Learn & Understand

Catching the Escaping Heat: The Economizer

The single greatest loss in most combustion is the heat that flies up the chimney in hot exhaust. Engineers fight this waste with heat-recovery devices — chief among them the economizer — that intercept the flue gas on its way out and steal back some of its warmth, cooling the exhaust and boosting efficiency.

Heat Going Up the Stack

Flue gas leaves a boiler hot — and that heat, once it exits the stack, is lost forever. Since it can be the largest single inefficiency in a combustion system, capturing even part of it pays handsomely. The temperature of the exhaust is a direct measure of this loss: the hotter it leaves, the more energy escapes with it.

The Economizer's Job

An economizer is a heat exchanger placed in the flue gas path, downstream of the main boiler, that transfers heat from the departing exhaust into the incoming feedwater or air. By preheating what enters the boiler using heat that would otherwise be lost, it reduces the fuel needed and cools the flue gas — a genuine something-for-nothing, recovering waste heat before it escapes.

Cooling the Gas

As flue gas passes through such a recovery unit and gives up heat, its temperature drops — and that temperature drop can be calculated from how much heat is absorbed and the gas's flow and heat capacity. The cooler the gas leaves, the more heat has been reclaimed, so the exit temperature is a scorecard for how well the recovery is working.

How Cold Is Too Cold?

There is a limit to how far flue gas can be cooled. Chill it too much and the water vapor and acids within it condense, corroding ductwork and stack — unless the equipment is deliberately built to survive condensation and recover even the latent heat. So heat recovery walks a line: extract as much heat as possible, but keep the gas above the temperature where corrosive condensation begins.

Calculating Exit Temperature

To find flue gas temperature after heat recovery, use the Flue Gas Temperature Calculator. Check the condensation risk with the Flue Gas Dew Point Calculator, and the draft it produces with the Stack Draft Calculator.