Bosch Geothermal Heat Pumps Applications Manual page 50

5 0 | Bosch Geothermal Heat Pumps
Clay soils tend to hold water very well and the water
content and thermal conductivity will not vary much over
time.
Good ground heat exchanger design involves selecting the
lowest soil thermal conductivity expected for the particular
soil type and installation location. Moisture and thermal
stability of soil will generally increase with depth in the
soil.
ASHRAE (2005) provides typical thermal conductivity
values for different soil types based on work by Salomone
and Marlowe (1989). Notice that the normal range
expected for each soil type is provided, as well as low
values that re ect worst-case scenarios for ground heat
exchangers, and high values that re ect analysis of
systems where maximum heat transfer rates are predicted
(Table 22). The low values are typically satisfactory unless
better data is available.
Thermal Conductivity Values per Soil Type
Soil Type Normal Range¹
Sands 0.35 - 1.45
Silts 0.50 - 1.45
Clays 0.50 - 0.95
Loams 0.50 - 1.45
Tab. 22 Thermal conductivity of soil types
Courtesy of American Society of Heating, Air-conditioning and
Refrigeration Engineers Inc., www.ashrae.org
¹) Btu/hr ft F
²) Reasonable values for use when no site or soil-speci c data
available
³) Moderately conservative values for minimum heat loss
through soil (use in soil heat exchanger or earth-contact cooling
applications). Values from Salomone and Marlowe (1989).
)
Moderately conservative values for maximum heat loss through
soil (use in peak heat loss calculations). Values from Salomone
and Marlowe (1989).
The thermal properties for sand and clay soils will typically
range from 0.3 to 1.9 Btu/h•ft•F°.
Thermal Properties for Sand and Clay Soils
Dry Density
Soil Type
(lb/ft²)
120
Coarse 100%
100
Sand
80
120
Fine Grain
100
100% Clay
80
Tab.23 Thermal properties of sand/clay soil types
Courtesy of American Society of Heating, Air-conditioning and
Refrigeration Engineers Inc., www.ashrae.org.
Data subject to change
Recommended Values for Design²
Low³ High
0.45 1.30
0.95 1.30
0.65 0.90
0.55 1.30
5% Moist
k
1.2 - 1.9 0.96 - 1.5 1.4 - 2.0
0.8 - 1.4 0.77 - 1.3 1.2 - 1.5
0.5 - 1.1 0.60 - 1.3 0.6 - 1.1
0.6 - 0.8 0.48 - 0.64 0.6 - 0.8
0.5 - 0.6 0.48 - 0.58 0.5 - 0.6
0.3 - 0.5 0.36 - 0.6 0.35 - 0.5
10% Moist 15% Moist
k k
0.93 - 1.3 1.6 - 1.2
0.96 - 1.2 1.3 - 1.6
0.60 - 1.1 0.6 - 1.2
0.4 - 0.53 0.8 - 1.1
0.4 - 0.48 0.6 - 0.7
0.35 - 0.5 0.4 - 0.55
Applications Manual
20% Moist
k
0.91 - 1.2 —
0.89 - 1.1 1.4 - 1.7 0.84 - 1.0
0.51 - 1.0 0.7 - 1.2 0.52 - 0.90
0.46 - 0.63 —
0.37 - 0.48 0.6 - 0.8 0.41 - 0.55
0.34 - 0.47 0.4 - 0.6 0.30 - 0.45
Bosch Thermotechnology Corp.
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