5   Designs of feedheaters

 

The aim of feedheater design is to provide a heat exchanger that will raise the temperature of the incoming feedwater to a specified outlet temperature, whilst drawing a predetermined amount of steam from the turbine cycle.

There are four groups of heat exchangers used in the feed system to fulfil this function:

  • High pressure feedwater heaters.
  • De-aerator heaters.
  • Low pressure feedwater tubed heaters.
  • Low pressure feedwater direct contact heaters.

The design of each type of heat exchanger is determined by the thermal, hydraulic and structural needs of its particular function in the feed cycle. The overall size of a heat exchanger is fixed by its thermal/ hydraulic design.

For a tubular heat exchanger to raise the temperature of the feedwater by a defined amount, the average overall heat transfer coefficient has to be found and used to determine the heat transfer surface area needed. From the tube size and feedwater flow, the tubenest diameter is found. With the tubenest diameter known, the heater diameter and the main structural details of the shell and water heater can be determined by use of the appropriate design standard. The length of the heater is a function of the heat transfer surface needed. A detailed explanation of thermal/hydraulic design is given in Section 6 of this chapter.

De-aerator head and DC heater sizes are determined by the need to accommodate the sprays or trays used to create sufficient water surface area to transfer the heat from the steam to the water, as needed by the cycle. The vessel diameter and length is determined by the need to contain the requisite number of sprays or trays.

There are long established British Standards for the structural design of feedheaters and these are used by the CEGB to specify a uniform design standard for all components, materials, welding and testing.

Until 1976, the standard used was BS1500 'Fusion Welded Pressure Vessels' supplemented by the ASME 'Boiler and Pressure Vessel Code', where appropriate.

In 1976 BS1500 was replaced by BS5500 'Unfired Fusion Welded Pressure Vessels' [11], which consolidated and expanded the previous standard. BS5500 has been used for all heaters ordered since 1976 and will be used for all future plant. For a specific heat exchanger size, the standard is used to determine the thickness of all pressure parts such as shell, tubeplate and water header, branches, etc. The materials, weld details and procedures, as well as relief valve capacity and vessel pressure testing, are also defined.

In certain specialist areas, such as heater internal design, the 'Standards of Tubular Exchanger Manufacturers Association (ТЕМА) 1978' [12] can also be applied.

The heat transfer coefficients are determined by a manufacturer's experience with a particular design of feedheater. However, a good approximation to the design values for surface type heaters can be obtained by reference to the 'Guide to the Design of Feed Water Heating Plant (BEMA) 1968' [13] or ТЕМА 1978' [12].

 

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