In terms of Efficiency-
Economizer-Increase the temperature of feed water
Air pre heater(APH)-Increase the temperature of combustion air
In terms of corrosion in boiler-
Economizer-The corrosion in boiler is due to the dissolve oxygen in the water. These dissolve gases will come out of water at higher temperature, which will be taken care in the Economizer automatically.
Air pre heater (APH)-Corrosion cannot avoid just by placing air preheater
In terms of ambient condition-
Economizer-No effect
Air preheater (APH)- if atmospheric conditions change than result into the change in the volume of combustion Air, which will affect the efficiency of system.
In terms of repairing/maintenance Cost-
Economizer-High cost in comparison with air preheater
Air pre heater (APH)-Low cost in comparison with economizer
In terms of heat transfer coefficient -
Economizer-higher 50 to 300 W/m2-deg.C
Air preheater (APH)- lower 10 to 50 W/m2-deg.C
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