What are the two main types of boilers?

The two main types of boilers are classified primarily based on the method of heat transfer and the design of the heat exchange surfaces. Fire-tube and water-tube boilers represent these fundamental categories.

In fire-tube boilers, hot gases produced from the combustion process pass through tubes, which are surrounded by water. The heat from the gases transfers through the tube walls into the water, generating steam. This type of boiler is commonly used for heating and low-pressure applications, where it is effective and efficient at transferring heat.

Conversely, water-tube boilers have water contained within the tubes, and the combustion gases flow around these tubes. This design allows for a higher pressure and temperature operation, making water-tube boilers suitable for industrial applications and high-demand steam generation. Their construction typically leads to faster steam production and greater efficiency in heat transfer in comparison to fire-tube boilers.

The other options, while types of boilers or classifications based on efficiency or fuel type, do not represent the primary structural classification of boilers. Each of those categories—electric and gas boilers, condensing and non-condensing, high-efficiency and low-efficiency—are relevant in specific contexts but do not encompass the fundamental design variations that define fire-tube and water-tube boilers.