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This is an open area accommodating the flame(s) from the burner(s). If the flames were allowed to come into contact with the industrial boiler tubes, serious erosion and finally tube failure would occur.The walls of the furnace section are lined with finned tubes called membrane panels, which are designed to absorb the radiant heat from the flame.This part is designed to absorb the heat from the hot gases by conduction and convection.Large boilers may have several tube banks (also called pendants) in series, in order to gain maximum energy from the hot gases.The longitudinal drum boiler was the original type of water-tube boiler that operated on the thermo-siphon principle (see Figure 3.3.5).
Cooler feedwater is fed into a drum, which is placed longitudinally above the heat source. The cooler water falls down a rear circulation header into several inclined heated tubes. As the water temperature increases as it passes up through the inclined tubes, it boils and its density decreases, therefore circulating hot water and steam up the inclined tubes into the front circulation header which feeds back to the drum. In the drum, the steam bubbles separate from the water and the steam can be taken off.The Oil fired 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.A further development of the water-tube boiler is the bent tube or Stirling boiler shown in Figure 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 industrial 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 industrial boiler:
* 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. source:news cz-boiler
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