Marine Boilers are one of the most important machinery onboard a ship. It is a device which produces steam for main propulsion or for auxiliary machinery.
To be precise, it is a closed vessel containing water or other liquid which is heated to produce steam which can also be used for cleaning, and heating of the sea water in the fresh water generator. Therefore, evaporate it to make it potable water. Steam generator is the other name for marine boiler; as it converts water into steam.
You will never see a ship without a boiler. In some ships, there are more than two boilers; especially for a steam powered ship which requires high temperature and high pressure steam. Meanwhile, ships carrying diesel engines as main propulsion systems, one or two smaller boilers can also run different ship’s machinery.
Marine Engineering Working of Boilers
The main work of a boiler is to produce high pressure steam. The feed water provided to the boiler drum exploits the heat of the energy by burning the fuel. The energy of the burning fuel is kept in reserve in the form of high temperature and pressure.
In the combustion chamber, inside the boiler, burning of fuel takes place and to achieve efficiency and combustion, air is supplied to this combustion chamber. The heat generated in the combustion chamber is then transferred to the water via a large surface area which allows the highest rate of energy transfer.
Types of Marine Boilers
There are two basic types of marine boilers, which are used in the maritime industry, namely:
- Water Tube Boilers
- Fire Tube Boilers
Water Tube Boilers
In the Marine Engineering concept, a water tube boiler is a type of boiler in which the water is heated inside tubes and the hot gases are around them. To be precise, the water is circulated in the tubes which is then externally heated by fire.
By the usage of this boiler, more steam is generated and the overall efficiency of the boiler is high. Water tube boilers are widely used as design of the boilers are quite suitable for application purposes. Water tube boiler consists of two drums, a set of water tubes, a feed water channel line and a combustion gas vent stack.
Parts of Water Tube Boiler
- Boiler shell: It is the external cylindrical part of a pressure container.
- Mud drum: It is a drum which is beneath a boiler and accumulates impurities such as mud and sediment.
- Steam drum: Collection vessel of water and steam.
- Strainer: It is used as a colander to hold up the solid elements while allowing the liquid supply.
- Water walls: It is used to extract heat from fuel to generate steam.
- Sight glass: On steam type boilers, a glass tube is used to give noticeable signs of water level in boilers.
- Furnace: The main aim of furnace is to give sufficient space for fuel particles to burn completely and to cool the flue gas to a temperature at which the heating surfaces can be operated safely.
- Safety valve: They are used to stop redundant force from the construction of a boiler.
- Feed check valve: The high force water flows along this valve, which releases to the boiler and supplies the water to the water type boiler.
- Steam stop valve: The steam flow supplied outside is controlled by steam stop valve.
- Burner: It is a type of device which is used for the outset of air and fuel together in the boiler at the preferred velocity. Burner is the pivotal equipment for the firing of oil or gas.
- Superheater: Used to increase the temperature of the steam.
- Economizer: They are the heat exchangers which are used to exchange heat with the fluid, normally water. They are also used to recover the heat of flue gas leaving the boiler by heating feed water.
Working of Water Tube Boiler
Thermal siphoning is the working principle of a water tube boiler. In this boiler, there are two drums, specifically steam, lower or mud drums. These two drums are connected through two tubes such as riser and downcomer.
Hot gases are generated when the fuel is burned which allows the hot gases to supply in the shell part of the boiler. The hot gases generated by the fuel will replace heat by water and the water gets changed into steam.
Increase in concentration takes place when the temperature increases. Understandably, the concentration of the steam will be less than the water. Hence in the steam drum, water and steam, gets split due to dissimilarity in concentration. The steam will flow upwards because of low concentration and the water will travel downward due to high concentration.
Water Circulation In Water Tube Boiler | Image Via: store.chipkin.com
Types of Water Tube Boilers
There are basically three types of Water Tube Boilers, namely:
Simple Vertical Boiler
A Simple Vertical Boiler is a type of boiler which generates steam at low pressure and in small quantities. It produces a pressure of almost 10 bar. Out of all the boilers, construction of this boiler is simple and straightforward.
Simple Vertical Boiler | Image Via: 4mechtech.blogspot.com
Stirling Boiler
The Stirling boiler is a bent tube, externally fired, stationary water tube boiler with natural circulation. It has a pressure range of maximum 60 bar and a steam production of about 50000 kg/hr. Efficiency of the Stirling boiler is up to 60% to 80%.
Stirling Boiler | Image Via: 4mechtech.blogspot.com
Babcock and Wilcox Boiler
Babcock and Wilcox Boiler is a horizontal drum axis, natural draft, natural circulation, multi-tubular, stationary, high pressure, solid fuel fired, externally fired water tube boiler, used to produce high pressure steam in ships for the function of other machineries and the production of electricity.
Babcock and Wilcox Boiler | Image Via: researchgate.net
Fire Tube Boilers
A fire tube boiler is a type of boiler in which hot flue gases flow inside the tube and water is surrounded by these tubes. Compared to water tube boilers, these are the low pressure boilers that cannot generate that amount of heat. Because of the less heat production, the overall efficiency of this boiler is lower. Fire tube boilers are one of the basic and old-designed boilers.
These types of boilers were famous in the 18th century, particularly appropriate for train engines. The combustion of heat and gas flows through the pipe surrounded by water. Most of the time, these boilers are considered for pressures up toward a highest of 250 psi and around 750 HP(Horsepower). The measurement of these boilers are done with their external diameter.
Parts of Fire Tube Boilers
- Safety valve: Safety precautions must be taken to prevent any loss; as the water at high pressure contains a lot of energy. Safety being the reason, safety valve must be installed to the boiler. When the pressure is above the specified limit, then the safety valve will be opened. A minimum of two safety valves are to be provided.
- Steam stop valve: In accordance with the requirement, a stop valve is provided to handle the steam control while the wet steam in the shell is transmitted to the superheater to reheat. The steam which was reheated is again transmitted to the shell in these circumstances; whereas the steam stop valve is operated to be sufficient enough.
- Feed check valve: Feed water is entered into the boiler through feed check valve and in addition to stop the feed water from returning, it is used as a non-return valve. To increase the efficiency of the boiler, following accessories are provided:
- Economizer
- Air preheater
- Superheater
The efficiency of the boiler will be low if operated without accessories.
- Pressure gauge: Pressure gauge is situated on the top of the shell. It shows the amount of pressure inspected inside the boiler. It indicates the pressure in various units like kgf/cm2 or K.pa or PSI. Presently, digital type instruments are used which senses the pressure by transducers and the signal is sent to the display unit.
Image Via: researchgate.net
- Water level indicator: The operator should be aware of the water level throughout the operation of the boiler. Otherwise, the boiler cannot handle the pressure and can cause an explosion. Hence, to avoid these problems, a water level indicator must be mounted to the shell of the boiler.
Types of Fire Tube Boilers
Cochran Boiler
Cochran Boiler is a fire tube boiler in which coal or gases are used as a working fluid, to generate steam and the steam is further used for various purposes. It is a multi-tubular vertical fire tube boiler having numerous horizontal fire tubes. In the Cochran boiler the heating surface is increased by the means of fire tubes which helps in attaining greater efficiency than a simple vertical boiler.
Cochran Boiler | Image Via: mechanicalbooster.com
Locomotive Boiler
The Locomotive boiler is a multi-tubular, horizontal and internally fired type boiler. This boiler is arranged horizontally and has multiple tubes. Internal combustion takes place and water is circulated naturally. The operation of the locomotive boiler is the same as the Cochran boiler but there is a difference in construction. Since it uses natural circulation and the operation is fired internally, it is known to be a fire tube boiler.
Locomotive Fire Tube Boiler | Image Via: green-mechanic.com
Lancashire Boiler
The Lancashire boiler is a horizontal type and an internally fired boiler as the furnaces used to present inside the boiler. It is a natural circulation boiler and generates low pressure steam. High thermal efficiency of about 80 to 90 %. The size of this boiler is roughly 7-9 metres in length and 2-3 metres in diameter.
Lancashire Boiler | Image Via: green-mechanics.com
Hot flue gases are produced by heating fuel which then heats the water surrounded by fire tubes. Inside of the fire tubes, the hot flue gases flow which heats the surrounding water. Transmission of heated water to steam takes place at high temperature. The wet steam and hot steam is segregated where the steam produced flows to the anti-priming pipe. Furthermore, the wet steam is superheated and dry steam is fed to the steam stop valve. The steam generated is used to operate a turbine which is then used for the generation of electricity.
The Lancashire boiler is used to operate locomotives, steam turbines, marines, etc. Also, they are used in various industries such as sugar industries, tire industries, textile industries and paper industries. The pros of the Lancashire boiler is that it is more reliable, it is easy to operate, it has about 80% thermal efficiency, less cleaning and maintenance effort. While the cons are, it requires more space, takes a lot of time for generation of steam and cannot generate high pressure steam.