Some water cooling examples :-

E3D produce some water cooled 3D printer items which look rather nice but at a cost :-  Titan Aqua (combined extruder & hotend  Kraken (hotend)

OPAM (One-Piece All-Metal) Water-Cooled Hot End Launches on Indiegogo  and a DIY version :-  Water Cool a 3D Printer Nozzle for Cheap and Easy! 

Although I already have water cooling kit that I used for astro, it's a bit big for hotend cooling so I've been accumulating parts for a lightweight version more suitable for this project.

1. Anself Ultra-quiet Mini DC12V Micro Brushless Water Oil Pump Submersible Water Pump Aquarium Fountain Pond Pump 240L/H 5W Lift 3M  This runs off 12v and therefore is easily controlled from the 3D printer control electronics and it is quiet and smaller than my previous one.  Can be used submerged or not.
2. 2m Clear Translucent Food Grade Silicone Tubing Milk Hose Beer Pipe Soft Rubber  I've got the 5mm bore and 7mm OD and two lots giving 4m which should be more than enough.
3. Aluminum Computer PC Radiator Water Cooler Cooling For CPU LED Heatsink 80mm  Possible heat exchanger.  There are also Chinese suppliers a few pounds cheaper.
4. Any old container as a reservoir.  With a large enough container and particularly a metal one, a radiator may not be necessary.  With a metal one I could add one of the spare CPU coolers I have   Or maybe use a finned heatsink in the water with CPU cooler outside.
5. 3D printed pipe connectors.

First thoughts for a dual nozzle cooler block.  To cater for standard heatbreaks the block will have M7 for 1.75mm filament or M6 for 3mm, threaded holes for the heatbreaks.  On the top side these will be opened up and tapped to 10mm for Bowden couplers (I also have M6 threaded 3mm Bowden couplers).  In order to enable the heater blocks (either V6 or Volcano) to be screwed onto the heatbreaks the cooler block needs to be divided up into three parts - two carrying the heatbreaks and another the cooling waterways and tube connectors (not shown in the diagrams below).  An external structural could be used to hold the three parts together.  The separate heatbreak blocks would also allow for moving up or down to level the nozzles.  The system used by the E3D Kraken might better but would need either Kraken heatbreaks or specially made ones from stainless steel.

A pair of Kraken heatbreaks would cost £24 which seems a lot for a couple of simple turned parts.  HERE is the engineering drawing.  I have a Chinese clone small lathe but doubt that would be up to turning stainless steel but I can try.  A plain part to go into the cooling block would make for a simpler block.  M6 SS bolts would simply need a groove cut between thread and plain shank and a hole drilled through the middle.

I've found an aluminium bar ½" x ¾" x 4" in my 3D parts drawer that I must have bought when I was thinking of making my own hotend blocks before.  That will take the threaded holes for the heatbreaks and Bowden couplers.  I also have a 1½" x 3mm aluminium bar that I could mill out for waterways.  I also have some rubber sheeting that was left over from roofing the observatory to use as a gasket.  Either another aluminium plate or 3D printed back panel with short pipes to take the water pipes.  I should be able to get the nozzles about 15mm apart or maybe even less.

Water pump delivered today and the tubing will go on the inlet and outlet connections with a little effort but it does mean that they are very unlikely to come off

I'm thinking I might go with a single nozzle to test the principle.  This will make for a much simpler system and take less time and effort to make.  I would like to try this out as soon as I can.

What's the point of a 3D printer if you can't make tea with it as well?

I guess the most obvious printer to try this on would be my Giant which wants the fastest extrusion for printing very large objects.  My Titan printer has an enclosure which can be heated so that is also a contender for water cooling.  Of course, there is nothing other than a relatively small cost to prevent my using water cooling on both.  High speed is a feature of the Titan too.  The Titan is currently working satisfactorily whereas the Giant in in the final stages of construction.

2 hours ago, Stub Mandrel said:

What's the point of a 3D printer if you can't make tea with it as well?

I could print a teapot   But although the hotend will be used at up to 300°C I don't envisage it producing boiling water!   It could, of course but...

I have 3mm filament stock as well as 1.75mm and an advantage of 3mm is that the water block could simply have an M6 hole right through as the 3mm heatbreak is M6 and I have an M6 threaded Bowden coupler with suitable PTFE tubing and a 3mm Bowden adapter for the E3D Titan extruder.  That's apart from the waterways of course.

8 minutes ago, Gina said:

I could print a teapot   But although the hotend will be used at up to 300°C I don't envisage it producing boiling water!   It could, of course but...

Yo didn't recognise a 'Utah Teapot'?

Nope.

I've found and ordered some aluminium bar 25mm x 15mm x 100mm which should do nicely for either single or dual extrusion.  Cut down, of course.

I have had another idea for the water jacket.  I have some thin copper sheet which could be cut to shape and edges bent up, soldered to another piece to make a flat water jacket which can be held against the hotend cooling block.  Small pieces of copper or brass tube could be soldered on for connecting the pipes.  If soldered well enough this will be watertight without any gaskets to leak.  I have a small gas blowlamp that I can use for soldering copper.  I have done this before with cooling a DSLR for astro imaging.

One of the pieces of aluminium has arrived - 1¼" x 5/8" x 4" but still waiting for the 25mm x 15mm x 100mm which should arrive tomorrow.

Been weighing things...  The extruder motor weighs 300g whereas the copper water block I used fro DSLR cooling weighs 120g.  So by going for Bowden feed I can save nearly 200g on the X carriage even with the relatively heavy 40x40x10mm copper water block.  I think I may try water cooling with the water block and save myself some work.

The other aluminium billet has arrived - 25mm x 15mm x 100mm.

Here is a photo of the water block and 12v pump.  Cooler part of hotend will be clamped to the water block with thermal grease between.  The tubing is silicone rubber and very flexible.  The water block is 40mm x 40mm x 12mm and although made of copper only weighs a little over 100g - much less than a direct feed extruder with motor.

Aluminium billet for heatsink - to be cut down to 40mm to match water block (or maybe a bit longer to take screws for a clamp for the water block) plus 3mm and 1.75mm Bowden feeds.

Have drill stand but my mains electric drill is in hiding and can't find it   Bench drill press is out in the shed protected by loads of snow!

interesting,
Have you managed to test it?

and what about TEC, have you tried to explore this way?

Missed this reply/query earlier.  See "GinaRep Giant Mk 3" 3D Printer where I use water cooling.

No, haven't tried a TEC.