Novena Heirloom – CPU cooling – Part 1

The production Novena laptop as envisioned by designer Bunnie Huang has an open-top layout with plentiful ambient air exchange. This means that a simple, passive heat exchanger mounted to the top of the CPU is adequate to the task of cooling the system. But when Bunnie and I first discussed the Heirloom design, we came to the conclusion that it would be a slightly more conventional clam-shell type layout.

Testing the heat sink assembly
Testing the fan-based heat sink assembly

In order to maintain the desired access to the system hardware, a portable Bluetooth keyboard was chosen to incorporate into the design. I integrated the keyboard into the Heirloom design by nestling it into the standard keypad location found in a typical laptop configuration. In this position, the keyboard hovers immediately above the CPU on the system board. The area behind the keyboard is occupied by a rear panel that includes the speakers, system switches and a lid closure sensor. This setup effectively seals up the enclosure so that ambient air is no longer sufficient for CPU cooling.

Initially, the task was left to me to find a way to resolve the cooling issue. Bunnie and I agreed it would be worthwhile to pursue a passive approach and avoid the use of a noisy, watt-hungry and space-hogging fan.  I began research in earnest and eventually found my way to a handful of companies specializing in CPU cooling systems. Correspondences with those firms led me to a lengthy round of dialog with engineers over how to manage the passive approach to the problem. This, combined with the quest for proper hinges for the case, became the major issues in the early development of the Heirloom design.

With this passive cooling idea in mind, I designed the speaker mounting arrangement so that there was a potential dual-use aspect to the speaker holes. The speakers are shock-mounted at a small distance below the Dieter hole array so that air flow through the same holes could potentially serve as supply air when the LCD is in the open position.

IMG_7814 (1 of 1)
The “Dieter” speaker hole array

An early prototype enclosure housing also included an experimental array of slots in the bottom to facilitate this top down air flow approach I came to appreciate from conversations with CPU cooling system engineers.

Slots visible in the case bottom during the finger slot milling operation
Slots seen in case bottom while milling finger slot

Many approaches were considered including “finning” the left side plate and connecting it to the CPU with a pair of heat pipes. This mass of aluminum at either side of the Heirloom enclosure seemed the logical candidate to dissipate heat from the system.The short length of attachment area available for the termination of the heat pipes effectively eliminated this idea. The fins may or may not have appealed . . .

The apparently obvious choice of the right side plate, with an uninterrupted surface to accept the heat pipe connection, was seen as too far away from the CPU for an effective solution.

Using the side plate as an exchanger
Using the side plate as an exchanger

I pushed through several other concepts punctuated by lengthy dialog with engineers including using an off-the-shelf heat sink material running along the back edge of the enclosure. The inability to provide a top to bottom air flow path through this heat sink proved to be its demise.

rear exchanger
A modified stock heat sink integrated into the back edge of the case

In the midst of this lengthy round of dialog over heat management using a passive approach, I was introduced to a new product promising to provide an interesting middle ground between passive and active solutions. A company acquired by Aavid, one of the firms whose engineering staff was very helpful, was developing a low-current, diaphragm style pump with a very small footprint. It is intended to be used with a typical on-CPU heat sink by enhancing the flow across it when ambient air is insufficient to maintain desired temperatures. Unfortunately, ongoing production issues have stalled the release of the product so I was never able to acquire one for testing.

Razor Jet
Diaphragm type air pump

My last attempt was an implementation of what one engineer described as a cold plate. This entailed using the peek plate as a mass for pulling heat from the CPU into the peek plate whose perforated surface would then serve to dissipate heat. One engineer indicated that this was a potentially useful solution. Another stated that it wasn’t technically a cold plate since there no fluid flow involved. And it does not address the issue of what to do with accumulated heat in the enclosure or even if that is actually an issue. I think I may still try it out at some point and see how it works.

cold plate exchanger
Using the peek plate as an exchanger – showing outline of SSD

So with the clock still ticking (yes, I’m old enough to remember that), I decided to consult with Bunnie and see if he might have some alternative thoughts about solutions to the challenge. After a quick update, he suggested that we might look at a fan-based solution. An upcoming trip to China would have fan-shopping added to the list.

Once again, I looked at various options for placement of fans, heat pipes and heat sinks and consulted with Bunnie. With the hinge issue still unresolved, it began to fall on Bunnie to come up with a plan while I wrestled with that.


NEXT: Designing a fan-based cooling system foe the Heirloom.


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