The Steam Generation Process
The process of steam generation starts when the feed water enters the steam drum through both internal tubes and the tubes that surround the furnace. The system of tubes that surrounds the furnace is known as waterwall or floor tubes. The feed water gets heated when passed through these tubes. In addition to this, there are large-bore downcomer tubes that pass outside the furnace and are used to circulate water between the drums, to which the tubes are attached from outside of the furnace.
The steam produced in the steam drum is known as wet or saturated steam. This steam cannot be used directly as it contains high amounts of moisture. To make this steam usable, it is first dried and heated with the help of a superheater located within the boiler. Once all the moisture content is removed from the steam, the superheated steam can be supplied to other systems. It is utmostly important to monitor and control the temperature of the superheated steam, or else it can cause damage to the systems to which it is supplied. For this reason, an implement known as an attemperator is used. The attemperator is a kind of cooler which is used to cool down the superheated steam to the right temperature. Thus, all the steam used in various system is taken out from the main steam drum of the boiler.
The energy from the fuel burning is used for various other purposes, the main two are as follows:
For heating feed water to produce steam
For superheating the steam from the boiler drum
To increase the efficiency of the boiler, the feed water that enters the boiler is pre-heated with the help of an economizer through which the feed water passes before entering the boiler. The exhaust gas of the boiler is also put to use by making it pass over an air heater, which heats up the combustion air entering the furnace. Thus, even the energy exhausted from the boiler is not allowed to go waste, which increases the overall efficiency of the system.
All these boilers are fitted with various safety fittings and control systems to monitor and control various aspects such as fuel oil flow rate, flow of combustion air, and feed water supply. All these systems should work in co-ordination to supply the amount of steam required by various systems. The boiler is also provided with various mountings to ensure safe operation of the boiler.
Types of Boilers
There are two main types of boilers. These are known as the basic boilers, the rest all are different versions of them. The main two basic boilers are:
Water tube boiler
Fire tube boiler
The design and arrangement of both the types is just the opposite. In water tube boilers, the feed water passes through the tubes and the hot gases are made to pass over them, while in fire tube boilers, the hot gases passes through the tubes and the feed water surrounds them.
Variant 03
Boilers are used on board the ship for producing steam.
There are two basic types of boilers in use in ships: the fire-tube boiler, and the water-tube boiler.
The fire-tube boiler consists of a cylindrical steel shell, which contains a furnace at the bottom.
The furnace is connected to a combustion chamber.
The combustion chamber is located in the middle part of the boiler.
The furnace, the combustion chamber and the tubes are all surrounded by water.
Boilers are now mainly used for auxiliary purposes on board ship.
Water-tube boilers are used for generating steam for main engines.
They have a steam drum at the top.
The steam drum is partly filled with water
Water drums are connected by banks of tubes.
The furnace is located at the bottom.
All system is contained in a fire-proof casing.
Gases are heated in the furnace and pass upward through the banks.
Gases transfer their heat to the water in the tubes.
The convection currents are set up and the water circulates round the system.
The heating surfaces of the boiler are the banks of tubes.
Superheaters are added to the system to increase its efficiency.
They are located between the rows of tubes.
Various valves and gauges are fitted to the boilers.
Safety valves are used to release any excess steam from the boiler.
The main stop valve is used to control the passage of steam to the engines.
Feed valves are used to add water into the boiler.
Water level indicators are used to show the water level.
Thermometers and pressure gauges show the temperature and pressure inside the boiler.
Drain valves are used to drain water from the system.
Salinometer valves are used to take water for testing.
Chemical dosing valves are used to add chemicals into the boiler.