Small mobile oven (Is this design feasible?)

[allanw]

Hi everyone, I have been busy doing some initial planning of a build I would like to start in the near future. Firstly, there are a few requirements that I have set for myself:

1. Must be small enough that I won't think twice about firing it up to cook a couple of pizzas.
2. Needs to be mobile.
3. Total width/depth must be less than 1000mm so it can fit through a gate at the front of the house.
4. Should be relatively lightweight to make moving it less difficult.

I have attached a drawing of how I would like it to look. This is what I am thinking so far:

Internal Diameter: 600mm (23.62in)
Dome Material: refractory castable
Dome Thickness: 40mm (1.57in)
Internal Height: 300mm (11.8in)
Insulation Material: Ceramic Fibre Blanket
Insulation Thickness: 100mm (3.93in)

Floor Material: Firebrick
Floor Thickness: 50mm (1.96in)
Insulation Material: Calcium Silicate Board
Insulation Thickness: 100mm (3.93in)

Flue Diameter: 150mm (6in)
Flue Height: Unknown
Flue Location: Inside oven just behind opening

Opening Height: 198mm (7.79in) based on 63% of dome height rule
Opening Width: 396mm (15.59in)

Estimated weight: 250kg - 300kg

Outer Dome Material: Considering fibreglass because it is a lot lighter than concrete render but also worried about the resin catching on fire when starting the fire at the opening. Note that the opening facade will be made of steel. Open to other suggestions.

Stand Material: Square Steel Tubing (painted)

I guess what I am looking for at this point is any possible details in the proposed build that might present a problem and that I should give further thought to before going any further. I appreciate all comments in regards to the design and choice of materials. Once I am confident that I am on the right track I will continue with accurate drawings of the inner dome and start giving some more thought to how I will build the form work for the dome.

[cobblerdave]

G'day
Mobil or movable?
I helped a bloke a few years ago to build a new oven on a brick pallet after he lost his first cob oven in a move.
That oven was moved onto a trailer and headed 900 mm south to Sydney.
And has as far as I know survived.
Regards dave

[allanw]

Hi Dave, looking at making it mobile. To be honest I hadn't really considered the possible effects of actually moving the oven around, but more so whether or not a 250kg - 300kg oven can be mobile enough to move around the yard or into the garage when big storms are on the way. It is good to know that an oven can survive such a long trip though.

[RandyJ]

Are you planning to cast the oven? Otherwise I am not sure how to get it that light with the stand. I know mine is all bricks and concrete and a little over built but I added up the weight and was a little over 8000 kg I know there is a lot of weight to shave from mine but have a hard time seeing less than 5-600 kg. So I hope you have all paved surface or I do not see it moving well.

Randy

[UtahBeehiver]

Allan,
Your proposed build in about the same size as DavidS from QLD, he is one of our old time members, see if he might have some suggestions on weight reduction on a cast oven. If he does not see this post, private message him.

[allanw]

Thank for your reply Randy, that is correct, I am planning on casting the oven. These are the weight estimates I have calculated (some of which are exact based on how much material will be used, some are best estimates):

Calcium Silicate Board 12kg (exact)
Floor Bricks 37kg (exact)
Dome 140kg (based on using 7 x 20kg bags of refractory castable)
Insulation Blanket 24kg (based on 2 x 7.6m rolls, yet to confirm exact length required)
Flue 5kg (based on 900mm flue, may end up being less)
Mortar 2kg (just a guess here but planning on building the dome in one piece so not a lot of mortar needed)
Metal Stand 30kg (based on 14m galvanised hollow square tube)

That brings it to a total of 250kg, excluding the castors, outer dome and some smaller pieces of steel to finish the stand. I am thinking that a fibreglass outer dome and some further optimisation the design might keep it under 300kg.

Thanks UtahBeehiver, I will check out DavidS's build and shoot him a message if he doesn't see this post.

[david s]

Hi Allan, Your plan seems doable but a couple of observations. 1. You don't seem to have included the weight of a supporting slab. in your calculations or are you placing the calsil directly on the stand? 2. I'd say one roll of blanket will be quite sufficient. 3. Don't know how fibreglassing against the lumpy surface of ceramic blanket will go. 4. If mounting the whole thing on castors make sure you have at least two of them lockable. 5. 140 Kg sounds like a lot. How thick are you making the dome? Remember that the heat takes around an hour/inch to penetrate. Most cast ovens are around 2" thick. All up my 21" oven weighs 250 Kg, my mobile version (which is the same size), that I roll on and off my trailer is around 180 Kg because I pinched as much weight wherever I could. Now in hindsight I'm sure I could manage to move 250 Kg but it might be too much for the thin floor on my trailer. See pics

[allanw]

Thanks for your observations David, they are much appreciated. Also thanks for providing me with photos of your mobile oven, it's good to see what is achievable within my weight range.

1. I was planning on placing the calsil directly on the stand, probably supported by sheet metal of an appropriate thickness but I hadn't yet factored in the weight of it.
2. Ok cool
3. Would fibreglassing a mould of the desired shape and then transferring it to the oven be ok once complete? Would it matter that some spots have an air gap between the insulation and the inside of the outer fibreglass dome? Alternatively, how thick did you make the render on your mobile oven?
4. Cool will do.
5. I am making the dome 40mm thick. When I was trying to work out the approximate weight I had trouble finding what volume a bag of refractory castable is good for. When I asked one of the retailers selling it the only answer I got was about 6 - 8 bags for a 600mm oven. I went with the middle ground and factored in 7 x 20kg bags. After reading your reply I went looking for this info again and found a datasheet that said 1900kg of dry castable required for one cubic meter. I calculated approx volume of the dome and worked it out to be just under 80kg of dry material. I am assuming that will be close to the finished weight since the water that gets added will all dry up. This brings the total weight more inline with your mobile oven which is good news.

How thick is the dome on your ovens? Also, the stand that you sit your mobile oven on when it's not in the back of your ute looks like it's made from L shaped steal beams. Could you please tell me what thickness you used and the dimensions of the L?

[david s]

Fibreglass img an outer shell is a fair amount of work, but would certainly reduce weight. You could glass over an exercise ball which can easily be adjusted for size by inflating or deflating it. Cover it with cling film or you may find the resin might react with the ball. There would be no problem having any voids between the glass she'll and the blanket. You could even make it a little larger so there is some space in there and fill it from a hole in the glass shell at the top with dry perlite, vibrating the outside until it's full.
I usually make the inner dome 2", but a little less for the mobile oven as I was wanting to reduce weight. Likewise the outer shell I usually make it 15 mm thick with random fibre reinforcing, but for my mobile I used 10 mm thick reinforced with fine chicken wire.

Trolley for oven is made from 50 x50 x 4 mm galv angle. Steel stand the oven sits on when not in trailer made from the same material.

[allanw]

I'll do a cost and weight comparison between fibreglass and 10mm reinforced render and consider my options for the outer dome. I'd certainly feel more confident rendering it as I have never worked with fibreglass before but the potential weight savings make it worth considering I think.

[david s]

At 10 mm thick the outer shell of mine used around 36 kg of dry material, add a little.extra for the water and chicken wire.

[allanw]

I've spent some time trying to come up with a good design and I am hopefully getting somewhat close. The following areas are where I am not sure if I have got quite right yet:

1. Under floor Insulation - Some searching of the forum indicates that calsil thickness of either 2 or 4 inches seems to be suitable. A fair amount of people using 2 inches appear to also combine with vcrete for under floor insulation so I am thinking in the absence of any other under floor insulation I probably need to go with 4 inches?
2. Dome Insulation - Dave, you mentioned in a previous reply that 1 roll of cermic fibre blank should be plenty but I am not sure if you meant that in regards to one roll being plenty for the 100mm of dome insulation I originally designed for or that 1 roll will produce less than 100mm of insulation thickness but that is ok?
3. Chimney flue size - I did some searching and used the flue calculator but I am not sure what the desired flow rate would be for an oven of this size. Based on the Giardino 60 oven, I have designed for a 5inch diameter flue at 300mm tall.
4. Dome shape - My dome shape is similar to that of the Giardino 60 although I have tried to smooth out the curve to be close to a constant curve without compromising too much on the ability to push the fire right up into the corner of the oven.

Total weight is looking to be in the 180 - 190kg range, with a 10mm render outer shell, excluding castors.

I'd appreciate any comments regarding the above items or anything else that I may have overlooked.

[david s]

Because your oven is small the main problem regarding heat loss is that the door opening area is a much larger percentage of the area of the oven than that of larger ovens. You could narrow down the width of the door opening, but that gives you an oven that's very difficult to work. The upside is that it takes very little fuel to fire so re-firing is not a problem. Insulating the oven conforms to the law of diminishing returns so 100 mm thick of blanket is overkill IMO.if the top of the outer shell is cold with 2" thick blanket another 2" won't do much, but it will increase your oven diameter and require a lot more material (weight) for your outer shell.
Regarding the flue, I think a 5" diam flue should be adequate for your oven, but at only 300m high will not produce a lot of draw. More importantly at that height it won't keep the smoke out of your face which is the primary reason for having a flue. Why not use the full 900 mm the pipe is manufactured to?

If you make your oven mobile use large diam wheels to make it easier to wheel around. I would not go smaller than 100mm and have at least two of them lockable. Also use galvanized steel for the stand and make sure you zinc paint the welds well.

[allanw]

Thanks Dave. I am adjusting my design now with a reduction to 50mm insulation. In regards to the flue, I hadn't yet been able to work out a suitable length so based it off the Forno Bravo Giardino 60 which has a 300mm flue length. If going longer is ok I am all for it.

I am planning on using 2 fixed castors and 2 swivel castors with brakes. I have found some that will do the job with a diameter of 125mm but I am still looking for larger ones. Definitely going to use galvanised steel. Looking at using a combination of 50 x 50 x 2 SHS and 50 x 50 x 3 angle. Thanks for the tip regarding the zinc paint, I wasn't aware that existed.

[david s]

Allan,
On re-reading your plan I want to offer an observation for your consideration. While I am all for pushing design parameters it helps to understand some limits. Most builders and manufacturers consider anything less than 50 mm for castable castings is too thin for strength and longevity. You can counter this to a certain extent by adding stainless steel needles (melt extract fibres) . If your main aim is to create a very light weight oven then you may be prepared to risk a thinner shell. Making it thicker also has the advantage of more thermal mass which is an advantage if roasting or baking. If you only want to use your oven to cook pizza then this advantage does not count. Designing an oven is very like boat design eg. make it light with a flat bottom will make it easier to haul, economy and great in estuaries but lousy in open water and via versa.

Regarding your castors and stand, I think you have it about right. The wheels will have a rated loading so make sure they well exceed the load you putting on them. The terrain you are wheeling the oven over will be the limiting factor for manouvreability. If it's smooth and level even 300 kg would be OK.

I usually make the outer cement shell of my ovens 15mm thick and only went to 10mm for my mobile because I was worried about the weight for the trailer. Keeping it down to 10 was way more difficult and I prefer 15mm every time.