SIEVE FILTER: Screen placement and orientation question.

[Catfishnut]

I'd like to open a discussion regarding the concepts of design for a sieve filter, primarily the orientation of the sieve screen and its shape and how the water should be directed at it.

I have seen quite a few samples in action, from an almost vertical screen to a flat, sloped screen, to a bowed screen to a rotational cylinder. Is there a "best way"?

I have a couple of ideas for my design, but I am not sure they are good ideas or not.

The first part of my design, which will not be changed as I have constructed it already, is a filter bag holder. This is going to be set down inside of a 22" diameter, 55 gal HDPE drum. The filter bag holder is hence a round disc of 1/2" polyethylene. I have cut ten holes in the outer perimeter of this disc and routered the edges with a Rabbetting router bit so that the filter bags will set down into the holes flush with the surface of the top of the filter holder disc. In the center of this disc, I will eventually cut open a large hole for the clean water that is strained out by the SS mesh to fall through to the bottom of the drum. I haven't cut this out yet as I am still pondering the design of the SS mesh sieve. The filter bags are 4"x14" nylon mesh, 150 or 200 microns

My first idea was to build a truncated cone from 200 micron SS mesh screen and bring the inlet water up the center of the cone and let it spill out over the entire surface of the cone using some sort of a fountain type head or a flared out PVC pipe. Just something to make the water fall fairly gently and evenly over the entire cone surface. At the bottom of the cone, the muck and the water that propels the muck down the sides of the cone would basically fall directly into the bag filters which can be later removed and cleaned. Some water and muck might stray around on top of the filter holder disc, but there will be dams to prevent it from falling directly into the clean water at the bottom of the drum.

My second idea is is simpler to construct. It would be a plastic cylinder, like a bucket, with the sides mostly cut out, leaving just 2 or 3 thin strips of plastic between the top and bottom to give support. The SS screen would be wrapped around the cylinder and fastened to the plastic ribs with slats of plastic and SS bolts/nuts. WIth this design, the water would come at the screen from the outer sides of the cylinder wall and directed against the screen. The strained water would go through and fall down through the center to the clean water at the bottom of the drum. Any muck and the water that propels it down the sides of the screen would land on the same flat, disc filter holder and the bag filters would catch the debris.

With the cone shape, I can get much more surface area, but it will be more difficult to assemble and maintain. Then again, would the slope of the cone, versus a straight vertical wall, lend to more plugging of the mesh with fines? Would it be better to have a vertical wall to shed the fines better? I can go either way here, but these two options are my only known options in a round barrel. Unless I make the mesh sieve perfectly horizontal, and I KNOW I don't want that!

So, the question is. What is better for operation? A sloped screen like a cone or a straight vertical wall like a cylinder or a slightly angled, flat screen with no curvature (like an A-Frame shape)?


Catfishnut

 

[Waterbug]

I assume for a minimum size filter the Cetus shape would be optimal because I assume they've done a lot more experimenting than I ever have or will.

I also assume the greater the slope the better the debris removal. But I'd bet the shape of the grid wires would have a big effect on the best screen angle. Perhaps manufacturers of these provide info on how to position each type of screen and water flow amount.

If you look at sieve filters used by industrial users, gold miners, etc., they all use different angles and shapes. So I think the best shape would have more to do with the actual filter. For example what if the screen can be huge? I don't think the curve would be a help, might even be a problem.

Most of the DIY sieves I've seen have tried to copy the Cetus exactly. I think that misses some advantages DIY has. For example Cetus has to ship these things so big and heavy is something they'd want to design away. And of course customers will always prefer a smaller footprint. DIY doesn't have to have those constraints.

So my thinking is there is no optimal screen shape/angle for all sieve filters. It just has to be right for your filter. For example I've used sieves just to remove pretty large stuff, about 1/32" and larger (about 800 microns) I'd guess. So for me almost horizontal shade cloth and window screen worked OK. Simple and cheap. And then I let other filters handle the smaller stuff.

Now that was long before I knew there were screens manufactured for this purpose, maybe I would have used one. But I think even in my next pond I'll probably just go with window screen and shade cloth. That the fines filter would have to handle the 300-800 micron stuff wouldn't be a big deal to me, I think. But I'll have to see what my next fish load actually is and whether I'd really need to keep smaller stuff out of the water column.