Sunlu had pre-production versions of their new 110 deg dryer at Formnext, it should be available soon. It’s double-walled and a bit bulkier than regular dryers.
I’m very new to the printing world but ramping up quickly. Researching dryers like we’ve all done, didn’t take long to realize they’re all relatively the same with the same shortcomings. Reading the OP’s review I was ready to make the leap but shipping the unit is almost as much as the unit itself.
I’d love to see/hear more about the mods you’ve done to the S2. Is your setup as simple as pumping outside air through your desiccant tubes into the bottom of the dryer? Not sure how long a wet pump is going to last running dry. How are you exiting the air from the dryer or are you circulating the air from the dryer through your desiccant tubes and back to the dryer vs drawing outside air?
It actually is that simple. I built a whole fixture but you don’t need to do that. The pump I use is an aquarium air pump so is designed to pump air for long periods.
The flow is low but aquarium pumps can move fair air. The Sunlu S2 has holes at the top for feeding filament but I don’t feed filament from it. I just use it to dry with so those become air exits (which is good because moist air is less dense and rises). There’s also the whole clamshell seam especially around the hinge. But the flow is low enough that there is no problem with air getting out.
Recirculating air back through desiccant would be effective with a caveat but with my setup it’s a one-way trip. My ambient humidity tends to run in the 40-50% range and that’s the water that has to be removed to make dry air. With recirculation, as you near end stages of drying the humidity in the dryer box can drop very low compared to ambient. But early in the dry when water is coming off the filament in greater amounts, the desiccant would be needing to remove more water than removing it from ambient air. Not sure which is the better way but the one way trip works great.
As to silica gel lifetime, I only have one trip through a charge of silica gel. My air dryer has a reservoir that holds 800g of beads and with my ambient humidity I completed drying 26 new spools of filament. I was needing to change out dryers during development so stopped before drying significantly slowed but estimate I could have done 30+ spools no problem. My second charge of beads is showing breakthrough now and exit air humidity is rising and slowing the time to reach the humidity I dry to for PLA and PETG HF, but it still gets there. It’s also nearing 30 spools dry but I also ran the first charge of beads through the filament dryer as a test which probably used up a lot of water capacity in that second charge.
When I first started on the journey, I started a thread to document it. It is mostly about filament drying but took a few turns and need to acknowledge @johnfcooley, @NeverDie, and @IslandBill for their contributions:
https://forum.bambulab.com/t/filament-drying-preliminary-results/
And the fixture that resulted is here:
I now do the first part of drying without dry air but with the door propped open since water still comes off and it should make the beads last longer. Once that moisture has come off and filament drying chamber humidity starts to come down, then I add dry air. Similarly for silica gel beads, those will get microwaved first get rid of the easy moisture and then into the filament dryer to polish the dryness level on down. I can’t hit very high temps in the filament dryer so this is an ongoing experiment. I’d like to restore the beads to full dryness but donkt know if I will be able to. But a special housing that can hit proper temperature fed with dry air might fix that too.
All I can tell you is it works for me. If any questions, feel free to ask or dm. The whole drying thing has been eye-opening for me. I started seeing moisture effects a few months after I got my X1C. Once I started drying this way, the only moisture effects I’ve seen were with a spool of PETG HF that I didn’t dry as an experiment just to see how it did straight from the bag. Had stringing, lifting, and a doughy surface (but not absolutely terrible) until I dried it. After that it printed beautifully.
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You could get the Sovol/Comgrow SH02. It has two main shortcomings - it’s noisy and it gets unreasonably hot on the bottom. The filament exit angle also sucks if you feed from it. But people made mods to address each issue, available on printables. I have two SH02 units besides the R3D D1 ones, and I’ve ordered the electronics for the quiet fan mod 
I’m considering this mod for the heat on the bottom: Raised feet for Sovol SH02 by Big B | Download free STL model | Printables.com
And this mod I’ve ordered parts for for the quieter fan https://www.reddit.com/r/Sovol/s/SyRVbkxe7D
There are also filament exit angle mods I might do too.
This dryer is easy to get on Amazon. (Don’t get the SH01, it’s really horrible.)
It sucks we have to go through all this nonsense to get a good dryer but this is the current market and is why I started this thread.
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Amazing. I realized 5 minutes after posting just how well documented you have your journey. Thank you for taking this on. You read so many arguments against the need to dry but my guess is it’s rare folks actually do the side by side comparative testing to see the results first hand.
Researching the dryers commercially available has truly tested my resolve. It’s clear 95% of the influencers out there are reviewing because the manufacture has compensated them. I just ordered the S2 against my better judgement but I’d rather spend my precious time down this rabbit hole you’ve started than watch another YouTube review
Thank you for your contributions to this amazing community.
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hopefully this won’t get pulled for advertising, but i think i’ve hit most of your requests with my dryer. all the parts are open source; no need to buy one. great list!