Steam boiler or simply a boiler is basically a closed vessel into which water is heated until the water is converted into steam at required pressure. This is most basic definition of boiler.
Working Principle of Boiler:
The basic working principle of boiler is very very simple and easy to understand. The boiler is essentially a closed vessel inside which water is stored. Fuel (generally coal) is bunt in a furnace and hot gasses are produced. These hot gasses come in contact with water vessel where the heat of these hot gases transfer to the water and consequently steam is produced in the boiler. Then this steam is piped to the turbine of thermal power plant. There are many different types of boiler utilized for different purposes like running a production unit, sanitizing some area, sterilizing equipment, to warm up the surroundings etc.
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Steam Engines are the second tier of engines in BuildCraft, above Redstone Engines and below Combustion Engines. They have an output of 1 MJ/t. They can use Vanilla Minecraft fuels to operate or EU with an Energy Link. Note that they still require a Redstone signal to run, as do Combustion Engines. You can supply this with Redstone or with Red Alloy Wire from RedPower - it may be more effective. Steam Engines will never explode unless the engine itself receives too much power. You can leave one running, for example, and it will never explode unless it is receiving power. The temperature is represented by the color of the central pylon. Blue means it's cool, Green means it's warm, Green - Orange toggle means it's beginning to get hot, Orange means that it is hot, and when its an orange -red toggle then it means it's near or in danger of imminent explosion, but its also where the engine runs most effectively.
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