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Unread postAuthor: POLAND_SPUD » Tue Nov 08, 2011 1:14 am

That way the motor could be isolated from the heat even over 72h and the air stirring assembly won't break the bank, depending on how much time technicians can spare for the fiddling needed to build it.
That's acctually how most high temperature fans or oven fans are built
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Unread postAuthor: jrrdw » Tue Nov 08, 2011 7:56 am

D_Hall wrote:Not sure what you mean with the defuser comment.


A ported tube to carry the heat from the burner and spread it through out the oven.

I think at this point we could come up with better idea's if we knew how big the oven is going to be and it's build configuration...
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Unread postAuthor: D_Hall » Tue Nov 08, 2011 9:52 am

Hotwired wrote:$75 looks fairly low to have a new fan for that temp.

How feasible would it be to improvise a propellor/impellor on a shaft which exits the oven to be chain/belt driven by an external motor.

That's what we've done in the past (and by "past" I mean like the past 20 years)

It sucks. I'm hoping for a better way.

jrrdw wrote:
D_Hall wrote:Not sure what you mean with the defuser comment.


A ported tube to carry the heat from the burner and spread it through out the oven.

I think at this point we could come up with better idea's if we knew how big the oven is going to be and it's build configuration...


Oven is a piece of 24" pipe inside of a 30" pipe with the gap filled with sand. It's 48" long. It's vertical. It's contents are about 18" in diameter and maybe 3" long. Four 3' long heating elements are welded to the inside of the pipe at 90 degree intervals.

The test requires minimal thermal gradients and several thermocouples are used to monitor the temperature in several locations.

At the end of the test, the lid is ejected at a couple hundred feet per second.

It's easiest to mount the fan to the lid. There are better ways from a thermal perspective, but build complexity ramps up very quickly.
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Last edited by D_Hall on Tue Nov 08, 2011 9:59 am, edited 1 time in total.
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Unread postAuthor: Gippeto » Tue Nov 08, 2011 10:14 am

http://www.caframo.com/hearth/hearth_pr ... dstove.php

Would have to separate the fan/motor from the generator portion and rig some mounts.

Bit over the $75 mark too, but that's retail...might get a volume discount from the manufacturer?
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Unread postAuthor: jrrdw » Tue Nov 08, 2011 5:02 pm

Put the heating elements in the sand and give it time to reach the temp needed. Once the thermocouples give the steady needed numbers your ready to do your thing.

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Unread postAuthor: Zeus » Tue Nov 08, 2011 5:21 pm

D_Hall wrote:No, but the fan needs to be at the top of it so I'm not sure how you'd do it with a simple magnetic link.


Is it set up like a ceiling fan? If so, it would be trivial to have a system like so.

Please excuse my using ASCII drawings, they are the most k3wl of diagrams and quite childish, but it's strangely accurate

Code: Select all
                  |
{motor}---[|]--{fan}
    shaft ^   |
             Oven wall



Having a magnet on each side of a C shaped bracket, one of which is driven by the motor, and the other is driving the fan. If it was started slowly, then it would work quite well (I think).
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Unread postAuthor: D_Hall » Tue Nov 08, 2011 9:13 pm

jrrdw wrote:Put the heating elements in the sand and give it time to reach the temp needed. Once the thermocouples give the steady needed numbers your ready to do your thing.

Thread cleaned.


Doesn't work that way. We run a temperature profile; not just a steady temperature. So we say things like, "One hour into the test the item will be at 100 +/- 1 degrees for ALL monitored points. It will then ramp up at a rate of 20 degrees per hour until it hits 200 +/- 1. At which point it will hold at 200 until...."

The temperature ramps and such are actually pretty easy to do. The hard part is getting an EVEN temperature throughout the oven while doing so. It means that it needs to have some pretty significant air flow inside.
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Unread postAuthor: jrrdw » Tue Nov 08, 2011 11:36 pm

I don't picture a fan working as good as every one wanting it to. Think about it, near the blades is going to be hot spot and further away will be cooler.

You can't run a temp profile when using radiant heat like my last idea? If the air isn't being blown around it would reach the same temp across the area, it would have to if heated evenly from all sides or at even spaced elements surrounding the chamber to be heated. What else could it do but rise to a constant temp if the heat is kept on...
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Unread postAuthor: Technician1002 » Wed Nov 09, 2011 12:23 am

D_Hall wrote:The temperature ramps and such are actually pretty easy to do. The hard part is getting an EVEN temperature throughout the oven while doing so. It means that it needs to have some pretty significant air flow inside.


I am not sure if this helps, but on some processes I work with, a distributed heating system can be designed in two stages. One stage predicts heat loss and provides BTU input to balance the loss. Consider this portion static heat loss. The heat is applied to the locations of heat loss such as chamber walls.

The BTU requirement is a simple linear compensation for the heat loss temperature gradient and varies directly by the differential temperature and R value. This is a caculated value and is set by the heat loss rate and modulated by the thermostat.

The second one is dynamic and is again distributed in proportion to the thermal mass of the chamber, air volume, and thermal mass of the load and internal components. On this section, the BTU is calculated to efffect Delta changes in temperature. A rate of rise and a fixed time is used to effect rapid temperature changes. After the program temp change, this is switched off and only the compensation circuit is left on. The thermostat modulates this. On larger ovens, this is divided into zones to reduce settling time. A typical installation has thermal heating blankets on thermal masses in the oven, thermal heated water or oil jacket for the chamber wall, and one or more convection air heaters for the change of the air volume temperature. This amount of complexity can be reduced for smaller environmental chambers and the precision of temperature required. The full bore treatment can settle a new temperature to +_ 0.1 C in about 20 minutes after the ramp interval. How fast of a response do you need?
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Unread postAuthor: D_Hall » Wed Nov 09, 2011 11:09 am

jrrdw wrote:I don't picture a fan working as good as every one wanting it to. Think about it, near the blades is going to be hot spot and further away will be cooler.

Yup. The fan is at the top. Hot(er) air will rise. The fan's job is to force that air back down where it will mix with cooler air.

You can't run a temp profile when using radiant heat like my last idea?

You can, but your system level frequency response takes a huge hit and as a result your control algorithms get more complicated. You're burying the heaters in a large thermal mass that must be heated before the chamber itself will start to see any temperature rise. Further, lacking a cooling system, it can be easy to over temp such a system if you unwittingly put too much energy into the system due to the slow response time of the system.


If the air isn't being blown around it would reach the same temp across the area, it would have to if heated evenly from all sides or at even spaced elements surrounding the chamber to be heated. What else could it do but rise to a constant temp if the heat is kept on...

If your oven has perfect insulative properties, that's a true statement. But this is the real world. Lacking fans, there will be a thermal gradient and the top of the oven will be 5 to 10 degrees hotter than the bottom of the oven (speaking from experience there). With the fans, there will still be a gradient, but it won't be as bad.


Technician1002 wrote:
D_Hall wrote:The temperature ramps and such are actually pretty easy to do. The hard part is getting an EVEN temperature throughout the oven while doing so. It means that it needs to have some pretty significant air flow inside.


I am not sure if this helps, but on some processes I work with, a distributed heating system can be designed in two stages. One stage predicts heat loss and provides BTU input to balance the loss. Consider this portion static heat loss. The heat is applied to the locations of heat loss such as chamber walls.

Makes sense and sounds sexy as hell, but I'm not sure that level of complexity is required. Also can't do the thermal blankets and such that you mention later in your post. There are visual requirements as well. If my specimen is hiding beneath a blanket, the camera can't see it.

As for required frequency response, I'm not the guy to ask. We've other guys that do this (and they do a fine job of it) but I just happen to think their fan system sucks balls. They don't *disagree* but they haven't found anything better either. I was hoping I could find something better.
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Unread postAuthor: jrrdw » Wed Nov 09, 2011 5:45 pm

Ok, someone mentioned magnetic fan. I was thinking (smell anything burning) that the fan could be driven by magnets alone. Using a north to south push to spin the fan. That could leave the heat protection out of it, also eliminate a needed power source that would be running a typical fan motor. Do magnets fail under heat conditions?

I don't know, what do you think? This idea if anything a experiment in perpetual motion... :roll:
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Unread postAuthor: Gun Freak » Wed Nov 09, 2011 5:51 pm

I've always thought about magnetic perpetual motion machines, but don't you think that if it would work, someone would have already made one?
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Unread postAuthor: mobile chernobyl » Wed Nov 09, 2011 5:52 pm

well perpetual motion = fail. And magnets - esp the needed neodymium based magnets for transferring the motion through a thick wall will totally fail around 350*C, so so they may lose some of their magnetism at the prolonged elevated temperatures in Dave's test environment.
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Unread postAuthor: jrrdw » Wed Nov 09, 2011 6:00 pm

Gun Freak wrote:I've always thought about magnetic perpetual motion machines, but don't you think that if it would work, someone would have already made one?


A jeweler (German I think) built a clock using temp change to wind the main spring. So delegate you had to make a good place for it, start it and never touch it again. The clock has a name but I don't remember it. It's the only PM machine that I know of the general public can get (if you can find one), or build your own...
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Unread postAuthor: mobile chernobyl » Wed Nov 09, 2011 6:09 pm

Almost PM, but not really when our relative notion of "400 days" is compared to estimated age (lol...) of the universe...

http://www.atmosclocks.com/history.html

It also uses barometric pressure and temperature deviations to "recharge" whatever mechanism is driving it....

I assume this is what your talking about?

Back to the topic at hand... This device will essentially "explode" after the cooking cycle is over? I saw that the lid will leave at a few hundred fps which makes it a little more interesting...
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