Evaporator design: minimal channel size.

Not sure if it belongs here or in the Overclockers section, but I want to reach the Prof, so I will try it here. Feel free to move it if appropriate.

We overclockers like to chill our CPU, which is basically a surface measuring a quarter of a square inch, putting out ~100W of heat. Designing an evaporator which is able to efficiently remove the heat is a hell of a job due to the enormous energy densities and space restriction: an evaporator made for mounting on the processor cannot be much larger than 2"x2.5"x2" (WxLxH).

Now, many creative designs exists. The spiral or maze-like channel machined into a block of copper is very popular. Gary's idea of a sunburst fin pattern with injection of the refrigerant at the edge (multiple injection points) of the block and suction at the center also has potential.

Just one thing bothers me. How to design the channels? We need to pack a lot of surface into a tiny package. Maximum channel cross-section is set by minimum refrigerant speed required.

But what about minimum channel size? There must be a limit on the minimal channel diameter. Take a 1/4" pipe as an example. We can split it in two parallel pipes with their cross-section area each half of the cross-section area of the 1/4" pipe. We can split it into 100 parallel pipes, the cross-section area adding up to the area of the 1/4" pipe. Maybe add a bit more to compensate the added friction.

The 100-pipes example provides much more refrigerant->metal surface area than the 1-pipe example. Refrigerant speed is also equal, assuming 100% perfect distribution. Pressure drop is a bit higher due to added friction.

But where is the limit? Can I make millions of channels, each having a few square microns cross-section area? If not, what limits the minimal channel size?

If you consider the current cap-tube scenario, you should consider the 5lb/hr rate for the 250btu/h application.

5 lbs an hour works out to be about .083lbs/minute.

I would figure the volume of the gas at the evaporating temperature, and then determine the refrigerant velocity within proposed "channels", or "tubing circuits". :)

We're only moving about 1.3 oz of refrigerant a minute. Heck, it's almost a moderate leak!

I'm thinking the ideal would be a solid copper hemisphere with a million tiny drill holes, all pointing to the center of the "ball", and a large hole down the middle for suction return.

The cross sectional area of the tiny drill holes must add up to the cross sectional area of the suction hole (actually a little more to compensate for friction), which must be sufficient for proper vapor flow.

The first question must be, "What cross sectional area is needed for proper vapor flow?"

Just some food for thought. Here are few designs that are easy to do. They could be done with drills and hole saws.

http://www.gatecom.com/~tmethod/blks123b.jpg

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Originally posted by herefishy I would figure the volume of the gas at the evaporating temperature, and then determine the refrigerant velocity within proposed "channels", or "tubing circuits". :)

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This would work out that the cross-section of 1/4" to 3/8" pipe would yield sufficient refrigerant speed. But when splitting our 'pipe' in two parallel parts, volume flow is halved, but refrigerant speed stays the same.

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We're only moving about 1.3 oz of refrigerant a minute. Heck, it's almost a moderate leak! [/B]

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Yup, and still it is hard to reach a low TD between chip and refrigerant.
Do a calculation on the energy density used in one of your normal applications, and then calculate the energy density of this particular application.
Preferrably do this with a comparable HFC refrigerant. NH3 has much better heat transfer properties than HFC refrigerants.

Prof, where are you?

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Originally posted by Gary
The first question must be, "What cross sectional area is needed for proper vapor flow?"

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If you mean vapour velocity: there are rules of thumb which indicate minimal refrigerant speed to ensure decent oil transport. Of course these values differ for rising lines and horizontal lines.

The vapour flow as is doesn't really worry me. Vapour wil flow through, eventually, albeit with increased friction due to the increased wall surface.

The real factor limiting channel diameter seems to be the oil flow and the forming of bubbles in boiling refrigerant. Bubbles cannot form in very small channels, so I envision that a boiling bit of liquid will blow a larger part of the wall surface dry.

Knowing how small the channels can be is important, since one of the major heat resistances in the chain is the refrigerant->metal resistance. Improve the metal->refrigerant surface area, and performance is likely improved.

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This would work out that the cross-section of 1/4" to 3/8" pipe would yield sufficient refrigerant speed. But when splitting our 'pipe' in two parallel parts, volume flow is halved, but refrigerant speed stays the same.

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If we split the flow between two parallel circuits whose cross sectional areas add up to the same cross sectional area as the original, then both total volume and velocity remain the same.

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If you mean vapour velocity: there are rules of thumb which indicate minimal refrigerant speed to ensure decent oil transport. Of course these values differ for rising lines and horizontal lines.

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If cross sectional areas are equal to suction line cross sectional area throughout, then velocity should remain constant throughout.

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Originally posted by Gary
[B]If we split the flow between two parallel circuits whose cross sectional areas add up to the same cross sectional area as the original, then both total volume and velocity remain the same.[B]

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Yes, but volume (actually mass flow)per pipe halves. Refrigerant velocity per pipe stays the same.

Hi DaBit. :)

I believe that the entire lineset (circuit) is so short on these things, that you're really getting down to splitting hairs.

like I've said before... you're only moving 1.3oz of refrigerant a minute @ -50 in these "micro-load" applications. I can't even good a good head pressure with 64' of .031" cap tube in a 100degF ambient. ;)

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Originally posted by herefishy
Hi DaBit. :)

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Hi again :)

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I believe that the entire lineset (circuit) is so short on these things, that you're really getting down to splitting hairs.

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No, absolutely not. Heat transfer from boiling HFC refrigerant to copper is not that good, and we do not have much surface area to work with.

An example (Using SI units :D):

Assumptions:
- boiling HFC to copper heat transfer coefficient @ -50: 1200W/m^2/K
- A maze-block with 45 cm^2 (~7 sq.in) refrigerant->metal contact area.
- 100% wall wetting.
- 100W load.

Then we have a TD of 100/(1200*(45/10000)) = 18K (~32F) between refrigerant and metal. Not exactly splitting hairs in my opinion...

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like I've said before... you're only moving 1.3oz of refrigerant a minute @ -50 in these "micro-load" applications. I can't even good a good head pressure with 64' of .031" cap tube in a 100degF ambient. ;)

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Even better: just adjust your captube for the lower head. Efficiency of reciprocating compressors increases with lower compression ratios.