What size and how many compressed gas cylinders do I need?

[d72hemi]

How do I figure out what size compressed gas cylinder, or multiple cylinders, is needed to flow dry Nitrogen (N2) at 100 PSI for 4 hours?

I have a project requiring me to flow dry N2 regulated at the cylinder at 100 PSI through a ¼" hose, through a cryogenic cooling system for 4 hours.  I have a 20 cf cylinder that can be pressurized to 2015 PSI,  but I do not have the knowledge to calculate how much time it will be able to supply a 100 psi flow.

Any help would be appreciated, Google has not been helpful with this problem.

Ian

[jar1292]

I guess it all depends on if the cooling system has a leak or exit for the pressure to release. if it is a 100% closed system then it would take a certain amount and then stay there till you manually release the pressure.  of course I dint know what cryogenic means i guess that maybe would tell me if it is a sealed system! as a welder with argon as a shielding gas tiging a weld at roughly 35-40 CFM (cubic feet minute)  it takes well over 4 hours to drain a a tank. but take it up to 100 CFM and it would not only ruin your weld but you would drain a 2000 psi tank in under 4 hours. i think i have never tried it. I guess it may come down to the fact of what will your regulator be, will it be PSI or CFM any gas supply store should be able to tell you what size you will need to keep a constant 100 PSI for 4 hours. just remember as if you have multiple tanks hooked into one exit the pressure stays the same in all as they cascade to the last one. price wise it would be cheaper to rent one big one versus five small ones for the same pressure. dint know if this helps... make sure you tell them about the 1/4 inch hose as it takes a lot less pressure to bring it up to 100 PSI than a 1 inch hose would just make sure your hose is rated to that pressure and the fittings are sound. a 1/4 hose with 100 PSI whipping around can leave a hell of a welt! if in questionable put a whip check at each end.

[d72hemi]

The N2 at 100 PSI is not going into a closed system, but is flushing a system into the ambient air. 

This cryogenic cooler will produce temperatures around -175°C and is used to cool the camera chip of a telescope. The instruction manual for the cooler states to flush the system with N2 at 100 psi regulated at the compressed gas cylinder. Think of it as blowing out contaminants from inside an A/C system. Disconnect a hose blow N2 into it at one end and watch it come out the other end after passing through the compressor and all other parts of the A/C system. 

I just need to figure out the size of the bottle I need to push 100 psi for 4 hours. Another way to think of it is just setting the regulator at the cylinder at 100 psi, and letting it flow for 4 hours. 

[2Gunz]

You are missing a piece of the puzzle to properly answer the question.

You need to know the flow rate.


I would say the easy way to do this is get a small bottle......  time it ........  then go from there.

Kinda a waste of money, but way cheaper then doing the math wrong, and frying the chip because you ran out of gas before you thought you would.

[d72hemi]

You are missing a piece of the puzzle to properly answer the question.

You need to know the flow rate.


I would say the easy way to do this is get a small bottle......  time it ........  then go from there.

Kinda a waste of money, but way cheaper then doing the math wrong, and frying the chip because you ran out of gas before you thought you would.

Do you know of a way to calculate/estimate flow rate?

I am not worried about damaging the chip, since it will be powered off. This N2 flush is intended to clear any contaminant out of the cooling system prior to recharging it. The estimate of required N2 is to ensure that everything is on site and ready to go during the daylight hours, minimizing potential downtime during the hours of darkness. I guess these are the types of issues you get when communication technicians are tasked with maintaining a telescope. It is a good thing I have experience working on automotive and housing A/C systems.

Ian

[67Charger440]

I understand what you are saying, but you still need to know the exit flow rate.  Put it this way.  I can keep 100 psi in a truck tire with a pin hole in it with a little compressor, but imagine the size it takes if you drilled a 1/4" hole.

here is a link.  

http://www.pipeflowcalculations.com/orifice/

Running rough numbers for you,  100 psi N₂ inside a systyem with a restricted outlet to maintain 100 psi and atmosphere outside yeilds the following

.001" orifice (bleeder hole) = .043 cfm = 10.3 ft³ total for 4 hours
.01" orifice = 1.68 cfm = 403.2 ft³
.1" orifice = 67.02 cfm = 16084 ft³

A large welders bottle of nitrogen (about 5 feet tall) holds about 200 ft³, limiting you to a .006" orifice to supply a sustained 100 psi for 4 hours.  It is the same as plugging the end of your hose and drilling a .006 hole in the plug.  Your regulator, though set at 100 psi, is trying to maintain 100 psi in the hose side of the system.  It will flow whatever volume it physically can to maintain that pressure.  It will burn .83cfm keeping up with your .006 inch  "leak" and maintaining 100 psi.

Yes, I am a mechanical engineer.

[67Charger440]

To address your situation a little more directly, your "instructions" seem to imply that the system will provide the restriction.  Yes, that is an unknown.  All the science in the world is useless if you cannot estimate the restriction inside the system.  The only way you can control it is to cap the outlet end of your once-closed-now-opened system with a restrictor of known size to control the max flow rate.  If you put a pressure gauge just before the restrictor it will read 0psi_g if the control restriction is bigger than the internal restriction.  If it starts reading pressure, the internal restriction is less that your control and you either have to accept it as restricted to the volume you can provide or go buy more N₂ and open it up to the next level of flow you can cover for 4 hours.  Repeat increases until the pre-restrictor cap reads roughly 0psi_g

[67Charger440]

If you are going to test it to determine flow, try on of these to get you close.  Direct the exit flow through a pipe and use this to measure the flow.

http://www.ebay.com/itm/Digital-Anemometer-Air-Flow-Wind-Speed-scale-Wave-Height-Meter-Beaufort-C-F-/300649159949?pt=LH_DefaultDomain_0&hash=item4600161d0d#ht_1189wt_1396

[d72hemi]

To address your situation a little more directly, your "instructions" seem to imply that the system will provide the restriction.  Yes, that is an unknown.  All the science in the world is useless if you cannot estimate the restriction inside the system.  The only way you can control it is to cap the outlet end of your once-closed-now-opened system with a restrictor of known size to control the max flow rate.  If you put a pressure gauge just before the restrictor it will read 0psi_g if the control restriction is bigger than the internal restriction.  If it starts reading pressure, the internal restriction is less that your control and you either have to accept it as restricted to the volume you can provide or go buy more N₂ and open it up to the next level of flow you can cover for 4 hours.  Repeat increases until the pre-restrictor cap reads roughly 0psi_g

It looks like missread the instructions, and did not provide you with all of the correct data.

I need to flush the system after evacuating it (.0001 torr), by using the regulator on the N2 cylinder to pressureize the system to 100 psi THEN opening the purge valve to vent the N2 for 4 hours. In this senerio, the opening in the redulator controls the flow of the N2 (mostly), since at that point the psi is not important. If this makes a differance in calculating/estimating the amount of N2 required I am all ears. However it looks like we will be performing a "guess and check" in a few hours.

Ian

[67Charger440]

That's a little different, but the fact remains the regulator will go to wide open until it meets the pressure you set it at.  Thus your flow through the system will only be limited by either the size of the purge valve opening, the internal restriction or if neither one of those is below the capabilities of the regulator flow, then the unrestricted flow of the regulator. 

If the point of the flush is to suck it down to near zero to rid it of any atmospheric content, then fill the void with 100 psig of N₂, the finally dump the N₂ but not allow air to re-enter by keeping N₂ flowing out of the system purge, which will in turn carry out the remaining miniscule trace of regular atmosphere...  Then you are home free.  You can use a flow meter like they use on argon bottles - they measure flow in cfm or cfh rather than the pressure like an air compressor or gas welding regulator.  Pick your flow (in CFM) and multiply it by 240 (minutes in 4 hours).  You will have your required quantity of N₂.

[Tilar]

I haven't read all of the replies so this may have already been covered, but A 1/4" fixed orifice will run 94 cfm of air at 100psi. Running it through a hose will have variables such as length of the hose, restrictions, air temperature and I'm sure there are more.