Copyright Goodheart-Willcox Co., Inc. 41 Learning Objectives After completing this chapter, you will be able to: ■ ■■ Identify the different types of boiler classifications. ■■ Differentiate the three boiler operating pressures. ■■ Describe the different types of fuels used for heating boiler water. ■ List the different types of materials used in boiler construction and the advantages and disadvantages of each. ■ ■■ Describe the difference between a wet-base and dry-base boiler design. ■■ Explain the difference between a fire-tube and water-tube boiler. ■■ Summarize what constitutes a condensing boiler design. ■■ Explain how latent heat is used in a condensing boiler. ■■ Describe how a “mod/con” boiler works. ■■ Define turndown ratio. ■■ Describe the advantages of an electric boiler. ■ Discuss the application of low NOx boilers. Technical Terms American Society of Mechanical Engineers (ASME) Annual Fuel Utilization Efficiency (AFUE) biomass condensing boiler cord dew point dry-base boiler fire-tube boiler flue heat exchanger heating load high-mass boiler high-pressure boiler hydrocarbon kilowatt (kW) liquefied petroleum (LP) low NOx boiler low-mass boiler low-pressure boiler medium-pressure boiler mod/con boiler modulating boiler Scotch Marine boiler turndown ratio water-tube boiler wet-base boiler B y definition, a boiler is an enclosed pressurized vessel in which water or other fluid is heated. The name boiler is deceiving. Some boilers are designed to simply heat water while others are designed to generate steam—only in the latter is there actual “boiling” action. The boilers discussed in this book do not actually “boil” but are used simply to heat water to a design temperature. The normal operating temperature of a hot water boiler can range from 90°F to 200°F. Boilers vary in size, shape, and complexity, Figure 3-1. Hot water boilers can be classified in several ways: ■ ■■ Operating pressure ■■ Type of fuel ■■ Construction material ■■ Wet- or dry-base ■■ Fire-tube or water-tube ■ Efficiency (conventional and condensing) Any particular boiler may fit a combination of these classifications, but for clarity’s sake, we discuss each quality separately as well as several other special types of boilers. 3.1 Operating Pressures Hot water boilers can be classified according to their normal operating pressure. The three common operating pressures are low pressure, medium pressure, and high pressure. It is important to remember that the water in a hot water boiler is not designed to actually boil, even though the operating temperature according to its classification may exceed 212°F. The boiling point of water at sea level is 212°F. As the pressure in some boilers (per classification) is increased, so is the water’s boiling point. 3.1.1 Low-Pressure Boilers By definition, a low-pressure boiler has an operating pressure of up to 160 psi and an operating water temperature of up to 250°F. However, most residential boilers are designed with a normal operating pressure of 12 psi and a normal operating