<div dir="ltr">All,<div> To add some information on liquid nitrogen boil-off / loss: </div><div><br></div><div>In 2007 the UC Berkeley Microlab did a survey of our nitrogen usage in order to confirm the accuracy of our metrology before moving to a recharge model for nitrogen use in labs not directly associated with the Microlab. From this analysis we discovered an 8% discrepancy between the amount of nitrogen we were consuming versus the amount of nitrogen we were being delivered. </div><div><br></div><div>This number is from a 9000 gallon tank, with purely gaseous use, that was being drawn on at the time at about 1800 L/m.</div><div><br></div><div>Danny Pestal</div><div><br></div><div>Facilities Manager</div><div>Marvell NanoLab</div><div>University of California, Berkeley</div><div>510-809-8600</div></div><div class="gmail_extra"><br><div class="gmail_quote">On Tue, Mar 31, 2015 at 6:26 AM, John Shott <span dir="ltr"><<a href="mailto:shott@stanford.edu" target="_blank">shott@stanford.edu</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div bgcolor="#FFFFFF" text="#000000">
Rick:<br>
<br>
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.<br>
<br>
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.<br>
<br>
Here is my understanding ... and I've tried to check these numbers
out with our LN2 supplier:<br>
<br>
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.<br>
<br>
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.<br>
<br>
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%.<br>
<br>
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.<br>
<br>
I trust that my colleagues will correct me if they believe that I've
badly misstated things here.<br>
<br>
Good luck,<br>
<br>
John<span class=""><br>
<br>
<br>
<div>On 3/27/2015 11:08 AM, Morrison,
Richard H., Jr. wrote:<br>
</div>
<blockquote type="cite"><span style="color:#1f497d">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?</span></blockquote>
<br>
</span></div>
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