Carlin burner with auber pid - issues holding temp

[Rocketdc]

I finally got my oven built and have been running a few test cycles to see how everything works. The primary issue I'm having involves holding the oven at 400 for 10 minutes or so.

I had the plumber set the burner to the higher end btu input of around 200,000 btu (I'm guessing) to help get the oven up to temp ASAP. The Carlin EZgas burner has a range of 50,000-275,000btu. As a result the burner cranks and hits 400F in probably under 2mins. The pid then shuts the burner off and I can watch the oven temp creep up to around 425-430 before starting to cool down. When the oven temp drops to 400 it sends a signal to the burner to fire again and the burner begins the 30 sec pre purge. By the time the 30 sec purge has finished the oven temp has dropped below 350 when the burner finally ignites again and quickly gets the oven back up to 400 and the cycle starts again.

Basically I'm getting a swing of around 80 deg from 350-430 and a wait time of well over 60 seconds (two 30 sec burner purges pre/post) in between each firing cycle to maintain the 400F temp for curing. This seems like a really inaccurate way to hold an oven at a particular temperature, but this is all new territory for me and the only other one I've seen was electric. An auber tech has helped tweak some of the parameters, but it hasn't really done much since the two 30 sec purges at the burner limits how fast it can be refired.

Since I have no experience with this setup I don't know if it's normal or some adjustments still need to be made at the burner or on the pid? Is it possible the btu input I have is too high and heating up too quickly? Would an 80 deg temp swing like I described negatively affect powder finish? Any other comments or suggestions?

[Rocketdc]

The oven is typical 2x4 metal stud construction, 20ga galvalume skin inside and out, rock wool insulation, 3.5'x7.5'x5.5' interior dimensions and sitting on the concrete slab. I assume having it sit on the concrete, instead of an insulated floor, isn't helping it retain the heat very well? Didn't expect it to drop so quickly when the burner shut off.

[34by151]

Some Pics of the oven and would help a lot

Bottom line if its rapidly losing heat its not a pid issue
Sounds like you have a good burner getting to temp quickly

Im not sure what the "30 sec pre purge" is about. The burner should be able to restart at any time
When it is off the fuel supply is removed
Start cycle is enable fuel then fore the igniter till combustion is detected
If no combustion after a set time the igniter will stop and the fuel shuts off
Id expect the ignition to take around 1-2 sec and be available immediately after shutdown

Can you explain the " 30 sec pre purge"?

Off the top of my head your issues may be related to the insulation, probe placement, vent placement and size, door seal and more

For reference my hold temp for hours. It takes over an hour to drop from 204c to below 200c
I generally leave the parts in the oven after cooking and crack the door a an hour or so latter
I mostly fire the oven late in the day and leave the stuff in overnight to cool

[rdj357]

I don’t recall the exact settings but I do recall reducing those purge times.

[Rocketdc]

Looks like the purge times were a factory setting. I can set them at 0, 10, 30 sec so I just reset them to 10sec which helps, but it's not much better because the oven still loses temp way too fast. If I can resolve the heat loss issue then I guess that resolves the refiring time since a few seconds wouldn't matter.

I ordered more gasket to help seal the door better, but I'm still at a loss as to why it drops temp so quickly. Most of the hotter areas outside the oven are around the smaller inset glass door.

Here's a few pictures, if it helps to see anything I left out I'll post some more.

[rdj357]

It sounds like a case of oversized burner more than heat loss. Heat is lost in walls and floor and also in parts you put in. The short cycling signals that to me. However I think that I have kine drilled around 175k on my 4’x3’x8’ and it works great hmmm

[Rocketdc]

It sounds like a case of oversized burner more than heat loss. Heat is lost in walls and floor and also in parts you put in. The short cycling signals that to me. However I think that I have kine drilled around 175k on my 4’x3’x8’ and it works great hmmm

The distributor I got the burner from also thought I should bring the btu down to 150k and see what happens. I'll resize that orifice and see what happens.

[34by151]

Congrats on a great build so far

I doubt the heat loss is the burner
Turning it down will only make it heat up slower and not change the heat loss

From your pics it seems like the heat is added from the top rear and the vent is in the lower front
None of this will contribute o the heat loss

Can you show where the the thermocouple probe is located
You may have the probe showing a false reading

I suggest you suspend a thermometer in your oven at different locations and test the oven
Compare these results to what your pid says

You will need to do this anyway to get the temps even
I suspended a bunch of cans to run the test
Once up to temp open the door and check with the non contact thermometer

Getting my oven to heat heat evenly was a bit of mucking about with the burner air flow
After that I did the final tune on the pid
As part of that was the final mount of the thermocouple probe
I ended up with 2/3rd's down and half way back

[Rocketdc]

Congrats on a great build so far

I doubt the heat loss is the burner
Turning it down will only make it heat up slower and not change the heat loss

From your pics it seems like the heat is added from the top rear and the vent is in the lower front
None of this will contribute o the heat loss

Can you show where the the thermocouple probe is located
You may have the probe showing a false reading

I suggest you suspend a thermometer in your oven at different locations and test the oven
Compare these results to what your pid says

You will need to do this anyway to get the temps even
I suspended a bunch of cans to run the test
Once up to temp open the door and check with the non contact thermometer

Getting my oven to heat heat evenly was a bit of mucking about with the burner air flow
After that I did the final tune on the pid
As part of that was the final mount of the thermocouple probe
I ended up with 2/3rd's down and half way back

Thanks, I took some of your advice along with a few others input when I was building it.

Here's some images with notations that show what's located where. The one thing I haven't done is test the heat reading's inside the oven with separate thermometers so I might as well try that next.

I also haven't run a baffle yet to guide the flame up the back wall or finished the oven venting to get it out of the building. I've run tests with the recirculating fan on and off and it doesn't seem to register any difference on the pid regarding the temperature drop.

[34by151]

You should not need the recirculating fan as the burner airflow will do that

I rarely turn mine on, just not needed
Ill clarify that though, the fan is needed for an element oven for a burner oven the air is already forced

Also most of my coating is on industrial parts. I make earthmoving equipment. So most parts a 12-20mm thick steel
When do I use the fan is when I have the odd artwork piece. It make take up some space but is thin
Think 1-2kg of metal vs 500-600kg
So the thermal mass is vastly different
In this case I run the fan opposite to the burner. IE fan starts with the burner shutdown

I was using my oven today, once its up to temp the burner is on for about 30sec then off for about 10min
Its getting ready for the 3rd 30sec burst when the cooking time is done

You can probably improve your airflow
From the looks of it the air coming from the burner will fist hit the opposite side wall and bounce back right at the thermocouple and the vent.
I have a simple baffle across the back of the oven
My inlet is 300mm
So the baffle is about 400 tall and 400 from the back wall
It extends to the other side where it meets in the corner (narrower as it extends across the back)

The aim of this is not to restrict the burner airflow and to direct the air up the back wall.
This make the air go evenly up the back wall as a wide curtain of air and loop back over the roof down the door
The parts then get a fairly even heat from all sides
It also pushes the cold air back to the burner inlet so as to mix the air
In other words it heats from the outside in

You want to have the air hit the thermocouple and vent last otherwise
The thermocouple is reading high and you venting hottest air

On my oven, The thermocouple is not in the line of fire of the airflow
Mine is a bit lower and roughly half way in, I'm roughly above where you have the vent
Its also not recessed just bolted though the skin

Also my internal skin is 3mm zinc steel, I had it in stock at the time and did not have another use for it. Otherwise I would have used 1.6 or 1.9mm sheet. My side walls and roof also have 50mm weld mesh attached to them with about 25mm gap to the wall. I use this to hang parts off the roof and on the side I use rods between the sides to hang parts

The mesh has 2 added benefits.
First it disrupts the airflow on the side walls mixing the air
Second it keeps the direct airflow away from the thermocouple

When I setup the airflow I used a bunch of wool string as telltales. I ran the burner fan with no heat, door closed and checked the flow looking though the window

You may want to do this test

My aim was to get the power to flow on all parts at the same time regardless of the position in the oven
I posted pics inside the other in another thread, if you cant find them let me know and do some more

PS you can also check the probe calibration by putting it in boiling water
I checked my calibration doing this first then by comparing to the probe in my gunblue bath at (320c)
I use the bath to blue excavator pins so the oven pid and thermocouple was checked that it matched the pid/termocouple in the bath

[rdj357]

I doubt the heat loss is the burner
Turning it down will only make it heat up slower and not change the heat loss

….

The advise to lower the capacity of the burner was not to address heat loss but to address short cycling. The controller reads air temp, if the burner is grossly oversized, it will heat the air quickly, cycle off, and the cold oven will absorb the heat causing the quick drop. I don’t know that his burner is *that* oversized but he’s put together a quality build so it certainly could be a factor. I also rarely use the recirc fan on my smaller oven.

[Rocketdc]

You should not need the recirculating fan as the burner airflow will do that

I rarely turn mine on, just not needed
Ill clarify that though, the fan is needed for an element oven for a burner oven the air is already forced

Also most of my coating is on industrial parts. I make earthmoving equipment. So most parts a 12-20mm thick steel
When do I use the fan is when I have the odd artwork piece. It make take up some space but is thin
Think 1-2kg of metal vs 500-600kg
So the thermal mass is vastly different
In this case I run the fan opposite to the burner. IE fan starts with the burner shutdown

I was using my oven today, once its up to temp the burner is on for about 30sec then off for about 10min
Its getting ready for the 3rd 30sec burst when the cooking time is done

You can probably improve your airflow
From the looks of it the air coming from the burner will fist hit the opposite side wall and bounce back right at the thermocouple and the vent.
I have a simple baffle across the back of the oven
My inlet is 300mm
So the baffle is about 400 tall and 400 from the back wall
It extends to the other side where it meets in the corner (narrower as it extends across the back)

The aim of this is not to restrict the burner airflow and to direct the air up the back wall.
This make the air go evenly up the back wall as a wide curtain of air and loop back over the roof down the door
The parts then get a fairly even heat from all sides
It also pushes the cold air back to the burner inlet so as to mix the air
In other words it heats from the outside in

You want to have the air hit the thermocouple and vent last otherwise
The thermocouple is reading high and you venting hottest air

On my oven, The thermocouple is not in the line of fire of the airflow
Mine is a bit lower and roughly half way in, I'm roughly above where you have the vent
Its also not recessed just bolted though the skin

Also my internal skin is 3mm zinc steel, I had it in stock at the time and did not have another use for it. Otherwise I would have used 1.6 or 1.9mm sheet. My side walls and roof also have 50mm weld mesh attached to them with about 25mm gap to the wall. I use this to hang parts off the roof and on the side I use rods between the sides to hang parts

The mesh has 2 added benefits.
First it disrupts the airflow on the side walls mixing the air
Second it keeps the direct airflow away from the thermocouple

When I setup the airflow I used a bunch of wool string as telltales. I ran the burner fan with no heat, door closed and checked the flow looking though the window

You may want to do this test

My aim was to get the power to flow on all parts at the same time regardless of the position in the oven
I posted pics inside the other in another thread, if you cant find them let me know and do some more

PS you can also check the probe calibration by putting it in boiling water
I checked my calibration doing this first then by comparing to the probe in my gunblue bath at (320c)
I use the bath to blue excavator pins so the oven pid and thermocouple was checked that it matched the pid/termocouple in the bath

Do you have a picture of the baffle you installed to redirect the burner flame?

[Rocketdc]

I doubt the heat loss is the burner
Turning it down will only make it heat up slower and not change the heat loss

….

The advise to lower the capacity of the burner was not to address heat loss but to address short cycling. The controller reads air temp, if the burner is grossly oversized, it will heat the air quickly, cycle off, and the cold oven will absorb the heat causing the quick drop. I don’t know that his burner is *that* oversized but he’s put together a quality build so it certainly could be a factor. I also rarely use the recirc fan on my smaller oven.

[34by151]

Do you have a picture of the baffle you installed to redirect the burner flame?

Here is a pic that shows the baffle.
The elements are not used for powder coating.
I use them with the circulation fan for baking paint.
The parts in the pic are painted and ready for baking.

All the seams are sealed with high temp sealant. In the end I dont think that was needed for coating but probably reduces the power used when baking paint.