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The evaporation of water from wood at high temperatures

Summary

When predicting the performance of wood drying processes, film coefficients for heat and mass transfer are among the parameters that need to be estimated. Customary methods such as the use of the Lewis relation were originally derived for vapour pressures which were small compared with atmospheric pressure. For high temperature drying of wood it is necessary to extend the associated psychrometric equation to one of more general applicability. Such extension is the subject of this paper. Essential to the analysis is the determination of the rate of change of vapour pressure with temperature adjacent to the wood surface. It is found that when the vapour pressure is not assumed small compared with atmospheric pressure, the above gradient contains a factor P - po where P is the atmospheric pressure and po is the free stream vapour pressure. At high temperatures and humidities, this factor becomes important and the film coefficient for mass transfer increases without limit.

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Abbreviations

c:

specific heat of mixture per unit mass of dry air, J/kg K

ca :

specific heat of dry air, J/kg K

cp :

specific heat of moist air at constant pressure, J/kg K

cv :

specific heat of water vapour, J/kg K

D:

molecular diffusivity — air/water vapour, m2/s

d:

film coefficient for mass transfer kg/m2s Pa

f:

mass flux, kg/m2s

h:

film coefficient for heat transfer, J/m2s K

k:

thermal conductivity of wet air, J/ms K

Le:

Lewis number

l :

latent heat of vaporisation of water, J/kg

P:

atmospheric pressure, Pa

p:

pressure of water vapour, Pa

q:

heat flux, J/m2s

Ra :

specific gas constant for air, J/kg K

Rw :

specific gas constant for water, J/kg K

T:

temperature, K

w:

moisture content of air, mass water.mass dry air

ɛ:

ratio of molecular weights, water/air

l :

liquid surface

o:

free stream

s:

wood surface

References

  1. Arnold, J. H. 1933: The Theory of the Psychrometer. Physics 4: July 255–262, September 334–340.

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  2. Lewis, W. K. 1922: The evaporation of a liquid into a gas. Mechanical Engineering 44: 445

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  3. Threlkeld, J. L. 1965: Thermal Environmental Engineering. Prentice-Hall, New York

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Hunter, A.J., Sutherland, J.W. The evaporation of water from wood at high temperatures. Wood Sci.Technol. 31, 73–76 (1997). https://doi.org/10.1007/BF00705922

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Keywords

  • Evaporation
  • Mass Transfer
  • Vapour Pressure
  • Film Coefficient
  • Free Stream