New member and first Portable 32" build

[NateQLDer]

Hey all,

I was recently (last two months) introduced to a WFO and it blew me away! where have these been all my life haha My friend had purchased a Jamie oliver portable one, it was amazing so naturally i wanted one. A quick Google showed them to be around 3800$ i nearly fell over! Now i consider myself fairly handy and love a good project so here starts my next obsession (much to my future wife's dismay).

So i have been researching like crazy and have decided on a 32" cast dome type oven, to be built on top of a steel frame with large casters to move from house to house. So far i have measured everything up and built the frame yesterday. From my calculations the weight of the oven should not exceed about 400kg and the steel frame i built should hopefully be good for way more than that ( only 30% bird shit/ 70% quality welds)

So i plan on laying 75mm thick Hebel for a base then 50mm vermiculite, then 75mm firebricks. The dome will be cast 2" thick with polypro fibres, using the sand castle technique. Then 2" of insulation matting, then 2" vermiculite then stucco coat for final appearance.

I do have a few questions so far, is there a specific size that my opening should be? i have heard of a rule for hight of the door but nothing about width? Also do you think for a portable oven that 50mm of vermiculite on top of the Hebel will be sufficient to insulate the base?

I am really looking forward to this build! I will have to wait at least 6 weeks for the next phase as the finance said not before the wedding, so its a waiting game!

Anyways thanks for having me and to Forno Bravo for setting up this forum! Wealth of info here. I have tried to post pics but they were too big so took screen shots and then they too small lol Any suggestions??

Cheers Nate

[NateQLDer]

Finally sorted the pic issue haha

Nate

[NateQLDer]

Update pics. Sorry for the delay, got married and the minister for war and finance cut funding until wedding was completed haha

[NateQLDer]

[NateQLDer]

[UtahBeehiver]

Nice start, there has been some interest on mobile bases lately so posting of your pics and other information will help these folks. Since you are doing a cast oven, I suggest you either contact David S on the Forum or review his post on cast ovens. He is our resident expert on cast ovens and also from Queenlands (Townsville?) He does this commercially and has a lot of insight on methods, supplies, etc. I am assuming you are place the cast on top of the cut fire brick floor, if not and placing on the hebel, this will be an issue as Hebel will become a heat sink. PS, congrats on the marriage and new sidewalk supervisor.

[david s]

Hebel is quite a good insulator as it's full of air, but that also reduces its strength. In addition, because it's made with Portland cement, direct contact with heat is out. I had only 1" of vermicrete between the cooking floor and the Hebel and it wasn't enough. See here https://community.fornobravo.com/for...ion#post398266
you don't say what material you are using for the dome casting, but if you are using a proprietary castable refractory then it will probably already contain the burnout fibres, ask or sieve some to reveal.

Regarding the oven mouth size, providing you make its height around 63% of internal height you can make it as wide or narrow as you like. The wider it is the better acces you have to the oven interior, but also greater heat loss. Around 400 mm is pretty good.

[NateQLDer]

Nice start, there has been some interest on mobile bases lately so posting of your pics and other information will help these folks. Since you are doing a cast oven, I suggest you either contact David S on the Forum or review his post on cast ovens. He is our resident expert on cast ovens and also from Queenlands (Townsville?) He does this commercially and has a lot of insight on methods, supplies, etc. I am assuming you are place the cast on top of the cut fire brick floor, if not and placing on the hebel, this will be an issue as Hebel will become a heat sink. PS, congrats on the marriage and new sidewalk supervisor.

Hey Utahbeehiver, Thanks for the congrats I had originally planned to have the cast dome sit on the cut bricks when then in my haste in prepping the pad i forgot to allow for the thickness of the dome and just went as wide as the inner diameter. So once i laid and cut the bricks i thought i could just have the dome overhand on to the Hebel. That said, now you mention that the Hebel will be a heat sink it will be getting punished by the heat transferred by the dome... Rethink in order. When i say this project will be mobile, its just for home use, portable is probably a better descriptor. As i rent i wanted to be able to move it house to house until i buy my own place and can build a permanent one.

David S, thank you also for your input, i have been meaning to pick your brains for some time. I have seen a lot of your post and learnt a lot ( but clearly i have missed some things in my haste haha). David i was thinking of making my own refractory mix, I think the ratios on this site have generally been quoted as 3:1:1:1 Sand, fireclay, portland, hydrated lime plus with 3% ss needles by weight and 3% fibres correct me please if thats incorrect. Thanks for the info on the door width, i have made it 400mm so good to hear you echoed that, gives me confidence.

So question now is how to rectify the current situation. I see couple ways forward...

1) i could strip off the bricks widen and thicken the vericulite pad two inches wider and another inch say deeper then relay more brick and recut or,

2) i could leave pad 2" thick and run new ring of vermiculite around the current pad and the dome would sit on that. Thickness would be 4" ( i have attached drawing of proposed changes)

Davis how thick do you think the vermiculite insulation layer needs to be? I chose 2" and 2" bricks as the hight of the table was getting up there and i didnt want a step ladder to see inside haha would option two work? Did your Hebel crack as a result of excessive heat as well as vibration from travel? mine won't be moving except once every couple years.

TIA for all the help lads

[NateQLDer]

I have limited time off so elected to go with option 2, hope i haven't made the wrong decision haha So i added a 3" border and packed it in place. The actual dome when cast will now have 10mm sitting on the bricks and the reminder o the vermiculite border to create a barrier between the dome and the Hebel.

[UtahBeehiver]

That will work, what ratio did you use? Support ratio should be about 5:1, dome insulation ratio 8 or 9 to 1When I meant interest in mobile ovens I should of said, moveable.

[NateQLDer]

That will work, what ratio did you use? Support ratio should be about 5:1, dome insulation ratio 8 or 9 to 1When I meant interest in mobile ovens I should of said, moveable.

Hey Mate, glad it should work, its about a 3" border and nice and solid today. I used 5:1 ratio for the mix, 6:1 was used for the original 2" pad. I get you now regarding the mobile ovens bit, I just wanted to better clarify in case david thought this was to be used as a business and carted around continually and that perhaps that and the 1" vermiculite layer he used might have contributed to the cracking Hebel. time will tell i suppose. Thanks again, will post pics again when i get to the next stage.

Nate

[david s]

I think you'll be ok with 2" of vermicrete under the floor. This is usually considered inadequate, but the Hebel itself is a pretty good insulator and the 2" of vermicrete should be enough to protect it from heat damage. The Hebel failure in my build was documented in "mobile deconstruction". I am not sure if it's failure was caused by heat or lack of strength or combination. However as you point out yours will not be subjected to the kind of stresses an oven gets on a trailer and you have double the amount of insulation between the Hebel and the floor bricks so you should be ok.
Regarding your castable mix, I have not tried the homebrew as a castable, but have recommended it to two people who have reported it successful. Must be four or five years ago now. I use a proprietary castable refractory because I make ovens and don't want to risk having lots of warranty issues, but because of cost if I were building a one off oven for myself I'd take the risk of using homebrew.
Your recipe sounds ok and the ss fibres are not essential (they are expensive) Add at least 2% by weight of dry material. They make handing difficult (not called needles for nothing) although their real name is "melt extract fibres" The poly propelene fibres however, are important. Most proprietary calcium aluminate based castables contain some kind of burn out fibres. The poly propelene fibres contained in the Novomesh 950 system of concrete fibre reinforcement , which is a two pack containing polyolefin and polypropylene fibres. Just use the really fine polypropylene ones and use half a handful for every 10 litres of castable,keep the others for reinforcing concrete. To gets these fibres to disperse into the mix properly you need to mix the stuff about double the time you would take to make the mix normally. I find it best to mix in a barrow so you can get the consistency right. It needs to be able to stand up vertically on its own unless you have an outside mould in which cars you can make it more fluid and vibrate it.

[NateQLDer]

I think you'll be ok with 2" of vermicrete under the floor. This is usually considered inadequate, but the Hebel itself is a pretty good insulator and the 2" of vermicrete should be enough to protect it from heat damage. The Hebel failure in my build was documented in "mobile deconstruction". I am not sure if it's failure was caused by heat or lack of strength or combination. However as you point out yours will not be subjected to the kind of stresses an oven gets on a trailer and you have double the amount of insulation between the Hebel and the floor bricks so you should be ok.
Regarding your castable mix, I have not tried the homebrew as a castable, but have recommended it to two people who have reported it successful. Must be four or five years ago now. I use a proprietary castable refractory because I make ovens and don't want to risk having lots of warranty issues, but because of cost if I were building a one off oven for myself I'd take the risk of using homebrew.
Your recipe sounds ok and the ss fibres are not essential (they are expensive) Add at least 2% by weight of dry material. They make handing difficult (not called needles for nothing) although their real name is "melt extract fibres" The poly propelene fibres however, are important. Most proprietary calcium aluminate based castables contain some kind of burn out fibres. The poly propelene fibres contained in the Novomesh 950 system of concrete fibre reinforcement , which is a two pack containing polyolefin and polypropylene fibres. Just use the really fine polypropylene ones and use half a handful for every 10 litres of castable,keep the others for reinforcing concrete. To gets these fibres to disperse into the mix properly you need to mix the stuff about double the time you would take to make the mix normally. I find it best to mix in a barrow so you can get the consistency right. It needs to be able to stand up vertically on its own unless you have an outside mould in which cars you can make it more fluid and vibrate it.

Hey David, thank you for you reply! Glad to hear the 2" should be ok. When you say its considered generally inadequate is that from the heat loss/ need more fuel perspective? will be in treating to see how it retains heat once completed i guess. In regards to the home brew, taking into consideration the 2" desired dome thickness (high dome design) with 32" inner diameter how many bags of proprietary cartable do you think i would need? if its likely to be 3 bags then it might just be easier to buy them and be done with it? I think they are circa 50$ per bag is that about right? One more thing if i could; can i cast the vent piece and entrance separately from GP concrete? or will that have to be refractory aswell? just not sure what temps are likely in that region? If still hot but not critical could perhaps make the home brew for that section??? sorry for all the questions and thanks again for all the advice!!

Cheers Nate

[david s]

First up ask as many questions as you can. We're doing this because we want to share experience and see people have success building their own ovens.
Regarding the insulation, whether it be under the floor or over the dome, generally 3" is what is considered adequate. Going more than that will give you a slight improvement but it's the rule of diminishing returns, so it becomes not worth the expense to overdo it. Remember that the greater the temperature the higher the heat loss and also that moist insulation doesn't work too well. After firing your oven around 30 times, in varying conditions you'll develop an understanding of the ovens performance and appreciation of how it all works, provided you're observant. I'm not sure exactly how much castable you'll require but you can calculate the amount using Vol = 4/3 x Pi x r3, work out the volume of the sphere using centre to outer of the inner shell (r= 16 + 2) the subtract the volume of the inner sphere using inner radius (16") but remember to halve it because it's a hemisphere. I think you'll need way more than 3 bags. The area not required by the oven mouth is approx the same as what you require for its surround, so work the volume out as if there's no door opening. You lose around 20% of the volume when you add water because the stuff compacts when you add the mixing water. It is also way easier to work the maths in metric rather than imperial, because 1000 cubic centimetres = one litre. You are correct the stuff is expensive. It costs me $20/bag in freight alone. If you're concerned about cost you might want to consider using homebrew for the whole thing.

I was catering for a housewarming party last weekend and the new owners had inherited a home built cast oven. On testing it out they couldn't make it work, so got me to do the job with my mobile trailerable oven. I lit their as well as mine on set up and four hours later their oven still had not burned the carbon off the inside. It was also so hot on the outside and under the floor that you couldn't hold your hand on the outside. This means that their oven is basically unusable with no insulation under the floor or over the dome. Impossible to reach pizza temperature lack of insulation means it will not hold heat pumped into it. It also used lots of wood to fire it.

[NateQLDer]

First up ask as many questions as you can. We're doing this because we want to share experience and see people have success building their own ovens.
Regarding the insulation, whether it be under the floor or over the dome, generally 3" is what is considered adequate. Going more than that will give you a slight improvement but it's the rule of diminishing returns, so it becomes not worth the expense to overdo it. Remember that the greater the temperature the higher the heat loss and also that moist insulation doesn't work too well. After firing your oven around 30 times, in varying conditions you'll develop an understanding of the ovens performance and appreciation of how it all works, provided you're observant. I'm not sure exactly how much castable you'll require but you can calculate the amount using Vol = 4/3 x Pi x r3, work out the volume of the sphere using centre to outer of the inner shell (r= 16 + 2) the subtract the volume of the inner sphere using inner radius (16") but remember to halve it because it's a hemisphere. I think you'll need way more than 3 bags. The area not required by the oven mouth is approx the same as what you require for its surround, so work the volume out as if there's no door opening. You lose around 20% of the volume when you add water because the stuff compacts when you add the mixing water. It is also way easier to work the maths in metric rather than imperial, because 1000 cubic centimetres = one litre. You are correct the stuff is expensive. It costs me $20/bag in freight alone. If you're concerned about cost you might want to consider using homebrew for the whole thing.

I was catering for a housewarming party last weekend and the new owners had inherited a home built cast oven. On testing it out they couldn't make it work, so got me to do the job with my mobile trailerable oven. I lit their as well as mine on set up and four hours later their oven still had not burned the carbon off the inside. It was also so hot on the outside and under the floor that you couldn't hold your hand on the outside. This means that their oven is basically unusable with no insulation under the floor or over the dome. Impossible to reach pizza temperature lack of insulation means it will not hold heat pumped into it. It also used lots of wood to fire it.

Hey David, once again thank you for your input. I calculated the area of my half dome previously but perhaps I made a mistake. I have done it again this morning and came up with 26.6 Litres for the dome 32" inner diameter and 2" thick. Using a Boral concrete ready reckoner says 108 bags to 1 Cu Meter (1000Lt) give bag estimate of 2.8 Bags? ... surely that cannot be right?even if i add 20% loss in volume to account for compaction it would only get an extra half bag or so. Does that sound right to you? at 50$ a bag and for four bags i don't mind its not a cost issue. I kinda wanted to make my own brew because i like to make things and to have made a refractory mix that stands up to the heat would be cool But for ease I'm leaning towards proprietary mix.

Once cast the dome will be covered in 2" ceramic insulation and another 2" vermicrete and then a layer of stucco. Im hoping this should be sufficient from the sounds of it to provide a well workable oven which maintains good heat integrity. What do you think? Sounds like that other oven that couple inherited had no insulation is that right?! seems crazy

ADDIT**

So i went out and grabbed a couple bags of refractory cartable from total refractory management here in Brisbane. They are fairly reasonable at 35$ a bag plus gst. High temp 1300 degrees high strength castable. Im making the sand mould now, but heres the question. should i cast the dome as one piece? or separate it into say two or three pieces to give a natural weakness?

Cheers Nate