Keep Your Systems Cool

To get started, you may want to take a look at my related recipe Select the Right Power Supplies for Your Servers. In that article, I show you how to calculate a system's power requirements, and the figures you arrive at there will plug in to the formula in this recipe for determining cooling requirements.

Elementary Computer Cooling Principles

Typical computers are cooled by what is called "forced convection" using fans to move air throughout the system. The size and speed and number of cooling fans will depend on how much heat you need to remove from your system. Generally speaking, a larger slower fan is more efficient at moving air than a smaller faster fan, and it will also be quieter.

You should pay attention to the direction of airflow (intake and exhaust) while making sure to always move hot air away from the computer and its components. The effectiveness of any forced air convection cooling system is dependent upon the ambient air temperature (the temperature of the air in the room), so environmental control of your computer room will be a factor.

Furthermore, since we cannot cool our system below ambient temperature, then the lower we get our ambient temperature, the cooler we can get the system without condensation formation. Cooler air is denser than hot air, which means we have to move less volume of air to move the same mass of air, and remember it is the mass of air that cools our computer, not the volume. So, the lower the ambient temperature, the more efficient our cooling system will be and the fewer fans we will need to move the necessary amount of air in and out of the computer system.

If you'd like more information on this topic, or to get a handle on related hardware available, both APC and Liebert have product lines dealing with the environmental control (or thermal management) of a computer room.

Thermodynamics 101

Once you know and understand the fundamental laws of thermodynamics, the study of how heat is moved, you can gain an appreciation of what is going on inside a computer and why it is happening. Essentially, the laws of thermodynamics state that no heat is gained or lost within a "system"—only moved from one point to another. Also, that adjacent "systems" will seek equilibrium. A "system" in this context is a temperature zone. For our purposes, the air inside the computer is one "system," and the ambient air is another.

Note: For the calculations in this article I use temperatures in degrees Celsius, but for the following example—it's irrelevant what scale the temperatures are measured in.

Next (and by way of example)—if the air inside the computer is 100 degrees and the air outside is 50 degrees, then the best we can hope to achieve is cooling the computer to 75 degrees and heating the air around it to 75 degrees. That's what the laws of thermodynamics tell us. While it is possible, using alternative cooling methods, to cool a system below ambient temperature, I don't recommend this. It introduces the hazard of condensation formation, which if not dealt with completely at all times, will cause a failure. Obviously, condensation inside an electronic computer system cannot be tolerated because electricity and water do not mix.

Now, how does knowing all this help you design your server? Well, the forced-convection cooling system uses air as its heat-transfer medium. All you need to focus on is moving the heat out of the system by exchanging the air inside the computer (the hot air) with the air outside the computer (the cool air). These are our "system boundaries," in thermodynamic terms. The more air you can move across those system boundaries, the cooler you can keep the computer.

Now let's look at how you calculate your air exchange rates. The first thing you will need to do is change the way you think about the airflow rates that our cooling fans deliver. You need to remember that it is not the volume of air that cools the computer, but the mass of air. You must first calculate the amount of heat energy to remove from the system, and then derive the volume of air it will take to do that. From there, you can figure out which cooling fans will do the job based on manufacturers' specifications.

Each unit of mass of air can hold a certain amount of heat before its temperature increases another degree. This is called the "specific heat" of the air, and it is a constant at a given temperature and pressure. The specific heat decreases as temperature increases, and the density of the air decreases as temperature increases. This means that as the system heats up you will have to circulate a higher volume of air to remove the same amount of heat as before. Now you can see why it is important to keep the air in and around your computer as cool as possible.

Breaking Down the Math of Airflow Calculations

The amount of heat energy to dissipate from the system is determined from the power requirements. You calculate this by taking your numbers from the power consumption calculations and then dividing by the efficiency rating of the power supply (a percentage expressed as a decimal). For example, if you have a server that needs 500 watts of power, and the power supply you are using has an efficiency of 67 percent, then the heat output into the system would be 746 watts. This is what you get when you divide 500 by 0.67. Now, the same server with a power-supply efficiency of 80 percent would yield only 625 watts of heat energy. Therefore, the efficiency rating of your power supply has a profound impact on the amount of heat introduced into the system.

Before you get started with your own calculations, you need to decide how close you want to keep the internal system temperature to the ambient temperature. This is an arbitrary decision, but a good rule of thumb is 10 degrees C. The lower your temperature tolerance, the more air you will need to moveand the requirements can jump up really fast.

Note: In practice, the system designer will usually run through a few iterations of the equation, trying different values for the temperature difference, and noting the effects on airflow requirements. When he has reached a result that is realistic and satisfactory, he can move on. Experienced engineers know that a good rule of thumb is 10 degrees, so I suggest this value also. Ideally, we would like to keep it exactly the same as ambient, but in reality that is impossible. We just need to decide how much hot air we are going to allow to hang around inside our computer. Therefore I used the word "arbitrary".

At this point, you know how much heat you need to remove from the system. You also have decided how much of a temperature rise you are willing to tolerate inside the system. Combined with the properties of air (our cooling medium) such as density and "specific heat," this will tell us the mass of air we need to flow across the "system boundary." From there, it is only a matter of converting the mass of air to a volume of air—which will help you determine the size and speed and number of cooling fans that are needed for the job.

Ideally, half of your fans should be intake and the other half should be exhaust. Remember, you'll actually need twice as many fans as your calculations indicate. That's because you'll need to get rid of just as much air as you bring in, in order to keep things moving properly. If there is inadequate exhaust flow in a system, the hot air will build up inside the computer and you will have problems. If you don't bring in enough air, you will not meet your temperature tolerance.

To switch a fan from intake to exhaust duty, simply flip it over and install it with the pitch of the blades pointing in the opposite direction. Some fans have a helpful arrow on them that indicates the direction of the airflow.

To save you a lot of trouble, I worked out the equation for deriving the volumetric airflow requirement for a given amount of heat energy. It is:

Volume of air = watts / (density of air x specific heat of air x temperature change x 60)

Note: 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 will give you the volume in cubic meters per minute, which we must then convert to cubic feet per minute to compare with most manufacturers' datasheets for their cooling fan products. Here, you will be working through your equation once, using sample values that will show you an example of how the numbers would look in an actual system design problem.

For the example here, the units I'm using are metric, but I'll show a conversion to standard units at the end. I'll also include Web sites that are good sources for finding unit conversions and thermodynamic value tables for the numbers you will need to plug into the equation.

• 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.

The main advantage that system builders bring to the table for their customers is the ability to design and build a custom computer. This can also be their greatest hindrance, if they are not careful when choosing components.

Optimum reliability and longevity are achieved by effectively meeting a computer's power and cooling requirements. By making use of the techniques outlined in this recipe — and in my previous recipe Select the Right Power Supplies for Your Servers — system builders can confidently compete with the "big boys," and field products of remarkable quality.

Proper system design is perhaps the greatest contributing factor to the minimization of failures once a computer is operating in the field. Heat-related troubles are often misdiagnosed by field technicians, and (aside from power problems), are among the most difficult to resolve.

By figuring your cooling requirements and planning appropriately, you, the system builder, will save money in the long run by not having to chase after and troubleshoot intermittent problems. This translates directly into more satisfied customers and new opportunities for future business growth.

DAVID GILBERT is the owner of Appalachian Computer Systems, a system builder based in West Virginia that specializes in multiprocessor SCSI RAID servers.