Properties of steam (2)

elements of mechanical

1. Elements of Mechanical Engineering Branch : Chemical Technology by: Mahendra Rajput

2. Subtopic: Types of steam Enthalpy of steam P roperties of steam

3. The steam during the steam generation process can exist in three different types i.e. wet steam, dry and saturated steam, and super heated steam. Wet steam : Both the water molecules and steam coexist to from a two phase mixture, called wet steam which will be in thermal equilibrium because both of them will be at the saturation temperature. 4.3 Types of steam

4. Dryness fraction of steam : A wet steam can have different proportions of water molecules and dry steam. The quality of the wet steam is specified by the dryness fraction which indicates the amount of dry steam present given quantity of wet steam and is denoted as x . The dryness fraction of a steam is the ratio of mass of the actual dry steam present in a know quantity of wet steam to the total mass of wet steam.

5. Mass of dry steam Dryness fraction, x = present in wet steam. Total mass of wet steam x = m s m s + m w The dryness fraction of wet steam is less than 1 and for dry steam equal to 1.

6. Wetness fraction : The wetness fraction of steam is defined as the ratio of mass of the water particles present in a known quantity of wet steam to the total mass of the wet steam. Wetness fraction= m w m s + m w wetness fraction=(1-x)

7. Priming : When wetness fraction is expressed in percentage, it is know as priming. Priming = 100(1- x ) (2)Dry Saturated steam : A steam at the saturation temperature corresponding to a given pressure and having no water molecules entrained in it is know as dry saturated steam.

8. (3)Superheated steam : When a dry saturated stem is heated further at the given constant pressure, its temperature rises beyond its saturation temperature. The steam in this state is said to be superheated.

9. We know non flow energy equation, Heat supplied; q=du + p dv q= (u 2 -u 1 ) + p (v 2 -v 1 ) = (u 2 + pv 2 ) – (u 1 + pv 1 ) But specific enthalpy; h= u + pv q= h 2 -h 1 = h Heat supplied = Change of specific enthalpy. 4.4 Enthalpy of steam

10. (1)Enthalpy of Liquid (h f ) h f = c pw (t f - 0) kJ/kg (2) Enthalpy of Dry Saturated steam (h g ) h g = h f + h fg kJ/kg (3) Enthalpy of Wet steam (h) h = h f + x h fg kJ/kg

11. (4) Enthalpy of Superheated steam (h sup ) h sup = h g + c ps ( T sup - T sat ) Degree of superheat : It is defined as the difference between the temperature of superheated steam and dry saturated steam at the given pressure. Degree of superheat = ( T sup - T sat ) Amount of superheat : It is defined as the amount of heat added during superheating of steam. It is also known as heat of superheat. Amount of superheat = c ps ( T sup - T sat ).

12. Thank You all