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How
They Work
http://si.wsj.net/public/resources/images/OB-FA306_ev_fro_EV_20091203182244.jpg
The heat pipes along TAPS, also known as
thermosiphons, work as freezers for the ground, without
requiring a power source. They operator purely off
temperature differential between the air and ground the
are buried in. Filled with anhydrous ammonia, the heat
pipes remove heat from ground in the winter, effectively
super-freezing it, which prevents it from thawing during
the warmer summer months. The following diagram explains
the general process taking place within a heat pipe.
http://www.faculty.virginia.edu/ribando/modules/ExtendedSurface/pipeline1.gif As seen in the above image, the heat
from the ground warms the working fluid (anhydrous
ammonia) which then vaporizes and rises to the top
of the heat pipe, where the cooling fins remove the
heat to the surrounding air and the ammonia
condenses, falling back to the bottom of the tube,
where the cycle repeats. This heat transfer will
only work when the air is colder than the ground.
During summer, when the air is warmer than the
ground, the ammonia sits in liquid form at the
bottom of the tube and nothing happens. If the heat
pipe didn't effectively super-freeze the ground
during the past winter, risk exists for the ground
supporting the VSM to melt, which can lead to
shifting of the pipeline. To see the physics behind
this heat transfer, click here.
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