Download Implimenting a Simple Heat Exchanger Unit with

Implementing a Simple Heat Exchanger Unit with
Commonly Available PC Cooling Components.
An Application note by Stephen Zajac
Introduction
Traditional heat exchangers are
complex units comprised of compressors,
radiators, and phase changes involving
chemical refrigerants. These systems have
several points for failure and are expensive
to build, making them impractical for
applications requiring small amounts of
cooling in a portable environment. If the
application requires heating, another
completely separate unit must be included.
This paper will discuss a method for
creating a heat exchanger comprised of
three distinct functional blocks; the two
mediums between which heat is being
exchanged; and one element to exchange
the heat between these two mediums. The
system can also heat the medium in
question simply by reversing the direction
of the current applied. Finally, A simple
implementation makes this system useful
is highly demanding environments.
An interesting analogy to voltage and
current in an electric circuit can be made,
with voltage representing temperature
difference, and current flow representing
heat flow. Max current (heat flow) is
obtained when there is a short circuit (no
temperature difference), and max voltage
(temperature difference) is obtained when
there is an open circuit (no heat flow).
Ohms law can also be considered with the
inverse of thermal conductivity being
analogues to resistance.
Basic Design
The core technology in this
application is a thermoelectric cooling
device. This unit creates a temperature
difference by moving thermal energy from
one side of the device to the other. The
function of the device can be changed
from cooling to heating simply by
reversing the direction of the applied
current, and thus changing the direction of
the heat flow.
The most important thing to
consider when using a thermoelectric
device is that the maximum temperature
difference is obtained when there is no
heat transfer through the device, and
maximum heat transfer is obtained when
there is no temperature difference across
the device. This is illustrated in figure 1.
Figure 1: Heat pumped (x-axis) vs.
temperature difference (y-axis)
The practical result of this concept
is that the more heat flow (aka the more
water in the system), the smaller the
temperature difference that can be
maintained. This means that every real
world cooling system will have a
minimum temperature that it can reach
based on the amount of water in the
system.
The next component to consider is
the water circulation system, which is the
medium to which heat is either applied or
removed from. The component that is
connected to the thermoelectric device is a
water block, which is fed water through
tubes and a pump system. With an
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Implementing a Simple Heat Exchanger Unit with
Commonly Available PC Cooling Components.
An Application note by Stephen Zajac
efficient thermal interface between the
thermoelectric device and the water block
it can be assumed that the temperature of
the thermoelectric device’s cold side and
the water temperature are the same. A
picture of this system is shown in figure 2.
Figure 2: Water block used as a method of
transferring heat to and from the TEM.
The final element to consider is the
heat sink applied to the hot side of the
thermoelectric device. This must be sized
large enough not only to remove the heat
pumped by the thermoelectric device, but
also the heat that it generates due to
resistive heating. Since there is always a
temperature difference across a heat sink,
it can not be assumed that the temperature
of the hot side of the thermoelectric
device, the heat sink, and the air are all the
same. Remembering that maximum heat
transfer is obtained when the temperature
difference across the device is minimum,
any temperature difference between the
hot side and the air adds to the overall
temperature difference, which reduces the
efficiency of the system. It is thus
advantageous to use the maximum cooling
potential available since this translates
directly to a more efficient system, where
as a transistor will operate essentially the
same at 30 °C or 50 °C, only with a
reduced lifespan. A proposed cooling
system is illustrated in figure 3. In this
system two large cooling fans (one
pushing air and one pulling air) are used to
move as much air across the heat sink as
possible. The total surface area of the
heatsink is about eight square feet, with
heat pipes used to transfer heat from the
base of the heatsink to the fins. Heat pipes
take advantage of evaporation and
condensation of a liquid in order to
transfer heat. The final aspect is a silver
paste interface used to thermally connect
the micro gaps between the surfaces of the
heat sink and thermoelectric device. With
all of these components in place, the
heatsink is able to dissipate 300 watts of
heat with only a 5 °F rise in temperature
from the thermoelectric device to the air.
Figure 3: Dual fan/heatsink cooling
system applied to the hot side of the TEM.
Real World Performance
A graph of temperature change
with respect to time for the cooling mode
is shown in figure 4. The temperature of
the water starts out at 120 °F compared to
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Implementing a Simple Heat Exchanger Unit with
Commonly Available PC Cooling Components.
An Application note by Stephen Zajac
the 80 °F air temperature, meaning that
there is actually a negative temperature
difference. This causes the system to
operate at peak efficiency until a water
temperature of 80 °F is reached. At this
point a temperature difference starts to
build up, reducing the cooling potential of
the system. The temperature difference
and heat flow balance out at around 50 °F.
At this point the amount of heat entering
the system through the air is equal to the
amount that is being removed by the
thermoelectric system. If a larger
temperature difference was required, than
one could simply reduce the amount of
water in the system, meaning less heat
would be absorbed by the atmosphere,
translating into less heat that needs to be
removed to lower the water temperature.
device and the thermoelectric effect. An
interesting note is that in this mode, the
heatsink is actually pulling heat from the
environment. The air outtake was
measured to be 2 °F colder than the air
intake. It would be appealing to turn off
the cooling fans to save power, however
they are actually increasing the
performance by providing room
temperature air to the heatsink. The point
at which the transition from both resistive
and thermoelectric heating to just resistive
heating can clearly be seen in figure 5.
Figure 5: Heating performance of a
thermoelectric system.
Figure 4: Cooling performance of a
thermoelectric system. Time (x-axis) vs.
Temperature change (y-axis).
A graph of the heating
performance of the system is shown in
figure 5. This again shows temperature
change with respect to time, only now the
water starts out at a temperature of 50 °F.
Initial heating performance is double that
of the cooling performance, since we have
heating due to both the resistance of the
Conclusion
To recap, this system consists of
three elements; a water block with a water
circulation system, a thermoelectric
device, and a heatsink with cooling fans.
Maximum heat transfer is obtained with
minimum temperature difference, which
makes efficient thermal interfaces very
important. There is no temperature
difference lost in the water block as long
as there is sufficient water flow, and added
temperature difference due to the heatsink
is about 5 °F. The final performance of
the system is that it can heat or cool 1 liter
of water in 10 to 20 minutes.
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