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  • Results 1 to 9 of 9

    Thread: QT/Moving bed construction question

    1. #1
      avorancher is offline Senior Member
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      QT/Moving bed construction question

      I'm going to build a 3'x4'x4' moving bed filter out of wood lined with either fiberglass or a rubber liner. I built a larger QT tank previously with a similar construction technique and it has worked well, but a trip to the big box store yesterday surprised me at how much 3/4 plywood has increased in price here in California in the past 10 years. So I'm looking at using something a bit cheaper this time. The tank will be 4' in depth. I plan on using 2x4 construction and the largest unsupported "span" for the wood sheeting will be 1' x 2'.

      The question is... will MDF board or OSB board be strong enough to support 48" of water with that rather short span between the 2x4 framing? It is an indoor tank and will not get wet, so waterproofing isn't a concern. The price of 15/32" OSB is about a quarter of the price of rough 3/4 plywood and I need to buy 3 or 4 sheets. It also needs to be thick enough to support 4 or 5 large bulk-head fittings and/or uniseals.

    2. #2
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      I would rather use 55 gallon barrels for the moving bed filters. I would only use a good grade of plywood for the the QT tank.

    3. #3
      icu2's Avatar
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      Quote Originally Posted by avorancher View Post
      I'm going to build a 3'x4'x4' moving bed filter out of wood lined with either fiberglass or a rubber liner. I built a larger QT tank previously with a similar construction technique and it has worked well, but a trip to the big box store yesterday surprised me at how much 3/4 plywood has increased in price here in California in the past 10 years. So I'm looking at using something a bit cheaper this time. The tank will be 4' in depth. I plan on using 2x4 construction and the largest unsupported "span" for the wood sheeting will be 1' x 2'.

      The question is... will MDF board or OSB board be strong enough to support 48" of water with that rather short span between the 2x4 framing? It is an indoor tank and will not get wet, so waterproofing isn't a concern. The price of 15/32" OSB is about a quarter of the price of rough 3/4 plywood and I need to buy 3 or 4 sheets. It also needs to be thick enough to support 4 or 5 large bulk-head fittings and/or uniseals.
      I think a lot of how much support you need depends on which of these choices you use. Fiberglass is going to need a lot less
      than the liner.
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    4. #4
      Zac Penn's Avatar
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      MDF will have less deflection than OSB in the same thickness so if the cost is the same then i would go MDF. I have built a 4' x 4' x 4' 2x6 tank with 16" OC spacing and used 7/16" OSB sheeting without much deflection. Your best bet would be to use liquid nails on each framing member and then use nails or screws to secure the sheeting to the framing while the glue cures. Oh, by the way when i built my tank I had open spans of 14.5" x 45" between the vertical studs so I am sure you will be fine with your construction method.
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    5. #5
      avorancher is offline Senior Member
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      Quote Originally Posted by Zac Penn View Post
      MDF will have less deflection than OSB in the same thickness so if the cost is the same then i would go MDF. I have built a 4' x 4' x 4' 2x6 tank with 16" OC spacing and used 7/16" OSB sheeting without much deflection. Your best bet would be to use liquid nails on each framing member and then use nails or screws to secure the sheeting to the framing while the glue cures. Oh, by the way when i built my tank I had open spans of 14.5" x 45" between the vertical studs so I am sure you will be fine with your construction method.
      Sounds good. I used liquid nails and screws when I did my larger QT tank and it hasn't budged (or bulged) for 6 years or so, but of course I also used 3/4 plywood which made it extremely rigid. MDF is a much better price than plywood, so I will probably go that route with a single coat of 4 oz. fiberglass cloth for waterproofing on the inside to keep the corners neat and clean. Thanks for sharing your experience with your tank.

      I would go with barrels or totes, but not enough room for 6 barrels or 2 totes in the filter house along with all the other equipment. Wood lets me customize to the space I have.

    6. #6
      BWG is online now Senior Member
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      What grade of plywood were you looking at? We can get 3/4 plywood cheaper than 3/4 MDF. 2x4 construction 12 inches on center and only 4 feet max there will be no issue using 1/2 inch sheathing.
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    7. #7
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      Plywood has a rough surface, while MDF has a very smooth surface and is very heavy; but the two have a significant difference when they become wet.

      Plywood has opposing layers of thin layers of real wood, glued together under pressure with good glue and retains all of its strength when it becomes wet (think plywood boat).

      MDF is a composite of sawdust that will rapidly loose strength and structure when wet (think cardboard). In addition, all holes rapidly reduce MDF strength and it will flake into chips even when dry.

      If your lucky, and keep the MDF dry your gamble will be fine for awhile. Plywood will probably last until you get tired of it.

    8. #8
      avorancher is offline Senior Member
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      Quote Originally Posted by BWG View Post
      What grade of plywood were you looking at? We can get 3/4 plywood cheaper than 3/4 MDF. 2x4 construction 12 inches on center and only 4 feet max there will be no issue using 1/2 inch sheathing.
      Must be a local Socal problem. In the 2 big box stores near me. 19/32 plywood sheathing is $35 and up. Only 3/4 was sanded cabinet grade and over $45. I can get 15/32 subfloor for around $25 so that might be sufficient, but all I saw on the shelf was pretty crappy.. twisted and full of knots. I'm wary of knots due to all the uniseals/bulkheads I'll be putting in. Around here 3/4 MDF is only $17, so significant difference.

      I don't want to be penny-wise and pound foolish on a nice pond, but also don't want to waste $$ on a fairly large overhaul of my filtration system.

    9. #9
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      Quote Originally Posted by BWG View Post
      What grade of plywood were you looking at? We can get 3/4 plywood cheaper than 3/4 MDF. 2x4 construction 12 inches on center and only 4 feet max there will be no issue using 1/2 inch sheathing.
      Supply and Demand. Southern California has a huge demand for building materials.

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