Oven Temperature Controls

Mark Buskell

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Join Date
Sep 2003
Location
Florida
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892
I am working on a 100' oven with 5 zones. Each zone has upto 24 burners. 12 top burners, 6 middle & 6 bottom. Each zone has 1 damper (0% closed to 100% open, 4-20 ma) and 2 combustion blowers (Siemens drives) to regulate the gas to low, medium and high.
We have a themocouple to monitor the temperature in the exhaust duct for each zone.

Normally at the plant they go through a trial and error method turning the burners and on till they get the desired temperature and save this in a recipe.

Now they have requested it would be nice to put in the desired temperature and have the computer decide which burners to turn on and off.

I am thinking we will need mutible rtd's or thermocouplers to even start doing this type of process. This may let me achieve an even temperature in each zone.

Other than that, I have no ideal on the best way to achieve this. Anyone out there got any suggestions.

I will not be able to check back in till tommorrow night. These 12 hour days are getting old.
 
Mark Buskell said:
I am working on a 100' oven with 5 zones. Each zone has upto 24 burners. 12 top burners, 6 middle & 6 bottom.
First of all let me ask you about, purge, low gas pressure, high gas pressure, UV detector etc, in other words safety aspects, are they already setup?, they have to be handled outside the plc.

Mark Buskell said:
Each zone has 1 damper (0% closed to 100% open, 4-20 ma) and 2 combustion blowers (Siemens drives) to regulate the gas to low, medium and high.
There are two ways to control air/gas ratio, one is mechanically, the other one is thru dampers one for gas and other for air (vfd in your case), use a PID output to drive your damper (4-20 mA).

Mark Buskell said:
We have a themocouple to monitor the temperature in the exhaust duct for each zone.
Use that termocouple as your PID´s PV, depending on the PLC you have to scale it.

Mark Buskell said:
Normally at the plant they go through a trial and error method turning the burners and on till they get the desired temperature and save this in a recipe.
No, try to tune your zone , use an option like the one explained at www.controlguru.com, if enough money use an autotuning software.

Mark Buskell said:
Now they have requested it would be nice to put in the desired temperature and have the computer decide which burners to turn on and off.
In my case all my burners are always on, what is controlled is amount of gas and air and controlling temperature as a final result. Use some kind of hmi to setup your temperature.

Mark Buskell said:
I am thinking we will need mutible rtd's or thermocouplers to even start doing this type of process. This may let me achieve an even temperature in each zone.
yes, one rtd or thermocpuple for each zone, in your case you will use 5 zones.
 
First of all let me ask you about, purge, low gas pressure, high gas pressure, UV detector etc, in other words safety aspects, are they already setup?, they have to be handled outside the plc.

All the safetys are set up and handled outside the plc.

This particular oven is cooking dough. The controls are ControlLogix with a ton of profibus devices (point i/o and the drives).

In baking this dough they like it to be more browner on the bottom side than the top. The conveyor goes down the middle of the burners with upper and middle burners on top and lower burners on the bottom side of the conveyor. The lower burners are also closer to the dough.

I agree that I could use a PID to control the dampers, but how does one decide which burners do turn on. If I need more heat, do I turn on burners 1 & 12 first (1 at front of zone and 12 at end of zone).

1....4................... 12
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^
^.. ^.. ^.. ^.. ^.. ^
(----------------------) Belt
^.. ^.. ^.. ^.. ^.. ^ Lower Burners

I am thinking that I will need more thermocouples to raise or lower the temperatures in the top or bottom to achieve the desired browning needed.
 
Mark, there is a new technology called pulse firing and It´s been promoted by www.kromschroder.com, this time instead of changing gas/air ratio to control temperature they use burst to turn on and off a burner (special burners), they have profibus interface so from your your CLX you can turn on any burner when ever you want.

Look for BCU480 unit at their web page
 
I cannot see a reliable way to guess which burners need to be turned on to raise the overall temperature of an oven that big.

You'd basically just have to cut it zones and say "I need the exit end to be a bit warmer to brown up our bread a bit more" or "I need the entrance end to bit a little bit cooler to provide the dough more time to raise before the cooking starts" and "I need the middle to be warmer to get the bread to be cooked more thoroughly".

Does an increase at Zone 1 really increase the exit temperature very much?
 
I have been working a lot of hours and this is the first time I have had time to check back in.

Update:
I have it working but still need to do more testing.
What I did is scale a PID from 0 to 900 degrees. The PV is the incoming temperature from zone 1. I scale the output from 0 to 100. When the incoming temperature starts exceeding the recipe setpoint temperature. The Pid starts outputing from the 0 to 100%.

So at 5% I turn zone 1 combustion blower to medium.
10% zone 2 combustion blower to medium.
and so forth.
At 30% I then turn zone 1 combustion blower to low.
and so forth through zone 5.
At 60% I then start turning off zone 1 middle burners.
At 65% I then turn off zone 2 middle burners.
and so forth.
At 80% i then turn off zone 1 bottom burners.
and so forth.

I ran it for 2 hours this morning and it controlled at 410 degrees. Mostly it needed just to lower the blowers.

I still need to tune my loop to match it to the oven heating and cooling times.

This seems to work OK for controlling the conveyor belt temp.
 
What about when you are running product?

When running product cold goes in and hot comes out. Therefore the faster the line flow and the more dense the product on the conveyor, the more heat will be required to maintain temperature. Do you monitor the rate of product going in?
Will it make a difference?
 
Peter Nachtwey said:
When running product cold goes in and hot comes out. Therefore the faster the line flow and the more dense the product on the conveyor, the more heat will be required to maintain temperature. Do you monitor the rate of product going in?
Will it make a difference?

Mark

Peter brings up something very important
YOU ARE HEATING DOUGH - if you could restrict heating to the dough only it would probably be easier.
You do need to know the rate of dough thru oven and what the BTU demanded by the dough is.

Line speed and if you can get it dough weight per unit time should be one of your algorithm controlling parameters.

The other thing is in these food outfits
Production is GOD - nothing else matters
After you get this thing all tuned up and running they are going to turn up the speed on the conveyer drive to make the production God happy.
Make sure you tell them not to do this
(they will anyway)
AND make sure you know what the rate was when you had it all running. They tweak the speed pot there goes their warranty.
Get some of that tape the QC people use to indicate tweaking pots.


Dan Bentler
 

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