What type of boiler operates with water inside the tubes?

A water tube boiler operates with water inside the tubes, which is a key distinguishing feature of this type of boiler. In water tube boilers, water is contained within the tubes that are heated externally by the combustion gases. This design allows for higher pressure and temperature operation, making them suitable for power generation and various industrial applications.

In contrast to fire tube boilers, where the combustion gases pass through tubes surrounded by water, water tube boilers use the tubes to contain water directly, allowing for more efficient heat transfer and quicker response to load changes. This also results in a lower water content in the system, which contributes to faster steam production and improves safety and efficiency.

While low pressure boilers can encompass different designs, not all of them are specifically categorized by the water tube configuration. Electric boilers utilize electric heating elements, and their operation does not involve tubes in the same manner as water tube boilers. Thus, the characteristics and functionality of water tube boilers are what make this choice the correct answer.