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Latent Heat

Latent Heat
Usage

Two of the more common forms of latent heat (or enthalpies or energies) encountered are latent heat of fusion (melting) and latent heat of vaporization (boiling). These names describe the direction of energy flow when changing from one phase to the next: solid liquid gas.

In both cases, the change is endothermic, meaning that the system absorbs energy on going from solid to liquid to gas. The change is exothermic (the process releases energy) for the opposite direction. For example, in the atmosphere, when a molecule of water evaporates from the surface of any body of water, energy is transported by the water molecule into a lower temperature air parcel that contains less water vapor than its surroundings. Because energy is needed to overcome the molecular forces of attraction between water particles, the process of transition from a parcel of water to a parcel of vapor requires the input of energy causing a drop in temperature in its surroundings. If the water vapor condenses back to a liquid or solid phase onto a surface, the latent energy absorbed during evaporation is released as sensible heat onto the surface. The large value of the enthalpy of condensation of water vapor is the reason that steam is a far more effective heating medium than boiling water, and is more hazardous.

Specific latent heat

The specific latent heat is the amount of energy required to convert 1 kg (or 1 lb) of a substance from solid to liquid (or vice-versa) without a change in the temperature of the surroundings — all absorbed energy goes into the phase change — is known as the specific latent heat of fusion. Likewise, the amount of energy required to convert 1 kg (or 1 lb) of a substance from liquid to gas (or vice-versa) without a change in the external temperature is known as the specific latent heat of vaporization for that substance. Tables of values for the two specific latent heats are published; values for some common substances are given below.

The latent heat for a different mass of the substance can be calculated using the equation:

Q = mL

where:

Q is the amount of energy released or absorbed during the change of phase of the substance (in kJ or in BTU),

m is the mass of the substance (in kg or in lb), and

L is the specific latent heat for a particular substance (kJ-kgm-1 or in BTU-lbm-1); substituted as Lf to represent as the specific latent heat of fusion, Lv as specific latent heat of vaporization.

In other words, specific latent heat is found when energy is divided by mass, or as represented as

Table of latent heats

The following table shows the latent heats and change of phase temperatures of some common fluids and gases.

Substance

Latent Heat

Fusion

kJ/kg

Melting

Point

C

Latent Heat

Vaporization

kJ/kg

Boiling

Point

C

Alcohol, ethyl

108

-114

855

78.3

Ammonia

339

-75

1369

-33.34

Carbon dioxide

184

-78

574

-57

Helium

21

-268.93

Hydrogen(2)

58

-259

455

-253

Lead

24.5

327.5

871

1750

Nitrogen

25.7

-210

200

-196

Oxygen

13.9

-219

213

-183

R134a

-101

215.9

-26.6

Toluene

-93

351

110.6

Turpentine

293

Water

334

0

2260 (at 100oC)

100

Latent heat for water

To calculate the latent heat of condensation in water in the temperature range from 40 C to 40 C the following empirical cubic function can be used:

Lwater(T) = 0.0000614342T3 + 0.00158927T2 2.36418T + 2500.79

with a determination coefficient of R2 = 0.999988

See also

Bowen ratio

Sublimation (physics)

Specific heat capacity

Enthalpy of fusion

Enthalpy of vaporization

References

^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6.

^ Clark, John, O.E. (2004). The Essential Dictionary of Science. Barnes & Noble Books. ISBN 0-7607-4616-8.

^ Textbook: Young and Geller College Physics, 8e, Pearson Education

^ Cubic fit to Table 2.1,p.16, Textbook: R.R.Rogers & M.K. Yau, A Short Course in Cloud Physics, 3e,(1989), Pergamon press

Categories: Thermochemistry | Atmospheric thermodynamics | Thermodynamics

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