CLOSED FEEDWATER HEATER

Closed feedwater heaters are shell-and-tube heat exchangers. They are basically small condensers which operate at higher pressures than the main condenser because bled steam is condensed on the shell side, whereas the fcedwater, acting like circulating cooling' water in the condenser, is heated on the tube side. 

It was shown in Chapter 2 that the temperature rise in each heater and cconomiser is equal for maximum cycle efficiency. Thus the heaters receive bled steam from the turbine at pressures determined roughly by equal temperature rise from the condenser to the boiler saturation temperature. They are classified as low pressure (LP) and .high rrcssttr~ (HP) heaters depending upon their locations in the cycle. The LP heaters arc usually located between the condensate pump and the deacrator, which is followed by the boiler ked pump. (BFP). The HP heaters are located between the BFP and the eeonomiscr, 

When bled steam entering H fccdwater heater is superheated, as in a HP beater, the heater includes a dcsupcrhcating zone where steam is cooled to its saturation temperature. It is followed by a condensing zone where the steam is condensed to a saturated liquid rejecting the latent heat of condensation. This liquid, called heater drain, is then cooled below its saturation temperature in a subcooling zone or a drain cooling zone before the drain is cascaded backward or pumped forward. 

Figure 8.13 shows the schematic diagram and the temperature profiles of a three-zone closed feedwater heater. There are, however, two-zone heaters that include a desuperheating and a condensing zone or a condensing and a subcooling zone. There are also single-zone heaters that include only a condensing zone. A drain-cooling zone, instead of being a part ofthe shell, may be located outside it. It is then called a drain cooler.