The document discusses boiler circulation systems and boiling phenomena. It covers: 1) Sub-critical and super-critical boiler systems and different circulation methods like natural, forced, and assisted circulation. 2) Features of boiling like nucleate boiling, critical heat flux, film boiling, and departure from nucleate boiling (DNB). 3) Special features of once-through supercritical boilers including their start-up system using a boiler circulation pump (BCP) to maintain minimum flow during low load conditions.

1. Boiler Circulation
Theory
Presented by
Nauman Salim Kalshekar
Manager-Operations
Nabha Power Ltd.

2. SH
Furnace
Eco
Water
Separator
BFP
BCP
WSDT
Eco
Furnace
SH
CONDENSER
9/9/2020 2
Sub-critical and Super-critical boiler
Water-wall design
Boiling Phenomenon
Different Circulation Methods
Special feature of supercritical boiler
Agenda

3. SUB-CRITICAL /
SUPERCRITICAL SYSTEM
Sub-critical & Super-critical system

4. SUB-CRITICAL /
SUPERCRITICAL SYSTEM

5. WATER-WALL DESIGN
Rifled Tubing

6. WATER-WALL DESIGN

7. NUCLEATE BOILING
• With nucleate boiling, steam bubbles are released at the tube surface while the water is
still wetting the surface.
• It is a type of boiling that takes place when the surface temperature is hotter than the
saturated fluid temperature by a certain amount but where the heat flux is below the
critical heat flux.
• In general, the number of nucleation sites are increased by an increasing surface
temperature.

8. CRITICAL HEAT FLUX & TRANSITION BOILING
CRITICAL HEAT FLUX
•The critical heat flux is the peak on the curve between nucleate boiling and transition
boiling.
•Critical heat flux describes the thermal limit of a phenomenon where a phase change
occurs during heating (such as bubbles forming on a metal surface used to heat water),
which suddenly decreases the efficiency of heat transfer, thus causing localised
overheating of the heating surface.
TRANSITION BOILING
•Transition boiling may be defined as the unstable boiling, which occurs at surface
temperatures between the maximum attainable in nucleate and the minimum attainable
in film boiling.
•The formation of bubbles in a heated liquid is a complex physical process which often
involves cavitation and acoustic effects, such as the broad-spectrum hiss one hears in a
kettle not yet heated to the point where bubbles boil to the surface.

9. FILM BOILING
•In film boiling, the tube surface is covered by a film of steam, which acts like a good
thermal insulator between the tube surface and the water.
•The result is that the heat transfer decreases, the tube can overheat and less steam is
produced.

10. DNB (DEPARTURE FROM
NUCLEATE BOILING)
Water wall with internal rifle tubing Water wall with smooth tubing

11. • Circulation system can loosely be defined as the combination
of equipments and processes for maintaining differential
pressure throughout water/steam path.
• For a sub critical boiler this system consist of Drum, down-
comer, Ring header, Evaporator, Riser tubes etc.
• For supercritical boiler it consist of BFP, Economiser and
super-heater.
BOILER CIRCULATION
SYSTEM

12. CIRCULATION
METHODS

13. Natural Circulation:
• Circulation of water takes place due to natural convection currents produced by the
application of heat.
• Saturated water flows down the unheated down-comer and receives heat in the riser
where a part of it gets converted into steam.
• The difference in densities of saturated water in the down-comer and the steam-water
mixer in the riser brings about natural circulation.
• It is applicable to all those boilers, which are operating at a pressure less than critical
pressure.
Forced / Once through Circulation:
• In a Forced circulation, the working fluid is forced through the boiler circuits by an
external pump.
Assisted Circulation:
• Circulation is assisted with the help of boiler recirculation pump.

14. CIRCULATION RATIO
It is the ratio of the mass flow rate of circulating water
Gfw (t/hr) to the rate of steam generation Gs (t/h);
Circulation Ratio = Gfw / Gs
It usually ranges from 4 to 30 for Natural Circulation
and 3 to 10 for Forced Circulation boilers.

15. Once-through boiler has start-up system including
BCP.
To maintain minimum flow and to equalize flow
to many parallel tubes, it is always necessary to
feed water to the boiler at rate of 25% load while
firing.
SH
Furnace
Eco
Water
Separator
BFP
WSDT
Eco
Furnace
SH
However, the steam generation is in low
conditions which is less than the rate of 25% load,
the heat loss is very high accordingly.
SPECIAL FEATURES OF
ONCE THROUGH BOILER

16. Once-through boiler has start-up system including
BCP.
However, the steam generation is low conditions
which is less than at rate of 25% load, the heat
loss is very high accordingly.
SH
Furnace
Eco
Water
Separator
BFP
WSDT
Eco
Furnace
SH
To recover the possible heat loss, hot water of
approx.20% load is returned to Economizer inlet
to accumulate the heat in the boiler water using
BCP.
BCPBCP
SPECIAL FEATURES OF ONCE
THROUGH BOILER

17. Once-through boiler has start-up system including
BCP.
During start-up of the boiler, approx. 5% load
flow is fed to the boiler continuously by BFP and
balance approx. 20% required to maintain 25%
load flow through economizer and furnace water
wall tubes is supplied by the BCP.
SH
Furnace
Eco
Water
Separator
BFP
BCP
WSDT
Eco
Furnace
SH
25% load flow
5% load flow
20% load flow
SPECIAL FEATURES OF
ONCE THROUGH BOILER

18. Once-through boiler has start-up system including
BCP.
The steam water mixture flows to water
separator (WS), where it is separated to steam
and water. The separated steam flows to the
superheater and water flows to water separator
drain tank (WSDT).
SH
Furnace
Eco
Water
Separator
BFP
BCP
WSDT
Eco
Furnace
SH
SPECIAL FEATURES OF
ONCE THROUGH BOILER

19. Once-through boiler has start-up system including
BCP.
SH
Furnace
Eco
Water
Separator
BFP
BCP
WSDT
Eco
Furnace
SH
The BCP takes the suction from
WSDT and re-circulates approx. 20%
load flow.
The recirculation flow joins the
approx. 5% BFP flow and approx.
25% is fed to the boiler.
CONDENSER
The excess 5% load flow is
discharged to condenser from WSDT
as per the water level in WS.
Indicated system is called “Start-up
system”.
SPECIAL FEATURES OF
ONCE THROUGH BOILER