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We have a pressure-swing adsorption air dryer that is identical to the one shown here on the NMR wiki (Balston 75-20, currently a Parker-Balston 75-A20): http://nmrwiki.org/wiki/index.php?title=NANUCairdrying_setup The dryer feeds a receiver/ballast tank. Our system appears to produce properly dried air; however, at a certain point in the cycle (presumably the vent portion) the pressure in the receiver tank declines to a level unsuitable for use with our NMR. This occurs even if the outlet of the receiver tank is shut, suggesting that stored air is backing out through the air dryer. We are struggling to find a manual/technical information for this air dryer to help troubleshoot our issues. It appears the issue could be resolved by a check valve placed at the tank input, although it seems unlikely that the dryer was designed to require such an element. Rather, it would seem that some internal element of the dryer (i.e. within the blue control box) is at fault--possibly the solenoid valve that controls venting/re-pressurization, or the assembly at the outlet (probably a shuttle valve with a bypass). 1) Assuming it exists, could someone possibly provide technical documentation for this air dryer? 2) Do most systems have an element to prevent backflow from the receiver tank to the dryer, or should a properly functioning dryer prevent this? Any more general thoughts or comments regarding our issue are welcome, as are clarifying questions. Thanks in advance for your assistance! A few additional details: 1) House air is being supplied to the drier at 7 bar (regulated). 2) The receiver tank will build to about 7 bar until one of the dryer solenoid valves switches (venting air is audible); pressure then declines to a fairly stable 4 bar. |
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A properly functioning dryer should not allow backflow. Once repaired, your system should not need an additional check valve. The dual solenoid valves in the control box are rebuildable. Rebuild kits should be available through a distributor representing the valve mfr. so long as you have the full model number from the actual valve. The push-to-connect fittings are notorious leakers, and the leaky fitting can be located by feel and sound. Often the leak is repaired by replacing the plasitc tubing. It is possible that one of the dessicant towers is fouled and needs to have the dessicant changed. Because the dryer has 2 identical sides, troubleshooting is easily accomplished by swapping components from one side to another to see if the problem moves. The manual has no schematic or pneumatic diagrams, but is available in .pdf form at this URL: http://www.daviscontrols.com/synergydownload/Balston/Manuals/68.pdf |
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Depending on your exact symptoms and age of your system, it could also be a stuck shuttle valve. We had that problem on an older model - it would work fine for days/weeks, and then suddenly lose pressure, but only when blowing though one of the columns. When it switched to the other column, the pressure went back up. So if this is what you see: There is a shuttle valve (brass Tee-fitting with three hoses connected to it) that has a movable plug. This plug has a small hole in it that allows the drying air to flow across the inactive column. This sliding plug can get stuck in one or the other position (either old O-rings or particulate from the columns, etc.). When this happens, instead of most of the air exiting the air drier, it goes through the inactive column and the exit pressure drops. The two symptoms of this are cycling pressure with column cycling (every 4-10 minutes, depending on your settings) and very loud venting that also cycles with the cycling. If this is your problem, take apart the shuttle valve, replace the O-rings and clean it out. (I never found a direct replacement.) -David Good post, David. The shuttle valve is aluminum in some units. - Jerry Hirschinger (Jul 23 '14 at 06:50) |
Please give the timing of the sequence of actions. How long does each state last? When venting air is audible, how loud is it? Does the regular, brief high-pressure venting still occur? Have you opened the box to determine where the air is venting? Perhaps there is simply a leaky connector. - Jerry Hirschinger (Jul 15 '14 at 09:52)