MULTIPLE PASS ECONOMIZER AND METHOD FOR SCR TEMPERATURE CONTROL

摘要

A gas temperature control system for maintaining a desired economizer outlet gas temperature across a range of boiler loads comprises a plurality of tubular configurations having surfaces that are in contact with the flue gas. Each tubular configuration, preferably, comprises a plurality of serpentine or stringer tubes arranged horizontally or vertically back and forth within the economizer, and each tubular configuration having a separate feedwater inlet. Heat transfer from the flue gas is accomplished by controlling the feedwater flow rates through the tubular configurations. In a temperature control system having two tubular configurations, the overall heat transfer capacity of the economizer may be reduced to maintain the desired economizer outlet gas temperature during low boiler loads by reducing feedwater flow through one tubular configuration and by overflowing the other tubular configuration, such that total flow of feedwater through the economizer is maintained substantially constant. Feedwater flow rate entering the economizer may be used to engage/disengage the economizer water internal proportion or bias system. Once engaged, the measured feedwater flow to the economizer is used to generate a proportioned or biased flow rate demand signal. The demand signal is then compared to the measured underflow proportioned or biased flow rate. If there is a difference between the demand signal flow rate and the measured flow rate, control valve(s) modulating the proportioned or biased flow is/are adjusted.