This document summarizes key concepts in basic steam engineering related to nucleate boiling in boilers. It describes the process of nucleate boiling and how variations in circulation can lead to temporary changes in water level known as shrink and swell. Key terms are defined, such as subcooled nucleate boiling, quality, annulus formation, thermal driving head, head losses, and boiler end points including the end point of circulation. Causes and limits of natural circulation in boilers are also covered.

1. Basic Steam Engineering 62b-201 Nucleate Boiling

2. 1. Describe the theory of nucleate boiling and how it applies to boiler operations 2. Describe how variations in circulation lead to shrink and swell Enabling Objectives

3. Boiling Process In A Generating Tube Tubes have a waterside (internal volume) and a fireside (external surface) LIKE BOILING WATER IN A POT, BOILING IN THE GENERATING TUBES IS A PROGRESSIVE PHENOMENON (i.E. Little bubbles, bigger bubbles, bulk boiling)

4. SUBCOOLED Nucleate Boiling Definition of SUBCOOLED : Below temperature ( T SAT ) at which water will change state to steam at a given pressure Bulk of boiler water is SUBCOOLED as it rises up generating tubes

5. SUBCOOLED Nucleate Boiling Bubbles Break Off From Tube Wall And Enter Main Stream Of Water And Collapse T BulkFluid <T Saturation POP!

6. Nucleate Boiling Steam bubbles & bulk fluid = T SAT Bubbles no longer collapse NUCLEATE BOILING SUBCOOLED NUCLEATE BOILING T SteamBubble = T BulkFluid

7. Quality Definition of quality: Quality of STM is the dryness or purity. It is the ratio of: Mass of STM in LBM Total mass of STM & WTR LBM “ LBM” is pounds of mass Expressed in % 5% quality is “wetter” than 90% X =

8. Steam Slugs At 5 % quality - steam slugs form NUCLEATE BOILING SUBCOOLED NUCLEATE BOILING STEAM SLUGS 5% QUALITY T SteamBubble > T BulkFluid

9. Annulus Steam slugs combine further up tube Form column of steam in center of tube Ring of water remains - adheres to tube inner wall Called annulus

10. Annulus Annulus Water carries away heat Critical to protecting tube from melting NUCLEATE BOILING SUBCOOLED NUCLEATE BOILING STM SLUGS 5% QUALITY ANNULUS STM COLUMN OF

11. Nucleate Boiling Ideally steam & water exiting generating tubes will be: Annular flow 20 % - 30% quality

12. Moisture Separators Chevron & cyclone type Remove water from steam to prevent moisture carryover Steam exits boiler at 99.75% (recall that steam exits tube at 20-30%)

13. Natural Circulation There is no pump to circulate boiler water We rely on natural circulation Termed accelerated due to slant in generating tubes Promoted by thermal driving head (TDH) Retarded by head losses (hl)

14. Thermal Driving Head Explanation: AVERAGE SPECIFIC VOLUME (VOL) OF WATER IN GENERATING TUBES > WATER IN DOWNCOMERS (i.E. On a LBM basis, the water in tubes takes up more space) Therefore => density of steam/ water mixture in tubes < water in DOWNCOMERS

15. Thermal Driving Head Therefore => weight of steam/ water in generating tubes < water in DOWNCOMERS Therefore => pressure on water in DOWNCOMERS > pressure on steam/ water mixture in generating tubes

16. Thermal Driving Head As water molecules exit generating tubes, heavier water fills void, causing Natural circulation (NC)

17. Thermal Driving Head TDH increases linearly with increase in quality Greater the steam demand, greater TDH Note: for the most part increase in firing rate is synonymous with an increase in quality

18. Head Loss TDH creates flow down DOWNCOMERS and back up generating tubes Actual mass flow rate of NC is difference between TDH and hl : M flow = TDH - HL

19. Head Loss As quality increases, larger steam slugs form Slugs impede flow of remaining water in tube, causing head loss

20. Head Loss Explanation: Specific volume of water in DOWNCOMERS stays the same as temperature increases Specific volume of steam/water mixture in generating tubes increases as temp increases As volume in generating tubes increases, hl increases exponentially

21. TDH Vs. Hl If TDH = hl, then flow in boiler stops!! Remember ==> M flow = TDH - HL

22. Flow Rate Stops... Is this bad?????

23. End Point Of Circulation Major concern! No cooling taking place Boiler is a heat exchanger Possible causes: High firing rate Reduction in steam drum pressure below minimum limit

24. Boiler End Points Capacity of a boiler is limited by: Design of boiler Its operation These limits are known as boiler end points

25. Boiler End Points There are 3 boiler end points : End point of combustion Moisture carryover End point of circulation

26. End Point Of Combustion Fuel rate at designed max with all burners lit Air at amount required for complete/proper combustion Steam drum pressure at set point

27. End Point Of Combustion End point of combustion is the only end point that is likely to be reached in a properly designed and operated boiler

28. Moisture Carryover Occurs when moisture separators are flooded and water is carried over into steam lines Always occurs just before end point of circulation

29. Film Boiling Definition: When excessive heat transfer rates and insufficient circulation causes a layer of steam to form on the surface of tubes

30. Film Boiling Film boiling is infrequent occurrence Causes: Poor boiler design Misalignment of burners (most frequent cause) Causes excessively high heat by flames impinging on furnace tubes

31. Departure From Nucleate Boiling Definition: Point where heating surfaces of tubes are abnormally hot If temp is high enough, heating surface may burn out or melt --this is known as the burnout point or critical heat flux Departure from nucleate boiling (DNB) is very close to this point

32. Departure From Nucleate Boiling Occurs prior to end point of natural circulation Causes: Reduction in steam drum pressure below lower operating limit High firing rate

33. Film Boiling & DNB A-B WATER HEATING B-S SUBCOOLED NUCLEATE BOILING S-C NUCLEATE BOILING C-D ONSET OF FILM BOILING D-E UNSTABLE FILM BOILING E-F STABLE FILM BOILING A B C D E D’ F DNB OR CRITICAL HEAT FLUX S LOG ( TSURFACE - TWTR/STM) LOG (HEAT FLUX)

34. Shrink & Swell During up power maneuvers, a temporary rise in boiler water level occurs -- called swell During down power maneuvers, a temporary drop in boiler water level occurs -- called shrink

35. Swell Increase in steam demand: More steam leaves boiler. Drum press starts decreasing. Steam bubbles in GEN tubes expand. Water level “swells.”

36. Swell Automatic combustion control system (acc sys) senses decrease in drum press and increases firing rate Drum press increases and regains SETPOINT Water level decreases as bubble size decreases

37. Swell Water reaches normal and gets slightly below normal Automatic boiler control system (ABC sys) will increase feed flow to match steam flow Water level achieves normal

38. Shrink During a down power maneuver shrink has the opposite effect: Throttle closes Steam drum press rises Water level lowers or “shrinks”

39. Shrink Acc system decreases firing rate Drum press regains SETPOINT Water level increases Water level reaches normal, and slightly above normal ABC sys decreases feed flow

40. Can the Effects of Shrink and Swell Be Avoided? YES, and the navy knows how but still doesn’t ! How and why not ?? Increasing boiler size reduces the fluctuations in level caused by shrink and swell (space limitation).

41. Summary Waterside & fireside SUBCOOLED nucleate boiling Nucleate boiling Annulus Quality of steam (equation) Steam slugs Film boiling DNB

42. Summary Boiler end points Causes and limits of natural circulation Thermal driving head Head loss Boiler power level & natural circulation (graph of TDH VS head loss) Shrink & swell

43. Any Questions...?

44. Basic Steam Engineering 62b-201 Nucleate Boiling