My guess is that the designers figured that no damage would occur to the glass at the relatively low temperatures when starting the process. You could switch to an open loop mode and simply set the output variable to 10% or some other reasonable value. The thermal inertia of the system would then limit the rate of temperature rise. Likewise when the operation was over you could just cut power and allow the oven to cool as fast as heat could escape.
In some cases precise closed loop control is not needed, or maybe the oven was designed with "fuzzy logic", which I don't hear of much today.
I find it funny how engineers sometimes lose face by pushing a sales type question to a sales guy, while a sales guy is perfectly okay to push an engineering question off to an engineer. Talk about a double standrard. Oh well, I guess that''s what we get for being so smart.
Your description of the process is quite similar to some kinds of heat treatment for metals, which I understand. So my assumption about what matters and what does not matter were fairly correct. Thanks for the additional information.
What I did not include was some detail on the process. The glass was loaded to a carrier completely outside of the oven. The carrier was moved into the oven. The heating process ran until the set temperature was reached. The carrier then exited the oven completely and moved into the air-cooling and tempering station. The oven was not actively cooled between cycles.
Rob, yes indeed. But not everybody is willing to tell others that "they have to wait for the boss to tell them" what to say. One time I was in that situation, with every member of our customers group asking me for the same "freeby." That was a commitment that I was glad to defer, although I am certain that they felt that I was "losing face" by having to pass a sale-type of question to our sales person.
It is probably the case that the temperatures not covered were not critical at all, and that on temperature rise full heat was totaally satisfactory, and that on cooldown, once the temperature got down to 200C there was no reason to leave the heat on, just let it coast down to ambient. Sometimes it happens that those who know a process very well don't realize that the uninformed don't have a clue as to what matter and what does not matter. And 9it is quite possible that the gentleman was waiting for a response from the home office and did not dare say anything for fear of committing to make expensive changes.
It may also be that the specified range was where the accuracy was guranteed, but that the range was a bit more, but perhaps not in the 1% accuracy area. That happens also. So sometimes what the unknowing may consider a major flaw is instead a detail not worth considering.
My wife slumps glass in a kiln and, in that process, the warm-up rate is not at all critical for the first couple of hundered degree. Perhaps that may have been the explanation but the resident engineer should have had no problem explaining if that were the case.
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