The development of boilers is divided into two aspects: pots and furnaces.
In the first half of the 18th century, the steam engines used in British coal mines, including Watt's early steam engine, used steam pressure equal to atmospheric pressure. In the second half of the 18th century, steam at higher than atmospheric pressure was used. In the 19th century, the commonly used steam pressure increased to about 0.8 MPa. In line with this, the earliest steam boiler was a large-diameter cylindrical vertical pot shell containing water. Later, it was changed to a horizontal pot shell, and the fire was burned in the brick furnace body below the pot shell.
As the boiler gets bigger and bigger, in order to increase the heating area, a fire cylinder is installed in the pot shell, and the fire is burned at the front end of the fire cylinder. The smoke comes out from the back of the fire cylinder and is discharged to the chimney through the brick flue. The external heating of the pot shell is called a fire tube boiler. At first, only one fire tube was installed, which was called a single fire tube boiler or Cornish boiler. Later, two fire tubes were added, which was called a double fire tube boiler or Lancashire boiler.
Around 1830, fire tube boilers appeared after mastering the production and expansion technology of high-quality steel pipes. Some fire tubes are installed in the pot shell, forming the main heating surface of the boiler, and fire (smoke) flows through the tubes. Installing as many fire tubes as possible below the water storage line of the pot shell is called a horizontal externally fired backfire tube boiler. It consumes less metal but requires a lot of masonry.
In the mid-19th century, water tube boilers appeared. The heating surface of the boiler is the water pipe outside the pot shell, which replaces the pot shell itself and the fire tube and fire tube inside the pot shell. The increase in the boiler's heating area and steam pressure is no longer limited by the diameter of the pot shell, which is beneficial to increasing the boiler's evaporation capacity and steam pressure. The cylindrical pot shell in this kind of boiler was renamed drum, or steam drum. The early water tube boilers only used straight water tubes, and the pressure and capacity of straight water tube boilers were limited.
In the early twentieth century, steam turbines began to develop, which required boilers with higher capacity and steam parameters. Straight water tube boilers can no longer meet the requirements. With the development of manufacturing technology and water treatment technology, bent water tube boilers have emerged. Initially, the multi-pot type was adopted. With the application of water-cooled walls, superheaters and economizers, as well as improvements in steam and water separation components inside the drum, the number of drums is gradually reduced, which not only saves metal, but also helps improve the pressure, temperature, capacity and efficiency of the boiler. .
Previous fire tube boilers, fire tube boilers and water tube boilers were all natural circulation boilers. Water vapor flows naturally due to different heating conditions in the ascending and descending pipes, resulting in density differences. While developing natural circulation boilers, once-through boilers began to be used in the 1930s, and auxiliary circulation boilers began to be used in the 1940s.