Cutting power vs heat output into coolant

Community Laser Talk
1

I was just wondering, how cutting power translates into coolant heating ?
I’ve never run my CDWG 80W (sustained) tube for long so no idea if my water bucket and aquarium pump will hold up.

I’ve received a large order of face shield PETG (0.5mm) parts to cut, and, according to internet (hehe) i’ll be cutting it at fast speed and low power. The job needs full days work, many days in a row but with frequent stoppages to load more material. I cant see one run to be more than 3 minutes long (to be clarified after testing), then unroling petg roll from the back into machine, magnets on, run again, rinse/repeat…

I know i will be buying proper chiller down the line, but will this kind of excercise unduely deplete my tube using just the water bucket ?

2

As long as you keep an eye on the water temperature and don’t let it get above about 22 degrees © you should be fine. Add frozen water bottles or a bit of ice once in a while as necessary.

3

If one was to “guesstimate some numbers”, it’s not hard to calculate the approximate heat input to the coolant.

I’ll toss some numbers out for fun.

My 80 watt red&black draws about 160 watts (measured) at idle. That includes the coolant pump.

At 65% “indicated output” on the Ruida controller my 80 watt tube runs at about about 24mA which is probably around 70 watts.

At that power level the machine draws about 610 watts. (measured)

So subtracting the idle power from the loaded power gives about 450 watts that presumably the tube and power supply are consuming.

Modern switch-mode power supplies are pretty efficient so if we assume an efficiency of 89% that gives us 400 watts of power delivered to the tube and 50 watts wasted in the power supply.

So, with 400 watts of input power and about 70 watts of optical power coming out of the “bright end”, that leaves us with about 330 watts of heat wasted in the tube that needs to be be dealt with by the cooling system.

One watt = aprox 3.412 BTU/hour so 330 watts = 1126 BTU/hour

One BTU is the amount of heat required to raise 1 lb of water 1 degree F.

IF the cooling system holds 5 gallons that’s about 42 lbs.

1126 BTU / 42 lbs = 26.8

So for every hour of operation at that power level, the water temp rises by about 26.8 degrees F.

But wait, there’s a little bit more. My “aquarium style” coolant pump (which I no longer use) draws about 26 watts. And since it’s a submersible pump ALL of that energy ends up in the water.

So that’s an additional 81.7 BTU/hr which adds an additional temperature rise of almost 2 degrees F/hour.

Unfortunately the coolant pump runs all of the time the machine is powered up so that energy is always heating water whether the tube is firing or not.

So to summarize:

In the above example, with the laser idle the coolant is being heated by the pump at a rate of about 2 degrees F / hour and with the laser firing it’s about 28.8 degrees per hour.

The math for all of that should be good. Plug in whatever power numbers you like for your particular machine and cooling system capacity. If you’ve got a “Kill A Watt” meter or similar it’s easy enough to measure idle and running power on your machine. If you’ve got one of those fancy laser power measuring tools then you can directly measure the output power of the tube, otherwise you’ll have to guess.

Got all that? There will be a short quiz after recess. :slight_smile:

2 Likes
4

Screw you teach, leave us kids alone :smiley:

On a serious note, what i’m interested in is does heat fall off with cutting with less power than maximum ?
I’ll be cutting very thin material, fast and light on power, but lots of it. Does lower tube output power lower thermal discharge into coolant ? By my intuition it should, but in practice ?

All i need to know if my tube is safe for couple of weeks of long but light load with a bucket of water, until i scrounge up the cash for a proper chiller (at which point all of this becomes irrelevant :smiley: )…

I’m at the point where i’ll be using rectal thermometer to check water temp for the thing… :smiley: Things are moving too fast to lose the order, and no time for high-tech solutions…

5

We don’t need no education
We don’t need no thought control
No dark sarcasm in the classroom
Teacher, leave them kids alone

Absolutely.
If you cut the input current to the tube in 1/2 the power consumed will be APPROXIMATELY half.
The voltage drop across the tube decreases with increasing current so the relationship between current and power isn’t very linear, but for the purpose of “back of a napkin” heat calculations it’s close enough.

Ahh, there’s the real question. The math is helpful but the REAL question has a lot to do with where you (and your laser) live.
Your results in Death Valley, CA will be rather different than in Frostbite Falls, MN
If you wake up in the morning and your coolant bucket is 40F then you can do quite a bit of lasering before temps get out of hand. If your bucket starts at 70F in the morning then you’re already screwed.

Here in my little patch of CA, in an unheated/cooled space I can run my laser much of the year with just the water bucket but there’s a few weeks in the summer when I need to drop some ice in the bucket. (I keep a couple of 2 liter water bottles in the freezer and drop one in when needed)

This summer I WILL be buying a chiller.

6

I live in Phoenix, AZ … my Laser are in the Garage … so I’m screwed already. :wink:

Btw, did anybody try a Waterchiller which you use normally for drinking water?

7

True. I live in Northern ireland with its rubbish weather, laser currently sitting in unheated commercial space (huge garage essentially). So 10-15C outside at the moment, and next week.

Dont think it would keep up with heat chucked out into the water ?

8

I have a CW-3000 attached to a 80W Laser.

Thinking if I put a 1/10 HP Fish Tank Chiller in line … about 1400 BTU.
If this works?
The CW-5000 has about 2800 BTU

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I keep the spare distilled water jugs inside my machine on the bottom left side.

10

@Stroonzo Where do you keep the cold

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11

I can’t find Dr Pepper 10 ANYWHERE so I’m stuck with this:

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