Tray Oven thermocouple project

Williyb

Member
Join Date
Sep 2003
Posts
35
We have a tray oven thats around 50' long. Its a direct fire oven that uses burners every foot or so, above and below the trays. This oven loads and unloads on the same end. Since we are to cheap to invest in a M.O.L.E to graph our lateral heat problems. I would like to come up with a way to graph our own temps with something less expensive. My idea was to use a roll of thermo wire and hook onto a tray and let it pull through the oven on sitting on a holder we set on each tray. Of course this would only work for half the revolution and would have to be moved over one foot at a time to graph the temps. Anyone have anything better to add or know of a company that sells something that would graph temps over time?

I plan on using a 5/04 and a analog card and let the trending produce our graph for us.
 
If you are thinking of using a thermocouple you will have problem getting an accurate point measurement, the measured temperature of a thermocouple is a function of the temperature difference beween the cold junction and hot junction, if you run a thermocouple through an oven then the temperature measured is a function of the temperature gradient inside the oven and not the temperature at the junction. A 3-wire or 4-wire PT100 RTD is not affected by this.

Just connect to a chart recorder, this will give you temperature against time.
 
Its a direct fire oven that uses burners every foot or so, above and below the trays.

How do you control the gas flow to these burners?
This obviously must have a temperature sensing device/controller, why not use this?

Gunner
 
Profile all lateral points simultaneously.

You'd probably be better off profiling the test points all at once. Just bundle a bunch of TC's together and get it all in one shot. It is a little more cumbersome, but it gets it done quicker. At least you could see the lateral axis at the same time. You just need to hand feed the TC bundle so it doesn't snag. Multi-channel strip chart recorders are inexpensive. How many points do you need to see total? 5, 7, 9, 11 ???

Also consider manual checks with a laser thermometer. We use them to do quick checks on thermal polyethylene film sealing bars. They work good and are inexpensive for quick checks. Grainger sells one for about $90.

Be aware of "heat soak" time. The time it takes the TC to stabilize. I'm guessing 5000 msec or so.

You may want to install permanent sidewall TC's in each zone, so you could do quick checks with your meter.
 
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Its not uncommon to have uniformity issues with gas fired burners. Are you concerned with uniformity within each heat zone, or is this an issue between heat zones? We typically survey equipment with a set up similar to what everyone has suggested so far. Ie, take a tray and basket (if you have baskets) and attach your thermocouples to them. Then feed your wire through as the tray indexes along. It may help to embed the end of the lead in a piece of steel or whatever, to show more. There are many cheap multichannel recorders that would work. So would something like softwaretoolbox's plot pak.

http://www.instrumentationactivex.com/Features/Packages/Plot_Pack/plot_pack.html

Thay still may cost as much as a recorder tho.
 
We've done similar studies on our solder wave machine. We used a DAQ unit with a thermocouple and an 'event marker' (aka toggle switch). We could watch the PCBs travel through the 3 pre-heat zones and the solder wave itself -- so, we just flipped the switch every time the PCB entered a new stage. I'm not sure if you can see into your oven, so that may not work for you. I'm assuming that your oven is causing some type of problem with your product? We always place the thermocouple beneath the most heat sensitive component on the PCB; if they are all rated the same we place it under the largest component -- since that will be the one with the most thermal lag. Whether you're doing a calibration or failure mode investigation, I would recommend taking your data with a typical thermal load and measure the temperature of the parts -- not the air. You may find that you need to 'balance' the temperature of each zone to get the optimal temperature at your product -- ie. 1st zone probably needs to be hotter than the last since the product is cold going in.
 
To answer your last question Spectrum controls makes a card that you can use either rtd's or thermocouples with your 5/04. They are available up to 8 channels

http://www.spectrumcontrols.com/ab-1746.html

I've used the cards and they are easy to set up and work well. Not sure if you want to use the trending program in logix to analyse the data though?
 
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To answer your last question Spectrum controls makes a card that you can use either rtd's or thermocouples with. They are available up to 8 channels

http://www.spectrumcontrols.com/ab-1746.html

I've used the cards and they are easy to set up and work well. Not sure if you want to use the trending program in logix to analyse the data though?
 
Infrared

Is there any way the inside of the oven can be viewed with an infrared camera? That would give you a quick and dirty idea if you have a problem before going through the expense of putting thermocouples everywhere. What is probably more import is to check how uniform the temperature of the material is when you take it out of the oven.
 
We bake many different types of muffins thru our oven. We have a problem with the color of the muffin is from the extreme right and the extreme left of our rack. There are about 38 racks and each rack is around 13' wide. Our burners have three tubes, one marked near one intermdiate and one far. Each has a flow adjustment. The gas is at 0 lbs and the air draws it thru a gas flow regulator that we use to adjust the air/fuel mixture. Our problem is trying to get the flame even from on end to the other and still keep it lit. Problem is I cant really see the flame on the other end very well and it keeps me from knowing at what point we are burning the muffin Since we have 5 different zones with top and bottom burners. Each Zone are not physically seperated, except for the air supplying the burner tubes.

My main concern is to get each burner the same in every one of the five zones. Each zone has a thermocouple and feeds a controller that regulates the air pressure of each zone for low and high fire. My second problem is we dont just use the same burners all the time for eavery product. Each product has a different profile, which calls for different burnmers to be on. Which I dont agree with, I need to be able to check the difference between each side and show it on a graph so they can understand the difference's in the profiles. Plus that would me to keep right burners working. We have about 70.
I want to set the Flynn flame controllers up so the SLC controlls them and not the operator.

We also have a 70' Gemini indirect fired oven that we have run a MOLE thru. This is just a recorder that samples temps with 10 thermocouple inputs. We are then able to download the results and show it on a graph. We use this info to know where we need to open or close the slot in our zone controls. I want the same for the direct fired oven, but was hoping I could just build my own. Last night I brought in my cheap food thermometer and wrapped it up in a oven mit then placed it in a enclosure. I then put that box in another enclosure and stuffed a bunch of oven mits around the first box. I sent this through the oven and found it only reached about 100 deg inside the inner mit. It didnt harm the cheap digital thermometer so I thought I could do the same thing with a 8 input data recorder using thermocouples.

Gunner,
I can contol each zone's heat but I cant control the uniformity of each burner without knowing this data since I only have 3 small windows to see thru. We use these side thermocouples but they are only on one side of the oven and are right next to a burner. Very inaccurate in my opinion. But this data only helps me if I can fix the root problem of keeping the burners lit. If I turn down my near adjustment I run the risk of the burner not staying lit, if I turn my far up I cant see how far to much is. I need a "window" to see whats going on in each zone.
Thanks for all the ideas,
Is there a limit on the length of the thermocouple wire?
 
for doing the temps once i would use your idea of a box with insulkation and a small datalogger with memory and after a run evaluate the outcome.
you could also run a steelpipe al the way and put in the inside a spring (electrician use them and on the end one simple thermocouple. so you can see what happen in the length of the oven.

I woulkd think your main problem is airflow over the product.

as thermal wire is inexpensive you could drill lots of holes and have a wire in each tube.
this way you can make a nice thermal graph and nice overview.

wire maybe very long as long as the resistance of the measuring equipment is very high.
 

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