Liquid Nitrogen store ventilation

[ad77]

what is a typical ventilation arrangement for a store room housing liquid nitrogen vessels (small vessels associated with a laboratory)?

any assiatnce will be appreciated.

 

[imok2]

adzz, Here is a good source of information:

http://www2.umdnj.edu/eohssweb/aiha/technical/ventilation.htm

 

[quark]

First thing is to check for existence of local codes. Nitrogen asphyxiation is very dangerous and it causes fatality, sometimes, without indication.

You should check with Dewar manufacturers about liquid N₂evaporation rates. 1 liter of liquid N₂ evaporates into 24.6 ft³ of gas.

Generally. oxygen level should not be less than 18%. If I consider a typical evaporation rate of 0.095l/day/dewar then N₂gas released into the room will be 0.095*700 = 66.5 liters or 2.35 cu.ft/day. Suppose if your room size is 1000 cubic feet then oxygen content should be 209.5 cu.ft and nitrogen will be 790.5 cu.ft. But, due to asphyxiation N₂ increases to 790.5+2.35 = 792.5 cu.ft and O₂ will be 207.5 cu.ft or 20.75%. There is a reduction of 0.2% approximately. This type of calculation can be used to get required no. of fresh air changes in the absence of codes.

Oxygen monitor is a must IMHO. The following links will be useful to you.

http://www.gla.ac.uk/seps/01-024.html

http://www.onc.ed.ac.uk/liquidnitrogen.htm

Regards,

 

[ad77]

thank you imok & quark - as always, information that is relevant.

 

[lilliput1]

ad77, imok & quark,
I have made ventilation calculations using excel spreadsheet but it should be verified. The Liquid N2 freezer manufacturer rep never got back to me to let me know the evaporation rate. Also I am not sure if my procedure - calculating the critical concentration is correct. If you guys can give me your email address I can email you the spreadsheet & you can let me know what you think.
lilliput1

 

[imok2]

lilliput1, I am not knowledgeable in this by any means, however if you are looking for evaporation rates for various size cylinders, I suggest you take a look here and peruse the site you may find it to your liking.

http://www.taylor-wharton.com/Pages/Liquid Cylinders/XL 60-240.htm

 

[CRG]

As Quark stated, “Oxygen monitor is a must IMHO.” I don’t know too much about the construction of a small Dewar; however, many large cryogenic storage vessels rely on maintaining a vacuum in the interstitial space. If there is a vacuum leak the evaporation rate will be much higher. Also, a oxygen sensor could monitor the area in case of a spill. One source that will have a lot of safety standards for this is the CGA (Compressed Gas Association.)

 

[quark]

Lilliput,

If the transfer of liquid nitrogen is automatic by level control or temperature control inside the freezer then you have to only deal with evaporation rates resulting from temperature difference. But our application(storage of master cell banks) includes opening of cryocan lids. I never did the detailed calculation for evaporation when the lid is open because this includes evaporation of liquid nitrogen and superheating of gaseous nitrogen. We only take care of evaporation when the lid is closed and don't have much problems. O₂ is in control.

I want to have a look at your spreadsheet. My id is builblock1@yahoo.com

PS: OSHA recommends oxygen level of 19.5%

Regards,