Boiler feedwater preheat

The optimum feed temperature to a deaerator is the boiling point of water at the deaerator pressure. Typically, 230°F to 240°F. However, in many of the plants, BFW to the deaerator is heated to 160°F to 180°F by heat exchange with process pumparound or product streams. 

The effluent from the deaerator (230°F) will then typically flow to the boiler s convective section, ECONOMIZER, zone. This usually consists of four to six rows of tubes at the top of the convective section, where the BFW is heated to 350°F to 450°F. If the boiler is firing high-sulfur fuel oil (2 to 4 percent sulfur), this may be a problem. The dew point of H SO in the flue gas may easily be as high as 450°F. 

The 230°F water inlet may cause the flue gas temperature to drop below 450°E This will precipitate a weak H SO mist, which will aggressively corrode the fins or studs of the economizer tube bank. If low sulfur (less than 100 ppm HjS is being fired), H SO corrosion should not be a problem. 

For boilers firing fuel oil (bunker), it s best to preheat the BFW externally to perhaps 350°F to protect the boiler s economizer convective tubes from corrosion. If a small amount of vaporization occurs in the preheat exchanger, this should not compromise the operation of the economizer, provided that there is only a single inlet to the economizer section. 

To estimate the efficiency of fuel to steam, three factors are involved. 

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Taking the heat capacity of water to be 4.3 kJ kg K , heat duty on boiler feedwater preheating is... 

The profile of steam generation is shown against the grand composite curve in Figure 16.35a. The process can support both boiler feedwater preheat and steam generation, b. Maximum superheat temperature... 

If the demand for LPS exceeds the amount of exhaust steam available, the PC opens the pressure let-down bypass. FC-1 serves to minimize the bypass flow. If the pressure controller (PC) opens the bypass and the let-down flow rate exceeds the set point of FC-1, the previously inactive (saturated) FC-3 becomes active and starts cutting back the LPS flow to the boiler feedwater preheater and thereby reduces the plant s demand for LPS. This is an energy-efficient response, because the energy recovered from the LPS supplied to the feedwater preheater is less than the energy content of the HP steam that is needed to produce that LPS. 

The collected condensate is supplemented by fresh feedwater under level control (FC-11) and is sent to the boiler feedwater preheater (HE-1). 

The ammonia synthesis loop uses two ammonia converters with three catalyst beds. Waste heat is used for steam generation downstream the second and third bed. Waste-heat steam generators with integrated boiler feedwater preheater are supplied with a special cooled tubesheet to minimize skin temperatures and material stresses. The converters themselves have radial catalyst beds with standard small grain iron catalyst. The radial flow concept minimizes pressure drop in the... 

The flue gas heat content preheats reformer feed. Likewise, the heat content of the process gas is used to produce superheated high-pressure steam (5), boiler feedwater preheating, preheating process condensate going to the saturator and reboiling in the distillation section (6). 

Description Gas feedstock is compressed (if required), desulfurized (1) and sent to the optional saturator (2) where some process steam is generated. The saturator is used where maximum water recovery is important. Further process steam is added, and the mixture is preheated and sent to the pre-reformer (3), using the Catalytic-Rich-Gas process. Steam raised in the methanol converter is added, along with available C02, and the partially reformed mixture is preheated and sent to the reformer (4). High-grade heat in the reformed gas is recovered as high-pressure steam (5), boiler feedwater preheat, and for reboil heat in the distillation system (6). The high-pressure steam is used to drive the main compressors in the plant. 

The flue gas typically exits the radiant box at 1800 to 1900 . A waste heat recovery (WHR) unit is provided to recover heat from this gas. Typically, this consists of a package unit containing a reformer feed preheat coil, followed by a steam superheat coil (if applicable), followed by a steam generation coil, followed by a boiler feedwater preheat coil. If combustion air preheat is used, the air preheat unit t5 ically replaces the boiler feedwater coil. The flue gas typically exits the WHR unit at about 300°F. 

In order to limit thermal stresses and to reduce corrosion of the steel vessel, the reflector temperature was regulated near 350°F. About 50 kw of heat conducted from the fuel core to the reflector liquid was removed by circulating the heavy water with a 30-gpm canned-rotor pump through a reflector cooler which acted as a boiler feedwater preheater. A jet was located in this high-pressure circulating loop, the suction of which drew a continuous stream of gas from the vapor space above the reflector to a catalytic recombiner so that the concentration of deuterium and oxygen gases in this vapor space could be kept below explosive limits. 

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