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Define: i) Dryness Fraction ii) Sensible Heat Of Water iii) Latent heat of vaporization iv) Superheated Steam

Mumbai university > Mechanical Engineering > Sem 3 > Thermodynamics

Marks: 4M

Year: Dec 2015,

1 Answer
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Dryness Fraction: The term dryness fraction is related with wet steam. It is definedas the ratio of the mass of actual dry steam to the mass of steam containing it. It is usuallyexpressed by the symbol ‘x’ or ‘q’. 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=$\frac{m_s}{m_s+m_w}$

Thus if in 1 kg of wet steam 0.9 kg is the dry steam and 0.1 kg water particles then x=0.9.

Sensible heat of water: It is defined as the quantity of heat absorbed by 1 kg ofwater when it is heated from 0°C (freezing point) to boiling point.It is also called total heat (orenthalpy) of water or liquid heat invariably. It is reckoned from 0°C where sensible heat is takenas zero.

If 1 kg of water is heated from 0°C to 100°C the sensible heat added to it will be

4.18 × 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.

Latent heat of vaporization: It is the amount of heat required to convert waterat a given temperature and pressure into steam at the same temperature and pressure. It isexpressed by the symbol hfgand its value is available from steam tables. The value of latent heatis not constant and varies according to pressure variation.

Superheated steam: When steam is heated after it has become dry and saturated, it iscalled superheated steam and the process of heating is called superheating. Superheating isalways carried out at constant pressure.The additional amount of heat supplied to the steamduring superheating is called as ‘Heat of superheat’ and can be calculated by using the specific heat of superheated steam at constant pressure ($C_ps$), the value of which varies from 2.0 to 2.1 kJ/kg K depending upon pressure and temperature

IF Tsup,Ts are the temperatures of superheated steam in K and wet or dry steam, then

($T_sup-T_s$) is called is called degree of superheat.

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