrained in the exiting fluidizing gas for return to the bed.

Disposed within vessel 12 above grid 26 is a steam turbine 32, supported by suitable means not shown. A tubular coil 34 is provided around the surface of turbine 32. The coil is connected to the exhaust outlet 36 of turbine 32 at one of its ends, and with the steam inlet line 38 of a second, lower pressure turbine 60 disposed, in this embodiment, in vessel 14. Steam is supplied to turbine 32 through a conduit 42 communicating with a suitable source, e.g. a steam boiler, not shown. The mechanical energy produced as a result of the transfer of energy from the steam to the turbine blades (not shown) is transmitted to the shaft 44 fixedly connected with the blades, and thence to the next succeeding stage.

In operation, a heated fluidized bed of solids is produced and maintained in vessel 12 by passing a suitable fuel, i.e. natural gas, into conduit 50 from a source 48 where it is joined by an oxygen-containing gas, i.e. air, from a source 49. The gases are burned, for example by means of a burner 52 placed in conduit 50. Granular solids are introduced into conduit 50 from a storage means 54 advantageously provided with a control valve 56. The hot combustion products issuing from the burner 52 transport the solids upwardly into vessel 12 through a conduit 22. The solids and gases pass through grid 26 above which the solids are maintained in the pseudoliquid state, or fluidized. Steam is supplied to turbine 32 and after contact with the blades, exhausts through exhaust line 36 into the heat exchange coil 34. The steam in coil 34 receives heat transferred from the hot fluidized solids surrounding it. The steam then passes into the steam inlet conduit 33 of the adjoining lower-pressure turbine. The operation of the turbine- fluidized bed vessels 14, 16, 18 and 20 is substantially identical to that described relating to vessel 12.

As shown in Figure 1, it is possible with the present invention to employ reheat as many as five times before the resulting advantages are counterbalanced by the capital costs involved. Thus a six-stage turbine system can be devised, employing five reheat stages, and terminating with a turbine which need not be disposed Within a bed of fluidized solids since its exhaust is not to be employed in a succeeding stage. The shaft from the last or lowest pressure turbine is connected to a suitable means for employing the mechanical power produced.

Another embodiment of the principles of my invention comprises a plurality of turbines within a single fluidized bed (not shown). Also, it is not essential, to obtain the advantages of the present invention, that the heat exchange means containing the exhaust steam be disposed about the surface, or be disposed in coil-like fashion. The embodiment presented, though, has the advantage of requiring minimum space and presents advantages regarding manufacture of the unit.

The advantages of heat transfer by heat fluidized solids can be obtained in systems which do not employ reheat. For example, the heat exchange coils shown in Figure 1 can be omitted and the exhaust 36 of turbine 32 connected directly with steam inlet 38 of turbine 40 in vessel 14. It is not, of course, necessary that a plurality of turbines be employed since the advantage of minimizing pressure loss by adding heat during expansion can be used with any single turbine.

As pointed out above, use of the reheat cycle in a system employing a plurality of turbines is desirable because of the resulting economies in the heat rate. Reheat by use of a heated bed of fluidized solids has been found to offset the disadvantages heretofore encountered in adopting the reheat cycle. Another advantageous embodiment incorporating reheat by means of a heated fluidized bed is shown in Figure 2.

Referring now to Figure 2, steam from any suitable source (not shown) is introduced into high pressure turbine 110 by means of a conduit 111 whereby contact between the turbine blades and the steam is effected. Resulting mechanical power is conducted from the turbine to a point of use by means of a suitable shaft or other means 112 attached to the blades. The exhaust from turbine 110, which is at reduced pressure, leaves the turbine through conduit 113 and is conducted through conduit 114 to a coil 115 maintained within vessel 116. Vessel 116 contains a heated bed of fluidized solids which is produced by passing a suitable fuel from a source (not shown) through conduit 118 into conduit 119 where it is mixed with air or other oxygen-containing gas entering conduit 119 by means of conduit 120. The resulting mixture is passed through a suitable burner means 122 in conduit 120 whereupon combustion occurs, and the hot combustion products are then passed into the lower portion of vessel 116. Solid particles of a suitable contact material are introduced into vessel 116 by feeding intoconduit 119 at a point past burner means 122 from a hopper 123 or other storage container. The combustion gases effectively transport the solid particles upwardly into vessel 116 through grid 124. Grid 124 is a conventional supporting means normally employed in apparatus designed to contain a bed of fluidized solids. The quantity of solid materials and gases entering vessel 116 are chosen so that particles will be maintained in the described pseudo liquid state above grid 124.

In the normal maintenance of a fluidized bed an interface occurs defining a lower dense phase and an upper dilute phase. Below the interface solids are maintained in the pseudo liquid state; above the interface, gases leaving the bed entrain a certain percentage of the fines and do, therefore, maintain a very dilute fluidized phase in this upper portion. Actually, larger particles appear -in the dilute phase also and since the velocity of the gas at this point normally is insufficient to maintain the larger particles suspended, they drop downwardly into the bed again. This phenomenon is known as hindered settling. The gases containing entrained fines pass upwardly into a cyclone separator 126 in the upper portion of vessel 116 where fines are separated and returned to catalyst beds by means of the cyclone dip leg 128 and the gases and any unburned fuel contained therein pass out of vessel 116 through conduit 130 and advantageously are employed in a manner to recover their sensible heat and to use any unburned fuel, for example, by being passed to a unit producing steam for the turbines.

The exhaust-steam from the first turbine stage 110 passes through coil 115 in indirect heat exchange relationship with the hot fluidized solids where heat is added to the steam. The steam then exits from the unit through conduit 132 and is conducted into the next stage 134 of the turbine system, that is, a lower pressure stage. Steam entering turbine 134 contacts the blades whereby part of the energy of the steam is converted to mechanical energy and the steam decreases in pressure and exhausts to a reheater 140 containing a bed of fluidized Solids. As shown in Figure 2, as many as six turbines can be employed, in series, having five reheat stages when reheat is accomplished by the use of a heated bed of fluidized solids.

While five separate units containing fluidized solids are shown in the drawing, it should be understood that the system can be simplified. For example, two or more of reheat coils 115, 138, 148, 160 or 166 can be located in a single bed. The size of the coils would, of course, be adopted with a view to obtaining proper residence 6 time of steam within the bed. Other embodiments of the invention might include either a single fluidizing gas source, or separate sources for each reheater unit.

'I claim:

1. Apparatus of the type described including in combination a vessel having an outlet conduit in its upper end portion and an inlet conduit in its lower end portion, a bed of heated fluidized solid particles within said vessel, a first steam turbine disposed within said bed of fluidized solids, a steam inlet means and an exhaust conduit connected to said turbine, a second steam turbine having a steam inlet conduit, a heat exchange means communieating with said exhaust conduit of said first turbine and disposed in contact with said bed of fluidized solids, an exhaust conduit of said heat exchange means communieating with said steam inlet conduit of said second turbine.

2. An apparatus combination of claim 1 including second and third vessels having an outlet conduit in their upper end portion and an inlet conduit in their lower end portion, a bed of heated fluidized solid particles within each of said vessels, said second turbine disposed within said second vessel, heat exchange means communicating with an exhaust conduit of said second turbine and disposed vvithin said bed of solids within said second vessel, a third steam turbine disposed within said third vessel, heat exchange means communicating with an exhaust conduit of said third turbine and disposed within the bed of solids within said third vessel, a steam conduit connecting said heat exchange means within said second vessel with a steam inlet conduit of said third turbine, a fourth steam turbine, and a steam conduit connecting the heat exchange means within the third vessel with a steam inlet conduit of said fourth turbine.

3. Apparatus of the type described comprising a vessel, an inlet conduit in the lower end portion of said vessel, an outlet conduit in the upper end portion of said vessel, a perforated grid disposed horizontally across said vessel in its lower portion, means communicating with said inlet means for the introduction of particle solids and fluidizing gases, a steam turbine disposed within said vessel above said grid, stream inlet means connected to said turbine, heat exchange means communicating with an exhaust conduit of said turbine and disposed within said vessel above said grid, a second steam turbine, a conduit connecting said heat exchange means and an inlet conduit of said second steam turbine.

4. Apparatus combination of claim 3 including second and third vessels having outlet conduits in their upper end portion and inlet conduits in their lower end portion, a perforated grid disposed horizontally across each of said vessels in their lower portion, means communicating with the inlet means of each of said second and third vessels for the introduction of particle solids and fluidizing gases,

I said second steam turbine disposed within said second vessel above the grid therein, heat exchange means disposed within said second vessel above its grid and communicating with an exhaust conduit of said second tubine, a third steam turbine disposed within said third vessel above its grid, a steam conduit connecting the heat exchange means in said second vessel with a steam inlet conduit of said third turbine, heat exchange means cornmunicating with an exhaust conduit of said third turbine and disposed within said third vessel above its grid, a fourth steam tuurbine, and a steam conduit connecting said heat exchange means within said third vessel with a steam inlet conduit of said fourth turbine.

References Cited in the file of this patent UNITED STATES PATENTS 1,086,755 Curtis Feb. 10, 1914 1,889,586 Grebe Nov. 29, 1932 2,129,413 Evans Sept. 6, 1938 2,689,112 Gilmore Sept. 14, 1954 2,690,051 Peskin Sept. 28, 1954 2,729,428 Milmore Jan. 3, 1956

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