(k) The above calculations neglected any

approach utilizing a digital computer. However, some

useful relationships can be obtained via a steady state

consideration of the thermal benefit of finning the pile. A

analysis assuming that the mean ground temperature,

1/4-inch-thick, 6-inch-wide fin of 4-feet length would

g

T , remains unchanged at a distance of ten radii from the

dissipate 6.2 Btu/lin ft hr at 20 F air temperature and 20.7

pile.

Btu/lin ft hr at 0F. The use of four fins would have the

The heat removed by the pile is:

effect of essentially doubling the heat transfer rate from a

cylindrical surface. The use of six fins, a number

commonly used, would reduce the freezeback time from

while the heat input from the soil is:

16.5 and 10 days to 12 and 6 days for ambient air

temperatures of 20 F and 0 F respectively.

(I) The above example assumed that the

air would be quiescent during the freezeback period; the

effect of an average wind of only 2 mph is considered

next:

(m) The freezeback time is reduced from

20 to about 12 days, representing a reduction of 40%

from the natural freezeback time without thermal pile

assistance. Had the temperature averaged 0 F, the

time would be reduced by about 60%o to 7-3/4 days.

These results are tabulated below:

Freezeback Time (Days)

Without Fins

With Fins (6)

20F

Wind

0F

20F

0F

For the sample problem above in which the average wind

speed was 2 mph, it is estimated that the permafrost

0 mph

16 1/2

10

12

6

temperature would be depressed from 28 F to about 19

2 mph

12

7 3/4

--

--

at the pile/soil interface.

5 mph

9 1/2

5

--

--

(2) Single-phase piles.

Natural Freezeback Time 20 days (Permafrost at 28 F)

(a) As previously described, the single

(n) As indicated by these calculations, the

phase system, or convection cell, operates by virtue of a

amount of surface area presented to the cold outside air

density gradient induced by temperature difference

is critical and thus it is essential that snow (which is a

between the above-ground (exposed to air) and the

rather good insulator) not be allowed to impede heat

below-ground portions of the pile. Such systems may

transfer from the pile. Heat transfer by the emission of

use a confined liquid, or gas, or ambient air as the heat

long-wave radiation from the pile will accelerate the heat

transfer medium within the pile. As the fluid extracts

transfer process while absorption of solar radiation tends

heat from the soil surrounding the pile, its density

to retard heat transfer. The use of high albedo paint to

decreases, thereby causing the fluid to rise and be

reflect the incoming solar radiation is a common practice.

replaced by overlying cooler fluid. Heat exchange to the

(o) Computation of heat transfer in

atmosphere is accomplished either through the pile wall

service. An indication of the magnitude of temperature

for liquid systems or by

depression below the mean temperature of the ground

surrounding the pile is useful in appraising the potential

adfreeze strength provided by the thermal pile. This

problem is best solved by means of a finite difference