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!
p (bar) | t s (oC) | v f (m3/kg) | vg (m3/kg) | u f (kJ/kg) | u g (kJ/kg) | h f (kJ/kg) | hg (kJ/kg) | s f (kJ/kg K) | s g (kJ/kg K) |
---|---|---|---|---|---|---|---|---|---|
0.0061121) | 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 |
. | . | . | . | . | . | . | . | . | . |
1.013252) | 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 |
221.23) | 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 = cw (tf - t0) (3)
where
hf = enthalpy of water (kJ/kg)
cw = 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 - hg - can be obtained from tables as above. The value depends on the pressure.
For saturated steam at standard atmosphere -2) - the specific enthalpy - hg - is 2676 kJ/kg .
The specific enthalpy of evaporation can be calculated from:
he = hg - hf (4)
where
he = specific evaporation enthalpy (kJ/kg)
Specific evaporation enthalpy for water at standard atmosphere is:
he = (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 = he 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:
hs = hg + cps (ts - tf) (5)
where
hs = enthalpy of superheated steam (kJ/kg)
cps = specific heat of steam at constant pressure = 1.860 (kJ/kg oC)
tf = saturation temperature (oC)
ts = superheated steam temperature (oC)
cps = 1.860 (kJ/kg oC) at standard atmosphere. Be aware that cps 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