🔍 Key Concepts

• Natural circulation in water-tube boilers (what makes water/steam move without a pump) • Effect of temperature on water density and how that creates flow • Relationship between heated riser tubes and cooler downcomers in a boiler

💭 Think About

• Ask yourself: what physical property must change when water is heated in the tubes so that it will rise on its own? • Which of the choices would actually create a continuous loop of flow (up in some tubes, down in others) without any mechanical pump? • Think about where the hottest water/steam mixture is in the boiler versus where the coolest water is. What difference between those two regions makes circulation happen?

✅ Before You Answer

• Identify which option deals directly with a physical property of water that changes when heated or cooled. • Check which answer could explain why flow continues even if the area, angle, or height of the tubes stays the same. • Make sure the choice you pick would still cause circulation if the boiler were perfectly level and the tubes all had the same size, as long as there is heating in some tubes and cooling in others.