US3040537A - Steam power generating apparatus - Google Patents

Description

June 26, 1962 s. M. ARNOW 3,040,537

STEAM POWER GENERATING APPARATUS Filed Sept. 28, 1960 5 Km J m w N 5 m1? 1% d m M 5V.

United States Patent 3,040,537 STEAM PGWER GENERATING APPARATUS Samuel M. Arnow, Philadelphia, Pa., assignor, by mesne assignments, to Baldwin-Lima-Hamilton Corporation, Philadelphia, Pa., a corporation of Pennsylvania Filed Sept. 28, 1960, Ser. No. 59,078 6 Claims. (Cl. 60-67) The present invention is directed to a modification of the steam power generating apparatus described and claimed in US. Patent 2,883,832 granted to me April 28, 1959, on an application filed May 3, 1956.

The steam power generating apparatus as described and claimed in my aforesaid patent is very efiicient when employed in a steam power generating plant which is operating at full load or at substantially full load. When, however, the plant is operating at less than full load, for example, one-half load, my previous invention becomes ineffective. The present invention is directed to means in the heat cycle for restoring effectiveness during periods of such reduced load and to apparatus for activating such means automatically during operation at the reduced load.

This invention relates to steam power generating apparatus, and more particularly to a steam power generating station in which turbines are utilized to drive electric generators, for example. In such plants the heat carried away by the combustion gases from the boilers represents one of the chief sources of efficiency loss.

It is an object of this invention to provide a steam power generating apparatus including an efficient means for tempering incoming air.

Another object is to provide an efiicient steam power generating apparatus arranged for absorbing heat from exhaust gases, for preheating condensate, and for tempering incoming air, all in combination.

Still another object of this invention is to provide a steam boiler having an air preheater and which includes a readily controllable incoming air tempering means for introducing air at a substantially constant temperature to the preheater, notwithstanding fluctuations of temperature of the air introduced into the tempering means.

Another object is to provide an efiicient means for utilizing hot exhaust gases for preheating the condensate at the most effective point in the steam power thermal cycle, without condensing ingredients of the exhaust gases or corroding the preheater.

It is a further purpose of the present invention to accomplish all of the foregoing objects irrespective of whether the steam generating plant is operating at full load or at substantially less than full load.

Still a further object of this invention is to provide means for feedwater heating and for air pre-tempering from exhaust steam withdrawn from turbine bleeds at different pressure bleed points according to loading.

Other objects and advantages of this invention, including the simplicity and economy of the same, and the ease with which it may be applied to existing steam power generating systems, will further become apparent hereinafter and in the drawing.

Referring to the drawing, the number 1 represents generally a boiler having a gas exit 2 connected through breeching 3 to a stack 4, the draft being induced by a blower or blowers designated 5. Combustion-supporting air is introduced into the lower portion of the fire box of the boiler 1 by way of a duct 6 by action of a blower or blowers 7, said duct passing through the gas exit 2 of the boiler and having there interposed in it a preheater 3,040,537 Pitented June 26, 19 62 8 which absorbs some of the flue gas heat, whereby the line 31.

air is preheated before entering the boiler.

Steam is conducted from the boiler 1 through a'conventional pipe 9 to drive a turbine 10 having an associated condenser 11. Leading from difierent bleed points of the turbine 10 are pipes 12, 13, 14A and 14B through which exhaust steam at different pressures is channeled to feedwater heaters, with pipes 12 and 13 feeding feedWa-ter heaters 15 and 16 respectively and with pipes 14A and 14B leading into a common pipe 14 which feeds feedwater'heater 17. Pipe 14B (and also, if desired, pipe 14A) is provided with a check valve 44 (and 43) for preventing flow of steam exhausted through pipe 14A from flowing back to the turbine 10 through the pipe 14B. Pipes 14A and 14B are each also provided with a control valve 45, 46 respectively whose function and manner of operation will be described hereinafter. It will merely be mentioned at this point that for greatest economy of operation the feedwater heater 17 should be connected to that bleed point nearest to the low pressure exhaust of the turbine at which the pressure and temperature of the exhaust steam are nevertheless sufiiciently high to avoid causing the stack temperature to be pulled down to a temperature sufficiently low to cause or to tend to cause condensation and corrosion.

The condensate from the condenser 11 is propelled, by a plurality of pumps diagrammatically shown as a pump 18, through a feed pipe line 19 which extends to a coil 20 in the feedwater heater 17, then, in accordance with my invention described in US. Patent 2,883,832, to a low level economizer 21 in the breeching 3. It should be understood that the coil 20, although conventionally referred to and illustrated as a coil, in practice is actually a system of tubes. It is further to be understood that a system of tubes well known in the art as used for heat transfer purposes, will hereinafter be referred to for convenience as a coil. The feedwater pressure in economizer 21 should be above atmospheric, to prevent leakage of exhaust gases into the feed water. Continuing on, the line 19 extends first to a coil 22 in the feedwater heater 16, then to a coil 23 in a feedwater heater 15, through a plurality of boiler feed pumps diagrammatically shown as a pump 25. From the pumps 25, line 19 extends through an econornizer 26 (which latter is located within the boiler), and finally to the boiler 1 in which the line 19 terminates. An additional feedwater heater 27, or a group of them, may also be provided, respectively fed by ascending pressure bleed points.

Provided in the air duct 6 is a water-to-air heat exchanger 30 which is set into the air inlet duct 6 and which is interposed in a closed water circulation pipe Pipe line 31 also includes a coil 32 located within the feedwater heater 17. The water in the line 31 is under positive pressure and circulated by a pump indicated at 33, such water being purified so that any possible leakage into the heater 17 would not contaminate the condensate. Also, since the water in line 31 is under positive pressure, there is no possibility of leakage of air into the line '31 at the air tempering heater 30 to contaminate the water. A conventional cascading system of heater condensate drains extends from heaters 27, 15, 16 and 17 back to condenser 11. This is a closed system sealed from the atmosphere, thus preventing air from entering heater 17. The rate of circulation is requested by a temperature-responsive valve 34 controlled by a thermostat 35 having its sensing element located within the air duct portion 36, which is downstream relative to the Water-to-air heater 30. Valve 34 is downstream relative to the pump 33.

It is important to observe that the feedwater heater 17 is connected to transfer heat through both the coils 32 and 21). Working at sub-atmospheric pressure, at the bleed point of lowest possible pressure consistent with stack temperature, the heater 17 is ideally suited economically for pre-heating the Condensate prior to its introduction into the low level economizer 21, because the work done by the steam in the turbine is at a maximum when bled off at this low pressure. Moreover, heat is concurrently transferred by heater 17 to the water in pipe 31, whereby both the low level economizer 21 and the tempering heater 30 are heated from a common, sub-atmospheric pressure source.

In operation, the temperature of the gases as they leave the air preheater of the conventional steam generating system is usually maintained at about 260 F and the air entering the preheater is desirably not less than about 100 F.; otherwise during low outside temperature periods the flue gases leaving the preheated may be cooled below the dew-point and the maintenance cost of the pre-heater would become excessive because of corrosion resulting from condensation. In order to avoid this difliculty, it is conventional to temper the air by increasing its temperature by about 60 F. before it enters the pre-heater. For 65 F. (annual average) incoming air, the temperature would be raised to about 125 F., which is safely above the desired minimum. Steam is used for this purpose and is usually withdrawn from one of the bleed points in the turbine. However, using such a method it was not practical to control the extent of preheating. Prior to my invention, as described in US. Patent 2,883,832, in order to safeguard against contamination of the steam by the air, the bleed point selected must have had a pressure above atmospheric pressure at all operating loads.

In accordance with the improved air pre-heating system of my aforesaid Patent 2,883,832, a significant gain in economy is achieved by withdrawingthe steam at the lowest pressure bleed point, which is below atmospheric pressure and in one practical case, for example, has a full load pressure of approximately only 5.7 pounds per square inch absolute. Notwithstanding the fact that the steam is withdrawn at a pressure below atmospheric, steam contamination is nevertheless avoided. As was before indicated, the withdrawal of the steam at such low pressure bleed point results in an increase in work done by the turbine before the steam is withdrawn.

Also, in accordance with my aforesaid Patent, 2,883,- 832, an even greater gain in economy is achieved by causing the feedwater heating circuit to pick up heat from the flue gases ahead of the stack. This heat, which includes the heat added by the induced draft fans would, in a prior art plant, have been lost up the stack. By employing the flue gases to assist in heating 'the feedwater circuit, I am able in my Patent 2,883,832 to dispense with the bleed point 40 of the turbine which otherwise would have been connected to an additional feed- Water heater interposed between the feedwater heaters 1 6 and 17. By normally not using the bleed 40 located intermediate of bleed points 13 and 14B increased work is done in turbine 10 by the steam which otherwise would have been extracted from the bleed point 40. In the present example, bleed point 40 is located at a point at which the pressure is about 14 pounds per square inch absolute (p.s.i.a.) at full loadwhereas the bleed lines 12 and 13 under normal full-load operating conditions are located at points at which the pressure is of the order of 54 and 22 pounds per square inch absolute (p.s.i.a.), respectively, as i indicated in the drawing.

The system described in my Patent 2,883,832 and also described thus far in my present patent application, achieves good economy of operation and functions very satisfactorily, provided the steam generating plant is operating at or substantially at full load.

To bring out the modification introduced by my pres- 4 cut invention, assume that a plant which has been operating at full load under the pressure and temperature conditions mentioned above and shown in the drawing reduces its operation to substantially less than full load, say one-half load When this occurs, the pressure at the lowest bleed point to which pipe 14B is connected is reduced from 5.7 p.s.i.a to about 2.8 p.s.i.a., and the temperature of the steam pipe 14B drops from 167 F. to about 139 F. This causes the water in pipe 19 leading up to the economizer 21 to drop from 164 F. to about 136 F. As a result, the 275 F. gas in breaching 3 in passing economizer 21 drops to a temperature below 200 F. and the temperature in stack 4 becomes say P. which is too low to avoid the possibility of condensation and corrosion.

In accordance with my present invention, the drop in temperature in stack 4 is sensed by a temperaturesensitive thermostat 60 having a pair of upper contacts and a pair 'of lower contacts and a bridging contactor or switch arm 61 so adjusted that when the stack temperature drops below a selected point, say F., the switch arm 61 of the thermostat, which had prior thereto been in the upper position, drops to the lower position, as shown in the drawing. This connects the lower contacts and opens the upper contacts of the switch. As a result, an electrical circuit from power lines L1, L-2 which had been previously closed through a motor-operated valve 46 in pipe 143 is now open, and a previously open circuit is now closed through the motoroperated valve 45 in pipe 14A. Motor-operated valves 45 and 46 may be assumed to be biassed, as by a spring or other means, normally to closed position and to be opened by motor when the electrical circuit therethrough is closed. Thus, when the switch arm 61 drops to the lower position to close the circuit through the motor valve 45, the valve 45 opens, while valve 46 whose motor is no longer energized now closes. This connects bleed point 40 to the feedwater heater 17 by Way of pipes 14A and 14, and shuts off pipe 14B. The pressure at bleed point 40, as previously indicated, is of order of 14 p.s.i.a. at full load at a temperature of about 210 F. At half load operation, the pressure at bleed point 40 is of the order of 7 p.s.i.a. at about 177 F.

It will be seen then that when the temperature in stack 4 drops, the temperature-sensitive thermostat 60 is eifective to switch the bleed connection from the lowest bleed point on turbine 10 to the next higher bleed point. By so doing, the water temperature in pipe 19 leading to the economizer 21 is raised again to the vicinity of 164 F. and the stack temperature is raised to 200 F. or higher. In this manner, the stack temperature is maintained well above the dew point, and condensation and corrosion are thereby avoided irrespective of whether the steam generating plant is operating at full load or half load.

In order to provide for satisfactory and efiicient operation at less than full load, as well as at full load, the Water seal shown in my aforesaid Patent 2,883,832 is replaced in the apparatus of the present patent application with a float-operated valve 62 which allows water to pass therethrough from right to left as viewed in the drawing but which does not permit the passage of steam therethrough from left to right. The change from a water seal, as shown in my Patent 2,883,832, to a floatoperated valve 62 is necessary because of the impractical length of water seal which would be required for a heater connected to a 14 p.s.i.a. bleed point.

For the purpose of indicating when the exhaust steam fed to heater 17 is being supplied from bleed point 40, a signal lamp may be connected across the terminals of water-valve 45.

In lieu of an automatic system for switching the bleed point from the lowestmost line 14B to the next higher line 14A, as just described above, the switchover may of course be made manually in response to the sounding of an alarm. The numeral 63 in the drawing represents such an alarm which is set off when the switch arm 61 of thermostat 60 changes positoin, either from the upper to the lower position or from the lower to the upper posi tion. The connections to valves 45 and 46 would be disconnected since these valves would now be operated manually. V

In the drawing, alarm 63 is represented as having two switches 64 and 65, which close electrically in response to current flow through the coil associated therewith. When closed, the switches remain closed until opened manually, as to shut oif the alarm. These switch elements are of course, not necessary when the operation is fully automatic, as first described above. In manualalarm operation, with the stack theremostat switch arm 61 in the upper position, switch 65 is in closed position (having been closed by the current through its coil before switch 64 is opened) and switch 64 is in open position, having been opened manually to shut off the alarm. When due to a drop in stack temperature, switch arm 61 drops to the lower position, current flows through lead 67, the alarm is actuated, and switch 64 is closed electrically by the current flowing through its coil. The alarm continues to ring until it is shut oif manually by opening switch 65. Switch 64 remains closed. When, due to a rise in stack temperature, the thermostat switch arm 61 rises to the upper position, current flow through lead 66, the alarm is again actuated, and switch 65 is closed electrically by the current flowing through its coil. As before, the alarm continues to ring until shut off by manually opening switch 64, switch 65 remaining closed.

It will be understood, of course, that for manual operation of valves 45 and 46, employing the operating alarm means 63 as described above, and also for the fully automatic operation first described, the stack thermostat 60 would be so set that the switch arm 61 would not move into contact with the upper contacts until the stack temperature had risen to a substantially higher temperature than that reached under say one-half load operation, so that the connection from the turbine to the feedwater heater 17 can be safely switched to the lowest pressure connection 14B.

The invention of the present application constitutes an important improvement to the basic system described and claimed in my basic Patent 2,883,832, in that it permits a steam generating plant employing my basic invention to be operated at less than full load, as well as at full load. This is unquestionably an important advantage, as many steam generating plants are at times operated at less than full load and at other times are operated at full load.

The present invention senses automatically that the load has appreciably changed by detecting the appreciable change in stack temperature. And, in the fully automatic form of operation, the apparatus provided by the present invention then automatically makes the necessary switch or change in steam supply, switching from one bleed point to another and thereby simultaneously effecting the required change in steam supply to the feedwater heater which controls both the stack econo-mizer and the air tempering heater.

While the preferred embodiment of this invention has been described in some detail, it will be obvious to one skilled in the art that various modifications may be made without departing from the invention as hereinafter claimed.

Having thus described my invention, I claim:

1. In a steam power plant; a boiler, including conducting means for exhausting combustion products, said conducting means terminating outwardly in a stack; a turbine driven by steam from said boiler; a condenser connected to condense steam from said turbine; a plurality of steam bleed points each at a different pressure point on said turbine, at least one of said pressure points being below atmospheric pressure at full load; a plurality of feedwater heaters having water coils therein; means connecting different bleed points to different feedwater heaters, the bleed point of lowest pressure being connected to a first feedwater heater, and other bleed points of increasingly higher pressure being connected to second, third, etc., feed-water heaters in that order; a low level economizer located in the path of flow of said combustion prod ucts; a feedwater pipe line extending from the condenser to a first water coil in said first feedwater heater, then to the economizer, then through the coils of said second, third, etc., feedwater heaters in sequence in that order, and then to the boiler; temperature-sensitive switch means positioned to be sensitive to the temperature in said stack; a first motor-operated valve in the connection from the bleed point of lowest pressure in said turbine to said first feedwater heater; a second motor-operated valve in the connection from the bleed point of next higher pressure in said turbine to said first feedwater heater, said first and second motor-operated valves being connected electrically to, and operative automatically in response to actuation of, said temperature-sensitive switch means for automatically switching the first feedwater heater connection from said bleed point of lowest pressure to said bleed point of next higher pressure, and vice versa.

2. The combination as claimed in claim 1 further characterized by the provision of signal means operative to provide an indication when the feedwater heater connection is connected to the bleed point of higher temperature.

3. The combination as claimed in claim 1 further characterized in that said combination includes a conduit through which combustion-supporting air is carried into said boiler, in that a water-to-air heating element is located within said conduit, in that a closed-circuit water pipe line connects said Water-to-air heating element to a second water coil in said first feedwater heater, said water pipe lines to said first and second coils in said first feedwater heater being free of fluid communications with each other and both being in heat exchange relation with said first feedwater heater.

4. In a steam power generating system; a boiler having an air intake means; a turbine driven by steam from said boiler; a condenser connected to condense the steam from said turbine; conducting means including -a stack for exhausting combustion products; a plurality of steam bleed points each at a diiferent pressure point on said turbine, at least one of which is below atmospheric pressure at full load; a plurality of feedwater heaters each having at least one water coil therein; means connecting different of said bleed points to different feedwater heaters, the bleed point of lowest pressure being connected to a first feedwater heater and the other bleed points of increasingly higher pressure being connected to second, third, etc. feedwater heaters in that order; means for pumping the condensed steam from the condenser through a first coil in said first feedwater heater and then at pressure albove atmospheric pressure through a low-level econornizer located in the path of flow of said combustion products; a preheater arranged to transfer heat from exhaust gases to the incoming air; an air tempering heater located in the path of flow of incoming air ahead of said preheater; separate water circulating means comprising a closed circuit sealed from fluid communication to said condensed steam and in heat conducting relation to said air tempering heater and to said first feedwater heater for transferring heat from said feedwater heater to said air tempering heater; and temperature-sensitive switch means positioned to be sensitive to the temperature in said stack for automatically switching the turbine connection to said first feedwater heater from said bleed point of lowest pressure to the bleed point of next higher pressure and vice versa, said automatic switching means including a first motor-operated valve in the connection from the bleed point of lowest pressure in said turbine to said first feedwater heater; a second motor-operated valve in the connection from the bleed point of next higher pressure in said turbine to said first feedwater heater, said first and 3,040,537 7 8 second motor-operated valves being connected electriized in that said water in said closed circuit is at a prescally to, and operative automatically in response to actuasure above atmospheric. tion of, said temperature-sensitive switch means.

R f i t t 5. Apparatus as claimed 1n clalm 4 characterized in e eremes C ted m the file of hls pawn that said first feedwater heater is switched to a higher- 5 UNITED STATES PATENTS pressure bleed point when the stack temperature is below 2,788,175 Bourek et a1, Apr. 9, 1957 a predetermined temperature. 2,807,013 Prough et a1 Sept. 17, 1957 6. Apparatus as claimed in claim 4 further character- 2,383,832 Arnow Apr. 28, 1959

Images