[labnetwork] LN2 vs N2 generation on site

[John Shott]

Rick:

In an earlier message, you had commented about the large volume 
difference between what you use and what gets delivered.  To me, 
something seems strange if you are getting 2.6M cuft per month but are 
using only a bit more than half of that.

I believe that large LN2 tanks are noticeably more efficient than that 
for gaseous usage and would expect that the amount that you use to be at 
least 90% of what is delivered.

Here is my understanding ... and I've tried to check these numbers out 
with our LN2 supplier:

A big tank that is just sitting there (that is with little or no usage) 
would lose something like 0.5-1.0% per day due to heat transfer and gas 
that ends up getting vented but not used.  However, for any system that 
is using a significant volume of product per day ... and yours clearly 
is ... I would expect that venting losses would be very close to zero.  
When you are using a significant volume of product, that would tend to 
reduce the pressure in the tank, the pressure building circuit kicks in 
... but that doesn't result in any loss because the liquid that is 
vaporized to build pressure goes into your tank.

There is certainly a significant loss anytime that you have a delivery 
... and, if your vendor is like ours, you pay for all of that nitrogen.  
I believe that the largest fraction of that consumption is the nitrogen 
that is "depressurized" from the tanker following your fill to reduce 
the tanker pressure from above your tank pressure down to the legal 
limit for them to drive again on the highways which is, I think, 10 
PSIG.  The nitrogen consumed during your fill that never goes into your 
tank including pre- and post-fill purges and this tanker "blow down" is 
significant.  Others may have a better number than I, but I count on 
losing at least 20,000 cuft of gas per delivery.  On average, we get 8 
deliveries per month of 600,000 cuft per delivery for a total delivered 
volume of about 4.8M cuft.  So, losing 20,000 cuft per delivery is only 
a 3% loss.  Of course, with a smaller tank, the per-fill loss percentage 
goes up assuming that they are still filling with a 53' trailer.

Note: you will have higher losses if you are using liquid withdrawal to 
any degree because there is a fair volume of nitrogen that is "lost" as 
gaseous nitrogen for every liquid gallon that you extract.  If your tank 
pressure is set up for gaseous usage with a tank pressure of 120-150 
PSIG, then any liquid usage is probably not terribly efficient because 
of flash losses, etc.  ... basically the gas that is vaporized in the 
process of getting things cooled down so that you get good liquid 
extraction.  I don't have much experience in that area so can't really 
offer a precise number for loss during liquid withdrawal but that is 
likely to be significantly lower than the 90% number that I think you 
should expect for gaseous withdrawal.  However, if your system is 
predominantly used as a source of gaseous nitrogen, I would expect that 
your overall efficiency should be closer to 90%.

How well are your flow meters calibrated?  Do you have flow meters on 
all possible usage?  Are your flow meters fully temperature compensated 
or are you measuring gas that is noticeably colder than room 
temperature? Anecdotally, we just sent our two main flow meters out for 
calibration:  when they came back, they are now giving readings that 
indicate that we are consuming more nitrogen than our vendor is 
supplying.  In short, I'm not sure that I trust the absolute readings 
that I get off these big flow meters for an absolute reading.  In short, 
if your tank is used predominantly for gaseous nitrogen, I would expect 
that your overall tank efficiency should be closer to 90% and would look 
carefully at whether I was monitoring all usage and whether my usage 
flow meters were giving me believable results.  If your liquid 
withdrawal and usage are significant everything that I have said may be 
off base because I expect that liquid withdrawal is an inherently lower 
efficiency process.

I trust that my colleagues will correct me if they believe that I've 
badly misstated things here.

Good luck,

John


On 3/27/2015 11:08 AM, Morrison, Richard H., Jr. wrote:
> Thanks for your comments so far and I looked forward to more 
> discussion. BTW after fees and surcharges I pay $1.23 per 100cuft and 
> I use 1.45million cuft per month (data from flow meters). The company 
> delivers the equivalent of 2.6million cuft so I lose to evaporation 
> and cooling 1million cuft per month, does this sound right or do I 
> have a big leak some place?

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