What type of boiler is classified as a Two Pass Boiler?

A Two Pass Boiler is characterized by the design that allows the flue gases to pass through the boiler tubes twice before being expelled. This design increases the heat transfer efficiency, as it allows for a greater surface area for the heat to exchange with the water.

The Horizontal Return Tubular (HRT) Boiler facilitates this two-pass configuration, where the combustion gases flow through the tubes in one direction and then are directed back through a return path for a second pass. This maximizes the heat extracted from the combustion gases before they exit the system, thus improving efficiency and performance.

In contrast, a Firebox Boiler typically has a more straightforward path for the flue gases and does not utilize a two-pass design. A Vertical Boiler employs a different arrangement, usually more compact and primarily designed for limited space and capacity without the two-pass functionality. A Natural Circulation Boiler refers to a boiler utilizing the natural convection of water, which can be independent of its pass configuration. Therefore, the Horizontal Return Tubular Boiler's design best fits the classification of a Two Pass Boiler due to its specific arrangement facilitating the two-pass flow of flue gases.