Page 251 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION 5.35

FIGURE 5.7 Latent heat of fusion for soils.
2. Air ground conversion factor N. This factor is necessary in order to approximate the
temperature of the ground when only the air temperature is known. The combined
effects of radiative, convective, and conductive heat exchange at the ground air junction
have been considered in the determination of the actual value. Refer to Table 5.12 for
values that have been established by experimentation under freezing conditions.
3. Air freezing index AF. The penetration of freezing temperatures into the soil is partially
dependent on the duration and magnitude of the temperature difference between the
air and the ground. The lower the air temperature and the longer the freezing persists,
provide a cumulative increase in the penetration of frost. The method used to express
the freezing temperature-duration measurement is in degree days Fahrenheit. In order
to find the value, the high and low temperatures for any 24-h period are averaged. Then
32 is subtracted from that number to obtain the freezing index for that day. If it is found
to be 10°F below freezing for 1 day, this amounts to 10 degree days. If the number cal-
culated comes to 1°F below freezing, and if this continues for 10 days, the figure would
also be 10 degree days. The freezing index should not be confused with the normal
degree-day measurement used by HVAC engineers for heating calculations.

TABLE 5.12 Air-Ground Conversion Factor
Surface type N Factor

Snow 1.0
Pavement free of snow and ice 0.9
Sand and gravel 0.9
Turf 0.5

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