This topic is a followup of this topic, to stay ontopic.

For the newcomers: I have built an R134a based water chiller which cools a water/methanol mixture, which in turn should cool the microprocessor in my PC. I am currently able to reach -16 °C under load, but I am pretty sure my current evaporator is not performing too well. It's leaky too. For more details see the previous topic.

So, I need a new heat heat exchanger. First I was after a plate heat exchanger, but I got a few answers from the manufacturers which all say that my system is too light for a plate heat exchanger.

Heat exchanger design parameters:

- Qmax=200W @ -20 °C
- Qnom=150W @ -20 °C
- Refrigerant used: R134a (~4kg/hour)
- Coolant used: 30/70 vol.% methanol/water mixture.
- Coolant flow: 600-1000L/hour, netto.

For this new heat exchanger I was thinking of a coaxial heat exchanger design using a 3/8" copper tube (evaporator) inside a 22mm (approx. 7/8") copper pipe.

I also want to equip the inner pipe with copper fins over about 2/3 of it's length, to increase the contact area with the coolant. Why not over the full length? Well, I need some space to allow the coolant to distribute and mix. Fin layout will be something like this:
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The 3/8" inner copper tube will give me a decent refrigerant speed (at the end of the evaporator around 750ft/min)

The 22mm outer copper tube carries the coolant and it guarantees a decent coolant speed, although I prefer it to be higher. But the next smaller size pipe is 15mm, and this imposes a too high flow restriction.

The piping itself will be bent into a circular/spiral like construction to save space. Refrigerant will enter on top, suction gas will leave at the bottom, so refrigerant and oil flow is supported by normal gravity.

Questions:

- Is this design OK, or can it be better?
- How long should I make the coaxial pipe construction? I think 3 .. 3.5 meters (10 feet) will be sufficient.