Submerged PC Cooling
Performed By:
Group 11
Geoffrey Clark
Chris Fitzgerald
Patrick Hague
Richard L. Roberts
Mech 690 Senior Design
August 3rd, 2007
Abstract
A vast portion of computer systems technology currently available in today’s market are based with air cooling in mind to dissipate heat, while only a small number use forms of liquid cooling. Liquid cooling offers more efficiency with transferable heat due to its densely packed molecules, while also using less energy in the process. Although there are many forms of liquid cooling, submersion will put forward the best results and also providing a larger cooling area. To mechanically accomplish this, a watertight tank is needed to house the selected non-moving components, to be immersed in dielectric fluid. The dielectric fluid provides for a form of liquid cooling that is safe to use with powered electronics equipment. Academically the focus of the project pertains to heat generation analysis and power savings methods by utilizing a passive system (fan-less).
Table of Contents
I. Abstract Page 2
II. Table of Contents Page 3
III. Introduction Page 4
IV. Procedure Page 7
V. Data Page 11
VI. Sample Calculations Page 15
VII. Results Page 20
VIII. Discussion Page 23
IX. Conclusion Page 24
X. Appendix
i. Appendix A – Components and Costs
ii. Appendix B – Data
iii. Appendix C – Charts and Graphs
iv. Appendix D – Drawings and Assembly
v. Appendix E - Photographs
vi. Appendix F – Formal Presentation
Introduction
In the present day most home and business computers are found to contain one of two forms of cooling available, air or liquid cooling. They are the only offered methods to the means of keeping our computers running for long periods of time without overheating. Air cooling is the more dominant type of cooling, utilizing forced or natural convection by use of fans and/or heat pipes. Liquid cooling, the more unconventional and rare, consists of primarily one type of cooling using a reservoir, pump, tubing and water blocks. Both have advantages and disadvantages with current technology, some which outweigh initial drawbacks. Deviating slightly from the two more standard forms of cooling is the possibility of a cooling technique utilizing immersion. However to immerse electrical components in a liquid that liquid MUST be dielectric or non conductive as to not damage them in any way.
Air cooling has been the foremost component with computers since the beginning. Firstly, by natural air convection and small heatsinks, the first pc’s were not able to pump out power and heat as they do today. As the technology evolves more and more heat is created through advancements in speed and overall performance. Forced convection has now become the forefront, processors and once integrated graphics chips are no longer able to be naturally cooled. They require this forced air convection or they will become damaged. Air is everywhere, inlet and outlet venting provides and endless supply of a cheap form of cooling and is perfect in heat removal.
Using a liquid to cool computers is a little bit more complex to set up and requires some monitoring of its activity. A setup requires the cooling components as mentioned above, in addition to a liquid. Most liquids are a formula that promotes a dielectric solution which is harmless to electronic components, reason being that if you were to spill this liquid at any time, your computer would not be harmed. Liquids have a superior ability to dissipate heat with much smaller comparable volumes compared to air. With a properly setup system processors and components can be run at lower temperature due to liquids properties, to the point where these components can be safely overclocked beyond their factory limits.
Between the two current options air is obviously the cheaper and more abundant method, but liquid cooling has many options not yet explored. The properties that can be applied far outweigh the downsides for the cooling systems setup needed. With newer more powerful personal computers being created today there is a need for the technology to either become more efficient in terms of power usage, or to develop more efficient methods of cooling.
Immersion is not a common practice with personal computers at all, by far the direct opposite. This process though is not limited to only a heat sink/fan or water block/pump. A method like this uses a non-conductive liquid such as mineral, vegetable or silicon oil (for starters) to completely cool the computer, not just single components. Although there are many forms of dielectric fluids, not all are applicable on the long term (Volkel 11). The potential for a liquid and its thermal properties are very great. Seeing what normal water cooled systems can accomplish with several ounces of fluid, imagine the possibility using several gallons. While a system like this is not available on the market today it is a feasible concept that can use a natural convection to properly dissipate the computers processor heat.
Why is this important? The cooling of microprocessors is extremely important. The integration of computer components into the simplest of household items raises design problems which need to be overcome in order to dissipate the heat produced by the electrical components. Without dissipating this heat, the computer processor will continue to heat up until it destroys itself or malfunction.
Major processor manufacturers are now building new factories to produce processors with a lower power output (Intel). The effect of this is that cooling demands are reduced. Although this defensive tactic on cooling is necessary, it also costs the companies billions of dollars to build plants and change manufacturing processes just to reduce heat output. If a cooling system can be designed without requiring a cooling fan, and be a fraction of the current size of heat sink and fan assemblies then many of the problems with computer system designs will be reduced. Processors will be able to take up less room using large amounts of power with more efficient cooling systems. The production costs of processors will be reduced due to the packaging and design for current cooling methods.
Procedure
The first thing that had to be done was to research everything that needed to be incorporated into the project. In order to dielectrically cool a computer requires a nonconductive fluid. Mineral oil is the fluid of choice because it has a low enough conductivity for it not to transfer any electricity from one component to the next. If the fluid was too conductive, transfer of electrons would occur, and the computer would short out. Mineral oil is also relatively cheap and obtainable. In order to keep the power consumption of the system lower than what it would have been with fans, there needs to be a passive cooling design in use. Therefore, a large heat sink was to be used to remove the heat from the enclosure. The material was specified for the enclosure to be acrylic. This decision was for aesthetic purposes only. The acrylic enclosure went well with the already available acrylic computer case. Enough computer parts were obtained by the project members to be able to assemble a working computer. Because of this there were very little initial purchased parts.
Once adequate research was performed it was time to start the design process. Prior to the creation of any SolidWorks drawing, a sketch of that part or design was produced. The first design was of the acrylic enclosure. The design of the enclosure needed to meet certain sizing specifications. After designing the enclosure and knowing the exact dimensions that the enclosure needed to be in order to house the motherboard, the enclosure heat sink was designed. The design for the enclosure heat sink including an opening which allowed for component cables to pass through the enclosure. In addition to that, it would have a track along the underside of the heat sink to fit on top of the enclosure to allow surface contact between the fluid and the bottom surface of the heat sink. Because the enclosure and associated heat sink design was so large there now needed to be a new method for mounted the CD-Rom and Hard Drive. To do this, mounting brackets were drawn up so that they would attach to the underside of the top case panel. Once attached the Hard Drive and CD-Rom could be fastened to the mounting brackets.
In order to solve the issue of fin sizing for the heat sink, calculations were performed. First the total heat dissipated from the various processors within the system along with the motherboard and RAM were calculated. By using the totaled value of heat dissipated by the computer, the fin sizing such as spacing, length, and the amount of fins were calculated. Free convection equations were used because the design utilizes passive cooling.
After determining what size heat sink was required to dissipate the total wattage displaced by the enclosure, COSMOS was used to model the heat sink and perform thermal analysis on the system. In Plot 1, the color gradient shows what the fluid temperature will do to the temperature of the heat sink, and moreover how well the heat will be dissipated through the sink.
Plot 1 - COSMOS Thermal Analysis
After the research, design, and calculation stages of the project were complete it was time to assemble, and manufacture the enclosure, heat sink and mounting bracket. First the enclosure was cut from stock into separate sheets of acrylic. Then using a system of braces and fish tank epoxy the sides of the enclosure were fit and sealed together.
The motherboard tracks were mounted and the enclosure was complete. The enclosure heat sink tracks could then be milled out. These tracks ensured the ability to have surface contact between the bottom of the heat sink and the mineral oil in the enclosure. The port which would allow the cabling to reach the power supply and the IDE components was then milled out. The enclosure heat sink fabrication process was then completed. This process entailed the facing off any remnant material to ensure proper heat transfer per our design calculations. As mentioned earlier there were little purchased parts from the start of this project. The parts that were purchased consisted of new heat sinks for both the video card and the CPU. These heat sinks were now properly installed onto their respective processors. Once the machining and fabrication was completed, it was time to assemble the entire system in preparation for testing. This included properly running, and attaching all cabling from the motherboard through the heat sink, and to their respective components.
In order to properly gauge how well the dielectric fluid transfers heat from the submersed components to the heat sink, we needed to collect data on idle temperatures of both the CPU and the fluid. The CPU temperature was obtained from the system BIOS in the PC Health menu. The Fluid temperature was obtained from a digital thermometer. Next was the stress test on the CPU, and video card. This test was done to determine how well the design works under maximum loads. Two programs were used in conjunction to apply the load. Those programs were Prime95, and Benchmark 2003. The program that was used to monitor and collect temperature information was called SpeedFan. After all of the idle and load temperatures were created, they were pooled together and put into graph form. This was done so that the data could be properly analyzed.
Data
In this section, some basic calculations were done to determine what fluid temperatures were acceptable to run in our final enclosure heat sink calculations. In Table 1, 2, and 3 the CPU, GPU, and Northbridge die temperatures were calculated respectively. In order to drive the heat transfer within the space required and with natural convection, a high temperature differential was needed and the fluid temperature was obtained from these calculations.
Below is a list of components used in the computer and their power output into the system. These specifications are essential to calculating the heat transfer equations in the tables below.
I. Computer Components
- Power Supply: Ultra X-Connect 500 Watt
- Motherboard: Gigabyte GA-7N400
- Processor: AMD Athlon XP 2000+
- Video Card: ATI x700 Pro
- Memory: Kingston DDR 400 512MB
- Hard Disk: Western Digital 20GB
- CD ROM: Generic Device
- Case: Pre-built acrylic with custom interior enclosure
II. Power Consumption of Original System
- Processor: 60.3 Watts
- Video Card: 33 Watts
- Memory: 3.2 Watts per stick
- Motherboard: 50 Watts combined
- Case Fans (5x 80mm): 2.16 Watts per fan.
- Video Card Fan: 1.92 Watts
- CPU Heat Sink Fan: 4.2 Watts
- Total Power Usage: 163 Watts
III. Power Consumption Within Enclosure
- Processor: 60.3 Watts
- Video Card: 33 Watts
- Memory: 3.2 Watts per stick
- Motherboard: 50 Watts combined
- Total Power Usage: 147 Watts (because these are maximum outputs, our factor of safety will be already applied)
IV. Mineral Oil
- Specific Gravity: 0.86
- Density (ρ): 860 kg/m3
- Viscosity, cSt @ 40°C: 34.5
- Thermal Capacity (Cp): 2000 J/(kgK)
- Thermal Conductivity (k): 0.15 W/(mK)
- Prandtl Number (Pr): 395.6
V. Component Design Specifications
- Processor Max Die Temperature: 90°C
Heat sink Properties
Thermal Conductivity (k) W/(mK) / Fin Thickness (t) m / Length of Base (b1) m / Width of Base (b2) m / Height of Base (Lb) m / Length of Fin (Lf) m / Number of Fins (N) / Fin Pitch (S) m
180 / 0.00159 / 0.06033 / 0.06033 / 0.00635 / 0.02857 / 18 / 0.00318
Mineral Oil Properties
Thermal Conductivity (k) W/(mK) / Velocity (U) m/s / Temperature (T∞) C / Viscosity (µ) m2/s / Prandtl Number (Pr)
0.145 / 0.001 / 25 / 0.0000345 / 400
Processor power (Q) W / 60.3
*Note: Fluid properties are estimated to be similar as motor oil
Base temperature of heat sink / 82.4213
Thermal resistance of base / 0.0097
Total thermal resistance / 0.9426
Surface area of one fin / 0.0035
Total surface area of heat sink / 0.0654
Reynolds number / 1.7487
Fin efficiency / 0.9677
m / 10.8185
Length of cross section / 0.0294
Nussault number / 3.3000
Average convection coefficient / 16.7483
Table 1 – Central Processing Unit Die Temperature Calculation