Minimizing liquid nitrogen cylinder venting from laser

[pMetal]

We are using liquid nitrogen cylinders to supply nitrogen assist gas for our fiber laser.
It is normal for the liquid nitrogen to always be boiling off, so the cylinder vents to the atmosphere periodically to reduce the internal pressure.

It is the peak of summer right now, so I was wondering if the venting is happening more than usual because our building is hot. And since nitrogen is expensive, I was wondering if we could reduce the amount of venting in a meaningful way by cooling the general area or the outside of the tank.

Do you know if this could be effective or cost-effective?

 

[BT Fabrication]

if not used for more then a day it will always vent off.

 

[pMetal]

if not used for more then a day it will always vent off.

Have you noticed if the venting happens more in the summer compared to the winter?

I am new to having it, so I don't know yet.

 

[hvnlymachining]

Yes it vents more in hotter weather, if you can keep the tank in a cooler location, and definitely out of sunlight it will help. Since liquid nitrogen boils at such a low temperature you have no chance at stopping it, just slowing it down. Every degree cooler helps. ( A tiny amount, but helps)

 

[dana gear]

Insolation blanket, cool location, it's going to vent. depending on withdrawal rate.
It's almost always cheaper than high pressure cylinders.

 

[memphisjed]

We switched over to pallets (aka batteries, aka cylinder cubes) of o2 from liquid because of this. Technically more per cubic foot yet we save a lot. Every time you hear venting it is just when it is quiet enough to not pass as background, and you happen to be there.
bonus is no having to deal with pressure boosting valves when the weather changes.

 

[BT Fabrication]

Have you noticed if the venting happens more in the summer compared to the winter?

I am new to having it, so I don't know yet.

Like others said, reducing the heat transfer into the dewar will reduce the off gassing, so if the shop or area its stored in changes all over it will vent more when warmer and less when its cold out. I think the boiling point of liquid nitrogen is somewhere around -185C. It all depends how much Cubic ft you use.
big 2000 psi tanks hold about 330CF per x12 in a pallet.
tanks of pressurized gas are nice for smaller use or even palletized with 12 tanks per or a large bulk tank for those times where it doesn't run for days at a time.
liquid is good if you need it refilled within i think about a 2-3 week time frame and is cheaper that way.
pressurized is a bit more $ but saves if not running 24/7

as for compressing the gas for re use, its a bit of an expensive thing to do with gasses as it takes alot of energy.

 

[pMetal]

Thanks for the replies. I will look into an insulating blanket for the cylinder.

The massive nitrogen needs of fiber lasers is something I was not expecting when I bought the machine. The 12-packs of high pressure cylinders are expensive and only last a few hours of use, and the liquid cylinders boil off if you don't use them constantly. Not the best situation! Ultimately I see that I'll need to find an on-site nitrogen generation system to be able to run the laser cost-effectively.

 

[DDoug]

Thanks for the replies. I will look into an insulating blanket for the cylinder.

The massive nitrogen needs of fiber lasers is something I was not expecting when I bought the machine. The 12-packs of high pressure cylinders are expensive and only last a few hours of use, and the liquid cylinders boil off if you don't use them constantly. Not the best situation! Ultimately I see that I'll need to find an on-site nitrogen generation system to be able to run the laser cost-effectively.

You seem to be in that grey area of too much usage for cylinders, and not enough for liquid.
Seeing how liquid is cheaper, what is the monthly cost for liquid even with the venting ?

 

[memphisjed]

The 12-packs of high pressure cylinders are expensive and only last a few hours of use,

A pallet should be the same use as one liquid bottle. Them gas people are smart like that. Lasers are expensive to run, but you shouldn’t be burning thru that much gas unless you have big (20kw or better and cutting thick material).
Do you have leaks anywhere? Hours seems really low. A heavy week on plasma is one pallet of oxygen, and they are hogs of gas comparatively.

 

[BT Fabrication]

local laser shop just invested in a massive bulk high pressure tank. they used to use a 12 rack of cylinders every 8-9 hours of cut time.
just need to spend the big $ to get a massive tank installed and run all the hard lines to it with the expansion evaporator gassifier piece beside it.

i think the small on site generators start in the 50K range.

 

[pMetal]

A pallet should be the same use as one liquid bottle. Them gas people are smart like that. Lasers are expensive to run, but you shouldn’t be burning thru that much gas unless you have big (20kw or better and cutting thick material).
Do you have leaks anywhere? Hours seems really low. A heavy week on plasma is one pallet of oxygen, and they are hogs of gas comparatively.

Apparently the fiber lasers use a ridiculous amount of nitrogen. Its a 4KW machine, so not giant but not small. No leaks, it just really uses that much. With a 2.5mm nozzle orifice (which seems to be the one for most of the materials I cut), at 200psi, it uses 1,302 CFH. And a 3mm nozzle uses 1,874 CFH. This is according to charts from the machine manufacturer.

So a 12-pack of 300CF high pressure tanks contains 3,600 CF theoretically. So with a 3mm nozzle, that theoretically would deplete a 12-pack of tanks in 1.92 hours. Crazy.

Just today I bought a high-pressure breathing air compressor at auction. I'm going to try going the CDA (clean dry air) route and see if it works out to cut my thinner steel parts with this regular purified air instead of nitrogen. I've read that it can work well, given that regular air is already 78% nitrogen and the main purpose of nitrogen in a fiber laser is just to blow the molten material out of the cut. I think the compressor's output meets the cleanliness and dryness requirements.

If that can work, I'll be a happy camper. Every time I hear the liquid nitrogen vent it stresses me out because I know I'm hearing money being thrown (blown) away.

 

[pMetal]

One more question: My gas vendor says that I can manually vent the liquid cylinder down to around 200psi, and then it will take a while until it reaches the 350psi automatic relief vent pressure. He says this will reduce the overall venting loss. To me this sounds wrong: Wouldn't I just be intentionally venting the same amount that it would be automatically venting on its own over the same total amount of time?

Wouldn't the evaporation rate be the same either way?

Gas vendor says that the way the physics works is that it boils off less at the lower pressure. I'm rusty on the physics here, but I think the opposite is true. Can anybody shed some light on this?

 

[Strostkovy]

Are you feeding the laser from the liquid port into an evaporator or from the gas port?
We have our nitrogen system down where we have a hose and regulator to push gas from the dewars that are about to vent into the vent port of the dewars we are drawing nitrogen from. If we have no dewars above 300 psi we have a high pressure bottle on standby to pressurize them. If all of our dewars are high pressure, we can connect the gas port of the dewar to the inlet of the vaporizer which very rapidly drops the pressure of the dewar.

 

[Halcohead]

One more question: My gas vendor says that I can manually vent the liquid cylinder down to around 200psi, and then it will take a while until it reaches the 350psi automatic relief vent pressure. He says this will reduce the overall venting loss. To me this sounds wrong: Wouldn't I just be intentionally venting the same amount that it would be automatically venting on its own over the same total amount of time?

Wouldn't the evaporation rate be the same either way?

Gas vendor says that the way the physics works is that it boils off less at the lower pressure. I'm rusty on the physics here, but I think the opposite is true. Can anybody shed some light on this?

You are correct that proactively venting the tank will not save you nitrogen, and will in fact increase the heating rate of the liquid nitrogen. The boiling temperature of nitrogen goes up with pressure. Venting the tank from 350psig to (~365psia) to 200psig (~215psia) drops the boiling point (and temperature) of the liquid nitrogen by 7deg C (13F) from -156C to -163C. Feel free to look at the reference here: https://webbook.nist.gov/cgi/fluid.cgi?PLow=175&PHigh=375&PInc=5&Applet=on&Digits=5&ID=C7727379&Action=Load&Type=SatT&TUnit=C&PUnit=psia&DUnit=kg/m3&HUnit=kJ/kg&WUnit=m/s&VisUnit=uPa*s&STUnit=N/m&RefState=DEF

I wouldn't stress about summer vs winter temperatures for the tank too much. Much, much more important is the quality of your insulation and eliminating any points where you have thermal shorts (uninsulated areas) between the nitrogen and ambient. If there is any part of your nitrogen tank or bulk-storage system that is frosting over or sweating, improving the insulation to that will make a big difference.

If you care to quantify the difference of ambient temperatures, summer vs winter temps will vary the ambient temp by ~30C, but since the LN2 is boiling at -160C, and the rate of heating of the nitrogen is proportional to the temperature differential, that change from freezing to very hot is only a 15% change in the rate of boiloff of the nitrogen. By contrast, pressurizing the tank from sea-level pressure (0psig, 15psia, -193C boiling) to 300psig (-156C boiling) makes a 39C difference in the liquid nitrogen temperature, reducing the boiloff rate by 20%.

Anyways, lots of words but I hope this helps some.

 

[Bill D]

You can always put the dewars in a walk in freezer. I bet the electricity to run the freezer will cost more then the vented N2. It might save money even if the cooler is not run just from the insulation
Bill D

 

[snowshooze]

I only have a 10 liter can.
But, here in Alaska, when it is cold outside, I store it there.
And when it isn't, I keep the vacuum thermos in an insulated box.
Anything you can do is good.
But, boil off it will.
There is no stopping that.
You just have to scribble it in as a consumable and tack it on.
Cost of business.

 

[zachmelo]

I used to run oxygen out of a large 300L dewar for my home glassblowing studio. In order to get a longer life from the tank when we knew we wouldn't be melting for a while, we would adjust the pressure builder valve.

I don't know if LN2 dewars have the same pressure building circuit, but by closing the valve it would stop liquid from making its way into an external pipe which would provide pressure to the dewar. The tanks are so well insulated that they required the external line to keep pressure up. They still vented after some time as ambient heat made its way through the walls of the dewar vessel, but closing the pressure building valve we got a lot longer shelf life for our dewar tanks.

This page shows a pressure builder valve:

Operation of a Portable Liquid Nitrogen Vessel - Middlesex Gases

Read more about Operation of a Portable Liquid Nitrogen Vessel, a news post from Middlesex Gases, the elite supplier of gases in New England.

www.middlesexgases.com

A quick Google search for "dewar pressure builder off" will tell you more about reducing the venting of your dewar by limiting flow through this loop.

I've since switched to a home built low pressure oxygen system, but that's off topic for the thread.