Air ejector
The air ejector draws out the air and vapours which are released from the condensing steam in the condenser. If the air were not removed from the system it could cause corrosion problems in the boiler. Also, air present in the condenser would affect the condensing process and cause back pressure in the condenser.
The back pressure would increase the exhaust steam pressure and reduce the thermal efficiency of the plant. A two-stage twin-element air ejector. In the first stage, a steam-operated air ejector acts as a pump to draw in the air and vapours from the condenser. The mixture then passes into a condensing unit which is circulated by feedwater.
The feedwater is heated and the steam and gases are mostly condensed. The condensed vapours and steam are returned to the main condenser via a drain and the remaining air and gases pass to the second stage where the process is repeated.
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Any remaining air and gases are released into the atmosphere via a vacuum-retaining valve. The feed water is circulated through U-tubes in each of the two stages. A pair of ejectors are fitted to each stage, although only one of each is required for satisfactory operation of the unit.
Heat exchangers
The gland steam condenser drains cooler and low-pressure feed heater are all heat exchangers of the shell and tube type. Each is used in some particular way to recover heat from exhaust steam by heating the feedwater which is circulated through the units. The gland steam condenser collects steam, vapour and air from the turbine gland steam system.
These returns are cooled by the circulating feed water and the steam is condensed. The condensate is returned to the system via a loop seal or some form of steam trap and any air present Is discharged into the atmosphere. The feedwater passes through U-tubes within the shell of the unit. The drains cooler receives the exhaust drains from various auxiliary services and condenses them: the condensate is returned to the feed system.
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The circulating feedwater passes through straight tubes arranged in tube plates in the drain cooler. Baffles or diaphragm plates are fitted to support the tubes and also direct the flow of the exhaust drains over the outside surface of the tubes. The low-pressure feed heater is supplied with steam usually bled from the low-pressure turbine casing. The circulating feed is heated to assist in the de-aeration process.
The bleeding-off of steam from the turbine improves plant thermal efficiency as well as reduces turbine blade heights in the final rows because of the reduced mass of steam flowing. Either straight or U-tube construction may be used with single or multiple passes of feedwater.
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