Which boiler type has water running through the tubes?

The water tube boiler is characterized by having water running through the tubes. In this design, water is contained within the tubes, while flue gases pass around them. This allows for efficient heat transfer as the combustion gases heat the tubes, which in turn heats the water inside. The design of water tube boilers is typically advantageous for high-pressure applications and allows for faster response to load changes due to the smaller volume of water in the tubes compared to fire tube boilers.

In contrast, fire tube boilers operate by having heat from the combustion gases pass through tubes that are surrounded by water. The water is on the outside of the tubes, which is the reverse of the water tube configuration. Cast iron sectional boilers consist of cast iron sections that contain water but do not focus on the tube design, and gravity-fed systems don't specifically pertain to water flow through tubes as seen in the water tube design.