Some notes on cooling and chillers


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    Default Some notes on cooling and chillers

    Hi All,

    It was a hot summer, and I found that my 50W laser's power would drop over the course of a few hours run due to temperature rise of the cooling water. It seemed to struggle to get through the same 6mm ply that was cutting like butter when it started, and was highly dependent on the temperature of the cooling water. I'd like someday to determine power vs temperature, but I don't have the ability to do it now.

    For a few months, I used ice or blue ice blocks in the 4 gallon reservoir to cool it down. Worked great at keeping the power up but was tedious in swapping out blocks or adding ice. A chiller was needed - and not a radiative like the '3000, because my work area was also pretty hot. I needed an actual compressor chiller. I could not find clear advice on sizing, etc, so I looked up the calculations and here are my notes from reading up on chillers, and my own estimates of chiller sizing. If nothing else, it will be useful to me to have written down somewhere.

    At full power, I see ~500W used by the laser. Probably 400-450W to the tube supply, and the other 50W to the fan, motors, etc. At 85% efficiency I estimate ~375-400W is delivered to the tube, of which 300-350W is heat and the rest as laser output.

    Let's call it 350W heat that must be removed. At 0.293 Watts per BTU/H, this means that I need about 1200BTU/h cooling capacity to keep the water at a constant temp. More BTU's and I can make it colder, less and the temperature will still climb but will climb slower than having no chiller at all.

    This puts me above the 1/10HP (~900-1000 BTU) range, but below the 1/4 HP range (~2300-2500 BTU/hr). But as luck would have it, I found a "new" Active-Aqua 1/4HP chiller and figured what the hell. I bought it. Sadly, the unit was a lemon as shown by a simple test, so it's going back to the seller. But.. I learned some valuable information that may be useful to you guys while checking it.

    With water circulating, and laser off I ran the chiller for about a half hour while using a meter to plot the temperature in the 4 gallon reservoir every minute. Here's what I got. Sorry for Fahrenheit scale, US chillers are all in BTU which relies on degrees F.

    Some notes on cooling and chillers-active_aqua-jpg

    The water is cooling consistently at about 0.6F/minute, or 36F/hr. The reservoir size is 4 gallons, which equals 4 * 8.33 => 33.3 pounds of water. So 33.3 pounds of water * 36F/hr => about 1200BTU/hr cooling capability. This is far below the rating for a 1/4HP unit! I should have been dropping the temperature of 4 gallons of water at about twice the rate that I did. An actual 2300BTU/hr chiller would have given me -1.15F/min without the external heat load of the laser tube. This chiller would barely keep up with the laser's heat output, when it should be able to do nearly double the cooling that it is doing.

    So valuable lesson, folks. It might be worthwhile to check your chiller to see if it's in spec! I probably just got a bum unit, but had I never sat down and mathed it out I would have never known. I will replace this one with a 2000 BTU/hr unit and see how that goes.

    Similar Threads:
    Last edited by electronrancher; 09-27-2015 at 05:49 PM.


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    Default Re: Some notes on cooling and chillers

    Just for curisity's sake, I did another test today with the chiller while the laser was running. Here's how it turned out.

    Some notes on cooling and chillers-active_aqua2-jpg

    I again run the chiller full out (setpoint = 40F) and at about 8 minutes in, I start a 5-minute laser blast at 22mA. We can see the water temperature profile now has two slopes - at the beginning and end it runs with no laser load so the temperature drop is similar to what we saw before: 37F/hr, or about 1230 BTU/hr, or 360W of heat being continuously removed. From 8-13 minutes the heat load of the laser reduces the slope to ~13F/hr, or about 430 BTU/hr or 125W of heat being continuously removed.

    Not as bad as I thought! The chiller is still working below spec, but at least there's some chilling capacity to spare even if the laser is running full blast. And for trivia's sake: Now we know that a generic 50W tube (850mm x 50mm, Automation Tech in Chicago) dissipates 235W while running at 22mA.



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    Default Re: Some notes on cooling and chillers

    Good information. I'm using that same tube with a 60 watt supply. I wouldn't run it at 22ma for a long time and also wouldn't chill it to 4 C. I'm targeting 15 to 20 C. Where did you get that set point?


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    Default Re: Some notes on cooling and chillers

    Yes, I never intended the chiller to actually reach that temperature. I just wanted to set it so low that the compressor was guaranteed to be running full blast so as to give as straight a cooling line as possible. 15C is probably the lowest I'd actually use, or higher in our thick southern humidity.

    18mA is also my "day to day" current setting, I just cranked it up to 22mA for the test to try to find the max dissipation. Honestly, I found the tube output power was really only 50W (49.6W) at 22mA so I figured that was the absolute limit of driving capability. The seller only lists the drive current as "~20mA", right? Not too descriptive.

    Have you checked your tube's output power by any chance? I'm curious.



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    Default Re: Some notes on cooling and chillers

    Ok that all makes sense.
    So you are saying that you can measure tube output by measuring heat added to the tank. Sounds possible but wouldn't it be better to measure power of the beam? It just seems like a more direct way to do it. Like heating up some object?


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    Default Re: Some notes on cooling and chillers

    No, you can't measure beam output by the waste heat produced by the tube. Remember, I was doing calculations for a chiller so that waste heat number is important to know how big a chiller to get.

    My discussion of tube output power was in response to your question about 22mA. At 22mA, my tube output is 50W, and the waste heat is about 235W.



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    Default Re: Some notes on cooling and chillers

    I am new to this forum. I own a little hobby 40w CO2 laser from China and I use a 5 gallon bucket with three 1/2 gallon frozen orange juice containers. I am running into laser tube failure after failure and was wonder if it is due to not being cold enough. The two gallons of water I have in the bucket stay between 55-65 degrees. The vendor tells me I should get 1500-2000 hours out of each tube. First is this an accurate number, and second, what temperature should the water be at for cooling the tube?



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    If that's 55F-65F, you're fine on temperature - that's colder than I use because I would get condensation running that cold.

    I assume you have a good water pump circulating this water, right? Might just want to check the output hose to make sure it's flowing nice and is not clogged.

    I am doubtful about getting 1000-2000h life from a generic tube, I'd expect that a nice RECI tube could get there, but not a no-name tube. Could be wrong though...

    But I would expect more than 300h from a generic tube, so if you're killing tubes very quickly you may be using too much current. Does your machine have a current meter to tell you how much current is flowing in the tube?

    And would you mind describing what you cut, and what is happening to these tubes? Just die? Get weaker? Explode?





    Quote Originally Posted by ieman43465 View Post
    I am new to this forum. I own a little hobby 40w CO2 laser from China and I use a 5 gallon bucket with three 1/2 gallon frozen orange juice containers. I am running into laser tube failure after failure and was wonder if it is due to not being cold enough. The two gallons of water I have in the bucket stay between 55-65 degrees. The vendor tells me I should get 1500-2000 hours out of each tube. First is this an accurate number, and second, what temperature should the water be at for cooling the tube?




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Some notes on cooling and chillers

Some notes on cooling and chillers

Some notes on cooling and chillers