Liquid Nitrogen

[Fahad3608]

Greetings,

we have two storage tanks of liquid nitrogen as an emergency back-up in our plant, and it was written in the vendor manual the storage capacity can supply the required nitrogen in a gaseous form for 8 hours with flow rate of 9000 SCFM and 24 hours with flow rate of 1300 SCFM. however, if you try to calculate it reversibly you will find different capacity values.

tank capacity= 53 m3 (we have two).

- for sure the volume of liquid nitrogen is different than the nitrogen gas but it shouldn’t be different when you calculate it back , and i have used the expansion factor 694 m3 of gas for each 1 m3 of liquid nitrogen ? is this expansion factor right ?
- what is the conversion factor to convert from m3 to SCF of liquid nitrogen?

Regards,

 

[Caloooomi]

What pressure are the tanks kept at?

What conditions are considered for "S"? 70 deg F, 1 atm / 0 deg C, 1 atm / 15 deg C, 1 atm, or something else?

expansion factor would be dependent on what temperature / pressure you are expanding to?

9000 SCFM is pretty big, i'm assuming it's being pumped?

 

[Fahad3608]

What pressure are the tanks kept at? 165 psig

What conditions are considered for "S"? 70 deg F, 1 atm / 0 deg C, 1 atm / 15 deg C, 1 atm, or something else? i don't know the vendor calculation basis

expansion factor would be dependent on what temperature / pressure you are expanding to? [highlight #FCAF3E]113 F ( ambiant) and 160 psig.[/highlight]

9000 SCFM is pretty big, i'm assuming it's being pumped? its downstream the ambian vaporizers

 

[LittleInch]

It would appear from simple scf calcualtions that the tanks are sized for the worst case event (9,000 for 8 hours). How the second flowrate has been determined is not clear, but given that one volume is half the other, maybe the lower flow is base don only having one tank available?

Who knows? We don't. All depends on what the specification called for and then the vendor has taken the worst case scenario to size the tanks.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Fahad3608]

Could you please share the calculations you made,

Thanks,

 

[LittleInch]

Sorry,

I got confused by the change in units. It actually works out quite well when you use 9000 sm3H...

Using the right units and your ~ 700:1 gas/liquid ratio, I agree your figures don't work.

however on a SCF basis, scenario A = 4.32 MMscf, scenario B = 1.87 MMscf

Turning this into m3 liquid This handy little site

http://www.airproducts.com/products...s/weight-and-volume-equivalents/nitrogen.aspx

gives 1.3MMscf for 53,000 litres of liquid N2, so 2.6MMscf for 106 m3 liquid N2. so scenario B works, but scenario A doesn't

Something's not right, but what it is I can't tell from here.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Fahad3608]

Thanks a lot for your explanation, but I think as you mentioned there is something wrong in their basis or the flowrate units.

 

[bimr]

Scenario A has capacity for 4.6 hrs at 9,000 scfm.

Scenario B has capacity for 33.4 hrs at 1,300 scfm.

 

[racookpe1978]

What temperature change (if any) are they making assumptions against?

Until ALL of the liquid has evaporated into gas, the 1:700 liquid-to-gas volume ratio is going to dominate, right?

 

[Fahad3608]

-as i mentioned, i am just like you , I am wondering about the basis they used and written in their manual.

-yes, right

thanks

 

[georgeverghese]

The thermal design datasheet for the vaporisers may give some clues to what the rates should be.

 

[Fahad3608]

yes, it is written " the flow rate is 1300 SCFM for 24 hours and 9000 SCFM for 8h/day"

 

[georgeverghese]

But what does the TEMA sizing datasheet day on flows and duty - do the flow and duty match up?

 

[Fahad3608]

It is ambient vaporizers, and its sized for Max. 9000 SCFM/8hrs and Min. 1300 SCFM/24hrs

 

[Caloooomi]

Were the units sized for the same approach temperature for the 8 hours and the 24 hours?
You are running the 24 hour units at less than a quarter of the 8 hour flowrate which is quite the derating - I'm assuming it is quite a high fin density on the units?

Do you have datasheets for the ambient vaporisers? Could you not just contact the company directly?

 

[Fahad3608]

I have the data sheet, but there is nothing crucial in it except what I have mentioned.
Yes, I have contacted the company and I did not receive any feedback yet

 

[Caloooomi]

do you have the surface area of the ambient vaporisers, model name?

 

[georgeverghese]

So the 9000scfm rate represents the max flow, while the 1300scfm rate is the turndown rate used for the design of these vaporisers.

So these rates need not both match up with the emergency LN2 tank inventory. One of these may while the other need not, or maybe neither do.

 

[georgeverghese]

The 8hour duration for the 9000scfm rate and the 24hr duration for the 1300scfm rate may be related to some emergency operating case in your plant and may not necessarily be translated to derive or match up with LN2 tank volume.