The cross drum boiler is a variant of the longitudinal drum boiler in that the drum is placed cross ways to the heat source as shown in Figure 3.3.6. The cross drum operates on the same principle as the longitudinal drum except that it achieves a more uniform temperature across the drum. However it does risk damage due to faulty circulation at high steam loads; if the upper tubes become dry, they can overheat and eventually fail.
The cross drum boiler also has the added advantage of being able to serve a larger number of inclined tubes due to its cross ways position.
Bent tube or Stirling boiler
A further development of the water-tube boiler is the bent tube or Stirling boiler shown inFigure 3.3.7. Again this operates on the principle of the temperature and density of water, but utilises four drums in the following configuration.
Cooler feedwater enters the left upper drum, where it falls due to greater density, towards the lower, or water drum. The water within the water drum, and the connecting pipes to the other two upper drums, are heated, and the steam bubbles produced rise into the upper drums where the steam is then taken off.
The bent tube or Stirling boiler allows for a large surface heat transfer area, as well as promoting natural water circulation.
Advantages of water-tube boilers:
• They have a small water content, and therefore respond rapidly to load change and heat input.
• The small diameter tubes and steam drum mean that much higher steam pressures can be tolerated, and up to 160 bar may be used in power stations.
• The design may include many burners in any of the walls, giving horizontal, or vertical firing options, and the facility of control of temperature in various parts of the boiler. This is particularly important if the boiler has an integral superheater, and the temperature of the superheated steam needs to be controlled.
Disadvantages of water-tube boilers:
• They are not as simple to make in the packaged form as shell boilers, which means that more work is required on site.
• The option of multiple burners may give flexibility, but the 30 or more burners used in power stations means that complex control systems are necessary.
Combined heat and power (CHP) plant
The water-tube boilers described above are usually of a large capacity. However, small, special purpose, smaller waste heat boilers to be used in conjunction with land based gas turbine plants are in increasing demand.
Several types of steam generating land based gas turbine plant are used:
• Combined heat and power
These systems direct the hot exhaust gases from a gas turbine (approximately 500°C) through a boiler, where saturated steam is generated and used as a plant utility.
Typical applications for these systems are on plant or sites where the demands for electricity and steam are in step and of proportions which can be matched to a CHP system.
Efficiencies can reach 90%.
• Combined cycle plant
These are extensions to CHP systems, and the saturated steam is taken through a superheater to produce superheated steam. The superheater may be separately fired because of the comparatively low temperature of the gas turbine exhaust. The superheated steam produced is directed to steam turbines which drive additional alternators, and generate electricity.
The turndown ratio of these plants is poor, because of the need for the turbine to rotate at a speed synchronised to the electrical frequency. This means that it is only practical to run these plants at full-load, providing the base load of steam to the plant.
Because of the relatively low temperature of the gas turbine exhaust, compared to the burner flame in a conventional boiler, a much greater boiler heat transfer area is required for a given heat load. Also, there is no need to provide accommodation for burners. For these reasons, water-tube boilers tend to provide a better and more compact solution. Because efficiency is a major factor with CHP decision-makers, the design of these boilers may well incorporate an economiser (feedwater heater).
If the plant is ‘combined cycle’ the design may also include a superheater. However, the relatively low temperatures may mean that additional burners are required to bring the steam up to the specification required for the steam turbines.