How to transfer heat better?

[mvtop]

Hi,

i"m making a evaporator coil to cool a conatiner.

a pic of the coil is in the thread "Getting Colder, need advice" not the final version but close enough for the discusion.

the container will be made of a stainless steel.
i'm wondering whats best way to improve the transfer of heat from the steel to the copper?
it will be well insulated. But i didn't know or could find a good way improve the bond between the two in order to improve the transfer.

could soft solder the coils to each other to improve that section, not sure if that would help the performance.

any thoughts on this.


thanks.
mvtop

[Yuri B.]

Hi. On my view, there should be not only maximal thermocontakt between the coil and the walls of the container it cools but also good thermo insulation between each single ring in the coil itself. I would have soft soldered the coil to container's walls but don't know what might be suitable for insulation, which should be rather porous and at the same time impermiable for dew water which will be forming on the coil.

[Yuri B.]

There are such thing as "liquid metal" - am not sure how it is called right in English. Sort of resin, sold in tubes, which when dried is hard as iron indeed. Must be costly, however. Am not sure whether the silicon grease which we apply to sensors or to compressor heaters will suit.

[Yuri B.]

"Liquid metal" and silicon in my last post refer, of course, to the "bonding" of the coil with container. Concerning the latter, I would tried to use one not of cylindrical (bar like) form, but the oval, flask-like one. In such a "flask" there would be proportionally more solution in thermo contact with the coil.

[mvtop]

thanks Yuri. B.

I thought about doing that, but mainly I didn't think you could solder coper to stainless and never had a reason too.
if so, that would be an excellent bond that would transfer nicely.

i'll try it and see how it bonds.

as for the insulation between the coils, I wasn't going to leave any gaps between the coils, so as to have a tighter wrap and more coil density. thoughts?

thanks
Mvtop

[Yuri B.]

Good morning Mytop. Evaporation will be inadeqaute if each ring of the coil is in contact with the neighbourings. There will be low superheat if any as a result. Why is it impossible to immerse the coil in the solution?

[RANGER1]

You can get heat transfer paste from a refrigeration parts supplier or electronics shop . They use it for silicon chip etc to help transfer heat from it .
After you fill in all the gaps etc use it liberally , then insulate it .
Stainless steel is not as good for heat transfer , direct contact would be best as Yuri B . suggested .

[Yuri B.]

One point. On my view, you will never get the adequate evaporation inside a single pipe coil. Make the collectors on both sides of the coil with some 3, 5, maybe more pipes i- see how the original coils are built. Refrigerant needs as more space as possible to expand.

[Peter_1]

You can solder it with a flux and thin/lead like plumbers use.
We've applied it already on steel and stainless steel. Apply enough décapant (I only know the French name)http://www.jerestaure.com/images/imagecache/300x231_cuivroxyd.jpg
I think copper flux is the English name.

[mvtop]

One point. On my view, you will never get the adequate evaporation inside a single pipe coil. Make the collectors on both sides of the coil with some 3, 5, maybe more pipes i- see how the original coils are built. Refrigerant needs as more space as possible to expand.

and good morning too.

I didn't understand this at all.
collectors on both sides of the coil, 3,5 + more pipes?

hmm?

Mvtop

[Peter_1]

I think Yuri tries to explain you must divide your flow over more tubes than just one like you did now.

We have eutectic systems with 30 m 1/2" pipe running without any problem. If it's a stable process, you can use your capillary tube. Disadvantage is that your cap tube must be selected for the system while freezing down the solution (phase change)
But a TEV will feed the coil much better at startup.

[Yuri B.]

It seems to be my delusion. Untill now thought ALL evaporators necessarily consist of several parallel coled pipes joined at the entry and the exit by "collectors". For better expanding the refrigerant. I was wrong, in a single piped coil it do its job not bad too.
We all learn here something.

[Gary]

In going from a larger tube to a smaller tube, there is more copper surface area for heat transfer, however...

The refrigerant must travel through a smaller pipe and over a greater distance, which slows down the refrigerant flow.

Possibly, the advantages of greater surface area would be outweighed by the disadvantages of reduced refrigerant flow.

This reduction in flow could be overcome by sending the refrigerant through two or more parallel paths (of equal distance). But then we run into the problem of ensuring that each path is fed an equal amount of refrigerant.

[mvtop]

Hi. On my view, there should be not only maximal thermocontakt between the coil and the walls of the container it cools but also good thermo insulation between each single ring in the coil itself. I would have soft soldered the coil to container's walls but don't know what might be suitable for insulation, which should be rather porous and at the same time impermiable for dew water which will be forming on the coil.

i was thinking about soldering the coils together once i get them bonded to the container, but apparrently this is not a good idea, according to your other post.

Insulating tape could be used to insulate the each coil from the other while still allowing a good bond to the container.


Ranger1.
i've used the heat transfer paste before but not in anything AC related. that will work nicely.


Thanks for the info

Mvtop