Copyright Goodheart-Willcox Co., Inc. Chapter 3 Boilers 47 Fire-tube boilers contain tubes surrounded by water. The more commonly found type of boiler heat exchanger, fire-tube boilers are used in both residential and commercial applications, Figure 3-11. Combustion gases from the burner pass through the inside of the tubes, which heats the water. These tubes contain baffles, designed to slow down the speed of the flue gases as they pass through the tubes. Without these baffles, the hot flue gases would pass through the boiler too quickly, failing to transfer much of the heat from the gases to the water. By reducing the speed of the combustion gases through the tubes, boiler efficiency is increased. Each set of tubes makes several passes through the boiler before exiting through the flue outlet, located at the rear of the boiler. For example, a three-pass boiler will have three sets of tubes that pass back and forth within the boiler three times, Figure 3-12. The most common type of fire-tube boiler used for heating applications is known as the Scotch Marine boiler. This type of boiler Burner Burner Gas valve Gas valve Combustion gases Combustion gases Water Flue Water Flue Dry-Base Boiler Wet-Base Boiler Goodheart-Willcox Publisher Figure 3-9. In a dry-base boiler, the area under the combustion chamber is dry. In a wet-base boiler, the water that is heated is located throughout the combustion chamber. A B Anton Moskvitin/Shutterstock.com Figure 3-10. Steel boilers such as this (A) have bundles of welded heat exchanger tubes (B) that are submerged in water. Goodheart-Willcox Publisher Milen Mkv/Shutterstock.com Figure 3-11. An example of a fire-tube boiler.