Page 116 - Introduction to Marine Engineering

Feed systems 103

a drains cooler for the heating steam supplied to the generator. From the
feed heater, the feedwater passes into the steam-to-steam generator.
Packaged feed systems are also available from a number of
manufacturers. With this arrangement the various system items are
mounted on a common base or bedplate. The complete feed system may
be packaged or a number of the items.

The condenser is a heat exchanger which removes the latent heat from
exhaust steam so that it condenses and can be pumped back into the
boiler. This condensing should be achieved with the minimum of
under-cooling, i.e. reduction of condensate temperature below the
steam temperature. A condenser is also arranged so that gases and
vapours from the condensing steam are removed.
An auxiliary condenser is shown in Figure 5.5. 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. Sea water passing through the banks of tubes provides the cooling
surface for condensing the steam. The central diaphragm plate supports
the tubes and a number of 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.
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 is shown in Figure 5.6. 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 sea water.

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.