7.2   Thermal/hydraulic design - part 1

 

Figure 3.56 shows a typical de-aerator and feedwater storage tank, the head of which heats and de-aerates the incoming condensate before it drains by gravity into the storage tank. The design uses spray nozzles to produce a fine film/spray to maximise the surface area of the water available to the steam for heat transfer and to minimise the distance that the oxygen has to travel to be released.

Any residual oxygen is released while the water is further heated as it passes over a series of perforated trays, which causes the condensate to fall as a continuous 'rainfall' from tray to tray.

The heat transfer coefficient in the fine film/spray zone is about five to ten times the value for the drop phase of approximately 14 kW/m2K. As nearly 90% of the temperature rise occurs in the film phase, the temperature difference between the water drops cascading from the lower trays and the steam is small. The additional trays, however, are needed to allow time for the residual oxygen to escape and also to heat the water to the saturation temperature equivalent to the prevailing pressure.

The steam flow path is shown by the arrows in Fig 3.56. A small flow of steam, along with the oxygen and non-condensable gases, is extracted by vents on the top of the head. The mixture of steam and oxygen, if vented directly to the condenser, would constitute a heat loss. To save this heat, a vent condenser is provided to heat the incoming feed by condensing the vapour and extracting the heat from the oxygen and non-condensable gases.

The oxygen and non-condensable gases are extracted from the vent condenser by venting it to the condenser, from where the gases are discharged by the main air extraction pumps.

The storage tank associated with the de-aerating function stores about 300 tonnes of feedwater in a tank of approximately 4.5 m diameter by 32 m long: it is accommodated on an elevated floor in an annexe between the boiler house and turbine hall.

Diffusers are provided to discharge the leak-offs and the HP drains into the tank, as indicated on Fig 3.56.

The steam coils used to boil and thereby de-aerate the tank content prior to unit start-up are also shown in Fig 3.56.

Unless provision is made to induce movement of the tank contents, stagnant areas of subcooled water will result. A distribution system is provided to prevent direct discharge of the incoming heated and de-aerated feedwater from the boiler feed pump suction pipe connection. By use of a distribution system, adequate mixing of the tank content is ensured.

Vertical baffles are also needed to prevent possible 'sloshing' of the tank content from end to end under conditions of abnormal steam flows across the tank surface. The transfer pipes between the tank and the de-aerator head are of generous size to allow flow of vapour to the head when the tank content boils due to a reduction in de-aerator tank pressure.

Section through a typical tray-only type de-aerator and associated storage tank

The de-aerating head shown in Fig 3.57 employs a different design philosophy, using trays only to obtain the de-aeration of the feedwater. The incoming feed is heated by a large vent condenser which contributes an appreciable proportion of the temperature rise over the heater. This reduces the heating required during transit through the trays.

The condensate is directed onto the top tray from where it cascades through the rest of the trays to flow by gravity into the storage tank. The steam/water flow paths are indicated on the figure.

The steam flow to the vent condenser is about 25-30% of the bled-steam flow: any oxygen and non-condensable gases are swept away in this vent flow. The vent condenser is vented to the main condenser, as indicated, and the gases are discharged from the main condenser by the air extraction pumps.

 

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