Single page Print

Asetek's Vapochill CPU cooling system


A Darth Vader suit for an overclocked PC
— 12:05 AM on March 17, 2003

Manufacturer Asetek
Model Vapochill Premium Edition
Price (street) US$569
Availability Now

THERE IS A PHRASE well-known to anyone who has ever tried to make his car go faster than the manufacturer intended: Speed costs: How fast do you want to go? Of course, it's all relative. You can spend $50 on a performance air cleaner or several thousand on a supercharger. The former will get you closer to the ideal: drawing as much air as possible into the engine. The latter, however, will get you past the ideal: putting more air into the engine than is otherwise possible.

Now that I've opened with a Shameful Car Analogy, let's move on to computers, specifically to overclocking. The prime enemy of the overclocker is heat. Pumping up the CPU voltage goes hand in hand with the quest for higher clock speeds, but higher voltage means substantially more thermal output from the processor. There are ways to deal with this heat: beefy, all-copper heatsinks with large fans, even water-cooling systems. Of course, the problem here is that the material we're using to cool the processor, be it air or water, is at best room temperature to start with. What we want is better than "at best"—a cooling supercharger. Enter phase-change cooling and the Vapochill.

Phase change 101
The key advantage of the Vapochill system over air cooling or water cooling is its ability to cool the processor below room temperature. It accomplishes this task using phase-change cooling, much like an air conditioner or a refrigerator. For the uninitiated, here's a primer on phase-change cooling.

First, it will probably help to understand some theory on how gases behave with respect to temperature, pressure and volume. Pressure and volume are inversely related; as pressure goes up, volume goes down, and vice versa. Take a turkey baster or a baby's aspirator and plug the end with your finger, then squeeze the bulb. As you squeeze, you're lowering the volume of the container, and at the same time, you can feel the increased pressure on your finger.

Temperature is directly related to pressure; as pressure increases, so does temperature. For example, if you have a volume of gas in a sealed container, and you heat that container, the pressure inside will increase. To look at it from the opposite direction, if you have a certain amount of gas and you compress it, you're decreasing the volume, thus increasing the pressure and the temperature. If you then reverse the process, you'll increase the volume, thus decreasing the pressure and the temperature.

Of course, a gas doesn't have to be a gas. Do things to it, and you can change it into a liquid. For example, if you lower nitrogen gas to -320 degrees Fahrenheit at standard atmospheric pressure, it will condense into a liquid. As you might expect, different gases condense at different temperatures.

Notice that in the example above, I used the caveat "at standard atmospheric pressure." That's because the temperature at which a gas will condense varies with regard to pressure; the higher the pressure, the higher the temperature at which condensation will occur.

So let's review: Assuming a constant volume, temperature goes up as pressure goes up, and temperature goes down as pressure goes down. A gas can be turned into a liquid if it gets cold enough, and the higher the pressure, the "less cold" the gas needs to be in order to condense into a liquid. Such a transition, from gas to liquid or vice versa, is called a phase change.

Now, let's expand on an earlier example. Suppose we have a gas in a sealed container that enables us to compress the gas within. Suppose also that the container and the gas inside is at room temperature. We compress the gas, decreasing its volume, which raises its temperature as well as its pressure. We could decompress the gas to its original volume, which would restore its original pressure and temperature. But what if, instead, we somehow cooled the compressed gas? Since the higher pressure means the gas will condense at a higher temperature, if we got the right gas, we could theoretically turn it into a liquid by cooling it to at or above room temperature.

Let's assume that such a gas is present in our sealed container, and we cool that gas down to a point where it condenses into a liquid. Now, we decompress the liquid to its original volume. What happens? Well, the pressure is no longer high enough to keep the gas in its liquid state, so it evaporates, or turns back into a gas. As the volume increases, the pressure decreases, and so does the temperature. Because we cooled the gas while it was still compressed, upon decompression it drops below room temperature. This ability to produce temperatures below ambient is the key advantage of phase-change cooling, whether it's in your air conditioner, your refrigerator, or the Vapochill system.

How the Vapochill does it
Of course, our example of one large sealed container isn't very practical for actually cooling anything, so the Vapochill system is necessarily more complicated. It basically consists of four parts: A compressor, a condenser, a throttling or expansion valve, and an evaporator.

As one might expect, the compressor compresses the gas, raising its temperature and pressure. Once compressed, the hot gas flows through metal tubing to the condenser. The condenser looks like a miniature radiator, and it draws heat away from the gas and disperses it outside the system. With an air conditioner, outside the system means outside the house. With a refrigerator or the Vapochill, outside the system basically means "the room it's sitting in." Once cooled, the gas condenses into a liquid and makes its way to the throttling valve.

In order for the system to cool effectively, there must be a distinction between the high pressure portion of the system which contains compressed gas and/or liquid, and the low pressure portion of the system, which contains gas waiting to be compressed again. This separation is accomplished by the throttling valve.

The throttling valve is a very small tube called a capillary tube. The high pressure liquid flows into this small tube, and from there to the evaporator. The small diameter of the tube restricts the flow of the liquid, allowing only so much to flow in a given amount of time. This moderates the evaporation process and keeps the high and low-pressure areas separated.

Once the liquid flows through the throttling valve, it goes to the evaporator. The evaporator has a larger volume than the throttling valve, so the liquid immediately undergoes a drop in pressure, evaporates to a gas, and loses a significant amount of heat in the process. The cold gas is in direct contact with the evaporator, so it draws heat away from the evaporator, cooling it.

In an air conditioner or refrigerator, the evaporator looks a lot like the condenser; it is a radiator through which air is blown or drawn. As the air passes through the cold evaporator, it's cooled, providing the refrigeration effect. With the Vapochill, the evaporator is a copper plate approximately the size of a quarter, which is in direct contact with the CPU core. Thus all of the Vapochill's cooling potential is concentrated on the core of the CPU.

On a more general note, regardless of the configuration of the evaporator, the cold gas "steals" heat from the evaporator, becoming warm gas in the process. It then flows out of the evaporator and through tubing that takes it back to the compressor to repeat the process.