Introduction and definition of 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 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)
H = enthalpy (kJ)
m = mass (kg)
h = specific enthalpy (kJ/kg)
Specific enthalpy is a property of the fluid and can be expressed as:
h = u + p v (2)
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!
- 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. vf - 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 - hf - can be obtained from tables as above. The value depends on the pressure.
For saturated water at standard atmosphere - 2) -the specific enthalpy - hf - 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)
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)
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)
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)
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