Hi guys,

I have built an R134a based water chiller which should cool a water/methanol mixture, which in turn should cool the microprocessor in my PC. I am currently able to reach -17 degrees C under load, but I have the idea that the evaporator is not performing optimal.

Some more information:
- Compressor: Danfoss NL11F
- Condenser: an old steel R12 unit well capable of handling the load
- Capillary: 2.4 meters of 0.8mm ID copper tube
- Evaporator: 6.5 meters of 1/4" copper pipe wound in a spiral, submerged in the water/methanol mixture.

I suspect that the 6.5 meters of 1/4" piping submerged in liquid just gives too much pressure drop. With 100kPa pressure at the compressor's service line the liquid temperature does not go below -16 degrees C (R134a boils at -26 degrees C at that specific pressure). I also suspect that liquid refrigerant is blown through the evaporator before it has the chance to evaporate.

If I want to build a better evaporator, I need to know a few things:

- What must be the minimal speed of the (gaseous) refrigerant in the suction line to ensure sufficient oil return? I assume these values are different for both vertical and horizontal sections.

- For a total load of 200W, how large an evaporation surface (copper tubing) do I need? I understand that this depends on the allowable dT between liquid and boiling R134a, but are there guidelines? The lower the dT, the better.

- Any other considerations for the new evaporator? I plan on building a coaxial heat exchanger type of evaporator (a thin refrigerant line within a larger pipe where coolant flows along the line)