Document pBJYKRKJZgjpmk88vwY5yXwyw

176 CHAPTER 9 1952 Guide tent within the ranges cited increases the conductivity by approximately 30 or 40 percent. At higher moisture contents the percentage increase would be less. Effect of Soil Characteristics. The thermal conductivity of the soil, at a given density and moisture content varies in general with the texture of a soil; being rela tively high for coarse-textured soils and relatively low for fine-textured soils. The mineral composition of the soils also affects the conductivity. Quartz tends to give high values, whereas minerals such as plagioclase feldspar and pyroxene, which are constituents, of basic rocks tend to give low values of thermal conductivity. These points are illustrat'ecj by the values in Table 3 which lists seventeen soils in approxi mate order of their magnitude of thermal conductivity from greatest to least for seven different density-moisture content conditions. Some of the .values in this table have been determined by extrapolation and are consequently approximate. Blank spaceS^n the table indicate that the density or moisture content, or both, are such that no-tests were possible for that condition or that no tests were sufficiently Table 3. Thermal Conductivity (k) Values of Soils in Approximate Order op Decreasing Values" Mean Temperature--Ifi F I I Moisture Content--% SoiD No. Son. Deaicitation 4 | 4 j 10 | 10 | 20 | 20 Dby Dbnbity-db fxr co rt 100 no 120 90 110 90 100 P4714 P4703 P4701 P4709 P46G4 . P4601 P4705 P4706 P47U P4704 P4713 P4502 P4503 P4708 P4602 P4710 P4505 Fine Crushed Quart* Crushed Quart* Graded Ottawa Sand Fairbanks Sand Lowell Sand Chena River Gravel Crushed Feldspar Crushed Granite Dakota Sandy Loam Crushed Trap Rock Ramsey Sandy Loam Northway Fine Sand Nortbway. Sand He&ly Clay Fairbanks Silt Loam Fairbanks Silty Clay Loam Northway Silt Loam 12.0 11.5 10.0 8.5 8.5 16.0 16.0 14.0 10.5 11.0 9.0=b 6.0 7.5 .5.5 7.5 6.5 5.0 6.0 4.5 4.5 4.5 4.0=b 6.5 5.5 6.0 22.0 13.5 13.0 0.5 10.0 9.5 7.0 * <= Btu per (square foot) (hour) (Fahrenheit degree per inch). 15.0 13.5 13dz 10.0 8.5 7.5 5.5 9.0 5.0 9.0 5.0 S.Orb 4.Ode 7.Orb 8.0 7.5 7.5 0.O 10.0 10.0 9.6 7.0=t close to permit a reasonable extrapolation of the data. Granular soils, particularly those with high quartz contents, head the tabulation or have the greatest conduc tivity at a given condition. Sandy loam soils are midway in the table and fine grained soils such as clay and silt loam are last. Estimating Thermal Conductivity. The four diagrams of Fig. 3 are presented to aid in the estimate of the thermal conductivity of any soil. . Two of. the charts are for sands or sandy soils, and two for silt and clay Boils. One of the diagrams for each type of soils is for the frozen, and the other for the unfrozen condition. It is expected that these charts will give conductivity values with a precision of 25 per cent. The effect of such factors as density, moisture content, freezing, or texture may be easily approximated by use of these graphs. Specific Heat of Soils Tests to determine specific heat were run on twelve soils. On five of the soils, tests were made at three or four mean temperatures varying from about 10.to 140F. The specific heat values of all twelve soils varied by only a small amount (about 0.01), and averaged 0.19 at 140 F. The specific heat values of the soils decreased with a decrease in temperature. The average value at zero F would be about 0.16. Values at temperatures Heat Transmission Coefficients of Building Materials 177 between zero and .140 F can be estimated by considering a straight-line relationship between the two values given.: . Surface Conductance The surface conductance of a wall is the combined heat transfer to or from the wall by radiation, convection and conduction. Each of the three portions making up the total may vary, independently of the others, thus affecting the total conductance. The heat transfer by radiation between two surfaces is controlled by the character of the surfaces (emissivity); the temperature difference between them, and the solid angle through which they see. each other. The heat transfer by-convection and conduction's Fig. 3. Determining Thermal Conductivity op Soils prom Density and Moisture Content . > controlled by the roughness of the surface, by air movement, and tempera ture difference between the air and the surface. The importance of the effect of temperature of surrounding surfaces on the surface conductance, due to the effect on radiation, is illustrated in Table 4, which applies to a vertical surface at 80 F, with ambient air at 70 F and effective emissivity equal to 0.83.4 In many cases, because the heat resistance of the internal parts, of the wall is high compared with the surface resistance, the surface factors are of minor importance. In other cases, e.g., single glass windows, the surface resistances constitute almost the entire resistance and are therefore very important. An analysis of various factors affecting surface conductance and the difference between surface and air temperatures will be found in Reference 5. (See also Chapter 23.) The convection part of the surface conductance is affected markedly by air movement. This is illustrated by Fig. 4, which shows the results of tests6 made on 12 in. square samples of different materials at a mean temperature of 20 F, and for wind velocities up to 40 mph. These con ductances include the radiation portion1 of the coefficient which, for the conditions of the tests, was about 0.7 Btu per (hr) (sq ft) (F deg). More