miscellaneous projects, etc.
A little while ago, a friend of a friend told me about someone who had built collapsible CR boxes and sent me some photos. (I don't know who built the originals, or I'd credit/link them.) It was a very clever design, but I wanted to do things a little differently so I built a few for the local mask bloc's fledgling air purifier lending library, with help & input from friends. There's still some modifications & improvements I want to try, but I feel good enough about the basic design to share it. Suggestions would be welcome!
This page is sort of formatted as assembly instructions, because I don't have construction photos. I'll try to get actual construction photos at some point, but for now I'll try to explain some of the construction tricks along the way, and you can probably figure out the rest.
CR Boxes are simple air purifiers made from box fans and furnace filters. Compared to most commercial purifiers they have a high Clean Air Delivery Rate, they're cheap, and you can often get the parts from a hardware store. Air purifiers can reduce pathogens, allergens, smoke, and other pollutants in the air.
CR Boxes are big and bulky - usually they're cubes about two feet on a side - and most of that volume is air. That's fine if you keep them in one place, but if you want to move them around (e.g. if you're lending them out for events) it's much easier if you can fold them up.
The sides of the box are made from furnace filters held together with duct tape hinges at three corners. There's two different types of hinge - one that allows 90 degrees of motion, and another that allows 270 degrees of motion.
Probably the most complicated part of this design is the 270 degree hinge, and it's not that complicated - just duct taping the edges of two filters together with a gap equal to the thickness of a filter.
Construction tip: The easiest way I've found to make this hinge is to stack the two filters you're going to connect with another filter in between them (making the gap you need), apply duct tape down the length of the edges, then remove the filter in between, fold the two out flat on the floor/table, and apply duct tape to the other side of the hinge... which is probably clear as mud; I'll try to get proper photos soon.
I've taken to drawing little arrows that align when the hinge is folded correctly so it's easier to see at a glance whether you're putting it together right. It'll also be obviously wrong if the fully unfolded sides form a rectangle instead of a square, but the arrows make it easier.
These are simpler to make, and connect the other two filters to the central pair.
Construction tip: I like to make these hinges by holding the two filters at right angles to each other (ask a friend for help if you can) and applying a couple of pieces of tape to the inside corner where they meet. Then I can fold them flat against each other and carefully fit a strip of duct tape down the right angle on the other side of the hinge.
Construction tip: For the top, I make another square tray just like for the bottom, then cut a hole in in that's slightly smaller than the fan and tape the fan on. Non-collapsible CR boxes with square fans are often made by taping the fan directly to the top of the filters, but that doesn't give you edges extending down to hold the filters in place. You could substitute other fans if you want, e.g. an array of good computer fans would probably be quieter than most box fans.
The way these round fans protrude through the inside of the coroplast top is a little inconvenient. With flat square fans, the filters can be stored tidily in between the bottom and top parts of the box when folded, but that doesn't work with this type of fan... yet. I think there's room for improvement here.
And that's it! I'd like to make a nice carrying bag to put this in for transport, but if nothing else this is a whole lot easier to store and to lug around than a fully assembled CR Box. Also, it takes up way less space in a closet or on a bike trailer.