Quote:
Originally Posted by Steve McVoy
The insulation helped a little, but still the max. temperature is 500 deg F.
The problem is leaks, allowing hot air to rush out the top and cold air to rush in the bottom. I will make a gasket for the door and plug all the holes in the inside of the oven.
|
Yep, 50' of stove-gasket and a pail of refractory cement will serve you well in this situation. Also consider throwing a test cadaver jug in while you're still in the test and measurement phase. (Unless I've missed the part where you may have already mentioned including one.) It may have beneficial or detrimental effect on the convection conditions. I'm certain that an unoccupied oven will behave quite differently to an oven with a large hollow glass object inside.
Another way to look at things is you're going to have a sizeable portion of the volume of the oven occupied by a vacuum, so on paper things will heat up faster with less energy input. Obviously in reality the thermal inertia of the glass envelope more than offsets the 35+ liters of vacuum in terms of required heat input.
Are you planning to measure the cool-down time as well? Definitely do this with some sort of test mass of glass. If it is
too well insulated you might wind up waiting 6 hours for things to come down to the point where you can safely retrieve them to ambient conditions without catastrophic thermal shock. My experience with glasswork consists mostly of laboratory style jointery in pyrex, and some mucking around with soda-lime pieces. I'm unfamiliar with the whole hot-to-not side of the thermal processing on CRTs and the thermal down-ramp might not be anything critical other than "Don't open door above 160C". Still worth considering in terms of "how long will things take on a per-tube basis" since the oven is a one-at-a-time bottleneck. (Or maybe three if they're 8" tubes and the evacuation system is engineered for it.) In the event that some
enthusiast¹ drives a van loaded with all of his dead tubes up from California to be a part of rebuilding them, if each tube requires ~8 hours to oven process he's going to have a LOT of free time to kill given any realistic number of tubes that can concurrently occupy a van.
The literature on the "Champion" plant has all the hallmarks of something you'd find in the text only advertisements at the back of magazines. "Make Money Rebuilding Picture Tubes In Your Garage! Send $5 for our full brochure and catalog to:" type of thing. Sure, we know it works, but its all very rough-and-ready procedures. It does mention "One tube per hour" for the oven, but I think that's a bit generous, even in ideal conditions. I can't find any specific documentation of how long it "should" take. Obviously, "When the vacuum is good enough you can tip-off" is precisely "how long" it takes, but they don't even give estimates for the various size tubes.
On a related note, what are the plans for servicing the pumps prior to the first live-fire testing? High vacuum pumps left idle for long periods, especially if exposed to atmosphere along the way, need servicing before they're ready to suck once more. At the
very least the oil will need flushed. That is to say not just a quick drain and fill job. More like drain and fill followed by a run for a few hours, then drain and fill again, pump to hard vacuum, maintain for 90 minutes, and keep the system airtight from that point on. An outright overhaul wouldn't be totally uncalled for, depending on the condition of the pumps.
I hope that no one here is interpreting my posts as doom-and-gloom nay-sayer "born to fail" etc... To the contrary, I very much want this project to succeed beyond all of our wildest dreams. I'm merely trying to ensure that things are considered from all angles so that the project doesn't wind up dying from a self inflicted foot-wound. If I'm pointing out something that might not be wise, I'm going to do my best to provide a proper explanation, and also a workable solution where applicable. I'm not just sitting on the sidelines trying to shoot everything and anything down for kicks.
¹ We're all mad here!
