Note: If you just want the "good stuff", jump down to the section on Convection.

 Heat 

Heat is vibration of molecules. Lots of vibration feels hot to the touch. No vibration feels extremely cold. Molecules are like teenagers with peer pressure. Blending in and being accepted is what counts. They like to vibrate as much as the next guy. They do not like to vibrate more than the next molecule, and they do not like to vibrate any less. If all the other molecules jump off of a bridge, then this molecule will do it too. If a group of molecules is vibrating like crazy (hot), and they go to school with some molecules just barely vibrating (cold), the whole school will, after some time has passed, end up vibrating half way in between (warm). 

 What’s the difference between molecules of metal and of air? Metal molecules only know their immediate neighbors because they don’t move about. When the neighbors’ 2 doors down vibrates a lot, our molecule doesn’t do anything different until his next door neighbor met the other neighbor then came over to "pass it on". Passing it on one by one is called conduction and there is more later. 

 Air molecules have the special quality of being free and of moving upwards as they get hotter. We like this a lot and we will take advantage of this. Air molecules also vibrate as heat, but they don’t have permanent neighbors. They just cruise around bumping into other air molecules, left and right. But, if they bump into a madly vibrating (hot) piece of metal, they start vibrating a lot too. And when this happens, it is time to head skyward. Up it goes until it bumps into other air molecules, transferring some of its vibration to the others it meets until its back to vibrating a little. When this happens it falls back down to repeat the process all over again. This air going up and down is called convection.

All of this talk about heat and not a degree to be found. Temperature is an objective (exact) measure of heat or vibration. Words like hot and cold are subjective (relative to one another). Since we know that heat is vibration, we can figure out that the least amount of vibration possible is to be perfectly still. Perfectly still is absolute zero degrees (on the Kelvin scale). There is no way to have "negative" vibration. There can be no less. On the other hand, there is no imaginable limit to how hard something can vibrate. This would be in the millions of degrees like the sun. Well, that’s for the scientists to worry about. We are doing some engineering, so let’s get real and worry about the range of temperatures we see here on earth: from 32F to 100F (120F in the desert!). 

Here’s what you need to know with respect to temperature. Heat will transfer from an object that is of higher temperature than an adjacent (right next to) object of lower temperature. Heat will not transfer if both objects are at the same temperature. (That’s why it is hard to cool a computer in a hot room.) 

Conduction: Through and Between Solids 

A very practical model of conduction is that of a CPU and a heat sink. The goal is to quickly move newly generated heat (made by electricity pulsing about the circuitry) up and out of the "chip". Here is a photo of an accessory which adds a fan into a heat sink. Modern CPU’s are ceramic packages topped with a metal (aluminum?) lid. An aluminum heat sink is held onto the CPU. Heat should flow through the CPU to it’s lid, then jump across to the heat sink bottom and through to the ends of the fins on the heat sink. 

 And now a word about thermal grease, paste or compound: Heat transfer is very efficient when 2 solids are in close contact with each other. Some of the spring-type clips used to hold the heat sink down are very strong, and this encourages good conduction. Think down to the microscopic level, where metals have minute scratches and imperfections. Heat conduction can not occur where there is no contact. Therefore, a special kind of paste or grease was made to encourage maximum conduction. It works to fill in voids and it may have a higher heat conduction factor. This comes with some good heat sinks or can be bought at cheaply Radio Shack (Cat. No: 276-1372). It should be used when there is a possibility that the CPU is overheating because the heat sink is not taking enough heat away. Caution; touching a heat sink and observing that it is fairly cool does NOT necessarily mean that it is working with good conduction. It is possible for the CPU to be very hot underneath it, but heat is not making its way across the boundary of the 2 parts. The best way to check this is to put an accurate thermometer on the CPU edge, then on the heat sink. If there is more than a few degrees difference paste should be used.

Following through with the above example, convection is the process of moving heat of off the heat sink fins and out into the air within a computer case. Convection is the heat sink surface heating the air which is closest to the surface, followed by the replacement of that newly heated air with some other nearby air which has not been heated yet. Repeat until cool. 

Natural Convection versus Forced Convection 

When the air heated by a surface rises up by itself, it is termed natural convection. If air is blown over a surface by a fan, it is called forced convection. (A kitchen convection oven really uses forced convection to cook food.) Heat sinks (like on CPU’s and on radiators) usually have fins or thin pieces of metal to help natural convection. "Fins are Natural, but Fans are Forced." 

Now we have finally come to the matter at hand; CCC and forced convection. Computers use forced convection for cooling because the fan in the power supply pulls air through the case. Also, most CPU’s have fans on their heat sinks, which is also forced convection. If we want to improve the cooling of a computer, we need to know about forced convection. Our objective is to take all of the heat created inside of the case and continuously move it to the outside of the case. 

Forced convection performance for computers is driven by the local velocity of the air, the mass flow rate of the air, and the difference in temperature between the components and the incoming air. Lets look at velocity or speed. If air is blowing fast across a component, then many molecules have a chance to grab some heat from the surface. If there is no velocity, then you are back to natural convection, which really has a little bit of velocity because some molecules are going skyward. 

The flow rate of air through the case is what takes the heat out of the case, the final leg of the journey when you imagine the heat being created inside the electronics, being conducted to the surface, then being moved into the air within the case by convection, and finally being exhausted out into the room. More air moving through the case means less temperature rise of that air, thereby increasing the efficiency of the forced convection. 

To understand why difference in temperature is important, lets imagine the 2 extremes. 

• If the outside air is kept at the same temperature as the inside, heat will not move between the air and the surface. Picture a cereal bowl upside-down over a CPU. The air and the part will just keeping getting hotter until the CPU fails. 
• If the outside air was really cold and it blew onto the hot part, the part would cool very rapidly (like a wind chill factor). 

So the bottom line is that a large difference in temperatures is good for cooling. Now you probably have sufficient background to feel comfortable pursuing advanced design modifcations. 

Further Information 

Heat transfer is most often used by Mechanical Engineers and Desginers. In difficult situations, Mechanical Engineers will investigated convection heat transmission by using theoretical, numerical, and empirical methods of analysis. An example of numerical methods is Finite Element Analysis. FEA results in cool graphics like this one I did of air flow over a hot plate: 

There is example temperature test data on the Fast, Cheap, and Easy results page. To find more information on the subject of Heat Transfer, use a search engine with keys words such as;  heat transfer convection conduction thermal thermo temperature

If I find any good sites that seem to have some staying power (so I don't have to keep fixing broken links) I will post them here. This site has nice 4 meg video .avi FEA files http://www.algor.com/ on computer fan airflow and on thermal gradients which are worth checking out.

You are welcome to link to this site, but please don't copy it.

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dclark@desernet.com