flame ROD

Jeff Elston

How dose a flame rod work?

<A HREF="http://www.heatinghelp.com/getListed.cfm?id=405&Step=30">To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"</A>

Bruce_17

in a monitor?

Norm Harvey

Do you mean a flame proving rod in a gas burner?

If so, its my understanding that the bulb and capilary are filled with a temperature sensitive material,.. when the flame from the newly lit pilot (on auto pilot systems)or the gas burners, etc,.. quickly heats the fluid inside the bulb and capilary the fluid expands making a switch to allow the gas valve to stay open.

If the switch is not made, the controller assumes the burners did not light, and rather than fill the house with gas, shuts the gas valve.

Much like a cad cell primary on an oil burner. it proves to the control that combustion has been established.

To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"

bob_50

A flame

rod receives an AC signal from the controler and rectifies it to a DC signal and sends it back to the controler.

Lurkin' Murkin'

With a flame rectification system, the presence of a flame actually allows current to be conducted from the sensing rod to a ground source. The ground source can be the burner surface itself (if conductive to ground) or a second rod (which must be larger in surface area than the sensing rod). Ions are formed within the flame that travel between the rod and ground, creating a dc current that the ignition control constantly monitors. Usually, there is a minimum microamp value that the control looks for, to keep the gas valve open.

Glen

yes - but

it is my understanding that it only rectifies a portion of the AC sine wave. So the controller looks for a modified AC/DC signal. And that's why we can't fool it (or even try!) using a variable DC power source.

Unknown

Jeff contact me

by e-mail with your postal mailing address and I will mail you a complete description.

Lurkin' Murkin'

I believe that the voltage applied to the flame rod is AC. The ratio of flame rod to the ground area is an important factor in how well the dc current is produced. A minimum 4:1 area ratio is recommended, assuming the rod and ground are optimally located. The only way to fool this system (in the absence of a flame) is with a resistor and diode in series with the rod to ground. Pure restance, shorting, or temperature increase should not fool a properly designed control.

Bob Harper

Timmie's got it

He can exlain this better than all of us and has the material packaged.

To clarify: The flame rod itself does not convert or rectify the signal. It is the flame itself which allows one phase of an AC signal to make it across the pond so to speak.

You send an AC signal from the module to the flame rod. If the ground has a mass at least 4x that of the flame rod, an attraction for positive ions is setup across the flame forming a bridge. These positive ions in the core of the flame are usually the carbon atoms just liberated during combustion milliseconds away from mating with O2 molecules. Free electrons are the negative ions floating near the tip of the flame and outer mantel and do not figure into this equation significantly. As these positive guys form a chain holding hands across the gap, one half of the AC sine wave is completed so the a narrow band of the positive phase makes it across. On an oscilloscope the wave would look faily flat but be above the baseline, thus "rectified" into a DC signal.

The module has been "trained" to look for a DC microampere flow through this circuit. Just remember, you are not generating voltage from heat as with thermocouples. You are simply completing an electrical signal. Since the current is in miullionths of one ampere, the contacts need to be kept very clean. Because you are using certain ionized portions of the flame, you don't adjust the flame rod's impingement by the flame as you would with a TC. There, you heat only the hot junction and need a 400F delta T WRT to the cold junction so you only heat the tip. Putting a TC into a combustion chamber seems a bit anathema because as the combustion process proceeds, most of the hot junction of the TC gets heated the millivolt production drops off a bit.

With a flame rod, you want a nice portion of the rod just into the blue flame. The tip does not have to be in the flame. The side of the rod can work. It is far more important that this rod be within the ionized zone of the flame. Should secondary airflows distort your flame, you can lose signal to no fault of the electronics. Vent restrictions, unwanted air flows, negative CAZ pressures all can contribute to loss of flame rectification.

Check out HVAC-TV for Timmie's demonstrations.

HTH