Question

Working difference between economizer and air preheater

Answer 1

1.economizer requires lesser space and surface area compared to the air heater due to the high tube side water heat transfer coefficient and larger U compared to the air heater. In addition,extended surfaces make them very compact. Air heaters have very low overall U and hence huge. (due to low air/gas side heat transfer coefficients) 2.Gas pr drop (and air pr drop) are much higher with air heater and hence adds to operating costs. Typically an economizer of a 200-250,000 lb/h has a gas side draft loss of 1 to 2 in wc, while an air heater can have a total air plus gas loss of 4 to 7 in wc, which means additional fan kw. 3.Corosion potential is minimum with an economizer if a suitable feed water temp is selected( above acid dew point), while corrosion is a concern in an air heater due to the lower tube wall temperatures. 4.NOx formation is much more with an air heater due to the higher combustion temp with air preheat. Today's package boilers use only an economizer as NOx is also a concern. 5.Heat flux in the furnace is higher with use of air heater. Air heaters are used when the fuel has a low heating value or has low volatile content and combustion is difficult-not in oil/gas fired boilers of today. They can have a lower stack gas temp compared to the economizer (due to the lower heat sink temp) but if operating costs are considered along with corrosion, certainly no in oil/gas fired boilers.
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