Impact of compressed nitrogen entering a room 4

[Coolman8]

I would like to know what will be the temperature and relative humidity of a room when compressed nitrogen (72CMH) at an absolute pressure of 3 bar, 28 deg & -50deg C dew point entering it at atmospheric pressure.

Any help would be greatly appreciated.

 

[dcasto]

sealed room, no vents?

 

[Coolman8]

There will be a leakage of 17l/s of air from the room. The nitrogen is used for fire suppression in the archive room. As this room condition is controlled between 16°C to 20ºC and RH between 45% to 60%.

I am wondering what will be the impact on the room condition when it enter the space. I suspect the low dew point by the nitrogen might cause a drastic drop in the temperature and RH of the room which may damage the collections inside but I am unsure on how to proof it.

Any advice on the calculations or information which may be useful for the calculations will be appreciated.

Thanks.

 

[JLSeagull]

Others can address temperature and dew point impact. However, the use of a toxic gas like nitrogen as a fire suppression agent seems unusual and dangerous. Assure that the room has oxygen deficiency detection as well as visual and audible alarms for low oxygen in addition to the NFPA fire suppression alarms, etc. Consider FM-200 or human-rated clean fire suppression agents.

 

[Coolman8]

There will be oxygen sensors in the space to ensure that the oxygen level is maintain at a volume of 15%. No oxygen illness on human is reported at this concentration moreover the rooms are used for archives. Any damages to these precious collections are the last thing we want to see.

Thanks a lot for the concern but I would appreciate if the reply is target only at the problem which is the room condition (temperature and RH) when the compressed gas enters the space.

Thanks in advance.

 

[dcasto]

The temperature drop on the N2 will be less than 1 degree C, going from 3 bara to 1 bara. The room would warm up if anything. The hummidity would drop.

You could do a simple ideal mixing of the room volume of air and N2 to see about what the composition, temperature, and humidity. Wosrt case would be the N2 pushing all the air out and not mixing. The fun part would be to figuire out the heat capacity of all the materials in the room and what the finale temperature would be, I suspect a lot cooler than the 28 C because the papers ans shelving have a significant mass compared to the N2

 

[Coolman8]

Hi dcasto

I must confess that I am not too sure about the calculations process. Could you help me with it? If we are using a property relation between the states:

P1/T1 = P2/T2
3/(273+28) = 1/(273+X)

Nitrogen entering the room will be -173°C. Is this right?

I don’t have a clue to the mixing of the room volume of air and N2, could you show me the calculations in achieving the final temperature and humidity?

Thanks in advance.

 

[jistre]

Hmm.

The gas is also expanding, Coolman8. I think the equation you used describes what happens in a closed volume as you modulate the energy level of the gas and the only way energy can affect the gas is by modulating the temperature and pressure. However, the gas we're talking about expands into the room and makes that equation invalid.

Unfortunately, I don't remember the exact equation off the top of my head, and my thermodamnamics book is at home. Just be aware that your temperature is going to be off if you use that equation.

 

[Coolman8]

Hi jistre

I believe that you are right to say that there is something wrong with the calculation. Could you please guide me to the right equation? To be honest, I am not very good in subjects relating to mixtures of gases and that is why I am having problems here.

Could anyone give me more pointers? Any help will be appreciated. Thanks.

 

[jistre]

We're both in the same boat here, buddy. Thermo's not something I use a whole lot anymore, so while I could tell that your equation was missing out on the expansion of the gas and warn you of that pitfall, I really can't steer you in the right direction without poring through my old thermo books. There's probably someone else around here much more experienced and qualified concerning compressed gases that can point you in the right direction.

 

[KiwiMace]

You have a two part solution, but there are more parts to your problem.

1. The adiabatic expansion of an ideal gas will give you the temperature of the expanded N2. 3 bar is bugger all. So dcasto's estimate is probably right.

2. You can consider the Nitrogen as dry air and use a psychrometric chart to determine the state of the mixed air. You can probably assume the Rh=0 to start, unless you are very worried about accuracy, where you can use the dewpoint, or a sharper pencil on your psych chart.

Plot your starting room air state point (temp and RH), and then your Nitrogen (temp as determined, RH=0).

Draw a straight line between the two points. Your final room state will lie on this line, and be determined by the volumetric ratio of each at the end condition.

I would assume first that the nitrogen injected into your space displaces the air within the space, rather than some funky blend and leak scenario. Backcheck that your mixed condition doesn't violate the constraint placed by the 15% O2 minimum.

PS I'm pretty sure that N2 isn't all that toxic. Hard to breathe maybe when thats all you've got. At 3bar, you are going to need another room 1/3 the size to put your cylinders into. This lacks a sense of realness, suspect even.

Most conservators will be super-happy that their artifacts are not on fire or soaking wet, and will gladly suffer a few minutes of dry air. There is a reasonable time constant associated with mass transfer that will prevent paper and the like changing moisture content over a short period. Interesting to understand that the moisture content varies in proportion to the RH not the absolute moisture (humidity ratio).

 

[dcasto]

I ran an EOS on N2 to get the temperature drop (delta H =0).
N2 at -50C dew point has nearly 0% RH. so simple mixing as KiwiMace states.

N2 is not poisionous, but Our regulations will not allow anyone in a room with less than 18% O2 (I'm not in my office, so thats approximate)

 

[Coolman8]

The 15% concentrate of oxygen is what I am concern about because I don’t think I can do a simple calculation to determine the conditions taking that into the account and I am not capable of solving complex mathematic problems as well. We are actually getting an expert to analysis the given condition using software and the manual calculation is for me to have a rough idea what the room conditions will be like.

Thanks guy ( dcasto, jistre & KiwiMace) for all the help.

 

[mbt22]

Nitrogen may not be toxic, but oxygen deficient atmospheres are deadly. I would be very scared of this system. I would absolutely not go into a room with this sort of system, unless it was deactivated and physically disconnected under a strictly controlled permit system.

11% - 16% oxygen causes "dimunition of physical and intellectual performance without the person being aware".

Is there an on-site BA-equipped emergency response team to rescue the person who has become confused due to low oxygen and hasn't escaped? A strictly enforced buddy system to raise the alarm from outside the protected area?

How rapidly does the gas mix? Are you going to get local areas of much lower oxygen concentration, remembering a couple of breaths of nitrogen are enough to render you unconscious.

Anything below 19.5% oxygen is considered hazardous. Below 11%, even for a very short time there is a risk of death.

References:

http://www.boc.ebcnet.co.uk/p/controllingRisk-inertGases.pdf

http://www.cganet.com/pubs/free/SA-17_1.pdf

http://www.cdc.gov/niosh/docs/2005-100/chapter5.html

http://www.chemsafety.gov/index.cfm?folder=recommendations&page=details&ReportID=31&show=all

There are lots of companies that specialise in fire suppression. I would consult them - simply flooding the room with nitrogen gives you an excellent chance of brain damaging or killing someone.

 

[stookeyfpe]

Sorry for the late post, but if this design is for installation in the US, it is in violation of NFPA 2001 for clean agent fire protection systems and the 2006 International Fire Code. No US code official will approve what is being proposed.

 

[jistre]

Is that strictly because nitrogen displaces oxygen in the air, and are other oxygen displacers banned as well?

 

[mbt22]

NFPA 2001 is available online without charge. It's not my field (petrochemicals, hence the nervousness about inert atmospheres), so I haven't read it.

http://www.nfpa.org/aboutthecodes/AboutTheCodes.asp?DocNum=2001&cookie_test=1

 

[Coolman8]

Thanks for all the concerns regarding the safety issue of using such a system for fire suppression.

This is an interesting link I came across while trying to find out more about this form of system.

http://datacenterjournal.com/index.php?option=com_content&task=view&id=1444&Itemid=41

Perhaps those who are interested in pursuing the danger of such an application could start a new thread and start arguing over it. Nothing lose but plenty to gain.

 

[itdepends]

There are plenty of fire suppression systems that use oxygen deprivation to put out fires- your humble CO2 extinguisher is one of them.

Such systems typically have self closing or automatic doors and provide a warning before activation to allow the occupants to exit the building. By their very nature they will drop the oxygen content in the room- otherwise they won't work.

 

[zerosum]

And CO2 is not as dangerous as N2.