Keep Your Systems CoolKeep Your Systems Cool

Calculate the amount of heat to be dissipated from the system. Going back to our previous example, if you have a system that consumes 500 watts of power and has an efficiency of 67 percent—that gives you a value of 746 for watts: 500 / 0.67 = 746 Look up the value for air density at your given temperature. The value you will use here for temperature is the temperature that you want the inside of the computer system cooled to. It is an arbitrary value decided upon by you, the system builder. Air density is a chemical property of atmospheric air that behaves according to certain laws of chemistry. All you have to do is look up the value you need from the table below and plug it into your equation here. This Table of Air Properties (from The Engineering Toolbox), which I found handy for looking up the properties of air at a variety of temperatures: For example, at 20 degrees C. (approximately 68 degrees F.), we find a value of 1.205. Next, look up the value for the specific heat of air at your given temperature. It must be the same temperature as you used in the preceding step for air density. You want to use the value for specific heat at constant pressure (often written as cp). Work with the same air properties table as mentioned above. The values in this table should get you close enough, but if you need a wider variety of temperatures for your application, you could search the Internet for other such tables, or find a textbook or engineering reference at the local library that covers this sort of information in more detail. For our example, you will need the value at 20 degrees C., which is 1.005. As was covered in a previous paragraph, plug in your value for the tolerated temperature change in the system. Remember this is an arbitrary value. As a rule of thumb, 10 degrees C. is a good starting point. So, if your ambient temperature is 10 degrees C. and you want to keep the inside of the computer cooled to 20 degrees C., then your numbers are right. Remember, your temperature tolerance plus your ambient temperature must equal the temperature you are using as your basis for the values of air density and specific heat—again, taken from The Engineering Toolbox's Table of Air Properties. Plug in your constant value of 60. (Note: Again, the number 60 is the number of seconds in a minute. This needs to be in the equation to convert our flow rate from a "per second" value to a "per minute" value). This constant is used to convert your resulting units from cubic meters per second to cubic meters per minute. At this point, your equation should look like this: Volume of air = 746 / (1.205 x 1.005 x 10 x 60) Once you work it through, you will come up with approximately 1.027 cubic meters per minute. Now multiply your result by the constant value of 35.31. This constant is used to convert cubic meters per minute to cubic feet per minute (or "cfm" for short). Our final answer, then, is about 36.3 cfm. This site features information on Hydrologic Conversions which I find handy when converting units. (Note: You can ignore the word "hydrologic," as it's not relevant to our discussion here). Basically, unit conversion calculations are an integral part of doing work like this. After you get tired of doing calculations with pencil and paper — an online calculator like this comes in quite handy and will save you a lot of time. Once you have this information, you can look at product datasheets for various fans and find what you need. For example, Sunon makes an 80-mm fan that flows 37 cfm at 2800 rpm. That would be perfect for this application (or a situation where we need to move about 37 cfm of air) — assuming you could fit a pair of them into your chassis. If you have a 1U rack mounted chassis, (for instance) then you will be limited to 40-mm fans instead. Sunon has a 40-mm fan that flows 8.9 cfm at 7200 rpm, so you would need eight of those — four for intake, four for exhaust — to meet your cooling requirements. The PDF file, DC Brushless Fans from Sunon, is the product specification sheet for all the DC brushless fans from the company. We'll use it here to cross reference part numbers for existing off-the-shelf products that will enable our system to meet our calculated airflow requirements for cooling purposes. Run through the equation again, using different values. This step is optional, for the most part, but you may need to try again if you end up with airflow requirements that are impossible to accommodate. This might happen with small chassis that have limited fan space and high heat, as in the example above.