3/24/02 - Team Article - By: Brian, Brad, pHaestus and Joe

Intro/Primer -

Welcome to one of the first breakdowns of the 4 main cooling technologies in use to cool computers. In this article the ProCooling team took different parts that they have the most experience in and break down the technologies into the basic principles behind them.   This is not an over the top physics class type article, this is meant to help you understand what the deal really is with these cooling systems. This will also break down a few very common misconceptions about cooling systems.

With that said, lets move onto a Primer of what we are going to cover-

Basic Principles of Cooling - ( pertains to all cooling technologies)

• Energy can not be created or destroyed - Heat is thermal energy, I think we all know that. What some people don't know is that heat can't just disappear. It needs to be converted or changed, or moved. The power that enters the core, generates a lot of thermal energy, that energy is moved via the path of least resistance to a transport medium.  That medium, whether it be air, water, vapor, plasma, or solid - absorbs the heat energy and holds onto it until it can pass it off to another medium.

Heat exchangers work very well since they use a medium that can absorb mass amounts of heat: ambient air ( or in some cases sea water for industrial purposes).

What does this all mean? 100% of the heat your CPU/Pump/Ambient air generate MUST go some where. No matter if you are talking about a Phase change, or Water or HSF, what comes out of the Heat exchanger, and the losses in the transfer to other mediums = 100% All cooling systems are simply heat pumps, some just do it faster than others.

• Going Below Ambient -  Going below ambient temp ( ambient air around the heat exchanger) is not possible with normal Heat Sink or straight water cooled systems. In order to drop below ambient you MUST either use a heat pump device such as a compressor (for phase change), or Peltier type units. One other technology that works very well is Evaporation cooling.  Its a step child of water cooling and Phase change.  It will be covered later in this article, but it uses the rather dry ambient air to lower your temps below ambient.   So lets re-cap, you can only go as cool as the air going through the heat exchanger with a straight water cooling system and that would be at 100% efficiency.  With a HSF you can only get as cool as the air going through the HSF, which would also be 100% efficiency.

• 100% Efficiency - Its impossible to hit 100% efficient :)  Its also pretty hard to tell what your system is really running at, but if you get figures that your system is 100% or above... time to re-calibrate some of your temp tools :)

3 Basic Types and Phases of Cooling

Stages: ( all numbers relate to all 3 systems in the appropriate location on each system.)

1. Initial Heat Transfer - At this point around 99% of the heat from the CPU is going to the Heat Sink device.  The only things that can affect this are air flow from the sides or under the CPU.  The thermal interface between the core and the block is critical, and is where Lapping and Thermal Paste comes into play.  This is the same across all cooling technologies

2. Heat Spreading/Heat Transfer - Heat Spreading is a critical aspect for any cooling system. Since the core is only 1cm x 1cm in some cases, its hard for any coolant to absorb that much heat in such a small area.  It also would lead to instant failure if the flow was disrupted at all. Heat spreaders vary by a huge amount between systems.

• Heat sinks have normally used either an all Aluminum, all Copper, or a Copper/Aluminum hybrid for heat spreaders.  The spreader is meant to take all the heat from the small core and get it to the entire fin base as fast and as easy as possible.  The material used and the interface to the fins are of the greatest importance.
• Water cooling  uses the Heat Spreader a bit differently. Since Copper is more efficient at transferring heat than water by a massive amount, it is more efficient to spread the heat out with a block than run water just to the core and back.   Effectiveness is largely dependent on block/pump/radiator/heatload. The block can also give a margin of safety, since it generally has a large mass of Copper it can act like a heat battery, and soak up heat if the flow is stopped.  This will only work till the copper reaches its heat soak point when overall temps will start to increase very rapidly.  ( the entire time frame for most blocks is no more than 5 - 60 seconds between a cut flow and a fried chip on high power CPU's. One of the parts that makes a H2O system different than a Phase Change system is the volume of coolant in is the exact same as the volume of coolant out, and the coolant remains liquid.
• In a Phase Change system the Block is yet again totally different in what it does. In a Phase Change setup, its called the Evaporator simply because that's what it does. This is the epicenter of the phase change.  The Liquid that enters the block/evaporator is very compressed, and has a very low boiling point. When it enters the evaporator, it boils off, and takes a ton of heat with it in gas form.  This can yield many degree below 0 temps with ease. The Evaporator has to be able to take the intense pressure and temp swings that happen on these systems, they also must be made very efficient in order to get the heat from the core to the evaporation chamber as fast as possible.  A system like this does not benefit as MUCH as water cooling systems with the heat spreader, but its still needed for a few reasons. The first one is temp shock. You don't want freon spraying on your core at -50C onto a core that is cooking at 80C, that would shatter the core instantly.  The heat spreader acts as a mediator to slow down the instant temp shocks, as well as mediate the movement of heat away from the core.

3. Hot Coolant Transfer - The Coolant that's just been run through the heat spreader, is now on its way to the heat exchanger. The Coolant in all cases has picked up ~90% of the heat from the heat source.

• A heat sink has no coolant transfer lines, but just moves the heat into the ambient air. Air flow in a case is critical to keep this air from reentering the HSF and defeating effectiveness of the heatsink.
• With Water or Phase Change, these lines just carry the heat in either liquid or gas form to the heat exchanger

4. Parasitic Heat Loss - This is the loss that happens to any un insulated surface that's either hotter than ambient or cooler than ambient. This is a good and a bad thing.  This is where ~10%+ of the heat in your system can be lost into the ambient air surrounding the heat source.  This is also where a heat spreader can absorb more heat from the surrounding hot air.

• With HSF's and H2O cooling, this is a good thing, and can give some added cooling for "free". This is also why its good to keep some air moving around the heat source even with watercooling. The bad in all this is cause if you have a heat source NEAR the heat source you are cooling, your cooling system will "suck" in outside heat to a point. So if you have some hot VRM's right next to your block, putting a HSF on the block will only encourage the system to pull heat from the air if the block is cooler than the heat being given off by the VRMS.  This is also where a HSF system will benefit from very good airflow.
• We talked about the 2 "room temp" systems but Phase Change presents its own problems in this area.  Since most Phase Change setups run WELL below ambient temp, they will cool the ambient air by pulling in any heat near it.  This will also cause condensation and possibly ice to form.  Insulation on a Phase Change system is critical for efficiency, and reliability. A good and safe rule to use in any cooling system that may run below the ambient temp, is to have any component that is running cold insulated.  Any exposed surface that is cooler than ambient WILL form condensation if its below dew point, and will soak up ambient heat in the process.

5. Exhaust/Waste Heat- This is simply where all the heat ends up. No matter what technology you are talking about, in a perfect system 100% of the heat would exit at that point.  Now of course reaching 100% efficiency on anything like this is not really possible so we take what we can get.

• In a Heat sink all the heat MUST be lost through the exhaust air as fast as possible.  There is so little mass in a HSF there is no other place for the heat to really go. A HSF  setup sends without a doubt the most heat to the fin surface, but also has the highest restriction on its size.  This is the single biggest factor that hampers a HSF from performing as well as a water cooling setup.  If you could make a HSF with as much surface area as most radiators it would perform just as good as any water cooling setup. ( depending on air flow of course).
• Water cooling and Phase change cooling have a few other factors in this. In water cooling I would venture a guess that a good deal of heat is lost in the hoses, reservoir, and any places its in contact with a case side or something. Given that silicone and vinyl are not good heat conductors, they do carry some of the heat to the surface where it is dissipated in the air. So with that said the Radiator on a H2O rig moves a lot of the heat but has a much easier job of it since there is a much higher mass between it and the core to loose heat at different stages.  Radiators also benefit from the almost unlimited size you can go since you are able to mount it anywhere.
• Condensing coils have the same things going for it except for that they are able to make much more efficient use of fin space, and small coolant volumes.   The evaporators only need to be a fraction of the size that a water cooling system needs to be to do the same amount of heat movement.

6. Cooling Air / Return Coolant- This differs with each setup, but in theory this is either the cooled coolant from the radiator / condenser or fresh ambient air in the case of a HSF.  The temp of all of them if its a perfect world should be at ambient temp. Once again we cant count on that happening too often ;)

• For a HSF this is mostly dependent on how well the airflow in the case is planned out. Since you want to get the hot exhaust heat that's coming out of the HSF as far away from the intake as possible. In a good setup the HSF would be sucking in cool air from outside the case and blowing that down onto the HSF.
• The water coming back from a radiator on a H2O rig really may not be much cooler than the incoming coolant into the radiator. This is a factor of how effective the radiator is for the volume/speed/type of coolant you are moving through it. Of course the closer to ambient the better.
• Phase change is far different than the others in this respect. The condensers main operation is to cool the hot gas that was boiled up from the evaporator. Once the gas is cooled down it re-condenses into liquid, so the flow back to the evaporator is a cooler, and liquid form. The temps of the return coolant varies between system, heat load, etc...

That should get you set for the rest of the article.   It is being broke up into 4 sections:

1. Phase Change Cooling - I know this is of great interest since many people simply don't know much about how this technology really works. Brian takes you though this and makes it pretty clear what happens where and how!
2. Hybrid Cooling Technology - pHaestus talks about the cooling technology that may not fit into any other single area on its own.  Part phase change, part watercooling, part air cooling. This includes heat pipes, and evaporative cooling.
3. WaterCooling - This is going to cover the many facets that are related to this topic. Joe will look at watercooling setup and design, what to look for in a rig, and also technologies that work well with water ( ie: Pelts).
4. HSF/Air Cooling - This is a topic many people "know" about, but brute force air flow is not always the best solution for any given situation. Brad will show you how to do it right.

Click the topic to jump to that part of the article. or just click below to go through the entire article *recomended*

Click here to get started - Phase Change Cooling Explained