What is the source point for gas in a gas recirculation system?

In a gas recirculation system, the source point for gas can indeed be any of the specified locations: the boiler outlet, economizer, or air preheater outlet.

Gas recirculation systems are designed to improve the efficiency of combustion by reintroducing exhaust gases back into the combustion chamber, thereby reducing the amount of fresh air needed and lowering emissions.

The boiler outlet is a common source because it contains flue gases that can be recirculated for further use or mixing with incoming air. Additionally, gases exiting the economizer, which preheats water before it enters the boiler, can also be utilized. Lastly, the air preheater outlet can serve as a source too, as it warms the incoming combustion air using waste gases, enhancing efficiency.

Since all these points can contribute gases to the recirculation process, the correct choice acknowledges that gas can be sourced from any of these locations, making "All of the above" the appropriate answer.