Some specific (I think) cooling questions

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  1. #1
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    Default Some specific (I think) cooling questions

    I am in the process of setting up and upgrading my 40W CO2 laser cutter and looking at the cooling system. I plan to use a PC water cooling system. But in order to make sure I plan the parts out right, I have a few questions that hopefully others already know:

    1. What is the volume of liquid the laser tube holds?
    2. What are typical temperatures of the water leaving the tube?
    3. Has anyone measured the temperature of the glass on the laser? Is it hotter at the output end of the tube?
    4. Would ammonia be safe to run through the tube? The reason being that it is more efficient at absorbing and releasing heat than water.

    Currently, this is my planned system:

    240mm radiator with 2 standard 120mm fans (fans may get replaced with thermopiles)
    Latex tubing to the laser to ensure the tubes don't collapse
    Flow direction will be from laser output to the rear of the tube (assuming that the output of the laser is the hottest part)
    Check valves on the input and output to maintain pressure (to prevent air bubbles)
    Flow switch on output as part of the interlock system
    Temperature sensor installed inside the radiator that will be connected to a little circuit to kill the laser power if the water is too hot. Part of the interlock system
    Reservoir (right now, it is a waterproof ammo box)
    Pump (12V PC style pump or submersible fountain pump?) One of the reasons I am asking the above questions...

    These things are planned just for gathering some information and probably won't stay a part of the system:
    Flow meter connected to the controller board (RAMPS1.4) (this might be only temporary to gather information about the system)
    Thermistor (temperature sensor) mounted to the laser tube

    Ideally I would like the system to be as self-contained as possible. I am also wanting to shoot for "not way too much" but of course, not too little either. Being relatively quiet and efficient is the goal. My thoughts are that moving the water too quickly will not allow the water to absorb the heat from the laser. And having too much volume of water would make chilling the water more difficult.

    If it isn't obvious, I am an engineer. lol. I know that I am overthinking things and much simpler systems will work by sheer brute force or by throwing tons of money at it to buy a chiller (overkill for a 40W system, I think.) But for me, this is part of the fun and if I can put together a self-contained, quiet, and efficient DIY system that is relatively cheap, then I think that could benefit others. So, if anyone out there already knows the answers to the above questions, it would be appreciated and save me some work taking a bunch of measurements.

    Thanks for your attention!

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  2. #2
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    Default Re: Some specific (I think) cooling questions

    1/ I am an engineer too.

    2/ I am not a laser expert.

    Personally I use a temperature controlled chiller, but I get where you are coming from... so, some basic engineering principles.

    a/ Your tube was designed for water (de-ionised, not tap) cooling, not any other form of cooling, water cooling, as used in auto-mobiles for a century, because, guess what, the "thermal mass" (the amount of heat required to raise a given volume of coolant by one degree) of water beats just about anything else hands down, and is literally orders of magnitude greater than any gas.... you tub itself has a coolant loop of a fixed volume BTW.

    b/ Ambient is a *****, so is delta tee, unless you are using actual chilling / refrigeration, then ambient is your floor temperature, and the smaller the delta tee, eg the closer your coolant gets to ambient, the less efficient the heat transfer from coolant to atmosphere.

    c/ put it all together and it means that unless your ambient is below 12 degrees Celsius, it pretty much doesn't matter what size radiators of fans you use, your collant is likely to get over 20 degrees C in a small closed loop.

    d/ one answer is to increase the thermal mass, a coolant loop with 50 gallons of water is soooooo much more stable than one with 5 litres... with 100% thermal transfer efficiency from an electric heating element, 100 watts will raise the temperature of 1 litre of water by 1 degree C in 40 odd seconds, so if your closed loop is 5 litres and ambient / start temp is 15 C and for tube longevity you don't want to go above 20 C then you can run at 100 watts for 40 x 5 x 5 = 1,000 seconds, or 16 minutes , which is like saying I walk into the shop at 9 am and by 9:15 am I'm away from one world of physics and into another where I have to get that thermal energy out of my coolant before I can go back to where I was at 9 am.

    Thankfully these things are easy and cheap to simulate in test rigs, aquarium heaters and so on to simulate loads, if it all goes tits up the worst that can happen is you mess up ten bucks worth of aquarium stuff, not an expensive laser tube.

    Liquid/liquid heat exchangers tend to be quite efficient, so you can go a dual loop system, with a very large mass of ordinary water in the secondary system, or example above, where you get 16 minutes at 100 watts, substitute 50 (imperial) gallons for 5 litres and you get 45 times the endurance for the same temp rise.

    I know someone who has submerged copper coils in the bottom of a 100 gallon aquarium that he got for a song because the glass was scratched up, it#s not portable or compact or elegant, but it works.

    Biggest caveat is there is a world of difference between a gauge and an instrument, the instrument is calibrated, which is why back in the day X device would have three oil pressure gauges, the majority was always right, and you also have to be careful what you are measuring, a sensor attached to the outside of a tube is not measuring the temperature of the liquid inside the tube, so for me the first order of business would be obtaining a calibrated sensor / gauge where the sensor is actually immersed in the tube / primary coolant circuit coolant and measuring actual coolant temp.

    If best accuracy of this set up is X degrees then you need to subtract X degrees from your maximum coolant target temp, so if max tube temp is 20 degrees C, which it should be for longevity, and sensor and gauge is accurate to plus or minus 0.5 degrees, and calibrated, then you need to shoot for 20 - 0.5 - 19.5, and the curve of tube gas temp vs permanent chemical change is *not* linear.

    In closing, I'd like to relate a conversation I over heard between a laser vendor, a laser purchaser / user, and a friend of mine, a retired marine engineer.

    The purchaser was looking for ways to deal with frost, the vendor said premium quality automotive antifreeze was a no no, the purchaser was looking at self draining and anti siphon systems, there was some to-ing and fro-ing, the vendor admitted that as far as the tube itself was concerned, all the coolant saw or touched was glass, the problem was, the anti freeze would damage, over time, chemically, the chiller being sold with the laser as standard equipment.

    My friend ended the dilemma by asking three questions.

    How much is the tube to replace? (2000 bucks)
    How much is the chiller to replace? (500 bucks)
    Which is easier and quicker to replace, and which one can you do without temporarily by running some sort of botch? (duh)

    One advantage to the overkill chiller, apart from the compact and portable and so on, is they pretty much all come with a two wire open/closed circuit sense connector, you can't turn the tube on without the chiller running, and with the coolant being within temp acceptable ranges, and it is once less thing to buy when you move up to a bigger tube.



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Some specific (I think) cooling questions

Some specific (I think) cooling questions