US3196844A

US3196844A - Method and apparatus for controlling a forced flow steam generator

Info

Publication number: US3196844A
Application number: US115834A
Authority: US
Inventors: Sulzer Jean-Th
Original assignee: Sulzer AG
Current assignee: Sulzer AG
Priority date: 1960-03-30
Filing date: 1961-03-27
Publication date: 1965-07-27
Anticipated expiration: 1982-07-27
Also published as: CH376129A; GB950212A; ES259284A1; BE601768A

Description

y 27, 1965 JEAN-TH. SULZER 4 METHOD AND APPARATUS FOR CONTROLLING A FORCED FLOW STEAM GENERATOR I Filed March 27, 1961 5 Sheets-Sheet 1 Fly. 1

REA/V1 2. \SULZER 5 y 7, 1965 JEANTH. SULZER 3, ,84

METHOD AND APPARATUS FOR CONTROLLING A FORCED FLOW STEAM GENERATOR Filed March 27, 1961 5 Sheets-Sheet 2 Fly. 2

P] i P! l y 27, 1965 JEAN-TH. SULZER 3,196,844

METHOD AND APPARATUS FOR CONTROLLING A FORCED FLOW STEAM GENERATOR Filed March 27, 1961 5 Sheets-Sheet 3 Fly. 79 e0 y 27, 1965 JEAN-TH. SULZER 3,196,844

METHOD AND APPARATUS FOR CONTROLLING A FORCED FLOW STEAM GENERATOR Filed March 27, 1961 5 Sheets-Sheet 4 Fly. 5 V

n/A/VFl 501. ZER

July 27, 1 JEAN-TH. suLzER 3,196,844 METHOD AND APPARATUS FOR CONTROLLING A FORCED FLOW STEAM GENERATOR 5 Sheets-Sheet 5 Filed March 27, 1961 1 1 1 1 1 11 11 1 1 1 11 1/ 1 11 1 11 1 1 1 1 0 1 1 1 "1 11 1, \Q 1 11 1 1 1 1 1 1 1 1 1 1 11 1 11 1111 1 1 111 1 11 1 1 11 1 1 1 1 1 1 1 7 7 11 1 1 11 1 1 11 1 I KII 1 1 M 1 1 1 1 1 1 1 1 111 1111 1 11111 1 1 1 1 1 1 1 1 7 1 1 1 1 7 7 1 1 1 1 1 1 1 u 1 SQ km mm \m% 5 United States Patent 6 Claims. ici. 122-448) The present invention relates to the control of a forced flow steam generator, particularly one operating at supercritical pressure, whereby one of the factors determinative of the output of the steam generator is adjusted either automatically or by hand to correspond to the required or desired output and one or a plurality of the other factors determinative of the output is or are adjusted by additional control means to correspond to the output to which the first factor is adjusted.

It is conventional to adjust the fuel supply to the furnace of a forced flow steam generator in response to the steam consumption, for example, of a steam turbine supplied by the steam generator and to conform the feedwater supply to the steam output by adjusting the feedwater supply to maintain a predetermined steam temperature. The rate of feedwater supply is increased upon an increase of the steam temperature at the end of the evaporating section of the steam generator and decreased upon a drop of said temperature. Alternatively, the steam consumption of the steam consumer can be used to control the feedwater supply and the fuel supply can be adjusted in response to the temperature of the operating medium after it has passed through a portion of the tube system of the steam generator. In motive power plants, for example in plants for propelling vessels, one of the factors determinative of the output of the steam generator, namely the fuel supply or the feedwater supply, can be adjusted by hand to correspond to the steam output required for producing the desired speed of the vehicle or vessel whereupon the other of the factors is adjusted to correspond to the desired output by controlling the other factor to maintain a predetermined temperature of the slightly superheated steam leaving the evaporating section of the forced flow steam generator.

The aforedescribed method is not very well suited for controlling a forced flow steam generator operating at very high pressures, particularly at supercritical pressures, because the differential quotient dt/di wherein 1' stands for temperature and i for heat content, is very low at the sections of the tube system of the steam generator where the water is converted into steam which sections are otherwise most suitable for obtaining a signal for controlling the feedwater supply or the fuel supply. Because of the low differential quotient dt/di ascertaining of the heat content of the operating medium by measuuring the temperature thereof is very inaccurate.

According to the invention the adjustment of a second factor determining the output of a forced flow high pressure steam generator after adjustment of a first factor determining the steam output to correspond to the desired output is effected in response to the specific volume or to the density of the operating medium flowing through a section of the tube system of the steam generator which is particularly suitable for the aforedescribed control method.

The density or the specific weight of the operating medium is the reciprocal value of the specific volume. These three values have a high differential quotient for the temperature dependence in the region in question.

Since density, specific weight and specific volume are equivalents in the control system according to the invention they have been generically termed internal characteristics of the operating medium.

An object of the invention is the provision of a combination of a plurality of elements for controlling the operation of a tubular high pressure forced flow steam generator operating under variable load conditions whereby the feedwater supply or the heat supply to the generator is adjusted either manually or automatically to correspond to the load on the steam generator, i.e. to the desired steam output thereof, and the supply not so adjusted is conformed to the adjusted supply by adjusting the supply which is not initially adjusted to correspond to the load, in response to an internal characteristic of the operating medium, which is either the specific volume or the density of the medium, preferably as the medium passes from the evaporating tube section of the steam generator to the superheating section.

It is a further object of the invention to provide an apparatus for continuously measuring changes of the specific volume of a flowing fluid whereby the change of the shape of a yieldable or deformable container filled with fluid of the type of the fluid whose specific volume must be measured or an equivalent thereof and placed in and subjected to the pressure and temperature of the flowing fluid is used for indicating changes of the specific volume of the fluid and may be used for producing control signals corresponding to the changes.

In a modification of the aforedescribed apparatus for measuring changes of the specific volume of a flowing fluid the container is formed by elastic bellows permitting a difference of the pressures inside and outside of the container and this pressure difference is used to actuate means acting on the container for altering the internal volume thereof to make the pressure within the container equal to the pressure outside of the container. The action of this pressure equalizing means is representative of changes of the specific volume of the fluid and the equalizing means may be used to produce control signals for actuating other control apparatus in response to such changes.

The novel features which are considered characteristic of the invention are set forth with particularity in the appended claims. The invention itself, however, and additional objects and advantages thereof will best be understood from the following description of embodiments thereof when read in connection with the accompanying drawing, wherein:

FIG. 1 is a graph to explain the method according to the invention.

FIGS. 2-4 show different circuit diagrams of steam generators according to the invention.

FIG. 5 is a longitudinal sectional view of a device for measuring the specific volume of the operating medium.

FIG. 6 is a longitudinal sectional view of a device for measuring the density or" the working medium.

In the diagram shown in FIG. 1, the temperature 2 and the specific volume v of water at 270 atmosphe e absolute pressure are plotted against the thermal cap 1 i. The diagram shows that in the region between 566 700 kcaL/kg. which is the heat content in the conversion part of the steam generator, the curve representing the specific volume v is very steep in comparison with the temperature curve 1 which is flat in this region. Accordingly, adaptation of the steam generator to different load conditions in dependence on the specific volume of the working medium or on the reciprocal values, namely density or specific gravity, is much more accurate than such adaptation in dependence on the temperature t.

FIG. 2 diagrammatically illustrates a steam power plant including a steam generator and a turbine and comprising a feedwater tank 1, a low pressure feed pump 2, a

water preheater 3, a high pressure feed pump 4, a rate of feedwater flow measuring device 5,

tubular heating sections

6, 7, 8, 9, a throttle member 10, a turbine 11 with a condenser 12, a condensate pump 13 and

fcedwater preheaters

14, 14. The water is evaporated in the section 7 and the produced steam is superheated in the

sections

8 and 9. Between the

heating sections

7 and 8 is a measuring station 15 where a measuring device 16, which will be described later, measures the density p of the Working medium. The measuring device 16 transmits a corresponding signal to a regulator 17, which transmits aregulating signal to a subsidiary regulator 19, which signal regulates the set point of the regulator 19. The regulator i9, which has a proportional (l) or proportional-integral (PI) characteristic, receives a signal from the flow measuring device 5, and in the event of deviations appropriately adjusts the rate of feedwater supply by means of the feed pump 4. The primary load or output adjustment of the steam generator is effected by a speed measuring device 2t) on the shaft of the turbine 11, which device acts on a load signal transmitter 21. The load signal transmitter 21 adjusts the rate of heat output of a furnace 22 in dependence on the speed of the turbine shaft, by acting on

throttle members

23 and 24 disposed in the supply conduits for the air and fuel. The load transmitter 21 also transmits a loaddependent set point signal to the regulator 17, adjusting the set point of the latter to correspond to the working medium density which is to be maintained at the load conditions in question. Furthermore, the load transmitter 21 transmits a load-dependent signal as a preliminary impulse to the regulator 19 which signal is added at an addition station 18 to the signal transmitted by the regulator 17.

In the arrangement shown in FIG. 2, the furnace 22 is adjusted primarily by the load transmitter 21 in dependence on the load on the turbine 11. If the fire intensity is increased while the feedwater supply remains unchanged, the density measurement at the measuring station 15 shows a drop in the density of the working medium since the density decreases with increased heat supply. in the event of such a reduction in density the regulator 17 and the regulating circuit consisting of the

elements

4, 5, 19 operate the feed pump 4 to increase the feedwater supply to correspond to the new load conditions. The signal superimposition at the adding station 13 effects a reduction of the reaction time of the regulating system in the conventional manner.

FIG. 3 shows a different embodiment of the invention. A feed pump 30 feeds the working medium through a throttle member 31 and a rate of flow measuring device 32 into heated

tube sections

33, 34, 35, 36, 37 which are arranged in series with respect to the flow of the operating medium. The water is evaporated in section 34 and the resulting steam is super heated in the

sections

35 and 36. The working medium leaving the section 37 flows through a pipe 38 to a steam consumer, not shown. The pipe line 33 is provided with a pressure measuring device 39 producing a signal which operates a load signal transmitter 49. As in the example illustrated in FIG. 2 the signal produced in the transmitter 40 adjusts the intensity of the heat produced in a furnace 41 by acting on

throttle members

42, 43 disposed in the air and fuel supply conduits of the furnace. A pipe line 44 including a rate of flow measuring device 45 branches oil the conduit interconnecting the feed pump 30 and the throttle member 31 and conducts feedwater to

points

46 and 47 where it is injected into the operating medium flowing from the heater 35 to the heater 36 and into the operating medium flowing from the heater 36 to the heater 37. A throttle member 48 is arranged in the feedwater pipe upstream of the injection point 46 and a throttle member 49 is arranged in the feedwater pipe upstream of the injection point 47. The throttle member 48 is adjusted by means of a regulator 50 in response to a temperature signal produced by a device 51 which is responsive to the temperature of the operating medium flowing between the

heating sections

36 and 37. The throttle member 49 is adjusted by a regulator 52 in response to a signal produced by a temperature sensitive device 53 placed at the outlet of the heating section 37. The rate of flow measuring device 45 actuates a regulator 54 which receives a set point signal from the load signal transmitter 40 and, when the rate of fiow measured by the device 45 deviates from the desired value, transmits a signal corresponding to the deviation to a regulator 55 which actuates the throttle member 31. The regulator 55 compares the signal received from the device 54 with the signal produced by the rate of flow measuring device 32. A device, 57 which will be described later, responds to the specific volume v of the working medium at station 56 between the tube sections 34- and 35. The device 57 produces and transmits a signal corresponding to the specific volume of the working medium to a regulator 53. The latter also receives a set point signal from the load signal transmitter 40. The signal produced by the regulator 58 is added at 59 to the signal produced in the regulator 54 and the resulting signal is fed to the regulator 55.

In the arrangement shown in FIG. 3 the furnace 41 is primarily adjusted as in the arrangement shown in FIG. 2. The adaptation of the steam generator to different load conditions is effected by the

regulators

54 and 58. The regulator 54 acts on the throttle member 31 in such manner that for each given load condition apredetermined quantity of cooling water to be injected at 46 and 47 is required to obtain a constant temperature downstream of the

tube sections

36 and 37. If, for example, the required injection quantity increases, the feedwater supply is increased, and vice versa. Since, however, such a regulation would be very slow owing to the long time required by the feedwater to travel through the steam generator, the signal corresponding to the specific volume of the work ng medium at the station 56 is included in the control signal fed to the regulator 55. 7 FIG. 4 shows another embodiment of the invention. Numerals 6t designate feedwater preheaters discharging into a tank 61 wherefrom the feedwater is forced by a feed pump 62 through a preheater 63, a throttle member 64, a rate of flow measuring device 65 into a heated tube section 66 and therefrom consecutively through

heated tube sections

67, 68, 69 and 70 of the steam generator. The. water is evaporated in section 67 and the resulting steam is superheated in the sections 68 to 70. A turbine 71 coupled to an electric generator 72 receives the steam produced by the steam generator. The exhaust steam of the turbine 71 is condensed in a condenser '73 and the condensate is fed by a condensate pump 74 to the first of the preheaters 60.

A pressure responsive device 76 is provided in live steam pipe 75 between the tube section 70 and the turbine 71. The device 76 produces and transmits a signal corresponding to the live steam pressure to a regulator 77 which also receives a signal from the rate of flow measuring device 65. The latter also transmits a signal to a load signal transmitter 78 which produces and transmits to the regulator 77 a set point signal corresponding to the rate of feedwater input which, in a forced flow steam generator, corresponds to rate of steam output. A pipe line 79 branches off from the feed pipe between the throttle member 64 and the rate of flow measuring device 65. A rate of flow measuring device 89 is interposed in the pipe 79 which feeds liquid Working medium to

injection stations

83 and 34,through throttle members 81 and S2 for cooling the steam passing from the first tubular 'superheater section 68 to the second superheater section 69 and for cooling the steam passing from the latter to the final superheater section 70. The throttle member 81 is operated by a regulator 85 in dependence of a signal produced by a device 86 which is responsive to the temperature of the operating medium leaving the heating section 69. The throttle member 82 is adjusted by a regulator 87 in dependence on a signal produced by a device 83 which responds to the live steam temperature. The regulator 87 maintains a preadjusted constant temperature at the outlet of the steam generator. The temperature maintained by the regulator 85 is load-dependent, the reguiator 85 receiving a set point signal through a signal conduit 89 from the load signal transmitter 78. The steam generator is heated by gases produced in a furnace 9%) whose burners are supplied with fuel and air by conduits provided with

throttle members

91 and 92, respectively. The latter are actuated by a regulator 93 controlled by a signal produced by a device 94, described later, which is responsive to the density of the working medium passing from the heating section 67 to the heating section 63. The regulator 93 receives a set point signal from the load signal transmitter 78 through a signal conduit 95. The latter receives a signal at an adding station 96 from the rate of flow measuring device 80 which signal is transformed in a signal transformer 97 before it is superimposed on the set point signal from the load signal transmitter 78. The signal produced by the pressure measuring device 76 may also be fed as a preliminary impulse to the regulator 93.

In the arrangement shown in FIG. 4, which is particularly suitable for supercritical pressure operation, the rate of feedwater supply is adjusted by the throttle men ber 64 primarily in response to the pressure in the conduit 75. The feedwater supply, which is a decisive factor for the steam output, is continuously measured by the rate of fiow measuring element 65 and actuates the load signal transmitter '78 which transmits a set point signal corresponding to the load conditions to the loaddependent regulating circuits of the control system. The regulator 93 adjusts the furnace 90 in response to the signal produced by the density measuring device $4 whereby the heat output of the furnace automatically satisfies the load on the steam generator. The fuel and combustion air supply to the furnace is so regulated that the produced heat maintains a density of the working medium between the

heating sections

67 and 68 which density is different for different load conditions. To comply with this requirement, the set point of the regulator 93 is adjusted by a load signal received from the transmitter 78. The set point of the regulator 93 is also influenced by the rate of flow signal received from the device St). If maintenance of the desired steam temperature requires an increased rate of cooling water supply to the injection stations, the heat output of the furnace 96 is reduced, and vice versa. The signal transformer $7 may act in steps, i.e., it abruptly alters the signal transmitted to the addition station 96 and therefrom to the regulator 93 if one or more predetermined limit values are exceeded.

FIG. 5 shows a device for measuring the specific volume of a working medium, which device may be used in the embodiment of the invention shown in FIG. 3. A pipe 1% has a widened portion Till accommodating a support 162 provided with passages for the working medium. A double bellows 1% closed by a cap 103 is fastened on the support N2 and encloses an appropriate quantity of working medium or an equivalent thereof. The cap 1% is connected to a rod 1494' whose free end forms a rack 105. At the point where the rod 1% is connected to the cap 1% the latter is provided with bores M96 permitting flow of working medium into and from the interior of the inner bellows. A roller 108 guides the rack M5 to effect meshing of its teeth with a pinion 107 connected to a shaft 11% which extends outside of the pipe portion 101 through a high pressure seal W9. A gear ill is mounted on the shaft lid outside of the pipe portion 101 and engages with a pinion 112 of a motor 113. The latter receives electric current from a current source 114 and is conrolled by contacts 115, 116 which cooperate with a diaphragm M7. The latter subdivides a chamber 118 inside the support 102 into two parts, one of which communicates with the interior of the outer one of the double bellows 104, i.e. with the annular space between the bellows, through a bore 119 and the other with the interior of the pipe portion lltll through bores 12b and 122. The lower end of the rod 1%, the roller 1&8 and the pinion W7 are placed within a protective sheath 1 2i extending downward from the support M2 and communicating through the bores 122 with the inside of the pipe portion lill.

The working medium sealed in the double bellows TM has substantially the same temperature as the working medium flowing in the pipe portion 101. If there is a difference between the pressure of the working medium sealed in the bellows 1M and of the working medium flowing through the pipe portion 101, the diaphragm H7 is deformed by the excess pressure, and the latter closes a circuit through one or the other of the contacts 115, 116 whereupon the motor 113 adjusts the vertical position of the cap W3 by means of the rack Hi5 until the pressures acting on the diaphragm 117 are equalized. The vertical positions of the cap hi3 and of the rod 104' and the angular position of the shaft lllti indicate the specific volume of the working medium flowing through the pipe 1%, ltill and may be used for mechanically actuating appropriate parts of a regulating system either directly or through a transmission. The angular movements of the shaft lit may be converted into a hydraulic or an electric signal by conventional means.

FIG. 6 shows a device for measuring the density of a working medium which device may be used, for example, in the control system shown in FIG. 2. A radioactive radiation transmitter 133 is placed in a straight portion of a pipe loop which is provided with thermal insulation 131 and radiation protection 32. A passage in a bend 134 of the pipe 130 and extending in the direction of the straight portion of the pipe 13b connects the latter with the outside and is closed by a plug 135'. For the purpose of greater permeability to radiation, the plug 135 has honeycomb bores 136. To reduce flow resistance adjacent to the passage a guide plate 137 is placed in front of the plug and has a small opening 133. Therefore, the plate 137 has to resist only the dynamic pressure of the working medium. Beyond the plug 135 is a radiation measuring device or detector 14% situated in a chamber formed in a radiation shield 139 and provided with cooling air ducts. The radiation protection 132 has a recess T41 adjacent the radiation transmitter 133 to reduce the screening effect. A radiation measuring device M2 provided with a radiation shield is placed adjacent to the recess 141. The signals produced by the two

radiation measuring devices

140 and 142 are fed to a comparison device 143 which eliminates disturbing influences and which transmits through a signal line 144 a signal free from disturbing influences and representing the density of the working medium.

This measuring system determines the absorption of the radiation of the radiation transmitter by the working medium. It is well known that this absorption depends on the density of the working medium. The intensity of the radiation received by the radiation measuring device 146 is considerably influenced by the desnsity of the working medium owing to the long absorption path in the pipe line 130. The intensity received by the radiation measuring device 1412, on the other hand, is practically independent of this density. By comparison of the two measured values it is possible to eliminate any disturbing influences affecting the two measuring devices and to obtain a measurement value which accurately corresponds to the density of the working medium.

The embodiments illustrated in the drawing are, of course, only examples, illustrating some possible applications of the invention and they can be altered within the scope of the invention. The method according to the invention may be performed in combination with regu lator circuits different from those shown in FIGS. 2-4. Measurement of the specific volume or density of the working medium which is effected at or near the conversion zone of the working medium in the illustrated and described examples may in principle be effected at other points of the steam generator. The measuring devices shown in FIGS. 5 and 6 may be replaced by any other devices for measuring specific volume, density or specific weight of the evaporated working medium. It is not necessary to measure absolute values. In most cases it is sufficient to determine the relative variations of these magnitudes. In the system shown in FIG. 6, the radiation measuring device 142 may be omitted and the required control signal be obtained solely from the measuring device 140.

In the examples illustrated in FIGS. 2-4, the measuring device for the density or specific volume is placed in the conversion zone of the working medium, i.e., in supercritical pressure steam generators between the evaporating part and the superheating part. In principle, however, a different arrangement of these measuring devices is possible. The measuring devices shown in FIGS. 5 and 6 have a particularly low measuring inertia and, in some cases, it may be preferable to place these devices at other points in the steam generating parts of the illustrated plants than at the locations shown.

Whereas the present specification describes examples using the invention for controlling forced flow steam generators and the claims also refer to steam generators, the invention can also be applied to plants using an operating medium other than water and steam.

I claim:

1. A method of controlling the operation of a forced flow steam generator operating at least at the critical pressure of water and under variable load conditions and having feedwater supply means and heat supply means, including the steps:

of adjusting one of said supply means to correspond to the desired steam output, of directly measuring the absolute density of the steam at a single location within the steam generator where a predetermined steam temperature is desired, and

of controlling the second supply means in response to the measured absolute steam density.

2. A method as defined in claim 1 wherein the second supply means is the heat supply means.

3. A method as defined in claim 1 wherein the second supply means is the feedwater supply means.

4. A control apparatus for controlling the operation of a forced flow steam generator operating at least at the critical pressure of water and under variable load conditions and having a tubular heat transfer system having an outlet, feedwater supply means connected to said heat transfer system for supplying feedwater thereto, and heat supply means adapted to supply heat to said heat transfer system for evaporating and superheating the operating medium passing through said heat transfer system, said apparatus comprising:

control means operatively connected to one of said supply means for adjusting the connected supply means to effect a supply corresponding to the desired steam output, and

a single, self-contained, flow unrestrictive, absolute density measuring device connected to said tubular heat transfer system at a location upstream of the outlet of said system,

8 said density measuring device being operatively connected to the second of said supply means and including means for adjusting said second supply means in response to the absolute density of the steam at said location.

5. A method of controlling the operation of a forced flow steam generator operating at least at the critical pressure of Water and under variable load conditions and having a tubular heat transfer system including an evaporating section followed by a superheating section and means for supplying feedwater and heat at variable rates to said heat transfer system, the method comprising the steps:

of adjusting one of said supply means to correspond to the desired steam output,

of directly measuring the absolute density of the steam at a single location between said evaporating and superheating sections, and

6. A control apparatus for controlling the operation of a forced flow steam generator operating at least at the critical pressure of water and under variable load conditions and having a tubular heat transfer system including an evaporating section and a superheating section connected to said evaporating section for receiving steam therefrom, feedwater supply means connected to said heat transfer system for supplying feedwater thereto, and heat supply means adapted to supply heat to said heat transfer system for evaporating and superheating the operating medium passing through said heat transfer system, said apparatus comprising:

control means operatively connected to one of said supply means for adjusting the connected supply means to effect a supply corresponding to the desired steam output, and a single, self-contained, flow unrestrictive, absolute density measuring device operatively connected to said tubular heat transfer system between said evaporating section and said superheating section, said density measuring device being operatively connected to the second of said supply means and including means for adjusting said second supply means in response to the density of the steam flowing from said evaporating section to said superheatmg section.

References Cited by the Examiner UNITED STATES PATENTS 2,217,637 10/40 Junkins 122-448 2,217,642 10/40 Luhrs 122-448 2,258,719 10/41 Saathoff 122-448 2,297,203 9/42 Decker 122-448 2,316,239 4/43 Hare 73-32 2,337,851 12/43 Junkins 122-448 2,635,462 4/53 Poole 73-32 2,754,676 7/56 Poole 73-32 2,804,851 9/57 Smoot 122-448 3,017,869 1/62 Profos.

PERCY L. PATRICK, Primary Examiner.

ALDEN D. STEWART, FREDERICK L. MATTESON,

]R., MEYER PERLIN, Examiners.

Claims

5. A METHOD OF CONTROLLING THE OPERATION OF A FORCED FLOW STEAM GENERATOR OPERATING AT LEAST AT THE CRITICAL PRESSURE OF WATER AND UNDER VARIABLE LOAD CONDITIONS AND HAVING A TUBULAR HEAT TRANSFER SYSTEM INCLUDING AN EVAPORATING SECTION FOLLOWED BY A SUPERHEATING SECTION AND MEANS FOR SUPPLYING FEEDWATER AND HEAT AT VARIABLE RATES TO SAID HEAT TRANSFER SYSTEM, THE METHOD COMPRISING THE STEPS: OF ADJUSTING ONE OF SAID SUPPLY MEANS TO CORRESPOND TO THE DESIRED STEAM OUTPUT, OF DIRECTLY MEASURING THE ABSOLUTE DENSITY OF THE STEAM AT A SINGLE LOCATION BETWEEN SAID EVAPORATING AND SUPERHEATING SECTIONS, AND OF CONTROLLING THE SECOND SUPPLY MEANS IN RESPONSE TO THE MEASURED ABSOLUTE STEAM DENSITY.