LN2 Cost Reduction

Have you considered installing an onsite N₂ generator instead of using LN₂?

Dear DVader1000,

Thanks for the advise. Currently, we are using storage tanks, delivered by LN2 trucks daily. We cannot install onsite N2 generator due to two reasons, one, there is no space for it, two, it requires quite high initial capital outlay, and at this tight financial situation, it will not get through. The problem is at the user end point, not at the source. Thanks for your input.

Best regards,

The best way to do this is to make sure you use "super insulation" on the LN2 storage vessel and do not pipe LN2 to the ovens. Pipe gaseous LN2 as needed from the dewar with an expansion pipe from the LN2 tank to the end use ovens.

good luck

Dear Aurizon,

Thanks for the suggestion. Our burn-in ovens require LN2 purge directly in order to achieve the temperature profile in time. Gaseous LN2 may not be able to do so.

Best regards,

Jack Leong.

Your problem is not well defined enough for me to help you here. The LN2 piping, I assume is vacuum jacketed. If not that is one way to solve the problem, but it is costly. This system should be analyzed by a consultant that can glean from your available information the source of your problem. You may actually have an optimum system, you may not. The information provided here is inadequate for anyone to comment intelligently. The venting gas at the ovens is called stay-full wherein the line vents gas that evolves from the heat leak of the line. If this is eliminated you will get a big slug of gas each time you fill or consume. That may not be all that bad if you are filling every 24 hours, but if you are filling every 1/2 hour it may. Again analysis is necessary. I can help you, I build transfer line systems for Liquid Helium which is literally hundreds of times more difficult than LN2. ability@ameritech.net is my e-mail address.

Assuming that the requirement is liquid N2 the vent is not the problem. But the constraints are different if you are only using the gaseous form.

For liquid N2 use: assuming that the existing situation can be improved, it would be via improved insulation and/or reduced surface area.

On the other hand, if these are burn-in ovens rather than temperature-cycling ovens, you are probably only buying liquid N2 as a source of dry N2 solely because it is more convenient to store than gaseous.
So, if your system always evaporates and heats the liquid N2 at the point of use, a partial solution is to replace the venting valve with an overpressure valve, and allow your ovens to take gaseous N2 from the pipe*. That will significantly reduce the area of pipe that is heating the stored liquid. The vent will only operate if there is a problem, as under normal conditions surplus N2 gas/vapour will back up along the pipe and bubble up through the tank (yes, the gas will heat the N2 in the tank, but the problem will be less than if the pipe is filled with LN2).
But in that case, the other question would be whether you need to use N2 at all, or whether dried air would do the job equally well (or maybe dried and oxygen-depleted)

*I don't know how standard this is, but I have seen systems that ensure sufficient gaseous N2 that the ovens don't need to handle the liquid at all - the end section of the pipe has a heater that is turned on if the pipe temperature falls below about 100K.

If you can provide more details, we can provide more targeted help.

Dear Physicist,

Thank you for your insightful response. Our ovens are temperature cycling ovens. We need direct LN2 delivery to achieve the temperature cycle fast. Gaseous N2 may not be practical in our case. The pipe we use is the vacuum jacketed type.

However, we do not see this problem (venting) with another piping system (which leads to another group of temp cycling ovens) which is flexible (Liggett?) and uses an on-line vacuum pump to create the vacuum insulation for the pipe. I don't see any vent valve there.

Best regards,

Jack Leong.

If your bleeding off that much LN2 I hope your room is well vented. If not your are probably not bleeding off that much. As was said earlier a proper system with vacuum insulated pipes would be my approach if I truly need LN2 at the chambers. How much are you consuming vs wasting by bleeding the line?

Where I have worked, we also had high-pressure CO₂ lines, so N₂ was almost always vented outside in the same way. But, even if you do vent N₂ to the room, there is little danger until you exceed 20% nitrogen concentration (very different from CO₂, of course)

_{Hello Physicist?:,}

You should tell if there is too much N_2.

The workers will seem over happy?

Why do you say that - the only physiological effect would be to oxygen depletion and reduction in CO₂ levels?

Hello Physicist?:

I was just trying to fit the 'Laughing gas' joke in. But it seemed not to work, sorry. And thinking.......it is N20?

Dear Sir,

The room is well ventilated and we have oxygen detectors and alarm.

The vent is built as part of the piping system and is purging N2 gas outside of the room especially when the temperature cycling burn in ovens are not running.

It is wasting about 20% of the overall LN2 consumption.

Best regards

Jack Leong.

Would a simple way to reduce consumption when the ovens are not in use be to close a valve right by the LN2 cylinder. If the storage in the pipe is sufficient that this does not help, you could in principle put enough back-pressure on the bleeder valve to return the LN2 to the tank (but you'd have to set it up very carefully, and (assuming the system is already half-way decent) it would at best reduce the overall usage by a couple of percent.

I believe that many companies that don't use the LN2 for cooling have evaporation coils near the tank, and then pipe the gas itself around - though I believe the main reason is to avoid the cost and safety implications of piping the LN2 throughout the site. There is still N2 wastage when there is none used, unfortunately.

Dear Physicist, thanks for your feedback. I am going to try out closing the valve right by the LN2 cylinder whenever the ovens are not running. Our production people have isolated some ovens that we can turn one of the main valve to the ovens off. I think this will help. I will know the results in a week. I hope I can share the good news with you all.

We do have the evaporation coils near the tank but the gaseous N2 generated from there is being used in the drying cabinets only but not for cooling in the temperature cycling ovens.

Thanks very much. Jack Leong.