Approximately how much hotter is the water that flows out of the economizer into the boiler?

• A. 50 F
• B. 100 F
• C. 150 F
• D. 200 F

Answer: (B)

The economizer uses heat from the hot exhaust to preheat the feedwater before it goes into the boiler, so the water leaving the economizer is noticeably warmer than the water entering it. In typical operation, the temperature rise is about 100 F, which is the common approximation used for this stage of heat recovery. The exact amount can vary with boiler load and the temperature of the flue gases, but 100 F captures the usual amount of preheating. A much smaller rise would indicate less heat recovery, while a much larger rise would require conditions outside normal operation.