Page 249 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION 5.33

FIGURE 5.5 Approximate frost depth in feet. (NFPA.)

have been verified under actual field conditions and found to be well within the accuracy
necessary for engineering design purposes. These assumptions are:

1. The soil is considered homogeneous throughout.
2. All pore water is converted to ice at 32°F.
3. Average thermal properties are applicable.
The depth to which the 32°F isobar will penetrate can be calculated by using the modi-
fied Berrigan equation, which is:

⋅
X = C 48 KNF (5.12)
L
where X = penetration of frost into soil, ft
C = coefficient, dimensionless (This is a function of the freezing index, the mean
winter temperature at the site, and the thermal properties of the soil.)
N = conversion factor, air index to surface index
F = air freezing index
SF = surface freezing index
2
K = thermal conductivity of the soil, Btu/ft /h/°F
F = air freezing index, degree days Fahrenheit/year
L = volumetric latent heat of fusion, Btu/ft 3
Each of the elements of the formula will now be discussed and defined.
1. Coefficient C. This is a general coefficient that considers the overall effect of the tem-
perature change in the soil mass around a buried pipe. It is a function of the site freezing
index, the mean annual site air temperature, and the thermal properties of the soil. To
find this coefficient, two factors must be determined. They are the thermal ratio and the
fusion parameter.

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