Elements of Mechanical engineering (18ME15/25) a VTU syllabus topic.
Module 1, Introduction to steam.

DIFFERENT STATES OF STEAM

•Wet steam

•Dry saturated steam

•Superheated steam

WET STEAM

•A wet steam is defined as a two-phase mixture of water molecules and steam in thermal equilibrium at the saturation temperature corresponding to a given pressure.•Both water molecules and steam will have same saturation temperature.•The dryness fraction of wet steam is less than one.

DRY STEAM

•A dry steam is the steam at the saturation temperature corresponding to a given pressure and having no water particles.

•The dryness fraction of dry steam will be unity.

SUPERHEATED STEAM

•A superheated steam is defined as the steam which is heated beyond its dry saturated state to temperature higher then its saturation temperature at the given pressure.

•The dryness fraction of superheated steam will be more then unity.

Temperature Enthalpy Diagram

DRYNESS FRACTION OF STEAM

•The ration of mass dry particles presents in a known quantity of wet steam is defined as dryness fraction. It is denoted by x, X=mg/(mg+mf )

•It is expressed as fraction or percentage. It indicates the quality of steam i.e., If dryness fraction (x)=1, the steam is stated as dry steam and if x lies between 0 and 1, the steam is stated as wet steam.

Saturation Temperature 

•The boiling temperature is known as the temperature of  formation of steam (or) saturation temperature(ts 0C)

Sensible Heat

•The amount of heat required to change temperature of one kg of water from 00C to saturation temperature is defined as sensible heat and is denoted by hf.

Latent Heat

•The amount of heat required to convert one kg of water at saturation of temperature into dry saturated steam at the same temperature, is known as latent heat of evaporation and is denoted by hfg.

Total heat of evaporation

•The amount of heat required to convert one kg of water form 00 C to dry saturated steam, is known as total heat of evaporation.

•It is the sum of the sensible heat(hf) and latent heat(hfg).

•It is denoted by hg and hg = hf+hfg

Amount of super heat

•It is the amount of heat required to convert dry steam at (ts) into superheated steam at Tsup.

Superheated Temperature

•If dry steam of temperature ts is heated further its temperature will rise and which is known as superheated temperature and is denoted Tsup.

Degree of super heat

•The amount of rise in temperature when dry steam is converted into super heated steam is termed as degree of super heat i.e.

•degree of super heat= Tsup- ts 0(C)

Enthalpy of Steam

•It is defined as sum of the internal energy and the product of the pressure and volume. It is heat content of the steam and is denoted by h.

        i.e. h = u +PV                                                               where,                

•h is enthalpy of steam

•u is internal energy

•P is pressure

•V is volume

Enthalpy of dry steam

•The amount of heat required to convert one kg of water from 00C to dry steam (at saturation temperature).

• hg = hf + hfg kJ/kg

Enthalpy of wet steam

•Wet steam contains water particles in it. If x is the quality/dryness fraction of wet steam, the enthalpy of wet steam is calculated by,

  hwet = hf +x hfg kJ/kg

Enthalpy of superheated steam

•The amount of heat required to convert 1kf of water (at00C) into super heated steam (at Tsup0C) is defined as enthalpy of super heated steam is denoted by hsup.

hsup = hf  + hfg + Csup(Tsup -ts)        (as hf + hgf= hg)

        = hg + Cps (Tsup -ts)                       

where,

Cps = 2.25 kJ/kg K. is specific heat of super Heated steam

Specific volume

•The volume(m3) occupied by unit mass (1 kg) of a substance is know as specific  volume (or)

•The reciprocal of density is known as specific volume.

•i.e. volume/unit mass is specific  volume.

•It is usually expressed in m3/kg.

Specific volume of saturated water

•The volume occupied by one kg of water saturation temperature at a given pressure is defined as specific volume of saturated water. It is denoted by Vf.

•Note: This value is available from steam tables, corresponding to the given pressure/temperature.

Specific volume of wet steam

•The volume occupied by one kg of wet steam at a given pressure is known as specific volume of wet steam.

If ‘x ’ is the dryness fraction of wet steam

Then x Vg is the specific volume of wet steam

 

Where, Vg is specific volume of dry steam

  (Obtained from steam tables)

Specific volume of superheated steam

•The volume occupied by one kg of superheated steam at a given pressure and super heated temperature is known as specific volume of superheated steam.

  It is denoted by Vsup.

  Vsup=Vg  Tsup/ts       Where, Tsup – Superheated temperature in K

 ts – Saturation temperature in K

Note: Tsup, ts must be in kelvin only.

   K = 0C + 273

External work of evaporation

•Whenever the volume increases from Vf to Vg, the piston inside the cylinder moves upwards. The work utilized to move this piston due to increase in volume is defined as external work of evaporation (work done).

• For dry steam It is  = 100 PVg kJ/kg
•For Wet Steam it is = 100 PxVg kJ/kg and
•For superheated steam it is  = 100 PVsup kJ/kg        
                                       
where, P is constant pressure in bar , x is dryness fraction

Internal latent heat

•The energy spent/ required is internal latent heat. Actually this much amount of heat is required to change the phase.

  It is given by,= (hfg – 100 PVg) kJ/kg               

Where, hfg= latent heat(total)

  100PVg= external work of dry steam

Internal energy of steam

•Actual energy stored in the steam is called internal energy. It is the difference between the enthalpy of steam and the external work of evaporation and is denoted by U.

For dry steam,            Ug = hg  - 100 PVg kJ/kg

For wet steam,            Uf = hf + x hfg -100 P x Vg kJ/kg

For superheated steam  Usup = hsup – 100 PVsup kJ/kg