Steam power plant
Steam power plant is arrangement that converts the energy of fuel into mechanical energy. Steam power plant consists of a boiler, a turbine, a condenser a feed pump
The feed pump supplies water to the boiler. The function of the boiler is the formation of steam from feed water. There are two basic types of boilers in use in ships: water-tube boilers and tire-tube boilers.
Water-tube boilers have replaced fire-tube boilers for generating steam for main engines. There are two basic types of water-tube boilers: the main boilers and the auxiliary boilers. The main steam boilers generate steam for main egines,driving the propellers, and the auxiliaries. The auxiliary boilers generate steam only for the auxiliaries and other ships needs.
According to the pressure water-tube boilers are classed as low, intermediate and high pressure boilers.A water-tube boiler consists of water and steam drums, water-heating tubes, a furnace, a superheater, an air-heater, an economizer.The furnace is
used for combustion of fuel and generating hot gases. The hot combustion gases from the furnace pass upward.
Heat is absorbed by the water and steam is formed.
The function of the superheater is the formation of superheated steam from saturated steam.
The economizer is used for heating feed water.
The function of the air-heater is heating combustion air. The superheater; the economizer and the air-heater are heat-transfer surface which cool the exhaust gases to the minimum possible temperature and increase the efficiency of the boiler.
Steam from the boiler passes to the turbine where the heat energy of steam is converted into mechanical energy. Steam from the turbine passes to the condenser where it is converted into water. The cycle is repeated.
Refrigerating plants
Refrigerating plant is an arrangement for eliminating heat. Refrigeration itself is based on the relationships between pressure and temperature for a refrigerant.
The refrigerating machine consists of four main parts: the compressor, the condenser, the regulator, the evaporator.
The refrigerant gas is drawn into the compressor at evaporator or suction pressure. Then it is compressed to the pressure necessary to liquefy it. After that the refrigerant is discharged at this pressure into the condenser and in the condenser it is condensed into liquid form. The liquid is then metered into the low pressure side of the system through the regulator or expansion valve. Here it is evaporated in the cooling pipes arid grids and the resultant vapour is drawn again into the compressor.
Since the refrigerant returns to the compressor in its original form the cycle of operations is continuous and unchanged.
Diesel engines
Four-cycle and two-cycle diesel engines are commonly used in marine practice. They may be single or double-acting, trunk-piston type, crosshead type, opposed-piston type.
In the four-stroke diesel engines the first
stroke is called the suction stroke. The piston moves downward and a charge of air is drawn in into the cylinder through the open inlet valve.
The second stroke is called the compression stroke. The piston moves upward. The charge of air is compressed into the clearance volume. At the end of the stroke the fuel injection valve opens and finely atomized fuel oil is sprayed in the combustion volume under high pressure. The fuel oil is ignited and burns generating hot gases. Hot gases expand and push the piston down on the third stroke.
The third stroke is called the working (expansion, combustion, power) stroke. The piston moves downward. Near the end of the stroke the exhaust valve opens and part of the hot gases escape.
The fourth stroke is called the exhaust stroke. The piston moves upward. All the products of combustion are forced out. The exhaust valve closes, and the cycle is repeated.
In the two-cycle diesel engine instead of an exhaust valve there is a ring of exhaust ports. In place of air inlet valves there are scavenging ports. Here one complete cycle requires only two strokes of the piston or one revolution of the crankshaft.