A cewa steam da ya fi shi dry saturated idan a wani tsari ta hanyar tsaftace ita ce mafi yawa da boiling point. Ba a zama da shawarar a yi steam kamar haka a fili ba, saboda steam yana da kyau a gane abubuwa masu ruwa. Saboda haka, steam da a yi a drum of the boiler yana da ruwa da kuma abubuwa masu ruwa. Idan abubuwan ruwa a cikin steam ya fi 7% by mass, maka dryness fraction of the steam yana cewa 0.93, wanda yana nufin cewa steam yana da 93% dry.
Enthalpy Enthalpy of wet steam yana nuna a matsayin product of specific enthalpy (hfg) and dryness fraction (x). Heat content of the wet steam and dry saturated steam are different. Dry saturated steam is having higher heat content (useable energy) than the wet steam.
Actual Enthalpy of Evaporation
Actual total Enthalpy of wet steam
Idan, hf Liquid Enthalpy.
Density of water is higher than that of the steam, so the specific volume of the water is far lesser than that of the specific volume of the steam.
Thus the droplets of water in the wet steam will occupy negligible-space and the specific volume of the wet-steam is less than that of the dry steam and given by the formula:
Actual specific volume = x vg
Idan, vg is the specific volume of dry saturated steam
Relation of enthalpy and temperature corresponding to different pressure range is graphically represented in Phase Diagram.
Idan ruwa a kan zama mai karfi daga 0oC zuwa saturation temperature at atmospheric pressure it follows the saturated liquid line until it has received all its liquid enthalpy hf and is represented by (A-B) on Phase diagram.
Any further heat addition results in change in phase to saturated steam and is represented by (hfg) on phase diagram i.e B-C
Idan heat an zama, then the liquid start changing its phase from liquid to vapour and then the dryness fraction of the mixture starts increasing i.e moving towards unity. In the phase diagram dryness fraction of the mixture is 0.5 at exactly mid of the line B-C. Similarly at point C on the phase diagram dryness fraction value is 1.
Point C is in the saturated vapour line, any further heat addition results in increasing the steam temperature i.e beginning of steam super-heating represented by line C – D.
Region towards left side of the saturated liquid line.
Region towards right side of the saturated vapour line.
Area between the saturated liquid and saturated vapour line is mixture liquid and vapour. Mixture with varied dryness fractions.
It is the Apex point where saturated liquid and saturated vapour lines meet. Enthalpy of evaporation diminishes to zero at critical point, it means that water changes directly to steam at critical point and thereafter.
Maximum temperature which liquid can attain or exist is equivalent to critical point.
Temperature 374.15oC, Pressure 221.2 bar, values above this are super-critical values and are useful in increasing the efficiency of the Rankine Cycle.
Flash steam is produced when water pressure is reduced from high to a low pressure, then the water is at higher temperature than that of saturation temperature at low pressure. Thus this excess heat energy is released at low pressure in the foam of flashing and the steam thus produced is “Flash Steam”.
