Example – 8” Concrete Wall
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Transcript of Example – 8” Concrete Wall
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Example – 8” Concrete Wall
Conductivity, k
BTU∙in/(h ft2 °F)
Resistivity
h ft2 °F/(BTU∙in)
Resistance, R
h ft2 °F/(BTU)
Conductance, U
BTU/(h ft2 °F)
~15 0.067 0.53 1.9
Permeability, µ
perm∙in
Diffusion Resistivity
rep/in
Diffusion Resistance, Z
rep
Permeance, M
perm
~3.2 0.31 2.48 0.40
perm = grain/(hr∙ft2∙in Hg)
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Review of Terms
1. -ity terms – material properties (k, µ)
2. Inverse of 1. (resistivity, diffusion resistivity)
3. -ance terms – 2. × thickness (R, Z)
4. Inverse of 3. (U, M)
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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
x
pw
d
d
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Series Moisture Transfer
rep
permZ
Z
P
x
pw
1,
d
dtotal
i
total
i
Z
Z
P
P
ΔP = water vapor pressure difference
Z = Diffusion resistance
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Saturation Vapor Pressure
• Function of temperature
Ref: ASHRAE Handbook of Fundamentals
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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
total
i
total
i
Z
Z
P
P
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Cautionary Notes
1. 1-D moisture and thermal transport
2. Assumed steady-state and equilibrium conditions
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Equilibrium Moisture Content
• Mass (or volume) ratio between moisture in material and wet (or dry) material
• http://www.forestprod.org/cdromdemo/wd/wd4.html
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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?