**2.1 Steam boiler history**

 The use of two-phase systems accompanied by a phase change to transform thermal energy into mechanical energy is old. It dates back to the first century with the invention of the aeolipile by the Greek mathematician Héron d'Alexandrie [17]. However, no practical system was built until the Italian architect and inventor Giovanni Branca designed a boiler. But, it is really only from the end of the seventeenth century that engineers developed modern steam machines. The first real steam machine was built by English engineer Thomas Savery in 1698; this machine was used for pumping water. The James Watt boiler, built in 1785, who was one of the first engineers to achieve the thermodynamic properties of steam, used the safety valve and valves to control the flow of water and steam in its boilers [18]. At the beginning of the nineteenth century, British engineer Richard Trevithick and American inventor Oliver Evans developed machines without condenser using high-pressure steam. Trevithick used this model steam engine to equip the first railway locomotive. Trevithick and Evans built road vehicles powered by steam [18]. The French engineer Marc Seguin (1781–1875) developed a fire-tube boiler, which in 1827 equipped George Stephenson's famous "Rocket" locomotive. The first improvement in Evans' boiler was the "Lancashire" fire-tube boiler patented in 1845 by British engineer William Fairbairn, in which the flue gases circulated through tubes inserted in the water tank, increasing the area through which heat could be transmitted. Fire-tube boilers, however, had limited capacity and pressure and sometimes presented a risk of explosion [17]. The first boiler with water tubes (**Figure 1**) patented in

**Figure 1.**  *First boilers with water tubes (Babcock and Wilcox).* 

 1867 by American inventors George Herman Babcock and Stephen Wilcox allowed a higher pressure than that of the fire-tube boiler [19]. In this boiler, the water passed through tubes heated from the outside by the combustion gases, and the steam was collected in a top drum. In the twentieth century, the water-tube boiler found wide applications due to advances such as high-temperature steel alloys and modern welding techniques, which made the water-tube boiler the standard boiler type for all high-capacity boilers.

## **2.2 Steam boiler classification**

 Steam boilers can be classified according to various parameters such as design (fire-tube or water-tube), depending on the support, circulation method of water, steam and water/steam mixture (natural circulation, forced circulation), and thermal power. These are fuel-steam generators; they consist of two separate compartments, one in which the fuel burns and the other in which the water circulates. But generally, they are classified in two categories: water-tube and fire-tube steam boilers.
