US3261332A - Vapor generator vapor temperature control - Google Patents

Description

- y 1966 w. a. SCHUETZENDUEBEL ETAL 3,261,332

VAPOR GENERATOR VAPOR TEMPERATURE CONTROL Filed June 4, 1964 3,261,332 VAPOR GENERATOR VAPOR TEMPERATURE CONTROL Wolfram G. Schuetzenduebel, Avon, and Richard D. Hottenstine and William H. Clayton, Jr., Windsor, Conn., assignors to Combustion Engineering, Inc., Windsor, Conan, a corporation of Delaware Filed June 4, 1964, Ser. No. 372,556 14 Claims. (Cl. 122-479) This invention relates to supercritical pressure reheat vapor generators and to means for controlling the vapor temperature therein.

It is well known that maximum efliciency of steam power plants consistent with the capability of the apparatus can be attained when the steam temperature of both the primary circuit and the reheat circuit is controlled at its maximum value. During low load operation, in particular, the temperature of the steam leaving both the primary and reheat circuits tends to drop off, with there being more of a tendency for the reheat temperature to drop than the temperature of the primary steam. This is due not only to the predilection of boiler designers to keep reheat surface away from furnace radiation, but also to the decrease in temperature of the steam delivered to the reheater at the lower loads. It is desirable to have not only means of temperature control for one of the temperatures, but also a steam temperature control which is effective .to balance the two steam temperatures. While any of the conventional modes of steam temperature control may be used to control the primary steam temperature, it is the control of the reheat steam temperature or the balance of these two, and therefore the relative heat absorption of the two circuits with which our invention is concerned.

Our invention in a narrower sense comprises the location of primary heating surface, preferably an economizer section, in a stream of combustion gases with reheat heating surface being located downstream therefrom. Hot fluid is circulated from the furnace circuits through this economizer decreasing its effectiveness so that the heat pickup of the reheating section is increased.

It is an object of this invention to provide in a supercritical reheat vapor generator an improved means of steam temperature control which is particularly effective at reduced load.

It is a further object to obtain this vapor temperature control in a supercritical pressure reheat unit by decreasing the effectiveness of the heating surface of a first circuit whereby more heat is therefore picked up in the second circuit.

Other and further objects of the invention will become apparent to those skilled in the art as the description proceeds.

With the aforementioned objects in view, the invention comprises an arrangement, construction and combination of the elements of the inventive organization in such a manner as to attain the results desired, as hereinafter more particularly set forth in the following detailed description of an illustrative embodiment, said embodiment being shown by the accompanying drawing wherein:

FIGURE 1 is a diagrammatic representation of a side elevation of a supercritical pressure reheat vapor generator, showing the location of the furnace section recirculating pump in the through-flow portion of the circuit; and

FIGURE 2 is a similar representation wherein the circulating pump for the furnace section is located in the furnace recirculating line.

In the illustrative embodiment of FIGURE 1 fuel and air are supplied to the furnace or combustion chamber United States Patent "ice 2 burning the fuel therein, passing gaseous combustion products through horizontal flue 4 and vertical flue 6 with the gases egressing through duct 8 to be conveyed through an air heater to the stack (not shown). Feedwater is supplied at high pressure to the vapor generator through conduit 10 passing through low temperature economizer 12 and thereafter being conveyed through the high temperature economizer 14. This heated feedwater is then conveyed through conduit 16 to the mixing vessel 18 from which it is passed .through the furnace section 20. The primary fluid passing through this section is heated by furnace radiation to a high temperature in the order of 800 F. and is conveyed from the outlet pipe 22 to the finishing superheater section 24, where it is further heated by the combustion gases passing thereover through flue 4. This superheated vapor is conveyed through outlet pipe 26 to a high pressure turbine (not shown).

The steam returning from this turbine enters as low temperature reheat steam through conduit 28 passing through a low temperature reheater section 30 and a high temperature reheater section 32. The reheated steam at high temperature is then conveyed through reheat outlet pipe 34 to the reheat turbine (not shown).

Furnace section circulating pump 36 is located in the through-flow portion of the furnace heating section and is operative to induce recirculation of hot fluid from the furnace section outlet pipe 22 through the recirculating line 38 to the mixing vessel 18. The basic recirculating system is described in US. Patent No. 3,135,252 to W. W. Schroedter. When pump 36 is operating the recirculated fluid through line 38 mixes with the incoming preheated feedwater passing through line 16 mixing in the mixing vessel 18. The fluid being pumped by circulating pump 36 is, therefore, at a relatively .high temperature.

As the gaseous products of combustion or flue gas pass through fines 4 and 6, they are cooled at each successive section by transferring heat to the heating surfaces located therein. It is evident, therefore, that the gas temperature entering each section is a function of the heat absorbed in the preceding sections in relation to the gas flow.

In order to increase the reheated steam temperature egressing from this unit, valve 48 is opened permitting a portion of the flow from pump 36 to pass through line 42 mixing with incoming feedwater in line 13. This operates to increase the fluid temperature entering the heating surface of the high temperature economizer 14 thereby decreasing the temperature head between the fluid heating surface 14 and the flue gases passing thereover. Because of this decreased temperature head less heat is picked up by the high temperature economizer section 14 and the gas temperature leaving this section is consequently higher. This higher gas temperature passing over the reheater section 30 results in a higher temperature r head or therefore more heat picked up in the reheat section. A portion of the excess heat in the gases entering the reheat section 30 remains in the gases leaving that section such that the gas temperature entering the low temperature economizer 12 is also increased. This is operative to increase the heat absorption in that section and therefore the fluid temperature passing through line 13, further emphasizing the increased fluid temperature entering the high temperature economizer 14.

The higher reheat steam temperature leaving the low temperature reheater 30 also enters the high temperature reheater 32 resulting in a higher reheat temperature leaving through the reheat outlet line 34.

Reheat outlet temperature sensing means 48 sends a control signal which is matched with the desired reheater outlet temperature at set point setter 50 whereby an error signal is sent to controller 52. This signal is representative of a desired temperature entering the high temperature economizer 14. This temperature entering the high temperature economizer is sensed by temperature sensing device 54 which sends a control signal to controller 52. The desired and measured temperatures are compared with this controller and the control signal sent through line 56 to operate valve 40 so as to obtain desired temperature entering the high temperature econornizer 14 and consequently the desired reheater outlet temperature.

This mechanism is therefore operative to vary the relative heat absorption of the primary and reheat circuits, and may be controlled to balance the primary and reheat outlet temperatures. Where our invention is controlled to balance temperatures, conventional steam temperature control may be used to control the temperature level. Our invention functions in the same manner, regardless of the system used to control it. For instance, the valve 40 may be controlled to equalize the primary and heat temperatures, with gas recirculation operating to maintain the primary or reheat steam temperature. Alternately valve 40 may be controlled to maintain the desired reheat temperature, with gas recirculation being controlled to maintain the desired primary temperature.

In order to make this control of maximum effectiveness, the surface should be arranged so that during operation without reheat steam temperature control the reheat section 30 operates at a very low temperature head, that is, the difference between the temperature of the gases in the flue 6 and the temperature of the reheat steam in the reheater 30 is very low. Although this will result in a high surface requirement for that section of the reheater, due to the low temperatures involved and the low pressure, it is relatively inexpensive heating surface. Due, however, to this low temperature head, any change in the gas temperature entering the reheat section will be very effective in changing the amount of heat absorbed.

Due to the decreased pickup in the economizer 14 additional heat must be absorbed elsewhere in the primary circuit. As previously described, a portion of this heat requirement is achieved in the low temperature economizer 12. If an air heater is used in conjunction with this unit, the increased gas temperature entering the air heater results in an increased preheated air temperature and, therefore, an increased heat release rate in the furnace. Additional increase as required must be made up by firing additional fuel, which must be done in any event in order to supply the additional heat being picked up now by the reheater.

The low temperature economizer 12 is not essential to the operation of our invention although it is very helpful in attaining maximum steam generator efiiciency. This heating section aids in reducing the flue gas temperature to avoid excessive stack losses and contributes to the increase in temperature entering section 14 thereby increasing the efficacy of the reheat steam temperature control.

It should furthermore be noted that the contribution of the high temperature economizer 14 is such as to make the arrangement effective for reheat temperature control even if the reheat section 30 were not placed downstream of the low temperature economizer 14. In such an arrangement passing the hot fluid through conduit 42, thereby increasing the temperature. of the fluid entering the heat exchange section 14, decreases the heat pickup in that section. This varies the relative heat absorption between the primary and reheat circuits. The decrease in heat absorption of the primary circuit must then be made up elsewhere, in order to maintain full steam temperature of the primary fluid. An increase in the furnace heat release would occur either by an increased air heater temperature, by increased fuel input or by both. This, in turn, increases the gas temperatures throughout the unit resulting in an increased gas temperature entering the reheat section 32 and an increased heat absorption therein.

It can therefore be seen that although the instant embodiment would be operative Without the inclusion of the low temperature reheat section 30' or the low temperature economizer 12, the efiicacy of this mode of steam temperature control is substantially increased by their incorporation. Regardless of whether heat exchange sections 30 and 12 are included, reheat temperature control is obtained by regulating valve 40 to vary the flow of the hot fluid passing through conduit 42 to heat feedwater entering the high temperature economizer 14. The regulation of valve 40 is performed in such a manner as to control the temperature of the reheat steam leaving through steam pipe 34.

It should be noted that in the embodiment of this invention the heat used to increase the temperature of the fluid entering the high temperature economizer is obtained from another part of the primary circuit; namely, the furnace wall portion. The importance of this as an aid to the efficacy of the steam temperature control can be best illustrated by considering the converse. If the temperature of the fluid entering the high temperature economizer were heated by something such as a separately fired heating unit, the effectiveness of the economizer would, of course, be decreased. However, since heat is being supplied to the primary circuit in this heater, less heat need be supplied in the furnace and therefore the firing rate of the unit would be decreased. This would, in turn, have the effect of decreasing the reheat temperature which tends to counteract the increased reheat temperature which we are trying to obtain by increasing the temperature of the fluid in the high temperature economizer.

The steam generator illustrated in FIGURE 2 is similar to that of FIGURE 1 except for the location of the recirculating pump 36. The recirculating pump in this embodiment is located in the recirculating line 38 whereby it pumps fluid from the furnace section outlet pipe 22 to the mixing vessel 18. Reheat temperature control is obtained by regulating valves 44 and 46 to produce flow through conduit 42 thereby increasing the temperature in the high temperature economizer 14. The automatic control signal operating through line 56 of FIGURE 1 must accordingly be modified to operate valves 44 and 46. Inasmuch as the fluid passing through line 42 in this embodiment is that leaving the furnace section, it is of a considerably higher temperature than the fluid passing through conduit 42 of FIGURE 1. In FIGURE 1 the fluid passed through this conduit is a mixture of the very high temperature fluid and the incoming feedwater passing through conduit 16. The amount of fluid therefore which must pass through conduit 42 of FIGURE 2 in order to obtain an effective control is somewhat less than the amount required in the embodiment of FIG- URE 1.

In a sub-critical unit a temperature of the fluids in sections such as those with which we are here concerned are limited to the saturation temperature of the Water therefore in such subcritical units the mode of recirculation such as herein described would not be effective to substantially change the temperature of the fluid entering the high temperature economizer 14 and would therefore be ineffective for steam temperature control. Our invention would however be effective to control steam temperature on a double reheat unit.

While we have illustrated and described a preferred embodiment of our invention it is to be understood that such is merely illustrative and not restrictive and that variations and modifications may be made therein without departing from the spirit and scope of the invention. We therefore do not wish to be limited to the precise details set forth but desire to avail ourselves of such changes as fall within the purview of our invention.

What We claim is:

1. A supercritical pressure vapor generator having a first circuit, a second circuit, fluid passing through each circuit, a combustion zone for the burning of fuel therein, means for conveying combustion gases from said combustion zone; means efifective to transfer heat from said combustion gases to said first circuit, including a section of said first circuit located in the flow path of said combustion gases; means effective to transfer heat from said combustion gases to said second circuit; means for transferring heat to the fluid entering said section of said first circuit from a portion of said first circuit downstream, with respect to the fluid flow, of said section of said first circuit whereby the effectiveness of said section for heat absorption is reduced, thereby increasing the relative heat absorption of said first circuit; means for determining the relative heat absorption of the fluids flowing through said first and second circuits; and means for regulating said heat transferred to said section of said first circuit from the downstream portion of said first circuit in response to said last-named means.

2. An apparatus as in claim 1 wherein said first circuit includes tubes lining the walls of said combustion zone, a mixing vessel and means for conveying fluid from said section to the mixing vessel, means for conveying fluid from the mixing vessel to and through said tubes, means for conveying at least a portion of the fluid passing through said tubes to the mixing vessel; a pump located in the last-named means; said means for transferring heat to the fluid entering said section comprising means for conveying fluid from the discharge of said pump to the inlet of said section; and throttling means located between said pump and said mixing vessel; and said means for regulating said heat transfer comprising throttling means located between said pump and said section.

3. An apparatus as in claim 1 wherein said first circuit includes tubes lining the walls of said combustion zone, a mixing vessel and means for conveying fluid from said section to the mixing vessel, means for conveying fluid from the mixing vessel to and through said tubes, means for conveying at least a portion of the fluid passing through said tubes to the mixing vessel; a pump located between said mixing vessel and said tubes; said means for transferring heat to said section comprising means for conveying fluid from said pump discharge to said section, and said means for regulating said heat transfer comprising throttling means located in said means for conveying fluid to said section.

4. A supercritical vapor generator having a first circuit and a second circuit, fluid passing through each circuit, a combustion zone for the combustion of fuel therein, and means for conveying a stream of combustion gases from said furnace; heating surface comprising a portion of said first circuit forming a section in the gas stream at an upstream location; and heating surface comprising at least a portion of said second circuit forming heating surface located in the gas stream at a downstream location; means for increasing the temperature of the fluid entering said section of said first circuit by transferring heat thereto from a downstream portion of said first circuit with respect to fluid flow whereby the heat absorbing effectiveness of said section is decreased, thereby increasing the relative heat absorption of said first and second circuits; means for determining the relative heat absorption between said first and second circuits; and means for regulating the temperature of the fluid entering said section of said first circuit in response to said last-named means.

5. An apparatus as in claim 4 wherein said first circuit includes tubes lining the walls of said combustion zone, a mixing vessel and means for conveying fluid from said section'of said first circuit to the mixing vessel, means for conveying fluid from the mixing vessel to and through said tubes, means for conveying at least a portion of the fluid passing through said tubes to the mixing vessel; a pump located in the last-named means; said means for increasing the temperature of the fluid entering said section comprising means for conveying fluid from the discharge of said pump to the inlet of said section of said first circuit; and said means for regulating the heat transfer comprising throttling means located between said pump and said mixing vessel, and throttling means located between said pump and said section.

6. An apparatus as in claim 5 wherein said first circuit includes also a low temperature economizer portion, iocated upstream of said section of said first circuit with relation to fluid flow, but located downstream with respect to gas flow, and also located downstream with respect to gas flow of said heating surface comprising a portion of said second circuit.

7. An apparatus as in claim 4 wherein said first circuit includes tubes lining the walls of said combustion zone, a mixing vessel and means for conveying fluid from said section of said first circuit to the mixing vessel, means for conveying fluid from the mixing vessel to and through said tubes; means for conveying at least a portion of the fluid passing through said tubes to the mixing vessel; a pump located between said mixing vessel and said tubes, said means for transferring heat to said section of said first circuit comprising means for conveying fluid from said pump discharge to said section; and said means for regulating said heat transfer comprising throttling means located in said means for conveying fluid to said section.

8. An apparatus as in claim 7 wherein said first circuit includes also a low temperature economizer portion, located upstream of said section of said first circuit with relation to fluid flow, but located downstream with respect to gas flow, and also located downstream with respect to gas flow of said heating surface comprising a portion of said second circuit.

9. A supercritical vapor generator having a primary circuit with fluid flowing therethrough, a reheat circuit with the fluid flowing therethrough, a furnace for the combustion of fuel therein, and a flue for the conveyance of combustion gas therefrom; a primary section of heating surface comprising a portion of said primary circuit located in said flue at an upstream location with respect to the gas flow; a reheater section comprising at least a portion of said reheater circuit located in said flue at a downstream location with respect to the gas flow; means for transferring heat to the fluid entering said section of the primary circuit from a portion of the primary circuit downstream of said primary section with respect to fluid flow; means for determining the temperature of the fluid leaving the reheat circuit; and means for regulating said means for transferring heat to the fluid entering said primary section in response to said tem perature determining means.

10. A supercritical pressure vapor generator having a primary circuit, a reheat circuit, a furnace for the combustion of fuel therein, a flue for the conveyance of combustion gases therefrom; a first section of primary heating surface comprising a portion of said primary circuit, located in said flue at an upstream location with respect to the gas flow; a second section of primary heating surface comprising tubes lining the furnace walls; a reheater section of heating surface comprising at least a portion of said reheater circuit located in said flue at a downstream location with respect to the gas flow; means for conveying lflllld from said first section to said furnace section during normal operation; and means for returning a portion of the fluid from said furnace section to said first section to increase the temperature of the fluid pas-sing therethrough; measuring means for determining a measure of the heat absorption in said reheat circuit; and means for regulating the quantity of fluid returned to said first section in response to said measuring means.

11. An apparatus as in claim 5 including also a third heating surface comprising a portion of said primary circuit, said third section located upstream of said first section with relation to the fluid flow but located downstream of said first and second sections with respect to the gas flow.

- 12. The method of operating a vapor generator comprising; burning fuel in a combustion zone and forming products of combustion, passing a fluid at relatively high pressure in convection heat exchange with the products of combustion in a convection zone, thereafter passing said [fluid at high pressure in heat exchange relation with the products of combustion in another zone, passing a lower pressure fluid in heat exchange relation with the products of combustion, returning a portion of the high pressure fluid from said other zones and mixing said returned fluid with the high pressure fluid entering said convection zone whereby the temperature of the fluid entering said convection zone'is increased and the heat absorption in this Zone is decreased, determining the relative heat absorption of the high pressure and low pressure fluids, and regulating the amount of fluid returned in response to the determined relative heat absorption.

13. The method of operating a vapor generator comprising burning fuel in a combustion zone and forming products of combustion passing a fluid at relatively high pressure in convection heat exchange with the products of combustion in a convection zone at an upstream location, thereafter passing said fluid at high pressure in heat exchange relation with the products of combustion in another zone, passing a lower pressure fluid in heat exchange relation with the products of combustion including passing the lower pressure fluid in heat exchange relation with the products of cornbustion at a downstream location, returning a portion of the high pressure fluid from said other zone and mixing said returned fluid with the high pressure fluid entering said convection zone at said upstream location whereby the temperature of the fluid entering said convection zone is increased and the heat absorption in this zone is decreased, the gas temperature entering the downstream location therefore being increased and the heat absorption at said downstream location being increased, determining the relative heat absorption of the high and low pressure fluids, and regulating the amount of fluid returned in response to the determined relative heat absorption.

14. The method of operating a vapor generator comprising burning fuel in a combustion zone and forming products of combustion passing a fluid at relatively high pressure in heat exchange relation with the products of combustion at a downstream location, subsequently passing said fluid in convection heat exchange with the products of combustion in a convection zone at an upstream location, thereafter passing said fluid at high pressure in heat exchange relation with the products of combustion in another zone, passing a lower pressure fluid in heat exchange relation With the products of combustion including passing the lower pressure fluid in heat exchange relation with the products of combustion at a location intermediate said downstream and upstream locations, returning a portion of the high pressure fluid from said other zone and mixing said returned fluid with the high pressure fluid entering said convection zone at said upstream location whereby the temperature of the fluid entering said convection zone is increased and the heat absorption in this zone is decreased, the gas temperature entering the downstream location therefore being increased and the heat absorption by the low pressure fluid passing through said intermediate location being increased, determining the relative heat absorption of the high and lO'W pressure fluids, and regulating the amount of fluid returned in response to the determined relative heat absorption.

References Cited by the Examiner UNITED STATES PATENTS 3,038,453 6/1962 Armacost 122-406 3,135,246 6/1964 Kochey l22479 3,186,175 6/1965 Strohmeyer l22479 KENNETH W. SPRAGUE, Primary Examiner.

Claims (1)

1. A SUPERCRITICAL PRESSURE VAPOR GENERATOR HAVING A FIRST CIRCUIT, A SECOND CIRCUIT, FLUID PASSING THROUGH EACH CIRCUIT, A COMBUSTION ZONE FOR THE BURNING OF FUEL THEREIN MEANS FOR CONVEYING COMBUSTION GASES FROM SAID COMBUSTION ZONE; MEANS EFFECTIVE TO TRANSFER HEAT FROM SAID COMBUSTION GASES TO SAID FIRST CIRCUIT, INCLUDING A SECTION OF SAID FIRST CIRCUIT LOCATED IN THE FLOW PATH OF SAID COMBUSTION GASES; MEANS EFFECTIVE TO TRANSFER HEAT FROM SAID COMBUSTION GASES TO SAID SECOND CIRCUIT; MEANS FOR TRANSFERRING HEATING TO THE FLUID ENTERING SAID SECTION OF SAID FIRST CIRCUIT FROM A PORTION OF SAID FIRST CIRCUIT DOWNSTREAM, WITH RESPECT TO THE FLUID FLOW, OF SAID SECTION OF SAID FIRST CIRCUIT WHEREBY THE EFFECTIVENESS OF SAID SECTION FOR HEAT ABSORPTION IS REDUCED, THEREBY INCREASING THE RELATIVE HEAT ABSORPTION OF SAID FIRST CIRCUIT; MEANS FOR DETERMINING THE RELATIVE HEAT ABSORPTION OF THE FLUIDS FLOWING THROUGH SAID FIRST AND SECOND CIRCUITS; AND MEANS FOR REGULATING SAID HEAT TRANSFERRED TO SAID SECTION OF SAID FIRST CIRCUIT FROM THE DOWNSTREAM PORTION OF SAID FIRST CIRCUIT IN RESPONSE TO SAID LAST-NAMED MEANS.

Images