*2.2.1 Fire-tube steam boiler*

 In this type of boiler, the flue gas passes inside submerged tubes in the water (**Figure 2**) [20]. These steam generators are widely used in industrial and commercial facilities, especially in the locomotives and marine applications. Modern fire-tube steam boiler can produce steam pressure up to 25 bars (low and medium pressure) and a flow rate of 1–25 t/h [19]. They can use natural gas, oil, or solid fuel.

The fire-tube steam boiler consists of a cylindrical tank, which contains tubes inside. These tubes collect the hot gases at the exit of the burner. Hot gases, accumulated in a first pass at the back of the steam boiler, are carried by a group of tubes submerged in water to a second pass at the front of the boiler. A second group of submerged tubes take the combustion gases to a third pass at the rear of the steam boiler; this third pass opens on the chimney for the evacuation of fumes to the outside. The heat transfer between the tubes and the combustion gases is mainly done by the convection mode. A typical example of this boiler is illustrated by **Figure 3**.

**Figure 2.**  *Principle of fire-tube steam generator.* 

*Numerical Simulation of the Accidental Transient of an Industrial Steam Boiler DOI: http://dx.doi.org/10.5772/intechopen.86129* 

**Figure 3.**  *Typical fire-tube steam boiler.* 

## *2.2.2 Water-tube steam boiler*

 It is a type of steam generator in which water circulates in tubes that are externally heated by flue gases (**Figure 4**) [19, 20]. They represent the vast majority of steam generators in service.

 These steam boilers are used in industrial and power plants to produce high steam pressure. They use gas, oil, or solid combustible as fuel [19]. A typical watertube steam boiler is illustrated by **Figure 5**. Generally, water-tube steam boilers have two or more tanks, the upper tank called collecting tank (drum) and lower tank called distributor tank. The hot gases produced by the burner are directly in contact with the evaporating tubes; inside of these, vaporization occurs. The steam thus generated is collected in the drum, and the excess water is returned to the bottom tank by non-heated pipes (downcomer). The heat transfer between the tubes and the combustion gases is mainly done by radiation. The flue gases can also be used in the preheating of combustion air and the feedwater.

The performance comparison of the two types of steam boiler is presented in **Table 1**.

**Figure 4.**  *Principle of water-tube steam boiler.* 

#### **Figure 5.**  *Typical water-tube steam boiler.*


#### **Table 1.**

*Comparison of the two types of steam boiler.* 
