Boiler Heat Recovery — Name the Device Identify the device used to heat the feedwater by utilizing sensible heat in the exhaust flue gases before they leave through the chimney.

Introduction / Context:
Enhancing boiler efficiency requires recovering as much heat as practical from the flue gases. One key accessory in the gas path is designed specifically to preheat feedwater, reducing the fuel needed to raise it to saturation. Correctly identifying and applying this device is central to energy-saving retrofits and modern boiler designs.

Given Data / Assumptions:

• Hot flue gases exit the boiler at elevated temperature.
• Feedwater is cooler than saturation temperature.
• A heat exchanger is placed in the gas path before the stack.

Concept / Approach:
An economiser is a feedwater heater that transfers sensible heat from exhaust gases to incoming feedwater. By raising feedwater temperature, it reduces the enthalpy rise required in the evaporator and superheater, thereby decreasing fuel consumption. This is distinct from a superheater, which raises steam temperature above saturation, and from safety or control fittings like fusible plugs and stop valves.

Step-by-Step Solution:
Locate the device in the flue-gas train: typically downstream of the evaporator/superheater and upstream of the air preheater or stack.Understand duty: increase feedwater temperature without changing its phase.Name the device: economiser.

Verification / Alternative check:
Boiler heat-balance calculations show a direct reduction in stack losses and fuel use when an economiser is installed and properly controlled.

Why Other Options Are Wrong:

• Fusible plug: a safety device to prevent crown-sheet overheating.
• Superheater: heats steam, not feedwater.
• Stop valve: isolates steam flow; no heat recovery function.

Common Pitfalls:
Confusing economisers with air preheaters; the latter heat combustion air, not feedwater. Both may be used together for maximum efficiency.

Final Answer:
economiser