TM 5-852-4/AFM 88-19, Chap. 4
direct mixing for air systems. Successful operation of
Volumetric heat capacity of thawed backfill
3
Cu = 80 [0.17 + (1.0 x 0.4)] = 45.6 Btu/ft F
this concept requires use of plumbing within the pile
Heat required to depress the slurry temperature
which physically separates the warm and cold fluid
to the freezing point:
columns. As air temperatures increase above ground
45.6 x 31 (40 - 32) = 11,310 Btu/pile
temperatures, the convective process is stopped,
Heat required to freeze slurry:
thereby preventing induction of warm ground
31 x 4,600 = 142,600 Btu/pile
temperatures. It should be noted that when ambient air
Heat required to depress the slurry temperature
is used as the heat transfer medium, summer winds may
from the freezing point to 23 F:
cause undesirable air flow within the pile and necessitate
29.6 x 31 (32 - 23) = 8,260 Btu/pile
the use of positive shutoffs.
Total heat to be extracted from the slurry:
(b) Although a potentially simpler,
20(11,310 + 142,600 + 8,260) = 3,243,000 Btu
essentially unpressurized system, the thermal efficiency
Time required for artificial freezeback excluding
of the natural convection, single-phase pile is less than
allowances for system losses
that for two-phase systems owing to the increased
3,243,000/225,000 = 14.4 hr (with losses allow
internal resistance associated with for the liquid-filled
20 hr)
pile: primarily the mass flow of the liquid with some
(c) The
maximum
operating
contribution due to the liquid side boundary layer thermal
temperature for the coolant is usually set 10 F below the
resistance in the portion of the pile exposed to air and for
desired in-situ permafrost temperature and the
the ambient air-filled pile: the low volumetric heat
temperature rise in the system should be fixed at 5 F or
capacity of the air at low rates. Some laboratory studies
less. Thus, for this example, the maximum temperature
reported by Johnson"' indicated that for liquid-filled (both
is (23 - 10) = 13F and a difference of 5 F would place
water-ethyl alcohol and trichloroethylene), 2-inch and 4-
inch model piles, a 5 to 15 F temperature difference
the lowest temperature at 8 F. The coolant freezing
point should be at least 10F below this minimum. If it is
between air and the material to be cooled was necessary
likely that the air temperature will fall below this freezing
to achieve any heat transfer over a range of air flow from
point during the refrigeration operation, then the low air
0 to 40 mph; this indicates the influence of inertia forces
temperature would establish the freezing point for the
which must be overcome to permit development of fluid
coolant.
flow.
(d) Allowing a temperature rise of 4F
(c) There are few heat transfer field
data available for this type of pile system at time of
in the refrigerant and selecting a 21 percent sodium
preparation of this publication.
chloride brine, the required circulation rate is:
(3) Forced circulation piles.
(a) In some cases it may be
225,000
necessary to install artificial refrigeration pipe or tubing
60 x 0.799 x 4 x 1.169 x 62.h/7.5 = 120.6 gpm
on the pile to accelerate slurry freezeback time and to
have such refrigeration available in the event that
(e) Using 3/4-inch black pipe on the
permafrost temperatures rise to unacceptable levels
pile, the rate of circulation is in the order of 1 ft/sec,
after construction.
The thermal pile technique is
which should be considered as an upper limit. This size
restricted to that period of the year when air
pipe or tubing is most commonly used.
temperatures are low and normally cannot be used to
(f) The
brine temperature will
accelerate freezeback during the summer period. The
average
11F which will result in a temperature
following example shows calculations required to
difference between the surrounding soil of (40 - 11) =
29F at completion of the refrigeration cycle.
determine the amount of heat to be extracted from the
ground.
(g) It is essential that temperature
(b) Example. It is assumed that the
sensors be used to insure that proper freezeback rates
average volume of slurry backfill for a group of piles is 31
and temperatures are obtained.
3
feet each.
The slurry is placed at an average
(h) Should the option of forced
temperature 40 F and must be frozen to 23 F. A silt-
circulation of a gas or liquid within a closed metal pile be
3
water slurry of 80 lb/ft dry weight and 40 percent water
considered, the computation procedures outlined above
content is used as backfill material, and an available
may be adapted, the case being technically comparable
refrigeration unit is capable of removing 225,000 Btu/hr.
to the freezing points used for stabilization of ground in
Calculate the length of time required to freeze back a
construction or for stabilizing foundations experiencing
cluster of 20 piles.
permafrost degradation. However, so far as is known
Volumetric latent heat of backfill
this approach has not been used except experimentally
3
L = (144 x 80 x 0.40) = 4,600 Btu/ft
in conventional foundation bearing piles in North
Volumetric heat capacity of frozen backfill
3
Cf = 80 [0.17 + (0.5 x 0.4)] = 29.6 Btu/ft F
4-127
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