Formation of Steam and its Properties

Matter can exist in any form, solid, liquid, vapour or gas. If a solid metal is heated, at some temperature it melts and becomes a liquid. Further heating transforms liquid metal into a vapour and ultimately a gas. Each change of state from one form to another is called a change of 'phase'. During change of phase, we find, either addition or removal of heat. The temperature at which the phase change occurs depends upon the substance and varies according to the pressure maintained.

`Gas' is the state of substance of which evaporation from the liquid is complete. Substance like air, oxygen, nitrogen, hydrogen etc., are treated as gases within the temperature limits of applied thermodynamics. They are fluids which cannot be liquified by the application of pressure at constant temperature. A 'vapour' is a partially evaporated liquid containing pure gaseous state together with liquid particles in suspension. Vapour may be easily condensed back to liquid state or it may be further evaporated to become a perfectly dry gas. Vapour used in engineering applications are wet steam, ammonia, sulphur dioxide, carbon dioxide etc.

Steam on account of heat energy contained by it, is used in steam engines of locomotives, water pumping systems, pneumatic presses and forges, etc., Steam is the main sources of energy in steam turbines used for power generation. It should be noted that Gas Laws which you have studied earlier do not apply to vapours.

FORMATION OF STEAM :

There are three distinct stages in the production of steam from water. They are explained as follows with the help of a temperature Vs enthalpy (total heat) diagram (Fig). It should be noted that steam is invariably formed at constant pressure.

Stage 1 : (OA) This is the warming phase in which temperature of water increases upto saturation temperature, tsat . Saturation temperature corresponds with the boiling point of water and varies with the pressure at which water is heated. Higher the pressure higher is the saturation temperature. The heat energy supplied to produce this temperature rise is called `sensible heat' or the 'liquid heat' (h) or liquid enthalpy. Sensible heat or liquid enthalpy is defined as the quantity of heat absorbed by 1 kg water when it is heated from its freezing point i.e, 0°C to boiling point.

Stage 2 : (AB) It is an evaporation stage. Water evaporates at constant temp., (tsat) and transforms into dry saturated steam. Point 'B' represents this state of steam. Line AB represents evaporation of water at constant temperature and of course, constant pressure. The state of steam between the extreme points A and B is wet indicating incomplete evaporation. For example point B' on the line AB indicates such a state. Partially evaporated steam will have water particles under suspension in it and hence called wet steam.

The heat energy supplied during evaporation is called 'enthalpy of evaporation’ or ‘Latent heat of steam' or 'Latent heat of evaporation 'L’. Its value depends on the pressure at which water is heated. Higher the pressure lower is the Latent heat.

Enthalpy of evaporation or Latent heat is defined as the heat required to convert 1 kg water at given pressure and temperature into steam at the same pressure and temperature.

Stage 3 : (BC) This phase begins only when evaporation of water is complete and all dry saturated steam is formed. Further supply of heat produces superheated steam which is accompanied by a rise in temperature. The amount of heat energy supplied in the superheat phase is called the 'superheat enthalpy’ given by Cp ΔT per kg. where Cp = mean sp. heat of steam and ΔT is degree of superheat.

Note that temperature change takes place only during the transfer of heat when a substance is in a single phase. Observe it in OA and BC stages. If a two phase mixture exists then the temperature remains constant until a complete change of state takes place (line AB).

Any intermediate point between A and B, say B' represents incomplete state of evaporation. That is, steam is wet at B' containing a few droplets of moisture under suspension. The heat supplied during the stage AB' is qL where q is a fraction called dryness fraction (q<1).

Point P in the diagram is a critical point at which latent heat is zero. Temperature corresponding to this point is called critical temperature (374°C).

DRYNESS FRACTION :

It is defined as the ratio of mass of dry saturated steam to the mass of wet steam containing it. Let M = mass of wet steam (kg) m = mass of water particles (moisture) in suspension (kg)

Then, (M—m) = mass of pure dry sat. steam (kg)

Dryness fraction, q = (M — m) /M (no units)

Note : If M = then

q = (1—m) kg

In other words, 1 kg wet steam contains q kg dry saturated steam, q being dryness fraction.

Quality of steam is expressed as percentage of dryness fraction.

For ex., if 10 kg wet steam contains 2 kg moisture, then,

q = 10-2 / 10 = 0.8

Hence quality of steam is 80%.