that the effort required to advance the pile is similar to

La = length of pile exposed to air, ft

(b) The addition of fins to the pile

that required for a closed-end pipe pile and may make it

impossible to advance the pile at all. Normally H piles

improves its heat transfer capability. An indication of this

improvement can be determined for a unit length of pile

require less additional effort in the spring as compared to

fall.

per fin by:

(d) The design engineer should

therefore carefully consider the period of installation with

respect to mobilization, transportation, work and

equipment efficiency, ground water and soil problems

when augering as well as the attendant freezeback time.

Since the problems associated with the installations at

different periods of the year will be reflected in the

quotations.

e. Heat transfer by thermal piles.

(1) Two-phase thermal piles.

(a) As previously noted, the two-

phase system operates on an evaporation-condensation

cycle wherein the vapor condenses on the inner walls of

the pipe pile and flows down the pipe walls to mix with

the liquid phase. The requirement for spontaneous

operation of the device is that the temperature in the

upper reaches of the interior wall must be colder than the

(c) For the case of unfinned piles, natural

saturation temperature of the vapor. The selection of the

convection (no wind), and assuming that turbulent

refrigerant should consider such factors as its vapor

conditions generally prevail, the equation q = hc A ∆T =

pressure, vapor density, and flammability. A refrigerant

hc A (Tv - Ta) (equation 9) is modified by introducing:

having a low vapor pressure at a given temperature will

tend to minimize the leakage potential and to simplify

sealing. A high liquid density at a given temperature will

tend to increase the gravity forces which remove the

liquid condensate after its formation on the upper walls of

the pile. Although the thermodynamics of the internal

pipe refrigerant are important, particularly the thermal

resistance of the condensate film which varies in

thickness along the interior pipe wall, the governing

resistance (exclusive of the freezing soil surrounding the

pile) may be assumed to be the air boundary layer on the

pile's exterior surface. This is particularly true for

conditions of heat transfer from the exposed portion of

the pile by essentially natural convection. On this

where:

assumption, rate of heat transfer to the exterior from the

exposed surfaces of a two-phase thermal pile may be

a is determined for the mean temperature condition, Tm

estimated using the following equation: further, assuming

that; the pipe and soil are in intimate contact along the

2

g

= acceleration of gravity, ft/sec

entire buried portion; the pipe relies solely upon heat

dissipation from its vertically oriented surface i.e., no

0

= coefficient of expansion for air, I/Fabs

horizontal piping connections at the surface) and; the pile

is of sufficient diameter so that the upward vapor flow

p

= air density, lb/ft'

and downward condensate flow do not impede their

mutual development:

cp

= specific heat of air at constant pressure,

Btu/lbm F

∆T

= TV - Ta, F

J = absolute viscosity of air, Ibm/ft hr

= refrigerant vapor temperature, F

TV

= ambient air temperature, F

Ta

= surface area of pile exposed to air per lineal foot,

Al

2

ft

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