Whether we have or haven't is not so much of importance, because, if we want we can
modify the boiler so as to set aside one section of the tubes to heat the water before it is delivered to the boiler proper. Look at the sketch on the next page. Here we have added a separate bank of tubes through which the feed water passes before it goes into the boiler drum. This bank of tubes is placed in the path of the gases traveling, towards the air heater and the stack. Most of the heat in these gasses has been absorbed in the boiler tubes but not all of it. They probably still have a temperature of around 600 degrees. By making them travel through this added tube bank still more of the heat will be absorbed, consequently the economy of the boiler as a whole will be increased. Hence, this bank of tubes is known as an economizer. With this arrangement, the water is first heated to a temperature of around 212 degrees in the feedwater heater by the exhaust steam from the feed pump, and then, in the economizer, the temperature is further raised to a point not very far below the temperature of the water in the boiler.