# Steam and Vapor Enthalpy

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/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*at standard atmosphere. Be aware that

^{ o }C)*c*varies with temperature.

_{ ps }### 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

Air, LNG, LPG and other common gas properties, pipeline capacities, sizing of relief valves.

### • Thermodynamics

Thermodynamics of steam and condensate systems.

## Related Documents

### Condensation of Steam - Heat Transfer

Heat transfer when steam condensates.

### Energy Accumulated in Heated Water - kWh

The amount of thermal energy stored in heated water.

### Evaporative Cooling

Evaporative cooling tutorial.

### Heating Water by Injecting Steam

Water can be heated by injecting steam.

### Latent Heat Flow

Latent heat is the heat when supplied to or removed from air results in a change in moisture content - the temperature of the air is not changed.

### Liquids - Latent Heat of Evaporation

Latent heat of vaporization for fluids like alcohol, ether, nitrogen, water and more.

### Saturated Steam - Properties - Imperial Units

Steam table with sensible, latent and total heat, and specific volume at different gauge pressures and temperatures.

### Saturated Steam - Properties for Pressure in Bar

Saturated Steam Table with properties like boiling point, specific volume, density, specific enthalpy, specific heat and latent heat of vaporization.

### Standardized Enthalpies and Entropies

Standardized enthalpies and entropies for some common substances.

### Steam Entropy

Basic steam thermodynamics - entropy diagram.

### Superheated Steam - Enthalpy

Enthalpy of steam superheated to temperatures above it's boiling point.

### Superheated Steam - Entropy

The entropy of steam superheated to temperatures above saturation points.

### Vapor and Steam

An introduction to vapor and steam.

### Water-Steam Mollier Diagram

Enthalpy-entropy diagram for water and steam.

### Wet Steam - Enthalpy

Wet steam, dryness fraction and enthalpy.

### Wet Steam - Quality vs. Dryness Fractions

Introduction and definition of steam quality and dryness fraction including calculating wet steam enthalpy and specific volume.