TYPES OF BOILERS

DIFFERENT TYPES OF BOILERS

1. A steam turbine had been the popular prime mover of the past to propel the ship.
2. As the fuel price raised up during 1973,it put an end to the rapid manufacturing of steam turbine engines.
3. As a result of this fuel efficient Diesel engine(motor ships) largely replaced steam turbine engines.

The diesel vessel still required some steam supply for auxiliary purposes:

1. Heating duties: ME fuel oil heater, purifier heater, oil tank heating, Air conditioning, sea chest.
2. Run turbine generator
3. run cargo pump turbines in tanker
4. Drive steam driven deck machineries like windlass etc
5. operate bilge, stripping and other steam driven pumps
6. Tank washing in tanker ships and general cleaning
7. For boiler shoot blowing
8. Fire fighting
9. ME jacket fresh water pre-heater
10. ME lube oil sump heater

BOILER:

• It is a closed pressure vessel in which water is heated to produce steam.
• A marine auxiliary boiler is designed to suit marine environment.
• Correct functioning of the boiler and uninterrupted steam supply is critical for main engine, auxiliary engine and generators.

CLASSIFICATION OF BOILERS:

This classification is based on various criteria. Lets study one by one

CAPACITY:

1. High capacity: 100000 kg/hr and above (for steamships)
2. Low capacity: 1000-100000 kg/hr (for motor ships)

For motor tankers 20000-80000 kg/hr

PRESSURE:

1. High pressure: 60 bars and above (for steamships)
2. Medium pressure: 17 bars to 58 bars (for motor tanker ships)
3. low pressure: 6 bars to 15 bars (for motor ships)

RELATIVE POSITION OF WATER AND HOT/FLUE GAS:

1. Water tube: Hot gases of combustion passes through the tubes and water surroundings the tube.
2. Fire tube: If water passes through tubes and hot gases are surroundings the tube.

USAGE:

1. Main propulsion boiler: for steam ships
2. Auxiliary boiler: for motor ships

TYPES OF FUEL USED:

1. HFO and gas: steam ships
2. Light diesel oil, HFO, gas: motor ships
3. Other types include: coal, electricity, exhaust gas

Modern marine boilers also uses electricity to produce steam

CIRCULATION TYPE:

1. Natural: for steam ships and motor ships
2. Forced: for motor ships

SHAPE:

1. Cylindrical
2. vertical
3. Tank type

TYPES OF BOILER
MAIN BOILER: 

• These are for the propulsion of the ship. 
• They are high pressure water tube boiler.
• Produces pressure beyond 100 bars and temperature 513 degree C

AUXILIARY BOILER:

Supports the propulsion in some way like heating.

DONKEY BOILER

• A boiler used for the hotel needs of the ship like ..e.g supplying hot water to the galley.

TANK BOILER:

• A boiler having large water carrying capacity. These are low pressure auxiliary boiler.

VERTICAL BOILER:

• Any boiler where the shell is upright and furnace is usually contained within the shell at lower half.

HORIZONTAL BOILER:

• This is also referred as cylindrical boiler.
• Boiler cylindrical shell lies across its length parallel to the structure of the ship or ground level.

EXHAUST GAS BOILER:

• Boiler operated by hot gas from engine or other exhaust source.

DRUM TYPE BOILER:

• They are water tube boiler.
• It contain one water drum and one steam drum.
• They are also known as bend tube boilers.

PACKAGE BOILER:

• Fully automatic, low capacity boiler packaged inside a box type casing.
• Capable of quick steam production and flexible in being positioned anywhere.

DIFFERENCE BETWEEN FIRE TUBE AND WATER TUBE BOILER:

Weight 

The relative weight of fire tube to water tube boiler for equivalent heating surface area, with water at working level, is approximately 3:1.

Space:

High outputs from water tube boilers of very small dimensions when compared to the fire tube type due to good circulation and ability to withstand force.

Mechanical Flexibility

The water tube boiler is not so sensitive to fluctuating pressures.

Steam rising

• Steam can be raised at a faster rate in water tube boiler than in a fire tube boiler.
• A normal specified time for raising steam in a water tube boiler is 4 to 6 hours from flashing to full pressure.
• The minimum time required depends upon the initial temperature of the boiler.

Safety 

• There is a rare possibility of serious explosion in a water tube boiler.
• The tube diameters are wisely limited and drums are protected from direct radiation or flame impingement.
• In case of a tube failure, the boiler content escapes at a rate determined by the tube bore. In a fire tube boiler, serious rupture of an overheated furnace instantaneously releases entire feed water content.

DIFFERENT TYPES OF AUXILIARY BOILERS:

1. CYLINDRICAL SCOTCH BOILER

• Design is 200 years old.
• Found on ships during 19th century.
• Fire tube boiler is robust and can be used with not-so-good quality feed water.
• Now welded construction.

Main components of fire tube boilers:

SHELL: 

• It is outer plating of the boiler.
• Depending upon the space it can be arranged horizontally and vertically.

FURNACE AND COMBUSTION CHAMBER : 

• It is an enclosed space for the fuel combustion.
• Adjacent to furnace is the combustion chamber.
• Combustion of fuel starts in the furnace and completed in the combustion chamber.

TUBE PLATES:

• Tubes are fitted on both ends of the tube plates.
• They hold the tubes in position.

FIRE TUBE:

• Carry flue gas from combustion chamber to boiler uptake.
• They support boiler front tube plate and combustion chamber plate.

STAY TUBES:
Combustion chamber stays provides support to front and back plating of combustion chamber.
Steam space stay provide support to front and back plating of boiler.
Girder stays provide support to the top of combustion chamber.

DESCRIPTION:

1. Scotch boiler consists of a cylindrical shell containing furnaces.These are about 1 m in diameter.The number of furnace depends on the diameter of the boiler.
2. Two furnaces are usually fitted for boiler shell having diameter up to 4 m while 3 for diameter exceeding this value.
3. Furnace is water cooled.It is constructed in the form of corrugated cylinder.It provides additional strength, increase heat transfer, higher endurance.
4. After leaving the furnace hot gas enter the combustion chamber.Water level should be maintained up to the the top of combustion chamber otherwise this would lead to overheating and distortion.
5. From combustion chamber gases pass through smoke tubes.
6. Internal access is provided by means of top manhole in the shell and by lower manhole.
7. Refractory prevents boiler shell from overheating and distortion.

2. COCHRAN VERTICAL BOILER:

1. It became popular on board from the time the diesel ships became a common sight.
2. It is a tank type boiler of vertical type suitable for producing relatively small amount of low pressure steam for auxiliary purpose.
3. The shape of furnace is hemispherical
4. The product of combustion pass from furnace into combustion chamber lined with refractory and then to smoke tubes then to smoke box
5. It does not require stays.
6. It can also be adopted for use as exhaust gas boiler.It became famous on cargo ships.
7. This type of boiler forms a robust unit and use open feed system and poor quality feed water.
8. Internal access are provided by a manhole at the top while handholes in the lower section of the shell.
9. Hinged smoke box doors give access to the tubes and tube plate in the front while a removable rear panel fitted to the combustion chamber gives access to the back tube plate.

3. COCHRAN SPHEROID BOILER:

1. It is a Japanese design.
2. Previous one was the riveted boiler with hemispherical furnace attached to the boiler shell by means of ogee ring connection. This has been replaced by welded design.
3. The furnace is water cooled and of spheroid shape.

4. PACKAGE BOILER:

1. They require steam pipes, water pipes, fuel supply, electrical connections and can be made ready almost immediately. 
2. They are suitable for smaller ships , where steam demand is less, but steam is needed quickly.
3. They come both in water tube and smoke tube.The one explained here is smoke tube boiler.
4. Hot gases from furnace passes to combustion chamber then enters the FIRST PASS of smoke tubes which conduct them to a FRONT CHAMBER. From here they are directed through a SECOND PASS of smoke tubes to a smokebox .The gases then leave the boiler.

5. SPANNER VERTICAL SMOKE TUBE BOILER:

1. It has unique tube designs where heat transfer efficiency was improved by creating a swirl of gas flow.
2. They have a special twist along the greater part of their length, only a short ortion at each ends being left plain to allow for expansion.
3. This is a vertical smoke tube boiler with better efficiency then cochran design.
4. Furnace is water cooled.
5. Only the floor is covered by refractory.
6. Gas from furnace passes to smokebox vis vertical smoke tube.