TABLE OF CONTENTS Economizers for boilers have been available for nearly 150 years, almost as long as boilers themselves. For modern shell boilers, high efficiencies have made it increasingly difficult to justify the use of an economizer, the final decision being based in terms of payback period, which is also heavily dependent on fuel prices. Watertube boilers, on the other hand, need an economizer section in the gas passes in order to obtain satisfactory efficiency. For this reason, the economizer is integrated into the overall design, normally between the convective super-heater and the air heater if fitted.
In shell boilers with a working pressure of between 7 and 17 bar the temperature of the mass of water in the boiler is typically in the range of 170-210 C. Allowing for, say a temperature difference of 30-50 C between the exhaust gases and the water temperature, the boiler exit gas temperature cannot be economically reduced beneath about 200-260 C, dependent on the operating pressure. It becomes necessary, therefore, to modify the process principles to achieve further heat utilization and recovery. By using an economizer this is done by conducting the feed-water supply to the economizer wherein the exhaust gas passes over tubes carrying the feedwater. The feedwater, normally at temperatures between 30 and 100 C, represents a further cooling medium for the exhaust gases and provides the potential for the extra heat utilization. This is shown in Figure 31.1.
