Page 248 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

5.32 CHAPTER FIVE

FROST CLOSURE OF VENTS

During cold weather, exposed vents on a roof may become wholly or partially blocked by
frost on the inside portion of the exposed pipe. This is due to the fact that in cold weather,
a current of warm, moist air rises through the plumbing piping when there is little or no
flow through the system. This upward flow of air is caused by the temperature difference
between the air outside the building and the air inside the pipe. Since the air inside the
building is warmer than the free air, the inside air is lighter, causing the upward current.
This is the so-called chimney effect. When this warm, moist air reaches the chilled surface
of the exposed pipe, moisture condenses on the colder surface of the pipe in the form of
droplets. If the correct conditions exist, these droplets will freeze. If this continues long
enough, the pipe will become blocked.
There are actually two phenomena that can occur. The more common is the formation of ice
in the form of an annular ring in the pipe interior. Another kind of blockage takes the form of a
frost cap on top of the pipe. This problem of frost closure was the subject of a study at the National
Bureau of Standards in 1922. The conclusions resulting from that study are still valid today.
The best method to prevent frost closure is to increase the size of the vent pipe just below the
roof level, allowing the warm air to bypass the side of the exposed pipe as much as possible. It
was shown that a 4-in (DN 100) pipe would not close up except under the most adverse condi-
tions. One of these conditions was observed in the vicinity of Niagara Falls, New York, where
the spray from the falls solidified on the pipes regardless of their size. The solution was to run
either an electric heat trace line or circulate hot water around the perimeter of the pipe to keep it
warm. For more unusual conditions, it was found that keeping the vent pipe as low to the roof as
possible was acceptable. Blocking by snow at that low height never occurred, because the snow
was porous enough to pass air and had a tendency to melt very quickly.

DEPTH OF FREEZE IN SOILS

In order to prevent the freezing of water in underground piping systems, the pipes must be
buried far enough below grade so that the soil used as backfill provides enough insulation
from the air temperature to prevent the freezing process from starting. This depth is called
the frost depth, which is the level to which the 32°F isotherm will penetrate.
There are various methods used to determine the frost depth in a particular area: the
local authorities such as the building department or fire marshal for information and advice
based on past experience; recommendations of fire insurance carriers; or the use of maps
such as the one shown in Fig. 5.5.
The following discussions will show how to calculate the frost depth and how to obtain the
required criteria in order to perform the calculations with an acceptable degree of accuracy.

DERIVATION OF THE BASIC FORMULA
FOR FROST DEPTH

The U.S. Army Cold Region Research and Engineering Laboratory (CRREL) has devel-
oped a practical method for calculating the frost depth of soil. The method used to calculate
frost penetration is based on heat transfer principles involving a phase change (from water
to ice) of pore water, or water held in the voids of soil. The actual freezing process under
these conditions is very complex and does not lend itself to a fixed mathematical solution.
In order to reduce the complexities involved for the exact determination of each value that
follows, several assumptions have been made. These assumptions have the effect of sim-
plifying the solution, but also introduce some slight error. All of the following derivations

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