*Condenser
*Extraction pump
*Air ejector
*Heat exchangers
*De-aerator
*Feed pump
Condenser
An auxiliary condenser. The circular cross-section shell is provided with end covers which are arranged for a two-pass flow of sea water. Sacrificial corrosion plates are provided in the water boxes. The steam enters centrally at the top and divides into two paths passing through ports in the casing below the steam inlet hood.
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Sea water passing through the banks of tubes provides the cooling surface for condensing the steam. The central diaphragm plate supports the tubes and several stay rods in turn support the diaphragm plate. The condensate is collected in a sump tank below the tube banks. An air suction is provided on the condenser shell for the withdrawal of gases and vapours released by the condensing steam.
The main condensers associated with steam turbine propulsion machinery are of the regenerative type. In this arrangement, some of the steam bypasses the tubes and enters the condensate sump as steam. The condensate is thus reheated to the same temperature as the steam, which increases the efficiency of the condenser. One design of regenerative condenser.
A central passage enables some of the steam to pass to the sump, where it condenses and heats the condensate. A baffle plate is arranged to direct the gases and vapours towards the air ejector. The many tubes are fitted between the tube plates at each end and tube support plates are arranged between. The tubes are circulated in two passes by seawater.
Extraction pump
The extraction pump is used to draw water from a condenser which is under vacuum. The pump also provides the pressure to deliver the feed water to the de-aerator or feed pump inlet. Extraction pumps are usually of the vertical shaft, two-stage, centrifugal type. These pumps require a specified minimum suction head to operate and, usually, some condensate level control system in the condenser.
The first-stage impeller receives water almost boiling at the high vacuum conditions present in the suction pipe. The water is then discharged at a
slight positive pressure to the second-stage impeller which provides the necessary system pressure at the outlet. Where the condenser sump level is allowed to vary or maintained almost dry, a self-regulating extraction pump must be used.
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This regulation takes the form of cavitation which occurs when the suction head falls to a very low value. Cavitation is the formation and collapse of vapour bubbles which results in a fall in the pump discharge rate to zero.
As the suction head improves the cavitation gradually ceases and the pump begins to discharge again. Cavitation is usually associated with damage but no damage occurs at the pump's low-pressure conditions. Also, the impeller may be designed so that the bubble collapse occurs away from the impeller, i.e. super-cavitating.
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