Principle Of Circulation
1. Principles of Circulation
Subcooled FW enters the drum, mixes with the circulating boiler water, and
attains saturation temperature instantly, as the boiling, circulating water is
several times the incoming water flow.
This circulating water picks up its latent heat progressively from the hot flue
gases to form steam as it goes around the evaporator circuits several times.
This steam is continuously separated in the drum by the steam separators. There
is a balance between the incoming feed water (FW) and the outgoing steam when
the system is properly functioning.
Circulation ratio is the water in circulation divided by the steam fl ow. In
other words it is the number of times the water has to go around the various
evaporator circuits before it is all converted into steam. Latent heat is added
to the circulating water at constant pressure and constant temperature.
There is no circulation in SC boilers as it is a forced fl ow arrangement. In
once-through (OT) subcritical boilers also there is no circulation. To take
advantage of the relatively low boiling temperature of water (critical
temperature is 374.1°C), the hottest portion of the boiler, namely, the
furnace, is encased in tubes carrying boiling, circulating water. The screen,
division wall, boiler bank (BB), and EVAP tubes also form parts of the
circulating system.
It therefore follows that the most important use of circulating water is
extracting high amounts of heat, particularly in the furnace, to keep the tubes
cool. This is only possible as long as the steam bubble formation on the inside
of tubes does not give way to a film of steam. In other words the departure
from nucleate boiling (DNB) does not set in.
It is important to remember that a boiler is not designed for circulation, but
for cooling the gases with ECON, evaporator, SH, and RH surfaces. It is then
checked for circulation.
Adequacy of circulation to prevent DNB is vital in all conditions of
operation—at all loads with all fuels and combinations. It means that the
velocities of steam–water mixture at all points are high enough to keep the
tubes wet with no DNB. This is the essence of circulation requirement, and
circulation check should be performed to verify that this condition is fully
met. Usually, changes to the supply and riser tube geometry that feed and
collect the water–steam mixtures, respectively, in the various circuits are
needed to remedy the defi ciencies. At times, other measures such as fitting of
ferrules, using ribbed/rifled tubes, and so on may also be needed.
2. Flow in Vertical and Horizontal Tubes
1. Tube A is bubble fl ow with low velocity and a few steam bubbles in a
predominant
water flow.
2. Tube B is emulsion fl ow where the steam bubbles increase and hence produce
froth.
3. Tube C is slug fl ow with fi ne bubbles coalescing to form big bubbles
almost fi lling the bore of the tube.
4. Tube D is wet wall fl ow where the steam fi lls the tube with an annular fi
lm of water cooling the tube.
5. Tube E is dry wall fl ow where the water fi lm is replaced by a thin steam
film that has poor cooling ability.
In a horizontal tube the fl ow patterns are different. Owing to the density
difference, all the steam bubbles migrate to the top of the tube and slide
along the tube wall.
1. At higher velocities (Tube A) of >1 m/s, the steam bubbles join together
and move
along with water, resembling wet water fl ow.
2. At low velocities (Tube B) of <0.5>250,000 kcal/m2 h or 92,000 Btu/ft2
h) cannot be avoided, particularly in the burner zone. There is a limit to
increasing the velocities at (higher) pressures >150 bar when the
circulation ratios are on the lower side.
Ribbed or rifl ed tubes are helpful in delaying the onset of DNB when compared
to smooth tubes, as they offer more wetted surface for adherence of water film.
The permissible steam by weight percentage (%SBW) for the same heat flux is
raised from a range of 20–40% level to a range of 70–90% level by the use of
ribbed tubes. Since they are expensive, they are employed around the burner
zone and mainly in high pressure and SC boilers.
To maintain wet wall flow or nucleate boiling under all conditions, the
following criteria must be satisfi ed for each circuit. A circuit is a set of
heated tubes of similar shape and heat input that allow upward fl ow of water.
1. Exit quality. SBW at the top of any circuit should be less than a specifi ed
limit depending on the drum pressure and the location of burners—whether at the
top or bottom—to prevent film boiling at the top of the circuit.
2. Minimum velocity. Water velocity at the commencement of the circuit should
exceed
a specifi ed limit, depending on the inclination of the tubes to prevent the
steam bubbles from adhering to the tube walls, causing overheating, and also to
prevent sludge accumulation.
3. Saturated water head (SWH). The SWH, the ratio of pressure loss (including
static head) to the pressure produced by a column of saturated water of the
same height, is required to be at a certain specifi ed minimum to prevent fl ow
reversal. The usual remedy for meeting this requirement is to increase the water
fl ow to the defaulting circuit.
