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Develop Filter Disk / False Bottom

I've had a few requests for 6" and 8" filter disks.
I think the 6" was for a carbon filter and the 8" was for a false bottom.

A little research shows that most folks make the false bottoms with round holes 3/32" on 5/32" centers. In metric that's about 2.38mm on a grid of 3.97mm. That seems to be about 30% open area and yet I hear that 50% open is more ideal.

I've seen a professional mash tun at the boiler factory with a false bottom that had slots instead of holes. Some talk about the grain bed not packing or something that I'm not sure about.
So if anyone can chime in here with advice I'd sure appreciate it.


  • Below is a sample from the people that made our filter disks. They can be made by acid etching or laser cutting.


    The false bottom that I saw at the boiler factory appeared to be about 25 to 35% open area but I didn't think to measure are to take a picture of it.

    268 x 290 - 25K
  • Hi Lloyd,

    AFAIK 30% is the right number.

    The commercial FB's I've seen were slotted, however the bottom of the slot was machined with a taper from narrow to wide in the centre of the slot back to narrow - note the slot itself was a consistent width, it was just the underside that was machined. Something to do with trying to suck the wort through....

    Compaction is a function of speed during draining and composition of the grain bed... the faster you draw off & the thicker the grain bed composition eg wheat / rye without rice hulls, the more chance of compacting & getting a stuck sparge.

  • Thanks @crozdog, makes sense.
    The next question is the slot width and length. I've seen more than one.
    About 1.5 to 2mm wide and as long as you like seems to be the rule but I can find no firm answers anywhere.

    I'm figuring on making both the 6" and 8" filter plates slotted so it should work for carbon filtering or false bottom filtering. Two birds, one stone kinda thing.

  • Seems like you guys have the filter plates sorted :D

    Do you think you can have the 8" filter plate made to hold the weight of a 50L keg full of grain/water? I'm thinking even when drained, the mash would still hold a fair weight.

  • Maybe, the SS plate cannot exceed 1.5mm thick or the clamp/gasket will not work properly.
    Should be OK. 1.5mm clamped between two 8" ferrules is really strong - if in doubt you can use high pressure clamps for even more clamping force.

    The holes or slots cannot be smaller than the thickness of the plate or we get into very expensive machining territory.

  • How many lbs of grain would you be putting in the keg? I lose 100 US gallons of water in 450 lbs (204.54kg) of grain from absorption. Or .22 US gallons per pound of grain (.484 gallons per KG of grain).

    Water weighs 8.33 lbs (3.78kg) per US gallon. / .22 = 1.83lbs of water per lbs of grain used. (4.026kg of water per kg of grain) So if you were mashing with 10lbs (4.54kg) of grain you will absorbs about 18.3lbs (8.31kg) of water. Figure 30% of the grain weight is removed by sugar extraction so the total weight of the grain and water left behind sitting on the false bottom would be about 25.3lbs (11.36kg) after you drained off your wort.

    Not sure what the real grain bill you are using that fits in a 50L keg but this above is an example of the basic math that would need to be applied to understand the weight the false bottom will need to support after you drain off the wort. When all the water is still in there with the grain it is really not an issue as the water really does not weight anything as far as the false bottom is concerned as is is supported by the fluid mass under the plate.

  • edited August 2013

    I'll ask the question on the beer forums mate, might get some additional info there with luck.

    edit. done. False Bottoms for 50l Kegs Under Development

    StillDragon Australia & New Zealand - Your StillDragon® Distributor for Australia & New Zealand

  • A 50l mash would have around 10kg grain which would absorb about 1l/kg with barley. Corn etc absorbs more

  • ok so based on croz's input. 10kg of water retained and 6kg of remaining barley = 16kg or 35lbs of weight that the plate would need to support after the drain off of the mash. Seams like with a margin for safety it would need to be built to support about 50lbs or 23kg without collapsing.

  • 1.5mm might distort slightly, but not significantly.

  • so you just turn it over every few batches...

  • Correct me if I am wrong, aint 10kg only ganna give ~5% abv in 50L?

  • edited August 2013

    LoO around about that. i was thinking beer when I typed that....dunno that you really want much higher gravity for a whiskey wash & am not sure on the alcohol tolerance of whiskey yeasts.

    Four thoughts on how to stop distortion:

    1. radial ribs could be welded to the underside.
    2. large false bottoms are sometimes made out of rows of triangular bar joined together. checkout fig 7 on page 6 of this pdf
    3. make the plates in quarters & interlock the edges so the joins form strengthening members eg image
    4. do what norcal does and have a mesh ring that sits under the FB to support the middle
    316 x 243 - 31K
  • Unless you are going to boil after to increase the density of the mash 5% is about all you will get. Unless you want to waste a lot of grain. You get diminishing returns above this point you add 30% more grain and get 10% higher yield. Now you could run 2 washes at 5% and then boil out half the water before you ferment like beer makers do to get high ABV beers but just adding more grain does not make much sense as you don't get back what you put in.

  • edited August 2013


    have a look at these pics of the machining on the bottom of the slot

    image image

    false bottom - base.jpg
    640 x 480 - 51K
    false bottom - top.jpg
    640 x 480 - 70K
  • edited August 2013

    possibly being made from perforated stainless steel sheet would make this super affordable.

    Perforated Stainless Steel Sheet @ Google Images

  • That's some great feedback and ideas so far, especially on strengthening the filter plate. I had the FSW (Fine Scotch Whisky Glenmorangie Clone ) in mind, where the author uses 6 litres of grain to 2 kg (1.58 Gallons to 4.4 lb) of water to mash in for a 20L fermenter. The majority of the fermentables come from 4-6 Kg of LME.

    I was thinking of mashing enough for maybe a 180L (8.8 - 14.2 Lb) batch?

  • Philter, have you got those around the right way?? 6l grain : 2l water would be a very dry mash which I doubt would drain & not provide any liquid to dissolve the LME. FYI, I usually use around 2-3litre:1kg.

    For beer I usually do the following: mash in with half my desired volume; mash out with the volume of all losses (dead space, evaporation, grain absorption etc) then sparge with half my desired volume.

  • Had this posted this afternoon on the beer channel, seems to agree with what i've been able to figure out snooping on the internet. The holes sound much simpler and just as good.

    Punkin, sorry but I thought you were mainly interested in the material to use. Most slotted lauter plates are in the 0.6 to 0.8mm region. I would not go above 0.8mm with this type of plate but calculate your gap to spacer dimension to yield around 18 to 25% open area. With drilled holes or perforated plates a maximum of 1.5mm is good. Again dont go above 1.5mm as the holes will block very easily. Those plastic false bottoms that used to be around (I still have one somewhere I think) had 3/32" holes on a 5/32 pitch (thats 2.4mm and 4.0mm) and a 35% open area. You needed a very course crush to be able to set the filter bed and then the runoff was just too quick. For a number of years I used a perforated stainless steel plate with 1.0mm holes on a 2.0mm pitch. I worked quite well, had an open area of 28% or so but was only 0.75mm thick and required multiple support points to take the grist load (I used inverted 6mm cap screws) but the nuts were above the plate and were a real pain when removing the spent grist. Wes

    StillDragon Australia & New Zealand - Your StillDragon® Distributor for Australia & New Zealand

  • That's a problem. 1.5mm SS304 plate is actually only 1.4mm thick.
    The holes or slot width must be larger than the plate thickness.

  • @crozdog said: Philter, have you got those around the right way?? 6l grain : 2l water would be a very dry mash which I doubt would drain & not provide any liquid to dissolve the LME. FYI, I usually use around 2-3litre:1kg.

    Bugger, that should have been 6L water to 2Kg grain.

    @Law_Of_Ohms said: Maybe he is making Crystal malt!

    Not me :))

    @Lloyd said: That's a problem. 1.5mm SS304 plate is actually only 1.4mm thick.
    The holes or slot width must be larger than the plate thickness.

    Use 2x plates at 1/2 thickness, and somehow interlock them together for the slots/holes to line up?

  • @Philter, that's brilliant. If it was well thought out you could "dial in" the desired open area. Certainly something to think about.

  • @lloyd, i know you've been busy with other developing products, but wondered if you have had a chance to consider this further?

  • Thanks for the bump @crozdog, I'd completely forgotten about this project.
    We're looking for an 8" filter disc that doubles as a false bottom, right?

  • Hi Lloyd, I think that is what you were originally looking at. I know that a lot of guys use sanke or Euro style kegs for their mash tun. These are 15+ inches in diameter, but I recall seeing 13" FB's being sold for this purpose

    personally I'm looking for a false bottom to go in a 200l drum to replace the large braid I've been using....

  • a 15" full false bottom on a sankey keg is more than worth any price increase over a smaller one I have used both, better efficiency, less likely to stick the mash, etc...

    here is one for a drum at Bubba's Barrels


    800 x 533 - 82K
  • The model with the element port raised base is pretty clever.

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