# Location to install thermocouple in bread cooking plant



## Signode (Oct 9, 2016)

Hi;


I am working for automation of temperature control system of a bread cooking plant. Currently burner is working on natural gas and LPG. Operator manually opens the NG valve to get the desired temperature. When the pressure of NG drops, operator manually opens the LPG valve to get the desired temperature.
I have installed two control valves one at NG and one at LPG which operates with 4-20mA signal. Fatek PLC has been installed to control the valves operation.



Now problem is that on which location thermocouple should be installed??
Initially i installed thermocouple at stack where exhaust air goes out but at this location a problem appeared that feed back doesnt come quickly. I mean if for example, system opens the NG valve to increase the temperature, the size of flame is increased but feed back comes after 8-12sec. when no feed back within 4-5sec, system more opens the valve.......................


Can anyone guide me where to install thermocouple and how to install. 

An image of burnner area is attached.


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## canbug (Dec 31, 2015)

That looks like an interesting problem. Not sure if you are able to but can you locate the sensor by drilling through the fire brick and get a little closer to where the product is? Maybe a couple of sensors and average them?


Tim.


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## MikeFL (Apr 16, 2016)

I'm with Tim. The thermocouple should be in the cooking area. 

You have NG as primary and LPG for backup as if NG supply is unreliable either due to poor performance of the utility or poor design in the plant where some other process causes an adverse drop in pressure.

Ultimately you want to operate the NG as primary and the more expensive LPG only in the event NG can't meet demand, right?

You need some logic "If NG status = wide open && temp < setpoint, throttle LPG..."

I know nothing about your PLC system or what input options are available.


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## Signode (Oct 9, 2016)

Hi TIM
Nice idea. I have "K" type thermocouple. At stack temperature is around 650c. At your suggested point (closed to product) I think temperature will be around 800-900c. So what do you think the installation of thermocouple closed to product will be ok?
Some people says that thermocouple may get tilt or damage because at upside of it, there will be flame.


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## canbug (Dec 31, 2015)

You need to stay within the thermocouples limits. I wouldn't think you need to monitor the hottest point, just a consistent point, therefore the sensor could be outside the main flame and when you know your output milliamps, you can now adjust your gas valve to keep that temp consistent in that zone.


Tim.


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## joebanana (Dec 21, 2010)

You want the thermocouple at the heat source because that's what you're monitoring. The temperature anywhere in the process is dependent on the source temp. So the closer to the source the better.


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## glen1971 (Oct 10, 2012)

Where is your operator getting the desired temperature? Why not mount it close to where your operator is getting their reading from and see how it works?


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## John Valdes (May 17, 2007)

I bet those fresh baked pita breads are delicious.
Do you have zones or just one heat source? Is this a long conveyor belt? From the picture it seems like TC's protruding from the sides of the oven would be the best.
But what source of heat will they control?
Seems like controlling heat in zones would be the best bet. How long is the oven?


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## Signode (Oct 9, 2016)

Hi
This time there is no feed back. Operator, as per his experience, opens the valve of NG, after specific time, start product in, see the product if cooked or not and then open/close the valve. So no idea about the temperature at cooking zone.
The conveyor is almost 20feet long. Like boiler burner, flam produces in oven, over the product. 
What I got, I would have make a drill in briks and install the TC over the product, under the flam. Kindly guide me as per your experience if existing K type TC would be ok or not.


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## Rora (Jan 31, 2017)

When performing fuel control for burner applications, it's highly recommended that you use several redundant sensors, especially considering that you are using thermocouples and not RTDs... the latter fails to max range, thermocouples fail to 0 C, which would cause both valves to open fully (could be worked around if your alarm trips somewhere between expected room temperature and 0 C).

Two sensors allows redundancy through hi-select control, three sensors gives you 2 out of 3 voting control w/ 2 sensor averaging (PLC picks the two readings which are closest to each other and takes the oven temp as the average).

As far as where to place the sensors, it actually doesn't matter too much assuming you tune with the required offset to reach the desired temp per the operator's assessment. Technically your measured variable here is the ambient temperature where the bread passes through, but if it is closer to the flame then you won't have as much lag time which will improve performance when changing temperatures. On the other hand, too close to the flame and you will pick up disturbances in the fuel flow if it causes flaring and such (decrease or disable derivative controller action if this occurs).

Don't really see any reason why you wouldn't drill holes in the brick. Like others said, just make sure the thermocouple is rated beyond the oven temp. May want to consider some kind of open-end well or mount that could be sealed into the brick, the sensors would then occupy the inner space (this would allow sensors to be serviced and prevent heat loss).


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## splatz (May 23, 2015)

I'd take measurements with thermocouples in various positions simultaneously, see where the temperature cools first when you lower the gas pressure. I would imagine that some low spot far from the flames will show the temperature drop fastest, but that's just a guess, it's better to measure than to guess.


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## Helmut (May 7, 2014)

Signode said:


> Hi;
> 
> 
> I am working for automation of temperature control system of a bread cooking plant. Currently burner is working on natural gas and LPG. Operator manually opens the NG valve to get the desired temperature.( where and what is he using to currently measure the tempaerature?) When the pressure of NG drops,( what's making the pressure of the NG drop?) operator manually opens the LPG valve to get the desired temperature.
> ...


See red


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## Signode (Oct 9, 2016)

Thanks to all for spending time to share nice ideas.


Helmut said:


> See red



Hi Helmut;
As i already stated, currently there is no any oven temp display for operator. Operator, as per his experience, opens the NG valve and after almost 20-25minutes, he passes the bread in oven. according to final product condition, he opens/closes the NG (manual) valve. So all operation is manual. This plant is situated at such area where Natural Gas pressure varies especially in winter. Some time sunddenly pressure drops and operator is known about it when he see the uncooked product. Thats the reason they want to have automation that when NG pressure drops, temperature drops, system open the NG valve up to 100% and if set pint temp not yet achieve, system open the LPG valve.


Checking of oven temperature at different points by thermocouples is not easy job. For which i will have to make holes in bricks.

As per Helmut idea, i think it will be better to install a pipe after making hole in bricks and then install TC in that pipe. *What other members comment? *



As i have already purchased TC input module of FATEK, so i can not move to RTD. For this I think I should use S type thermocouples (0-1700C) which limit is beyond the flame temp (almost 1200C). Two TCs will be installed at two different places to get AVG temp. *What do other members think if it will suffice the purpose or not?*


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## Rora (Jan 31, 2017)

Signode said:


> As i have already purchased TC input module of FATEK, so i can not move to RTD. For this I think I should use S type thermocouples (0-1700C) which limit is beyond the flame temp (almost 1200C). Two TCs will be installed at two different places to get AVG temp. *What do other members think if it will suffice the purpose or not?*


You could do averaging with two sensors, just make sure there is something programmed to handle one of sensors failing... i.e., find out what the difference between the readings are during normal operating conditions. Let's say it's 10 C... have a condition that the temperature must be within, say, 20 C, before the reading from both is averaged. If the readings are too far apart, select the highest temperature (hi-select) instead and trigger an alarm for the potentially failed sensor.


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## Helmut (May 7, 2014)

Signode said:


> Thanks to all for spending time to share nice ideas.
> 
> 
> 
> ...


That kinda makes sense now. He's not adjusting valves to control temp, he is adjusting the valves to achieve a finished product, with the temperature being the unknown variable. 

I would install a thermocouple with a chart recorder or a data logger. You'll need to take readings over a period of time when the product is at it's best and go from there. 

I would start with one thermocouple, middle of the oven, to get readings, and go from there. 

Sounds like a fun opportunity to modify a oven. 

Good luck with it.


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## Helmut (May 7, 2014)

Even better yet would be install the thermocouple and let him run the oven manually for a set time.... a couple days...and plot readings. 

You're in Pakistan?


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## MechanicalDVR (Dec 29, 2007)

From what I've seen two or three spots need to be averaged. 

Good luck trying to get spots where you are reading ambient and not radiated temps from the flame.


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## Helmut (May 7, 2014)

MechanicalDVR said:


> Good luck trying to get spots where you are reading ambient and not radiated temps from the flame.



I think a thermocouple in a pipe would take that out of play, don't you?


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## MechanicalDVR (Dec 29, 2007)

Helmut said:


> I think a thermocouple in a pipe would take that out of play, don't you?


Depends on the proximity of the flame.


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## Wiresmith (Feb 9, 2013)

it sounds like you are using a controller, if it has a derivative adjustment you could increase that slightly and tune it to compensate for the lag


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## gpop (May 14, 2018)

In a idea world the sensor would be as close to the product as possible. In a perfect world the sensor would be inside the product. 

No one cares if the top of the oven is hot next to the flame what we care about is how much heat is reacting with the product. 

You may find controlling the oven in its current configuration near impossible especially if its a single stage/single burner. If the operator is adjusting the whole oven to control the browning in the end stage then its going to be a constant battle as you are adding heat at the start where it is not required to adjust the result at the end.


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## Helmut (May 7, 2014)

gpop said:


> In a idea world the sensor would be as close to the product as possible. In a perfect world the sensor would be inside the product.
> 
> No one cares if the top of the oven is hot next to the flame what we care about is how much heat is reacting with the product.
> 
> You may find controlling the oven in its current configuration near impossible especially if its a single stage/single burner. If the operator is adjusting the whole oven to control the browning in the end stage then its going to be a constant battle as you are adding heat at the start where it is not required to adjust the result at the end.



I think this project is in Pakistan. 

If so,there is probably 2 guys outside pedaling a bike, that makes the conveyor move.


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## LGLS (Nov 10, 2007)

Helmut said:


> I think this project is in Pakistan.
> 
> If so,there is probably 2 guys outside pedaling a bike, that makes the conveyor move.



That's racist and insensitive.


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## Navyguy (Mar 15, 2010)

I tend to agree with the notion that multiple sensors are required. This is not new; there have been productions ovens for decades. Go visit where they make wonder bread or some place like that.

Secondly, while not specific to this situation I have been involved in trying to install sensors in fire brick and other similar materials; it does not work. The slightest flaw in the brick or refractory will act like a chimney and the temperature will soar through there and cook anything you put there.

The best option would be an optic sensor https://www.omega.ca/en/resources/fiber-optic-temp


Cheers
John


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## Signode (Oct 9, 2016)

Helmut said:


> Even better yet would be install the thermocouple and let him run the oven manually for a set time.... a couple days...and plot readings.
> 
> You're in Pakistan?



When initially I installed TC at stack and let HMI to record the temperature, I got Avg 580c.
Now as I already have K type TC, so first i will try to get readings after installation of this TC then will move further.


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## MikeFL (Apr 16, 2016)

Maybe consider monitoring NG gas pressure if the TC doesn't work for you.


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## Rora (Jan 31, 2017)

Placing it in a pipe or in such a way that the element isn't exposed to the air would result in fairly significant delay as the material insulating it would have to equalize with any temperature changes. It could actually be slower than it's current placement in the chimney.

Sounds like drilling the brick isn't going to work, so that leaves you with either infrared or fuel pressure. Infrared would require minor tuning according to the emissivity of your target inside the oven. Monitoring fuel pressure would give you the quickest response, however, to do it correctly the controller would need to calculate the mass flow based on the gas pressure/atmosphere differential and gas temperature in order to estimate the actual amount of fuel being provided.

Since you already have it installed in the chimney, you might as well try the easiest option that Wiresmith suggested and increase the derivative action, see if you cant get it to an acceptable level of performance?


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## gpop (May 14, 2018)

you can install it in the oven as long as it has a 4" nipple welded to the outside of the oven and the probe is long enough to go through the brick. We have done it for years on dryers that reach 1600 F. The nipple allows for the tc connector to be in a cooler area and because the tc threads on to the nipple it stops air flow. We use to jam the nipple with fire cement then insert the tc but over the last few years we skipped that and never had a problem on the connector side. 

The probe may need to be replaced about once every 2 years as the tip will fail. It seems to last longer on gas then it did on oil fired but i would always keep a spare.


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## MikeFL (Apr 16, 2016)

IIRC in a pizza oven the TC is on the side 3/4 of the way back and 1/4 of the way up from the deck.


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## FTC (Oct 10, 2016)

Couldn't you just use a volume chamber directly before the burner train to reduce your fuel issues. I assume your fuel pressure is still lower than the utilities supply pressure. Temp measurement is always going to be slow and should be used as a set point for your fuel flow control.


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## Signode (Oct 9, 2016)

Hello;


By continuous efforts, i have succeeded to control the temp of oven.
When i installed a "Ceramic covered K Type" TC in the middle of oven and seen on what temp loafs were cooked, it was around 700c. But hence temp feed back was as slow as it was on the stack.
I did one experiment, cut the 2" piece of ceramic cover and get opened the TC tip as shown in below image and installed back the sensor and found very good results as required. Temp feed back was as quick as it required by PID loop.
I did also control the speed of chain of oven by PLC by making a speed curve proportional to temp and finally found required results. As the gas pressure suddenly inc/dec, system keeps an eye on oven temp and regulates the gas control valve and chain speed to get the desired cooked product.


Thanks a lot those people who helped me.


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## MikeFL (Apr 16, 2016)

Signode said:


> Hello;
> 
> 
> By continuous efforts, i have succeeded to control the temp of oven.
> ...


Now if you want to refine the process further, you can measure the color of the bread coming off the line like we do here.


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## Signode (Oct 9, 2016)

MikeFL said:


> Now if you want to refine the process further, you can measure the color of the bread coming off the line like *we do here*.



Hello MikeFL;
Nothing attached with your post. Do you want to show some video?


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## Alexander1989 (Sep 17, 2019)

Signode said:


> Hi;
> 
> 
> I am working for automation of temperature control system of a bread cooking plant. Currently burner is working on natural gas and LPG. Operator manually opens the NG valve to get the desired temperature. When the pressure of NG drops, operator manually opens the LPG valve to get the desired temperature.
> ...



Wow looks incredible !


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## MikeFL (Apr 16, 2016)

Signode said:


> Hello MikeFL;
> Nothing attached with your post. Do you want to show some video?


I didn't attach anything to that post. By "here" I meant in the USA.

We use online (read that as "on-the-line") colorimeters for everything from baking McDonald's hamburger buns to feeding Perdue chickens, vinyl siding, paper, glass, etc. In the case of baking bread, the feedback from the colorimeter is used by a PLC to adjust conveyor speed and cooking temperature.


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## paulengr (Oct 8, 2017)

Ovens are notoriously difficult to control because it is naturally an integrating system. This means for instance when you fire the burner it heats up the refractory so it takes longer than you expect to get to temperature then when you try to stop, it keeps going from the heat stored in the oven walls. Then things go completely crazy when the thermal mass changes...like when someone loads or unloads it.

Second problem is it is anything but linear. 90%+ of heat transfer in all industrial processes is by radiation not convection or conduction. So heat transfer is proportional to the 4th power of temperature delta. So doubling the gas (flame) far more than doubles temperature. And since it’s not really convection measuring air except maybe on the walls of the firing chamber is pretty much pointless. You check stack temps for environmental reasons not control,

For this reason most oven controllers are sliding mode or PID but measure wall temperature and operators have to mess with it to get consistency.. Often with PID if you get it to work at all derivative gain is super high like 10x higher, with some P and I. The goal is to stabilize what is effectively an almost out of control system so it doesn’t chase it so much with just enough PI to gradually bring the average in line.

This of course doesn’t get into issues like working with metals where pyrometers start acting crazy because of absorption wavelengths.



Sent from my iPhone using Tapatalk


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