What is the meaning of marine boilers?
Boiler is a pressure vessel used on ship in which the water is heated to evaporated and generated the steam and unit is so arranged that the generated steam accumulated in it.
Boilers are one of the essenstial equipments onboard ships. It’s purpose is to provide heating to the the main diesel propulsion engine, to the bunker F.O. tanks, to make it less viscous for transfer purposes as well as easy ignition.
Material used:
Solid drawn mild steel is generally used for the construction of superheater support tubes. Tube temperature for convection superheater sections is considered to be final superheat temperature + 30oC. For radiant heat, higher temperature is considered.
Boiler is an apparatus to produce steam. Thermal energy released by combustion of fuel is used to make steam at the desired temperature and pressure.
The steam produced is used for:
•(i) Producing mechanical work by expanding it in steam engine or steam turbine.
•(ii) Heating the residual Fuel for combustion purposes & for transferring
•(iii) Heating for accommodation & Domestic water.
Boiler Design Requirements:
1.The boiler must be safe to operate
2.The boiler must be able to generate steam at the desired rate and pressure
3.The boiler must be economical to operate
For economy of operation and to generate steam at the desired rate and pressure, a boiler must have the following attributes: –
1.Adequate water and steam capacity
2.Rapid and positive water circulation
3.A large steam generating surface
4.Heating surface that are easy to clean on both water and gas sides 5.Parts accessible for inspection
6.A correct amount and proper arrangement of heating surface
7.A firebox for efficient combustion of fuel
Boilers Properties:
(i) Safety: The boiler should be safe under operating conditions.
(ii) Accessibility: The various parts of the boiler should be accessible for repair and maintenance.
(iii) Capacity: Should be capable of supplying steam according to the requirements.
(iv) Efficiency: Should be able to absorb a maximum amount of heat produced due to burning of fuel in the furnace.
(v) It should be simple in construction .
(vi) Its initial cost and maintenance cost should be low.
(vii) The boiler should have no joints exposed to flames.
(viii) Should be capable of quick steaming up and loading.
Working:
source:Marine notesA boiler is used to heat feed water in order to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced. All boilers have a furnace or combustion chamber where fuel is burnt to release its energy. Air is supplied to the boiler furnace to enable combustion of the fuel to take place. A large surface area between the combustion chamber and the water enables the energy of combustion, in the form of heat, to be transferred to the water.
A drum must be provided where steam and water can separate. There must also be a variety of fittings and controls to ensure that fuel oil, air and feedwater supplies are matched to the demand for steam. Finally there must be a number of fittings or mountings which ensure the safe operation of the boiler.
In the steam generation process the feedwater enters the boiler where it is heated and becomes steam. The feedwater circulates from the steam drum to the water drum and is heated in the process. Some of the feedwater passes through tubes surrounding the furnace, i.e. waterwall and floor tubes, where it is heated and returned to the steam drum. Large-bore downcomer tubes are used to circulate feedwater between the drums. The downcomer tubes pass outside of the furnace and join the steam and water drums. The steam is produced in a steam drum and may be drawn off for use from here. It is known as ‘wet’ or saturated steam in this condition because it will contain small quantities of water, Alternatively the steam may pass to a superheater which is located within the boiler. Here steam is further heated and ‘dried’, i.e. all traces of water are converted into steam. This superheated steam then leaves the boiler for use in the system. The temperature of superheated steam will be above that of the steam in the drum. An ‘attemperator’, i.e. a steam cooler, may be fitted in the system to control the superheated steam temperature.
The hot gases produced in the furnace are used to heat the feedwater to produce steam and also to superheat the steam from the boiler drum. The gases then pass over an economiser through which the feedwater passes before it enters the boiler. The exhaust gases may also pass over an air heater which warms the combustion air before it enters the furnace. In this way a large proportion of the heat energy from the hot gases is used before they are exhausted from the funnel.
References: Introduction to Marine Engineering by D.A.Taylor
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