Two solutions exist for recovering the waste heat from boiler flue gases. Conventional economizers preheat boiler make-up or feed water. Condensing economizers recover both latent and sensible heat from the flue gas, and are able to raise boiler efficiencies to over 90%.
Conventional stack economizers are more effective where a smaller volume of water needs to be heated to a higher temperature. They are commonly used to preheat boiler feed water. Conversely, condensing economizers are better suited to heating a larger volume of water to a lower temperature. They can be used to preheat process water, in addition to preheating boiler make-up or feed water.
Commonly-accepted rules of thumb estimate that conventional stack economizers can recover about 5% of input fuel energy, and condensing economizers can recover up to 10% of input fuel energy.
III Equipment Options
Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers. Condensing economizers are available as small as 50 hp, and a single condenser can be used on multiple boilers. Both direct contact and indirect contact units are available. Materials of construction need to be carefully considered for economizers operating in the condensing mode.
1. Equipment Manufacturer Database
2. Boiler Burner Consortium