Crumbling Combuster

[arty]

Hi everyone- newbie here looking for an answer.
I have a VC Federal Large, and several times in the seven years I have used it, the combuster has crumbled and clogged the baffle plate.
What is causing this?

 

[BrotherBart]

One of two things:

1. You are putting unseasoned wood in when you reload with the combuster still engaged. The steam coming out of the wood is causing a rapid drop in the temperature of the combuster and causing "thermal shock" which causes it to crumble.

2. You are running to hot of a fire with too much air and flames are getting to the combuster. It is made to burn gases, not have direct contact with flame. It is called "flame impingement".

 

[arty]

It isn't unseasoned wood. I dry it for one year stacked outside, then move it inside for the spring and summer, to finish.
I usually run it with the air doors fully open during waking hours, and it consumes 2 or 3 4"x20" logs per hour.
Should I bank it down?

 

[BrotherBart]

It isn't unseasoned wood. I dry it for one year stacked outside, then move it inside for the spring and summer, to finish.
I usually run it with the air doors fully open during waking hours, and it consumes 2 or 3 4"x20" logs per hour.
Should I bank it down?

Yep. Houston, we have found the problem. When the surface temp of the stove gets to 500 degrees damp it down (cut back on the intake air) and let the cat provide the heat.

That stove has got to be getting REALLY hot.

 

[Mo Heat]

It's also possible to simply overheat your cat. You can use one of these to prevent that, but they're pricey.

 

[arty]

The manual that came with the stove said the combuster should run in the 600 to 1400 range, and we usually have it at 600 to 800. Rarely will it ever exceed 1000, and I bank it back then.
The baffle seems to be a poor design for keeping the flames out.

 

[BrotherBart]

When you say six to eight hundred are you talking about the surface temperature of the stove or the temperature of the combustor? Most cat combusters will light off at five hundred degrees. The surface temp of a stove is generally about half of what the temp is in the firebox. With a cat you get the surface temperature up to around 250 before you engage the cat. If I remember correctly on the Dutchwest you turn the handle to engage the cat. So, if you are running six to eight before engaging it then you are overfiring the hell out of the stove and the combuster when that thing lights off. And saturating the combuter with flame in the process.

Just my two cents worth.

Mo Heat is the cat expert here and I ask him to correct me if I am wrong. It has happened on occasion. Luckily not with Enron stock.

 

[arty]

There is a gage on top of the stove, with the probe going thru the refractory to the combuster.

 

[Mo Heat]

If it is 'crumbled' it sounds like probably thermal shock. 'Flame impingement' usually tends to peel the catalyst coating off the substrate, although at high temperatures, I guess anything is possible. Brother Bart pretty much covered the root causes of TS above.

Physical shock, on the other hand, usually occurs during transport. It can occasionally occur with extreme expanding or contracting of things near, or in contact with your cat (or cat can), or when something warps from over heating. I don't think it typically results in 'crumbling', though. More likely, it breaks into fairly discrete pieces.

Here is a description of what causes thermal shock from a catalyst maker:

http://www.appliedceramics.com/default.aspx?id=48&action=view&msgid=15

Here is a lot of other good info for cats on the same site:

http://www.appliedceramics.com/default.aspx?id=48

These guys (IIRC), or some other cat maker web site, once had photos of different cats that had been destroyed by different types of stresses. It was very informative, but I can't seem to find it. If you can find those photos and compare them to the damage you witnessed, you might know more.

I once spoke to a Condar Company guy (they made cats for Vermont Castings at that time, maybe still do) and he told me that the most likely reason for cat failure in VC stoves was indeed thermal shock. He didn't specifically mention moisture, though. Interesting, he suggested that reloading the stove with a bunch of 'cooler' temperature wood was likely responsible. I suppose wood can be a cooler temperature for any number of reasons. Any of which could result in cooler exhaust gases 'thermal shocking' a much hotter cat element. Allowing newly loaded wood to char on all sides and really get burning can help reduce this risk.