Hello AMMRLs,
Thanks for the great summary, Josh. I am spoiled here at NANUC as
I've not had many of these issues except for one. Our 800MHz system
requires the lower dewpoint air than all our other systems so a
special drier was needed for that. We went with a Balston PSA
(Pressure Swing Adsorbtion) Drier and we were witnessing a swing in
lock level when the drier would switch from one tank to the other.
We also believed it was a pressure fluctuation but was assured by the
supplier that that was not likely. We then read some literature ( my
apologies I cannot find it now ) that found that it was not pressure
that was causing the fluctuations but the adsorbtion of O2 gas as a
side reaction to the desired adsorbtion of H2O. There was an
introduction of a O2 concentration gradient as the drier swung from
one tower to the next. Because of O2's paramagnetic properties we
could see this fluctuation in the lock. The solution is the same as
the pressure solution, make sure there is plenty of ballast to mix
the gases evenly before it travels to the NMR.
Sincerely,
Deryck Webb
------------------------------------------------------------------------
------------
Deryck Webb - NMR Technologist
NANUC - Canada's National High Field NMR Centre
Room 103 Email: deryck_at_nanuc.ca
NANUC Bldg. Phone: (780) 707-0857
University of Alberta Fax: (780) 492-9174
Edmonton, Alberta Cell: (780) 707-0857
Canada T6G 2E1 AIM: derycknmr
http://www.nanuc.ca
On Nov 2, 2006, at 11:44 AM, Josh Kurutz wrote:
> Hi everybody,
>
> In responding to my query about compressed air systems, a number of
> you used the phrase "you touched a nerve," or something similar.
> Indeed, it appears that I touched the squid giant axon of NMR
> facility manager nerves by probing this subject!
>
> I'm not including the compendium of responses in this email because
> it is so large - over 23 pages of 12-pt text. This is so much text
> I fear it will appear as a result in all kinds of unrelated archive
> searches. (Don't laugh just because this "summary" is so verbose!
> The full set of unsummarized responses is even yet still larger.)
> Unless there's demand to include it in the AMMRL archive, I'll just
> post the full set of responses on the "Reports" page of our
> unofficial site (homepage.mac.com/jkurutz):
>
> http://homepage.mac.com/jkurutz/FileSharing18.html
>
> In this summary, I'll describe the general lessons I've learned
> since posting my query. then I'll describe my own situation, a
> little about how I ended up needing to make this decision, and how
> we've decided to proceed.
>
>
> ** THE BASICS:
> Your standard compressed air system consists of, in order:
> - One or more compressors that suck air from their surroundings and
> feed it to...
> - A big tank, also called a "ballast," "receiver," or "wet trap."
> - A device to remove oil, if the compressor is lubricated
> - Optional: A refrigeration-based dryer
> - A desiccant-based dual-tower air dryer
> - Optional: A nitrogen generator/separator
> - Recommended: A second big tank, aka "ballast," "dry receiver,"
> "dry trap."
> - Particle filters, maybe more oil condensers, maybe a color-
> indicating desiccant
>
> ** AIR COMPRESSORS in this context come in three main designs:
> piston (aka "reciprocating"), screw, and scroll (which spins one
> set of deep spiral-shaped grooves against another to move gas into
> the center). Scroll units rely on a teflon seal and require no oil
> in the air line. Piston and screw compressors come in "oil-free,"
> "splash lubricated," and "pressure-lubricated" flavors.
>
> Here are some key points to consider:
> - Oil-free units eliminate the risk of oil contamination in the air
> line.
> - Oil-free units generally run hotter than their lubricated
> counterparts because oil helps distribute heat away from moving parts.
> - Oil-free units must be replaced much more frequently than
> lubricated ones, probably because they run so hot.
> - Scroll compressors are considered the premium units because they
> are so quiet and contaminant-free. They are frequently the only
> option for facilities that must place the compressor in the same
> room as the spectrometer. However, they need to have their tip
> seals changed every 3 to 4 months, and the whole device wears out
> after just 2 to 5 years (even the salespeople admit to 5 years)!
> - Piston compressors are generally reliable, but they're really
> noisy. This doesn't seem to be a problem if there's sufficient
> ballast air space in the line and the compressors are off in a
> distant room. However, I didn't hear from anyone with a 600+ MHz
> system or anyone with a cold probe saying they observed no
> vibration from their piston system.
> - Piston compressors are generally not designed to run a 100% duty
> cycle. They need down time to cool off. I couldn't find any good
> data to show whether the on/off cycling of these compressors
> yielded problems in NMR spectra. The mechanical engineers say there
> should be no problems from acoustic noise in the line or compressor
> switching because you generate a supply of ~110 psi and regulate
> down to ~80, so fluctuations shouldn't be propagated downstream.
> Several AMMRL respondents told me that mechanical engineers didn't
> realize how sensitive NMR spectrometers are to pressure
> fluctuations. I think it all depends on the timescale on which the
> pressure regulators respond to pressure changes, and I've found no
> specs or non-handwaving arguments on this subject.
> - Screw compressors are much quieter than ones with pistons, but
> not quite as quiet as scroll units.
> - Oil-lubricated screw compressors have a reputation among some
> people as being even longer-lived than piston compressors
> - There is much on the web that indicates screw compressors are
> more "green" than pistons because they consume less energy. This
> may be marketing hype, but there is no prevailing counterargument
> claiming pistons are more green than screws.
> - Heat given off by the units was a big subject, but hard to pin
> down. Everyone with scroll compressors said they "run hot" compared
> to pistons, probably because of the difference in lubrication
> strategies. I got mixed messages when comparing screw compressors
> to the other types. The rep selling piston compressors said there
> was no spec available on how much heat his units gave off, but said
> they were cooler than screws. The rep selling screw compressors
> said they were about the same as pistons because you're compressing
> the same amount of air in the same time, ergo they should produce
> about as much heat; plus, he agreed to give me a spec on how much
> heat they should give off (15,000 BTU/HR for a 19 SCFM system prior
> to air dryer at 110 psi).
> - There are a couple of good websites that discuss various aspects
> of different compressor types, but they don't touch on the heat issue:
> http://en.wikipedia.org/wiki/Gas_compressor
> http://process-equipment.globalspec.com/Industrial-Directory/
> piston_compressor
> http://www.engineeringtoolbox.com/air-compressor-types-d_441.html
>
> - The NMR facility managers who appeared most satisfied with their
> systems (aside from those who use reliable house air or, best of
> all, dry N2 boiloff from a big outdoor liquid N2 tank) use dual
> pressure-lubricated screw compressors. They're very quiet, though
> not as supremely quiet as scroll units. Oil lubrication helps
> extend their lives to 20+ years of potential use. They need regular
> filter changes, but this is uncomplicated enough that university
> maintenance staff can be trained how to keep them going.
>
> ** DUAL/MULTIPLE COMPRESSORS
> Whatever the compressor type, it is important that the initial
> receiver tank be equipped with two or more compressors. It is very
> common for systems to be configured with "dual" compressors sitting
> atop one receiver tank, but this is normally done to reduce the
> duty cycle on each compressor. In our context, we need two
> compressors so we'll always have one working in case the other
> gives out. This must be explained explicitly to the sales reps
> because its not "normal" for them. You can also configure things so
> each compressor has its own tank, but the sales reps all said this
> was unnecessary.
>
> ** DRYERS
> - Virtually everyone who said something about their air dryers is
> using a "dual-tower" desiccant drying system. Here, compressed air
> passes over a bed of desiccant to achieve a low dew point while the
> other tower "purges," i.e. some compressed air is sent in reverse
> direction over the "wet" desiccant to dry it out. After awhile, a
> control panel will switch roles of the towers.
> - Several people have refrigerated dryers on their systems to take
> our most of the moisture before the air gets to the desiccant
> dryer. There doesn't seem to be a big advantage to this unless
> it'll help reduce the load on the desiccant dryer. However, it's
> standard equipment for many compressors, and the technology is
> quite reliable, so there's no big incentive to exclude a
> refrigerated dryer unit.
> - Many expressed concern that switching between towers induced a
> momentary pressure fluctuation that would affect data collection,
> so they put a big (30 to 200 gal) tank after the dryer to dampen
> them. This sounds like a very sensible idea.
> - Some units have control panels that monitor the dew point of the
> air produced to determine when to switch between towers. This is
> supposed to reduce its power need or something like that.
>
>
> ** NITROGEN GENERATORS
> We won't be getting a nitrogen generator with this system because
> we don't need to go that low in temp, but I'll include a discussion
> on them for the record. Many of you with Bruker systems described
> having a nitrogen "separator" that enriches N2 to about 98% of the
> compressed "air." This seems to work well for just about everybody,
> and few have reported problems with their units. Some said they had
> molecular seive-based generators, but these seem geared more for
> systems demanding less gas flow than most spectrometers
>
> ** SOLIDS NMR NEEDS
> The Varian Inova Installation Guide says that the dew point for a
> CP/MAS system needs to be an incredibly miniscule 80K (-193 deg C,
> -315 °F)! Yet many of you responded that you're doing lots of
> solids NMR with dew points of approximately -80 deg. C by using a
> nitrogen generator/membrane accessory. I'd like to hear more from
> people with solids setups and what your dew points really are on
> operation spectrometers.
>
> ** SEPARATION OF SUPPLIES FOR DIFFERENT LOADS
> A few labs reported that they split their loads:
> A) High-flow/low-pressure/very dry needs such as VT are hooked up
> to more costly N2
> B) Low-flow/high-pressure loads such as antivibration legs are
> hooked up to compressed air.
> This seems to make the most sense if you're running solids, which
> seems to require a very high flow rate of enriched N2 gas.
>
> **!! SPECIAL NOTE FOR COLD PROBES
> - I received one or two mentions from people hooking up their cold
> probe pneumatics to their high-pressure supply. This sounds OK, but
> it still leaves your cold probe vulnerable to failure or shutdown
> of the air system. If you're configuring a new cold probe to run
> with the common air system, I RECOMMEND YOU DON'T DO IT! We had big
> problems with our cold probe when our house air started
> occasionally dipping down to 55 psi.
> !!- To get around the problem, we hooked up our cold probe
> pneumatics to a cylinder of dry compressed air last year, and we've
> had ZERO problems despite having to shut down our compressed air
> system a number of times for various reasons. We've been configured
> like this for over half a year, and our cylinder is still over 2/3
> full.
>
> ** SIZING THE SYSTEM
> - Getting a properly-sized system is very important, and it seems
> to be trickier to figure out than one might think. The spectrometer
> manufacturers provide specifications for how much air is required
> for one spectrometer in a typical setup. In the U.S., the standard
> unit is SCFM, standard cubic feet per minute. What these specs
> don't tell you about is the extra load imposed by the air dryer
> and, if you have one, the nitrogen separator/generator. These can
> be huge! One facility reported their 10 SCFM compressor system lost
> 1-2 SCFM to the dryer, and another 5-6 to their N2 separator, and
> they ended up with insufficient air for their spectrometer, and
> they had to upsize their compressors!
> - One other tricky thing is that the specs you have for air use, in
> SCFM, may be given for different pressures. For instance, the
> Varian Inova Installation Guide says normal operations will demand
> 1.6 SCFM at 45 psi during sample eject. But if you have a CP/MAS
> unit, it will require 2.8 SCFM at 90 psi. The Kaeser compressor
> product literature has a table o' multipliers you can apply to your
> SCFM requirements at different pressures to gauge what the
> normalized flow should be at 100 psi.
>
> ** BRAND CONSIDERATIONS
> - Because this message is intended for peer discussion, not
> publication, per se, I'll frankly describe the following
> recommendations I garnered unscientifically from my review:
> - The happiest managers seem to have Kaeser-brand screw
> compressors. The parts are a bit more expensive in the U.S. because
> of the German origin, but they're apparently worth it.
> - One person said they've seen or worked with both Kaeser
> (pronounced "Kay-zer") and Ingersoll-Rand screw compressors and
> find that the I-R's aren't made quite as well as the Kaeser's.
> - Some people found I-R compressors to be fine for their purposes.
> - One said the Ing-Rand control panel was very touchy. It's very
> fancy and does great things when it's operational, but it's prone
> to total failure if exposed to power fluctuations. I haven't read
> the document, but this person reported the I-R warranty requires
> the control panel be on a power conditioner, else the warranty is
> void.
> - One person also said their I-R compressor had a problem allowing
> oil vapor into their lines, even with filters in the line.
> - Quincy was a compressor brand that came up when talking with our
> vendor. Though no one owning this brand responded to my AMMRL
> query, I found two instrumentation facilities happily using Quincy
> piston compressors. In each case, however, their compressors were
> on different floors than their magnet rooms. This looks like a
> great brand for this type of compressor.
> - Atlas-Copco got mixed reviews for their oil-free scroll
> compressors, but this may be due to the intrinsically low life of
> this type of compression mechanism.
> - One person reported they had Sullair compressors and had problems
> with them.
> - Two people reported having Jun-Air systems, and they like them OK.
>
> - Vast numbers of respondents love their Balston or Parker/Baltson
> air dryers. This appears to be the best default choice in dryers.
> Kaeser systems come with kaeser dryers, and they seem to work fine,
> too.
> - Most everyone loves their Hankinson air dryers. I've had some
> trouble w/ ours, but it's 12+ years old, and it may have been
> fouled by our bad house air. I wouldn't mind having another one.
> - The name Wilkerson came up a couple of times with respect to
> dryers and filters, and it seemed their products were OK.
>
> - People seemed happy with Parker N2 generators
>
> ** OUR SYTEM & FUTURE PLANNING
> We have three Varian Inovas (2x600, 500), one 600 with a cold probe
> and carousel sample changer. One 600 needs 1.5 LPM air to cool its
> shims. All are equipped with pneumatic antivibration legs and FTS
> VT systems that heat and refrigerate the VT air. Our cold probe's
> pneumatics are on a cylinder of high-grade compressed air. We do
> biomolecular work, but a fair amount of it is in the -5 to 25
> degree C range, so we need to use the FTS chillers frequently. We
> had been in the habit of leaving the FTS units on all the time to
> achieve stable 25 °C sample temperatures, but we're reluctant to do
> that now. The spec dew point for these chillers is -60F. There is
> faculty interest in adding solids capability to the 500, which
> would dramatically increase our compressed air needs. Getting an
> extra-capacity system now won't cost much more than one that just
> meets our current needs, so we're trying to pretend that we've got
> a solids system.
>
> Adding up all of our needs yields a total of between 7 and 8 SCFM
> with the solids system and two samples being ejected. We're
> shooting for a compressor system capable of 18-21 SCFM that can
> deliver ~15 SCFM with a dryer, but no N2 generator. We'd like to
> hit that dew point of -100F, but won't complain if we get -70.
>
> ** OUR SOLUTION
> We've decided to go with a Kaeser system of oil-lubricated dual-
> screw compressors (SX-6, SCB-5) feeding a 60 gallon wet receiver,
> which then feeds a Kaeser desiccant dryer with a 60 gallon dry
> receiver. The pair of 5 HP compressors should deliver 21 CFM _at_ 110
> psi, but that should get cut to ~13 CFM of -100 deg F dew point air
> after the Kaeser dryer (KADW-20). If you detect flaws with this
> setup, especially if you have experience comparing the heat output
> of screw and piston compressors, please let me know now before we
> take delivery.
>
> ** COST
> Price is always a touchy subject, but I thought I'd give you the
> ballpark figures (+/- 10-20%) so you know what order of magnitude
> we're dealing with. Of course, your system's costs would depend on
> your sizing, dew point needs, and your vendor's whim, so these
> figures cannot be used for the precise computations on which sales
> people depend. Each of these compressors is approximately $5K, the
> dryer is approximately $1.5K, and with all the tanks, filters,
> gauges, etc., the whole thing will run approximately $15K without
> installation. We received a quote on a Quincy system that also
> looked very good and was approximately $1500-$2K cheaper, but we
> ended up being turned off by the noise and potential vibration
> issues with the pistons.
>
> ** OUR SAD STORY
> Last year, our cold probe started having intermittent "low cooling
> power" faults. It turned out - after spending long periods of time
> watching pressure gauges - the pressures of the compressed air,
> which feeds the cryogenic pneumatic valves, was fluctuating.
> Sometimes it dropped much lower than the regulated pressure (down
> to ~55 psi), shutting some of these valves for a few seconds, thus
> leading to faults observed on the cryobay window. After more gauge-
> staring, we determined the difficulty lay in our aged house air
> compressor. We got around the problem by hooking up the cryo system
> to a gas cylinder, whose gas we filter and regulate down to 80 psi.
> This arrangement has been great.
>
> Early this year, I walked in to the spectrometer room and found all
> three spectrometers' temperatures were high but unregulated and
> fluctuating a little. Near the magnet with the cold probe, I
> smelled charred plastic. Turned out that all three FTS chillers had
> frozen shut, depriving VT air to all the spectrometers. Those of
> you familiar with Inovas will know that they're configured in a way
> that VT air loss leads to catastrophic overloading of the heater in
> the probe. On a cold probe, this melts the expen$ive plastic VT/
> tuning knob assembly.
>
> So our house air was failing us both by giving inconsistent
> pressure, but also by sometimes being so wet that water would
> condense and ice up in the VT air line. After talking with the
> administration, we were given approval to get our own air system.
> That's when we started shopping and I began talking to so many of you.
>
>
> **!! THANKS !!**
>
> Thanks so much to all of you who responded, including Andrew
> Fowler, Andy Staley, Andy King, Andy Soper, (alphabetical order,
> you see), Carlos Amezcua, Craig Butts, Dave Gindelberger, David
> Vander Velde, Deane McIntyre, Elwood Brooks, Eric Paulson, Dr.
> Sukenick, Herve Bizot, Jeff Ellena, Jerry Hirshinger, John Witte,
> J. Kotesh Kumar, Ken Fishbein, Ken Osbore, Klaas Hellinga, Mark
> Edgar, Martha Morton, Rich Shoemaker, Robert Honeychuck, Robin
> Kinnel, Sara Kunz, Tom Stringfellow, Walt Niemczura, Bill Kearney,
> Wing-kong Kwan, and Xavier Lemercinier.
>
> It's been most educational. I hope this compiled result helps those
> of you making similar decisions now, and those who are reading this
> as an archive entry in the future.
>
> - Josh
>
>
>
>
> Josh Kurutz, Ph.D.
> Technical Director, Biomolecular NMR Facility
> University of Chicago
> Gordon Center for Integrative Science, room W123C
> 929 E. 57th St.
> Chicago, IL 60637
> Office: (773) 834-9805
> Spectrometer Room: (773) 702-4052
> Cell: (773) 315-5732
> Fax: (208) 978-2599
> nmr.bsd.uchicago.edu
> homepage.mac.com/jkurutz
>
>
>
>
>
Received on Mon Nov 06 2006 - 17:49:25 MST