Air/Gas assist

[Steve Baker]

We've been thinking in terms of Nitrogen gas assist for our lasersaur -
but I've been trying to figure out how much gas we might need and it seems
like it might be a lot.

100 cubic feet of nitrogen comes out at about $25 from our local
supplier...and leasing the tank comes out at $80/year with a $100 initial
outlay for a pressure regulator.

Can anyone comment on how much nitrogen we might get through per hour of
cutting?

-- Steve

[Justin Krull]

Try this out. Someone else might want to double check it, but I'm pretty sure I've got the calculations right.

Regards,

Justin

[Justin Krull]

Just to make this clear:

I labelled the inputs, but there wasn't enough information in your email to fill it out for you, so you'll have to change the inputs for your case. Let me know if you have any questions.

Justin

[Steve Baker]

That sounds wrong somehow...

As populated, the spreadsheet says that you have 1,500 cubic meters of gas
for 59 hours of continuous usage. To get 1,500 cubic meters of gas
through a 4mm exit hole within 59 hours, the gas would have to be escaping
at something close to a thousand mph!

-- Steve

-- Steve

[Jonathan Buford]

What pressure is it at for the calculation of volume vs. the working pressure in the tube prior to exiting? Expansion after the exit could provide the difference.

[Steve Baker]

It occurs to me that the M5 hole that we're told to drill into the end-cap
at the bottom of the lens tube isn't the limiting factor. The "one touch
coupling, male 90 degree elbow" (MSELL6-M5) in the BOM for connecting the
inlet hose to the bottom of the lens tube has a much smaller hole. So if
20psi is set on the tank regulator - then that's the pressure in the hose.
The pressure inside the lens tube must be less.

-- Steve

[David Armstrong]

4mm exit hole is far to large , 1.5mm - 2.00 is a far better nozzle size , given the beam dia , theirs no requirement to go larger than 3x focused beam size

your just wasting gas , and not getting it to where you need it in the kerf area

 

some cases iv’e used 1mm nozzle , but a compromise is 1.5mm , rather than keep changing it all the time

 

we run our O2 at 2 bar & Nitrogen at 3 bar  ( measured at a solinoid valve outlet on the machine rather than at the bottle .

 

also use a slightly larger pipe size than the orifice we use 5mm id tube , then let the head do it’s work 

and I cant see any reason for any greater gas flow that this , concidering the low power lasers used in Lasersaur

 

dont waste the gas ! 

 

hope it helps

Dave

 

[Justin Krull]

We've got a compressible flow nozzle here. It'll require a bit more logic. Thanks for the sanity check. I'll update the spreadsheet later today.

Justin

[David Armstrong]

i made mine to take a standard of the shelf nozzle , so they just screw in and can be changed in seconds

but i’m using my own design head , which is a bit more bulkier than needed on a lasersaur

 

the way i have it is that the gas from the bottle is at a slightly higher rate than the outlet say 3 bar , then i regulate it at the solinoid valve down to the working pressure of 2 bar

if the nozzle dia is small then the gas cant escape faster , so therefore holds the constant pressure through the nozzle orrifice , and because the bottle regulator is set higher , then essentialy the feed pressure cannot drop and stays consistant through the pipework

 

if that makes sense

 

Dave

[Justin Krull]

Here's the updated spreadsheet. It's invalid for low Mach numbers of gas coming out of the nozzle, so it will spit out "invalid" for those right now. At high enough (>28 psi) regulator pressures, the flow through the nozzle is choked at Mach 1.

[Justin Krull]

I fixed a couple of mistakes from the last version and it is also now a valid approximation over the whole range for a convergent nozzle. Choked flow happens at ~13 psi, not 28 psi. It should be plenty good to use for rough estimates of tank life. I'll validate it with my tank this weekend. Let me know how this works for you.

[Steve Baker]

So you're still saying that the gas comes out of the nozzle at close to
the speed of sound(!) - and that a typical gas cylinder (say 100 cubic
feet) will last much less than an hour?

That seems to make this whole idea of gas assist a bit of a non-starter
for me - if I'm going to need GIGANTIC gas storage facilities and spend
many hundreds of bucks a month on refills?!?

Is that really what people do for cutting regular kinds of materials
(plywood, acrylic) on a 120W machine?

-- Steve


Justin Krull wrote:

>>> *From:* Justin Krull <JTbi...@gmail.com>
>>> *Sent:* Thursday, December 13, 2012 3:35 PM
>>> *To:* lase...@googlegroups.com
>>> *Subject:* Re: [lasersaur] Air/Gas assist

>>>
>>> We've got a compressible flow nozzle here. It'll require a bit more
>>> logic. Thanks for the sanity check. I'll update the spreadsheet later
>>> today.
>>>
>>> Justin
>>>
>>>
>>> On Thu, Dec 13, 2012 at 7:24 AM, David Armstrong <
>>> david6.a...@ntlworld.com> wrote:
>>>
>>>> 4mm exit hole is far to large , 1.5mm - 2.00 is a far better nozzle
>>>> size , given the beam dia , theirs no requirement to go larger than 3x
>>>> focused beam size
>>>> your just wasting gas , and not getting it to where you need it in the
>>>> kerf area
>>>>
>>>> some cases iv’e used 1mm nozzle , but a compromise is 1.5mm , rather
>>>> than keep changing it all the time
>>>>
>>>> we run our O2 at 2 bar & Nitrogen at 3 bar ( measured at a solinoid
>>>> valve outlet on the machine rather than at the bottle .
>>>>
>>>> also use a slightly larger pipe size than the orifice we use 5mm id
>>>> tube
>>>> , then let the head do it’s work
>>>> and I cant see any reason for any greater gas flow that this ,
>>>> concidering the low power lasers used in Lasersaur
>>>>
>>>> dont waste the gas !
>>>>
>>>> hope it helps
>>>> Dave
>>>>
>>>>
>>>
>>>
>>
>>
>

-- Steve

[Justin Krull]

We decided to get an air compressor because, even if we just used it for testing to save money and switched over to nitrogen for production runs, it would be worth the 100-200 bucks to have for other uses. This is the one we got:

http://www.amazon.com/California-6310-Oil-Free-6-3-Gallon-Compressor/dp/B00889ZYPQ

It's extremely quiet because it's a piston compressor, but is definitely cheaply constructed (made in China may/may not be a coincidence).

[Ward Elder]

I use a normal air compressor. Without the air the lens will be pooched in no time!
Thank you
Ward Elder
Eldersoft
(204) 791-7754

----- Original Message -----
From: Steve Baker [mailto:st...@sjbaker.org]
Sent: Friday, December 14, 2012 11:57 AM
To: lase...@googlegroups.com <lase...@googlegroups.com>
Subject: Re: [lasersaur] Air/Gas assist

[Justin Krull]

It looks like a 100 cu ft tube would last quite a lot longer than an hour. Make sure you're using the V2 spreadsheet:

[Justin Krull]

Steve, you might have been putting the water volume of the tank (100 cu ft=2.832m^3) into the gas volume. Doing that gives a lifetime of a bit less than an hour, but is incorrect.

[David Gustavson]

How can the gas tank water volume be over 2 m^3?? That's a lot bigger than any gas bottle I've seen. Maybe it's ft^3??

Sent from Dave's iPhone 5.

<Gas Tank Life (14-DEC-2012).xlsx>

[Justin Krull]

Because pressurized gas takes up a lot less volume. The 2 m^3 is an estimate of the volume when all the gas is let out of the tank to standard room pressure.

[Justin Krull]

Nevermind, very sorry! The 2.8m^3 is way too large of a number, so Steve is correct. It would last under an hour. Thanks David and Steve. 2.8m^3 is the pressurized gas volume. The water volume of the tank is much less.

The spreadsheet is fine, the input number was just incorrect. I added time in minutes to the spreadsheet and changed the input water volume to the water volume of a 100 ubic foot cylinder. Here are some more realistic estimates:

[Steve Baker]

I knew about the compressed versus uncompressed volume thing. Sure, the
volume of a "100-size" nitrogen tank is pretty tiny. However, the
defining fact (and what makes it "a 100-sized tank") is that if you take
all of the gas out and put it in a room, it occupies 100 cubic feet!

So...yeah...the spreadsheet predicts that this medium-sized tank will run
out in well under an hour...and the biggest tank I can get delivered from
the local AirGas dealership won't last even half a day!

My problem is that if this calculation is correct - then the cost of
nitrogen-assist is truly horrifying!

I frequently cut designs that take one or two hours at a stretch, and I
fully expect to keep my lasersaur busy for 8 hours a day for months.

But if it's costing me maybe $30 to $40 per hour for nitrogen, then that's
a non-starter! The sheer number of cylinders I'd have to keep on hand and
the fact that I'd have to switch them out mid-cut would mean that I'd have
to lease at least a dozen large cylinders and get them refilled every
week!

But this spreadsheet also says that the gas comes out at about the speed
of sound! That's ungodly fast!

Does it really take gas moving that fast to do the job of "nitrogen assist"?

-- Steve


Justin Krull wrote:

>>>> *From:* Justin Krull <JTbi...@gmail.com>

>>>> *Sent:* Thursday, December 13, 2012 3:35 PM
>>>> *To:* lase...@googlegroups.com
>>>> *Subject:* Re: [lasersaur] Air/Gas assist

-- Steve

[Steve Baker]

Really? I'm skeptical.

My local hackerspace has a 60W ULS laser cutter - with no air assist and
no lens or mirror cleaning jets of any kind. The lens gets manually
cleaned whenever it looks cloudy...maybe every 10 to 20 hours of use - and
it's lasted at least a year.

-- Steve

>>>> but i’m using my own design head , which is a bit more bulkier than

>>>>> some cases iv’e used 1mm nozzle , but a compromise is 1.5mm ,

>>>>> rather
>>>>> than keep changing it all the time
>>>>>
>>>>> we run our O2 at 2 bar & Nitrogen at 3 bar ( measured at a solinoid
>>>>> valve outlet on the machine rather than at the bottle .
>>>>>
>>>>> also use a slightly larger pipe size than the orifice we use 5mm id
>>>>> tube

>>>>> , then let the head do it’s work

>>>>> and I cant see any reason for any greater gas flow that this ,
>>>>> concidering the low power lasers used in Lasersaur
>>>>>
>>>>> dont waste the gas !
>>>>>
>>>>> hope it helps
>>>>> Dave
>>>>>
>>>>>
>>>>
>>>>
>>>
>>>
>>
>
>
> -- Steve
>
>

-- Steve

[Ira Burton]

I assume just about any positive pressure system is sufficient to keep smoke off the lens, and nitrogen is not needed for that.

Some will use nitrogen to help keep fires from flaring up around the cut on some materials, it also does not require that volume of nitrogen.  

I think the rate in this spreadsheet is specifically to help channel the heat through the cut, which does require some significant volumes.