62B-201-110 AUG 00

Basic Steam Engineering

62b-201Nucleate Boiling

62B-201-210 AUG 00

1. Describe the theory of 1. Describe the theory of nucleate boiling nucleate boiling and how it applies to boiler operationsand how it applies to boiler operations

2. Describe how variations in 2. Describe how variations in circulation lead to shrink and swellcirculation lead to shrink and swell

Enabling Objectives

62B-201-310 AUG 00

Boiling Process In A Generating Tube

• Tubes have a Tubes have a waterside waterside (internal volume) (internal volume) and a and a firesidefireside (external surface) (external surface)

• LIKE BOILING WATER IN A POT, LIKE BOILING WATER IN A POT, BOILING IN THE GENERATING BOILING IN THE GENERATING TUBES IS A PROGRESSIVE TUBES IS A PROGRESSIVE PHENOMENON (i.E. Little bubbles, PHENOMENON (i.E. Little bubbles, bigger bubbles, bulk boiling)bigger bubbles, bulk boiling)

SUBCOOLED Nucleate Boiling

• Definition of Definition of SUBCOOLEDSUBCOOLED::

• Below Below temperature (temperature (TTSATSAT) at which water ) at which water

will change state to steam at a given pressurewill change state to steam at a given pressure

• Bulk of boiler water is SUBCOOLED as Bulk of boiler water is SUBCOOLED as

it rises up generating tubesit rises up generating tubes

SUBCOOLED Nucleate Boiling

• Bubbles Break Off From Tube Wall And Enter Bubbles Break Off From Tube Wall And Enter Main Stream Of Water And CollapseMain Stream Of Water And Collapse

POP!

TBulkFluid<TSaturation

Nucleate Boiling

• Steam bubbles & bulk fluid = TSteam bubbles & bulk fluid = TSATSAT

• Bubbles no longer collapseBubbles no longer collapse

NUCLEATE BOILING

SUBCOOLEDNUCLEATE BOILING

TSteamBubble = TBulkFluid

62B-201-710 AUG 00

Quality

• Definition of quality:Definition of quality:

• Quality of STM is the dryness or purity. It is Quality of STM is the dryness or purity. It is

the ratio of: the ratio of: Mass of STM in LBM Mass of STM in LBM

Total mass of STM & WTR LBMTotal mass of STM & WTR LBM

• ““LBM” is pounds of massLBM” is pounds of mass

• Expressed in %Expressed in %

• 5% quality is “wetter” than 90%5% quality is “wetter” than 90%

X =

Steam Slugs

• At 5 % quality - steam slugs formAt 5 % quality - steam slugs form

NUCLEATE BOILING

SUBCOOLED NUCLEATE BOILING

STEAM SLUGS 5% QUALITY

TSteamBubble > TBulkFluid

62B-201-910 AUG 00

Annulus

• Steam slugs combine further up tubeSteam slugs combine further up tube

• Form column of steam in center of tubeForm column of steam in center of tube

• Ring of water remains - adheres to tube Ring of water remains - adheres to tube

inner wallinner wall

• Called Called annulusannulus

62B-201-1010 AUG 00

Annulus

• Annulus Annulus

• Water carries away heat Water carries away heat

• Critical to Critical to protecting tube from meltingprotecting tube from melting

NUCLEATE BOILING

SUBCOOLED NUCLEATE BOILING

STM SLUGS

5% QUALITY

ANNULUS

STM

COLUMNOF

62B-201-1110 AUG 00

Nucleate Boiling

• Ideally steam & water exiting Ideally steam & water exiting

generating tubes will be:generating tubes will be:

• Annular flow Annular flow

• 20 % - 30% quality20 % - 30% quality

62B-201-1210 AUG 00

Moisture Separators

• Chevron & cyclone typeChevron & cyclone type• Remove water from steam to Remove water from steam to

prevent moisture carryoverprevent moisture carryover• Steam Steam exits boilerexits boiler at 99.75% (recall at 99.75% (recall

that steam that steam exits tubeexits tube at 20-30%) at 20-30%)

62B-201-1310 AUG 00

Natural Circulation

• There is no pump to circulate boiler waterThere is no pump to circulate boiler water

• We rely on natural circulationWe rely on natural circulation

• Termed Termed acceleratedaccelerated due to slant in generating due to slant in generating tubestubes

• Promoted byPromoted by thermal driving head (TDH)thermal driving head (TDH)

• Retarded by Retarded by head losses (hl)head losses (hl)

62B-201-1410 AUG 00

Thermal Driving Head

• Explanation:Explanation:• AVERAGE SPECIFIC VOLUME (VOL) OF AVERAGE SPECIFIC VOLUME (VOL) OF

WATER IN GENERATING TUBES > WATER WATER IN GENERATING TUBES > WATER IN DOWNCOMERS (i.E. On a LBM basis, the IN DOWNCOMERS (i.E. On a LBM basis, the water in tubes takes up more space)water in tubes takes up more space)

• Therefore => density of steam/ water mixture in Therefore => density of steam/ water mixture in tubes < water in DOWNCOMERStubes < water in DOWNCOMERS

62B-201-1510 AUG 00

Thermal Driving Head

• Therefore => weight of steam/ water in Therefore => weight of steam/ water in generating tubes < water in generating tubes < water in DOWNCOMERSDOWNCOMERS

• Therefore => pressure on water in Therefore => pressure on water in DOWNCOMERS > pressure on steam/ DOWNCOMERS > pressure on steam/ water mixture in generating tubeswater mixture in generating tubes

62B-201-1610 AUG 00

Thermal Driving Head

• As water molecules exit generating tubes, As water molecules exit generating tubes, heavier water fills void, causingheavier water fills void, causing

Natural circulation (NC)Natural circulation (NC)

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Thermal Driving Head

• TDHTDH increases increases linearlylinearly with increase in with increase in qualityquality

• Greater the steam demand, greater Greater the steam demand, greater TDHTDH

• Note: Note: for the most partfor the most part increase in firing increase in firing rate is synonymous with an increase in rate is synonymous with an increase in qualityquality

62B-201-1810 AUG 00

Head Loss

• TDHTDH creates flow down creates flow down DOWNCOMERS and back up DOWNCOMERS and back up generating tubesgenerating tubes

• Actual mass flow rate of NC is Actual mass flow rate of NC is difference between difference between TDHTDH and and hlhl::

MMflowflow = TDH - HL = TDH - HL

62B-201-1910 AUG 00

Head Loss

• As quality increases, larger steam slugs As quality increases, larger steam slugs

formform

• Slugs impede flow of remaining water in Slugs impede flow of remaining water in

tube, causing tube, causing head losshead loss

62B-201-2110 AUG 00

TDH Vs. Hl

• If TDH = hl, thenIf TDH = hl, then flow in boiler flow in boiler stops!!stops!!

Remember ==> Remember ==>

MMflowflow = TDH - HL = TDH - HL

62B-201-2210 AUG 00

Flow Rate Stops...

• Is this bad?????Is this bad?????

62B-201-2310 AUG 00

End Point Of Circulation

• Major concern!Major concern!• No cooling taking placeNo cooling taking place

• Boiler is a heat exchangerBoiler is a heat exchanger

• Possible causes:Possible causes:• High firing rate High firing rate • Reduction in steam drum pressure Reduction in steam drum pressure

below minimum limitbelow minimum limit

62B-201-2410 AUG 00

Boiler End Points

• Capacity of a boiler is limited by:Capacity of a boiler is limited by:• Design of boilerDesign of boiler

• Its operationIts operation

• These limits are known as These limits are known as boiler end boiler end points points

62B-201-2510 AUG 00

Boiler End Points• There are 3 boiler end pointsThere are 3 boiler end points::

• End point of combustionEnd point of combustion• Moisture carryoverMoisture carryover• End point of circulationEnd point of circulation

62B-201-2910 AUG 00

Film Boiling

• Definition: Definition: • When excessive heat transfer rates and When excessive heat transfer rates and

insufficient circulation causes a layer insufficient circulation causes a layer of steam to form on the surface of of steam to form on the surface of tubestubes

62B-201-3410 AUG 00

Shrink & Swell

• During During up powerup power maneuvers, a maneuvers, a temporary rise in boiler water level temporary rise in boiler water level occurs -- called occurs -- called swellswell

• During During down powerdown power maneuvers, a maneuvers, a temporary drop in boiler water level temporary drop in boiler water level occurs -- called occurs -- called shrinkshrink

Swell

• Increase in steam demand:Increase in steam demand:• More steam leaves boiler.More steam leaves boiler.

• Drum press starts decreasing.Drum press starts decreasing.

• Steam bubbles in GEN tubes expand.Steam bubbles in GEN tubes expand.

• Water level “swells.”Water level “swells.”

62B-201-3610 AUG 00

Swell

• Automatic combustion control system (acc Automatic combustion control system (acc sys) senses decrease in drum press and sys) senses decrease in drum press and increases firing rateincreases firing rate

• Drum press increases and regains Drum press increases and regains SETPOINTSETPOINT

• Water level decreases as bubble size Water level decreases as bubble size decreasesdecreases

62B-201-3710 AUG 00

Swell• Water reaches normal and gets slightly below Water reaches normal and gets slightly below

normalnormal

• Automatic boiler control system (ABC sys) Automatic boiler control system (ABC sys)

will increase feed flow to match steam flowwill increase feed flow to match steam flow

• Water level achieves normalWater level achieves normal

62B-201-3810 AUG 00

Shrink

• During a down power maneuver During a down power maneuver shrink has the opposite effect:shrink has the opposite effect:• Throttle closesThrottle closes

• Steam drum press risesSteam drum press rises

• Water level lowers or “shrinks”Water level lowers or “shrinks”

62B-201-3910 AUG 00

Shrink • Acc system decreases firing rate Acc system decreases firing rate

• Drum press regains SETPOINTDrum press regains SETPOINT

• Water level increasesWater level increases

• Water level reaches normal, and slightly Water level reaches normal, and slightly

above normalabove normal

• ABC sys decreases feed flowABC sys decreases feed flow

62B-201-4010 AUG 00

Can the Effects of Shrink and Swell Be Avoided?

• YES, and the navy knows how but YES, and the navy knows how but still doesn’t !still doesn’t !

• How and why not ??How and why not ??

• Increasing boiler size reduces the Increasing boiler size reduces the fluctuations in level caused by shrink fluctuations in level caused by shrink and swell (space limitation).and swell (space limitation).

62B-201-4110 AUG 00

Summary

• Waterside & firesideWaterside & fireside

• SUBCOOLED nucleate boilingSUBCOOLED nucleate boiling

• Nucleate boilingNucleate boiling

• AnnulusAnnulus

• Quality of steam (equation)Quality of steam (equation)

• Steam slugs Steam slugs

• Film boilingFilm boiling

• DNBDNB

62B-201-4210 AUG 00

Summary

• Boiler end points Boiler end points

• Causes and limits of natural circulationCauses and limits of natural circulation

• Thermal driving headThermal driving head

• Head lossHead loss

• Boiler power level & natural circulation Boiler power level & natural circulation (graph of TDH VS head loss)(graph of TDH VS head loss)

• Shrink & swellShrink & swell

62B-201-4310 AUG 00

Any Questions...?

62B-201-4410 AUG 00

Basic Steam Engineering

62b-201Nucleate Boiling