Scratch Build – In Progress Project: Hush!

Monkey Puzzle · 21 May 2011

Taster pics:

Update Links:

http://forums.bit-tech.net/showpost.php?p=2042374&postcount=2

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http://forums.bit-tech.net/showpost.php?p=2342319&postcount=125

http://forums.bit-tech.net/showpost.php?p=2342321&postcount=126

http://forums.bit-tech.net/showpost.php?p=2384857&postcount=137

http://forums.bit-tech.net/showpost.php?p=2384860&postcount=139
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Hello all. I've started making a large passive radiator/case. I've been working on it for a while, but since the design is quite complicated I thought I'd wait until it got to a recognisable, likely to work(!) stage before starting the build log.

I have had an innovatek Konvekt-o-matik before selling it on members' market, but from what I've heard you really need a shedload of them to handle high heatloads. Plus, they're expensive (~£80/6tubes), bulky and the design is flawed in my opinion - aluminium, made for 8mm ID tubing, and by their design the more of them you add the larger the pressure loss - 8mm inlet splits to 8mm tubes running in parallel.

So I figured I'd make my own;

copper,
designed for 7/16" or 1/2" tubing,
minimise pressure loss by matching resistance of the tubes to 1/2" tubing,
massive amount of surface area (since it's the equivalent of an Aga it needs to have headway for extra heatload),
wide fin spacing and compact enough to be self-contained within a case.

The case is going to be approximately 45cm wide x 40cm deep x 46cm tall + height of castors. So it's slightly smaller than a mountain mods UFO (45x45x45)

The top, far side of the case (the non-window side panel on a normal case) and the bottom will be made up of a large finned copper radiator, hopefully with enough surface area and passive airflow to run completely passive, with air rising up through the case. I wish I was competent enough with google sketchup to draw a detailed plan, but it just seems a nightmare because of the design, so I'm afraid I've largely stuck to hand-drawn sketches. Here's a very rough idea of what the passive radiator element of the case will look like.

Initially I was going to use 30 metres of standard 15mm outer diameter half-hard pipe used in home plumbing and flatten it as in normal watercooling radiators, but after buying a small sample (a gentle elbow bend) and trying to flatten it I realised it's very tricky to get an even inner channel, and it would take forever to flatten 30m of the half-hard stuff...

After scouring around to find a cheap source of soft copper pipe I found on ebay what was listed as 9m of soft 15mm outer diameter, 0.4mm-thick-walled copper pipe used as gas lines in boats and motorhomes, and being the only bidder got it dirt cheap. . I picked up an adjustable pipe cutter (3-22mm) for a few quid and measured and cut 130cm lengths. Turns out I was sold around 14m, so had 10 x 130cm, and an offcut.

Unfortunately, though slightly easier than the the half-hard copper, the walls of the stuff I got was 1.5mm so it was still a nightmare, and still came out uneven in cross-section and took ages to flatten. I also found it's expensive stuff to get any more - about £80 for 25m. I'll find a good use for it later on though.

So, time for plan B. I managed to find a site selling 10m x 6mm outer diameter, 4.8mm inner diameter soft microbore copper tubing, so I picked up 6 rolls for ~£42 inc p&p

I then unrolled them, measured and cut into 48 lengths - 16 x 120cm, 16 x 125cm and 16 x 130cm - the difference is because the 48 tubes will be arranged as so, so the outer tubes need to be a bit longer:

The microbore copper is so soft it's very easy to bend and straighten, though it's tricky to get 130cm lengths completely straight. It work-hardens and quickly becomes difficult to bend, though it can be annealed again by sticking it over a gas hob.

Anyhow, I had originally planned to use some of the 15mm pipe I had for the end pipes distributing the water to the 6mm copper tubes. It has a 12mm inner diameter so is a pretty good match for either 7/16" or 1/2" tubing.. After straightening one of the 130cm lengths I bent it to a hook shape over a rolling pin, which was quite tricky. I then cut it to a rough length and measured out the 48 x 6mm holes for the microbore pipe to connect to.

The pipes will have to do a bit of bending at the ends, but hopefully with the small bore pipebender I got this won't be too much trouble, though in reality it'll probably be very very frustrating, since the pipe needs to be bent after the copper fins have been pushed into place.

After lots and lots of drilling here are the radiator end pieces. This would have been so much easier if I had a bench drill and a vice, but as I didn't at this stage I had to improvise:

Plan for pipes joining distributor end tube - side view

After all that drilling my plans changed (always the best way for a project to go horribly wrong). I managed to source some cheap copper sheet for around a quarter of the going rate from shopping around, and for a small extra charge the seller was even willing to cut it with a metal shear into lots of 395mm x 50mm strips for the heatfins. So the dimensions of the whole thing changed, and the drilled pipes were now the wrong size. Ho hum.

I now plan to instead use a plenum akin to those in standard pc radiators. They serve a purpose in allowing a reservoir of water so that water going down the tubes doesn't cause unequal flow between pipes at the inlet end compared to the other. More importantly, it would be easier to make rather than many cramped, fiddly bends for the tube ends.

Here's a picture of the copper. It had been left lying around in a scrapyard for God knows how many years, and was a bit scratched here and there, but should clean up nicely enough.

In the pic are a 39.5cm x 45.5 cm x0.9mm copper sheet for the side wall the tubes go through, 63 of 39.5cm x 5cm x 0.9mm copper strips to be used as heatfins, and 16 of 7.5cm x 39.5cm strips (additional 39.5cm x 5cm heatfins have since been cut from them). There's also some copper bosses (solid cylinders) that I nmay put to good use.

After cleaning

Monkey Puzzle · 17 Jul 2009

I then needed to drill 48 x 6mm holes in each of the copper fins and the copper wall. This took a while.

I initially tried using G-clamps and my bench drill and ran into several problems. As the holes were drilled, they pushed a cusp through, deforming the clamped stack. Whilst the cusp/sleeve from drilling is actually useful for soldering and heat transfer, it introduces inaccuracy in the drilling. So I made a jig for putting the copper strips in for drilling.

JIG FOR DRILLING

Sadly the bench drill I have is only 180W, and so lacks the torque to drill large metal holes, so I switched to using an 810W hand drill in a heavy duty drill stand, which allows accurate vertical drilling.

PICS OF DRILLING

After doing some reading up on natural convection and passive heatsink design, I came across a problem sheet set for engineering students on how to optimise the fin spacing for a passive radiator, from a book by a couple of heat transfer professors, and even better, the software it ran on was freely available on the web. So, using the software, I adjusted the parameters to model my heatsink as best I could.

Passive heatsinks rely on natural convection, and this requires the free movement of air over the fins. The fins are much more effective spaced much further apart than in air-cooled heatsinks (~2mm for a Thermalright Ultra Extreme) or even the most sparsely-finned watercooling radiators (~1fin/3.125mm or 8fpi for an RX XSPC radiator).

The simulator models a given width, height and depth of passive heatsink at a given input heatload, and plots the heat transfer of a given fin, and the total heat transfer of all fins combined, at varying fins spacings.

So, whilst the heat transfer for a single fin increases up to a certain point, increased fin spacing means fewer heatfins overall. There's a balance between the two, giving an optimal spacing for a given heatload:

From the simulator there's also another interesting trend - as the input heatload decreases, the optimal fin spacing (fin-pitch) increases, which means I need to optimise the heatfin spacing for the air-water delta T I want to aim for...

The simulator mathematically models a heatsink made from two copper plates held at a set temperature with heatfins that run perpendicular between them, so it's not exactly what my heatsink design is, and in adapting it to model my design I'm not entirely sure how to adapt mine to it, since my design has 48 6mm outer diameter, 4.8mm inner diameter tubes running through the heatfins. I'm unsure as to whether I should adapt it so I equalise the inner heatpipe surface area (4.8mm)to the end-heatplate- to-heatfin surface area in the model, or the outer heatpipe (6mm) surface area... Hope that makes sense!

I altered the parameters to assume just two end heatplates as in the original design, to give a conservative estimate of the performance, and the heatfin spacing (1 heatfin per 10mm, so about 9.1mm between each heatfin). This gave around a 300w heat transfer for a delta of 10C between the air (20C) and the heatpipe/water temperature (30C). But as I say, this is hopefully the worst-case scenario (though the model uses copper-copper joins rather than soldered joints...). A point to note is that the model only calculates the heat transfer from the heatfins - it excludes the heat transfer from the copper tube surface area (~11,000cm^2) and the copper wall (~3,600cm^2).

PIC WORSTCASE SCENARIO (spacing of 10mm on the x-axis)

Adjusting the model to equalise the end plate-to-heatfin surface area in the model with the tube-to-heatfin surcace area gives silly numbers (~560W heat transfer at a 0.5C Delta T, 400W for a 0.4C Delta T).

The real performance will probably lie somewhere inbetween - whilst the best case scenario is probably largely correct in terms of more accurate surface area for the water to transfer heat to the pipes and fins, the model assumes continuous copper joints, and inaccuracies in hole size and loss from soldered joints (~96% tin/ 3.5%Silver/0.5% copper solder) will no doubt lower performance.

Anyhow, enough hypotheticals, here are some pics of where the project is up to at the moment.

After drilling the heatfins I found the holes were marginally too small, using a digital Vernier, in th order of a few hundredths of a mm.

PIC OF DIGITAL VERNIER

So I decided to erode them down slightly by putting them in a drainpipe full of vinegar and harpic toilet cleaner (since it's hydrochloric acid based). I must say, looking through household detergent ingredients for the strongest acid in the supermarket made me feel like a terrorist!

PIC OF TUBES IN DRAINPIPE

PIC OF TUBES OUT OF DRAINPIPE

I then used a microbore pipe bender to make the bends in the pipe. It's a handy little tool, but unfortunately they put far too much paint on it, meaning the measuring bits and the 6mm tube channel was too small, meaning the tube wouldn't get equal pressure around it when bending and would deform too much for my liking. After a quick bit of paintstripping with Nitromors I bent the tubes for insertion into the copper wall. This was a bit fiddly; in order for the bends to line up exactly with the drilled holes I needed to know amount of length the bend took. After a few annoying mishaps, annealing and restraightening I got the tubes bent accurately.

Monkey Puzzle · 17 Jul 2009

After inserting all the tubes I could then start putting the heatfins on.

I ran into a bit of a problem when putting the fins on - they're a tight fit and the smallest angle off horizontal, or pipe angle off vertical means they can be tricky to get on. It's a bit of a trade-off between tight fitting fins with minimal gaps (less solder and better heat transfer) and ease of putting the thing toether. The fins are pretty tight, and in gently hammering them ona few of the tubes were pushed too far down, which you can se in the pic. Luckily, using the pipe cutter without the cutting blade inserted it's possible to grip the tube and push it back through the heatfins.

PIC OF PIPE CUTTER MINUS BLADE

There's only 11 heatfins on in various temporary positions (finished article will have 74 fins);

PICS WITH HEATFINS

Monkey Puzzle · 17 Jul 2009

Roughly how the removable motherboard tray will sit (looks a little tighter height-wise than it will be due to the top pipes being a little bent down atm). The PSU will sit behind at the bottom, fan facing down.

A sheet of 2mm styrene that was lying around:

Heatgun:

Bending:

A quick and dirty tray to bath it in. So quick and dirty that it leaked, so it has an outer box lined with plastic sheet a mattress came in - now it's in the styrene tray to avoid cutting up the plastic lining:

Deoxidising:

Shiny:

Making solder slinkies.

After cutting to solder rings - not sure how many are in the bag, probably a thousand or so.

Blowtorch - gets up to temperature okay.

Various bits for putting fins on and soldering:

Fin in place with 9mm thick wooden spacers for straightening the fins (they need to be hammered gently into place and deform a little in the process)

Solder rings put around the pipes - it's quicker to put them on like this, with a fin above, near the pipe ends, as it made putting the fins on a lot quicker.

Solder rings after tightening with needl-nosed pliers (surprisingly quick and easy).

A freshly soldered fin on top: The fins are a bit rainbow coloured from scale slowly being removed by the weak acid bath - hopefully it'll all go - the bottom fins are quite pink. It was pretty disconcerting to see the shiny copper scale up and get covered with burnt flux, as it didn't seem to get removed at first. I'm a bit concerned about the acid possibly attacking the solder joints - I may switch to cleaning it only at the end when all in place and using a fine wire brush to clean the pipe before soldering.

Unfortunately I couldn't correct the bend that got pushed too far through (3rd from the left at the bottom). A few of the first fins are a little bent as well, though they'll be mostly out of sight behind the motherboard tray.