Ammrlers,
I requested info from people with experience with nitrogen gas packs and
nitrogen generators. Summaries and quotes below. We are probably going
to go with an oil-less compressor/N2 generator system sized to provide
gas when everything is at maximum flow. Not sure yet the exact size,
cost (expensive!), or location.
kas
1) Several people replied that they used the boil-off from a large
external tank, and were happy with that system. That is the system I
have now, and I am also very happy with it. However, our large tank is
going away due to concerns about costs and waste, so I have to find an
alternative.
2) Gas packs use the boil-off from special liquid nitrogen dewars to
produce the pressurized gas. The good news is that it is clean, dry
nitrogen. The bad news:
expensive- often several dewars are needed to provide a continuous supply
in addition, for best results the dewars have to be plumbed together
the supplier I talked to actually tried to talk me out of the system, he
said that all the current customers had problems or were unhappy
> I tried a gas pack in this lab, but I couldn't stabilize the
> pressures. I went through liquid nitrogen a lot faster than I would
> have thought.
> No, but we have done it temporarily during house N2 outages. Pro: no
> dryers, very low maintenance. Con: cost and delivery of LN2,
> floorspace to store, connect, and roll around gas packs, gas packs
> are magnetic iron, must construct a supply header and purchase gas
> packs. Like: gas packs require very little maintenance. Dislike:
> wrestling gas packs, adjusting pressure builder regulators. If you
> get different gas packs from a supplier each week, you will need to
> check the pressure builder regulator on each one every week. Setting
> the pressure builder regulator is a headache. If you keep and
> refill your own gas packs, then the pressure builder regulators will
> not need reset every week. If you construct a header to connect
> several gas packs at once, then they will all go empty at the same
> time unless you set the pressure builder regulators to a different
> delivery pressure on each gas pack. The header consists of a
> manifold with a check valve at each supply connection. The number of
> supply ports on the header depends on how many gas packs are
> available. The number of gas packs needed depends on the gas usage
> rate and your resupply interval. When I did it, I set the lowest one
> to 20psi above the spectrometer eject pressure, then each other pack
> was set to a different higher pressure, separated by 5psi. The
> header check valves were 1psi, so the highest pressure pack would
> empty first, and on down the line. Header output pressure was
> reduced at each spectrometer by its own local regulator to the proper
> pressure for that spectrometer.
3) Nitrogen generators separate out the nitrogen from the air. They can
be fed clean, dry compressed air, or the generator can include a
compressor as part of the unit. The systems do need power, so when the
power goes out, so does your gas. A backup of some kind (even
cylinders) is recommended.
> I use a nitrogen generator in one lab, where I have just one
> solutions spectrometer. It is great (Terranova is the vendor). For
> several instruments I would use the nitrogen generator or several, if
> you don't have enough flow for a solids instrument. It is much
> simpler to administer and has very little maintenance cost (higher
> front end cost). Remember with a nitrogen generator you throw away
> 20-30% of your gas flow and will see a pressure drop. To protect
> from pressure fluctuations, it may be necessary to have a holding
> tank (40Gal) in line.
> Pros - an almost unlimited supply of dry nitrogen gas. Cons - not
> always reliable because the gas line is connected to several labs and
> the buffer tank is exhausted very quickly if the generator stops;
> initial cost and ongoing maintenance of the generator and it's
> associated equipment (compressor, dryer, separator).
> a nitrogen membrane separation cartridge on an existing compressed
> air line (although this does not provide dry nitrogen). Pros - no
> maintenance, no ongoing costs, reliable. Cons - if you want dry
> nitrogen you need to buy a dryer too.
>
> This department has an air-fed Stirling Cryogenics StirLIN-1 liquid
> nitrogen generator from which we tap off part of the nitrogen gas
> before it reaches the cryogenerator and I also have a DIC Separel
> cartridge for separating nitrogen from compressed air.
>
> Membrane systems are
> nice and quiet, but I'm not sure they can provide enough N2 for solids.
> "Dual tower" systems can provide N2 on up to an industrial scale. We have
> a Parker DB-5 that's enough for 500, 600, and 800 (all liquids only) with
> some spare capacity, although to get ~90 psi out we're exceeding specs.
> The tower systems are quite noisy, though, and you'd want it in some sort
> of mechanical room and not the facility.
>
> The real problem is providing enough compressed air to drive the N2
> generator, either type. You need at least double the output flow and at
> least 20 psi over the output pressure, sustained.
> We killed a reciprocating compressor supposedly sized
> correctly, and an "adequate" scroll compressor overheats after ~20
> minutes. Here, we managed to kill 2 "industrial" scale recips - they never
> had a problem until we added our system in line, but they would never
> unload so one would run forever and never switch to another. A rotary
> screw compressor, sized appropriately, is what you'd need/want - they're
> designed to run under this sort of load, and the only downtime seems to be
> for routine maintenance.
4) Additional thoughts:
> Gas usage by the spectrometer is fairly constant if the sample never
> ejects or inserts, so if you take your eject/insert air from a
> different source, then your gas pack refills will be more consistent
> from week to week, and you'll save lots of N2. Eject air does not
> pass through the coil, so it doesn't have to be dry NMR air. Any
> garage compressor, followed by fluid separator, particulate filter,
> and oil vapor(carbon) filter cartridges will work well for eject
> air. To do this, you have to go inside each spectrometer to isolate
> the eject air to a separate supply.
--
Karen Ann Smith Director, NMR Facility
Group Leader, Analytical Chemistry Services
Department of Chemistry and Chemical Biology
Clark Hall MSC03 2060
1 University of New Mexico Albuquerque, NM 87131-0001
karenann_at_unm.edu 505.277.4031 http://nmr.unm.edu
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Received on Tue Jul 12 2011 - 09:53:15 MST