Fan mounted on the custom base.

Novena Heirloom – CPU cooling system – Part 2

Bunnie and I discussed ideas for a CPU cooling system design while he was preparing a trip to China, so he decided to look for a viable fan while he was there. I had a centripetal fan in the shop that seem potentially usable, but it was too large for the available space. Although the small cooling device I had located earlier might have been useful with a typical CPU heatsink, a standard fan would require moving the heat from the CPU to a different location. So we returned to using heat pipe technology. Bunnie designed an evaporator plate that cleverly takes in the CPU and the adjacent IC with a single machined component. A single heat pipe then makes its way over to the fan assembly through a few bends.

Bunnie installs heat pipes for the CPU cooling system
Bunnie installs heat pipes for the CPU cooling system

A bit of correspondence regarding possible layouts for the fan/heatsink assembly led us to a place between the SSD and the battery which is as close as we could get to the CPU. Bunnie had already tracked down an interesting copper heat sink and was then able to design the condenser plate – also machined from copper but with a fancy plated finish.

The simple fixturing for CPU cooling heat pipe glue ups.
The simple fixturing for heat pipe glue ups.

We cobbled together a simple setup of the the case filled to the brim and Bunnie began working up a software testing environment to check it out. He then hooked up the fan to the Novena system.

Blue tape. Prototyper’s tool.

It quickly became apparent that some firmware work was needed to get the fan to operate with proper hysteresis instead of cycling on and off. So Bunnie got his Xobs on the phone and got the CPU cooling system working properly.

Testing the CPU cooling system in the fully closed laptop environment.
Testing the fully closed laptop environment.

The system was then run under extreme conditions to test the effectiveness of the CPU cooling system. It is slightly overkill for this computer so it worked quite nicely in testing.

Bunnie using his phone-based thermal imager.

So now we had a complete system but no way to mount the fan into the case. The fan’s normal mounting points were misplaced for convenient use. I noticed that the fans had a removable metal cover plate on one face and it occurred to me that this might be the key. I could simply make a replacement for this plate that would bind everything together.

New base and original cover plate.

Three iterations later I had a CNC machined aluminum mounting plate that uses mounting locations on the peek array below. It provides a place for thermally mounting the heat sink and additional real estate for a potential plenum assembly.

stack of new fan bases
A stack of new fan bases

When I first considered this idea, Bunnie and I both commented on the fact that the extant cover had a slight bulge in the area around the fan blades. I examined it to find that there was no immediately obvious reason for the shape as there was plenty of clearance for the blades without it. And since the fan exhibited other examples of possible legacy design details, I concluded it would be reasonable to ignore that feature. This assumption would soon come back to bite me.

The fan with metal cover plate removed.
The fan with metal cover plate removed.

The first example of the CPU cooling fan assembly looked to be fine so I sent off the plates to have them anodized in red. But when I had my assistant Darrell begin assembling the remaining fans we discovered that the fan housing castings were inconsistent in how flat a surface they presented to the base plate. It was enough to possibly be an issue with the fan’s efficiency so I set up a lapping operation on the lathe and had Darrell begin to flatten them. As the law of unintended consequences loves to hide away in my shop, we soon discovered that the removal of enough material to properly flatten the fan housings led to binding of some of the fan’s blades against the new bases.

 

Back to the CNC mill with the fan plates for clearance cutting . . . Fortunately, I had a fitted fixture from previous operations that made it easy to set up for this job.

So now, many months after initially considering how to keep the Heirloom version of the Novena cool, we now have a functional CPU cooling system thanks to the genius of Bunnie and a bit of effort on my part. I still have to asses whether to route the condenser air out of the case. But given the nature of the Novena system, it is actually unlikely that the system will make regular use of this cooling system since the very act of accessing Bunnie’s amazing hardware as intended will allow enough ambient air exchange to passively cool the system. But for any who find themselves using their computer as a conventional laptop, the CPU is protected.

 

Leave a Reply