Dry Air Responses

The question

We are purchasing a new CP-MAS probe and need a source of dry air. Our house air (80-90 psi) is sometimes wet or oily and we need to clean it up. A company called Kaeser sells a Desiccant Dryer KADW-15 Wall Mount (15 SCFM, up to 150 PSI, -40 C dew point) with two desiccant columns and filters for about $1600. We have a similar one for FTIR and it works good (except for the occasional pressure release which one kind of gets used to.) http://us.kaeser.com/Products_and_Solutions/Compressed-air-treatment/Drying/Heatless-desiccant-dryers/default.asp#0 I would be interested to know if there are any experiences out there with this unit or with alternative solutions. If there are sufficient responses, I will share with the group.

Here are a few excerpts from the responses to the questions raised. I have deleted the sources.

Well ! We use oil free compressor which does not need oil removing filters. Also we have conected Nitrogen Generator to this compressed air, hence first moisture is removed and then oxygen is also removed. This helps two ways. One can do low temperature experiment too, and for high temperature experiments there are less chances of oxydation of working decoupler / observe coil. Though it is expensive solution to the problem of moisture but long lasting.

I recommend removing oil and liquid water before the gas reaches the dryer. Use a consolidating filter; e.g. Balston. Depending on how much oil and water collects, you can use an automatic or manual drain. If the filter is out of sight, get an auto drain.
Pressure transients from the dryer switching will show up in your MAS rate unless you install a generous ballast tank between the dryer and the MAS controller. I don't know how much volume you'll need, but I'd choose at least 20 gal|80 liter. If you have many feet|meters of piping, you don't need much additional ballast volume.

We purchased a similar system (Great Lakes Air, Model GRM15) from Compressed Air Systems for doing CPMAS (I think Compressed Air Systems may have changed their name to ATLAS COPCO). It has similar specs (15 SCFM, up to 150 PSI, -40 C dew point) with two desiccant columns and filters, and the cost was $1256 (including desiccant and filters). I have been very happy with its performance. More info on the model can be found here: www.glair.com

We have used Balston products for years with absolutely no problems. A model number 75-20 air dryer is attached to one of our solid state NMR instruments and a model 75-10 on another. The latter provides a lower volume of air and caused an unacceptable drop in pressure when switching between drying towers when we had VT air going full blast as well as the drive and bearing air. We got around that by placing a ballast tank (Balston model 72-012) after the dryer, rather than buying a larger dryer to supply adequate volume.

We have a similar unit from Parker Balston (Model 64-20, Max Inlet pressure 125 PSIG). It gives enough pressure for my two solid state NMR instruments. It works reliably. We have it in a neighboring room, so the loud switching between the two columns does not bother us. I have not noticed instabilities due to the switching. Our liquid's facility, which has a similar unit, does have trouble with that, so they put a large buffer tank after the dryer.
In addition I have for each spectrometer just at the spectrometer inlet point a Filter set: RTI Eliminizer Combo (3P-060-P04-DCP).

We have used Hankinson desiccant air dryers for a long time. They have worked well for us, but I'd bet other brands do pretty much the same thing. At times when I have compared prices, I came away thinking that Balston's dryers were overpriced compared with their competitors.
The main thing that limits the reliability of the dryers in our installation is big surges of oily water (like a quart or more at a time) that we can get in the air lines on humid summer days. I have installed a steam trap in front of the dryer's filter package, because the relatively small filters supplied with the dryer can't drain that much water fast enough to prevent some water entering the desiccant columns. Over time, the steam trap's drain fills up with goo and I have to clean that out. Water and oil getting into the alumina columns will deactivate them. However alumina pellets are cheap and it doesn't take that long to empty and refill the columns with fresh alumina if the dryer is laid out for ease of maintenance. The older Hankinson dryers weren't so great in that department but the new ones are much better, with quick-disconnect plastic tubing connections.
The alumina pellets always carry some alumina dust and you will certainly want to keep that out of your probe's air bearing, since alumina is very abrasive. I use a finer size filter after the Hankinson-supplied post filter. I think many people use a small disposable cartridge filter in their CP/MAS air line also.

Balston and Hankison also make similar products. I haven't heard anything bad about any of these. You want an oil condensing prefilter and a post column dust filter on all of these brands. Make sure the oil condensing filter has an autodrain--less work especially with Missouri humidity changes. Grainger has the complete setup (stock #6F571). These are around 135 lbs, so we mounted my new lab Hankison to 3/4" plywood and then mounted this to the studs.

I really love the ones from Balston. I use a wall-mounted twin-tower version. Haven't checked the latest models, but need to do so soon for another machine. The air we have sounds just like yours, yet we haven't had to change anything but a couple of the pre-filters in over 15 years -- truly maintenance-free. I'd highly recommend them!!

About this time last year, I posted a review of compressed air systems at NMR facilitites, and I think you might be interested:
http://chemnmr.colorado.edu/ammrl/archives/October-2006/24.html
You can also downlod the summary I wrote up on our facility's "Reports" page:
http://homepage.mac.com/jkurutz/FileSharing18.html
We ended up choosing Kaeser system (air dryer, dual oil-lubricated screw compressors, and minor accessories) because our colleagues seemed to have the best results and fewest problems with them for the cost. P.S. When configuring our new system, we included an extra tank downstream of the air dryer to dampen pressure fluctuations caused by the air dryer switching sides, brief power outages, and the like.

We have 2 magnets that we use in Solid State MAS mode, so we need clean dry air all of the time. I have had good luck with Balston air dryers, they also have a unit that switches between 2 desiccant towers. The advice I wanted to offer does not relate to the model of dryer, but to your problem with compressor oil in the house compressed air. We could never get our dryers to work for more than a few months until we put in a trap before the desiccating units to remove the compressor oil. Apparently this oil incapacitates the desiccating resin very quickly and irreversibly and can go on the damage the switching mechanism that directs air from one tower to the other. I would highly recommend installing an oil trap with a drain and replaceable oil filters, it will greatly extend the lifetime of any resin based drying unit.

make sure you have a suffiently large volume "ballast" tank after the drier column to keep pressure constant when columns switch otherwise you may crash your rotors. A cheap tank can be had by scavenging one from a new (don't use and old one or it will be rusty / dirty) home compressed air tool system for about $70.(simply remove the compressor unit) e.g. see: Jobmate 3G compressor at:
http://www.canadiantire.ca/browse/product_detail.jsp?FOLDER%3C%3Efolder_id=1 408474396674167&bmUID=1196146005836&PRODUCT%3C%3Eprd_id845524443295410&assortment=primary&fromSearch=true

We use an oil-free compressor from Jun-Air that dries the air before storage in the tank. The compressor is located remotely as it is rather loud and the air transported to the solid-state NMR spectrometer in a 2 inch pipe to allow the required pressure and flow to be maintained. Compressor is reasonably expensive, but reliable

I ended up getting an oil-free compressor(http://www.quincycompressor.com/qrds.html) along with a standard air dryer (http://www.hankisonintl.com/Products/RefrigeratedCompressedAirDryers-HPR.htm).

We use a Kaeser model KAD-45 for all of our house air as a backup for the house nitrogen. This system is probably a lot more expensive than you would want. Our NMR spectrometer probably uses the most gas flow of the instruments. We get a dewpoint of about -70°F during regular use. A dewpoint of -40°C/°F is usually the minimum required for NMR probes, so you should consider a dryer that can do better than that. We also use 5 inline Kaeser filters for oil and particulates. This system usually works well, but I wouldn't want either the dryer or the compressor in the spectrometer room. The blow off may occur quite often if you want to maintain the dewpoint with reasonable flow. On the KAD-45, you will suffer hearing damage if you are exposed to the purge air for very long.

It depends on what sort of MAS system you have (e.g 14 mm diatmeter spionner vs 2.5 mm have very different requirements). Perhaps do a calc based on pressures you need for operation and contact a supplier to get an "engineers" estimate for free?. I no longer do MAS work in my new job so will have to look at old stuff I have if I can find it. Larger is better.

We use a Balston air dryer that works great. I have not tried the one you are specifically asking about.

We used a dry nitrogen drive system which eliminated all of the problems (maintenance, supp;ies,electrical power, etc etc) with dirty wet or poorly regulated air supplies.  The down side is that you need to have a cryogenic tank of some size on the site..  We used the high pressure boil off (200psi) regulated to whatever we needed (50psi or so) to operate two NMR spectrometers  Bruker AC200 for liquids and MSL300 for solids work.  The supply was more than adequate as we used gas from a large tank which was used to supply emissions testing.  I have always said that smaller tanks dedicated exclusively to an NMR facility are also workable.

I've been using Twin Tower Engineering (www.airdryers.com) for the last 4 years and have been very please with their performance.

two scroll compressors NO OIL
one small refigerator dryer without rewarming heat exchanger
one Zander Al2O3+ zeolithes load in thermodynamic cycle self regenerating colums with frequent short recycling = loose 40% air but dew point down to -80°C

We have 2 different sizes of Kaeser dryers here. The dual tower regenerating dryers are a good design in general. Our KADW-15 is very reliable. I have had good luck also with Balston wall-mount dryers of similar design. I would put automatic blowoff valves on the filter/separator bowls and the receiver tank to avoid liquid accumulation.

We did NOT use compressed gas cylinders.  We used the boil off from a LIQUID nitrogen cryogenic tank (several thousand liter).  Many larger research complexes have these on site.  One could also use 160 loter LIQUID nitrogen portable cryostats delivered as needed.  Clean and totally dry..

We use a Pneumatic Products Corporation Model 60LDE.MEC. House air is 85psi in and 80psi out. The air is used on at least 7 NMRs, an IR, and an EPR. Its been trouble free for years, but the physical plant has a dryer upstream so it probably doesn't have to do much.

We use the bleed off gas from a high pressure liquid nitrogen Dewar. The tank lasts about 30 days, and the cost is much less than the cost for dry compressed air.