Temperature control

[bradal]

"You can stand in the shadow of a tall building in the day and its still light all around you. This is because air molecuels diffuse the sunlight."

Hmmm...Not because the light is reflecting off of other sources, ie buildings, vehicles, windows????

 

[TConnolly]

Its just an everyday example intended to contrast what happens in a vacuum - push any analogy too far and it obviously fails. However, reflection is the point, light is reflected by air molecules, thats how it gets diffused. Its a no brainer that convection doesn't exist in a vacuum furnace, but failure to recognize that radiant energy does not diffuse, hence it is completely dark in any shadow, is why sometimes a technician doesn't understand how he can have a cold spot in a vacuum furnace when something very close nearby can be over a thousand degrees, or why the outside parts of a stacked load get hot but the center parts in the stack stay cold.

Its also why lunar landing hoax advocates get their photographic analysis wrong.

 

[danw]

What is design intention for heating the control thermocouple in a vacuum furnace? Radiant energy from heaters? Or radiant energy from the load?

Dan

 

[alessio]

I am working on metal heating application. I will be using 6 heaters, 3 phase. I read a lot of good posts here, but i can't find the answer on combination of 5 SSR and 1 PWM(phase width modulation - 4-20mA) PID regulation.

My goal if it is possible: I must reach 80°C in chamber, so at the start of the day all heaters will get on command. When the temperature in TIC100 reaches 70°C , the process should decrease the power of PWM. If the temperature reaches 75°C, the 2nd SSR will go off etc.

I have already made some calculations and there should only 1 SSR will be ON + 1 PWM at medium power at the begging of the process after it reaches the full temeprature. When the application will work at 100% there will be all 5 SSR on + 1 PWM at 50% of power. Then only the PWM will changes.

So i planning to use 1 analog input for temperature input. 1 analog output(4-20mA) for controlling 1 PWM. 5 DO for controlling SSR. 1 DI for controling over heat(T>100°C). Temperature amplitude is 80°C+/-5°C(75-85°C).

Is this possible or should I turn to full SSR(6 SSR) or full PWM(6PWM). Programming PID regulation will be used.

Depending on the PID otuput the SSR will be turned ON. At he 50% for example there will be 3 SSR ON and 1 PWM at low power. And if the value goes up 51%,52% only the 1PWM increase power. This should be done with another PID regulation. So there will be two PID regulation. One for 5 SSR and 1 PID regulation for 1 PWM. If you can see any problem please let me know.

 

[Roy Matson]

As Alaric says it's hard to measure the temperature of a vacuum as there is no gas to conduct the heat, a shiny probe would measure less radiant heat than a dull probe. You need to measure the temperature of the object you are trying to heat. A vacuum chamber I am familiar with for melting steel has a normal flourescent fitting just a few inches away.
Roy

 

[alessio]

PWM(phase width modulation - 4-20mA) = 3 phase pulse width modulation.

 

[Matthias von Zorn]

Why not just use one loop, and use the PID output for both sets of heaters? So, if you want 100% pid to = 50% PWM power, scale the pid output to be 0 = 0% and 100% = 50%; and, then build a table that at certain pid outputs the SSRs come on. So, 0% = 0SSR, 20% = 1SSR, ..., 80% = 5SSR. Dont forget to put some kind of swing latch in there so they dont cycle too fast.

matt

 

[kamenges]

I agree with matt. I don't think you want two PI loops working on this. It will be very difficult to get the switchover right as different DO controlled SCRs come on and off line.

The only good reason to control only one of the SCRs is control resolution. I think matt has the right idea depending on where your resolution bottleneck is. If you have high control side resolution (the PI uses floats or doubles in the calculation) then you can use matt's suggestion directly. Just do what is effectively a modulo on the control signal and send that out to the analog SCR. The others are turned on and off at the modulo transitions.

An even better solotion if you know your process well enough and can model it is to drive the DO controlled SCRs completely open loop based on the calculated heat load. Then let the phase controlled SCR adjust all the time for errors in the model. This tends to get away from the transition issues while giving you the best control resolution. Also, since a significant portion of the heating is open loop it will tend to be more stable.

Keith

 

[dahnuguy]

Vaccum is sometimes counter intuitive for some people, but if you think about exclusively radiant energy transfer for a minute then it makes sense. Without a gas to diffuse radiant energy shadows are completely black. You can stand in the shadow of a tall building in the day and its still light all around you. This is because air molecuels diffuse the sunlight. In a vacuum it would be completely black, so its possible to have hot and cold spots very near each other in a vacuum furnace.

We recently discussed this on a different vacuum furnace temperatur problem in this thread: http://www.plctalk.net/qanda/showthread.php?t=39387

I am just reading along with a question......When I read the vacuum part a bunch of bells went of ..........of course you have covered all my thoughts about lack of convection etc............My question is.....Why not insulate the shell and then run hot oil or some heat conducting liquid through the shell? This would create a thermal mass and heat everything in sight all around from every angle but unless there is also a network of plates or tubes running inbetween each layer , I do not see how you can get even exposure.

Of course I doubt the machine produces a perfect vacuum.

The atuoclaves I worked on got down to 25 or 26 "H2O sometimes on a good day with new seals. And the last few inches took a long time.

So the inability of the machine to produce a true vacuum would actually help here.

So wood is kiln dried in a vacuum huh?

Never saw that process..........why does it need a vacuum?

 

[milldrone]

So wood is kiln dried in a vacuum huh?

Never saw that process..........why does it need a vacuum?

dahnuguy,

Only the woods that have specific temperature related problems, and woods that benefit from moisture gradient problems (provided the gain exceeds the added cost). Vacuum drying is always more expensive than conventional drying (heating + airflow) unless there are specific defects that can offset the added cost.

In a vacuum water vapor pressure drops so the same amount of drying can happen at a lower temperature. Sometimes the drying temperatures in a vacuum kiln are around 40 degrees Fahrenheit. This helps with certain woods that develop drying defects that occur at higher temperatures. Some of these defects happen at temperatures that are around 80 degrees and higher. So without vacuum the process in some parts of the world cannot be avoided.

Here is one manufacturer of vacuum dry kilns' photo gallery

http://www.vacuumkilns.com/content/category/5/16/35/