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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/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!

Steam and Vapor Enthalpy
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 o C) 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

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.

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## Citation

• The Engineering ToolBox (2003). Steam and Vapor Enthalpy. [online] Available at: https://www.engineeringtoolbox.com/steam-vapor-enthalpy-d_160.html [Accessed Day Month Year].

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12.8.9