US3186175A - Heat absorption balancing system for a steam generator having a primary steam circuit and a reheating steam circuit - Google Patents

Description

June 1, 1965 c. STROHMEYER, JR 3,186,1 75 HEAT ABSORPTION BALANCING SYSTEM FOR A STEAM GENERATOR HAVING A PRIMARY STEAM CIRCUIT AND A REHEATING STEAM CIRCUIT Filed Jan. 14, 1963 4 Sheets-Sheet l aqa RADIANT ssc'nou 45 Fig.!.

INVENTOR. Charles Strohmeyer,Jr.

his ATTORNEY June 1965 c. STROHMEYER, JR 3,186,175

HEAT ABSORPTION BALANCING SYSTEM FOR A STEAM GENERATOR HAVING A PRIMARY STEAM CIRCUIT AND A REHEATING STEAM CIRCUIT Filed Jan. 14, 1963 4 Sheets-Sheet 2 CONVECTION SECTION 47 RADIANT SECTION 45 INVENTOR.

I Charles Strohmeyer, Jr. F lg. 2. BY

his ATTORNEY June 1, 1965 c. STROHMEYER, JR 3,186,175 HEAT ABSORPTION BALANCING SYSTEM FOR A STEAM GENERATOR HAVING A PRIMARY STEAM CIRCUIT AND A REX-[EATING STEAM CIRCUIT Flled Jan. 14, 1965 4 Sheets-Sheet 3 K70 TURBINE g) I83 I84 I85 2 5|. 186 I87 I90 -Is4 wk V ll- ITo mi 1-H6 97 Q I72 I4I H68 !74 I I9 -Ios I42 /I75 I76 /I95 E O E I09 E E p :09 P I 9 P P '09 P?- I2I I23 I05 I07 I96 :3 I22 I we /I27 /I78 II 53 IIo :45 I28 2! at M A R FUEL FLOW AIR FLOW l-REHEAT 2 REHEAT I- 0:1 TEMPERATURE TEMPERATURE F Ig. 3.

INVENTOR. Charles Sfrohmeyer, Jr. BY

his ATTORNEY June 1,1965 c. STROHMEYER, JR 3,186,175

HEAT ABSORPTIO A STEAM N BALANCING SYSTEM FOR GENERATOR HAVING A PRIMARY STEAM CIRCUIT AND A REHEATING STEAM CIRCUIT 4 Sheets-Sheet 4 Filed Jan. 14, 1963 INVENTOR. Charles Strohmeyer, Jr.

United States Patent 3,186,175 HEAT ABSORPTEQN BALANQING SYSTEM FQR A STEAM GENERATQR HAVHQG A PRIMARY STEAM CIRQUIT AND A REHEATING STEAM (IIRCUIT (Shartes Strohmeyer, Jn, Wyomissing, Pa., assignor to Gilbert Associates, lino, Reading, Pa. Filed Jan. 14, 1963, Ser. No. 251,358 2 tilaims. (Cl. 60-73) This invention relates to devices and systems for improving the heat absorption balance among the variouscircuits of a steam generator throughout the operating load range when delivering steam to a turbine driver for powering an electric generator, and where steam from intermediate turbine stages is returned to the steam generator for reheating.

In such a plant, high pressure superheated steam from the steam genera-tor is admitted to a high pressure turbine. At some intermediate point in the turbine steam flow path, steam is exhausted to a conduit which conveys the steam to the steam generator where it is passed through reheating heat absorption conduits located in the heating medium. After reheating, the steam again enters the turbine to perform further work as the steam expands. The turbine may have a single steam reheating stage in which case, after performing its useful work, the steam exhausts from the turbine at low pressure to a condenser.

Steam may be reheated in more than one stage. In such case, the pressure of each succeeding reheating stage is lower than the upstream reheating stage pressure.

Steam exhausted to the condenser is condensed and pumped back to the steam generator feedwater inlet through a regenerative feedwater heating cycle wherein steam is extracted from the turbine steam flow path at intermediate pressures to successively raise the temperature of the feedwater before it enters the steam generator.

The feedwater passes through a preheating section, a generating section and a super-heating section. The high pressure circuit from the economizer inlet to the superheater outlet is called the primary steam circuit. Steam returned from the turbine to the steam generator for reheating and return to the turbine is called the 1st reheat steam circuit and the 2nd reheat steam cincuit.

Where there is one stage of reheating, the primary steam circuit operates in parallel with the 1st reheat steam circuit. The primary and 1st reheat outlet steam temperatures should be maintained at values which will produce maximum plant efficiency throughout as large a portion of the load range as is practical to produce maximum operating economy. The steam genera-tor is normally designed to produce specified outlet steam temperature for each of the primary and reheat circuits at a minimum control load. For loads above this point, some additional means for control is necessary to properly proportion heat input to each circuit to produce the specified steam temperature. For loads below the control load, steam temperature will depart from the specified value without means for further correction. Known systems for controlling steam temperatures of both circuits throughout at least a portion of the load range are expensive and are sometimes wasteful of power for driving auxiliary equipment as is the case where gas recirculation fans are used to proportion heat input to various steam generator components. Where there are two stages of reheating, the steam temperature control problem becomes more complex and more expensive to achieve both with respect to capital investment in equipment and cost of plant operation.

An object of this invention is to provide an efficient and economical means for the aforementioned steam generator of the reheat type whereby heat may be transferred from a reheat steam circuit to the primary steam circuit in varying relative amounts throughout at least a portion of the load range to balance heat absorptions in the circuits'exchanging heat to maintain specified outlet steam temperatures in both circuits.

' A more specific object of this invention is to provide said means for a steam generator having a primary circuit and a single steam reheating stage.

A further specific object of this invention is to provide said means for a steam generator having a primary c rcult and two steam reheating stages.

A still further specific object of this invention is to provide a system for control of feedwater flow, heat input and outlet steam temperatures for said steam generators where they are of the once-through type.

A still further specific object of this invention is to provide an economical and efiicient heat exchanger for exchanging heat between reheat and primary steam circuits.

Other objects and advantages of this invention will become more apparent from a study of the following description taken with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of a steam generator and steam turbine cycle having one stage of steam reheating, and embodying the principles of the present inventionj FIG. 2 is diagram of a steam generator and steam turbine cycle having two stages of steam reheating, and embodying the principles of the present invention;

:FIG. 3 is a diagram of a feedwater flow, heat input and outlet steam temperature control system for FIGS. 1 and 2; and,

FIGS. 4a. and 4b are an arrangement for a heat exchanger to transfer heat from the reheat steam to the primary steam.

General description of present invention In a steam-electric generating plant consisting of a steam generator, a steam turbine of the reheat type and an electric generator driven by the steam turbine, coordination of the steam generator and turbine with respect to steam temperatures is desirable to obtain the maximumoverall plant efficiency.

FIG. 1 illustrates such a plant. The steam generator 1 shown in dash and dot outline is of the once-through type. {High pressure superheated steam from steam generator 1 flows through conduit 2., through fiow control valve 3 to the high pressure turbine 4. After releasing energy preforming work, the steam in the high pressure turbine is reduced in pressure and temperature and exhausts through conduit 5 to low'temperature reheater 6 and high temperature reheater 7. After the steam is elevated in temperature, it is returned through conduit 8 duit 159 to pump 2i] which raises the pressure of the fluid to the Working pressure of the steam generator. Pump 20 discharges through conduit 21 to high pressure heating system 22 which receives steam from intermediate turbine stages through conduit 23. High pressure heating system 22 discharges through conduit 24 to the steam generator I preheating section'or economizer 25. Turbines 4, 9 and it drive electric generator through shaft 56.

In steam generator 1, the feedwater entering econo- I) mizer flows through the economizer and discharges through conduit 26 to the inlet header 27 which feeds fluid to the furnace waterwalls 29. Flow passes through waterwalls 29 and discharges to outlet header 30. Flow passes from outlet header 30 through conduit 31 to the inlet header 32 which feeds fluid to the convection pass wall enclosure 33. Convection pass wall enclosure 33 discharges to outlet header 34 and through conduit 35 to primary superheater 36. From the primary superheater, fluid passes through conduit 37 to the radiant superheater 40. The discharge of the radiant superheater 40 passes through conduit 41 to the final superheater 42. The final superheater 42 discharges to conduit 2.

The steam generator 1 has combustion furnace 28. Conduits 43 supply fuel to burners 44. Combustion air is supplied to burners 44 (not shown). The combustion air and the fuel from burners 44 are fired in the furnace 28. Heat released in furnace 28 is transmitted to heat absorbing circuits 29, 7 and 4t) mostly as radiant heat. This section of the steam generator is denoted as the radiant section 45. The hot gases from combustion pass and are cooled over heat absorbing circuits 33, 42, 6, 36 and 25 and exit from the steam generator at 46. This latter portion of the steam generator is denoted as the convection section 47. Heat is transferred to the circuits 33, 42, 6, 36 and 25, mostly by convection.

In order to maintain specified steam temperatures in conduits 2 and 8, the circuits 25, 29, 33, 36, 40 and 42 must be arranged in proper balance with circuits 6 and 7 so each group will receive its proportionate and balanced 7 share of heat input from the combustion products in furnace 28. If the balanced condition exists at one design control load of steam generator 1, then at other loads, this balanced condition is disturbed. As load is decreased, the ratio of heat absorption increases in the radiant section 45 and decreases in the convection section 47. The reverse is true as load is increased. Therefore,

design steam temperatures cannot be maintained over a wide load range unless the proper division of radiant and convection heat input to the primary and reheat steam circuits can be maintained throughout the same load range.

A once-through steam generator can be constructed so that there is no fixed division point between steam generating and superheating duties between economizer 25 circuit including conduit 5 reheaters 6 and 7, their interconnecting conduits and, conduit 8. Spray water increases the mass flow of reheat steam and in effect bypasses fluid around the high pressure turbine 4. This I lowers overall plant efliciency.

Other known means of reheat steam temperature control include multi-pass arrangements for gas flow wherein dampers regulate gas flow over the various heat absorption conduits to balance steam temperatures. This is expensive and wasteful of effective heat transfer surface; also, control response is slow. Gas recirculation,

where exit gas at 46 is returned to furnace 28 through 'gas ducts using a fan to provide the required pumping head, is also used to control absorption distribution between the radiant and convection sections. Gas recirculation increases convection section absorption and decreases radiant section absorption. Gas recirculation is expensive to install and consumes considerable horsepower to pump and recirculate hot gases through the flow path. Gas recirculation is not shown on FIG. 1.

Another method is the use of tilting burners, where 1- the direction of burner flame is raised or lowered in a vertical furnace so as to increase heat input to the convection section as the burners are turned in the direction of the convection section. While this may be used as a means of balancing temperatures, availability of such burners is limited.

A drum type steam generator having recirculation circuits in the generating section has similar characteristics for primary and reheat steam temperature control. Since the evaporation and superheating duties are rigidly divided by the steam drum, artificial means must be employed to upset the natural balance. Spray water can be injected into the superheater to lower primary steam temperature without loss of plant efficiency. This transfers some of the evaporating duty to the superheater. In such case, the superheater is designed for specified outlet steam temperature at the partial control load. For higher loads, the ratio of superheater absorption increases, requiring spray water injection to lower outlet steam temperatures to the specified value. Firing rate is adjusted to maintain steam pressure, and spray water in the superheater is regulated to control steam temperature.

Therefore, the present invention overcomes the past difficulties with respect to controlling steam temperatures at the outlet of two parallel and separate fluid flow paths of a steam generator, and where both flow paths receive heat from a common heat generating source as furnace 2S, and where one fluid path is operated at a substantially higher pressure than the other, said one fluid flow path discharging to the steam inlet of a prime mover, said other fluid flow path receiving steam from an intermediate stage of said prime mover for reheating and return to the same said prime mover.

More specifically, the present invention relates to a new and novel system and devices to maintain constant outlet steam temperatures for both fluid flow paths for at least a portion of the load range without the need for alteration of the relative position of the firing means or of the gas flow path through recirculation means and without the use of spray water injection into either of the flow paths as the basic element of proportioning heat transfer to the fluid flow paths. This is accomplished by distributing the heat absorption surface of each fluid flow path in the steam generator in a Way which will produce an approximate balance between flow paths, and providing means to exchange heat between flow paths externally from the common heat generating source flow path when the balance in temperatures between fluid flow paths is disturbed. Thus, the common heat generation source combustion rate can be regulated to control steam temperature at the outlet of one fluid flow path and the outlet steam temperature of the second fluid flow path can be regarded by exchanging heat with the first fluid flow path.

FIG. 1 shows one of many possible arrangement combinations for achieving the broad structural objectives. Heat exchanger 48 is used to exchange heat from the reheat circuits 6 with the generating section fluid flow from economizer 25. The low temperature reheater 6 discharges through conduit 49 to heat exchanger 48, wherein the reheated steam passes over and gives up heat to tubes 5-3. The cooled steam exits through conduit 51 to high temperature reheater '7 and from thence to turbine 9 through conduit 8. A portion of the fluid from conduit 26 passes through conduit 52 to heat exchange tubes in heat exchanger 48. Valve 53 regulates the amount of flow through conduit 52. Tubes 50 discharge to conduit 54, conduit 31 and to inlet header 32 in series.

Flow through conduit 52, tubes 59 and conduit 51 bypasses furnace water-wall circuits 29. This provides hydraulic head necessary to pass fluid through conduit 52, tubes 50 and conduit 54'. The small reduced flow through circuit 29 does not significantly reduce the heat transfer to these circuits. The total heat to header 30 remains s,1se,175

the same. The lesser mass flow will have a higher enthalpy. The enthalpy of fluid from economizer 25 is also increased as it passes through tubes 59.

When fluid is passed through tubes 56, the combined enthalpy of the fluid is increased as it enters header 32. Heat exchange rate in heater 48 is regulated by control of flow through valve 53.

The reheater is constructed of both convection and radiant heat transfer surfaces. For full load operation, cooling of steam in heat exchanger 43 is necessary to maintain outlet'steam temperature from high temperature reheater 7 at specified value (say 1000 F.). Flow through valve 53 is controlled to maintain specified temperature in conduit 8.

As load is decreased, less heat is absorbed in low ten perature reheater 6. This lowers steam temperature in conduit 3. Flow through valve 53 is reduced to main.- tain specified steam temperature in conduit 8. Firing rate of burners 44 is controlled to maintain specified temperature of the superheater outlet steam in conduit 2. (say 1000 F).

Thus, radiant and convection heat absorption can be balanced between the two flow streams by adjustment of firing rate in burners 44 and flow rate through valve 53. Relatively small flows will be required through valve 53. Their order of magnitude may range from 5 to 20 percent of rated primary steam how. The sh ll of heater 48 is designed to Withstand the low pressure fluid from the exhaust of turbine 4.

it should be noted that when there is excessive absorption in circuits 6 and 7, heat transfer to tube 5% reduces the furnace heat input required to produce specified steam temperature in conduit 2. This amplifies the effectiveness of heat exchanger 43 for controlling steam temperature in conduit 8.

In the case of a double reheat steam-electric generating plant, as shown in FIG. 2, there is the problem of balancing three steam temperatures exiting from the steam generator. The same numbers have been used for denoting identical parts in FIGS. 1 and 2. The element numbers for the reheat circuit of FIG. 1 have been applied to the second reheat circuit in FIG. 2. In PEG. 2 the heat exchanger 43 is installed in the second reheat circuit.

In FIG. 2, steam exiting from superheater d2 flows through conduit 2 to very high pressure turbine 4. Turbine 4 exhausts through conduit 57 to convection reheater 58, through conduit 59 to radiant reheater 6h. Reheater 6t discharges to conduit d1 which transports the steam to high pressure turbine 62. Turbine 6?. discharges through conduit 5 to reheater 6, through conduit 49 to heat exchanger 48, from heat exchanger .3 to conduit 51 and reheat-er 7, back to turbine 9 through conduit 8. The remainder of the cycle is the same as for FIG. 1. Valves 63 and 64 control the flow of reheat steam to turbines 62 and 9 in times of upset or emergency.

In accordance with the above discussion of steam temperature control for FIG. 1, primary steam temperature for FTG. 2 in conduit 2 may be controlled by means of variations in firing rate for the once-through type of steam generator illustrated. The 2nd reheat outlet steam temperature in conduit 8 may be controlled by varying flow through valve 53 as described above for FIG. 1. The outlet steam temperatures in conduits 2 and 8 are in effect bias-ed one with the other by means of heat exchanger 43.

There remains the problem of controlling the lst reheat outlet steam temperature in conduit 61. This may be accomplished by one of several methods presently available. In all cases the primary and 2nd reheat steam temperatures will be biased as a unit with the 1st reheat steam temperature. The biasing method will depend upon the particular means available to the individual steam generator manufacturer.

devices and structures of this invention.

Biasing may be achieved by means of damper controls (not shown). In such case, part of reheater 53 is arranged parallelly with superheater 36 in the gas flow path so that the gases from combustion furnace 28 will enter sections 58 and 36 at the same relative temperatures. Absorption between sections 58 and 36 is proportioned by means of dampers which regulate ratio of gas flow between the two parallel sections, The 1st reheat outlet steam temperature is controlled by means of the damper action, the primary steam temperature is controlled by means of regulating firing rate of burners 44 and 2nd reheat steam temperature is controlled by means of regulating flow through valve 53.

Biasing may be achieved by means of gas recirculation. In such case, hot exit gas from convection section 47 is drawn off from conduit 46 through conduit 65. Fan 66 provides the head required to recirculate gas through furnace 28. As gas recirculation is increased, a greater percentage of the heat absorption is transferred to the convection section 47. The 1st heat steam temperature is controlled by means of regulating gas recirculation flow, primary steam temperature is controlled by means of regulating firing rate of burners 44 and 2nd reheat steam temperature is controlled by means of regulating flow through valve 53.

Biasing may be achieved by means of varying elevation of the combustion zone. In such case, the fuel and air flow from burners 44 are diverted up or down through angle 68. This in turn affects the absorption in the furnace 28 and gas temperatures entering convection section 47. The 1st reheat steam temperature is controlled by means of regulating the angle of flame discharge from burners 44, the primary steam temperature is controlled by regulating firing rate and the 2nd reheat steam temperature is controlled by regulating flow through valve 53.

The arrangement of heat transfer surfaces shown on FIGS. 1 and 2 are representative. Other arrangements are possible which will employ the general principles, For example, Where gas recirculation is employed as a principle means to bias the primary and 2nd reheat steam temperatures with the 1st reheat steam temperature, radiant sections 60 and 49 would preferably be minimized or eliminated and the reheating surface of sections 53 and 42 would be increased correspondingly.

It is also evident that heat in both the 1st and 2nd reheat circuit could be exchanged with the primary circuit. In such case, flow from conduit 26 would be divided between two heat exchangers arranged in parallel to that shown for 48, each heat exchanger receiving and discharging flow from one of the two reheat circuits. It is also evident that heat exchanger 48 could be used in conjunction with the 1st reheat, using supplemental means as described above for control of the second reheat steam temperature.

Therefore, the present invention as it relates to heat exchanger 48 offers great design flexibility. The heat exchanger 48 arrangement may be adapted to suit the many commercial types of steam generators in combination with a wide variety of steam turbine cycles and steam conditions for primary and reheat steam.

FIG. 3 is an integrated feedwater flow, fuel flow, air

flow and steam temperature control system for the arrangement shown on FIG. 2. The same numbers are used for denoting identical parts in both FIGS. 2 and 3. The control system shown on FIG. 3 is electrically integrated and is of the O-10 volt direct current operational amplifier type, such as manufactured by the Hagan Chemicals and Controls, Inc., Hagan Center, Pittsburgh 30, Pa.

The load demand signal is the required output of the steam generator. This may be in the form of steam flow as measured from primary steam flow in conduit 2, or in the form of steam flow as measured from turbine 4, 1st stage pressure, or in the form of load requirement 7 established by an external source as a central dispatch ofiice, or in the form of generator 55 load in kilowatts, or in the form of AC frequency output of generator 55, or in combinations thereof or other equivalent means. The specific form of the load demand signal is not a part of this invention other than that it exists.

Turbine 69 is a composite of turbines 4, 62, 9 and 11, as shown in FIG. 2-. In FIG. 3, the circuit 76 transmits the load demand signal and converts it to a O to 10 volts DC. current in circuit 72. In all cases voltage increases the load signal and converts it to a to volts DC. current in circuit 72. In all cases voltage increases proportionally to increases in required output of the controlled variables, such as feedwater fiow, fuel flow,

etc. Increases in the load demand signal increase the feedwater flow, fuel and air flow controlled variables and vice versa for decreases in the load demand signal.

Fluid at 73 is drawn to steam pressure transmitter 74 through conduit '75. Transmitter 74 converts fluid pressure to electrical voltage in output circuit 76. Circuit 76 feeds to reset amplifier 77 where the steam pressure signal is compared to set point. Set point adjuster 78 establishes the value to which steam pressure shall be controlled. Increases in steam pressure above set point decrease the feedwater flow, fuel and air kow controlled variables through the integral action until the pressure returns to set point and vice versa for decreases in steam pressure. The integral action accumulation persists after pressure has returned to the set point. Circuit 79 transmits the output of reset amplifier 77 to amplifier 80 where the circuit 79 signal is added to the circuit 72 signal. Derivative action from rate of voltage change in circuit 72 is incorporated in amplifier 80. The derivative action is only sustained as long as a rate of change in circuit 72 voltage persists, after which the derivative action is cancelled after a short interval of time. Means are provided i amplifier 81) for adjusting the reference level of output voltage in circuit 81. Circuit 81 feeds to amplifiers 82 and 83.

Thermocouple 84 in primary steam conduit 2 in rc- 1 sponse to temperature generates a potential in circuits 85 and 86 when referenced to cold junctions in transmitter 87. Transmitter 87 sends out a variable voltage in circuit 88 proportional to changes in steam temperature in conduit 2. Circuit 88 feeds to amplifiers 39 and 90. The circuit 88 voltage is compared with set point voltage 91 to 89. Increases in steam temperature above set point decrease the output of amplifier 39 in circuit 92 and vice versa for decreases in steam temperature.

The reset action of amplifier 89 is similar to that of amplifier 77. Derivative amplifier 9% is responsive to voltage changes in circuit 88. Increasing temperature from 87 decreases the voltage output in circuit 93 for the duration of the change and vice versa for decreasing temperature. The signals of circuits 92 and 93 are totalized in amplifier 94. The output circuit 95 feeds to amplifiers 82 and 83.

In amplifier 82, the signals from circuits 81 and 95 are added together. Means are provided for adjusting the gain of each. Means are also provided for varying the reference level of the output voltage in circuit 96. Circuit 96 feed to reset amplifier 97 which is similar to amplifier 77. Means are provided to measure fuel flow in conduit 43 (shown in FIG. 2). Where oil is the fuel, orifice 98 is provided with pressure taps 99 and 101i. Taps 99 and 100 feed to transmitter 101 shown in FIG. 3 which converts proportional increases in fuel flow to proportional increases in control voltage in circuit 102. Circuit 102 feeds to reset amplifier 97. Amplifier 98 increases or decreases the output voltage in circuit 103 until the signals in circuits 96 and 102 are equal. If the actual flow as measured in circuit 102 is below the required flow as measured in circuit 96, the voltage of the 193 circuit will increase to open valve 108 until a balance is achieved between circuits 96 and 102. Circuit 103 feeds to electric-pneumatic converter 104 where proportional increases in the control voltage in circuit 103 are converted to proportional increases in pneumatic air pressure in conduit 1115. Conduit 105 feeds to valve positioner 106 which is provided with cam means to produce a given valve 108 opening for each given air pressure in conduit 1115. Positioner 11% supplies power air to valve operator 107 to change the position of valve 168. Conduit 109 supplies control air to converter 104. Conduit 1111 supplies power air to positioner 196 and valve operator 107.

Feedwater flow is measured across nozzle 111 in conduit 24. Pressure conduits 112 and 113 across nozzle 111 connect to transmitter 114. Transmitter 114 converts differential pressure across nozzle 11 to electric voltage in a way so that proportional increases in flow increase electric voltage proportionally in circuit 115. Circuit 115 feeds to reset amplifier 116 which functions the same as 97 to balance actual feedwater flow in circuit 115 with required feedwater flow in circuit 117 by opening and closing feedwater flow control valve 118. Circuit 119 corresponds to circuit 103, electric-pneumatic converter 120 to 194, conduit 121 to 165, positioner 122 to 1116, valve operator 123 to 107. Amplifier 83 subtracts the circuit 95 signal from the circuit 81 signal. Means are provided for adjusting the gain of each. Means are also provided for referencing the level of the output voltage in circuit 117. Also, the output gain from amplifier 83 may be biased with respect to the output gain from amplifier 82.

In FIG. 2, air is supplied to burners 44 through air supply duct 124. Damper 125 controls the flow of air. Lever arm 126 connected to damper operator 127 through connecting link 123 positions damper 125. Air flow is measured through air foil 129. Taps 139 and 131 sense differential pressure through the air foil 129. Conduits 130 and 131 connect to transmitter 132 shown in FIG. 3 which converts proportional increases an air flow as measured by differential pressure in conduits 130 and 131 to proportional changes in electric voltage in circuit 133. Circuit 133 feeds to optional ratio control 134 which ratios the air flow signal from 132 to amplifier 136 so as to maintain the proper fuel-air ratio in the burners 44 as measured in an oxygen meter taking samples from the flue gas in conduit 46 and sending the oxygen quantity measurement signal to circuit 135 after passing through a reset amplifier with set point (not shown). Ratio control 134 feeds to reset amplifier 136 through circuit 137 (134 could be omitted for purposes of simplification). Fuel flow measurement in circuit 102 is compared with air flow measurement in circuit 137 and the output signal of 136 in circuit 138 is varied until there is the proper balance as described for amplifier 97. Thus, air flow is made to follow fuel flow with a fixed ratio between the two.

Circuit 138 feeds to amplifier 139. The signal in circuit 96 which is the base signal and which reflects rapid change in the load demand signal is added to the circuit 138 signal in amplifier 139. The gain of the signal in the circuit 96 may be adjusted for calibration purposes. Also, means are provided for varying the reference level of the output voltage in conduit 149. Circuit 140 feeds to bias amplifier 141 and bias setter 142. The bias amplifier permits adjustment of air flow ratio with respect to fuel flow. Amplifier 141 feeds through circuit 142 to electric-pneumatic converted 143 which is similar to 104.

The pneumatic signal from 143 passes through conduit 144 to position 145. Positioner 145 feeds air to power piston operator 127 to drive it to the correct position. A cam and lever associated with the piston operator 127 .and positioner 145 controls the supply of air to the piston from positioner 145. The piston will assume a predetermined position for each control air pressure variation in conduit 144. Thus, piston position may be characterized as desired with air pressure in conduit 144. Piston op- 9 erator 127 drives air flow control damper 125 through connecting link 128.

Thermocouple 146 is located in the 1st reheat outlet steam conduit'61. The circuits from 146 to circuit 166 are the same as from 84 to circuit 95. Element 149 corresponds to 87, 162 to 91, 16b to 89, 161 to 90 and 155 to 94. When lst reheat steam temperature rises above the set point in 162, voltage decreases in circuit 166, decreasing gas recirculation flow and vice versa for steam temperatures below set point. Load demand trans mitter '71 sends a signal to amplifier 167 through circuit 72 which increases gas recirculation flow proportional with load decrease. Maximum gas recirculation is required at minimum control load. Circuit 166 adds to the circuit 72 signal described above. Means are provided for referencing the output of 167 in circuit 163.

Gas recirculation flow is measured across orifice 169 in conduit 67 shown in FIG. 2. Pressure taps 170 and 171 sense the differential pressure across orifice 169. In FIG. 3, taps 170 and 171 connect to flow transmitter 172 which functions similar to element 132.

The voltage increase in circuit 173 is proportional to gas flow increase in conduit 67. Actual gas flow measured in circuit 173 is balanced against required gas flow in circuit 168 and the output signal in circuit 175 is increased or decreased until circuits 1'73 and 168 are balanced. Increased voltage in circuit 175 increases gas recirculation fl-ow. Electric-pneumatic converter 176 is similar to 143 and 194. The action of positioner 177 and piston operator 178 is similar to elements 145 and 127. Piston movement is transmitted through link 1811 to lever arm 181 which rotates damper 182 in the gas recirculation conduit 65 shown in FIG. 2.

Thermocouple 183 is located in the second reheat outlet steam conduit 8. The circuits from 183 to circuit 1'94 are similar to those from 84 to circuit Q5.

Element 1% corresponds to =87, 188 to 89, 189 to 90, 190 to 91 and 193 to 94. The difference is that when 2nd reheat steam temperature rises above the set point in 190, voltage increases in circuit 194, increasing opening of valve 53, passing more primary circuit flow through the heat exchanger 48. Since heat input to the primary steam circuit is correspondingly increased, firing rate should be decreased accordingly. The reverse happens when 2nd reheat steam temperature falls below the set point. Circuit 19 feeds to electric-pneumatic converter 195 which is similar to 194. The control action from converter 195 to valve 53 is similar to that from 1& to valve 108. Positioner 1% corresponds to 106, valve operator 197 to 107, valve 53 to valve 108. Circuit 19 1 also feeds to amplifier 82. The gain of the circuit 194 signal may be varied before it is subtracted from the signal-s from circuits 81 and 95 in amplifier 82 Thus, as 2nd reheat temperature rises, firing rate is decreased and heat transfer from the 2nd reheat steam circuit to the primary steam circuit is increased.

It is also obvious that there is flexibility with respect to the arrangement of the control system. Alternatively, the thermocouples 8 3 and 183 could be interchanged with respect to their locations in the steam piping so that 2nd reheat steam temperature would control fuel and air flow direct and primary steam temperature would control valve 53. In such case, the circuit -95 connection to amplifier 83 would be omitted.

In order to adapt FIG. 3 to the FIG. 1 arrangement, elements 57, 53, 60, 61 and 63 are omitted. The system from thermocouple 146 to piston operator 17S and connecting links including the circuit '72 connection to amplifier 167 are not required. The 2nd reheat control circuit would be used for the 1st and only reheat steam circuit.

In the case where the steam generator has a 1st and 2nd reheat steam circuit and a heat exchanger 43 is provided for each reheat circuit to exchange heat with the primary steam circuit as described above, the control for regulating flow through each heat exchanger would be the same as 10 that shown in FIG. 3. Two systems would be required to operate in parallel, each having duplicate components to those shown in FIG. 3.

FIGS. 4a and 4b show an arrangement for heat exchanger 48 which can be used with FIGS. 1 and 2. Reheat steam enters the heater she'll through conduits 49 and flows back and forth across tubes 59. Bafiies 198 direct the how of reheat steam from the core 230 out toward the shell and 'bafiies 199 direct the flow of reheat steam from the shell to the core. The reheat steam exits through conduits 51. The core diameter, baffle locations and shell diameter are arranged to produce reheat steam velocities in the vicinity of ft. per second across the tubes 59 at full load flows. The tubes 50 are arranged concentr-ically around the shell, as shown in FIG. 4b. Flow through the tubes 56 is multipass. Primary circuit fluid from conduit 52 discharges to feeder tubes 201 which sup ply the multipass tube circuits 59. The mu'ltipass arrangement for each tube 50 circuit is contained within heater shell 48. Gnly the inlet and outlet of each multipass tube circuit protrudes through the heater shell. The connec tions 201a are broken off to show a clearer picture of the arrangement. They too supply the individual multipass tube circuits 5%. The outlet of the multipass tube circuits connect to discharge tubes 2M2. Tubes 292:: are broken ed for purposes of clarity. Tube 202 and 202a connect to conduit 54. Tubes 50 are finned on the exterior surface to increase the heat'transfer surface on the reheat steam :side. Primary circuit fluid iiows through the inside of the tubes. The heater shell 48 has an inner jacket 263 in the vicinity of the reheat steam inlet which is separated from the external shell sufficiently to pass cooling steam from conduits 2 34 between the jacket and the shell. Cooling steam may be obtained through conduits 204 and flow control valves 205' from conduit 5 of FIGS. 1 and 2.

' The heat transfer in heater 48 is very eflicient compared with heat transfer to the superheaters in the steam generator. The transfer rate for the heater 48 can be as high as B.t.u. per square foot of external heating surfiace per hour per degree of log mean temperature differential compared with about 6 to 10 B.t.u. to the same base for heat transfer which occurs in the superhe-aters in the steam generator convection section 4'7. To minimize the total length of the high pressure tube 50 circuits, the tubes are finned on the outside to raise the heat transfer rate from the reheat steam (containing superheat) to the tube metal so as to be more nearly equal to the transfer rate from the tube metal to the internal fluid. Thus, the high pressure material is minimized.

Thus, heat absorbed in low pressure reheater 6 can be effectively and efficiently transferred to the primary steam circuit in heater 4-8. This reduces the heat absorption duty required for the primary circuit to support full load specified temperature operation which offsets to a great extent the added cost for the heat exchanger 48. Nor- .mally some surplus reheating surface as is employed in 1 FIGS. 1 and 2 is required for the reheat circuits to pro vide somecontrol range for reheat steam temperature below design rating. The efficiency gains as a result of elimination of reheat spray water and reduced cost of control equipment, as a result of the present invention is evident.

In a 450,000 kilowatt steam electric generating plant having 3500 p.s.i.g. and 1000 F. primary steam and two reheats at 1025 and 1050 F. respectively, the heat exchanger 48 inlet reheat steam would be about 380 p.s.i.g. and 900 F. reducing the steam temperature about 75 F. through the unit before the reheat steam exits through conduits 51. The primary fluid temperature entering would be about 600 F., and would exit at about 700 F. Reheat steam flow through heater 48 would be about 2,240,000 lbs/hr. and primary fluid flow would be about 300,000 lbs/hr. The finned tube surface required would be about 5,000 sq. ft.

FIGS. 1 and 2 are not intended to be limiting with respect to location of heat exchanger 48. For example,

''i. exchanger 43 could be located in the reheat conduit where the steam temperature was sufliciently high (as 800 F.) to produce economical rate of heat transfer er square foot of transfer surface. Precise location of heat exchanger 48 and connecting conduits will depend upon the specific requirements of each application. Also, the conduit 52 can connect to the primary circuit upstream of preheater 25 instead of to conduit 26 without significant change in the performance of the system. For the same reason, conduit 54 can discharge to some downstream location other than conduit 31, as shown in FIGS. 1 and 2.

Thus, it will be seen that I have provided eflicient systems for a steam generator having a primary steam circuit and a reheating steam circuit, which systems Will improve the heat absorption balance throughout at least a portion of the load range for maintaining specified outlet steam temperatures in both circuits; furthermore, I have provided a novel apparatus and system for accomplishing the above, whereby heat may be transferred from a reheat steam circuit to the primary steam circuit in varying relative amounts to balance heat absorptions in the circuits exchanging heat; also, I have provided a novel apparatus and system for controlling outlet steam temperatures for a steam generator having a primary circuit and one reheating stage; also, I have provided a novel apparatus and system for controlling outlet steam temperatures for a steam generator having a primary circuit and two reheating stages; also, I have provided a novel system to integrate steam temperature control with control of steam generator feedwater flow and firing rate; furthermore, I have provided a novel heat exchange apparatus for exchanging heat in the reheat steam circuit with the primary steam circuit.

I claim:

1. A steam-electric generating plant and under variable load conditions comprising a steam generator having a high pressure primary circuit for flowing fluid therethrough, said primary circuit having heat absorption conduits located serially in preheating, generating and superheating sections of said steam generator, a steam consumer turbine for driving electric generating means, conduit means for interconnecting said steam generator primary circuit outlet and said steam consumer turbine, said steam generator including reheating heat absorption conduits, additional conduit means for conveying steam, after partial use in said steam consumer turbine from said steam consumer turbine to said reheating heat absorption conduits and for returning said partially consumed steam, after reheating, to said steam consumer turbine, a combustion furnace for generating hot products of combustion, and duct means arranged serially for conveying said hot products of combustion over and among said steam generator heat absorption conduits, a bypass conduit, said bypass conduit connected to the primary circuit at both ends and bypassing at least a portion of said preheating and generating section heat absorption conduits for establishing hydraulic head to flow a portion of the primary circuit fluid through said bypass conduit from an upstream to downstream point of the primary circuit, an auxiliary heat exchange means located in said additional conduit means and adapted to exchange heat with fluid flowing through said bypass conduit, flow control means in said bypass conduit, said reheating heat absorption conduits and said auxiliary heat exchange means being adapted for regenerative heating of the primary circuit fluid, said regenerative heating per unit of primary circuit flow con tinuously increasing throughout at least a portion of the upper load range as said plant load increases and at essentially constant primary circuit and reheating heat absorption conduit fluid temperatures at outlet points.

2. A steam-electric generating plant and under variable load conditions comprising a steam generator having a high pressure primary circuit for flowing fluid therethrough, said primary circuit having heat absorption conduits located serially in preheating, generating and superheating sections of said steam generator, a steam consumer turbine for driving electric generating means, conduit means for interconnecting said steam generator primary circuit outlet and said steam consumer turbine, said steam generator including reheating heat absorption conduits divided into low temperature and high temperature components, interconnecting conduit means for flowing fluid serially from said low temperature to said high temperature components, additional conduit means for conveying steam, after partial use in said steam consumer turbine, from said steam consumer turbine to the inlet of said low temperature component and for returning said partially consumed steam after reheating in said low temperature and high temperature components to said steam consumer turbine, a combustion furnace for generating hot products of combustion, and duct means arranged serially for conveying said hot products of combustion over and among said steam generator heat absorption conduits, a bypass conduit, said bypass conduit connected to the primary circuit at both ends and bypassing at least a portion of said preheating and generating section heat absorption conduits for establishing hydraulic head to flow a portion of the primary circuit fluid through said bypass conduit from an upstream to downstream point, an auxiliary heat exchange means located in said interconnecting conduit means and adapted to exchange heat with fluid flowing through said bypass conduit, flow control means in said bypass conduit, said reheating heat absorption conduits and said auxiliary heat exchange means being adapted for regenerative type heating of the primary circuit fluid, said regenerative type heating per unit of primary circuit flow continuously increasing throughout at least a portion of the upper load range assaid plant load increases and at essentially constant primary circuit and reheating heat absorption conduit fluid temperatures at outlet points.

References ited by the Examiner UNITED STATES PATENTS 3,111,936 11/63 Brunner 122-479 FOREIGN PATENTS 1,225,893 2/60 France.

137,548 4/30 Switzerland.

JULIUS E. WEST, Primary Examiner.

ROBERT R. BUNEVICH, Examiner.

Claims (1)

1. A STEAM-ELECTRIC GENERATING PLANT AND UNDER VARIABLE LOAD CONDITIONS COMPRISING A STEAM GENERATOR HAVING A HIGH PRESSURE PRIMARY CIRCUIT FOR FLOWING FLUID THERETHROUGH, SAID PRIMARY CIRCUIT HAVING HEAT ABSORPTION CONDUITS LOCATED SERIALLY IN PREHEATING, GENERATING AND SUPERHEATING SECTIONS OF SAID STEAM GENERATOR, A STEAM CONSUMER TURBINE FOR DRIVING ELECTRIC GENERATING MEANS, CONDUIT MEANS FOR INTERCONNECTING SAID STEAM GENERATOR PRIMARY CIRCUIT OUTLET AND SAID STEAM CONSUMER TURBINE, SAID STEAM GENERATOR INCLUDING REHEATING HEAT ABSORPTION CONDUITS, ADDITIONAL CONDUIT MEANS FOR CONVEYING STEAM, AFTER PARTIAL USE IN SAID STEAM CONSUMER TURBINE FROM SAID STEAM CONSUMER TURBINE TO SAID REHEATING HEAT ABSORPTION CONDUIT AND FOR RETURNING SAID PARTIALLY CONSUMED STEAM, AFTER REHEATING, TO SAID STEAM CONSUMER TURBINE, A COMBUSTION FURNACE FOR GENERATING HOT PRODUCTS OF COMBUSTION, AND DUCT MEANS ARRANGED SERISLLY FROM CONVEYING SAID HOT PRODUCTS OF COMBUSTION OVER AND AMONG SAID STEAM GENERATOR HEAT ABSORPTION CONDUITS, A BYPASS CONDUIT, SAID BYPASS CONDUIT CONNECTED TO THE PRIMARY CIRCUIT AT BOTH ENDS AND BYPASSING AT LEAST A PORTION OF SAID PREHEATING AND GENERATING SECTION HEAT ABSORPTION CONDUITS FOR ESTABLISHING HYDRAULIC HEAD TO FLOW A PORTION OF THE PRIMARY CIRCUIT FLUID THROUGH SAID BYPASS CONDUIT FROM AN UPSTREAM TO DOWNSTREAM POINT OF THE PRIMARY CIRCUIT, AN AUXILIARY HEAT EXCHANGE MEANS LOCATED IN SAID ADDITIONAL CONDUIT MEANS AND ADAPTED TO EXCHANGE HEAT WITH FLUID FLOWING THROUGH SAID BYPASS CONDUIT FLOW CONTROL MANS IN SAID

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