Problem with reducing flame current only when the appliance is cold. 3

[krstill]

I was wondering if anyone could help me with a situation I have started experiencing.

The appliance will lit correctly, the flame current will rise almost immediately to approx. 6.0 μA.
After 20-30 seconds, the flame current will start reducing over a 30-40 second period to just above the drop-out level of 1.0 μA
After approx. 30 seconds the flame current will start to increase and continue to above 6.0 μA. These appliances will then continue to run without fault.
This behaviour will continue on these units, so every morning from a cold start, same problem.

A basic description of the appliance is as follows:

A rectangular metal fibre burner, positioned 60 mm from the underside of a flat bottomed tank which has a 2 degree draft angle to the rear.
Flue gases exit the combustion chamber at the rear into channels running around the side and rear walls of the tank before exiting through a flue.
The appliance has an ignition electrode and a seperate flame detection electrode.

The flame appears to be stable (visibly) during the reduction and recovering flame current.

 

[Compositepro]

It seems you may be getting condensation on the insulator of the flame detector which then dries out. You may not be getting a good draft in the chimney until the chimney warms up, and this can be allowing moist exhaust gases condense on the flame detector insulator.

 

[macmet]

I wonder if you may just be losing some heat to the surroundings. After running a little while, everything heats up, and no longer absorbs as much heat from the flame, so it returns to "normal".

....but I am not really sure I understand your system. I've only worked on larger systems, where heating the surrounding mass was very important.

CompPro's idea also seems reasonable.

 

[krstill]

Many thanks for taking the time to reply to my post. Compositepro, you hit the nail on the head, 'condensation on the insulator'.

I increased the length between the mounting face and the tip of the ceramic, 3mm of ceramic is now exposed to the combustion chamber instead of being within the insulation surrounding the chamber and 'voila' I now have a stable flame current. Sometimes you just can't see the wood for the trees!!

This is a perfect example of the worth of forums and forumites like this and how they can help people in their hour of need!!

Many thanks again.

 

[rmw]

And thanks to you krstill for coming back and posting the result. All too often folks who devote their time to offering help never know what came of it. Thanks from all of us who post regularly.

rmw

 

[berkshire]

I will second rmw's remark,
It is galling to post an answer, only to find the original poster never even acknowleges that they found it, or that it was useful.
B.E.

The good engineer does not need to memorize every formula; he just needs to know where he can find them when he needs them. Old professor

 

[Compositepro]

Thanks for the feedback. A clearly stated question with all of the relevant information is the main ingredient of solving a problem.