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Transcript
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 1 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 2 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. 3