Originally posted by Gary
What stops the water from going straight down the center, rather than around and between the coils?
Sorry, this is a detail I forgot to mention, and of which I did not take a photograph. I blocked most of the inside of the inner coil with a capped piece of 22mm (~7/8") pipe since I noticed this flaw myself. This prevents flowing of the liquid directly through the center to the other side. There is approximately 5mm (0.2") space between the 22mm pipe walls and the inner coil.

Why not make all three coils coaxial, with the second coil wrapped around the first and the third wrapped around the second?
This is a loss of water/pipe surface contact area since the coils fit too well inside each other.

The second reason why I didn't do that was the length of the coils. To keep refrigerant distribution and heat load per coil the same, I thought it might be a good idea to keep the lengths of the coils approximately equal.

Why not drill three 1/4 inch holes in the end caps and bring the tubes out through those holes, so that all brazing can be outside the evaporator, similar to what you did with the cap tubes?
Bending the 1/4" pipe in small radiuses is still a problem for me. I do not have the tools, and using a cylindrical thing to bend around only works when you have enough space to work.

Besides, the end caps are soldered with normal (plumbing) solder. The melting point of the brazing material used and that solder differs ~500 °C / ~1000F, which is enough to leave the brazing intact while flowing the SnAg solder.

I did the captubes externally so I can easily alter the direction of the evaporator. Initially I will place it horizontally, but if required I want to be able to mount it vertically. And I want the 3/8" -> 3x captube distributor to be vertical to ensure an as good distribution as possible.

I pressure-tested the finished evaporator at 21 bars (~300psi), and I left a test pressure of 5 bars (72 psi) in it for 36 hours. Test pressure came out unaltered, so I can reasonably assume that the evaporator does not leak.