Steam Boilers, Engines, Nozzles and Turbines
The theoretical indicator diagram of a simple steam engine is based upon the assumption that

There is no pressure drop due to condensation
The expansion (or compression) of the steam is hyperbolic
All of these
Steam is admitted at boiler pressure and exhausted at condenser pressure

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Steam Boilers, Engines, Nozzles and Turbines
Blow off cock in a boiler is used to

Control the flow of steam from the boiler to the main pipe and to shut off the steam completely when required
Put off fire in the furnace of the boiler when the level of water in the boiler falls to an unsafe limit
Increase the temperature of saturated steam without raising its pressure
Empty the boiler when required and to discharge the mud, scale or sediments which are accumulated at the bottom of the boiler

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