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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 - super-heating

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/m2)

v = specific volume (m3 /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!

Steam and Vapor Enthalpy
p
(bar)
t s
(oC)
v f
(m3 /kg)
v g
(m3 /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 oC (212 oF).

The specific enthalpy of water (in SI units) can be calculated from:

hf = c w (tf - t0 ) (3)

where

hf = enthalpy of water (kJ/kg)

c w = specific heat water (4.19 kJ/kg. oC)

tf = saturation temperature (oC)

t0 = refer temperature = 0 (oC)

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 - hf (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 - tf ) (5)

where

h s = enthalpy of superheated steam (kJ/kg)

c ps = specific heat of steam at constant pressure = 1.860 (kJ/kg oC)

tf = saturation temperature (oC)

t s = superheated steam temperature (oC)

c ps = 1.860 (kJ/kg oC) 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 = 1E-4 J/kg
  • 1 Btu/lbm = 2326 J/kg
  • 1 cal/g = 4184 J/kg

Related Topics

  • Gases and Compressed Air

    Properties of air, LNG, LPG and other common gases. Pipeline capacities and sizing of relief valves.
  • Thermodynamics

    Calculate heat, work, temperature and energy. The thermodynamics of steam and condensate systems. Water and Ice properties.

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