TABLE OF CONTENTS The preboiler system of a typical utility steam generating unit (Fig. 2-1) includes a steam surface condenser, several closed tube-in-shell feedwater heaters, a deaerating feedwater heater, and sometimes an economizer. For industrial systems, feedwater heaters, with the exception of the deaerator, are often omitted unless the steam drives a turbine. The preboiler circuit condenses the turbine exhaust steam and prepares the condensate for return to the boiler. The condensation process significantly improves the efficiency of a unit, as is outlined in greater detail in Supplement 2-1 at the end of this chapter.
Preboiler/Boiler/Afterboiler Schematic
The potential for contaminant introduction to a steam generating plant is greatest in the preboiler system, especially at the condenser or via condensate return from an industrial process. This makes chemistry control and monitoring of condensate and feedwater extremely important.
A variety of materials have been used for feedwater heater and condenser tubes. The most common include 90 10 and 70 30 copper nickel, 304 stainless steel, carbon steel, Admiralty metal (70% copper and 29% to 30% zinc depending on whether 1% arsenic or tin has been added for increased corrosion resistance), and Monel (70% nickel, 30% copper). In previous years, copper alloys were widely selected due to the excellent heat transfer properties of these materials. Recently; as copper corrosion and its effects on downstream components have become better recognized, the use of copper alloys has declined. Stainless steel is becoming favored for feedwater heater tubes; and stainless steel, titanium, or...
