boilers. The higher the operating pressure the higher the flue gas
temperature leaving the boiler. Economizers are not practical below 50 psig
(345 kPa) steam pressure.
c) Control of both feedwater and gas temperatures leaving the
economizer at varying loads.
1.75 to 2.75 inch O.D. Normally, bare tube or cast-iron-covered tube are
provided for coal or high sulfur oil. Finned tubes can be used for natural
gas, No. 2 fuel oil, gas or low-sulfur No. 6 oil.
Design Considerations. Minimum feedwater and metal temperatures
vary according to the percent sulfur content of the fuel. Flue gas
temperatures must be maintained above the acid dewpoint to prevent condensate
and corrosion (see Figure 37). Feedwater temperature must be held at least 50
to 75 degrees F (27.8 to 41.7 degrees C) below that of the boiler saturated
temperature to prevent steaming in the economizer.
Economizer Corrosion Controls. Under conditions where a sulfur-
bearing fuel is being fired and the feedwater is not of sufficient
temperature, a Feedwater Preheat System is to be incorporated into the
economizer design and become part of the installed package. The Feedwater
Preheat System is to be capable of elevating the inlet feedwater temperature
to the minimum required inlet temperature. The system is also to be capable
of controlling the exit stack gas temperature by the elevation of feedwater
temperature. The system shall be operational during all boiler loads. The
system shall consist of the following components: feedwater preheater,
feedwater control valve with feedwater temperature sensing device, and stack
gas control valve with gas temperature sensing device.
Heat Balance and Fuel Savings. Economizers will affect the overall
plant heat balance and require careful evaluation in its application.
Economic analysis and heat balance should be made to determine whether to omit
the economizer and heat the feedwater with exhaust steam and/or to install an
air preheater. Where economizers are installed as accessory or retrofit items
in the flue gas system, the recovered heat may be calculated and expressed as
an increase in overall steam generator efficiency and further evaluated.
Include the following accessories and equipment for
a) If sootblowers are required, economizer shall be designed with
tube spacing and depths of banks suited for cleaning with sootblowers. The
external surface may be cleaned with the same type of equipment used to clean
Temperature and pressure indicators on feedwater outlet.
Temperature and pressure indicators on feedwater inlet.