Masonry Magazine August 1981 Page. 17
Exterior Surface Temperature
The maximum exterior surface temperature may be determined by using Step 16 in Technical Notes 43B. The maximum exterior surface temperature may be approximated by using Equation 4.
To max = (α/0.98) × Cc × Co × 160°F (4)
where:
To max = Maximum exterior surface temperature, in °F.
α = Absorptivity of the brick masonry used.
Cc = Glazing transmittance correction factor:
single glass = 1.212
double glass = 1.000
triple glass = 0.825
Co = Orientation correction factor.
For a thermal storage wall system with double glazing constructed with brown brick having an absorptivity, α = 0.78, the maximum exterior surface temperature for a South orientation may be determined by using Equation 4.
To max = (0.78/0.98) x 1.00 x 1.00 x 160°F
To max = 127°F
Fluctuation
The exterior surface temperature fluctuation, ΔT, of the brick thermal storage wall is simply the maximum exterior surface temperature minus the minimum exterior surface temperature. Thus, for the various thicknesses of brick thermal storage walls, the exterior surface temperature fluctuation, ΔT, would be:
8-in. wall, ΔT = 127°F - 54°F = 73°F
12-in. wall, ΔT = 127°F - 51°F = 76°F
16-in. wall, ΔT = 127°F - 48°F = 79°F
24-in. wall, ΔT = 127°F - 44°F = 83°F
Interior Surface Temperature
The simplified heat transfer equations may be used to determine the interior surface temperature fluctuation once the exterior surface temperature fluctuation is approximated. The interior surface temperature fluctuation may be used to predict the maximum and minimum interior surface temperatures.
Fluctuation. The amount of allowable interior surface temperature fluctuation, Ti, is another criterion for selecting the thickness of the brick thermal storage wall. The interior surface temperature fluctuation, Ti, of the brick thermal storage wall may be determined by using Equation 5, if the maximum and minimum exterior surface temperatures are known.
Ti = ΔT x e^(-x√(π/αP)) (5)
Values for interior surface temperature fluctuations for typical brick masonry construction may be obtained from Fig. 4 for specific exterior temperature ranges. The temperature fluctuation for the 8-in. thick brick wall as determined by using Equation 5 is:
Ti = 73°F x e^(-(5/12)√(3.14/(0.024x24)))
Ti = 73°F x e^(-0.67√13.68)
Ti = 73°F x e^(-0.67 x 3.7)
Ti = 73°F x e^(-2.48)
Ti = 73°F x 0.083
Ti = 6.06°F or approximately 6°F
Similarly, for the other wall thicknesses:
12-in. wall, Ti = 2.9° F or approximately 3°F
16-in. wall, Ti = 1.4°F or approximately 1°F
24-in. wall, Ti = 0.3°F or approximately 0°F
Minimum. The minimum interior surface temperature, Timin, may be determined by subtracting one-half of the interior surface temperature fluctuation from the sum of the exterior minimum surface temperature and one-half the exterior surface temperature fluctuation as given in Equation 6.
Timin = Tomin + 1/2ΔT - 1/2Ti (6)
The minimum interior surface temperature for the various face brick thermal storage wall thicknesses are determined to be:
8-in. wall, Timin = 54 + 36.5 - 3.0 = 87.5°F
12-in. wall, Timin = 51 + 38.0 - 1.5 = 87.5°F
16-in. wall, Timin = 48 + 39.5 - 0.5 = 87.0°F
24-in. wall, Timin = 44 + 41.5 - 0.15 = 85.35°F
The calculated minimum interior surface temperature should not be more than one or two degrees less than the interior design temperature. If this limitation is exceeded, a thicker wall may be required.
Maximum. The maximum interior surface temperature, Timax, may be approximated using a similar equation to that used to approximate the minimum interior surface temperature, except that the value of one-half the interior surface temperature fluctuation is now added. The maximum interior surface temperature, Timax, may be determined by using Equation 7.
Timax = Tomin + ΔT + Ti (7)
The maximum interior surface temperatures for various face brick thermal storage wall thicknesses are calculated to be:
8-in. wall, Timax = 54 + 73 + 6 = 133°F
12-in. wall, Timax = 51 + 76 + 3 = 130°F
16-in. wall, Timax = 48 + 79 + 1 = 128°F
24-in. wall, Timax = 44 + 83 + 0 = 127°F
The maximum interior surface temperature should