Masonry Magazine June 1981 Page. 17
where:
r = The average thermal resistivity of the storage media, in (°F. ft² hr)/Btu in.
rᵢ = The thermal resistivity of each component, in (°F. ft² hr)/Btu in.
tᵢ = The thickness of each component, in inches.
Thus consider again the 14-in. grouted hollow wall constructed of a 4-in. wythe of face brick, a 4-in. grouted space, and a 6-in. wythe of grouted hollow brick. The nominal thickness and respective average thermal resistivities of each component would be:
t₁ = 4 in., r₁ = 0.11 (°F ft² hr)/Btu in.
t₂ = 4 in., r₂ = 0.08 (°F ft² hr)/Btu in.
t₃ = 6 in., r₃ = 0.10 (°F ft² hr)/Btu in.
Substituting these values into Equation 5, the average thermal resistivity of the wall, r, is determined to be:
r = [(0.11 × 4) + (0.08×4)+ (0.10×6)]/(4+4+6)
r = [1.36 °F ft² hr)/Btu in.]/14 in.
r = 0.097 (°F. ft² hr)/Btu per inch of thickness
Thermal Conductance. Thermal conductivity and thermal resistivity refer to the value of heat loss for one inch of thickness. The thermal conductance is the value of heat loss for a specified thickness. The average thermal conductance for a one foot thickness of the storage media is used in the simplified equations of heat transfer for determining effective thermal storage. The average thermal conductance for a one foot thickness of the brick storage media may be determined using Equation 6:
C = 1/(r x 12 in./ft)
(6)
where:
C = The average thermal conductance of the storage media for one foot of thickness, in (Btu/hr/°F/ft²)/ft.
The 4-in wythe of face brick, 4-in. grouted space and 6-in. wythe of grouted hollow brick is calculated to have a thermal conductance per foot of thickness of:
C = 1/[0.097 (°F. ft² hr)/Btu in. x 12 in./ft]
C = 0.859 (Btu/hr/°F/ft²)/ft
Thermal Diffusivity. Typically the storage capacity of a material is represented by the amount of heat which can be stored in the material, the heat capacity of the material. The heat capacity may be determined by using Equation 7:
B=cxp
(7)
where:
B = Heat capacity, in Btu/cu ft/°F.
Typical values for the heat capacity of various brick are provided in Table 1. Thermal diffusivity is a function of the heat capacity and thermal conductance per foot of material thickness. The thermal diffusivity of a material may be determined by using Equation 8:
δ = C/B or δ = C/(cxp)
where:
δ = Thermal diffusivity, in ft²/hr.
(8)
The values of thermal diffusivity for typical brick masonry are provided in Table 1. The value of thermal diffusivity may be used to provide the designer with a better concept of heat storage in the passive solar energy system thermal storage component. The use of the thermal diffusivity in simplified heat transfer equations may provide a more rational approach for selecting the thickness of thermal storage walls. The rational approach for selecting thermal storage wall thickness is provided in Technical Notes 43E.
The average value of thermal diffusivity may be determined by using Equation 9:
δ = C(CXP)
(9)
where:
δ = Average thermal diffusivity, in ft²/hr.
Thus for the 14-in. thick wall assembly constructed of 4-in. solid brick wythe, a 4-in. grouted space, and a 6-in. wythe of grouted hollow brick the average thermal diffusivity may be determined using Equation 9. C, for this wall was determined to be 0.859 (Btu/hr/°F/ft²)/ft, p was determined to be 125 lb/cu ft and c₁ 0.22 Btu/lb°F. Thus, the average thermal diffusivity would be:
δ = 0.859 (Btu/hr/°F/ft²)/ft/(0.22 Btu/lb/°F x 125 lb/cu ft)
δ = 0.031 ft²/hr
The value of the average thermal diffusivity is useful in simplified heat transfer equations, however if precise values are desired, each component in section should be analyzed individually.
Emissivity. The emissivity of a surface is its ability to radiate heat to the surroundings. This is the basis of heat retrieval in passive solar energy systems as discussed here. The radiant heat from the surface of the brick masonry is what causes the useable natural flows of thermal energy: i.e., surface to air conduction, convection between surfaces and radiation to surfaces and the air, which heat the interior spaces of the building. Typical values of emissivity for brick are provided in Table 1. Exposed brick masonry allows the use of various