Explain the difference between fire-tube and water-tube boilers, focusing on their applications and operational characteristics.

The fundamental difference between fire-tube and water-tube boilers lies in which medium passes through the tubes: hot combustion gases in fire-tube boilers and water (or steam-water mixture) in water-tube boilers. This design difference dictates their respective applications and operational characteristics. In a fire-tube boiler, hot combustion gases from the furnace pass through tubes that are surrounded by water. The heat from the gases is transferred through the tube walls to the water, generating steam. Fire-tube boilers are generally simpler in design and less expensive to manufacture than water-tube boilers. However, they are limited to relatively low pressures and steam capacities, typically below 250 psi and 50,000 lbs of steam per hour. They are commonly used for smaller industrial heating applications, such as providing steam for process heating or space heating in buildings. In a water-tube boiler, water circulates inside the tubes, and hot combustion gases flow around the outside of the tubes. The heat from the gases is transferred through the tube walls to the water, generating steam. Water-tube boilers can operate at much higher pressures and steam capacities than fire-tube boilers, often exceeding 3000 psi and several million pounds of steam per hour. They are used in large power plants, chemical processing facilities, and other applications requiring high-pressure, high-volume steam. Water-tube boilers also offer better circulation and are more responsive to load changes than fire-tube boilers. Because the water volume is smaller compared to the steam generation rate, water-tube boilers generally require higher levels of water treatment and more sophisticated control systems. For example, a small dry cleaner might use a fire-tube boiler to generate steam for pressing clothes, while a large coal-fired power plant would use a water-tube boiler to generate steam to drive a turbine and produce electricity.