Formation of Steam

i. Water in Vapour State

If we heat water in a pan placed over a furnace, the temperature of water gradually increases. As the water reaches the boiling stage, vapours will start emanating from its surface. This is water in vapour form and we call it steam. The steam is thus defined as ‘Water in Vapour State’. It is known that, if water is to be boiled in an open pan, it has to be heated up to 100 o C. The pressure above the surface of water in an open pan is that of the atmosphere. Thus the temperature of raising steam at atmospheric pressure is 100 o C. If the pressure above the surface of water is changed, the temperature of raising steam would also change.If this pressure is increased, the temperature of boiling of water also increases.

This phenomenon can be very well understood, if we observe the working of pressure cooker in our home.The weight placed over the lid of pressure cooker helps in increasing the pressure in the vessel. This increases the temperature of boiling of water in it and the vegetables in the cooker get cooked fast in the vessel. Conversely, if we decrease the pressure on the surface of water, its temperature of boiling decreases. It means that water will boil at a lower temperature under reduced pressure or in the vacuum. The above principle is true for all the fluids which could be vaporized.

ii. Saturation Temperature & Pressure

Water, like other volatile materials can thus exist as vapour at different temperatures and pressures. This temperature at which water is converted into vapours at a given pressure is called as ‘Saturation Temperature’ of raising steam. The saturation temperature of steam at atmospheric pressure is thus 100 o C. The corresponding pressure of raising steam is known as the ‘Saturation Pressure’ of steam. The saturation pressure of water vapours generated in an open pan would be thus equal to one atmosphere. If the steam is to be raised at a higher saturation pressure, it has to be then generated in a closed vessel. The saturation temperature of steam at the higher pressure would be more and it shall also contain more heat content in it. To make use of steam as a heating medium and also to work out the amount of steam required for a particular milk processing activity, the saturation temperature and the saturation pressure of steam must be known.

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