A boiler in which water passes through tubes surrounded by hot combustion gases is called a __________.

A boiler in which water passes through tubes surrounded by hot combustion gases is referred to as a watertube boiler. In this design, water circulates through tubes that are heated externally by the combustion gases. This setup allows for efficient heat transfer and is particularly advantageous for high-temperature and high-pressure steam applications. The water contained in the tubes absorbs heat from the combustion gases, which leads to the generation of steam.

This type of boiler operates effectively under varying load conditions and is known for its safety features, including the ability to withstand higher pressures without risk of rupture. Additionally, watertube boilers are often used in larger industrial applications, making them a vital component in steam generation for processes requiring significant amounts of energy.

The other options, while related to boilers, refer to different designs or functions. For instance, a firetube boiler has hot gases passing through tubes that are surrounded by water, which is the opposite of a watertube boiler. Steam boilers generally refer to any boiler that produces steam; thus, it is a broader term. A condensing boiler is specifically designed to capture and reuse heat from combustion gases, typically to improve energy efficiency, but does not necessarily fit the description given in the question.