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Flue gas temperature sensors

Application message:

Heat Recovery from Flue Gas Steam Boilers. Without heat recovery, 20 to 40% of steam boiler energy consumption is lost through flue gas. With a heat exchanger, this loss can be reduced to 15%.Heat from flue gas can be recovered by three types of heat exchangers:

Economizer:
is a heat exchanger in the flue gas duct that operates at the highest temperature level. Flue gases are cooled there to about 130 °C and the heat can be used in another operating process. It is often used to preheat boiler feed water to tens of degrees above tap water temperature. The boiler feed water is passed through the economizer in the flue gas duct for this purpose.

Flue gas condenser:
is a heat exchanger in the flue gas duct that operates at a lower temperature level. Heat recovery takes place via condensation of the water vapor in the flue gases. Condensation of the flue gases takes place at temperatures below approximately 58°C (dew point). The heat can be used in another operating process but mostly for heating the make-up water for the steam boiler.

Air preheater:
is a heat exchanger specifically designed to preheat the plant's suctioned burner air. The burner normally draws burner air from outside air. The outside air averages about 12° on an annual basis and about 5°C during the heating season. This air can either be passed directly through the flue gas duct (air/air heat exchange for larger plants) or indirectly heated (water/air heat exchange for smaller plants). See measure Making steam with preheated combustion air for the use of hot air from the boiler house.

Notice:
Applying a fan in the flue may be necessary because the natural draft in the chimney is no longer sufficient. The recovered heat can be used to preheat tap water to 50 to 60°C. Under water supply decree 852/2004, it is mandatory that hot tap water does not contain legionella. Therefore, a minimum temperature of 60°C is required. The temperature of 60°C cannot always be achieved with heat recovery. Then the water must be additionally heated and the payback time becomes slightly longer. This measure is applicable if the hot flue gases are currently discharged outside. In addition, at least 2 meters of free space is required around the flue gas duct of the steam boiler to incorporate a heat recovery system.

Environmental aspects:
A natural gas-fired burner is present in the steam boiler. If the recovered heat from the flue gas is useful in the steam cycle, the steam boiler can save up to 15% in natural gas consumption.