Nitrogen Gas Absorbtion Into Air

I read the pop-up before clicking and my immediate thought was: How many people will respond before pointing that out?

I know approximately 78% by volume but the oxygen depletion equipment we use goes off at 18.8% oxygen. I want to know the dispersal/absorbtion as I am looking at how this interacts with the detection equipment in use.

It would depend upon the volumes of the two aread and the ventilation arrangements. It is too complex to approach theoretically, and experimentation would seem to be more fun.

When you inject nitrogen into your cabinets, one of two things will occur:

1. Your cabinets are sealed and you create internal pressure. This causes a dilution of the "air". The dilution is somewhat immediate and the ratio of nitrogen in this new mixture is based on how much nitrogen you injected.

2. Your cabinets allow for air to leave as nitrogen is introduced. This is called inerting, and can also be referred to as displacing. If the cabinets are then sealed, no "air" will exist, only nitrogen (if properly done).

You are kidding me, aren't you?!?

A more effective design is to have a small, but positive N2 flow though the cabinet. This is a well known process in the petrochem industries where a N2 'blanket' is introduced above the level of flammable liquids in closed tanks.

Nitrogen is cheap enough...you will need a decent regulator at the source cylinder, set to marginally above atmospheric pressure (approx 1psi/ 6kpa) should do it. Pipe the N2 via a suitable rotameter, placed so it is easily visible for monitoring, and drill a very small bleed hole (< 2mm) somewhere in the cabinet for the gas flow to exit.

Where you see no flow in the rotameter after commissioning, your bleed hole is too large and you will have to increase the N2 pressure accordingly...or the bleed hole is too small and will need to be drilled slightly larger. You only want to see a minimal N2 flow in the rotameter. You also need to make only fine adjustments to the regulator, otherwise you could blow off the cabinet doors! Problem solved...

All very sensible, except "Where you see no flow in the rotameter after commissioning, your bleed hole is too large ..." - can you talk that bit through, please?

Sure....where the regulator is set close to atmospheric pressure as suggested, and where the size of the bleed hole is too large, the pressure drop across the bleed hole may render the suggested diff.pressure too low for an effective positive nitrogen flow across the bleed hole area, and the N2 regulator discharge pressure will have to be increased to compensate, resulting in consumption of a much increased volume of nitrogen to keep the cabinet pressurised above atmospheric. ie if the bleed hole was drilled at 12mm or 1/2" in a cabinet with a void of say 1cubic metre, N2 consumption may be as high as one cylinder per 24hr cycle (the N2 consumption can be calculated exactly where the void and the pressure drop are known, ergo, the regulator pressure and bleed hole diameter can be calculated to suit a desired consumption of N2.). To go from nil N2 flow to a substantial flow requiring perhaps regular replenishment of N2 cylinders, may not be a very attractive solution, so I suggested a minimal bleed .....

... but why would the rotameter not indicate any flow?

Errmmm.........if only I had bothered to actually do the calcs, instead of visualising them while I typed...why didn't he pose the question directly in the first place ? Oh well led indeed! Perhaps he's in the corporate legal environment ? Did I type this crap out loud?

Quite correct JohnDG, the rotameter will indicate flow when the bleed hole is large...my mistake.

A greater definition of the objective is needed: what O2 residual can be tolerated? 1 ppm? 1 ppb? What material is used for the cabinet seals (BUNA rubber is highly "porous" to oxygen - oxygen diffuses through quickly.). IF you assume a given overall composite pore size, a known value of nitrogen purity and flow rate, etc.(not sure what the etc. is right at this moment), then you can calculate the final steady state O2 residual from Graham's Law of Diffusion of dissimilar gases. Note that even with a positive outflow of nitrogen (at X purity), there will still be an influx of oxygen that must obey the laws of physics. You will need a solid-state O2 sensor/transmitter if you really want to know how well you are doing.

I think that I had better clarify things as we appear to be going slightly off-track, although I am finding the comments very interesting.

I am not worried about the atmosphere inside the cabinets, that will be 99% nitrogen which is what is required.

What I want to know regards the opening of a cabinet door and the dispersal of the nitrogen into the room. If I have an oxygen depletion monitor in front of the door the alarm will go off after 30 seconds, while this is correct is there a formula that can be used to calculate the dispersal of the nitrogen into the air and give a time response?

Thanks

I think your question is unanswerable. If the cabinet door is very small compared with the cabinet size, then it will take longer to lose your N2. If the door is the same size as the wall, then it will lose N2 very quickly. Pretty obvious really. If there is air disturbance in the cabinet vicinity, then this will also affect it. What else can we tell you?

1. "we appear to be going slightly off-track"; 2. "will be 99% nitrogen which is what is required"; 3. "calculate the dispersal of the nitrogen into the air"

1. Generally, the need to exclude oxygen is to mitigate potential oxidisation/ corrosion and flammability/ explosion. The installation of cryogenic equipment has not been mentioned, and the OP 's posting stated only minimal info.
2. According to the OP, there no change in the actual mol weight of O2 in the cabinet, only a lesser % by volume owing to the introduction of pressurised N2.
3. Only relevant when considering N2 release as creating an asphyxiant atmosphere in the machinery space. No statement was made, but all manned machinery spaces must be suitably ventilated, and in-house rules should prevent manned access to a machinery cabinet until it is also ventilated/ purged with air.

Maybe what you will need is a cabinet air lock, depending on the size of the object(s) to be removed per instance, and a manipulator arm that allows you to work inside the cabinet without opening anything but the air lock.