Masonry Magazine October 1981 Page. 16
Fluctuation.
The exterior surface temperature fluctuation, AT, is the maximum exterior surface temperature adjusted for convective losses minus the minimum exterior surface temperature. Thus, for the wall thicknesses being considered:
8-in. wall, AT 114-54=60°F
12-in. wall, AT-114-51= 63°F
16-in. wall, AT 114-48=66°F
24-in. wall, AT 114-41= 73°F
Interior Surface Temperatures
Once the exterior surface temperatures of the vented thermal storage wall are approximated, the interior surface temperatures and temperature fluctuation are determined using Equations 4, 5 and 6. This is the same procedure as for the unvented wall, as provided in Technical Notes 43E.
Fluctuations.
The interior surface temperature fluctuation may be determined by using Equation 4.
TAT x Kal
(4)2
where: T, Interior surface temperature fluctuation, in °F.
AT Exterior surface temperature fluctuation, in °F.
w Wall thickness, in feet.
Constant 3.14.
8
- Thermal diffusivity, in ft³/hr.
= Constant 24 hr for daily cycle, in hours.
For the 8-in. thick vented thermal storage wall:
T60 xe(12) √3.14/10.024×24)
T, 12.65°F, or approximately 13°F
Similarly, for the other wall thicknesses:
12-in. wall, T, 6.10°F, or approximately 6°F
16-in. wall, T, 2.93°F, or approximately 3°F
24-in. wall, T, 0.68°F, or approximatley 1°F
Minimum.
The minimum interior surface temperature, Ti, may be approximated using Equation 5.
Ti min To min+2ATT,
and thus for the various wall thicknesses:
8-in. wall, Ti min 54+30.0-7.5=76.5°F
12-in. wall, Timin 51+31.5-3.0=79.5°F
16-in. wall, Timin 48+33.0-1.5=79.5°F
24-in. wall, Tmin 44+36.5-0.5=80.0°F
Maximum.
The maximum interior surface temperature may be determined by using Equation 6.
Timor Tomin+AT+T
and
(6)¹
for the various wall thicknesses of the vented thermal storage wall being considered:
8-in. wall, Ti mar 54+ 30.0+7.5=91.5°F
12-in. wall, Timar 51+ 31.5+ 3.0=85.5°F
16-in. wall, Ti maz 48+33.0+1.5=82.5°F
24-in. wall, Timar 44+ 36.5+0.5 = 81.0°F
Thermal Storage Wall Thickness
The criteria for aiding in the selection of thermal storage wall thickness for vented walls are the same as for unvented walls, except the available energy now includes the energy provided by the convective loop in addition to that provided by radiation. The amount of radiant energy available may be approximated by using Equation 7.
q = € 0.174 x (Ti mar+ T)/2
+459.6]
(T+ -459.6)/10 459.6) /10
(7)
where: q. Amount of radiant thermal energy, in Btu/sq ft/hr.
e
-Emissivity of brick masonry, usually 0.93.
T, Interior design temperature, in °F, usually 72°F.
For the various thicknesses:
8-in. wall, q, 12.07 Btu/sq ft/hr
12-in. wall, q, 10.52 Btu/sq ft/hr
16-in. wall, q, 8.98 Btu/sq ft/hr
24-in. wall, q, 8.47 Btu/sq ft/hr
The average amount of convective thermal energy obtained from the system when the convective loop is operating requires knowing the average temperature of the surfaces surrounding the airspace. The exterior surface temperature of the unvented thermal storage wall may be used to approximate the surface temperature on one side of the airspace. The interior surface temperature of the glass, the other side of the airspace,