Control Circuit Help

[Little Bill]

I have a customer that I'm doing a job for not related to my question. He ask, in addition to the job I'm already doing, if I could design him a "simple circuit". Well I'll just post a direct quote from him.

Basically I may be in need of a circuit (or whatever is needed) to control a desired motor (on/off) based upon tempurature readings from a simple thermostat or thermometer? I want to control the temerature in a combustion enclosure within just a few degrees. The motor would feed the fuel to the combustion chamber, hence the need for on/off control

I have, in my previous job designed a few small circuits, but have mostly worked on already built circuits. His requests lack a lot of information to this point, ie: voltage of circuit, type of temperature measurements, etc.
The only thing that comes to mind from what I've dealt with would be a temperature controller such as an Omron controller that uses either a thermocouple or RTD and low voltage (24V) control.
From the limited info I have, do any of you have a easy or fairly simple design for this application? He said he would provide more details later. I just wanted to find out if this is something that I even want to fool with. Any input on this would be appreciated.

 

[rcwilson]

On/off fuel control for a combustion chamber will probably not control within a few degrees. It also might blow up and hurt somebody.

Here's a scenario: motor runs, fuel flows, ignition occurs and chamber heats up to above setpoint. Controller turns motor off, fuel stops, flame goes out. As temperature drops , the controller turns motor/pump back on and fills the hot combustion chamber with a fuel/air mixture that ignites off the hot surfaces and blows the combustion chamber and the building apart.

Suggest the client look at a UL listed burner control system that has pilot lights, ignitors, purge timers, flame safety devices etc. You don't want the liability.

 

[beanland]

Danger & Avoid on-off

Danger & Avoid on-off

I agree, a simple on-off control will work only for a thermal process with huge thermal mass or where the burner is small relative to the heat load so transition time is slow.

If tight control is needed, a throttling control is needed, that varies the fuel & air flow smoothly. However, electric heat would be safer and easier to control if the process is small, the thermal mass low, and the tolerances tight.

 

[ptonsparky]

We worked with some pretty large burners fueled by sawdust. Our on/off was only at the high or low end limits and equipment failure . In between they controlled temperature, both air and fuel delivery, with VFDs. The natural gas fuel alternative was via modulating valve.

 

[RichB]

rcwilson--we called the "lighting off the backwall" in the Navy--pretty "impressive" when done a 1200lb super heated system on a carrier

 

[Little Bill]

Thanks Guys! That's why I decided to post this here. It didn't sound right to me and I didn't want to commit to anything without knowing more. I asked the guy to find out all the details and then get back to me. I will let you know when I find out from him.

 

[Jraef]

Combustion controls have somewhat complex safety considerations even above and beond what's already been mentioned. It's an industry all on it's own. Take a look at "Fireye", they are one of the standards of the industry (used to be part of Allen Bradley but now separate).).

 

[Little Bill]

Totally Different Concept!

Totally Different Concept!

I told my customer I didn't want any part in blowing his shop up and he sent me a reply that is totally different than the way he described his request originally. Perfect example of not saying what you mean, or mean what you're saying.:?
Anyway, here is what he told me about his project:

Ok, not an explosive fuel. It would be wood pellets used in BBQ grills. When temp drops in grill chamber, auger is turned on to place more pellets into the fire. The equation is not known, so it would have to be adjustable to keep grill at desired temp. Example: Grill box drops 5 degrees, so the switch turns on auger for 10 seconds (unknown part) to add proper amount of fuel. Since the temperature reactions to added fuel would be different for each enclosure, the time that the auger is turned on would need to be adjustable. So, the temp would need to be "settable" and the power on/off to the auger would need to be adjustable.

Now, let's regroup and think about this. It doesn't sound too hard, but I'm not a designer. Any thoughts here?

 

[ptonsparky]

They make grills just like that. Might be more reasonable to just buy one. Unless you work really cheap.

 

[__dan]

This is what he wants. Its much cheaper and easier to buy the right product than to try to homebuild something several times wrong.

The lambda control is an excess oxygen sensor in the flue gas. If you can copy these guys, great. If not, just buy the right product and walk away with pride.

http://www.woodboilers.com/product-detail.aspx?id=40

 

[Little Bill]

This is what he wants. Its much cheaper and easier to buy the right product than to try to homebuild something several times wrong.

The lambda control is an excess oxygen sensor in the flue gas. If you can copy these guys, great. If not, just buy the right product and walk away with pride.

http://www.woodboilers.com/product-detail.aspx?id=40

I'm just trying to check on what a customer wants. I don't know what (yet) that he is definitely going to do with this. He surely would know there is a product like that but maybe wants a larger version, I don't know. But if wants it, and I could possible make it, and he's got the money, well.....
Thanks for the link. I will take a look at it in depth later.

 

[kwired]

Ok, not an explosive fuel. It would be wood pellets used in BBQ grills. When temp drops in grill chamber, auger is turned on to place more pellets into the fire. The equation is not known, so it would have to be adjustable to keep grill at desired temp. Example: Grill box drops 5 degrees, so the switch turns on auger for 10 seconds (unknown part) to add proper amount of fuel. Since the temperature reactions to added fuel would be different for each enclosure, the time that the auger is turned on would need to be adjustable. So, the temp would need to be "settable" and the power on/off to the auger would need to be adjustable.

5 degrees is like pocket change at the point of combustion. You would not add very much wood at all and you could have a temperature surge of 100 degrees or more at the point of combustion. The increase in temperature will not be as instantaneous as with a gas fuel but likely will be sudden when it does happen.

 

[__dan]

5 degrees is like pocket change at the point of combustion. You would not add very much wood at all and you could have a temperature surge of 100 degrees or more at the point of combustion. The increase in temperature will not be as instantaneous as with a gas fuel but likely will be sudden when it does happen.

Draft also is a big parameter/ variable. The wood burning appliances with manual draft controls require attendance, tweaking of the draft control, make up air damper, for combustion control.

The Froling has a computer that runs Belimo motorized operators for primary and secondary makeup air dampers and a variable speed draft inducer. There's a lot to it. Their cord wood boiler is beautiful.

 

[George Stolz]

Sounds like someone got a little too excited watching pitmasters. :lol:

Tell him happy hunting, why waste your time on this. :happyno:

 

[Jraef]

Actually, this is what's called a basic "bang-bang" control loop. Not very different from just having a resistive heater element controlled by a contactor. The output would be crude and maintaining 5 deg. tolerance would be difficult without using PID. But to use a PID control loop you would have to be pretty good at programming the controller with only an on-off control output. The lag time to affect the outcome would present a significant challenge.

And if you didn't understand what I just said, you should avoid getting involved. Things like this tend to seem a lot simpler to the end user than they are in reality.

 

[kwired]

Actually, this is what's called a basic "bang-bang" control loop. Not very different from just having a resistive heater element controlled by a contactor. The output would be crude and maintaining 5 deg. tolerance would be difficult without using PID. But to use a PID control loop you would have to be pretty good at programming the controller with only an on-off control output. The lag time to affect the outcome would present a significant challenge.

And if you didn't understand what I just said, you should avoid getting involved. Things like this tend to seem a lot simpler to the end user than they are in reality.

Not to mention that the OP's customer will have a heart attack when he finds out how much it will cost after purchase, necessary peripheral devices, enclosure to house it all, and time to program and fine tune it all. Sounds like he is trying to save a buck vs. purchasing a grill that already has similar controls.

 

[ptonsparky]

I am having second thoughts on this. I just started playing with a couple CLICK Plcs, have a spare AB drive connected to an 18' grain bin fan, a 10 color HMI, another CH VFD rated 1/4 hp. All with Modbus capabilities, of which I need to play with a bit more. I'll need to add an analog card and some thermocouples to average the temp across the grill area...about $4250 for parts. My cost. It will be a learn as I go, parts of it anyway, so I will give a cut rate on programing. Surely what I learned working on 20 Million BTU sawdust burners will relate...somehow.

It would be interesting that is for sure. Doing the same thing over and over is not a challange and if the guy can handle a few burned steaks, go for it.

 

[Little Bill]

Actually, this is what's called a basic "bang-bang" control loop. Not very different from just having a resistive heater element controlled by a contactor. The output would be crude and maintaining 5 deg. tolerance would be difficult without using PID. But to use a PID control loop you would have to be pretty good at programming the controller with only an on-off control output. The lag time to affect the outcome would present a significant challenge.

And if you didn't understand what I just said, you should avoid getting involved. Things like this tend to seem a lot simpler to the end user than they are in reality.

I understand the PID, I had to work on huge ovens and small heat presses that had this type of controller. However, I didn't design or build these, I just worked on them, programming and replacing controllers, thermocouples etc. There is more to the customers application that I don't know.

Not to mention that the OP's customer will have a heart attack when he finds out how much it will cost after purchase, necessary peripheral devices, enclosure to house it all, and time to program and fine tune it all. Sounds like he is trying to save a buck vs. purchasing a grill that already has similar controls.

I am just trying to complete a job that I quoted him on, and he hit me with this. I figured it was the least I could do to just see what he proposed and if it was possible (cost and ability) to do this. I really don't know this guy and just trying to accommodate a new customer.

 

[beanland]

A tad complex

A tad complex

This is a case where knowing your limits is a good thing. I would politely tell the client that this is a bit more complex that you feel comfortable tackling alone and you would like to bring an expert on controls in to work. Offer to get a quote, be willing to build what the controls engineer designs, but otherwise, get out of the loop.

 

[billsnuff]

link to pellet stove manual

link to pellet stove manual

http://www.pelletking.com/pellet-stove-manuals/Breckwell-BigE-Pellet-Stove-Manual.pdf

see page 25 for schematic

enjoy :slaphead: