A feedwater heater is used in a conventional power plant to preheat boiler feed water. The source of heat is steam bled from the turbines, and the objective is to improve the thermodynamic efficiency of the cycle. The most common configuration of feedwater heater is a shell and tube heat exchanger with the feedwater flowing inside the tubes and steam condensing outside. (See Boilers and Shell and Tube Heat Exchangers.)
Figure 1 depicts the temperature profiles for a high-pressure feedwater heater which receives superheated steam extracted from a high-pressure turbine.
If sufficient superheat is available, it is possible to make use of the large temperature difference by specifying a separate section within the heater in which desuperheating occurs with a dry wall. This gives a higher heat flux than if condensation occurs, and also allows the possibility of raising the feedwater outlet temperature above that of the steam saturation temperature. The steam condenses almost isothermally, and the condensate is subcooled below the saturation temperature.
In the subcooling zone heater surface is assigned to extract heat from the condensate (drains) from the condensing zone.
A heater may have neither a desuperheating zone nor a drain cooling zone.
Figure 2 shows, in schematic form, the general arrangement of a three-zone heater. The shell contains a bundle of tubes (normally U-tubes). Two tube passes are almost always used. The feedwater inlet and outlet nozzles are connected to a channel on one side of the tube plate.
Figure 2. Typical airangemcnt of a three zone feedwater heater. (From Process Heat Transfer, 1994, CRC Press.)
In the condensing zone, the tubes are supported by plates or grids of rods. The desuperheating and drain-cooling zones are contained within the shell by a shroud or wrapper, and are usually well baffled to both support the tubes and promote a satisfactorily high shellside heat transfer coefficient. Sometimes other types of a baffle support, based on some form of grid or array of rods, are used to minimize the risk of tube vibration.
High pressure units are sometimes of the "header-type" construction. This is a specialized design in which the feedwater inlet and outlet headers take the form of separate cylindrical vessels which penetrate into the heater shell. Each tube is individually welded onto the headers, and the headers are welded to the shell. There are usually four tube passes.
Feedwater heaters can be located either horizontally or vertically. The horizontal orientation is more common, but vertical heaters are sometimes preferred.
A feedwater heater must be equipped with a vent to allow removal of non-condensing gases.
Thermal design of a feedwater heater requires an economic optimization of many factors, including material and operating costs.
Two publications which describe feedwater heaters, and their design, in some detail are those of BEAMA (1968) and HEI (1984). These documents provide performance charts which can be used to estimate the surface area requirement. However, a computer program is required to achieve an optimized design. The paper by Clemmer and Lemezis (1965) presents a design logic which is suitable for implementation in a computer program. Further background information can be found in the publication by EPRI (1984).
Special attention must be paid to avoidance of (a) wet-wall conditions in the desuperheating section, in order to avoid erosion/corrosion problems and (b) excessive pressure drop in the drain cooler, which could cause flashing, and consequent tube damage.
Pressure loss in the desuperheating zone causes a reduction in the saturation temperature of the steam condensing zone. This in turn causes a reduction in the temperature difference in the condensing zone. Design of the two zones is therefore a compromise between the need to maintain a high heat transfer coefficient in the desuperheating zone, while avoiding an excessive reduction in the overall mean temperature difference.
BEAMA (1968) Guide to Design of Feedwater Heating Plant, The British Electrical and Allied Manufacturers' Association Ltd., London.
EPRI (1984) Symposium on State-of-the-art Feedwater Heater Technology, Report No. CS/NP-3743, EPRI, Palo Alto, California.
Clemmer, A. B. and Lemezis, S. (1965) Selection and Design of Closed Feedwater Heaters, ASME Paper 65-WA/PTC-5, ASME. Winter Annual Meeting, Chicago, November 7-11, (1965) ASME, Vol. 79, No. 7, 1494-1500.
HEI (1984) Standards for Closed Feedwater Heaters, 4th edition. Heat Exchange Institute, Cleveland, Ohio.
Hewitt, G. F, Shires, G. L., and Bott, T. R. (1994) Process Heat Transfer, CRC Press.