Economizers

Introduction

Flue gases from large boilers are typically 450 - 650°F.  Stack Economizers recover some of this heat for pre-heating water. The water is most often used for boiler make-up water or some other need that coincides with boiler operation.  

Stack Economizers should be considered as an efficiency measure when large amounts of make-up water are used (ie:  not all condensate is returned to the boiler or large amounts of live steam are used in the process so there is no condensate to return) or there is a simultaneous need for large quantities of hot water for some other use.

The savings potential is based on the existing stack temperature, the volume of make-up water needed, and the hours of operation.

Economizers are available in a wide range of sizes, from small coil-like units to very large waste heat recovery boilers.

Where economizers are installed, good deaerating heater operation is essential. Because oxygen pitting is the most common cause of economizer tube failure, this vital part of the boiler must be protected with an oxygen scavenger

How They Work

Boiler stack economizers are simply heat exchangers with hot flue gas on one side and water on the other. Or, in direct contact condensing units, the make-up water is in direct contact with the flue gases.

Economizers must be sized for the volume of flue gas, its temperature, the maximum pressure drop allowed through the stack, what kind of fuel is used in the boiler, and how much energy needs to be recovered.

Economizers designed for natural gas only, would likely plug-up if installed on a coal boiler and would face increased risk of corrosion if installed on an oil-fired boiler.

Some units are designed to keep the flue gases above condensation temperature, and others are made of materials that resist the corrosive effect of condensed flue gases.

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