7 De-aerating feedheaters
Water from the condenser (condensate) which has been heated by the LP feedheaters is supplied to the de-aerator to be further heated and de-aerated preparatory to being fed to the boilers by the boiler feed pump, via the HP feed train.
De-aerators on modern large units are designed to provide feedwater at exit from the storage tank with not more than 5 micrograms per kilogram (μg/kg) of oxygen.
De-aeration is achieved by the application of Henry's Law which states that the quantity of gas dissolved in a given quantity of solution is proportional to the partial pressure of that gas over the solution. When this law is applied to the removal of oxygen from feed water, where the atmosphere above and around the condensate contains no oxygen, then the dissolved oxygen will escape to that atmosphere in an attempt to achieve equilibrium.
To construct a de-aerating heater to release the maximum amount of oxygen from the incoming con¬densate, the following factors have to be considered:
- The time for the dissolved oxygen to travel to the surface of the water.
- A dwell time is needed for the steam to heat the condensate and so increase the equilibrium pressure of the dissolved oxygen in order to release it.
- The surface tension of the water.
- The time taken for the diffusion of the oxygen from the water into the steam atmosphere.
The storage tank associated with de-aeration has to meet the following needs:
- Store approximately 7-10 minutes' worth of CMR flow, as defined in Section 3.3 of this chapter.
- Accept the leak-off flows from the boiler feed pumps, when needed. At low loads, when the flow through the feed pump would be less than about 25%, a special leak-off system which discharges to the de-aerator tank is used. Chapter 4, Section 14.7 of this volume describes the system and how it functions.
- Accept the HP heater drains.
- Heat the tank content from cold to provide hot de-aerated water for unit start-up.
The following examples of different manufacturers' de-aerators show how they achieve these objectives by different design philosophies.