Explain: i) Dry saturated steam ii) sensible heat of water iii) Total Enthalpy of steam iv) Dryness fraction.

Mumbai university > Mechanical Engineering > Sem 3 > Thermodynamics

Marks: 8M

Year: May 2016

1 Answer

Dry saturated steam: Saturated (dry) steam results when water is heated to the boiling point (sensible heating) and then vaporized with additional heat (latent heating). If this steam is then further heated above the saturation point, it becomes superheated steam (sensible heating).

Sensible heat of water: It is defined as the quantity of heat absorbed by 1 kg of water when it is heated from 0°C (freezing point) to boiling point. It is also called total heat (or enthalpy) of water or liquid heat invariably. It is reckoned from 0°C where sensible heat is taken as zero. If 1 kg of water is heated from 0°C to 100°C the sensible heat added to it will be 4.18 X 100 = 418 kJ but if water is at say 20°C initially then sensible heat added will be 4.18 × (100 – 20)=334.4 kJ. This type of heat is denoted by letter hfand its value can be directly read from the steam tables.

Total Enthalpy of steam: total enthalpy is enthalpy of water vapor and enthalpy of water droplets which is commonly denoted as $h_fg$

Dryness fraction: The term dryness fraction is related with wet steam. It is defined as the ratio of the mass of actual dry steam to the mass of steam containing it. It is usually expressed by the symbol ‘x’ If

$m_s$=Mass of dry steam contained in steam considered and $m_w$=weight of water particles in suspension in the steam considered

Then x=$\frac{m_s}{m_s+m_w}$

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