Patent Number: 048470430
Section: description

Referring to FIG. 1, a typical prior art jet pump is illustrated. The jet pump includes an inlet I, a mixing section M and a conical diffuser D. Diffuser D terminates in a tail pipe T. A jet J drives the pump. Typically water W is supplied to the jet J at a pressure. This pressure is typically the ambient pressure of the nuclear reactor (for example, 1020 psi), plus the additional head necessary to drive the jet pump. For example, total dynamic heads in the range of 625 feet are utilized in addition to ambient reactor pressure. A nozzle N in jet J serves the function of converting available static head in the water W to dynamic head. This dynamic head manifests itself in the high velocity of the water W being discharged from the nozzle. As is well known, surrounding water W.sub.s is entrained. It is entrained into the inlet I and the mixing section M. Within the mixing section M momentum transfer occurs. That is to say, the high velocity, low volume water admitted from jet J mixes with the low velocity water W.sub.s. Such mixing is usually complete at the end of the mixing section M. Typically the water at the end of the mixing section M has a flat velocity profile as indicated by the arrows 14. The function of the diffuser D is to reduce the velocity and increase outlet pressure. Accordingly, diffuser D conically expands. After the conical expansion, a tail pipe T may be utilized for discharge. Having set forth in a simplified format, the invention herein can now be summarized with respect to FIG. 2. Thereafter, and with respect to FIG. 3, the principles involved in the steam acceleration of the discharged water stream can be fully explained. Referring to FIG. 2, feedwater W.sub.f at 340.degree. F. is introduced interior of a water nozzle 20. At the same time, saturated steam at 545.degree. F. is introduced interior of a steam nozzle 30. Steam nozzle 30 is provided with a converging/diverging surrounding annular discharge to nozzle 20. Fluid feedwater W.sub.f issuing from nozzle 20 is joined by steam S issuing from steam nozzle 30 through converging/diverging, concentric nozzle 32. An explanation of how the velocity is added to the steam flow may best be seen by referring to FIG. 3. Referring to FIG. 3, feedwater W.sub.f inflows at pipe 24 into a chamber 20. Chamber 20 is configured with a water nozzle 22 at the end thereof. Nozzle 22 discharged axially of the jet pump. See FIG. 2. At the same time, saturated steam at 545.degree. F. and 1020 psi is introduced through pipe 34 to a steam chamber 30. Steam chamber 30 discharges at a converging/diverging nozzle 32. This converging/diverging nozzle is concentric around water nozzle 22. Thus, steam S discharging from the converging/diverging nozzle passes in the same direction as feedwater W.sub.f in slightly converging path. It should be understood that the steam is accelerated to a very high velocity. As is well known, the steam in passing through the converging/diverging nozzles has its pressure (1,020 psi) reduced nearly to the saturation pressure of the exhaust of the water W.sub.f from the nozzle. Assuming that feedwater is discharged at a temperature of 340.degree. F., a pressure in the range of 120 psi will be realized at the discharge of the converging/diverging nozzle 32. Acceleration of the steam through the converging/diverging nozzle will cause the steam to reach speeds in the range of 2,700 to 3,000 ft./sec. Steam flow will be supersonic, and will be moisture-bearing--that is, containing moisture particles. (Moisture-bearing steam is commonly termed "wet steam".) The water jet emerging from water nozzle 20 will likewise have the same static pressure value as does the steam leaving converging/diverging nozzle 32, that is, about 120 psi. The dynamic head representing the pressure reduction between feedwater supply pressure at introduction to water nozzle 20 (viz. 1250 psi) and discharge from water nozzle 20 (viz., 120 psi) is about 2900 feet. This corresponds to a bulk average discharge velocity from water nozzle 20 of about 425 ft./sec. When the wet steam S condenses to the stream of passing feedwater W.sub.f, the high momentum of the steam molecules and moisture particles will be transferred to the water jet. Such transfer is produced by a shear force acting at the interface between the water jet and the wet steam flow. This shear force will accelerate the jet as indicated by velocity vectors 50 at the discharge of the nozzle 38. Nozzle 38 has, typically, for the specific application here described, an exhaust flow area of 85%, approximately, of the exhaust flow area of water nozzle 20. Typically, the bulk average velocity of the fluid stream issuing from the discharge end of the nozzle mixing section will be 525 ft./sec. Remembering that the discharge velocity profile 50 of the stream W.sub.f mixed with the steam had a higher velocity gradient at the edges than at the center, it will be seen that fluid velocity ultimately developed in the driven flow W.sub.s at the sidewalls 60 near the exit of the mixing section M will have a higher velocity. This mixing-section M sidewalls region higher velocity is known, from testing done by General Electric, to lead to important performance increases in the jet pump diffuser D. This performance improvement is the result of the fluid streamlines adjacent the diffuser sidewalls 75 being enabled over a long path length downstream into the diffuser, to avoid development of the condition known as "flow separation". (Flow separation develops when the streamlines adjacent a wall and flowing against an adverse pressure gradient are slowed to the point they can no longer remain attached to the wall. At this point, the streamlines will turn away from the wall, and a (momentary or possibly permanent eddy will form downstream of the point of flow separation.) From the point of flow separation onward, the flow in the diffuser is no longer that of a gradual velocity-reducing flow-field. Flow losses develop, because energy is removed from the main flow to drive the eddy, and because the main flow velocity leaving the diffuser exit will be higher, causing higher exit velocity losses resulting from failure to convert dynamic head to static pressure. Simply stated, by having a discharge the jet apparatus J.sub.s with a high velocity profile on the exterior, a more favorable velocity profile 70 is established at the exit of the mixer. Accordingly, an improved performance is produced by diffuser D. It will be realized that the introduction of steam S into the feedwater W.sub.f produces useful work on feedwater W.sub.f. It also produces contact heat exchange, that is, virtually total conservation of all the thermal energy initially present in stream S. This contact heat exchange raises the temperature of the fluid discharge from the nozzle J.sub.s. At same time, the overall temperature of the water passing out of the jet pump is also raised. This combination of useful work together with virtually total thermal energy conservation produces well known thermal efficiencies in a steam power plant, such as that boiling water steam power plant schematically illustrated in FIG. 5. Referring to FIG. 5, a conventional FWDJP boiling water reactor is illustrated. A reactor vessel contains a core C. Core C heats upwardly flowing coolant which thereafter passes through steam separators 100. Separated wet steam thereafter passed through steam dryer 102. The resulting effluent--dry, saturated steam--passes out a line 103 where it drives a turbine 110. Turbine 110 drives a generator 120 which in turn puts out power on lines 130. Steam exhausted through turbine 110 passes out line 104 to a condenser 108. Coolant schematically illustrated by arrows 109 condenses discharged steam interior of condenser 108 to a pool of condensate typically residing at approximately 2 psi absolute interior of the condenser. A condensate pump 114 takes suction upon the condensate and discharges at a line 116 to a condensate preheater 118. Condensate preheater discharges to a feedwater pump 126. Feedwater pump 126 provides the balance of pressure head required to inject condensate--now termed feedwater--into the reactor, plus the additional dynamic head necessary to power the jet pump 160. In the invention herein disclosed, a bypass line 170 diverts dry steam from line 103 as it passes to turbine 110. Steam in line 170 is typically throttled at a steam valve 172 and introduced at a line 174 to the jet pump steam chamber 30 (see FIG. 3). It will be understood that the configuration of FIG. 5 is preferred. That is to say steam line 170, throttle valve 172 and inlet steam line 174 are all configured exterior of the reactor vessel. It can be understood, however, that a configuration such as that shown in FIG. 2 could as well be utilized. For example, wet steam discharged from steam separators 100, or alternatively, dry steam discharged from steam dryer 102 could be ducted directly in a line interior of the reactor to steam chamber 30. Referring to FIGS. 6A and 6B, the construction of a steam-assisted jet pump with multiple nozzles can be simply illustrated. Three steam water nozzles assemblies are shown powering the steam-assisted jet pump. Specifically feedwater W.sub.f is passed out water nozzles 20a, 20b, and 20c. Similarly, jets, of steam peripheral to the water jets are likewise shown at 30a30b, and 30c. Otherwise, the resultant operation is analogous. It will also be understood that alternative applications for boiling nuclear power reactor coolant recirculation exist. The beneficial action of the invention (to supplant, increase, or simply augment the capability of a conventional of FWDJP jet pump-based coolant recirculation system) is gained without the steam expansion in steam nozzle 32 undergoing the pressure expansion so extreme as to produce supersonic velocities downsteam of steam nozzle 32. It will also be understood that steam nozzle 32 under such applications may not exclusively possess a converging-diverging flow passage area characteristic, but instead may be optimized for the particular application at hand. It will also be understood that the invention is not necessarily limited to applications involving a single jet pump nozzle 38. (See FIG. 6A). It will also be understood that the invention has potentially significant application to securing forced circulation in the secondary side (steam plant side) of the steam generators of such nuclear power reactor types as dual cycle BWRs, pressurized light water reactors, heavy water rectors of the CANDU type, liquid metal reactors, and certain gas-cooled reactors. It will also be understood that the invention has potentially significant application to recirculating water in many types of fossil fueled boilers.