Steam is used on ships for the following purposes:
- As working medium for driving steam turbines where steam turbines are used for propulsion.
- For heating of fuel oil (oh high viscosity) in tanks to reduce viscosity.
- For geysers in accommodation spaces (for domestic use).
- For air conditioning system.
- For heaters of purifiers and fuel oil service system.
- Other areas, depending on the type of vessels.
The boilers, where the steam generated is used for main propulsion plant, are called Main boilers. Boilers used for generating steam only for other auxiliary systems are called Auxiliary boilers.
Presently the cargo vessels and other large vessels are fitted with diesel engines for propulsion (because of their higher thermal efficiency) and hence on these vessels, the auxiliary boilers are fitted for producing steam required for the auxiliary system.
AUXILIARY BOILER PLANT:
The working principle and operation of the auxiliary boiler plant is as follows;
The feed water required for the boiler plant is stored in feed water tank. To reduce the hardness of water (which causes deposits in the pipeline) chemicals are added to the feed water.
The feed water pumps (rated at 12 bar discharge pressure for a working steam pressure of 7 bar) draw feed water from feed water tank and supply the same to the Auxiliary boiler. The same water is circulated by boiler water circulation pumps (Discharge pressure of about 3 bar) through the exhaust gas economiser and the steam/water mixture generated is re-circulated continuously between Auxiliary boiler and economiser. The steam generated in the process is collected in the steam space of the auxiliary boiler from where the steam is supplied to various consumers (systems).
The steam after supplying the heat to the consumers becomes hot water (condensate) and this water is cooled to about 50-60 degree Celsius in an Auxiliary condenser (using sea water as the cooling medium). This cooled water returns back to the feed water tank.
Only exhaust gas economiser is used for raising steam while the ship is on a voyage. When the ship arrives in a port, (when the main engine is not running and exhaust gases are not available) only the auxiliary boiler is used and the required heat is supplied by an oil burner.
The difference between Boiler & Economiser:
The economiser has no steam space and hence it is called an economiser and hence steam is not drawn to the consumers from the economiser.
The Aux-Boiler collects steam from economiser and steam space is provided in the Auxiliary boiler from where steam is supplied to various systems.
Dump valve and dump Condenser:
When the steam demand is less in the systems, the steam pressure inside the boiler tends to rise, as the pressure rises by 0.1 to 0.2 bar above the normal working pressure, the dump valve opens and the excess steam is dumped into the dump condenser using seawater as cooling medium and the condensate is returned back to the feed water tank.