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I doubt anyone can tell you conclusively if this is overkill or not. I hope it is. My thinking on this is that avoiding rust is worth a little bit of hassle and cost to try to keep dry air in the engine. Plus it's kind of fun to muck around with these engine dryer systems, at least for me.
I have used a dehydrator for more than 10 years. My last three oil analysis 9/22, 4/23, and 9/23 report 0% water. I am awaiting the results for the latest oil change.
I had a C-140 once upon a time. I got concerned about moisture in the crank case one cold morning when I saw frost covering the inside the oil filler tube. I guess frost was all over the inside of the case. so, thats a data point to consider.
The mattress air pump is unfiltered air, but it only runs for a few minutes to purge the bulk of the humidity out of the crankcase. The small aquarium pump pushes air through a filter, then the desiccant, then another filter. I will send an oil sample to avlab at the next change and compare results to previous samples.
I have a 12volt sealed battery to power a mattress inflator. I suggest you use a HEPA filter for it. If using 3/8" fuel tubing it should it will be around 20-25 liters/min and that is an easy breeze but will purge a 4 cylinder in 3 min, but I leave mine on for 5.
I don't know what the filtering capability of aquarium air stones is, but they are designed primarily as diffusers, not filters. To be effective at filtering silica, you'd need at least a HEPA filter (particles down to 0.3 µm) on the outlet hose of the desiccant chamber.
My homemade dehydrator uses 7.5 pounds of silica gel and typically runs between 3 to 5% relative humidity in the desiccant chamber...much more than enough to keep air being pumped into the engine well above the dew point
how long between dry outs does 7 pounds last? What’s your process to dry that much out?
The Mojave Dehydrator uses orange indicating silica gel desiccant without colbalt chloride. It saturates with moisture to 15% by weight. I run my dehydrator on my O-360 (RV-7) 100% of the time. I regenerate the desiccant about every 2 months by 'baking' it in the oven at 225F for an hour or two. The system uses 2 lbs that are contained in vented bags for less mess in handling. The air is filtered twice before entering the breather on my engine.
Excuse me, I misspoke. I actually use 15 lbs of desiccant in the chamber. They come in 7.5 lb containers so I use two at time. Depending on ambient relative humidity, they last about 2-3 months. Longer in the winter, shorter in the summer. The stuff turns pink when is reaches saturation, and as it turns pinker I see the humidity in the desiccant chamber go up. I have two additional containers of dessicant, so when the 15 lbs in the chamber turns pink, I pour it out, replace it with the other 15 lbs, and take the pink stuff home to cook in the oven. I put it in a roasting pan at 250°F for about two hours, stir it every so often, and it turns it back to blue. The desiccant I have now has held up fine in this routine for the last 3 years. I use that much (15lbs at a time) so that I don't have to change it out as often. My wife has actually taken over that cooking job, based on her observation that I tend to spill silica beads around the kitchen. Those little suckers do escape quite readily and then bounce a long way on the tile floor of the kitchen. (I've also tried to demonstrate that I don't know how a vacuum cleaner works, but she isn't falling for that one).
Are you using a 'closed loop system' where you input into the oil filter neck and return from the breather tube? Seems like you are getting saturated beads as fast as some setups with quite a lot less. Are you running this 24/7 or on a timer for a couple or so hours a day?
It runs 24/7/365. Input through the oil dipstick tube. I was using a rubber stopper from the hardware store. It worked great, but I thought a 3D-printed screw-in from Scott Card would be more elegant. It is, but kind of a PITA to screw in. I'd guess that I could decrease amount of silica or decrease change frequency by decreasing air pump volume from 255 gph, but I don't know what the minimum effective air volume would be and this system as-is works to my satisfaction.
No, I think that's exactly correct. Recirculating the dried air in a closed-loop would certainly result in less bead change, and I found that to be true back when I was using such a closed loop, but AFAIK that would be the only advantage to closed-loop. I opted to accomplish the same end by just buying more silica to make it less of a PITA to hook the thing up after a flight. That degree of perceived PITA will vary widely from pilot to pilot. I chose the easier but more expensive route, bearing in mind that I live in less-than-steamy Minnesota.
My experience too, although I pump the air in the opposite direction. I'm assuming that the direction of the circuit wouldn't matter.
Dan, Mine pushed dry air into the dipstick location. It has a HEPA filter for both the high rate purge phase (5min) and dehydration phase(60min). When I started a recirculation system was used but it contaminated the desiccant with VOC's and they made a big stink when regenerated. Conclusion: bad idea to recirculate. Now, it runs on timers with purge.
+1. Stunk out the house when I tried recircing. Not great. I use a 1.75L bottle of silica one way in through the dipstick. I also put plastic pill bottles over the ends of the exhaust pipes to cap off air infiltration. The desiccant will last about 1 week in summer and 3 in winter, here in the frozen north, before drying. Used to dry in the oven but now I now use an old crock pot on low with the lid just cracked to dry the beads. Left on over night the beads are dry the next day and no worry about over heating and turning them black and useless, which has happened to a few on our field who use a microwave for drying.
My dehydrator is home-made. It uses 15 lbs of desiccant beads and pumps dry air in through the dipstick tube (via one of Steve Melton's printed screw-ins). I don't use a filter on the output hose, although I concede that theoretically it's not a bad idea (any 0.3 - 0.5 µ filter...like a HEPA filter). I've had several oil analyses since I started using the dehydrator and no untoward silica traces have been reported by Blackstone. I prefer connecting to the dipstick tube mainly because it's so much simpler to hook up after putting the plane away.
My dehydrator was originally a 5 lpm oxygen concentrator. The zeolite beds were replaced with 4 lb of silica beads, 2lb in each column. It self purges so no need to recirculate to extend bead life, no baking out the moisture and no stink. Although it worked with the original control board I ended up changing that out for one that gives control of the timing of the purge cycle and also allows the run time to be scheduled. It has a built-in discharge RH monitor using a TI HDC1080 humidity sensor with a measurement range from 0 to 100%. With the purge cycle set to 1 minute it is delivering air with below 0.1% RH less than a minute after startup. I run it for 15 minutes every 4 hours. I don't remember what I paid for the concentrator but they occasionally show up used for fairly cheap (~$100). The original unit had filters built-in so time will tell whether they are fine enough to prevent silica dust in the discharge. The unit connects to the dipstick tube.
Hi Cal, I love this idea - how are you purging the beads? Is there some kind of heater in the concentrator?
Hey Webb. I made one of these that incorporates a cheap digital hydrometer from Amazon that bottoms out at a 10% indication. Mine is a closed loop system that pulls air from the breather, runs it through the chamber with the hydrometer, then through the desiccant chamber, then back into the dipstick tube. With this setup, the gauge is reading air as it comes out of the engine before it gets to any kind of drying agent.
Concentrators use a "Pressure Swing Adsorption" cycle. Two cannisters are used. At any given time one is dehydrating air while the other is being purged. Flow through the active (dehydrating) cannister is restricted at the discharge end to maintain a pressure inside the cannister of about 3 atmospheres (~30psig). Meanwhile the cannister being purged is vented to the atmosphere (0 psig). The silica gel adsorbs more moisture at the elevated pressure and releases it when the pressure is relieved, hence the name of the cycle. Every 60 seconds valves redirect the flow so that the roles of the two cannisters are reversed and the one that was previously active is now being purged. Because two cannisters are used there is no interruption in the flow at the outlet.
Hmmm...in this part of the country, especially in the spring, it's very common for the temperature to cross the dew point every day, as one can see from dew on the grass etc in the mornings. My hangar isn't insulated....same condensation cycles are happening in my engine, I guess. It's no issue whatsoever to just leave my airplane on the dehydrator 24/7 and prevent (theoretically) any condensation in my engine rather than relying on the idea that flying will boil off what has already accumulated. It takes 30 seconds to attach or detach.
What the data can we gain if I removed the oil dip stick during a cold morning preflight and I see frost inside the tube leading down to the oil kidney on my C-85? The only frost in the hangar is inside my oil kidney.
On your C-85, does the crankcase oil level come up the fill tube high enough to block the upper portion of the tube from the crankcase air? In other words is the air you see in the tube separated from crankcase air?
I get what you're saying, but the way my dryer is set up, it's a closed loop. Once I permanently attach the breather tube downspout, it will end right above the exhaust and there won't be room to get a hose from the dryer on it. I suppose at that point I could go just pump dry air in the oil filler without circulating it, and I don't doubt your observation that dew is likely forming in an engine just like it is on a car hood or whatever, but I think that even with a Lycoming, which are known for rusting cams because of their location relative to where the oil tends to fling around in there, if a guy flies every few days, there will be a nice oil film protecting all the crucial stuff.
