I’ve been working on an air dryer that I believe will fix up the cheap commercial filament dryers but hitting small roadblocks so it’s taking longer.
I just discovered two things - those little hygrometers don’t have any seal to the front glass and if you pressurize the space behind them, they leak. Solution is to just smear silicone seal on the cover glass and squeeze/pressure it into the plastic/glass interface to get them to seal up. But at the low flow rates needed, those leaks are a killer.
The other issue is the pump I purchased thinking it would have enough pressure only has enough pressure hooked directly to an air stone. Start plumbing it around through bottles of desiccant and hygrometer housings and that pressure gets used up pretty fast. I took the outlet tube that would normally feed to a filament dryer and stuck it in a glass of water. After the apparatus and whatever leaks and flow resistance, I managed an exit pressure of about an inch of water column pressure - well down from the around 10-12” the pump manages directly.
After sealing the hygrometers, I am getting about 2” water column pressure and those measurements are without desiccant bottles in the chain. Close but not quite.
Using an aquarium pump seemed perfect at first but they produce “high” volume at low pressure. I need low volume at a bit higher pressure with all the tubing, bottles, and hygrometers in the line.
So just ordered a pump over twice as powerful so hopefully that puts it over the top. The pump I started with was nice and small but only manages 10kPa (1/10 atmospheric pressure). Its replacement is rated at 22kPa (about 1/5 atmospheric pressure).
Aquarium pumps don’t have or need much more pressure than that since aquariums aren’t terribly deep. They are optimized for the wrong thing in this case. I’ll have to go to a regular small air pump of some kind if this doesn’t work. But since the small air pump almost works, I’m hoping this new one will put it over the top.
Everything I’ve found about silica gel and Drierite/gypsum/calcium sulphate says that combination ought to work really well at drying the air and should have a fair amount of capacity to hold water. The silica gel grabs the excess water and dries the air somewhat, while the Drierite wrings it out nearly totally dry.
So closer but hit a small roadblock. The new pump should be here in a couple of days, though, and the test fixture is ready. Just no data yet.
I’m having a tough time visualizing what you’re trying to build. Are you using a pump to create a pressure variant? If so, have you tried a simple air exchange? If the interior is warm, moisture will simply outgas from any material within. Then, exchanging the moist air with outside air should be more than enough to cause desiccation. If you recall from your effort with the Sunlu S2, the dominant flaw identified was that they did not exchange moisture with the exterior.
And found more air leaks. The test fixture is just a 3D printed base set up to hold two bottles of desiccant and three hygrometers and the air pump. There are printed-in chambers behind the hygrometers and holes for fittings to plumb it all together. Here’s a version working on in Fusion but it shows the basics.
Anyway, the version I’m using now was 2 walls and gyroid infill in hopes that 2 walls would be sufficient. Turns out it’s not air tight with the filament I’m using. I won’t increase the walls but if I switch to honeycomb infill, that should add a lot more resistance to air flows in the model. Gyroid is just open for any kind of flows. I think a coating of something to seal the inside of the hygrometer chambers should block most or all of the air leaking into and out of the model with honeycomb infill being the backstop.
It’s amazing how much air was leaking out. As I was hunting for the leaks I could blow into the hygrometer chambers with the sealed in and sealed up hygrometers and feel and hear air escaping. Not through any seams or fittings - through the model. It was a lot more than I expected.
Once I get those chambers sealed it might still work with this original pump.
Hi Olias, I am using a pump and it does create a pressure gradient - it has to or else air won’t flow. All this is is a double column of desiccant. I think silica gel and calcium sulphate should compliment each other for this.
I have tried simple air exchange in the Sunlu filament dryer but that only works well on dry days. I may not understand your question/comment though.
That the S2 doesn’t exchange air is definitely an issue. But simple air exchange on humid days isn’t enough to lower filament water content and can add water. The air exchange in the dryer needs to be dry air in to push moist air out. That’s where this comes in - to make dry air. The goal is to have the pump in this push humid ambient air through the desiccant and into the filament dryer where it displaces the moist air carrying water from the filament. It gives air exchange to get rid of moisture but does it with dry air ready to hold more filament moisture.
This takes care of modifications to the filament dryer which didn’t really work well anyway except on dry days. My Sunlu S2 mod actually works great on dry days but this should turn all days into dry days and only would need a hole or fitting added to any filament dryer to let the dry air in.
It’s limited by the capacity of the desiccant. It can only remove so much water but calculations indicate that the quart bottles I’m using should hold enough desiccant to make this feasible. How quick the desiccant gets used up will be a function of ambient humidity.
Here is that same image annotated to show flow. The big circular ports facing front are where the hygrometers go and it’s those chambers behind the hygrometers that are leaking air through the walls. That’s the areas I need to seal.
The bottles for the desiccants fit in those big holes and each has an inlet tube that extends to the bottom of the bottle and an exit at the top that collects the hopefully “dried” air. The last arrow on the far right is where it would connect up to a filament dryer.
The AMBIENT hygrometer just sees the air straight from the pump to know what’s going in. The SILICA hygrometer reports the humidity between the silica gel and gypsum desiccants to both know how well the silica gel is doing and what the inlet air humidity is into the gypsum. And the DRIERITE hygrometer reports the final humidity of the air leaving the unit. Desiccant state should be immediately apparent from the numbers given by the 3 hygrometers.
Yep. Very familiar with those. They sure are air dryers but are way too big, too noisy, too much volume, and too power hungry for this application. Maybe in a production environment where there are rows of filament dryers but I’m shooting for home use.
They work on a different principle - it’s pressure that causes the humidity to go up to then get stuck on molecular sieve style materials. They are constantly popping and hissing as they alternately pressurize and depressurize the two chambers to remove water.
When I was in grad school we had a small one of those we plumbed into a cabinet because it was so obnoxious out in room proper. They do the job but I’m not interested in any kind of pressure-operated air dryer. This one does use pressure to move air through but not in the same way - it’s very low pressure and the flow is very small. Totally different except for the desired end result - dry air.
I spudged out the hygrometers to get access to the chambers behind them and painted the walls in thick superglue with a cheap model paintbrush.
It looks like the real issue was the roof of each chamber. The roofs were printed on top of support and there’s fair texture to the first layer. Hopefully the superglue fills the pores and seals this all up. But just above that roof is gyroid infill for a few layers which doesn’t block air flow.
The pump I was using has a pretty drastic tradeoff between pressure and flow. With a “big” flow caused by the leaks into and out of the model, the flow rate ballooned and the pump could barely produce the pressure to overcome the resistance to flow.
Hopefully if all the leaks can be sealed, there will be enough pressure to push the air through the desiccant properly and provide the necessary flow into the filament dryer.
Here’s a shot of one chamber upside down as the superglue cures. You can see the texture. With just a few roof layers, this was probably letting lots of air into the model to then escape from all the little pores and holes. This was part of the problem. The other part was the hygrometers not having their glass faces sealed. What little exit pressure I had doubled when I rubbed the silicone seal into the edge where the case and cover glass meet.
