Masonry Magazine August 1981 Page. 19
Radiant Heat Source Temperature
The surface temperature of the radiant heat source should not be more than 20°F greater than the interior design temperature. This is because a radiant heat source more than 20°F warmer than the interior design temperature may cause discomfort. The surface temperature of the radiant heat source should never be much more than 90° F. For the various thicknesses of face brick thermal storage walls, analyzed by calculation, the 8-in. wall might well result in uncomfortable radiant heat being supplied to the interior of the building.
Thermal Storage Wall Thickness
Consideration of the time lag, interior surface temperatures and the amount of radiant heat available aids the designer in selecting an appropriate thermal storage wall thickness by applying these considerations with empirical procedures. The amount of radiant heat available may be approximated by considering the average interior surface temperature and substituting this value into Equation 8.
q, = 0.174 x € x [[(T, max + T, min)/2 + 459.6] - (T; + 459.6)4]/108 (8)
where: q, Amount of radiant thermal energy, in Btu/sq ft.
€ Emissivity of brick masonry, usually 0.93.
Using Equation 8, the average amount of radiant heat available from the 8-in. thick thermal storage wall would be:
q, 0.174 x 0.93 x [[(98.0 + 83.0)/2 + 459.6] - (72+459.6)/108
q, 0.162 × [(550.1)-(531.6)]/10
18.97 Btu/sq ft.
Similary for the other wall thicknesses:
12-in. wall, q, 17.36 Btu/sq ft.
16-in. wall, q, 15.76 Btu/sq ft.
24-in. wall, q, 13.65 Btu/sq ft.
The interior surface temperatures, time lag and average amount of available radiant heat are summarized in Table 1. The 12-in. or 16-in. thick thermal storage wall analyzed for Washington, D.C., in January, should provide the best performance. Although the 8-in. wall supplies the greatest amount of radiant heat, it does not provide a sufficient time lag and the temperature may exceed the comfort level. The 24-in. thick wall offers temperatures within the comfort range, but the time lag will probably be excessive and the average amount of radiant heat available is low. Thus, the designer should select either a 12-in. or 16-in. thick wall, depending on the interior temperatures, time lag and average amount of radiant heat available which best satisfies the requirements of the particular design.
From Reference 5.
TABLE 1
Face Brick Thermal Storage Wall Calculation
Summary for Washington, D.C. in January
Unvented Wall
Wall
Thickness
(in.)
T
(F)
T
(F)
Time
Lag
(hr)
(Btu/ft³)
8
98.0
83.0
6
19.0
12
92.5
85.5
9
17.4
16
89.5
85.5
12
15.8
24
86.0
85.0
18
13.7
*Brown brick, a 0.78
METRIC CONVERSION
Because of the possible confusion inherent in showing dual unit systems in the calculations, the metric (SI) units are not given in the examples. Table 13 of Technical Notes 4G provides metric (SI) conversion factors for the more commonly used heat transmission units.
SUMMARY
This Technical Notes provides information which may be used to assist the designer in the selection of unvented thermal storage wall thicknesses, and to provide the designer some basic information on how the wave of thermal energy penetrates a thermal storage wall. The information presented is based on simplified heat transfer equations and the necessary assumptions so that the equations may be performed by hand calculations. The information is not intended to accurately predict actual performance, but only to provide a comparison so that the designer may more judiciously select the appropriate system type and thermal storage wall thickness. The decision to use the information and concepts presented in this Technical Notes is not within the purview of the Brick Institute of America, and must rest with the designer or owner of a specific project.
REFERENCES
1. ASHRAE Handbook and Product Directory, Fundamentals Volume 1977, American Society of Heating. Refrigerating and Air-Conditioning Engineers, New York City, New York.
2. Brick Masonry for Thermal Storage, by Stephen S. Szoke, McLean, Virginia, Presented at "Passive Solar Building Construction Program", Madison, Wisconsin, November 1980.
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