boiler circulation natural circulation free type natural circulation (straight tube boilers) ...

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Boiler Circulation Boiler Circulation Natural Circulation Natural Circulation Free Type Natural Circulation Free Type Natural Circulation (Straight Tube Boilers) (Straight Tube Boilers) Accelerated or Express Circulation Accelerated or Express Circulation (Bent Tube Boilers) (Bent Tube Boilers) Forced Circulation (Controlled Forced Circulation (Controlled Circulation) Circulation)

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Boiler CirculationBoiler Circulation

Natural CirculationNatural Circulation Free Type Natural Circulation (Straight Free Type Natural Circulation (Straight

Tube Boilers)Tube Boilers) Accelerated or Express Circulation (Bent Accelerated or Express Circulation (Bent

Tube Boilers)Tube Boilers) Forced Circulation (Controlled Forced Circulation (Controlled

Circulation)Circulation)

Boiler CirculationBoiler CirculationNatural

CirculationAccelerated

Circulation

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Natural CirculationNatural Circulation

In natural circulation the force of gravity In natural circulation the force of gravity available to produce the flow comes from available to produce the flow comes from the difference in densities of the fluids in the difference in densities of the fluids in the downcomer (downflow) and riser (up the downcomer (downflow) and riser (up flow) circuit. flow) circuit.

Ideally the fluid in the downcomer is water Ideally the fluid in the downcomer is water at or slightly below saturated temperature at or slightly below saturated temperature undiluted by steam bubbles undiluted by steam bubbles

Whereas the fluid in the riser circuits Whereas the fluid in the riser circuits contains a mixture of water and steam contains a mixture of water and steam bubbles at saturated temperature.bubbles at saturated temperature.

Natural CirculationNatural Circulation

The mixture in the riser circuits by virtue The mixture in the riser circuits by virtue of its higher temperature and the of its higher temperature and the presence of steam is lighter and hence presence of steam is lighter and hence less dense than the water in the less dense than the water in the downcomer circuits. downcomer circuits.

By virtue of the difference in their By virtue of the difference in their densities the lighter steam and water densities the lighter steam and water mixture rises to the steam and water mixture rises to the steam and water drum as it is displaced by the heavier drum as it is displaced by the heavier more denser water.more denser water.

Natural CirculationNatural Circulation

In any given natural circulation systemIn any given natural circulation system The circulation will increase with increased The circulation will increase with increased

heat inputheat input The increase in heat input causes a larger The increase in heat input causes a larger

percentage of steam in the steam and water percentage of steam in the steam and water mixture in the riser circuits, causing the mixture in the riser circuits, causing the mixture to become lighter in weight (less mixture to become lighter in weight (less dense).dense).

The increased difference in density causes the The increased difference in density causes the forces of gravity to be greater which in turn forces of gravity to be greater which in turn increases the rate of circulation.increases the rate of circulation.

Natural CirculationNatural Circulation

In a bent tube boiler the circulation rate is In a bent tube boiler the circulation rate is increased above the rate found in a increased above the rate found in a straight tube boiler by a means of straight tube boiler by a means of combination of two methodscombination of two methods Increasing the difference in densities between Increasing the difference in densities between

the downcomer and riser circuits by increasing the downcomer and riser circuits by increasing the heat absorbing abilities of the riser circuitthe heat absorbing abilities of the riser circuit

Reducing the frictional and impact losses in Reducing the frictional and impact losses in both the downcomer and riser circuitsboth the downcomer and riser circuits

Natural CirculationNatural Circulation

Increasing the difference in densities is Increasing the difference in densities is accomplished in the bent tube boiler by accomplished in the bent tube boiler by utilizing smaller generating tubes. utilizing smaller generating tubes.

The smaller the tube, the greater the The smaller the tube, the greater the ratio of surface area to volume of water ratio of surface area to volume of water within the tube and therefore the within the tube and therefore the greater heat absorbing characteristics of greater heat absorbing characteristics of the tube. the tube.

Natural CirculationNatural Circulation

The downcomer circuits are generally The downcomer circuits are generally comprised of larger tubes which are comprised of larger tubes which are usually placed away from the heat of usually placed away from the heat of combustion or outside the boiler casing. combustion or outside the boiler casing.

Thus keeping this circuit cooler and Thus keeping this circuit cooler and further increasing the difference in further increasing the difference in density between the riser and density between the riser and downcomer circuitsdowncomer circuits

Natural CirculationNatural Circulation

A Marine Boiler operates at 600 psiaA Marine Boiler operates at 600 psia The superheater outlet steam The superheater outlet steam

temperature is 875 °Ftemperature is 875 °F Feed water entering the steam and Feed water entering the steam and

water drum is at 350 °Fwater drum is at 350 °F What is causing circulation in the What is causing circulation in the

boiler?boiler?

Natural CirculationNatural Circulation

The water in the steam and water The water in the steam and water drum at 600 psia is saturated and drum at 600 psia is saturated and has a corresponding saturation has a corresponding saturation temperature of ___________?temperature of ___________?

Natural CirculationNatural Circulation

The water in the steam and water The water in the steam and water drum at 600 psia is saturated and drum at 600 psia is saturated and has a corresponding saturation has a corresponding saturation temperature of ___________?temperature of ___________?

486.2 °F486.2 °F

Natural CirculationNatural Circulation

How do we find the Density (How do we find the Density (ρρ) of the ) of the water?water?

Natural CirculationNatural Circulation

How do we find the Density (How do we find the Density (ρρ) of the ) of the water?water?

Density (Density (ρρ) = 1/Specific Volume () = 1/Specific Volume (νν)) Specific Volume for the feed water Specific Volume for the feed water

entering the boiler @ 350 °F entering the boiler @ 350 °F

Natural CirculationNatural Circulation

How do we find the Density (How do we find the Density (ρρ) of the ) of the water?water?

Density (Density (ρρ) = 1/Specific Volume () = 1/Specific Volume (νν)) Specific Volume for the feed water Specific Volume for the feed water

entering the boiler @ 350 °F entering the boiler @ 350 °F ννf f = 0.017799 ft = 0.017799 ft33/lbm/lbm

Natural CirculationNatural Circulation

How do we find the Density (How do we find the Density (ρρ) of the ) of the water?water?

Density (Density (ρρ) = 1/Specific Volume () = 1/Specific Volume (νν)) Specific Volume for the feed water Specific Volume for the feed water

entering the boiler @ 350 °F entering the boiler @ 350 °F ννf f = 0.017799 ft = 0.017799 ft33/lbm/lbm ρρ = 56.18 lbm/ft = 56.18 lbm/ft33

Natural CirculationNatural Circulation

Specific Volume for the saturated Specific Volume for the saturated boiler water in the steam and water boiler water in the steam and water drum.drum.

Natural CirculationNatural Circulation

Specific Volume for the saturated Specific Volume for the saturated boiler water in the steam and water boiler water in the steam and water drum.drum.

Saturated water temperature is Saturated water temperature is 486.2 °F486.2 °F

Natural CirculationNatural Circulation

Specific Volume for the saturated Specific Volume for the saturated boiler water in the steam and water boiler water in the steam and water drum.drum.

Saturated water temperature is Saturated water temperature is 486.2 °F486.2 °F

ννff = 0.02013 ft = 0.02013 ft33/lbm/lbm

Natural CirculationNatural Circulation

Specific Volume for the saturated Specific Volume for the saturated boiler water in the steam and water boiler water in the steam and water drum.drum.

Saturated water temperature is Saturated water temperature is 486.2 °F486.2 °F

ννff = 0.02013 ft = 0.02013 ft33/lbm/lbm ρρ = 49.68 lbm/ft = 49.68 lbm/ft33

Natural CirculationNatural Circulation

We have a density difference ofWe have a density difference of ρρ350 °F 350 °F – – ρρ486.2 °F 486.2 °F == 56.18 lbm/ft56.18 lbm/ft33 – 49.68 lbm/ft – 49.68 lbm/ft33

6.5 lbm/ft6.5 lbm/ft33

Forced CirculationForced Circulation

In a forced circulation boiler, the In a forced circulation boiler, the water is pumped through the water is pumped through the generating tubes, insuring a positive generating tubes, insuring a positive flow in one direction at all times, flow in one direction at all times, regardless of the rate of heat regardless of the rate of heat transfer to the water.transfer to the water.

Forced CirculationForced Circulation

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Steam SeparationSteam Separation

The operating pressure has an effect The operating pressure has an effect on the natural tendency of steam on the natural tendency of steam and water to separate.and water to separate.

In the separation of steam and In the separation of steam and water, the limiting velocity of a water water, the limiting velocity of a water particle conveyed in steam and the particle conveyed in steam and the force of gravity both vary directly force of gravity both vary directly with the differential of densities of with the differential of densities of the water and steam.the water and steam.

Steam SeparationSteam Separation

Density difference between Density difference between saturated water in a 600 psia boiler saturated water in a 600 psia boiler and a 1200 psia boiler.and a 1200 psia boiler.

Steam SeparationSteam Separation

The density of water at saturated The density of water at saturated pressure (600 psia).pressure (600 psia).

ρρ = 1/v = 1/vff

ρρ = 1/0.02013 = 49.68 lbm/ft = 1/0.02013 = 49.68 lbm/ft33

The density of steam at saturated The density of steam at saturated pressure.pressure.

ρρ = 1/ = 1/ννgg

ρρ = 1/0.76975 = 1.299 lbm/ft = 1/0.76975 = 1.299 lbm/ft33

Steam SeparationSteam Separation

The density of water at saturated The density of water at saturated pressure (1200 psia).pressure (1200 psia).

ρρ = 1/ = 1/ννff ρρ = 1/0.02232 = 44.8 lbm/ft = 1/0.02232 = 44.8 lbm/ft33

The density of steam at saturated The density of steam at saturated pressure.pressure.

ρρ = 1/ = 1/ννgg ρρ = 1/0.36245 = 2.759 lbm/ft = 1/0.36245 = 2.759 lbm/ft33

Steam SeparationSteam Separation

Density difference for the 600 psia Density difference for the 600 psia boiler boiler

49.68 lbm/ft49.68 lbm/ft3 3 – 1.299 lbm/ft– 1.299 lbm/ft33

ΔρΔρ = 48.381 lbm/ft = 48.381 lbm/ft33

Density difference for the 1200 psia Density difference for the 1200 psia boiler boiler

44.8 lbm/ft44.8 lbm/ft3 3 - 2.759 lbm/ft- 2.759 lbm/ft33

ΔρΔρ = 42.04 lbm/ft = 42.04 lbm/ft33

Steam SeparationSteam Separation

End PointsEnd Points

As the rate of steam generation of a As the rate of steam generation of a boiler increases, a condition is boiler increases, a condition is reached where further increase in reached where further increase in the steam rate is impossible. This the steam rate is impossible. This point is referred to as the end point point is referred to as the end point for the particular operating factor for the particular operating factor which determines the limit of steam which determines the limit of steam generation.generation.

End PointsEnd Points

End Point of CombustionEnd Point of Combustion End Point of Moisture Carry-OverEnd Point of Moisture Carry-Over End Point of Water CirculationEnd Point of Water Circulation

End Point of CombustionEnd Point of Combustion

The end point of combustion is limited The end point of combustion is limited by the amount of fuel which can be by the amount of fuel which can be burned properly and efficiently in a burned properly and efficiently in a given boiler. This in turn is limited by given boiler. This in turn is limited by the following:the following: The amount of air which can be forced into The amount of air which can be forced into

the furnacethe furnace The ability of the burner apparatus to mix The ability of the burner apparatus to mix

properly air and fuelproperly air and fuel The volume and shape of the furnaceThe volume and shape of the furnace

End Point of Moisture Carry-OverEnd Point of Moisture Carry-Over

The end point of moisture carry-over The end point of moisture carry-over occurs when the moisture content in occurs when the moisture content in the steam leaving the boiler reaches the steam leaving the boiler reaches a point where such moisture may be a point where such moisture may be harmful to the superheater, steam harmful to the superheater, steam piping or turbines. The specifications piping or turbines. The specifications for naval machinery limit the moisture for naval machinery limit the moisture content of the steam leaving the content of the steam leaving the saturated steam outlet to 0.25%saturated steam outlet to 0.25%

End Point of Water Circulation End Point of Water Circulation

The end point of water circulation The end point of water circulation occurs when the volume of water occurs when the volume of water supplied to the generating tubes is supplied to the generating tubes is not sufficient to prevent the not sufficient to prevent the generating tubes nearest the furnace generating tubes nearest the furnace from becoming dry and overheating.from becoming dry and overheating.

End Point of CombustionEnd Point of Combustion

What happens if you reach the End What happens if you reach the End Point of Combustion?Point of Combustion?

End Point of Moisture Carry-OverEnd Point of Moisture Carry-Over

What happens if I reach the End What happens if I reach the End Point of Moisture Carryover? Point of Moisture Carryover?

End Point of Water CirculationEnd Point of Water Circulation

What happens if I reach the End What happens if I reach the End Point of Water Circulation?Point of Water Circulation?

End PointsEnd Points

End Point of CombustionEnd Point of Combustion End Point of Moisture Carry-OverEnd Point of Moisture Carry-Over End Point of Water CirculationEnd Point of Water Circulation