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 flow. 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 deficiencies. 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 flow with low velocity and a few steam bubbles in a predominant

water flow.

2. Tube B is emulsion flow where the steam bubbles increase and hence produce froth.

3. Tube C is slug flow with fi ne bubbles coalescing to form big bubbles almost filling the bore of the tube.

4. Tube D is wet wall flow where the steam fills the tube with an annular film of water cooling the tube.

5. Tube E is dry wall flow where the water film is replaced by a thin steam film that has poor cooling ability.

In a horizontal tube the flow 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 flow.

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 rifled 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 satisfied 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 specified 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 specified 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 specified minimum to prevent flow reversal. The usual remedy for meeting this requirement is to increase the water flow to the defaulting circuit.