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Example – 8” Concrete Wall

Example – 8” Concrete Wall. perm = grain/(hr∙ft 2 ∙in Hg). Review of Terms. -ity terms – material properties ( k , µ ) Inverse of 1. (resistivity, diffusion resistivity) -ance terms – 2. × thickness ( R , Z ) Inverse of 3. ( U , M ). Governing Equation For Diffusion.

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Example – 8” Concrete Wall

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  1. Example – 8” Concrete Wall perm = grain/(hr∙ft2∙in Hg)

  2. Review of Terms • -ity terms – material properties (k, µ) • Inverse of 1. (resistivity, diffusion resistivity) • -ance terms – 2. × thickness (R, Z) • Inverse of 3. (U, M)

  3. Governing Equation For Diffusion • w water vapor flux [M/t/A, kg/s/m2] • µ permeability [perms∙in, perm = grain/(hr∙ft2∙in Hg)] • Permeance [ng/(s·m2·Pa)] • p is water vapor pressure • x is distance along flow path • Water diffuses from high vapor pressure to low vapor pressure • Permeability is a function of temperature in materials • Very ugly non-linear relationship

  4. Series Moisture Transfer ΔP = water vapor pressure difference Z = Diffusion resistance

  5. Saturation Vapor Pressure • Function of temperature Ref: ASHRAE Handbook of Fundamentals

  6. If Condensation Occurs • Set vapor pressure to saturation pressure at most likely point • Divide wall into two sections • Use relationship on each side of condensation • Recalculate vapor pressures

  7. Cautionary Notes • 1-D moisture and thermal transport • Assumed steady-state and equilibrium conditions

  8. Equilibrium Moisture Content • Mass (or volume) ratio between moisture in material and wet (or dry) material • http://www.forestprod.org/cdromdemo/wd/wd4.html

  9. Review moisture transport • Moisture transport • Liquid flow • Vapor diffusion • Air movement • Capillary action • What driving force is responsible for each one? • What do you need for each transport method?

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