Copyright Goodheart-Willcox Co., Inc. 48 Hydronic Heating: Systems and Applications was notably used for marine service on ships because of its compact size as well as its efficiency in operation and ability to work in any type of water. Advantages of fire-tube boilers include their simple design and the fact that they require less stringent water treatments. In addition, they are easy to clean, typically compact in size, and well suited for space heating as well as for indus- trial process applications. The disadvantage of these types of boilers is that they require extensive time to raise the water temperature due to the large ratio of water volume to heating surface. Because of this, fire-tube boilers do not respond quickly to changes in the building’s heating load. The heating load is defined as the heating requirement necessary to maintain a desired temperature in a building. This topic is covered in detail in Chapter 13, Building Heating Loads and Print Reading. In a water-tube boiler, water is contained inside tubes within the heat exchanger, and the combustion gases from the burner pass around the outside of these tubes. Heat from the flue gases is transferred to the water through the walls of the tubes, Figure 3-13. The more tubes added to a boiler, the greater vol- ume of heating surface, thus increasing the boiler’s volume and output. This design is the exact opposite of the fire-tube design. Water-tube boilers are used for both residential and commercial applications and can provide a faster response to heating load than fire-tube boilers. In addition, water- tube boilers are available in a greater range of sizes than fire-tube boilers and can reach very high temperatures. The disadvantages of water-tube boilers are higher initial capital costs, difficulty of cleaning, and their sometimes-daunting physical size. 3.3.3 Copper Copper-tube boilers are considered low-mass boilers because of their lightweight design and minimal volume of water they contain, Figure 3-14. The heat exchanger on a copper-tube boiler is constructed as a series of fins sur- rounding the tubing, Figure 3-15. These fins are designed to increase the surface area of the heat exchanger, thus DID YOU KNOW? The Most Famous Scotch Marine Boiler The Titanic had 29 boilers: 24 double-ended and 5 smaller single-ended. The larger boilers were 15′-9″ in diameter and 20′ long. The smaller were 11′-9″ in length. All had three corrugated furnaces of 3′-9″ diameter, 159 furnaces in total, and a working pressure of 215 pounds per square inch (psi). Chimney or flue Flue gases Water supply Water return Heat from combustion Fire tubes Goodheart-Willcox Publisher Figure 3-12. Fire-tube boilers have tubes that make several passes through the boiler before they exit through the rear of the boiler. Water return Heat from combustion Water supply Water tubes Chimney or flue Flue gases Goodheart-Willcox Publisher Figure 3-13. This illustration shows the configuration of a water- tube boiler.