When any liquid is heated at constant temperature, it starts changing in vapour condition. A vapour is a mixture of gas and liquid particles in suspension and it can be liquefied by moderate changes of pressure or temperature. The process of vapour formation from liquid state has some definite characteristics to follow in steam engine,
Characteristics of Steam formation.
01) The heat supplied during the process of vapourization changes its state gradually from liquid to gaseous state.
02) The vapour is said to be dry saturated when the process of vapourization takes place at constant pressure and temperature. In other word we can say when the liquid is completely evaporated it is called dry saturated.
03) The vapour is called superheated when the heat is added to the dry saturated vapour state. At superheated state, the temperature will rise gradually.
04) During the process of superheating a vapour at constant pressure, the volume will increase approximately in proportion to the absolute temperature, which indicates that the vapour is approaching the state of perfect gas.
05) The temperature at which the evaporation process is taking place is increase when the pressure on the liquid surface is increased
06) The latent heat of vaporization decrease with the increase of pressure and temperature but the sensible enthalpy increases under this condition.
07) The total heat (Enthalpy) of dry saturated vapour increases with increase in saturation temperature. In lower range of pressure and temperature, the rate of increase in sensible enthalpy is greater than the rate of decrease in latent enthalpy with the increase in pressure.
After certain saturation temperature the rate of increase in sensible enthalpy is less than the rate of decrease in latent enthalpy and hence, the enthalpy of dry saturated vapour decreases. This will continue to happen until the saturation temperature is reached when the latent enthalpy becomes zero and the enthalpy of dry saturated vapour becomes equal to the sensible heat.
This saturation temperature is called as critical temperature and the corresponding pressure is called critical pressure and the state of substance is called its critical state.
08) Steam can not exist as saturated vapour above its critical temperature. At any temperature higher than the critical temperature (374.14 degree centigrade) the vapour can only exist as a gas without any effect of pressure.
At critical temperature, the specific volume of dry saturated vapour becomes equal to the specific volume of the liquid from which it is being formed.
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