Structural strength and sheetmetal 1

[Kevin84]

Hi,

Since October 2007, I'm designing a new, quite interesting, project.
This project is the development of a new, custom-made spiral freezer. Dimensions of this spiral freezer are 200" x 180" x 120" (in metric units: 5000x4600x3000mm).

Ok, so far, no problems. For those of you who don't know what spiral freezers are, I've attached some pictures (In the uploaded zip file, you'll find a schematic version of a spiral freezer (blokschema1.jpg to blokschema4.jpg)) which, hopefully, will give you a clue of all the components and the purpose of spiral freezers (= freezing foods / beverages. The spiral allows the dimensions to be quite small, while the time that the product is submitted to the cold, is as great as possible).



Actually, this kind of freezers are made up out of 3 “main “ components:
1) An insulating frame, made up out of panels, which are welded together. (Temperatures within the machine will drop to -100°C, while the temperature on the other side is +10°C.
2) A conveyer belt, capable of turning around the drum.
3) The drum, which ensures that the turn radius of the belt is guaranteed. The drum is colored yellow in the 3D drawings.


The drum (+ it’s motor and reducer) will weigh about 2,5 ton, which is quite heavy.
Certainly when you know that all of its weight will have to be supported by some sheet metal.

Since It’s quite hard to describe, how an insulated panel is made in our company, I’ve attached a sketch (Zip file: detail_sheets.jpg). As you can see, normally we have 2 x 1.5mm (0.06”) thick metal sheets. In between those metal sheets, we inject polyurethane, and pressurize it to about 50kg/m². This gives our metal sheets additional strength and prevents the sheet metal from bending due to its own weight. But still, these metal sheets cannot support the two and a half ton heavy drum.
In normal circumstances, I would have increased the strength by building a frame around it, or welding in/on some I-beams. In this situation, it’s impossible to weld these things inside, or outside the machine. Welding some beams between the 2 metal sheets is also impossible, because this beam would conduct the cold to the outside of the machine. (? ice would start to build up and this is prohibited by law (-100°C may cause burn wounds etc))
So, how do I reinforce these panels? Do note that the minimum thickness of the polyurethane foam needs to be around 90 to 100mm.

I’m able to make the metal sheets thicker (max. 5mm or 0.2” thick), but this is still not “strenghty” enough.

The machine will rest on 6 legs, attached to the side of the machine. As you can see, the sheet metal will certainly bend.

Finally, my question:
Are there any tips or tricks to add a lot of structural strength to sheet metal?
Welding some other metal sheets onto the outer (1.5mm thick) sheets is an option, but a min. distance of 90mm between two sheets is necessary. Welding beams underneath the panels isn’t an option. (Costumer (and especially my boss) doesn’t want it)


I’m getting despaired, so I’m hoping that you guys will be able to help me out…
Kevin

 

[Kevin84]

Here are photos:

 

[hydtools]

Honeycomb core

Ted

 

[swearingen]

Is it possible to corrugate the sheet metal? As long as the corrugations line up, you can maintain your thickness requirement and gain significant stiffness.



If you "heard" it on the internet, it's guilty until proven innocent. - DCS

 

[drawoh]

Kevin84,

Is there any particular reason why your structure must be sheet metal? I design in sheet metal to save weight and to cover up components. This is important if something is portable and/or mounted in an aircraft and/or exposed to dust and EMI/RFI. How much of this matters with your spiral freezer

A space frame out of steel tube, angle or WF is something a civil/structural engineer would understand completely, and it would allow you to get at everything. If you have to cover stuff, the covers can be kept simple, non-structural, and removeable.

The cross-sectional area of a sheet metal structure at the base should be enough to hold your load, if you can prevent buckling. A lot of gussets would prevent this, but the analysis could be a lot of work. Is it worth it?

JHG

 

[TomFin]

How about a combination corrugating or forming the sheet-metal
so that it will add stiffness across the plane that you need it. With this added stiffness you might be able to cookie cut some material out of the sheet, ergo weight reduction.

Tom

Failure is a prerequisite of successful design

 

[Kevin84]

thanks everybody for the replies so far.

Honeycomb structure would be to expensive, i'm afraid. It would be a nice solution though.
Corrugative sheet metal might be an solution as well, I'll look into it.

Drawoh, I couldn't agree more with you, basicly. Normally seen, I would have dimensionized a ridged frame, which is capable of supporting the load.
However, the tubing of the frame is quite expensive. It costs about 1,2€ / m. Drilling in some holes or perhaps soms machining, adds to the cost of one single tube.
Since we have a workshop, with some high end lasercutters, we try to keep the costs as low as possible by using sheet metal. We cut, bend and weld in our own workshop, and by doing so, our price is lower than that of our "rivals".
As you've said, sheet metal and structural strength, don't add up to well. So, in stead of using some glassfiber based insulating material (don't know what's the english name), we use presurized Polyurethan foam. This adds a whole lot of strength and allows us to design very cheap machinery.
While the strength in, for example in a tunnel freezer (our main products), is comming from the foam, the sheet metal (stainless) allows our clients to rinse the machines quite easily. Also Belgium and European law states that the pieces of the machine needs to be, if possible, welded together in stead of being bolted together. Each bolt contains some edges, which are not rinceble. Welds seal up the edges and lock out the bacteria.

As you see, we use the sheet metal to lower our product's cost.
In this case, however, the foam is incapable of supporting the load, and will break, causing the sheet metal to bend.

Temperatures withing the freezer will drop to -100°C (almost -150°F). As the external temperature is higher (+10°C / 50°F), we don't wont any ice to build up on the outside. the foam layer is thus also an isulation material.


Ok, why it is impossible to use a tubed frame on this product:
1) If I should mount the frame inside the freezer, it wouldn't pass the hygienic inspection. (Underneath the tubings bacteria may still grow. Second, tubes inside the cold zone, well... due to the extreme cold and high temperature variations (rinced with steam, 15 minutes later -100°C) they just burst open (they litterly crack, not kidding)

2) Mounting them outside isn't really eye-appealing.
And other manufactures succeed in creating , what appears to be, tubeless spiral freezers.

Since i'm unable to open the uploaded zip file, i'll add the files with hyperlinks.
Machine schematic 1
Machine schematic 2
Machine schematic 3

and how a insulation panel is built up:
panel

Either this reaction has enlighted you all, or i've confused everyone

Thanks for the positive feedback so far.

Kind regards from Belgium,
Kevin

 

[KENAT]

Can't open the links right now but, if you can't corrugate the sheet can you locally bend, crease, dimple, joggle etc to increase I?

KENAT, probably the least qualified checker you'll ever meet...