US2902830A - Steam power plants - Google Patents

Description

Sept. 8, 1959 w. LENZ STEAM POWER PLANTS 4 Sheets-Sheet 1 Filed June 28, 1956 FIG. 2

m elvnr MLeIZ/Z Sept. 8, 1959 w. LENZ STEAM POWER PLANTS 4 Sheets-Sheet 2 Filed June 28, 1956 Sept. 8, 1959 w. LENZ STEAM-POWER PLANTS 4 Sheets-Sheet '5 Filed June 28, 1956 FIG.5

II I Sept. 3, 1959 w, LENZ 2,902,830

STEAM POWER PLANTS Filed June 28, 1956 4 Sheets-Sheet 4 INVEN TOR United Sttes Patent 2,902,830 STEAM POWER PLANTS William Lenz, Bergisch-Gladbach, Germany Application June 28, 1956;-Serial No. 594,535

Claims priority, application Germany July 2, 1955 11 Claims. (Cl.'60-- 73) It is known that the thermal efficiency of a steam circuit between a steam generator and a steamturbine can be substantially increased if-the average temperature of the energy conversion is kept as high as possible. For this reason isothermal expansion is aimed at a temperature which is equal to the highest temperature arising in the circuit. It can be approximated by multiple intermediate superheating during the expansion, as represented by a sawtooth curve in the entropy-total heat diagram. Technically, however, a repeated return of the steam from the turbine to the boiler and from the latter back to the turbine would cause sohigh a pressure loss in the v steam that a large part of the thermal gain woul-dbe lost. Also the plant would be made muchmore expensive and its construction would be so complicated and'so-diflicult tosupervise that its operation would be'rnade much more Furthermore, a high starting pressure is also necessary for multiple intermediate superheating. It now the steam is superheated to the final temperature in the boiler dilficulties are met with in the last part of the sup erheater and in the pipeline from the superheatertothe turbine because these are at the same time exposed to high pressure and temperature stresses. In addition, in the case of superheaters heated b y ilue gases difficulties arise ,due to the high temperature of theflue gases which lead to high temperature stresses in the tube walls, while the great wallthicknesses which the additional temperature stresses require, may lead to non-uniform heating of the heat transfer. surfaces. For thesereasons, in order to achieve adequate cooling of the superheater tubes high steam speeds are necessary which in turn again resultindisw advantageous increased pressure losses.

To overcomethe'hrst mentioned disadvantage it has already been proptos'edttoefiect the intermediate superheating not in the steam generator itself 'butin aheat carrier which is brought to a high temperature in the! steam generator and which transfers the heat to the steam in the intermediate stages, the intermediae: superheater beingmounted near the turbine.

According to the invention, theadvantages above set out ofisothen'nal expansion in as finely divided as:pos-.

sible-asequenc'e'of stages are ensured by using for-all the superheating steps divisional currents of heat carrier workinginparallel, whichcarrier is used also forfheating the steam to the turbine entry temperature-immediately-before it:enters the turbine. A heat carrier is used of;

which the boiling point atnormal pressureisabove-its highest operating'temperature.

Such fluid heat carriersare inthernsel-ves known. To

' increase-thehe'at transfer, they arecirculated bya plnnp 'and=aswell ashaving a low vapour pressure they have 1' transfer, a not too'high specific weight in-order t'o-ke'ep i temperature. 7

i tem e tu e- 55 'Qnli h l ltii tq his nerhe er. do

the pressure losses low and also have no chemical, aflinity for steelat the temperatures arising, of which material the heatingvsurfaces, pipelines, intermediate,superheaters and circulating ,p'ump consists. Also they must notqreact with water, as contact between. the heat carrier and water can" easily occur Where there. are leakages orv where damage occurs. i

Molten metals such as lead or alead bismuth orlead magnesium alloy have been found best, as such: heat carriers, butmolten' salts canalso, befused, pressure losses in the circuit which have tobe provided by the circulating pump can be heptvery low,: aswith lig .ds only relatively 10w speeds are nec essa ry 'inorder; with good, heat conductivitv'to lattaina high figurevofrfheat transfer. 'Fdnthislreason 'andon account ofi the small vo m of t e q it ef ne y requ r ment of th pump'fare substantially smaller than thesaving in relabe relatively small and the intermediate superheaters caja iecmp ito theturbineu entryfternperatulfe; but only to a temperature ,whiehflies below thelowest temperature of the heat (:attieii- This; again, is determined by. the, temperature risefin theZintermediate superheating. In

i ie r l eb stie he hi hes .p sssut -i m F W iimP i-il fir? L 1,. ob n b a ath hi hsstt rsr e 4 steam arising 'iri'the'steam'generator is relatively lo vand in s it g i h P e ure issan be ra s wl hea rsuffaclesj Fel e ei w sa r bfy m taial w h m derate" walhthicknesses, while at the, temperatures to which the ,heat carrier must be raised there is practically no frais dbres ur t e e a sgt wallihickaes s fromhthe steam generator to the ;tu rb 1' ne, can beponstructed withrelatively thin walls; Wallt peratures of he na l n r' m lfor th -he a. er arefllen it is ftriuefhi gher by afcertain aniountth H i hem i P W t ise a aby s' ei i nt h r h nl e -l1.i hest=.stea rt mpert ir a n 'p' in t hi= l9 l q Q PQWG J it h Ste au h glh ul r pe heat emperatur 6 di e tly, be o e the rS gu d Madeline f. l the t rbi y a. une ea t r s:ava by :1 mea ie hi h Pru 8.1 .2 and-,ternperature str sses arise together in th ubes qarrysfi HereLhW i en mb d am .en uhstantially smaller than in the steam generator ca beusedand there is e r sk fidposits. Forflthipsrre on a in; hewa lth ekess ther q e th m rat r s res an heake low. I

Asr'all the intermediate ,superheaters. according to the inventionare arranged; in parallel between.the pipeline for the hotheatcarrierv andfthe retur'nrpipeline for: the cooled heat carrier, so that each intermediate superheater is traversed bya' divisional curreritfof the'h' rier, only 'a single pipeline'for'tliehot heateat-reread asin gIe return pipeline for the cooled heat carrier is necessary. If the intermediate superheaters are traversed by the steam and the heat carrier in countercurrent, it is then possible to superheat the steam each time to the inlet steam temperature. Countercurrent flow can be obtained by providing a steam-carrying pipe with a concentric pipe about it and leading the heat carrier in the intermediate space in the opposite direction to the steam or by letting the heat carrier flow in a cross direction over the steamcarrying pipes but directing it by walls in such a way that overall the heat transfer takes place in countercurrent.

In order to keep the pressure loss on the steam side as low as possible the steam carrying pipes are made straight and the exit speed of the steam from the last row of rotor blades before each intermediate superheating stage is transformed with as little loss as possible into pressure by suitably shaped guide blades. In this way the speed is reduced to the value necessary for the transfer of heat. These guide blades at the same time take care of the distribution of the steam between the individual tubes of the intermediate superheater. The supply of the heat carrier is effected as uniformly as possible over the Whole periphery of the turbine in spirals, and the outflow of the heat carrier can take place in a similar manner at the outer periphery.

Further, from a particular value of steam pressure on, no further intermediate superheating is effected. At higher pressures, on the steam side the value of heat transfer is so high on account of the great density of the steam that only relatively small heating surfaces, that is, short tubes, are needed in order to transfer the heat necessary for the intermediate superheating, so that only small pressure loss is suffered. The stated steam pressure is chosen either so that during the expansion from this pressure down to the condenser pressure the saturated steam curve is reached or even downwardly crossed or so many intermediate superheating stages are provided that the exhaust steam of the turbine only has a certain degree of superheat.

The above mentioned temperature at which the steam leaves the steam generator can be substantially lower than the lower temperature limit of the intermediate superheater stages. Heating from the exit temperature from the steam generator up to the lower temperature of the intermediate superheating stages can be effected in this case in two ways. One possibility is to carry the whole return flow of the cooled heat carrier from the intermediate superheaters to the first superheater at a place at which the superheater heated by the heat carrier has the same temperature as the subdivided current of the heat carrier, and making this superheater sucha size that the collected total return flow of the heat carrier heats the steam from the exit temperature of the steam generator at which it enters the steam pipeline leading to the turbine up to the lower temperature of the intermediate superheaters. Theother possibility is that with intermediate superheating eifected at so low a pressure that the steam at condenser pressure is still superheated, the superheat of the steam leaving the turbine is used to heat the live steam and the rest of the heating up, up to the lower temperature of the intermediate superheating stages, is effected by the total return flow of the heat carrier. The remainder of the superheat in the exhaust steam is taken up in a water preheater traversed by the whole or part of the feed water.

The steam generator necessary for supplying the steam circuit according to the invention is a heat transferring device which consists only in part of Water or steam cooled heating surfaces and in part of about the same size of heating surfaces which are cooled by the liquid heat carrier. The rest of the heat transferring surfaces in the steam generator is taken by an air preheater for an air temperature of at least 400 C. In order not to make the heating surfaces of the air preheater too large, a twopart construction of the air preheater is advisable between the parts in which a small water preheater is incorporated. Particularly with high pressures, the residual evaporating and superheating heat only suffices for a part of the heat take-up in the combustion chamber. The rest of the heating surfaces of the combustion chamber is formed by heating surfaces over which the heat carrier flows and by the contact and radiation heating surafces between the combustion chamber and the air preheater.

It is known that the heating surfaces of steam generators and therefore similarly as regards the heating surfaces used in the invention for heating of the heat carrier, can be considerably reduced if pulsating combustion is used.

As pulsating combustion takes a very short time, as is in itself known, very high combustion temperatures are reached with small burner volumes but it requires an intensive cooling of the burner nozzles. Hitherto burners have been cooled by tubes which are under full boiler pressure.

On account of the small diameter of the burners and the necessity to cover them on the inside with a heat insulating layer, in order to be able to draw off the ash .in molten condition, it is very diflicult to rectify any leakages which may develop in these cooling tubes.

According to the invention the burners are cooled by the heat carrier, which is not under pressure. As the heat carrier not under pressure is cooled the burner can simply consist of a double jacket of sheet metal on the inside of which the heat insulating layer is provided. Such a burner can not only be made more simply and be more reliable in operation than a burner constructed of cooling tubes but it is also more convenient for dismounting and simpler to repair if a leakage should develop, the likelihood of which is, however, much less than in the case of a burner for pulsating combustion provided with cooling tubes under full pressure.

The following are, by way of example, data for the circuit:

Temperature of the live steam and of intermediate Figure 1 of the accompanying drawings is a diagram showing the layout of the plant for the case in which the intermediate superheating is only carried to such a value that at the end of the expansion the steam is no longer superheated.

Figure 2 is the corresponding entropy-total heat chart.

Figure 3 shows diagrammatically the layout of another plant in which the exhaust steam fromthe turbine is at the lower temperature of the intermediate superheating stages. 1

Figure 4 is an entropy-total heat chart corresponding to Figure 3, and

Figure 5 is a diagrammatic section of a boiler used in the plant according to the invention.

Fig. 6 is a section through a heat exchanger for superheating or reheating the steam.

The stages marked with capital letters in Figures 1 and 3 have the same capital letters indicating the corresponding state of the steam in the chart in Figures 2 and 4. In the charts, K indicates the critical point for water vapour. From the fact that the live steam isobars are above the point K, it will be seen that the live steam is above the critical condition. In Figures, P and 3 the water circuwith lat-ion is indicated by the thin solid line, the steam circulation by the thick solid line and the heat carrier circulation bythe thick broken line.

'In Figures 1' and 2, at A the steam enters with low sup'erheatthe first superheater heated by the heat carrier. The "first divisional current of the heat carrier at high temperature enters this superheater at C, while at B the total return flow of the heat carrier from the intermediate superheaters enters this preheating superheater. The entire 'return flow of the heat carrier flows through the first superheater from B "to 'A, where itleaves, after having raised the steam temperature to the lower temperature of the intermediate superheating stages. From A the heat carrier returns to the steam generator. From A to B the steam is raised to thelower temperature of the intermediate o'r reheater superheating stages, from B to C to the top live steam temperature and is then expanded from th'eto'p liy'esteampressur'e 'atC to the pressure of the first intermediate superh'eating or reheating stage at D;

where it has reached the lower temperature limit-f the intermediate superheatirig stages. In the first intermediate superheater or reheater between D and E it has its temperature again raised 'to the live steam temperature b'y'the' second divisionalcu'rrent of heat carrier, it is expanded further to P, where the second intermediate super-- The heat exchangers, a's'shown in Fig. 6, have straight extending-throughout steam tubes which are welded between plates 11 and, 12. One of the plates. 12 is rigid ly screwed with the outer mantle 13 of the heat exchanger and pipe connection 1 of the turbine. To the other platell a connecting member 15 is joined by screws- --16.' Thefresilient'seal between the plate 11 and the exterior mantle 13 is" brought about by a ring seal 17 of about imangular profile which can be tensioned by means of a pressure ring 18 and screws 19.

"*Tlie'diquidhat ca'rrie'r flows through the heat exchanger owing to the built-in guiding walls 20 in coil shape 'andis delivered or drained by connections 21 and 22' Because of thestraight steam tube It the resistance to flow of the heat exchanger for the steam is very slight.

The condensate formed in the condenser a is forced by the condensate pump b through" a bleed-off preheater c, is broughtto full pressurein the'feed pump d and is forced through the feed water preheater 2 into the section of the boiler provided with the steam generating heating surfaces 7. These advantageously lie in that part of the furnace in which the burners g are located. The steam then flows through the steam pipe h to the entry A of the above mentioned first superheater heated by the heat carrier.

The return flow of the whole of the heat carrier is effected by a circulating pump 1 which directs it through a preheater k in the walls I surrounding the upper part of the furnace and then through further heaters having radiation and contact heating surfaces In and thence through a pipe n to the entry to the intermediate super heaters. The divisional currents flowing out of the intermediate superheaters are collected in a pipe 0 and led at B into the first superheater. It will be understood that the steam expansions above referred to take place in a turbine and the output of the turbine is transformed into electrical energy in a generator p. The flue gases are reduced to the desired discharge temperature in a high temperature part q of an air preheater and a low temperature part r, between which the feed water preheater e is located.

Figure 3 shows the layout and Figure 4 the corresponding entropy total heat chart for the case in which the exhaust steam from the turbine is still at the lower temperature of the intermediate superheating stages. The intermediate superheating stages coming between the first two and-the last two are omittedfrom Figure 3 and reference letters are therefore omitted from the corre sponding stages in Figure 4'. Here the expansion from M to His down to condenser pressure but only down to the lower temperature of the intermediate superheating stages. The first'part of the superheat of the exhaust steam is used in a-heat exchanger s to bring the live steam from condition Ato conditionA', at which it now enters the first superheater. The exhaust steam then flows through a heat exchanger t in which the whole or a part of the condensate after outflow from the bleed-off preheater c is further heated. It is advantageous, in order to obtain the same temperature ditrerence's in the flue gas heated feed water preheater e as in the first exampleyonly 'to branch ofi apart of the feed water at the point P and after preheating this in the preheater t to send it direct into the boiler part '1 without passing through the preheater e. The more rapid heating up of the other part ofthe feed water by the preheatere gives less desirable temperature differences for the preheater e and the air' preheater q and therefore requires larger heating surfaces for'this, but thesame outflowgas temperature can nevertheless be obtained as in the former example. On account of the preheating of the feed water to as great an extent as possible in the bleed-off preheaters, only a part of the residual superheat and exhaust steam'can be extracted with the preheated feed water. Accordingly at Q part of the condensate is branched 'off and is' preheatedin-a third heat exchanger heated by the exhaust steam up to about the same temperature at which theother part comes out of the bleedoif'pr'elieate'rs, whereuponit is addedf-atR to the bleed-off preheated part. Less 'ble'ed-oif steam can then indeed be-used," while more steam'does its full work in the turbine and the sup'erheatin the exhaust steam is fully utilised. The pressure loss in the exhaust steam-in the three heatexchangers' s,"t, u results in acertain energy loss but this is less than the heat recovery.

. Between the two limiting cases in which the exhaust steam has no superheat and in which it-hasthe lower temperature of the intermediate superheating stages, any desired intermediate stage is'possible. The expansion in the turbine is then carried after one of the intermediate 'su'p'erheating stages down to condenser pressure and for example, the heat exchanger s is made smaller or entirely omitted while the he'at'exchanger t for feed water may also'be reduced in size or even omitted.

"FighreS diagrammatically illustrates in Verticallongitudinal section a boiler plant with pulsating combustion provided with burners cooled in accordance with the present invention, in which a heat carrier not under pressure is used. In Fig. 5 the path of the liquid heat carrier is indicated by dash lines and leads over the cooling mantle 10 of the pulsating burners.

The shaft fonn furnace 3 with a conical slag discharge hopper 4 is covered in its lower part with cooling tubes 5 in which steam is generated under the prescribed pressure and at a temperature which is somewhat above the corresponding saturated steam temperature. Through a pipe 7 the condensate reaches the preheater 8 and thence is returned into the boiler. The fluid heat carrier not under pressure which is cooled down in the intermediate superheaters through a pipe 9 reaches the burner cooling jackets 10 and thence passes into the radiation heated section 11, the contact heated section 12 and a pipe 13 to the intermediate superheaters, where it gives up a part of its heat content to the steam and heats the latter to the turbine inlet temperature. Differing from this arrangement, the burner cooling can also constitute the final stage in the heating up of the heat carrier, if the higher temperatures arise in the burners themselves.

I claim:

1. A steam circuit comprising piping, a steam boiler having a furnace, a multi-stage steam turbine, intermediate superheaters for superheating the steam in the circuit between one turbine stage and the next, a first superheater at the first turbine stage for superheating live steam from said boiler to the first turbine stage, means for circulating a fluid heat carrier through said intermediate superheaters arranged in parallel to effect the heating thereof and through said boiler to heat the heat carrier, the heat carrier being under low pressure and having a boiling point at that pressure which is above the highest temperature employed in the plant, the parts of said boiler in which water is heated and steam is generated surrounding at least part of said furnace and the remainder of the heating surfaces of the boiler serving to heat said heat carrier, said boiler including further heat exchange devices through which flue gases from said boiler pass after heating said steam and heat carrier for heating of materials for ancillary services to the boiler by the flue gases whereby the flue gases are cooled to a lower temperature than the lowest temperature of said heat carrier during its circulation.

2. A steam circuit as set forth in claim 1 wherein each of said intermediate superheaters comprises steam tubes and means for conveying said heat carrier along the outside of said tubes in counter-current flow to the flow of the steam.

3. A steam circuit as set forth in claim 2 wherein said intermediate superheaters comprise said steam tubes each surrounded by a concentric tube through which said heat carrier flows in a counter-current direction.

4. A steam circuit as set forth in claim 2 comprising guide walls in each said intermediate superheater located to guide said heat carrier in a path which crosses said superheater tubes but which is overall in counter-current to the steam flow within the tubes.

5. A steam circuit as set forth in claim 1 where said furnace incorporates burners operating under pulsating combustion, said burners being provided with coloing jackets through which said heat carrier is circulated.

6. A steam circuit as set forth in claim 5 wherein the heat carrier on its return to the boiler first passes through said cooling jackets.

7. A steam circuit as set forth in claim 5 wherein the heat carrier in its passage through the boiler last passes through said cooling jackets.

8. In a steam power plant including a steam boiler and at least one multi-stage steam turbine, a pipe system for steam and water, a superheater at said turbine for superheating steam from said boiler, reheaters arranged between the stages of said turbine, a pipe system for a liquid heat carrier, said boiler including a combustion chamber lined with one set of tubes in which the steam for said multistage turbine is generated and another set of tubes in which said heating medium is heated, said sets of tubes being arranged in two systems built into the boiler of which the one tube system disposed'where the flue gases in the combustion chamber are cooler serves for generating steam for operation of said turbine and the other tube system disposed adjacent the high combustion temperature portion of the combustion chamber contains said liquid heat carrier comprising a non-evaporating liquid which flows through said pipe system, passes through said superheater and reheaters and serves for heating the steam in said superheater and reheaters before and between the turbine stages.

9. A steam power plant according to claim 8, in which each said steam reheaters disposed in front of each turbine stage has straight tubes for conducting the steam around which the heat carrier is conducted in countercurrent or transverse current.

10. A steam power plant according to claim 8, and including a preheater between said boiler and said superheater and, in which the liquid heat carrier is heated to the required hot steam temperature and the pipe system therefore is divided between the superheaters and the reheaters arranged in parallelism, and the parallel portions of said pipe system are recombined into a single pipe to heat the steam in said preheater.

11. The power plant of claim 8, in which the heat carrier is selected from the group consisting of a molten salt, lead, lead bismuth mixture, and lead manganese mixture.

References Cited in the file of this patent UNITED STATES PATENTS 1,723,302 Ruths Aug. 6, 1929 1,782,220 Wanamaker et a1. Nov. 18, 1930 1,965,427 Nerad July 3, 1934 2,158,856 Emmet May 16, 1939 2,593,963 Biggs Apr. 22, 1952 2,595,822 Uggerby May 6, 1952 2,644,308 Downs July 7, 1953 2,653,447 Heller Sept. 29, 1953 FOREIGN PATENTS 220,960 Great Britain Nov. 6, 1924 246,593 Great Britain Feb. 1, 1926

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