Project: Hush!

13 Mar 2006
Project: Hush! - Updated 16/5/2022

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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; :)

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:eek:). I managed to source some cheap copper sheet for around a quarter of the going rate from shopping around:cool:, 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




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13 Mar 2006
Project: Hush! - Updated 4/4/10

I then needed to drill 48 x 6mm holes in each of the copper fins and the copper wall.:eek: 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.







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.




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.:cool: 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.



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!





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.




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13 Mar 2006



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.



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








I've just got the solder and flux and need to make a watertight box tonight to deoxidise the copper in a vinegar bath before, and after soldering them with a propane/butane blowtorch, since it'll oxidise all the surrounding copper. Anyone know of a place I can get lots of cheap, relatively strong acid? Not sure where to get it and any regulations - was told by a car bits shop that they can't sell battery acid (~35% sulphuric acid) these days. I could probably make do with distilled vinegar but it'll be expensive for the amount I need and take a long time as it's weak.

Any questions just ask. :)
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24 Mar 2008
High Wycombe
Very interesting stuff!! You clearly know what your doing when it comes to this!! :D

Will follow this with much interest, don't know anyone who has made their own radiator!

Be very interesting how it compares to off the shelf products too
12 Nov 2008
wow, a lot of effort has gone into this! Your calculations say it all.
Im looking forward to this project ;)
22 Dec 2008
Good shout, but more importantly well executed. I'm curious as to how you're going to solder all of these, one joint at a time? You're a patient man if so

Impressed by the design. Bit late now, but if you fill copper pipes with salt and cap the ends you'll find bending them easier. Sand works too, as does water, salts just the most convenient.

Will the case be the only radiator, or will you have one or two conventional active ones present as well?
13 Mar 2006
Thanks for the nice comments. :)

Good shout, but more importantly well executed. I'm curious as to how you're going to solder all of these, one joint at a time? You're a patient man if so

Impressed by the design. Bit late now, but if you fill copper pipes with salt and cap the ends you'll find bending them easier. Sand works too, as does water, salts just the most convenient.

Will the case be the only radiator, or will you have one or two conventional active ones present as well?

The plan for soldering is going to be to bath in vinegar, then brush flux on around the joints and put rings of solder around the joints of maybe 5 fins at a time and then blowtorch the fins, before deoxidising in a vinegar bath again.

The blowtorch is supposed to get copper pipe up to around 650C, but the heat is going to be dissipated, which is what the thing's designed for after all! So hopefully the heat will spread but also get slightly above the melting point of the solder (~217C). It's not really something I'll know until I put the torch to it - I can't test what temp it'll get to and how easily on scraps of copper. Even if the plan works out, it's going to take a long time to solder...

No plans for other radiators - this should hopefully be sufficient. One advantage of switching to using a plenum (hopefully CNC cut by a friend from a block of acrylic) is it allows the possibility of splitting the radiator into two loops in the future without too much hassle. :)

Will also have to see what the pressure drop is going to be like - it's designed for the tube resistance to match that of 1/2" tube (resistance of 49 tubes of 4.8mm internal diameter in parallel is equal to 1/2" ID) but the water still needs to go through 120-130cm of tube and a few gentle 90 degree bends. The plan at the moment is to have an XSPC Edge, MCW-60 and use a 10W DDC for quietness, but I may be able to mod it to 18W should the need arise (haven't opened the DDC to check if moddable).

Just measuring up a 2mm sheet of perspex atm to bend it and make the watertight innards of a shallow (~6cm) bath...
22 Dec 2008
resistance of 49 tubes of 4.8mm internal diameter in parallel is equal to 1/2" ID

I wouldn't know where to start calculating that, it's going to be flow dependent for one thing. Assuming laminar flow? I had a brief play with numbers, and the closest I could come was the resistance of the 49 tubes is about half that of 1" inner diameter tubing based solely on ratio of cross sectional areas. Flow down 4.8mm tubes is going to be pretty turbulent at the best of times, which is great for heat transfer but ruins me mathematically.

How big is the copper structure? It's very hard to judge from the pictures. If possible, which it looks like it might be, Id suggest (after vinegar) putting a ring of solder around each and every joint, then sticking the entire structure into a household oven at 230 degrees. If it fits, all you have to do is check every couple of minutes and the entire thing will solder itself together for you. if it doesn't fit, befriending the local pottery group and borrowing their kiln would be an alternative.
Heating it with a blowtorch will require a massive blowtorch or considerable ingenuity. Soldering a heatsink is, as you've noticed, not so easy. How does sitting it on an electric hob sound? Keep all the copper reasonably hot, turn a blowtorch on the part you're soldering. Heat wont dissipate anything like as fast if the temperature gradient is less
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