Feedwater heaters

Some of the earlier BWR units had feedwater heaters having copper alloy tubes. The environment of high oxygen and neutral pH water led to high copper concentrations in the feedwater and to undesirable deposits on the fuel and inlet fuel nozzles (20). In some instances, the copper deposits resulted in an increase in core pressure drop and necessitated plant power reduction. The copper alloys were eliniinated from the feedwater system in subsequent plants and most existing plants. 

Simplified Cycle. A simplified fossil steam cycle appears in Figure 19. The water accumulates in the bottom of the condenser, called the hotweU. It goes through a feed pump to pressurize it. The pressurized water passes through one or more feedwater heaters, which raise the temperature. The water then enters the boiler where heat from the fuel converts it to steam. The steam expands through the engine, usually a turbine, which extracts work. In the middle of the turbine some of the steam is extracted to supply heat to the feedwater heater. The remainder expands through the turbine and is condensed. The rejected heat is carried away by the condenser coolant, which is usually water, but sometimes air. The condensed steam then returns to the... 

In fossil fuel-fired boilers there are two regions defined by the mode of heat transfer. Fuel is burned in the furnace or radiant section of the boiler. The walls of this section of the boiler are constmcted of vertical, or near vertical, tubes in which water is boiled. Heat is transferred radiatively from the fire to the waterwaH of the boiler. When the hot gas leaves the radiant section of the boiler, it goes to the convective section. In the convective section, heat is transferred to tubes in the gas path. Superheating and reheating are in the convective section of the boiler. The economizer, which can be considered as a gas-heated feedwater heater, is the last element in the convective zone of the boiler. 

Feedwater heaters also may be considered as BOP equipment (with the exception of economizers), especially in utility power plants. Blowdown receivers and flash steam and heat recovery (FSHR) also may be considered BOP equipment. 

Feedwater heaters of various types, sizes, and shapes have been employed since the earliest days of boiler plant design. Their primary purpose is to increase the thermodynamic gains of the steam cycle and reduce problems of thermal shock in the boiler. Typically, they may be classified as HP or LP, open or closed FW heaters. Some types of open FW heaters additionally provide FW mechanical dearation. 

Fuel Handling Area 11. Reactor Coolant Pumps (4) 21. High Pressure Feedwater Heaters... 

The pre-boiler equipment, consisting of feedwater heaters, feed pumps and feed lines, is constructed of a variety of materials, including copper, copper alloys, carbon steel, and phosphor bronzes. To reduce corrosion, the makeup and condensate must be at the proper pH level and free of gases such as carbon dioxide and oxygen. The optimum pH level is that which introduces the least amount of iron and copper corrosion products into the boiler cycle. This optimum pH level should be established for each installation. It generally ranges between 8.0 and 9.5... 

Si = Ssat.liq + Vsat.iiq-p- i — Psat By an energy balance on the feedwater heater ... 

At point 3 the pressure must be such that the steam has a condensation temperature in feedwater heater I of 195 degC, 5 deg higher than the temperature of the feed water to the boiler at point 1. Its saturation pressure, corresponding to 195 degC, from Table F.l, is 1399.0 kPa. The steam at point 3 is superheated vapor at this pressure, and if expansion from P2 to P3 is isentropic,... 

Now we can make an energy balance on feedwater heater I to find the mass of steam condensed ... 

The condenser and throttle valve The pump and feedwater heater... 

The high-pressure, feedwater heaters are prone to leaks at the tube-to-tube sheet joints. Lining or over-laying the tubesheet with Alloy 600 and using tube materials with 1% Cr / 0.5% Mo can mitigate this risk considerably88. 

The matter of different investment criteria for energy users versus regulated utilities is exemplified by comparison of the feedwater heater project (41) to the cement kiln heat recovery project referred to above. The utility chose to replace the de-... 

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