Steam Temperature Control

In package boilers, steam temperature is controlled typically from 60% to 100%. If a large load range is desired, then the superheaters may be accordingly designed. In some applications, steam temperature may be required to be maintained at even 10% load. Convective superheaters, due to their low tube wall temperature and heat flux at lower load, are preferred in such cases. There are numerous methods for steam temperature control.

1. Desuperheaters using demineralized water for spray between stages. It may be

noted that spraying downstream of the superheater is not recommended as mixing of water and steam may not be good, and if the steam is used in a steam turbine, it can damage the blades. In addition, this practice allows the steam temperature as well as the superheater tube wall temperature to run up hotter and then controls the final temperature. Figure 3.29 shows the arrangement of superheaters with intermediate spray. In case demineralized water is not avail- able, steam can be condensed and used as shown in b. A portion of the feed water from the economizer inlet is used to condense the required steam quantity in a separate heat exchanger as shown. This is called spraying with sweet water. The steam-side pressure drop in the first stage of the superheater and the pressure drop in the parallel path, namely, the heat exchanger plus control valve, has to be equal for the sweet water to be injected into the desuperheater. In order to have

a good margin for the control valve loss, the exchanger is located several meters above the desuperheater.

2. A portion of the steam between two stages of superheaters may be taken and cooled inside the drum and then mixed with the other portion to maintain the final steam temperature. This is shown in Figure 3.30. A heat exchanger located

126 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Heat exchanger

Control valve

Drum

TC Desuperheater

Control valve

Superheater 1 Superheater 2 (a)

(a) Interstage attemperator with spray water and (b) condensed sweet water.

inside the drum cools the superheated steam. The amount of steam to be cooled is determined by the final steam temperature, and control valves facilitate that. Chapter 6 illustrates the sizing of this type of exchanger located in the steam drum with an example.

3. One may use the combustion air to cool the steam between stages as shown in

Figure 3.31. Drainable superheaters should be used with this concept; else condensed water in the exchanger can cause flow problems. Finned tube bundle is preferred for the exchanger if its duty is large as the exchanger can be com- pact. Based on the final steam temperature required, the damper air flow is adjusted.

4. The superheaters may be located as shown in the flue gas path in parallel with

another heating surface such as economizer (Figure 3.32). Damper may be modu- lated to obtain the desired steam temperature at a given load. One should ensure that the superheaters are oversized so that bypassing flue gases lowers the steam temperature. This system is not seen in package boilers, but large utility boilers use the concept due to the large space availability in such designs.

5. Three-stage desuperheating may be adopted in some high steam temperature applications to lower the tube wall temperature before spray (Figure 3.33). If a two-stage desuperheater is used, the tube wall temperature ahead of the desuperheater can be much higher compared to the three-stage desuperheater. The three-stage desuperheater system, however, is more expensive and adds to the cost of controls. This system is generally seen in steam generators or waste heat boilers where a large amount of import steam is required to be superheated as discussed earlier or when sudden surges are expected in flue gas flow as in some kiln-based heat recovery boilers discussed in Chapter 4.

Steam Generators 127

Mud drum

Steam inlet

header

Steam outlet

Steam drum Control

attemperator

Primary superheater

Control

valves

Secondary superheater

Attemperator bypass

FIGURE 3.30

Steam flow through the drum exchanger is adjusted to maintain the final steam temperature.

Flue gas

Final steam

FIGURE 3.31

Combustion air cools steam in a finned exchanger for steam temperature control.

128 Steam Generators and Waste Heat Boilers: For Process and Plant Engineers

Flue gas Superheater

Dampers Furnace

Economizer

FIGURE 3.32

Gas bypassing using dampers to control final steam temperature.

Three-stage superheating with two attemperators. (a) Two-stage superheater with interstage de superheater. Max. tube wall temperatures are 547°C and 519°C in each superheater. (b) Three-stage superheater with two desuperheaters. Max. tube wall temperatures are 544°C, 498°C, and 419°C in each superheater. Spray water flow is higher.

6. It is desirable not to use desuperheaters beyond the final superheater and just before the steam turbine, though some consultants suggest this to lower the cost of the boiler. Plant engineers should avoid this scheme if possible. There are two main reasons for this. The steam temperature and hence the tube wall temperature will be high at certain loads (exceeding the desired final steam temperature) depending on whether the superheater is of convective or radiant design. Hence, the life of the superheater is impacted. Also, if the mixing of water and steam is not effective, water droplets can find their way into the steam turbine thus affecting its performance. Hence, interstage attemperation is always preferable to downstream attemperation, or one should have the performance evaluated at all operating loads and accept the variations in steam temperatures without desuperheater control.

Steam Generators 129

Steam Temperature Control