Roasting Drum Material

[alex66]

Can anyone address the type of metals that roasting drums are made of and their benefits or shortcomings.

Is stainless steel worth considering ?

Thank you.

Alex

 

[sainak]

Could you please explain how big is that and where do you want to apply. Even standard steel can be used but you have to consider extension after warm. It will self lubricated by the oil released from the bean so dont worry about rust (if you use it often).
Most of big machine are using regular steel, small toys machine 1lb-3lb and even little bit bigger using stainless steel.
Stainless steel it is suitable but in order to process this material (bending, welding, turning and so on) you need someone with all necessary tools.
Laminated plate in stainless steel are not available every were expecially those with small holes used for drum. If you are not intrested about holes just a standard plate is much more easy.
Bye

 

[tlowing]

To expound on sainak's response: There are several considerations when choosing drum material. With these I'm assuming a traditional drum roaster where the drum is used to agitate the beans as well as transferring heat. Stainless steel and cast iron appear to be common.

http://roasterz.com/?q=node/8

1)Thermal Conductivity: How well does the material transfer heat when heat is applied?

2) Perforations: To enhance convection heat transfer you may want perforations (small holes) in the drum to help the heated air pass through. It is too hard to make so you need to buy it from a manufacturer.

3) Workability: How hard is the material to shape and weld? You need special tools, materials and expertise to weld stainless steel and cast iron is usually cast to shape.

4)Thermal Expansion: How much will the material expand when heat is applied? If it expands too much you may stress weld and damage the bearings if the drum is rotated on an axle with journal bearings. In that case you may want to consider thrust bearings.

5) Corrosion resistance: How much will the material oxidize under normal operation? In most cases the outside of the drum will be of a resistant material to prevent corrosion under normal conditions or it will develop a protective oxide layer. The inside of the drum will be protected by the bean oils.

Further Reading

http://en.wikipedia.org/wiki/Thermal_conductivity

http://en.wikipedia.org/wiki/Thermal_expansion

http://en.wikipedia.org/wiki/Heat_transfer

http://en.wikipedia.org/wiki/Welding

http://en.wikipedia.org/wiki/Corrosion

http://en.wikipedia.org/wiki/Journal_bearing

http://en.wikipedia.org/wiki/Thrust_bearing

 

[sainak]

Thanks tlowing for those very important considerations
If we talk about the external body of the drum, Cast Iron is commonly used for the "back plate" and the "frontal plate" as well, other materials can be used for the rest of this structure.
Cast Iron has different charactestics of extension under thermal stress conditions than FE or stainless steel it may result the internal drum (which is not cast iron) expand more than the external structure can permit with tragical consequences.
Cast Iron is usually cast to shape
It's true and price will be very high for single piece or just few.
After cast, those parts need to be process with milling or lathe machine for final and better accuracy.

Perforated plates for internal drum are available in the market with different thickness -from FE to AISI 304 different protective treatment
(forget about zinc coated plate!!)
Different hole diameter and shape's even flower shape instead of holes -Flowers rotary drum!!! a new revolutionary design!!

:lol:

Bending 2-3mm thick plate to shape your internal drum is not a big deal but according to dimensions you may need some good work shop.

Welding it's a serious matter, metals often may be distort in this operation- your perfect round shape drum it will assume the shape of an egg.
Not that much but something like that.

They are welded then process with lathe machine to ensure circumference and all other measure

Build the external structure and internal drum with same type of material (stainless steel ??)probably reduce risk and less surprise with extension related problems. if you mix different metal with big differences in terms or thermal conductivity and expansion, it will be difficult to calculate those factor and reactions in working condition.
For a drum with medium dimensions you can also consider a non very hard steel.... it's more easy to work, weld and shape.
An appropriate external treatment to prevent oxidation for high temperature.
Appropriate thick plate nothing small or thin that may result at the end too delicate.
For very small drum dimension I would say stainless steel it's one solution.