Steam and Vapor Enthalpy

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 - 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/m²)

v = specific volume (m³/kg)

Part of the water vapor - steam - properties can be expressed in a table as:

• 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:

h_f = c_w (t_f - t₀)         (3)

where

h_f = enthalpy of water (kJ/kg)

c_w = specific heat of water = 4.19 (kJ/kg.^oC)

t_f = 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 - 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)

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 ^oC)

t_f = 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 = 1,000 J/kg
• 1 erg/g = 1E-4 J/kg
• 1 Btu/lbm = 2,326 J/kg
• 1 cal/g = 4,184 J/kg

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