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

2. Boiler Circulation Natural Circulation Accelerated Circulation

3. Natural Circulation
In natural circulation the force of gravity available to produce the flow comes from the difference in densities of the fluids in the downcomer (downflow) and riser (up flow) circuit.
Ideally the fluid in the downcomer is water at or slightly below saturated temperature undiluted by steam bubbles
Whereas the fluid in the riser circuits contains a mixture of water and steam bubbles at saturated temperature.

4. Natural Circulation
The mixture in the riser circuits by virtue of its higher temperature and the presence of steam is lighter and hence less dense than the water in the downcomer circuits.
By virtue of the difference in their densities the lighter steam and water mixture rises to the steam and water drum as it is displaced by the heavier more denser water.

5. Natural Circulation In any given natural circulation system
The circulation will increase with increased heat input
The increase in heat input causes a larger percentage of steam in the steam and water mixture in the riser circuits, causing the mixture to become lighter in weight (less dense).
The increased difference in density causes the forces of gravity to be greater which in turn increases the rate of circulation.

6. Natural Circulation
In a bent tube boiler the circulation rate is increased above the rate found in a straight tube boiler by a means of combination of two methods
Increasing the difference in densities between the downcomer and riser circuits by increasing the heat absorbing abilities of the riser circuit
Reducing the frictional and impact losses in both the downcomer and riser circuits

7. Natural Circulation
Increasing the difference in densities is accomplished in the bent tube boiler by utilizing smaller generating tubes.
The smaller the tube, the greater the ratio of surface area to volume of water within the tube and therefore the greater heat absorbing characteristics of the tube.

8. Natural Circulation
The downcomer circuits are generally comprised of larger tubes which are usually placed away from the heat of combustion or outside the boiler casing.
Thus keeping this circuit cooler and further increasing the difference in density between the riser and downcomer circuits

9. Natural Circulation A Marine Boiler operates at 600 psia
The superheater outlet steam temperature is 875 °F
Feed water entering the steam and water drum is at 350 °F
What is causing circulation in the boiler?

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

11. Natural Circulation
The water in the steam and water drum at 600 psia is saturated and has a corresponding saturation temperature of ___________?
486.2 °F

12. Natural Circulation
How do we find the Density (ρ) of the water?

13. Natural Circulation How do we find the Density (ρ) of the water?
Density (ρ) = 1/Specific Volume (ν)
Specific Volume for the feed water entering the 350 °F

14. Natural Circulation How do we find the Density (ρ) of the water?
Density (ρ) = 1/Specific Volume (ν)
Specific Volume for the feed water entering the 350 °F
νf = ft3/lbm

15. Natural Circulation How do we find the Density (ρ) of the water?
Density (ρ) = 1/Specific Volume (ν)
Specific Volume for the feed water entering the 350 °F
νf = ft3/lbm
ρ = lbm/ft3

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

17. Natural Circulation
Specific Volume for the saturated boiler water in the steam and water drum.
Saturated water temperature is °F

18. Natural Circulation
Specific Volume for the saturated boiler water in the steam and water drum.
Saturated water temperature is °F
νf = ft3/lbm

19. Natural Circulation
Specific Volume for the saturated boiler water in the steam and water drum.
Saturated water temperature is °F
νf = ft3/lbm
ρ = lbm/ft3

20. Natural Circulation We have a density difference of
56.18 lbm/ft3 – lbm/ft3
6.5 lbm/ft3

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

22. Forced Circulation

23. Steam Separation
The operating pressure has an effect on the natural tendency of steam and water to separate.
In the separation of steam and water, the limiting velocity of a water particle conveyed in steam and the force of gravity both vary directly with the differential of densities of the water and steam.

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

25. Steam Separation
The density of water at saturated pressure (600 psia).
ρ = 1/vf
ρ = 1/ = lbm/ft3
The density of steam at saturated pressure.
ρ = 1/νg
ρ = 1/ = lbm/ft3

26. Steam Separation
The density of water at saturated pressure (1200 psia).
ρ = 1/νf
ρ = 1/ = 44.8 lbm/ft3
The density of steam at saturated pressure.
ρ = 1/νg
ρ = 1/ = lbm/ft3

27. Steam Separation Density difference for the 600 psia boiler
49.68 lbm/ft3 – lbm/ft3
Δρ = lbm/ft3
Density difference for the 1200 psia boiler
44.8 lbm/ft lbm/ft3
Δρ = lbm/ft3

28. Steam Separation

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

30. End Points End Point of Combustion End Point of Moisture Carry-Over
End Point of Water Circulation

31. End Point of Combustion
The end point of combustion is limited by the amount of fuel which can be burned properly and efficiently in a given boiler. This in turn is limited by the following:
The amount of air which can be forced into the furnace
The ability of the burner apparatus to mix properly air and fuel
The volume and shape of the furnace

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

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

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

35. End Point of Moisture Carry-Over
What happens if I reach the End Point of Moisture Carryover?

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

37. End Points End Point of Combustion End Point of Moisture Carry-Over
End Point of Water Circulation