US3443550A - Two-section heat recovery steam generator - Google Patents

Description

May 13, 1969 E AY ETALY 3,443,550;

TWO-SECTION HEAT RECOVER!" STEAM GENERATOR Filed May 5, 1967 Sheet of 2 F|G.l F|G.2 '(PRIOR ART) (PRIOR ART) /-l2 wla r r 8 I i d OUT C- oMMM BURNER T T 44A INVENTORS'.

HOWARD F. MAY, JOHN M. KOVACIK,

THEIR ATTORNEYL May 13, 1969 H, MAY ETAL 3,443,550

TWO-SECTION HEAT RECOVERY STEAM GENERATOR 7 Filed May 5, 1967 Sheet 6 of 2 g A q 3 E o w w 0 WE m '5 9 3; L l l (F? F ll-l w 6 n 3'- TN k e z m k D4 1-: G x e m 2 e o g- 1' 1: Rr' N m '2 x1 fl/ u a x g 5 1 i 2 o 5 w a e 58- Vu. a w 1 n-x 2 9 r e 95 G E 4 so a D lNVENTORS HOWARD F. MAY, I JoHNMKovAcm, BY M THEIR ATTORNEY.

United States Patent 3,443,550 TWO-SECTION HEAT RECOVERY STEAM GENERATOR Howard F. May, Scotia, and John M. Kovacik, Burnt Hills, N.Y., assignors to General Electric Company, a

corporation of New York Filed May 5, 1967, Ser. No. 636,529 Int. Cl. F2211 1/02 US. Cl. 122-7 Claims ABSTRACT OF THE DISCLOSURE Exhaust fired heat recovery steam generator having parallel flow paths, one path disposed on the hot downstream side of an exhaust firing burner and one path disposed on the cooler upstream side thereof.

BACKGROUND OF THE INVENTION The present invention is related to steam generators. In particular, the invention is related to steam generators of the heat recovery type in which exhaust heat from an engine such as a gas turbine is used to generate steam. Exhaust heat recovery steam generators are known to the art, and many schemes for the same have been proposed. In general, the two main areas where economies might be achieved by improvements in steam generators are in operatiton and in construction. Economy of operation implies improved efiiciency of the steam generator and economy of construction implies improved steam generation capacity for a given capital or hardware expenditure. The heat recovery steam generator of the present invention is primarily directed at the latter economy, that is a relatively economic structure, as compared to the prior art, for a given steam generation capacity.

In prior art, unfired exhaust heat recovery steam generators, the gas which supplies heat to generate steam is generally passed once through the steam generator after exhausting from its first use and single firing. Exhausting at a temperature on the order of 300 F., the gas still contains more thermal energy than the ambient air. This gas also contains unused or excess air which would otherwise be available to combine with fuel for combustion.

It is known to the art to fire the exhaust gases, that is, to combine additional fuel with the excess air so as to add heat thereto. In operation, the prior art fire-d steam generators generally pass the boiler water and vapor once through the steam generator. The result is that the boiler water or vapor being heated only sees a gas temperature drop from, say 1300 F. to 300 F. or 1000. By the present invention, boiler water sees a gas temperature drop of substantially more than 1000 even though the initial and exhaust temperatures are the same 1300" and 300".

Accordingly, it is an object of the present invention to provide a fired exhaust heat recovery steam generator having an optimum arrangement of components for the generation of a given amount of steam.

Another object is to provide such a steam generator having an improved steam generation capacity for a given amount of heat exchange surface operating at a given maximum temperature.

Another object is to provide such a steam generator, operating between the same temperatures but providing a greater heat exchange from the exhaust gases to the water.

Other objects, advantages and features of the present invention will become apparent from the following description of one embodiment thereof when taken in connection with the accompanying drawing.

Patented May 13, 1969 Briefly, the present invention is practiced in one form by a heat recovery steam generator having an economizer, evaporator and super heater disposed in the path of gas turbine exhaust gases. A burner is located upstream of the economizer, evaporator and superheater to add heat to the gases. Upstream of the burner, and disposed to absorb heat from the unfired gas turbine exhaust, is a preevaporator which is in parallel with, and part of the same flow circuit as the evaporator.

DRAWING In the drawing:

FIG. 1 is a schematic diagram of a fired exhaust heat recovery steam generator, known to the prior art,

FIG. 2 is a schematic diagram of an unfired exhaust heat recovery generator, also known to the prior art,

FIG. 3 is a more detailed schematic diagram of a fired exhaust heat recovery steam generator as in FIG. 1, and

FIG. 4 is a schematic diagram of a fired exhaust heat recovery steam generator according to thepresent invention.

DESCRIPTION Referring now to FIG. 1, showing a fired exhaust heat recovery steam generator generally indicated at 2, an economizer 4, evaporator 6, superheater 8, and burner 10 are shown disposed Within a conduit or stack 12 which is in turn disposed in the path of exhaust gases from a heat engine such as a gas turbine. A drum 14 is located external to the steam generator stack 12 and is disposed to receive feedwater from the economizer 4 and in turn to discharge feedwater into the evaporator 6 by means of a pump 16. The evaporator 6 receives water from drum 14 and returns water and steam thereto through a circuit. The drum 14 also feeds steam to the tubes of superheater 8 from which steam is fed to its point of use.

Typically, the conduit 12 receives exhaust gases from a turbine and diffuser (not shown) and through a turning elbow 40 which directs the gases to turn from a generally horizontal flow direction to a generally vertical one. Such turning of the gas fiow results in pressure and flow disturbances or turbulence in the conduit immediately downstream of the turning elbow 40. To help remedy this, turning vanes 42 and screens 44 are included in the gas path to guide the gas flow to the burner 10 and to provide for a more uniformly flowing exhaust gas.

Referring now to FIG. 2, wherein like numerals designate like elements, an economizer 4, evaporator 6, and superheater 8 are again shown disposed within a stack or conduit 12 in the path of gases exhausting from a gas turbine. An external drum 14 and pump 16 are also included and serve the same purpose as in the fired steam generator mentioned above. The unfired steam generator of FIG. 2 does not include a burner but simply uses the heat of the exhaust gas without further firing.

It will be apparent that for a given exhaust temperature, the capacity which is provided by the unfired steam generator arrangement of FIG. 2 is limited and can be increased either by raising the gas temperature or by increasing the quantity of hot gas. This is the function of the fired exhaust heat steam generator. The fired steam generator adds fuel to the unused or excess air of the exhaust gas and thereby adds to the quantity of heat in the gas for steam generation.

Referring now to FIG. 3, a fired heat recovery steam generator similar to that in FIG. 1 is shown, with the evaporator section 6 in greater detail. Starting at drum 14, a drum outlet line 18 leads from the drum 14 to the pump 16 which in turn discharges through pump discharge line '19 into an evaporator inlet header 20. Evaporator inlet header 20 communicates with a plurality of 3 evaporator tubes 6 which are disposed in parallel and which in turn communicate with evaporator return header 22. Return header 22 leads through return line 24 back into the drum 14. In short, evaporator tubes 6, headers and 22, lines 18, 19 and 24, and drum 14 constitute a circuit A.

In a typical application of the steam generator of FIG. 3, gas turbine exhaust approaching the burner at approximately 900 F. is heated by means of the burner to approximately 1300 F. and is then passed over the superheater, the evaporator, and the economizer tubes, finally exhausting at a temperature of approximately 300 F., to generate a given amount of steam in the steam generator. This provides a heat exchange corresponding to (1300-300) or 1000".

Referring now to FIG. 4, and with FIG. 3 as a reference point, a heat recovery element or pre-evaporator 26 is disposed within the stack 12 upstream of the burner 10 and downstream of turning elbow 40. Pre-evaporator 26 communicates with a pre-evaporator inlet header 28 and a pre-evaporator outlet header 30. The pre-evaporator inlet header 28 is connected by an inlet line 32 to pump outlet line 19 on the discharge side of pump 16. Preevaporator outlet header is connected by return line 34 to return line 24 immediately upstream of the drum 14. That is, evaporator tubes 26, headers 28 and 30, lines 18, 32 and 34, and drum r14 constitute a circuit B. Lines 32 and 34, headers 28 and 30, and evaporator tubes 26 of circuit B are connected in parallel with lines 19 and 24, headers 2'0 and 22 and evaporator tubes 6 of circuit A.

It will be seen that the pre-evaporator 26 in FIG. 4 is essentially a number of the evaporator tubes 6 of FIG. 3 moved into a different position before the burner 10.

As one example, assume that in the FIG. 3 steam generator there were 14 rows of steam generator tubes 6 and that in the FIG. 4 steam generator there are eight rows of evaporator tubes 6 and six rows of pre-evaporator tubes 26. That is, that within the stack 12, the same total steam generator structure or hardware exists in both cases. We have found, however, that, using the pre-evaporator arrangement in FIG. 4 and a burner firing temperature of 1300 in each case, the arrangement of FIG. 4 provides a capacity increase of approximately over the FIG. 3 steam generator.

The foregoing FIGURE 4 embodiment, while presently preferred, is not the limit of our inventive concept. The pre-evaporator 26 and its flow circuit B need not necessarily be connected with flow circuit A. If desired, a separate circuit B could be made to include a separate drum 14, pump 16, etc. In fact, there could be, upstream of burner 10, not only a pre-evaporator 26, but an economizer and a superheater. The net effect of such an arrangement would be to have an unfired exhaust heat recovery steam generator and a fired exhaust heat recovery steam generator in series (with respect to gas flow). This would provide a substantially increased steam generation capacity, but at a correspondingly increased hardware cost.

By way of comparison with the illustrative temperatures given for the steam generator of FIG. 3, gas turbine exhaust in FIG. 4 at 900 upsteam of the pre-evaporator is cooled by the pre-evaporator to 600 R, which is its temperature approaching the burner 10. The burner raises the temperature to 1300 which after passing over the superheater, evaporator tubes, and economizer reduces to 300 exhaust. This provides a heat exchange corresponding to (900-600) and (1300-300) or 1300. The foregoing temperature and number of rows, given in connection with the FIG. 3 and FIG. 4 steam generators, are examples only and not to be construed as limiting in any way the inventive concept herein disclosed.

We have found that the present invention approaches an optimum arrangement for providing improved boiler or steam generation capacity with a minimum of steam generation hardware. A single steam generator, as in FIG. 1 or FIG. 2, provides a given steam output capacity for a given amount of steam generator hardware. The optimized arrangement of this invention provides a substantially increased steam generation capacity without a corresponding increase in structure or capital cost.

An additional feature of the present invention is one of flexibility. All boilers should be fired at their maximum temperature, thus keeping at a minimum their required heat transfer surface. As applied to the present steam generator, any additional capacity of steam flow desired in addition to the basic capacity of the fired steam generator and over a range of from zero to about 40% thereof, can be met by controlling the size of the pre-evaporator. This gives the manufacturer more flexibility in that the same fired steam generator can be installed with various size pre-evaporators to provide different capacities for different requirements. The maximum firing temperature desired, as, for example, 1300", can thus be maintained over an output range of 40% since the basic fired steam generator is operating at full capacity, keeping the steam generator at its minimum possible size. This economy of manufacture results in economy of structure even though the pre-evaporator, which maybe of varying size on different units, will not always result in the optimum arrangement. Another feature of the present invention is that burner operation is improved because of improved gas flow. As discussed above in connection with the FIG. 1 prior art steam generator, how vanes and screens are generally required to provide gas flow distribution to the burner. In the present invention, the gas flow distribution to the burner is improved by the introduction of pressure drop in the gas turbine exhaust by the presence of the pre-evaporator. The pre-evaporator thus serves as an incidental aid to the combustion process in the burner and obviates the screens 44 of FIG. *1.

It will be appreciated that an exhaust heat recovery steam generator has herein been described which permits of substantial economies of manufacture and structure and which has an improved steam generation capacity for a given amount of heat exchange surface. The improvements realized by the present invention are the result of a more effective use of the exhaust gases as a heat source and as a supply of excess air.

It may occur to others of ordinary skill in the art to make modifications of the present invention which will remain within the concept and scope thereof and not constitute a departure therefrom. Accordingly, it is intended that the invention be not limited by the details in which it has been described, but that it encompass all within the purview of the following claims.

What is claimed is:

1. A heat recovery steam generator including an economizer, an evaporator, and a superheater disposed in the path of hot gaseous products of combustion, and

a burner disposed in said path upstream of said economizer, evaporator, and superheater to reheat said gaseout products, wherein the improvement comprises,

a heat recovery element disposed in said path directly upstream of and adjacent said burner to absorb heat from said gaseous products, said heat recovery element defining a fluid flow path connected in parallel with at least part of the flow path through said economizer, evaporator and superheater.

2. A heat recovery steam generator as defined in claim 1 further including a drum located external to said path, said evaporator disposed in, and forming a part of, a circuit to receive fluid from and to return fluid to said drum,

said heat recovery element comprising a pre-evaporator disposed in and forming a part of a parallel circuit to receive fluid from and to return fluid to said drum.

3. A heat recovery steam generator as defined in claim 2 in which said evaporator includes a discharge header and a return header and a plurality of parallel evaporator tubes extending therebetween, and

said pre-evaporator similarly includes a discharge header and a return header and a plurality of parallel pre-evaporator tubes extending .therebetween.

4. A combined heat recovery steam generator including a fired steam generator and an unfired steam generator, said fired steam generator having a burner and disposed downstream of said unfired steam generator with respect to the flow relative thereto of hot gaseous combustion products, said unfired steam generator having heat recovery elements directly adjacent and in line with said burner to bathe the flow of gas thereto,

5. In a heat recovery steam generator having a stack connected to a source of combustion-supporting waste heat gas and containing a plurality of substantially horizontal heat exchange tubes connected with pumping means to recircula'te fluid to and from an external steam drum, the improvement comprising:

a first group of evaporating tubes connected in parallel flow relationship with each other and in series flow relationship with the steam drum and pumping means,

a second group of similarly connected evaporating tubes spaced upstream in the stack from the first group, and

a burner assembly disposed in the stack between the first and second groups and directly adjacent the second whereby the second group precools the gases before firing and distributes the fiow to the burner assembly.

References Cited UNITED STATES PATENTS 2,547,589 4/1951 Marshall 122-7 2,926,493 3/1960 Poole et al. 3,314,231 4/1967 Hochmuth 6039.18

FOREIGN PATENTS 186,665 9/1956 Austria.

CHARLES J. MYHRE, Primary Examiner.

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