UNIT 1. MARINE BOILERS
Boilers can be found on every type of a ship. A boiler is used to produce steam from feed water. Main boilers generate steam for ship’s propulsion and auxiliary boilers provide steam for the various ship’s needs.
The main parts of a boiler are: a furnace where fuel is burnt to release its energy; a drum where steam and water can separate; downcomer tubes which join the steam and water drums and are used to circulate feed water between them; an economizer which is used to heat the feed water; a superheater where steam is heated and ‘dried’ and water is converted into steam and an air heater which warms the combustion air before it enters the furnace.
UNIT 2. CLASSIFICATION OF MARINE BOILERS
Marine boilers are classified in different ways:
1. By the purpose they are designed for marine boilers are classified as main and auxiliary boilers.
The main steam boilers generate steam for the main engines, driving the propeller and for auxiliaries. The auxiliary boilers generate steam only for the auxiliaries, heating of oil and fuel tanks and other ship’s needs.
2. By their structure boilers are classified as the fire-tube boilers and the water-tube boilers.
In the fire-tube boiler, or a Scotch boiler, the gases flow through the tubes and heat the water that surrounds the tubes. In the water-tube boiler feed water passes through the tubes surrounded by hot gases.
3. By the principle of water and steam water mixture motion the boilers are classified as natural and forced circulation boilers.
4. By the way they are supplied with air boilers are classified as natural draft, forced draft and supercharged boilers.
UNIT 3. WATER-TUBE BOILER DESIGN
In the water-tube boiler the gases pass around a number of small diameter tubes through which steam-water mixture circulates.
The furnace is the place where air and fuel are mixed for the combustion that heats the water in the drums, tubes and headers. It is lined on the floor and walls with refractory material which prevents the heat loss due to radiation.
The furnace is surrounded on all sides by tubes. They provide the main heat transfer surfaces for steam generation.
Superheaters are essentially a bank of tubes used to heat the steam above the saturation temperature. Economizers are heat exchangers which are fitted in a boiler to increase the efficiency of the boiler. Air heaters are used to heat combustion air and to recover more of the heat energy from the exhaust gases.
There may be main and auxiliary water-tube boilers; low, intermediate and high pressure boilers; single-drum, two-drum and three-drum boilers; horizontal and vertical boilers and single-flow and double flow ones.
UNIT 4. FUEL OIL
The quality of fuel is determined by its elementary composition, calorific (heat) value, viscosity, specific gravity, flash point, ignition temperature, pour point and some other characteristics.
Fuel oil is composed of carbon, hydrogen, oxygen, sulphur and nitrogen.
The calorific value of fuel oil is the heat available from that fuel when it is completely burned. There are two values: Higher Calorific and Lower Calorific ones.
The practical method of determining the calorific value is by means of a calorimeter.
Viscosity is the resistance of oil to flow. Heavy oils usually have high viscosity and light oils — low viscosity. Viscosity is obtained by a viscosity meter.
Specific gravity is a relation between the weights of a definite volume of oil and water. The specific gravity is obtained by means of a hydrometer.
Flash point is the temperature at which the fuel is ignited with a small open flame.
Ignition temperature is the minimum temperature required to ignite fuel oil.
Pour point is the lowest temperature at which the fuel can be easily handled.
Such fuel characteristics as carbon content, ash content, water content, mechanical impurities, total acidity and alkali content are also very important.
Feed water is the water supplied to a boiler to generate steam or hot water. The impurities in the boiler remain in the form of the salt deposits. They may lead to the following problems: scale, foaming, priming and corrosion.
The chief salts forming scale are those of calcium and magnesium. Magnesium sulphate forms a soft, sludgy mass, which can be removed by blowing down. Calcium sulphate produces a hard brittle scale. Alkalis or phosphate are sometimes put into marine boilers to prevent scale.
Foaming may be the result of dirty water, certain types of organic matter, oils in solution, etc. Oil adheres to the tube surfaces and causes overheating and rupture of tubes.
Priming may be due to improper construction of boiler or extremely high ratings. It can be prevented by maintaining the water at the lowest level. If foaming and priming occur, the blow off is opened several times, and fresh water is added.
Dissolved gases, oxygen and carbon dioxide in feed water may react with the metals in the boiler and lead to corrosion. These contaminants should be controlled or removed by a deaerator.
For efficient boiler operation a chemical analysis of the feed water must be run at least once a day.
UNIT 6. THE FURNACE
A furnace provides steady burning of fuel.
The furnaces of marine water-tube boilers are lined with refractory firebrick backed up by the insulating bricks.
Water walls are used in water-tube boilers to contain the heat of the furnace. Water walls consist of a number of tubes and extend along the sides of the furnace setting.
To provide the efficient combustion process of burning oil fuel, a certain set of conditions is necessary: maintenance of high furnace temperature, provision of continuous oil fuel and air supply and removing products of combustion.
The oil is first heated in steam or electric fuel oil heaters. This reduces its viscosity and makes it easier to pump, filter, and finally to atomize. The heated oil passes through the burners where it is atomized; this process breaks it up into a fine spray of droplets presenting a very large surface area of oil to the combustion processes.