Steam and Vapor Enthalpy
Vapor and steam enthalpy, specific enthalpy of saturated liquid, saturated vapor and superheated vapor.
When a liquid evaporates its go through a process where
 the liquid heats up to the evaporation temperature
 the liquid evaporate at the evaporation temperature by changing state from fluid to gas
 the vapor heats above the evaporation temperature  superheating
The heat transferred to a substance when temperature changes is often referred to as sensible heat . The heat required for changing state as evaporation is referred to as latent heat of evaporation .
The most common vapor is evaporated water  steam or moist.
Enthalpy
Enthalpy of a system is defined as the mass of the system  m  multiplied by the specific enthalpy  h  of the system and can be expressed as:
H = m h (1)
where
H = enthalpy (kJ)
m = mass (kg)
h = specific enthalpy (kJ/kg)
Specific Enthalpy
Specific enthalpy is a property of the fluid and can be expressed as:
h = u + p v (2)
where
u = internal energy (kJ/kg)
p = absolute pressure (N/m ^{ 2 } )
v = specific volume (m ^{ 3 } /kg)
Part of the water vapor  steam  properties can be expressed in a table as:
For full table with Enthalpy and Entropy  rotate the screen!
p (bar)  t _{ s } ( ^{ o } C)  v _{ f } (m ^{ 3 } /kg)  v _{ g } (m ^{ 3 } /kg)  u _{ f } (kJ/kg)  u _{ g } (kJ/kg)  h _{ f } (kJ/kg)  h _{ g } (kJ/kg)  s _{ f } (kJ/kg K)  s _{ g } (kJ/kg K) 

0.006112 ^{ 1) }  0.01  0.0010002  206.1  0  2375  0.0006  2501  0  9.155 
0.010  7.0  0.0010001  129.2  29  2385  29  2514  0.106  8.974 
.  .  .  .  .  .  .  .  .  . 
1.01325 ^{ 2) }  100.0  0.001044  1.673  419  2507  419  2676  1.307  7.355 
.  .  .  .  .  .  .  .  .  . 
220  373.7  0.00269  0.00368  1949  2097  2008  2178  4.289  4.552 
221.2 ^{ 3) }  374.15  0.00317  0.00317  2014  2014  2084  2084  4.430  4.430 
 s is the steam entropy
 suffix  f  referrer to saturated liquid
 suffix  g  referrer to saturated vapor  steam
Internal energy  u  can be calculated from (2) and is often omitted in tables. v _{ f }  change very little and is also often omitted.
 ^{ 1) } referrer to absolute vacuum.
 ^{ 2) } referrer to water boiling at standard atmosphere.
 ^{ 3 } ^{ ) } referrer to water critical point. For pressures above the critical point there is no definite transition from liquid to vapor.
Specific Enthalpy of Saturated Water
Specific enthalpy of saturated water  h _{ f }  can be obtained from tables as above. The value depends on the pressure.
For saturated water at standard atmosphere  ^{ 2) } the specific enthalpy  h _{ f }  is 419 kJ/kg . At standard atmosphere  1 bar (14.7 psi)  water starts boiling at 100 ^{ o } C (212 ^{ o } F).
The specific enthalpy of water (in SI units) can be calculated from:
h _{ f } = c _{ w } (t _{ f }  t _{ 0 } ) (3)
where
h _{ f } = enthalpy of water (kJ/kg)
c _{ w } = specific heat water (4.19 kJ/kg. ^{ o } C)
t _{ f } = saturation temperature ( ^{ o } C)
t _{ 0 } = refer temperature = 0 ( ^{ o } C)
Specific Enthalpy of Saturated Steam
Specific enthalpy of saturated steam  h _{ g }  can be obtained from tables as above. The value depends on the pressure.
For saturated steam at standard atmosphere  ^{ 2) }  the specific enthalpy  h _{ g }  is 2676 kJ/kg .
The specific enthalpy of evaporation can be calculated from:
h _{ e } = h _{ g }  h _{ f } (4)
where
h _{ e } = specific evaporation enthalpy (kJ/kg)
Specific evaporation enthalpy for water at standard atmosphere is:
h _{ e } = (2676 kJ/kg)  (419 kJ/kg)
= 2257 (kJ/kg)
Example  Energy to Evaporate Water
The energy to evaporate a certain amount of water can be calculated as
Q = h _{ e } m (4b)
where
Q = evaporation energy (kJ)
m = mass of water (kg)
The energy to evaporate 5 kg of water at atmospheric pressure can be calculated as
Q = ( 2257 kJ/kg) ( 5 kg )
= 11285 kJ
Specific Enthalpy of Superheated Steam
The specific enthalpy of superheated steam can be calculated from:
h _{ s } = h _{ g } + c _{ ps } (t _{ s }  t _{ f } ) (5)
where
h _{ s } = enthalpy of superheated steam (kJ/kg)
c _{ ps } = specific heat of steam at constant pressure = 1.860 (kJ/kg ^{ o } C)
t _{ f } = saturation temperature ( ^{ o } C)
t _{ s } = superheated steam temperature ( ^{ o } C)
c _{ ps } = 1.860 (kJ/kg ^{ o } C) at standard atmosphere. Be aware that c _{ ps } varies with temperature.
Common Units for Specific Enthalpy
 1 kJ/kg = 1000 J/kg
 1 erg/g = 1E4 J/kg
 1 Btu/lbm = 2326 J/kg
 1 cal/g = 4184 J/kg
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