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Thread: DIY laser chiller?

  1. #1
    Join Date
    Oct 2021
    Posts
    8

    Default DIY laser chiller?

    Refrigerated chillers for higher power machines (like >100W CO2) get pretty expensive, and I'm annoyed that they are pretty noisy and dump heat back into the room where the HVAC has to, once again, process the heat to get it outside the building (warm climate, we usually need to get rid of heat). The CW series isn't designed to be put outside, either.

    I've pushed around ideas of a large window AC unit, rip out the evaporator fan, and build a heat exchanger around the evap. There are a number of DIY builds documented online doing this for wort chillers (brewing), but they physically bent the evap tubing so it hangs upside down below the inside side and dipped it into a container. That is a big risk of having the copper tubing kink, and is a pretty bad "large Z-shape" form factor that sounds awkward. If I were to do that, I'd rip out the fan and build an acrylic tank in place around the evap core and use a bunch of E6000 to seal it all up.

    But then I'm wondering "why is this in a window at all?" Well, I could place the whole unit outside with just coolant lines. The front of the unit wasn't designed to be outside in the sunlight and rain, but most of that will be removed anyways. Still, there's not a clear mounting solution, and the water cooling lines may get pretty long.

    I had a better thought- what about a mini-split? Run refrigerant lines to an indoor evap side hacked to be a heat exchanger. That sounds pretty awesome. The mini-split's outdoor side is designed to go outdoors and the mounting solution is already worked out. Can be on the roof, on an outside wall, or on a ground pad.

    Now, I do see a clear prob that the conventional systems are not only either on or off, but once it's cycled off, it cannot be cycled back on for a few minutes as the back pressure on the high side will stall the compressor motor, which will result in failed restarts and can damage the motor. A laser needs consistent temp for consistent performance, and my calcs show an unreasonable mass of coolant would be needed for thermal mass if you have to shed like 2KW of heat over several minutes of cycle time with +/1C variation. The answer might be to build a system that has a variable recirc bypass to mix some or all the return flow back to the load mixed with some chilled coolant around the evaporator.

    So if the laser needs 20C, the tank can be chilled to 5C or 10C and we can calc the thermal mass needed so as the compressor reaches the lowest temp specified and cycles off, we're like 70% recirc flow and 30% chilled water flow, and the tank thermal mass only needed to be sized so it will not exceed 20C before it can restart the compressor. Rather than be sized so the tank is chilled to 19C but with so much thermal mass it only varies +/-1C before the compressor can kick back on.

    The alternative would be an inverter-driven mini split that can vary its compressor speed. However, I did thorough research- they're more expensive, and can't actually run at ANY speed, just full down to, like, 40% and below that must cycle off and then there is still a restart prob. If you were rastering with the laser, that could happen often. Recirc water bypass regulation will fix that, but at that point you don't have much need for the inverter drive, it just adds cost and complexity.

    Anybody done this?

  2. #2
    Join Date
    Apr 2012
    Location
    Beaverton, OR
    Posts
    292

    Default

    You don't actually need a chiller with a CO2, you just need to keep the head and electronics cool below the spec, I think about 90f for coherent. For this all you need is a fan/radiator, pump, and reservoir. Unless you are running on a very hot day you won't have to worry about being too hot. There is a slight loss in power running at higher tube temps but it's pretty insignificant.

    Commercial chillers use an internal bypass valve to turn cooling on/off. An AC unit that's big enough to sink that kind heat is going o be huge. I have a big ole chiller for the Rofin. 3HP, will handle 10.5kw, that's about 35000BTU.

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