Powder Coat Oven Control PID

[julianf]

Could someone clarify -

When using oil burners for powder oven I'm guessing you don't just fire the flame into the bottom of the oven, like you might gas?

Oil burners are not the cleanest of flames and I can't think pumping muck into the oven is a good idea?

 

[printcom]

When using oil burners for powder oven I'm guessing you don't just fire the flame into the bottom of the oven, like you might gas?

That was the first question I asked when planning this build, it appears that is exactly what you do do. I test fired the oven this evening I am pleased to say it got up to set temperature 180c in just over 7 minutes. the PID which I have set as a On/Off controller with a control period of 30 seconds to cater for the burners purge cycle was cycling the temperature between 174c and 181c. I will try to raise it so as to keep the temperature within the 180-200c range. As I have still got a few exhaust leaks and its worse than running your car in a garage with the door closed so I did not do as much as could.
Tomorrow I will seal the leaks and powder coat my first part.

 

[minimutly]

That was the first question I asked when planning this build, it appears that is exactly what you do do. I test fired the oven this evening I am pleased to say it got up to set temperature 180c in just over 7 minutes. the PID which I have set as a On/Off controller with a control period of 30 seconds to cater for the burners purge cycle was cycling the temperature between 174c and 181c. I will try to raise it so as to keep the temperature within the 180-200c range. As I have still got a few exhaust leaks and its worse than running your car in a garage with the door closed so I did not do as much as could.
Tomorrow I will seal the leaks and powder coat my first part.

This will **** you off so much in no time - you must have a medium between the burner and the oven - and a flue to takevaway the fumes. It could even kill you....

 

[julianf]

Our heating here is from a domestic oil boiler. They're never exactly clean inside.

 

[printcom]

This will **** you off so much in no time - you must have a medium between the burner and the oven - and a flue to takevaway the fumes. It could even kill you....

Yep your right, after spending quite a bit of time experimenting with flues. baffle plates, air flow settings and extractor fans (all of which made a positive difference) I cant get this run clean enough. I spat my dummy and have ordered a Eco flam Max gas 40. I will update you when it arrives - hopefully tomorrow.

 

[printcom]

New toys turned up, all I got to do now is work out how to set it up and get a few gas fittings.

 

[Ross365]

I wonder whether you could learn a lot about the way your system behaves by starting off with a really simple controller, like a capillary switch (£10-15)?
Sometimes, doing a quick/cheap/cheerful experiment gives you a far better insight than you would get from days of talking, and thinking and calculation.

 

[timgunn1962]

Most PID controllers can be used in on/off control mode: ie they can work just like a thermostat, but use the analog temperature sensor (usually a thermocouple or Pt100 sensor) and give you control over both the setpoint and the switching hysteresis. With the digital display, you can see how much the process is overshooting.
I’d buy the controller, sensor, etc and have a play. For the burner you are using, I’d use a controller with a relay output because I think the burner wants to see a Volt-Free contact. Check with the burner manual.
NEVER buy a PID controller without reading the manual FIRST, unless you have a decent amount of experience with commissioning process control systems. If you can’t download the manual from a site that allows you to do so without signing up to a lifetime of spam, buy a different controller. If you don’t understand the manual, or at least most of it, buy a different controller.
The reason for this is that you are going to need to make it work. If you run into difficulties, you can ask for help on here, or elsewhere online, with a link to the manual. Someone who understands controllers generally can probably take a look at the manual and hopefully point you in the right direction. If it’s a hassle to do so, they are unlikely to access the manual and you’ll be on your own.

 

[minimutly]

You're still going to be filling your oven with fumes......

 

[graffian]

I thought they double walled the sides and had the burners in the sides. On gas ovens that is.

 

[timgunn1962]

I wouldn't necessarily see the direct firing of the oven interior as a problem with the gas burner. It should run clean enough not to cause any problems with soot, etc. Condensing water vapour might be a problem when loading with cold stuff.

I'd expect the biggest challenge to be getting adequate stability. The burner only seems to run down to 17 kW, so I'd expect it to heat fast. My guess is that it'll need some thermal mass adding to damp out the saw-tooth temperature fluctuations that seem almost inevitable with a combination of low thermal mass, powerful heat source and relatively long output cycle time.

 

[minimutly]

I wouldn't necessarily see the direct firing of the oven interior as a problem with the gas burner. It should run clean enough not to cause any problems with soot, etc. Condensing water vapour might be a problem when loading with cold stuff.

I'd expect the biggest challenge to be getting adequate stability. The burner only seems to run down to 17 kW, so I'd expect it to heat fast. My guess is that it'll need some thermal mass adding to damp out the saw-tooth temperature fluctuations that seem almost inevitable with a combination of low thermal mass, powerful heat source and relatively long output cycle time.

Depends on the size of the oven and how well it's insulated I guess. Starting and stopping will increase the level of contaminants I would say. I really would build a stainless drum, with the burner in the middle, into a chimney, with an outer jacket, and a blower picking up that heat and pushing it into the oven, maybe even have the drum in the bottom of the oven, glowing red. This would give some elastic to the control system.
If you don't do this, you realise when you open the oven, you will be dumping two or three cubic m of carbon monoxide straight into the workshop? Unless you only open it cold of course.

 

[printcom]

After a few interesting days trying to decipher the burners manual and waiting for a Manometer to turn up I think I have got it sorted - ish.

I have used the existing PID set as an ON/OFF controller and the SSR as an un powered circuit breaker. I have set the temperature to 190c and am finding the variation is around 10 degrees with the recirculation fan on. 15c+ with it off? The biggest cause of the temperature variation is the burners 20 second purge time every time it starts up (as yet I have not found a way around this) From cold it takes around 3 minutes to get the oven up to temperature (that’s at the lowest burner setting) with almost no noticeable fumes – I will borrow the CO meter from her training room tomorrow and test things out.

Overall I am well pleased, yes it cost a bit more but at least her indoors will have enough power to use the washing machine when the ovens on….. and I can breath

I have added a few photos - just a bit of tidying up and the jobs a good un.

 

[carbon]

The biggest cause of the temperature variation is the burners 20 second purge time every time it starts up (as yet I have not found a way around this)

Surely this will be controlled by the burner control module and is there to check that the fan is spinning correctly before adding the fuel

 

[julianf]

Surely this will be controlled by the burner control module and is there to check that the fan is spinning correctly before adding the fuel

I don't think that's the reason for it.

I think it has two function -

To clear the firebox of (potentially) explosive vapours, and, on oil burners (which I know this is not) to get the pressure up, so, when the solenoid opens, there is a sharp start to the spray, rather than the dribbling start you would otherwise get.

I've owned very old oil units that don't have this pre ignition cycle - no solenoid either. Evreything just starts up at the same time, pump, fan, ignition electrodes. But none of the modern ones do this.

 

[8ob]

I am no ovenist and my addle brain is struggling with this design, surely it would benefit from the oil burner indirectly heating the oven. A simple steel steel baffled case within the oven that the oil burner fired into/out of would act as a heating element/store. Without the direct air movement within the oven would that not both save on fuel and be a cleaner environment for the coated items it wouldn't take much to add it to the original design.

Bob

 

[printcom]

julianf is correct the pre-purge is to expel any unburnt fumes before ignition. Unfortunately there is too much oven area to clear and all that 20seconds is doing is to reduce the temperature even further by pumping cold air in. I am not sure if it can be adjusted or not, I will make some enquiries next week.
8ob, I tried a baffled case with the oil burner, I too thought it was the obvious answer. Whilst it dealt with the fumes issue it made the temperature almost uncontrollable. I was having to set the thermostat to 160c to stop the temp rising to more than 200c - I ended up with a variation of nearly 50c.
I have spent a bit more time testing today and I am not sure this thermocouple is overly accurate - it tends to jump around a bit and as they are not expensive I will get a replacement.
Overall i am pleased, its been a learning experience (still is) and the end result is a usable oven, hopefully with a few more tweaks it will be an excellent oven. Thanks for all you input it really has been useful.