US3126053A

US3126053A - Brunner

Info

Publication number: US3126053A
Authority: US
Priority date: 1959-02-02
Publication date: 1964-03-24
Anticipated expiration: 1981-03-24
Also published as: DE1097459B; FR1245988A; GB913558A; CH365094A; NL129713C; NL237451A

Description

March 24, 1964 BRUNNER 3,126,053

' CONTROL SYSTEM FOR HEAT EXCHANGER Filed Jan. 29. 1960 2 Sheets-Sheet 1 7 Jnvemor:

ALF/950 BA U N M $424, i /1M March 24, 1964 Filed Jan. 29, 1960 A. B'RUNNER 3,126,053

CONTROL SYSTEM FOR HEAT EXCHANGER 2 Sheets-Sheet 2 Jnventor:

ALFRED BR N R United States Patent 3,126,053 CUNTRGL SYSTEM FGR l-EAT EXfIGER Alfred Brunner, Winterthur, Switzerland, assignor to Sulzer Freres, Soeiete Anonyme, Winterthur, Switzerland Filed Jan. 29, 1%0, Ser. No. 5,518 Czarms priority, application Switzerland Feb. 2, 1959 6 Claims. (Cl. 1654lt This invention relates to a heat exchanger having a plurality of local-temperature signal generators positioned at individual points of the heat-transfer surface, a meanvalue signal generator producing a signal that represents the mean temperature of the heat-transfer surface, and means for controlling the temperature of the heat-transfer surface.

The thermal stress-resisting capacity of heat-exchange surfaces is essentially limited by the physical properties of the material comprising the heat-exchange surface. With a view to keeping the size of the heat-exchange surface to a minimum, the maximum operating temperature is selected as close as possible to the highest permissible temperature for the material involved. This selection is complicated by the fact that due to factors that can be controlled only with difficulty, hot spots may occur that are substantially above the mean working temperature of the heat-exchange surface. It has been found, particularly in the case of heating surfaces of steam generators that are divided into separate tubes, that the operating conditions may be affected by many uncontrollable factors. For example, there may occur a breakdown of individual burners, a slagging of individual tubes, or stoppage in the secondary-air supply line with the result that certain parts of the heat-exchange surface are heated unevenly, either temporarily or for an extended period of time, or are subject to variations in the supply of working medium, so that certain portions of the heatexchange surface are exposed to greater thermal stresses than others. When a single point of the heat-exchange surface is utilized for regulation of the operation of the entire heat-exchange surface, appreciable temperature variations throughout the steam volume may result if an irregularity or failure causing change in heat application happens to occur precisely at the point chosen for regulation of the heat exchanger. Unless these temperature changes are discovered promptly, they may cause damage, particularly when the spot in question is relatively underheated and thus has a low temperature that is not representative of the entire heat-exchange surface, with the entire heat-exchange surface then being brought to a higher operating temperature.

For these reasons, it has been proposed, in the case of forced-circulation steam generators with a heat-transfer surface, which in the zone of evaporation and initial superheating is divided into parallel tubes, to assign temperature signal generators to a rather large number of parallel tubes with the temperature signal generators actuating through a blocking device means for increasing the quantity of the working medium to be heated and flowing through the system, and with the blocking device permitting pulse transmission only from the signal generator that is responsive to the highest temperature prevailing at a given moment.

Although dama e due to excessive thermal stresses can be avoided in this way, a drawback exists in that the signal generator signaling the highest temperature will actuate the means for controlling the temperature of the heat-exchange surface even when this peak temperature is not in the maximum-temperature range that is to be avoided for stability reasons. And even though the local peak temperature sensed may be due to afailure, operation in the other parts of the heat-transfer surface will be governed by this hot spot, which is not at all representative of the over-all condition of the heat exchanger. In the case of steam generators, for instance, this results in a mean temperature of the working medium leaving the heating surface that is incompatible with the temperature required for boiler or turbine regulation.

My invention makes it possible to overcome the drawbacks described. It is characterized by the fact that both the mean-value signal generator and the local-temperature signal generator which signals the highest instantaneous temperature level are able to act upon the means for controlling the temperature of the heat-exchange surface, but with the local-temperature signal generator which signals the highest temperature, however, being prevented from so acting by a blocking device that is operative so long as the highest temperature level is below a preset limit.

In accordance with the invention, the means for controlling the temperature of the heat-exchange surface are responsive to a mean temperature of the heat-exchange surface even where there are local temperature differences between comparable points of the heat-exchange surface so long as the highest local temperature is outside of the danger zone. Only when the highest local temperature, as determined at a given instant, exceeds a predetermined boundary value does the signal generator involved become free to actuate the means for controlling the temperature of the heat-exchange surface. In this way, considerably improved regulation of the heat exchanger is achieved.

The invention is particularly advantageous Where the heat exchanger presents a plurality of parallel-connected heat-exchange tubes. The mean-value signal may then be appropriately formed of a reading of the temperature which represents the combined temperature of the working medium discharged from the individual tubes.

In another preferred embodiment of the invention, the mean-temperature signal generator and the local-temperature signal generator signaling the highest temperature value are both adapted to act upon one and the same means for controlling the temperature of the heat-exchange surface-for example, upon means controlling the flow of heat or the radiation of heat from the heating medium to the tube wall. However, the two signal generators may also be made to actuate different means for controlling the temperature of the heat-transfer surface.

In a further practical embodiment of the invention, the action of the mean-value signal generator may be interrupted for as long as the local-temperature signal generator signaling the highest temperature value is acting upon the means for controlling the temperature of the heattransfer surface.

In some cases, it may be advisable in heat exchangers equipped with individual parallel tubes to assign more than one local-temperature signal generator to the individual heat-exchange tube. Moreover, it may prove advantageous to provide for adjustment of the temperature limit which establishes the actual connection between the local-temperature signal generator signaling the highest temperature and the means for controlling the temperature of the heat-exchange surface. It may prove particularly advantageous to provide means for adjusting this boundary value as a function of some operating parameter of the exchanger or of the plant of which it is a part.

The invention and additional features related thereto are explained below in greater detail in terms of the embodiments shown in the drawing, where:

FIG. 1 shows diagrammatically an arrangement in accordance with the invention for a heating surface divided into parallel tubes of a steam generator with means for injection of a colder working medium into the feedline of the heating surface;

FIG. 2 shows diagrammatically an arrangement similar to that of FIG. 1, except that here a blocking device of diflerent design is provided in the connection between local-temperature signal generators and injection means; and

FIG. 3 shows diagrammatically a heating surface of a steam generator with two different means for controlling the temperature of the heat-exchange surface.

In the arrangement of FIG. 1, the heat exchanger constitutes a superheater surface of a steam generator, which may be operated with forced circulation of the working medium. The heat exchanger may be located either in the radiant-heating section or in the convection-heating section. The steam flows through line 1 into header 2 and then through the parallel tubes 3a, b, c, and d, into outlet header 4 and into discharge line 5. For regulation of the final steam temperature or of the heat-transfer surface, respectively, an injection line 6 with a regulating valve 7 is connected to line 1. Some colder Working medium water, for example-may be sprayed into line 1 through line 6.

The cross section of the passage of valve 7 is controlled by a mean-value signal generator monitoring the mixed temperature of the working medium in line 5. To this end, the mixed temperature is determined in known manner by measuring the thermal expansion of a section of line with a measuring rod 8. A particular temperature of the wall of line 5 or of the steam flowing through the line is reflected in a certain position of a knife edge 9, which pivots at one end of the rod and whose free end is supported, with interposition of a spring 10, on a piston 11 that slides in a cylinder 12. An inlet opening 13 and an outlet opening 14 for a pressure medium are provided in the wall of cylinder 12. Parts 9-14 comprise a meanvalue signal generator. Through a pressure line 15, the cylinder volume closed oif by the piston is connected with the pressure chamber of a servomotor 16 in such a way that the pressure transmitted through line 15 acts upon the underside of a piston 18, loaded with a spring 17.

The principle of operation of the arrangement described is such that temperature measured on tube 5 corresponds to a certain well-defined pressure in line 15 and consequently to a certain Well-defined position of piston 18. Attached to piston 18 is a bar 19 which under the operating conditions depicted by the drawing abuts against a rail 20. Said rail pivots on

bearings

21 and 22 and through a rod 23 is linked with the stem of valve 7. A compression spring 24 is provided to assure that rail 20 follows all motions of bar 19. When the temperature of the working medium in line 5-that is, the mean temperature of the heat-exchange surface formed by the tubes 3rises above a preset level, injection valve 7 is opened, and vice versa.

Local-

temperature signal generators

25, 26, 27 and 28 are constructed in the same way as the mean-value signal generator comprising parts 9 to 14, described above, and are assigned to each of the tubes 3a, b, c, and d. Said local-temperature signal generators are connected through

signal lines

29, 30, 31 and 32 with

servomotors

33, 34, 35 and 36, which in turn are constructed in the same way as servomotor 16. However, the springs of

servomotors

33, 34, 35 and 36 are more strongly prestressed than the spring 17 of servomotor 16.

The piston rods of servomotors 33 to 36 are likewise able to act upon rail 20 and consequently upon injection valve 7. However, the arrangement is such that only the servomotor with the longest piston-rod travel is able to act upon the rail, and that said travel must be longer than the stroke of servomotor piston 19, which is controlled by the mean-value signal generator. Thus, the arrangement described operates as a blocking device, with the localtemperature signal generator signaling the highest temperature being inoperative so long as this highest temperature value is below a boundary value representing the diiference between the initial tension of spring 17 of 4 servomotor 16 and the initial tension of the springs of servomotors 33 to 36.

As a result of the arrangement described, the highest temperature on any tube must differ by a certain minimum amount from the mean temperature downstream from the outlet header before the latter temperature will surrender control to the highest tube temperature then prevailing. In this way, the superheat temperature is controlled with a satisfactory degree of regulation by the mean temperature at the discharge line so long as no tube damage due to the failures mentioned at the outset is able to occur. When the temperature at an individual tube reaches the predetermined allowable boundary value, rail 20 becomes disengaged from piston rod 19 and is raisedthus increasing the amount of medium injected-by the piston rod of the servomotor that is connected to the temperature signal generator signaling the highest temperature.

The operating principle of the arrangement of FIG. 2 is similar to that of FIG. 1. The weaker prestressing of the servomotor spring, as described above, here corresponds to the addition of a small signal At to the temperature signal formed by mean-value signal generator 41 on the basis of the mixed temperature in the discharge line 40 of the heating surface formed by three parallel tubes 42a, b and c of a steam generator. Assigned to each of the three tubes is a local-

temperature signal generator

43, 44 and 45 whose signals are transmitted, together with the mean-value signal that has been increased by the amount At, to comparator 46. The latter determines the signal that corresponds to the highest temperature at the various measuring points and, on the basis of that signal, controls, through signal line 47, a proportional-integral regulator 48, which in turn actuates the servomotor of injection valve 49. An advance signal is added to the output of regulator 48, said advance signal being taken, by means of temperature signal generator 50, from a point located in closer proximity to the point where the working medium enters the heating surface, and being fed, through a proportional regulator 51, into signal line 52, which leads to the servomotor. Here, too, the principle of operation is such that the highest local temperature (t t or t must differ by a predetermined minimum amount-namely, by At-frorn the mixed temperature measured at line 40 in order that regulation of injection may take place as a function of the instantaneous peak value of the local temperatures.

The signal At that is added to the signal from the meanvalue signal generator may be varied by means of handwheel 54.

In the embodiment according to FIG. 5', two different means are provided for controlling the temperature of the heat-exchange surface, namely a device for regulating the firing rate, and the aforementioned device for regulating injection. In the final section of parallel tubes 61a, b and c, which form the heat-exchange surface,

temperature signal generators

62, 63 and 64 are arranged. These are connected through

signal lines

65, 66 and 67 with a device 68 forming the mean value of the incoming signals. The proportional-integral regulator 69 is controlled through signal line '70 on the basis of the instantaneous mean value. Regulator 69 acts upon injection valve 71 in such a way that when the required value of said mean temperature, as introduced through signal line 72, is exceeded, the injection valve is opened more widely, and vice versa. By means of temperature signal generator 73 and propor tional regulator 73a, an advance signal is simultaneously fed into the signal line between regulator 69 and valve 71 so as to meet changing operating conditions at the earliest possible moment.

Assigned to each of tubes 61a, b and c is a local-

temperature signal generator

74, 75 and 76 which through

signal lines

77, 7 8 and 79 is connected to a comparator 80. Signal lines 66 to 68 likewise lead to said device. The signal signaling the highest of all measured temperatures,

as determined by the comparator, is fed through signal line 81 into a device 82 which establishes the difference between the highest value involved and the boundary value preset by means of handwheel 83 on device 84. The principle of operation is such that as soon as the amount of the difference between the highest temperature at any measuring point and the preset temperature boundary value exceeds zero, device 82 controls, through proportional-integral-diiferential regulator 85, the fuel valve 86 of burner 87 so as to reduce the quantity of fuel admitted, and with it the thermal stresses of the heat-exchange surface.

By contrast to the embodiments of FIGS. 1 and 2, here temperature control of the heat-exchange surface by means of the mean-value signal generator does not become inoperative when control is exercised on the basis of a local-temperature peak value. It should further be noted that the characteristics of the individual regulators are selected so that temperature control of the heat-exchange surface by adjustment of the firing rate overrides, as it were, control by variation of the amount injected.

The invention is not confined to the embodiments described. The heat-exchange surface may also comprise a number of tubes larger than that shown in each of the diagrams, and it is not necessary that each tube be equipped with a local-temperature signal generator; it will sufiice if every second or third tube of a heating surface with parallel tubes is so equipped. In the case of heatexchange surfaces that are constructed as superheaters of steam generators, it is further possible to locate a portion of the heat-transfer surface in the radiant section and an adjacent portion in the convection section of the steam generator, with heat transfer taking place largely through flue gases. In such an arrangement, it is primarily the tubes located in the radiant-heating section that are endangered, for which reason it is advisable to place the local-temperature signal generators likewise into that part whereas the mean-value signal generator may be assigned to the convection-heating part.

The mean-value signal generator might also be positioned in an intermediate zone of the entire heat-exchange surface-in other words, in an area where the temperature of the working medium to be heated has not as yet reached the final value. Moreover, the heat-exchange surface need not be formed of individual parallel tubes; the invention is quite generally applicable to any configuration of exchange surface presenting local-temperature signal generators at various points. In steam generators, the invention might also be applied to heating surfaces other than superheater surfacesfor example, to heating surfaces located in the evaporation zone.

In the case of steam generators, and in the case of heatexchange surfaces serving other purposes, means other than those described may be utilized for controlling the temperature of the heat-exchange surface. Thus, any method whereby the heat flow from the heating medium to the heat-exchange surface can be reduced-such as lowering the temperature of the heating medium, or reducing the rate of flow or pressure of the heating medium-may be employed. Other methods include the displacement of the flame relative to the heat-exchange surface, variation of carburization of the fuel, and screening of the exchange surface, all of which will reduce or control the radiation of heat onto the heat-exchange surface. Moreover, methods increasing the heat transfer from the heatexchange wall-such as lowering the temperature of the medium to be heated, increasing its rate of flow or pressure, and raising the specific heat of the medium to be heated-may be employed. Finally, and generally, any means improving the radiation of heat may be resorted to.

The boundary value for the highest local temperature of the heat-exchange surface, as predetermined in each case, need not necessarily be a constant value. Adjustment of the boundary value as a function of an operating parameter of the heat-exchanger or of the plant of which it is a part-in the case of steam generators, for example, as a function of boiler load-will prove particularly advantageous. In this instance, the boundary value for peak loads of short duration may be at a higher level than in the normal, lower load range since the stresses to which the tubes are subject at excessively high temperatures are also a function of the duration of heat application and are determined largely by the creep strength of the heatresistant materials utilized.

In the case of heat-exchange surfaces that are divided into individual tubes, it is further possible to provide more than one local-temperature signal generator per tube. Of practical value is a warning device producing acoustic or optical signals as soon as the permissible temperature boundary value is exceeded.

Nor is the invention confined to the specific method of determining the temperatures involved. These temperatures may be reproduced by signals also in such a way that the temperature of the heat-absorbing medium is measured at a given point on one of the sides of the heat-exchange surface.

Having described my invention, I claim:

1. A heat exchanger comprising a heat-exchange surface, separate means to develop signals representative of the temperature of each of a plurality of locations on said surface, means to develop a signal representative of a mean temperature of said surface, temperature control means for influencing the temperature of said surface, means coupling said separate and mean temperature-representative signal developing means to said temperature control means, said coupling means having an input from each of said signal developing means and an output to said temperature control means, said coupling means connecting to said output the input signal thereto of extreme value, and means to introduce a differential between the signal from said mean temperature signal generating means on the one hand and the signals from said separate temperature representative signal developing means on the other hand.

2. A heat exchanger according to claim 1 wherein said heat-exchange surface comprises a plurality of parallel-connected heat-exchange tubes through which a medium circulates.

3. A heat exchanger according to claim 2 wherein said mean temperature representative signal developing means is responsive to the temperature of the mixed medium discharged from all of said tubes.

4. A heat exchanger according to claim 2 wherein at least two of said separate temperature-representative signal developing means are assigned to each said tube.

5. A heat exchanger comprising a heat-exchange surface, separate means to develop signals representative of the temperature of each of a plurality of locations on said surface, means to develop a signal representative of a mean temperature of said surface, temperature control means for influencing the temperature of said surface, means coupling said separate and mean temperature-representative signal developing means to said temperature control means, said coupling means having in one class an input from each of said separate signal developing means and having in another class an input from said mean temperature-representative signal developing means, all of said inputs being in parallel, said coupling means further having a common output channel to said temperature control means, and biasing means associated with the inputs of at least one of said classes to isolate from said output channel signals from said separate temperature-representative signal developing means differing by less than a specified amount from the signal from said mean temperature-representative signal developing means.

6. A heat exchanger comprising a heat-exchange surface, separate means to develop signals representative of the temperature of each of a plurality of locations on said surface, means to develop a signal representative of a mean temperature of said surface, temperature control References Cited in the file of this patent UNITED STATES PATENTS Sprague Aug. 15, 1933 Eglofi Sept. 26, 1933 Conison Dec. 21, 1954 Profos July 30, 1957 Biehn Sept. 17, 1957 FOREIGN PATENTS Great Britain Aug. 14, 1926

Claims

1. A HEAT EXCHANGER COMPRISING A HEAT-EXCHANGE SURFACE, SEPARATE MEANS TO DEVELOP SIGNALS REPRESENTATIVE OF THE TEMPERATURE OF EACH OF A PLURALITY OF LOCATIONS ON SAID SURFACE, MEANS TO DEVELOP A SIGNAL REPRESENTATIVE OF A MEAN TEMPERATURE OF SAID SURFACE, TEMPERATURE CONTROL MEANS FOR INFLUENCING THE TEMPERATURE OF SAID SURFACE, MEANS COUPLING SAID SEPARATE AND MEAN TEMPERATURE-REPRESENTATIVE SIGNAL DEVELOPING MEANS TO SAID TEMPERATURE CONTROL MEANS, SAID COUPLING MEANS HAVING AN INPUT FROM EACH OF SAID SIGNAL DEVELOPING MEANS AND AN OUTPUT TO SAID TEMPERATURE CONTROL MEANS, SAID COUPLING MEANS CONNECTING TO SAID OUTPUT THE INPUT SIGNAL THERETO OF EXTREME VALUE, AND MEANS TO INTRODUCE A DIFFERENTIAL BETWEEN THE SIGNAL FROM SAID MEANS TEMPERATURE SIGNAL GENERATING MEANS ON THE ONE HAND AND THE SIGNALS FROM SAID SEPARATE TEMPERATURE REPRESENTATIVE SIGNAL SDEVELOPING MEANS ON THE OTHER HAND.