Cooling is not as tricky as you might think. There is 3 ways to approach this, only 2 really matter for a low scale 40w setup. First one is the cheapest and easiest. The higher the temp of water the worse your performance will be, from what I understand anything in the 30C range is about the most tolerable it gets. Lower is better. But not frozen...from what I've been told a very experienced cutter, he found 8C was the highest power he achieved.
1. Use a 5 Gallon resovoir system which gives you a fairly large space of water to heat up before you need to tend to it. Add about a cup of anti-freeze to the mix of DISTILLED water. You don't want ANY minerals in the water that might build up in your system. The anti-freeze works to keep algae and other ickies from growing in your water.
2. Use a smaller resovoir system (or even closed loop) and install 1 or more radiators found in CPU cooling systems with 120mm fans attached. This will continously cool your water system to ambient room temperatures, but with a tiny resovoir it will be difficult to add things like ice-packs to drop the temps if the ambient is quite hot.
3. Using an industrial coolant system. Overkill and unless your cutting A LOT, this is a very expensive option to take. You can also explore the idea of Peltier cooling but it is extremely expensive electricity/BTU wise compared to an industrial cooler.
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HOW TO COOL THE BLACKTOOTH 40W LASER TUBE