Homemade evaps.

[labrat23]

Well this is my first thread here at XS. I've been a member of ocforums for a while, but since I've decided to do a phase change system, I started reading up on them over here. This is pretty much a quote of my post over there, so tell me what you think.

I finally managed to squeeze some time in on the milling machine where I work to make some evaps. Had to sneek in on Saturday (shh). Anyway, the machine has the old prototrac control system, which I think was the precursor to cnc. Here are the pics.
http://home.comcast.net/~labrat23/evap1.JPG
http://home.comcast.net/~labrat23/evap2.JPG
The block is 1/2" thick and 2" square. The chanels are .300 deep leaving .195 for the base. I intend to have the cap tube pumping into the center and the suction line hooked to the recess in the corner at the end of the channel

I made two because I had the material there, and once I made one, all I had to do was sit and watch it make the second. That is until I broke my 1/8" end mill. You can see on the second one where the 1/8" cut ended at the bottom of the second ring. To finish it, I put in a 3/16" cutter.

Now I'm faced with the question: which one would be better? I think the one with a narrower channel would cause higher velocities and better heat transfer per inch of surface area, but the other has more area. Also, I could stack them, drilling all the way through the recess at the end of the channel of the top block. The refrigerent would be routed the past the center of the top block into the center of the bottom block and have the refrigerent flow through the top block towards the center. Hope this sounds clear enough.

[gclg2000]

man that looks awesome.

NIce work

[matttheniceguy]

Nice looking block. Since it is only 1/2" high I would say stacking the two together is a really good idea, although I don't know how well heat would transfer from the lower block to the upper one.

[gloatlizard]

well done

[sharp]

I love this design:D ,i made similar with milling machine.For this one you must have cnc.With thicker wall and one more turn,it would rock. :banana:

[wdrzal]

I love this design:D ,i made similar with milling machine.For this one you must have cnc.With thicker wall and one more turn,it would rock. :banana:

That design could be made on a manual mill, all you need is a turntable.

[hatemi]

I woudl use both of them. And use a drill bit that is slightly larger than the chanels and use it to make the walls have more surfece area and to ad turbuleces.

[wdrzal]

I would put a bottom tap in a chuck and follow the pattern it give the walls more surface area.

[labrat23]

As good as the drill bit sounds, it also sounds like a ton of work and the risk of scrapping the block. The walls on the first one are thick enough, but the second doesnt have enough material to play with. One slip and it would punch through the walls, short circuiting my refrigerant flow.

I like the bottom tap idea but i dont have a bottom tap yet. Before I would do this, I would figure out what kind of boiling is going on. Since the block temperature should be fairly close to the refrigerant temperature, I doubt there would be any film or jet nucleate boiling. I would guess that it is free convection boiling, i.e. the liquid flows smoothly with vapor boiling off the top as opposed to bubbles forming between the liquid and the walls of the block. Having said that, I would optimize the fins to exchange heat to a vapor flow. The problem with ultra tiny fins, like the threads of a 8-32 tap, is that they are too small to protrude out of the boundary layer of the refrigerant flow. In order to protrude out of the boundary layer, - or more correctly increase the area of the boundary layer - the threads would have to be at least the size of a 1/2-13 tap, which I cant get into the channels.

Onething about threading the walls is that the size of the thread has no effect on the surface area; it is the angle of the thread that matters. Copper is soft enough that it might be possible to make my own tooling to cut some small fins in the side of the fins. I would think min-fins sized to fit three or four on the main fins would be about right. I would just cut the tool on the milling machine out of mild steal and then heat it blue and drop it in a cup of oil to harden it.

Anyway, I'm getting carried away and creating a lot of work for myself. I might just use a 8-32 tap if I run the numbers and find that the boundary layer is thin enough to make it worth it.

Thanks for the responses guys.

That design could be made on a manual mill, all you need is a turntable.
I've done that before. The ncn or prototrac circle cuts are fairly accurate and good enough for most applications, but a turntable is simply a jewl to have and doesnt even compare to the cnc cuts.