Download Heather Brown: Cooling

Cloud Chamber
Cooling Analysis
Heather B. Brown
December 4, 2006
Motivation
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From experience, we know that the bottom of the chamber
must be cooled to a rather low temperature, generally as cold
or colder than dry ice (-70 deg C).
Dry ice is easy to acquire but entails maintenance every few
hours and does not provide a flat surface.
Since chambers have been made successfully and
consistently with dry ice, the next step is to devise a
perpetual cooling system to provide constant entertainment.
The continuous cooling would ideally be provided indirectly
through an electrical outlet.
Thermoelectric Module (TEM)
How they work
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Thermoelectric modules are solid state devices (no moving parts) that convert electrical energy into a
temperature gradient. They are inefficient and little power is produced.
They are typically 1.5 inches square (40mm x 40mm) or smaller and approximately 0.25 inches (4mm) thick.
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Initial heat sinks used to cool the
ceramic hot side
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The hot side was the motivation
for heat sinks.
Theoretically, with a Delta T of 70
deg Celcius, cooling the hot side
would further cool the cold side.
The heat sinks are the same
length and width as the TEMs.
Ice water was circulated through
the heat sinks as Figure 1 shows
to cool the ceramic hot side of
TEMs.
Result: These heat sinks did not
have enough cooling power
needed for the TEMs (cool side
reached maximum -11 deg C).
Figure 1. Schematic setup with TEMs
Figure 2. Top and bottom views of heat sink.
Heat Sink / Fan Design
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A CPU cooling fan was
purchased from Fry’s to cool
TEMs.
This heat sink / fan combo
consumes 2.4W of power and
has optimum operation at 12V.
Its dimensions are 83 X 73 X
61 mm.
Result: This attempt at cooling
the ceramic hot side was the
worst. The lowest temperature
reached with the TEM was +15
deg C.
Figure 3. Upside down view of the “Copper X478.”
The TEM hot side sits on the copper side.
Solid Copper Heat Sink Design
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Purchased from the same
company as the TEMs so we were
hoping for better results.
Was hooked up directly to our
water pump with rubber tubing.
The dimensions of this all copper
liquid heat exchanger are 89 X 64
X 12.7 mm.
This was the first all copper heat
sink we used.
Result: It did not transfer the cold
from ice water as well as we
needed. The minimum
temperature reached with the
TEM cold side was +5 deg C. This
was the end of our TEM usage.
Figure 4. (Top) All copper constructed
liquid heat exchanger made by the
same company that sold us the TEMs.
(Right) Yay! No more TEMs!
Liquid Nitrogen Cooling
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Purchased from MSC
distributors, this all aluminum,
2-fin system provides better
heat exchanging than copper.
The ‘recommended’ liquid to be
used with this plate is ethyline
glycol (typical antifreeze for a
car or CPU).
Dimensions are 279.4 X 198.12
X 19.05 mm.
We glued polyethyline tubing
into the fittings and tested ways
to create a flow of liquid
nitrogen through the cold plate.
This was by far the most
expensive item purchased for
the cooling team.
Figure 5. All aluminum cold plate used with liquid nitrogen testing.
Liquid Nitro Experiment
Competency
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We achieved the lowest
temperatures
experienced yet (avg. 18 deg C) in various
placements of the cold
plate (i.e. the plate
exhibits the same
behavior in many
different positions).
Figure 7. Horizontal transfer of liquid
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Figure 6. Vertical transfer of liquid
With a direct flow of liquid nitrogen into the
plate, the temperature went below -70 deg
C (thermometer’s measuring limit).
Liquid Nitro Experiment
Incopetency
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Proper fittings and funnel would
reduce or eliminate the leakage
found at the unification of tubing and
cold plate and provide a safer
method of transferring the liquid
nitrogen.
We now understand that the cold
plate must be cooled to roughly -60
deg C before trying to circulate the
liquid nitrogen due to the plate being
too warm and rejecting the liquid.
The pump used was made for 3 V
but needs at least 20 V to work
constantly with the liquid.
Figure 8. Unsafe method of transferring liquid
Conclusions
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TEMs are completely incapable of providing a low
enough temperature.
Liquid Nitrogen was definitely the best method
used so far because of the temperature results
attained.
A more independent circulating system would
need to be devised to continue using the liquid
nitrogen. A manufactured chiller would be the best
idea.
Future Plans
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www.thermo.com has many types of
chillers, circulators and baths.
One example of a circulator/bath
combo is the Neslab ULT-80 and it
operates from -80C to +10C.
Ultimately, a similar apparatus
would be the most effective for
achieving our desired temperatures
constantly.
Figure 9. Neslab ULT-80; Work area (L X
W X D) in cm is 13.7 X 17.8 X 24.1; weighs
336 lbs.; 4 gallon bath; cooling capacity
250W at -70C; costs $13,533 + Tax