Anyone know anything about SS flue pipe?

[Albedo]

Thanks Alycidon.and yes it does help:001_smile:

 

I haven't checked your links yet... but with regard to keeping it in the garage etc.

 

I tested customers logs that had been in the garage when I delivered mine. As you say, these logs that had been 'indoors' for ages tested at the same 18% MC as mine on the outside.

 

I accept that they may retain their summer MC on the inside.

 

Next winter I will take some of these logs and cut them in half and see. Also do some tests this summer to see what the best achievable MC is. I have done none of what you suggest but it does make sense.

 

It raises the question as to how long a summer MC can be maintained through the wood, through the winter.

 

 

Edit: I have cut logs in half before to get the internal MC, but only my outside stored ones and it was 18%. I have not done the above test before which is different.

 

My moisture meter is poised and at the ready... there is science to be done.

Edited June 23, 2012 by Albedo

[Albedo]

I just went and tested a few of my outdoor ones that are over a year old and nice and dry..as in protected from rain. I'm getting 16% on the outside. I am too lazy to cut one in half right now but will get around to it.

 

This thread is not all about MC however so I'm happy for you to leave me to play around with it for the next 6 months:001_smile:

Edited June 23, 2012 by Albedo
too many howevers

[Le Sanglier]

Ok, been keeping an eye on this thread, so now I am jumping in, I have two woodburners and have encountered many problems and think that I picked up some info along the way. 1;single skin pipes will always clog up, the water in the smoke condenses quickly to form sludge, angles make it worse, I know because this happened to mine until I replaced the whole lot; built an external chimney, lined it and capped the top. Only sweep it 3 times a winter instead of every week!

The key is to get the smoke out before it cools.

[openspaceman]

 

As Stereo says, you've got to dry it artificially and then avoid ambient MC to get and keep 15% MC.

 

Even then the energy used to do the drying is always more than necessary to just evaporate the water.

 

Some of the water in wood is "free" water and some weakly "bonded", as wood dries the free water migrates out quite easily and the wood does not change size or shape much. After that (aka the fibre saturation point) the moisture removed is associated with the cell structure and its loss causes the structure to shrink. most of the shrinkage is tangential, which is why you see radial splits in a round log, then a lesser amount of radial shrinkage and very little longitudinal shrinkage. The ratio of the tangential to radial shrinkage is an indication of how stable the wood in a plank will be and is why woods like oak are better quarter sawn to avoid the differential.

 

As wood dries below 25% mc wwb this bonded water is in equilibrium with moisture in the surroundings. As the capacity of the air to absorb moisture goes up ( i.e. its relative humidity goes down) moisture migrates through the wood and is evaporated. There is an equilibrium established between the wood and the air. In my house the rh in summer is around 55% and the temperature now is 18C, the equilibrium moisture content of wood will be around 10% on a wet weight basis on a wet day with no heating in the winter it will be around 15% ( given time for the moisture to migrate through the wood to the surface)

 

An interesting side effect of the water bound to the cell structure having a weak bond is that there is a small energy change in addition to that needed to evaporate the water, this shows up in a hysteresis in the equilibrium moisture content depending on whether the relative humidity is increasing of decreasing.

 

For most of our burning purposes these effects are insignificant and drying to the fibre saturation point is the most bang for buck.

If we make some simple assumptions, like ignoring the ash produced and allowing a flue gas exit temperature of 150+C to avoid chimney problems then a simple formula for the energy we release and have available for heating will be:

 

hardwood 5.2kWh/kg oven dry wood-0.75kWh/kg of moisture

softwood 5.5kWh/kg dry wood -0.75kWh/kg moisture

 

So a cubic metre of scots pine weighing 1 kg green with 0.4kg of dry wood in it and 60% mc will yield about 1.88kWh from the stove if burned efficiently, dry it to 30% mc wwb and it should give 2.6kWh, a 17% difference.

 

As others have pointed out the advantages of drying below the fibre saturation point are largely to do with clean burning rather than energy gains. Whilst advocating burning wood as dry as possible I don't think much of a burner that cannot cope with 25-30% mc wwb wood.

[Stereo]

I think the idea of the Esse is that it burns so slow that it makes the most out of the fuel and needs little tending during the day. If I have super dry dead elm which has been barn stored and is about 14-15% in the middle, I can chuck a big log in at 7am and won't need to refuel until the afternoon.

 

The issue with that is that if the wood is not perfectly dry it just smoulders endlessly and clags the thing up.

 

On the flue side, the top part (past about a meter high) of single skin on mine gets seriously clagged after a month or so. The 5 meters or so of stainless above that does not, it just gets that fine dust which is a doddle to clean. Last summer I dismantled the lower section myself sue to aforementioned sweeping issues and the SS from the cieling up needed a quick brush and was mirror shiny on the inside. The single skin bend just below and the pipe below that needed attacking with a rotary wire brush and chisel.

 

I think the flue is just wrong and was done on the cheap.

[T14EES T14UNK]

This is what I have:

 

Chimney sweep brush, chimney rods, flue brush

 

Can these have a drill attachment on them to spin it as you add pieces and extend further up the pipe.

I thought about getting some but I seem to remember them being a lot more than these you have.

[Alycidon]

I tested customers logs that had been in the garage when I delivered mine. As you say, these logs that had been 'indoors' for ages tested at the same 18% MC as mine on the outside.

 

Had they been split ?, bark does not allow moisture out.

 

I accept that they may retain their summer MC on the inside.

 

Next winter I will take some of these logs and cut them in half and see. Also do some tests this summer to see what the best achievable MC is. I have done none of what you suggest but it does make sense.

 

Once November gets here if your wood is kept in a barn then MC will have drifted up but at 18% or so I would be happy to send that out.

 

It raises the question as to how long a summer MC can be maintained through the wood, through the winter.

 

A few weeks, just depends on the weather conditions, damp fog and mist is the worst. MCs constantly move and reflect the weather.

 

Edit: I have cut logs in half before to get the internal MC, but only my outside stored ones and it was 18%. I have not done the above test before which is different.

 

My moisture meter is poised and at the ready... there is science to be done.

 

I am happy to send softwood out at up to 20%, as its less dense at 15% it burns very hot and very fast, customers get a bit twichy so addign a bit of damp slows the combustion down a shade.

 

A

[openspaceman]

The key is to get the smoke out before it cools.

 

The key is to burn smokelessly, which means maintaining a flame.

 

Failing that you are right, the smoke needs to be exhausted before it can condense onto the flue walls. This means keeping the temperature above the dew point of any condensible tars in the smoke (just about impossible) and water (a bit more than 100C) all the way up the flue. As the flue has a thermal mass it's always going to be cold enough at start up. A thin SS flue which is insulated will get to temperature faster than a brick flue.

 

I've seen single skin ss perforated by corrosion from being run too cool, the solution was to insulate it.

 

It's the need to prevent condensation in the flue that makes one reason a woodburner less efficient than a condensing gas boiler.

[Stereo]

I've seen several installations recently with double wall ss right down to the stove. Are there not temp issues with this? I couldn't do it on the esse as you can't sweep from inside the machine but many burners you don't need an access plate. I'm thinking of changing our villager a flat for a smaller, more efficient one and running ss right down to it would allow me to put in a nice big oak lintel I have but which would currently be too close for single skin flue.

[openspaceman]

I've seen several installations recently with double wall ss right down to the stove. Are there not temp issues with this?

 

Most commercial boiler have insulated ss directly above the heat exchanger, the reason is that the heat exchanger is designed to take out all the heat it can from the flue gases and then discharge them at the lowest temperature that's acceptable, otherwise they are wasting heat up the flue. They will typically have a thermocouple in the flue at this point and modulate the chip feed to keep this near a set temperature, often about 150C.

 

Many wood burning stoves simply do not have enough heat exchange area ( the stove body) to extract enough heat, hence the need for higher spec ss for solid fuel stoves compared with gas boilers, also because of the increased likelihood of a chimney fire.