Description

I made this model last year and it worked reliably for quite a few months and now I decided to publish the design. I'm sure some other people may benefit from it.

Warning: this is not a very simple project to complete, so please be advised that some basic electronics skills are needed (might be acquired along the way, no soldering required). Please exercise patience and curiosity. I sincerely hope you'll like it.

Why I designed it

Well, the most important reason is that I have two wonderful cats, and during the period of heating in the winter the air becomes just too dry (<20%) and the cats start collecting static electricity in their fur, which makes cuddling painful for them and for me.

I also don't like super dry air from a health perspective, my mouth and nose are getting too dry and it's just not a pleasant feeling. I prefer relative humidity of 40-60%.

The reason why I was frustrated with the off-the-shelf air humidifiers is two-fold:

• The cheaper ultrasound ones are super-sensitive to the water quality and result in an enormous amount of dust that covers everything in the room. it is also very sensitive to the location, as the water particles can create pools on the floor next to the humidifier
• The more expensive air washers are just ridiculously expensive, especially the more powerful ones, that can evaporate like 300-400 ml of water per hour. At the same their tank is frequently very small so that you have to refill them once or twice a day. If you have several of those in your apartment or a house, it becomes a noticeable chore.
  With the three-section humidifier I was able to achieve a stable 40-46% humidity in the room where I spend most of my time (16 sqm) and roughly 35% elsewhere in the apartment (another 30 sqm).

I'm considering printing another one for this winter season, I liked the result.

Hardware

First of all, you will need some engineering skills to make sure the stepper rotates with the right speed, I'll explain why I chose the parts I did, but I won't elaborate on how to actually make it work, you can use countless Youtube videos to work it out for yourself. What you are looking for is a steady rotation of a stepper so that evaporating disks make one full rotation per minute. That should be more than enough.

Second, apart from the printed parts, you'll need some gear:

• 120 mm computer fans, one fan for each section, I thoroughly recommend the quietest ones you can find/afford, this is the noisiest part of the assembly, I used BeQuiet SilentWings3, those are 18 dB at the maximum performance, also they are relatively inexpensive
• 3 pieces of standard 608RS bearing + 1 additional bearing per section. 3 sections would require 6 bearing.
• some m3x10, m3x16 bolts (hex cup head, standard for 3d printing)
• Attaching each fan and a shroud base requires 4 m3x35 bolts, depending on your fan, you may need longer ones, so if you don't have the bolts, it might be safer to buy m3x40 or even m3x50. Again with a standard hex cup head.
• A NEMA17 stepper + some electronics to power and drive it, more details later
• Or, as an alternative to NEMA17, I also designed a variant with a 28BYJ-48 stepper, but I never really tested it, so you may try to use it, I'd be interested to know the outcome. Again, it requires a different set of electronics to drive it. Search Youtube for more details on how exacly to do it, there are plenty of tutorials.
• Two power adapters: 5V for the electronics and 12V for fans.
• You would need a large plastic container for storage, like those you can find in IKEA. More on choosing the right container below.
  And… that's about it for the hardware.

Filament

As for plastic, roughly you might need 1 kg of filament per section. There is A LOT of 3d-printing to do. You may experiment with colors as well. I recommend PETG. There's no specific requirement for the plastic other than it should be safe and not degrade from the constant contact with water.

Layer height should be 0.2 (with a 0.4 nozzle). It's enough to have 15% infill for most parts, see comments below, int he Assembly section.

Electronics

Okay, here is just what I used, you should consider your options based on the availability of the parts.

So I used NEMA17 stepper with a TMC2209 driver, controlled by a ESP32 microcontroller. I used Esphome project to program the microcontroller.

The sole purpose of the setup is to make sure that the motor rotates with the constant speed, so that's rather simple. The choice of TMC2209 is dictated by the silent operation of the stepper that results from using it, as it does a very clever microstepping even in the simplest of the scenarios etc.

It is advisable to have a gradual acceleration during the motor start to reduce mechanical stress on the gears and section connectors.

But again, consult Youtube or other resources for more info.

Container selection

When choosing dimensions for the container, please remember that it needs to cover only a half of the height of the model. So it might be 150 millimeters in height or taller. Remember that the larger the bottom area of the container, the less frequent water top-ups are needed. I use 50(w)x30(l)x25(h) container and it works well for me. I need to top it up once every two or three days. If you have the container that fits the model height, you may use it as a summer storage.

I recommend transparent plastic, it's a lot easier to pour water and the check the water level visually this way.

Descaling

After a season of use, there will be heave scale on the bottom and the sides of the container. I use just a concentrated lemon acid to clean it up, same applies to evaporating disks and legs and joints of the model, just pour a generous doze of crystallised lemon acid (it's rather cheap) into the hot water (50-60 Celsius), Leave it rotating for half an hour, that should be enough for a complete descaling.

Printing and assembly

The model is designed for layer height: 0.2

Infill: 50+% for the legs, gears, section connectors (via the bearing), 15% for the rest

You may use a Fusion model or a large STL file to understand how parts fir together.

First you need to print and assemble the gear block

• 1 leg (print horizontally with supports on build plate only),
• 1 leg base,
• 1 leg base connector,
• 1 gear holder,
• Larger gear + a hex stem
• Smaller gear ( print vertically, with supports on the build plate)

Secure the gear holder to the leg using 4 m3x10 bolts. You won't be able to do it once you insert the smaller gear into place.

Put a bearing into the gear holder, insert a shorter end of the smaller gear into it, then secure the smaller gear with the NEMA17 rotating rod on the other side, secure the motor to the gear holder with 4 m3x10 bolts.

Assemble the larger gear by inserting the hexagonal stem into the larger gear base.

Insert the bearing into the other bearing hole on the gear holder. Put one more bearing onto a longer axis of a larger gear. Now you need push the larger gear into the bearing in the gear holder (at an angle), and then push the bearing on the other side of the larger gear into the leg so that the larger gear is rotating freely along the central horizontal axis of the air purifier.

You will also need:

• 2 leg connector bolts for the first leg
• 4 leg connector washers for the entire model
• 1 section end connector (a part to put into the disks block rod from the side through the bearing for the last section)

Then print and assemble a block of disks, one for each section. You would need:

• 12 disks (print one by one, lying flat on the print bad, be careful when removing them from the plate, there is a risk of deformation)
• 1 disk rod
• 11 disk spacers
• 2 disk spacer-fixator (with an hole in it)

Put the fixating spacer onto the very end of the rod and secure it a m3x16 bolt, the bolt is supposed to push against the plastic, so don't over-tighten it. Don't drive the bolt deep into the rod, it may deform the internal space. Next, put the 12 disks on the rod interleaved with the 11 spacers, there should be just enough space to secure the disks on the rod with the fixating spacer of the other end. Please pay attention, all disks should have their pattern oriented in the same direction.

Next, for each section you will need:

• 1 leg (print horizontally with supports on build plate only)
• 1 section roof
• 2 leg connectors (supports on the build plate)
• 2 leg bolts
• 1 shroud base
• 1 shroud
• 1 section interconnector (to connect the disk rods through a bearing, print horizontally with supports on build plate only, with 100% infill, if it doesn't fit, you may reduce scale by a few percent and reprint)

First connect the roof and the legs, secure the roof with the m3x10 bolts. Press the bearing into its socket in the middle of the leg. Put the model on the roof, CAREFULLY pull the legs apart just enough to insert the assembled disks on the rod, connect the larger gear stem (or the connector from the previous section) with one side of the rod, insert the section connector (or an end connector for the last section) through the bearing to secure the disks on the central axis. Check CAREFULLY that disks do move without touching anything by rotating the small gear manually. Please pay attention that the pattern of all disks in all sections should be oriented in the same direction. Once the section assembly is complete, install the section leg connectors (using washers where necessary) and secure them with the corresponding bolts so that legs are fixed against each other.

Once all sections are assembled, put the air humidifier on its legs, check that all the disks can rotate without touching anything by carefully rotating the smaller gear manually.

Next, install the fans. You need to check which way the fan is rotating (they only rotate one way), and install it so that the air is pulled up. Each fan should be covered with the shroud base and secured to the roof by four longer m3 bolts (up to m3x50, but m3x35 might be enough, depends on the thickness of your fan).

Then you meed to connect all the fans to the 12v power supply.

Next you need to assemble the driver for the stepper, please remember to use gradual acceleration if it's supported by the technique you use. Please pay attention that the rotation direction should be such that the spoon-like pattern on the plates “scoops” the water. The idea is that once water level goes down, we should still maximise the amount of water on the disks, the water is supposed to flow from the edges towards the center when the disks rotate. The correct rotation of the disks looks like as if the spoon pattern “scoops” the water when it rotates.

And then you just add a little bit of a solid grease (the 3d printer variety will do) onto the gears. Remove te excess grease, there shouldn't be an visible chunks remaining.

Well, that's it! By now you already tested it, so… Just put the assembly into the container, Pour tap water into the container up to approx. 1 cm below the bearing level. Mark the water level with a marker or an electrician's tape so that you can see the maximum water level easily.

Some tips

• From experience, fan performance (air flow per hour) has a very limited effect on the overall humidity of the room, prefer the quiet fans, even if they don't produce the absolutely maximum air flow for its size.
• After the refill, WATER LEVEL SHOULD NOT REACH BEARINGS. it will cause corrosion and then they would need to be replaced.
• Apply some 3d-printer grease to the gears once a season.
• Before starting the season, especially of you see the traces of rust on the bearing, you may consider replacing the bearings. I'd recommend using an 608RS or 608RS2 variety as they have at least some protection against the water environment (occasional drops of water).

Legal notice

This model is in public domain, can be used freely. The remixing is welcome (fusion model is included), but attribution is required. Feel free to use for commercial purposes, please keep the attribution.