I think I may have asked this a long time ago, but I must have not got an answer because I don’t remember it.
Anyway, The manual that came with my coffee roaster advises that if I roast more than one batch I should wait for a period of time for it to cool before I roast another batch. The machine runs for about eight minutes at 300-400 degrees, and is made of glass and plastic.
Wouldn’t it be physically more stressful for it to heat up and cool down twice than to just go ahead and do two consecutive batches? I’m under the impression that temperature chancge is harder on materials than just being hot, because of the expansion and contraction involved. It appears the problem is deterioration of the plastic parts.
The machine is an iRoast, by Hearthware. It does a pretty good job, by the way.
Please, clue me in.
Peace,
mangeorge
I think it’s more of an engineering question than a thermodynamics question.
Not necessarily. It might be that the extended time at a high temperature would cause warping, more so than the stress of temperature changes. IANAP, so take this with a grain of salt.
Oh.
I like big words.
Big words and shiny things, that’s for me. And smelly stuff.
Good thinking, Cap. Warping is, I think, a result of temperature change, but localized and uneven. Expansion and contraction at different rates over the structure.
Thanks,
mangeorge
How fast does it heat up? It could be that the roaster only reaches 3-400 degrees in the end, but spend some time at other temps. If this is true then not letting it cool would result in overcooking the beans.
I know nothing about roasting coffee though, and warping sounds like it could be it to.
A certain amount of the effects of heat are cumulative, also. Think about it this way: If a recipe calls for baking something at 400 degrees for 20 minutes, I don’t think you’ll get the desired effect if you bake it for 10 minutes, take it out and let it cool off, then putting it back in for another 10 minutes.
But I could be wrong, as I am neither a thermodynamacist nor a baker.
I think that’s it. You’ve got a time/temperature curve that determines the roasting of the beans. If you start from ambient, take 4 minutes to raise the temperature to 400 degrees, hold it for 4 minutes, then let it cool you’ll get a different amount of roasting than you would if the starting temperature was 400 degrees. The controls may be time based.
In addition to possibly screwing up the roasting by starting too hot, it’s also possible that running it twice in a row will result in a higher peak temperature the second time, which could possibly be hot enough to damage the roaster.
I was going to suggest that the designers of the device may have relied on a timer rather than a thermometer to gauge the correct operating temperature. If so they assumed that the inside chamber would start from an ambient temp of around 70 degrees and heat up in T amount of time to produce a max temp of 400 degrees.
If T is always constant then any any ambient temperature significantly higher (or lower) than 70 degrees is going to overheat the beans.
I base this theory on my past as a junior research analyst spending many minutes of study with my sister’s Easy-Bake Oven.
[Nitpick defensive stance] Yeah,Yeah I know. A lower starting temp won’t burn the beans[/Nitpick defensive stance]
Depending on the circuitry inside, the consideration may be having to cool off electronic components, not the mechanical stuff. Semiconductors degrade over time with heat soaking, and the 8 minute window is probably how long any insulation can be guaranteed to protect the circuitry.
When all else fails read the instructions.
The manufacture printed them for your informations. Ignoring them could void any warranty on the machine.
That’s the way it was designed, to be operated on an intermittent basis.
It’s borderline engineering/economics. The machine is made for use in the home, not in a commercial setting where it would see near-constant use. That level of use would require more durable (and more expensive) materials and components; the casual home-roaster would rather purchase a cheaper device.
Toasters are my favorite example of this – you can purchase the expensive top of the line model that’s regulated by electronics and sensors, with all sorts of settings and slots for various toastables…or you can get a cheaper two-slot toaster that’s regulated by a simple bimetallic strip.
(My first thought concerning the OP, however, was that the instructions had to do with properly cooking the beans rather than just letting the machine cool.)
Well lecture me now why don’t you! 
But it does seem pretty obvious that if I hadn’t read the manual, I wouldn’t know about the advisory. I do, in fact, wait the 20 minutes between batches.
The manufacturer’s concern is for the durability of the roaster, and it does have a pretty good electronic temperature control circuitry. There are three time/temp stages in thr roast profile. The electronics are in the bottom of the unit, out of the heat. I get very good results, but the chamber is showing definate signs of heat stress.
My OP is from plain old curiosity about the stress’ on the roaster. I can roast coffee beans in a skillet (not that one ;)) if I want to. Too lazy.
Bolding mine
You are correct, however, the OP was requesting help on understanding what was it about the design that prompted the manufacturer to issue that warning.
Most of the responses addressed possible design limits.
Your “because it says so” response though hit the nail right on the head. :rolleyes:
TWIMC
Overall appliances manufactured for home use, by and large, have similar precautions, warnings, and limitations imposed on their use.
Such products are NOT designed and made for continuous use due to considerations of COST.
You would choose the household over the commercial product almost every time because of the lower price.
An observation of the obvious. A warning is not a reason. I can warn someone not to cross a busy street on foot. My warning is not the reason that they shouldn’t do it. They shouldn’t do it because the traffic is dangerous.
And an explanation of possible design limitations, created by cost savings, was requested by the OP.
Not really relevant to the discussion. There are probably people on this board who have commercial grade appliances in their homes. In fact, though not worded as such, the OP was exploring the differences between his appliance and a possible commercial application.
I am one of those.
If you really want to know, buy one of each type. Ship them to me and I will disassemble them, compare all the components and return them in working condition with a full report of the differences in materials, design, and operational parameters.
Short of knowing what the exact differences are it is a matter of educated guesses in a mad mad world.
No one mentioned Heat Transfer…?
One more time:
I was interested in the different heat effects of running the thing for several batches with versus without a cool down period betweem batches.