I was wondering if you could be of assistance.

I am curious as to where a flame sensor should be positioned in the flame.

I have read much literature which describes where it should be positioned but I am wondering how can the white powder coating be avoided.

My belief is that a flame sensor that is glowing red hot will be kept clean and free of white powder.  

Can you provide me with any information on flame sensors, such as how hot the flame sensor should be, or what the white powder consists of, and where it comes from.

The flame sensors need to see the flame, not to stay in contact with the flame, they need to see the flame all time, to adjust you can check the output signal and check for maximum reading, most sensors has a connector to inject clean air to maintain clean the flame sensor view. You can find more information at honeywell: http://europe.hbc.honeywell.com/products/ecatdata/pg_c7027-35-44.html

maybe he meant 'flame rod'

Take the "V" out of HVAC and you are left with a HAC(k) job.

There are infra red flame sensors that only need to get a "look" at the flame,  but I think you are referring to the flame sensors commonly in gas furnaces and equipment.

These types of flame sensor use a process of flame rectification to sense that the flame is lit,  and they must be actually engulfed in the burner flame to function.

Flame rectification uses the fact that a flame will rectify an AC voltage to DC and allow that DC current to flow through a flame to detect a flame.  

Commonly, an AC voltage is applied to the flame sensor with a wire coming from the ignition module.  When the flame sensor is engulfed by a flame, The AC voltage is rectified and a DC current, commonly 4-12 microamps, flows from the ignition module through the wire to the flame sensor,  through the flame to ground on the furnace chasis.

The ignition control module has a circuit to detect that DC current, and commonly closes a relay when the DC current is detected that verifies that the burner is properly lit.  If the burner fails to light or is extinguished for any reason,  that DC current disappears and the control module takes suitable action to turn the burner off.

Flame rods or ionization probes work on the principle that when an AC voltage is applied between the rod and ground a suitable flame and correct placement will cause the AC to be superimposed with a DC component (ie shift the zero axis positive or negative) The flame sensor amplifier responds to this DC component. The magnitude of the DC and hence reliability of the flame operation is determined by a number of things including 1) the rod must be placed in an area where good combustion is taking place 2) The flame must be firmly "anchored" to the burner diffuser which is ground 3)the surface area of the efficient ground compared to the flame rod itself must be in the ratio of 4:1 or better (ie thin rods work best usually made from kanthal.

I wouldn't worry too much about a white deposit, a black carbon is worse because if it tracks thru the carbon to ground you lose all of the DC component and usually you will lockout during purge anyway as the amplifier will detect a leakage AC current. You need a microamp meter to set the rod correctly and if it needs to be bent do it early because once heated for some time it can become quite brittle. The microamp reading needed will depend on the control used. Honeywells only need a few microamps whilst others like Siemens a lot more. They are better to set up because they have a greater margin before it fails.

Also be aware of the flame lifting off the diffuser as it goes to high fire because you lose your ground. If this is a big issue and as long as the flame is still stable and not blowing off a Thick earth rod can be secured to the diffuser projecting into the flame so keeping the ground intact. Also be aware of a DC current reversal when the ignition spark is on and too close to the flame rod.

Hope this helps,
Rod

Rod Nissen.
Combustion & Engineering Diagnostics
 
 

Hi friends from Ecaudor. I'm serching for a diagram circuit for a Flame rod or ionization probe detector. I need to detect the presence of flame with a PLC but i need to make the interface betwen the flame sensor and the PLC.

Fertronic,

Do Ecuador's fire protection codes and industrial insurance regulations allow flame safety control from a PLC?

Why not use a commercially available flame safeguard system (that includes flame detection) that will signal its status to the PLC with either a discrete contact or digital communications?

Dan