Masonry Magazine August 1981 Page. 16
Technical Notes
Technical Notes 43 may be used as the minimum exterior design temperature. The actual expected interior design temperature should be used, usually ranging from 68 to 72°F. Design temperatures selected in this manner usually result in a conservative temperature gradient because when they are used in steady-state analysis the heat loss through the thermal storage wall is not accurately predicted.
The total resistance of the thermal storage wall assembly may be determined by using Equation 2.
R, = f + R + R + R + R + f (2)
where: R, = Total resistance of the thermal storage wall system, in (°F. ft² hr)/Btu.
f = Thermal resistance of the exterior air film, in (°F. ft² hr)/Btu, usually taken as 0.17 (°F ft² hr)/Btu.
R = Thermal resistance of the glazing material, in (°F. ft² hr)/Btu.
= Thermal resistance of the airspace between the glazing material and the brick thermal storage wall, in (°F. ft² hr)/ Btu, usually taken as 0.97 (°F. ft² hr)/ Btu if the airspace is between 4 in. and 4 in.
R, = Thermal resistance of any night insulation which might be used, in (°F. ft² hr)/Btu.
R = Thermal resistance of the brick thermal storage wall, in (°F. ft² hr)/Btu.
f = Thermal resistance of the interior air film, in (°F ft² hr)/Btu, usually taken as 0.68 (°F ft² hr)/Btu.
The total thermal resistance of an 8-in. thick brick thermal storage wall system constructed without night insultation would be determined by using Equation 2 to be:
R, = 0.17 + 1.45 + 0.97 + 0.88 + 0.68
R, = 4.15 (°F. ft² hr)/Btu
Similarly, for the other wall thicknesses:
12-in. wall, R, = 4.59 (°F ft² hr)/Btu
16-in. wall, R, = 5.03 (°F ft² hr)/Btu
24-in. wall, R, = 5.91 (°F ft² hr)/Btu
The minimum temperature on the exterior face of the brick thermal storage wall may be determined by using Equation 3.
To min = T; - [[(f/R) + (R/R)] × (T; - T.)] (3)
where: To min = Minimum exterior surface temperature of the brick thermal storage wall, in °F.
T; = Interior design temperature, in °F.
T. = Exterior design temperature, in °F.
Consider that these various wall thicknesses are to be constructed in Washington, D.C. The exterior design temperature may be obtained from Table 1 of Technical Notes 43, where it is given as 23°F. Using Equation 3, the minimum exterior surface temperature of the brick thermal storage wall may be determined.
To min = 72°F - [(0.68/4.15) + (0.88/4.15)] × (72-23)]
To min = 53.58°F or approximately 54°F
Similarly:
12-in. wall, To min = 50.65, or approximately 51°F
16-in. wall, To min = 48.23, or approximately 48°F
24-in. wall, To min = 44.47, or approximately 44°F
Maximum
The method used to determine the maximum exterior surface temperature of a thermal storage wall requires the use of complex hour-by-hour calculations using the solar energy transmitted through the glazing material, both direct and reflected, and the temperature of the exterior ambient air. An hour-by-hour energy balance with the hourly transmitted energy and the hourly exterior temperatures known would result in a good estimation of what the temperature of the exterior surface of the thermal storage wall would be. This type of procedure can only be feasibly accomplished by computer simulation and thus, a simplified hand calculation would require the judicious selection of an assumed thermal storage wall maximum exterior surface temperature.
On clear winter days, the maximum exterior surface temperature for a black surface, a = 0.98, behind glazing, may range from 140 to 180°F. A typical average value of 160°F may be used for these calculation procedures unless more accurate information is available to the designer. This value is applicable when double glass is used. To approximate the temperature which might occur when single or triple glass is used, the average temperature should be multiplied by the glazing correction factor, Co, provided in Table 3 of Technical Notes 43D. If the glazing material is other than glass, the correction factor should be obtained from the glazing manufacturer. Correction factors for absorptivity and orientation are also needed. The absorptivity. a, for surfaces other than black may be used to determine the correction factor for absorptivity. The correction factor for absorptivity is simply the absorptivity of the brick divided by 0.98. The correction factor for orientation, Co, is 1.00 for walls facing South. For walls other than South, the correction factor for orientation