# Fluidized bed filter question??



## boognishmofo (Oct 2, 2011)

I want to get opinions in a design for a fbf made from PVC. Most of the designs I have seen look more involved than I think they need to be. My idea would be a price of the precut PVC they sell at home depot. I believe the largest they have is 4" diameter, might be 6. It is 2' high and I would cap the bottom and put a screw on lid. Now here is where Im not sure if I need to run the input pipe from the top through the middle and leave a small gap from the bottom or if I can just run it through the bottom and put the output on the top. Once the bed was started it would keep the sand from clogging up the input correct?


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## Rick_Lindsey (Aug 26, 2002)

Would it stay unclogged when you turned it off if you have the input on the bottom?

-Rick (the armchair aquarist)


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## boognishmofo (Oct 2, 2011)

I think you would just have to get it started again. I have read that if you hook a garden hose up to it the sand will stay suspended for several minutes. Just thought it would be easier to make if all I needed was a bulkhead on the top and bottom.


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## BillD (May 17, 2005)

to me the best designs are conical which means the water flow dissipates as you get away from the inlet so the sand does not follow the water out the outlet. So lets say if you started with a 2 " pipe went to 3' and then 6", the sand would tumble bacK to the base once it got up high enough that the water flow could no longer carry it. I have seen them done with a single piece of straight pipe with a reducer where the inlet is.


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## boognishmofo (Oct 2, 2011)

Conical sounds easy enough. I found a couple rough pics googling fluidized bed omitting the word filter.















I understand now the reason one of the designs the output was a little offset from the input. If it is tall enough and the flow is just enough to get it bubbling a little dont you not have to worry too much about the sand blowing out the top. [/img]


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## BillD (May 17, 2005)

The diagram demonstrates why the conical design is ideal. The sand grains need to be separated at all times for it to work optimally. the size of the sand grains will also affect the necessary flow rate.


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## moto_master (Jun 23, 2006)

I tried this a few years ago. It "worked" but I wasn't satisfied with the design. I retired it to sit in the garage a while back, but now I'm looking to build a new one. If you're still wanting to do it, here are pictures of my design, and what was wrong with it.










This is the whole FBF. It came from a pump in my sump, through the FBF, and up to the tank. I included a ball shut off valve, but after using it I realized that I needed a one way check valve. Every time I turned the power off I had to reach to the back of my sump (which there wasn't very much room) and turn the valve off before turning the power off. If I had a power outage, all the water in the pipes that was above the water level in the sump would drain back into the sump, carrying the sand down with it. The sand would either get clogged in the small pipe, or it would make it all the way to my pump and clog the impeller.










Here is my "cone". A series of reducers to take the pipe from 3/4 inch to 4 inch. This was not nearly enough space for a gradual conical expansion. It did not hold much sand before it was up in the 4" pipe. The pump that I had was too strong, and it would blow all the sand out the return line. So I had to restrict the flow, which still blew out the smaller particles of sand, but left me with the larger sand. Even though the 4" pipe is about 14" long, the flow of water caries the sand all the way up it. This is why a conical design is crucial.










For the top I used a 4" screw top cap, and drilled a hole just big enough for the 3/4" pipe to fit through snugly. I PVC glued the pipe into the lid. This actually worked, until with moving it to much it started to separate and leak. I also had problems with water leaking from the threads of the cap. I used that sealing tape on it and tightened it down with a wrench, which was hard because it's difficult to hold a smooth round pipe from turning while you screw the cap on. The pipe glued into to cap started leaking because I was having to take the cap off every time I had an accident with the power shutting off. The sand would flow down into the tubes, and my pump was not strong enough to push it out.

I'm planning on building another FBF, but I'm scrapping the PVC idea. You just can't get enough conical expansion. And an absolute must is a one way check valve.


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