I'm building a system using a microcontroller (like an Arduino maybe) to automatically control the temperature of a kettle of liquid being heated by an LP gas burner with a PID algorithm to turn the burner on and off.
I'm having difficulty figuring out how to sense presence/absence of the flame so I can deactivate the igniter and then monitor for flame blowout. I looked at flame sensor rods as used in furnaces (rectifier type), but can't find anything on how to interface one to the microcontroller.
I considered using a thermopile, but they seem to be used mostly for pilot flame, so I'm not sure if would work for the main flame. I looked at a fire sensor (transistor based) for the Arduino, but it doesn't detect natural gas flame, so I'm not sure if would work for LP.
I need a solution.
Safety is not a big concern since this will never be used indoors, and will always be monitored constantly while in operation. A common approach for this is to manually ignite the burner and monitor a thermometer in the liquid. The burners are periodically adjusted using a ball valve. I'd just like to make the process a little easier, but have no intention of leaving it unattended. So I guess you could say that adding a
flame detector is just a secondary extra safety measure.
Thanks everyone for your input. I've began analyzing an electric alternative. I'll need to go 220V with around 5500W elements in the liquid, but it is definitely doable. I can use a SSR to feed the power and regulate the temperature. It's definitely a cleaner and simpler alternative, so I may go that way. Thanks again for being so responsive.
Now, I need to finalize on the microcontroller selection. I'm thinking maybe an Atmel. I see there are some opinions already posted on this. Anyone have specific suggestions on models, and rational for the proposed selection?
Best Answer
There are three common flame detectors used for fuel-gas heating systems: Flame Rectification, Temperature Sensing using a thermocouple, UV sensing.
UV sensors are not reliable when used outdoors unless the sensor and the flame pot are completely shielded from outside light. One of the systems that I had to troubleshoot several years ago suffered from exactly that problem - worked when it was on the ground, didn't work when it was up in the air on a lighting truss.
Flame Rectification needs a high-voltage AC source, usually in the 160 Vac range. Not difficult to generate but . . .
Another problem with Flame Rectification is when (not if) the flame-sensing rod becomes contaminated with combustion products from dirty fuel-gas. Easily fixed with some steel wool or other abrasive pad but a maintenance hassle non-the-less.
Temperature Sensing is both inexpensive and reliable. The only problem is ensuring that the thermocouple tip is always within the flame cone.
There are a lot of safety concerns if you are building this for a commercial application. Certification is a hassle and is always expensive. You would need to study the relevant regulations and design both your hardware and software to meet those regulations. Pay particular attention to the requirement for fail-safe operation.