Theoretical Analysis And Experimental Research On The Performance Of Low-pressure Heater And Deaerator With Water Jet Steam Injector

At present, energy demand is rising dramatically for human life and industrialproduction. But fossil resources are being exhausted. Facing this contradiction, savingenergy and reducing consumption is imminent in power plant. The surface heater ismainly used to heat feed water in low-pressure heating system of the power plant. It hassome weaknesses, such as high terminal temperature difference, scaling easily,containing copper in the condensed water, large system and so on. The condensed waterinto the deaerator needs to be heated at temperature of104℃again by the steam.Therefore, this paper focuses the core issues of national invention patent “ejectedthermal deaerator of the industrial boiler”(patent number: ZL2009101038522). Thetheoretical analysis and experimental research on the low-pressure heater and deaeratorwith water jet steam injector is carried out. The performance of water jet steam injectoris researched by experiment. The paper presents the method of the flash deoxidation.The water is heated above the temperature of110℃by two-stage steam. Then the highpressure saturated water into the atmospheric deaerator is directly flashed to remove theoxygen. The performance calculation model for water jet steam injector is establishedaccording to one-dimensional conservation equations. The parameters distribution isobtained by numerical simulation method. Based on the theoretical model andexperimental results, the main factors of the injector performance are analyzed.The experimental research indicates that operating parameters have importantinfluence on the performance of water jet steam injector. The entrainment ratio of theinjector decreases with the increase of inlet water pressure and temperature, andincreases with the increase of inlet steam pressure. Similarly, the outlet watertemperature decreases with the increase of inlet water pressure, and increases with theincrease of inlet steam pressure. The resistance coefficient increases while inlet waterpressure and temperature increases. There exists an optimum back pressure valuecorresponding to the maximum of outlet water temperature. The outlet temperature canreach110℃when the back pressure of the injector is0.12MPa. Therefore, it is able tomeet the temperature requirements of feed water into the deaerator. The experimentalresults also show that the two-stage injector has good heating performance. The outletwater temperature can increase87℃, and the average temperature rise of two-stageinjector is23%more than that of single-stage one. So the two-stage injector can be used in conditions of insufficient heating.Based on the certain simplifying assumptions and empirical formulas, theperformance calculation model for water jet steam injector is established by the globalmodel. Then the outlet water pressure under different operating conditions is obtainedaccording to the programming calculation on the basis of the theoretical model. Theresults show that the calculated results are consistent with the experimental results. Theoutlet water pressure of the injector increases firstly, and then decreases while inletsteam pressure increases. The boosting performance of the injector will be weakenedwhile inlet water temperature increases. The outlet water pressure increases with theincrease of inlet water flux at the initial stage. When inlet water flux reaches a certainvalue, the outlet water pressure little changes.Exergy analysis model of injector is deduced in view of available energy point.The influence of the operating parameters on economical performance is analyzed onthe basis of exergy analysis model. The results indicate that exergy efficiency ofinjector increases with the increase of inlet water temperature and entrainment ratio. Aminimum of exergy efficiency occurs as inlet steam pressure changes. The relationshipbetween exergy efficiency and pressure ratio is approximatively linear. According to thecontrast, the average exergy efficiency of the two-stage injector is21%more than thatof the single-stage one. It is also found that the exergy losses of each componentdecrease with the increase of entrainment ratio. The amounts of exergy losses in thefirst-stage mixing chamber play a large and critical part in this process while the exergylosses of the water nozzle are the lowest.The influence of the structural parameters on the injector performance is analyzedby numerical simulation method. The calculated results show that the entrainment ratioof injector increases at first and then decreases with the increase of area ratio. Thereexists a maximum value of entrainment ratio corresponding to the area ratio m=6.25.For a given water jet steam injector, there exists a maximum value of entrainment ratiowhile the dimensionless distance between throat and spout L=6. It has not enough timeand length to eject the steam due to short distance. The phenomenon of the reflowingwill appear if the distance between throat and spout is too long.This paper presents a lower-pressure heater and deaerator with water jet steaminjector to instead of the surface heater in power plant. The steam is ejected by thecondensed water in injector and heat the water at nearly saturated temperature. The hightemperature and saturated water will flash to remove the oxygen in the deaerator after the water pressure is boosted in the diffuser. Compared to the traditional atmosphericthermal deaerator, the steam consumption will be reduced by this method. According tothe contrast, the thermal efficiency of singe-stage recuperative heating system withwater jet steam injector is more1.29%than that of heating system with surface type. Itis confirmed by the industrial test of6MW turbogenerator in Yongrong power plant.Therefore, the lower-pressure heater and deaerator with water jet steam injector hasgood foreground for industrial application.