Cooling Thoughts

[tholly]

Hey folks .. thought I'd share some thoughts on what I think the direction should be on optimal and practical component cooling. Emphasis on "practical" and I will qualify myself as perhaps worthy of introducing this topic based upon 1) My area of training is in chemical engineering and 2) I have done considerable work over the years on various heat transport critical projects.

 

I. I'll simply state that below ambient cooling and its associated condensation problems will be hard pressed to become reliable with today's exposed circuitry.

 

II. Next, non-closed loop liquid cooling solutions all seem to require higher maintenance than air cooled choices. The stories that can be found online of extreme fouling when the liquid coolant is not maintained and fully understood are very troublesome. Electrolysis between dissimilar metallurgy and conflicts between various additives can be very complex and require chemical stability tests and monitoring that is costly and impractical.

 

III. Closed loop liquid cooling solutions are much more practical. However, they ultimately have some limitations that are difficult to overcome. Forced circulation of a liquid over the heated surface does result in higher heat transfer coefficients than is realized with air cooling ... but the air cooling does still occur at the radiator and the benefit provided is a greatly increased surface area in which to cool the heated liquid. The problem is it takes some delta T (temperature difference) to drive heat from say the CPU into the liquid coolant and some additional delta T to drive heat from the heated liquid into the ambient air. Heat transfer coefficients, surface area, air and liquid flow rates, power dissipation rates, and physical dimensions all enter into a very complex solution of the heat transfer performance. What is simple is the realization that two delta T heat transfer events are occurring and thus there is no way for this system to be superior to a properly sized solution that requires only a single delta T heat transfer event.

 

IV. The single delta T solution is the incorporation of heat-pipes that contain refrigerant that experiences a phase change at the point of heat input. Phase change (boiling) heat transfer coefficients are typically an order of magnitude greater than those of forced liquid circulation. Larger heat transfer coefficient equates to smallest possible equipment sizing to handle a specific heat load. I have enquired into >> if the heat pipe systems currently offered contain refrigerant << and have had no luck with return correspondence with the manufacturers. This does not surprise me in the least because typical sales personel are often not qualified to discuss the technical aspects or the business environment punishes them severely if the topic is considered "breaking ground" ... how dare them reveal in house secrets? A simple Google on "heat pipes" provides an easy and fun introduction into their capabilities.

 

My guess is the current air cooled solutions that have copper or other tubing contacted via heat transfer paste with the CPU's heat conductive cover are simply using forced conduction type of cooling. "Forced" because the air is being pushed with a fan and "conduction" because there is no phase change occuring internally. If today's systems were using refrigerants ... would not the sales literature be expounding upon the virtues of this benefit?

 

Another step upwards in performance beyond heat pipes with phase change is the usage of a thermosyphon type exchanger design. This arrangement further increases the heat transfer coefficient by establishing a circulation within the tubing that contacts the heat source. Obviously today's coolers have not evolved to this level of performance because a physical "loop" would be readily apparent in the design. A Google on "thermosyphon" can introduce one to the concept; however, to fully grasp the design criteria requires the study of detailed thermodynamics at the college level.

 

So ... I share my above thoughts with you all in the hopes that cooler manufacturers will somehow get wind of this conversation and accelerate their evolutionary development cycle of commercially available component coolers. Smaller, quieter, higher performing, and extremily reliable cooling solutions are possible. I have too many ongoing projects to champion this concept much beyond discussion here ... but some of you enthusiasts might can take it and run with it ... I simply want to be able to buy the end product and bolt it on to my machine and enjoy the benefits.

 

-- Tom