De-aerator
The de-aerator completes the air and vapour removal process in the condenser. It also functions as a feed heater, but in this case, it operates by direct contact. The feedwater is heated almost to the point of boiling, which releases all the dissolved gases which can then be vented off. The incoming feedwater passes through some spray valves or nozzles: the waterspray thus provides a large surface area for contact with the heating system.
Most of the feedwater will then fall onto the upper surface of the de-aerating cone where it is further heated by the incoming steam. The feedwater then enters the central passage and leaves through a narrow opening which acts as an eductor or ejector to draw steam through with the feed. The feedwater and condensed steam collect in the storage tank which forms the base of the de-aerator.
The heating steam enters the de-aerator and circulates throughout, heating the feedwater and being condensed in its turn to combine with the feedwater. The released gases leave through a vent connection and pass to a vent condenser or devaporiser. Any water vapour will be condensed and returned. Thedevaporiser is circulated by the feedwater before it enters the de-aerator.
The de-aerator feedwater is very close to the steam temperature at the same pressure and will if subjected to any pressure drop, 'flash off into steam. This can result in 'gassing', i.e. vapour forming in the feed pump suction. To avoid this problem, the de-aerator is mounted high up in the machinery space to give a positive suction head to the feed pumps. Alternatively, a booster or extraction pump may be fitted at the deaerator outlet.
Feed pump
The feed pump raises the feedwater to a pressure high enough for it to enter the boiler. For auxiliary boilers, where small amounts of feedwater are pumped, a steam-driven reciprocating positive displacement pump may be used. Another type of feed pump often used on package boiler installations is known as an 'electrofeeder'.
➤ Some Typical Marine Diesel Engine
This is a multi-stage centrifugal pump driven by a constantspeed electric motor. The number of stages is determined by the feed quantity and discharge pressure. Steam turbine-driven feed pumps are usual with high-pressure watertube boiler installations.. The two-stage horizontal centrifugal pump is driven by an impulse turbine, the complete assembly being fitted into a common casing.
The turbine is supplied with steam directly from the boiler and exhausts into a back-pressure line which can be used for feed heating. The pump bearings are lubricated by filtered water which is tapped off from the first-stage impeller. The feed discharge pressure is maintained by a governor, and overspeed protection trips are also provided.
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