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[kiwimodeller]

After reading recently that drops of water on a ceramic burner can cause it to crack a friend experienced it happen and so decided to convert the burner on his vertical boiler to stainless steel gauze. Having done so it appeared to work fine at first but then after a few minutes running on the bench it flashed back and began burning at the primary air holes. Subsequent playing around with different sizes of opening in the mesh, single and multiple layers of mesh and different positions of the jet have not fixed the problem. Sometimes it will flash back soon after being lit, other times it will run for 10 minutes and then flash over. The flame is a nice blue colour and does not appear unstable and the exhaust has been removed from the funnel so water no longer gets in to the burner. Any ideas on how to fix this are welcome as is discussion on whether the cracking of the ceramic in the burner of a vertical is really a problem or whether he should go back to ceramic. Thanks, Ian.

[gondolier88]

Advise your freind to stick with ceramic and find and remedy the problem of water finding it's way into the burner- thats far more worrying!

Greg

[kiwimodeller]

Greg the water was only condensate from having the exhaust exiting inside the funnel. That is fixed by moving the exhaust but the magazine article he has read says you are better to have stainless gauze rather than ceramic so he wants to do that if he can get it to work. Also it is not that easy in this country to find replacement ceramic material. Cheers, Ian.

[gondolier88]

Hi Kiwi,

As I said he should really look why he has water being carried over- does he have a condensate/oilseparator?

Ceramic is very easy to come by- find your local gas service engineer and ask him if he has any old gas fire radiants- the backs of these are solid ceramic- use a fine coping saw blade to cut to the required shape and drill 1.5mm holes at 3mm intervals allover it.

Greg

[gondolier88]

PS= failing that Forest Classics supply ceramic burners.

Greg

[Bee]

Too many holes! If you look at a ceramic burner the area of hole to solid is very small. Gauze is radically different so there is no back pressure through the gauze. Are you sure the idea is to use gauze directly as a replacement. A classic burner has say a tube with slits (narrow orifices are more difficult for the flame to flash back through) and the steel if used is heated by the flame to glow and disperse radiant heat just like ceramic does in a domestic gas fire.

[Proteus]

Bruce sells ceramic for it page 50 of his downloadable catalog. and will post abroad if you cannot get it local, but you need to sort the problem out first, when was the boiler last tested?

http://www.pollymodelengineering.co.uk/sections/bruce-engineering/price-list.asp

Proteus

[Bernhard]

Hi........in this boiler i have made one in brass work great,,never eny problems,,,

sorry rung photo...it is the Cheddar boiler

Regards Bernhard

[Steamer]

The velocity of the gas coming into the burner is too slow.   It needs to increase beyond the flame speed.   To do this I would experiment with reducing the area of nozzle/port entering the burner tray.   If the diameter is decreased slightly....its a  non linear activity.....I would decrease it a .005 on a 1/16 ID burner tube and try it, the gas velocity will increase.  If that increase results in the gas velocity traveling faster than the flame front velocity, it can't flash back.  Its an experimental activity...and It sounds like your close already....But you will need to make SURE that it won't flash back.  That will take some testing

The ceramic burner was doing this for you by forcing the gas up a million tiny little holes....

BE CAREFUL!  Playing with gas is dangerous and can hurt you or your loved ones.....if you don't feel confident in doing the modification , find someone who can.....OK HS lecture over   Have fun!

Dave

[stuart177]

I understand your distress with normal ceramic burners. They are basically surface combustion off cuts from products designed to radiate heat – not to provide high temperature gases to heat a boiler. A well designed ceramic burner projects the heat generation outside the burner itself and as a result keeps the exhaust flue gas hot enough to evaporate errant condensate as well as keeping the burner body relatively cool. This combination minimizes the risk of excess moisture in the flue system getting anywhere near the burner; if it does the risk of ceramic fracture is virtually zero.

Have a look at the burner offered at www.miniaturesteammodels.com . It has all the above features without having to go through the trial and error process of tuning your SS gauze version.

[kiwimodeller]

Thanks for that, he has gone back to ceramic and taken the steam exhaust away from the funnel so no condensate will drip on the ceramic. Bench tests are fine but the big test is our first sailing day this weekend. We have built quite a few ceramic burners for horizontal flues and never had a problem, it was only the burner for the vertical boiler that cracked which may just have been a one off or it may have been the condensate drips that did it. Unless we have continuing problems we will not bother to experiment further with the mesh. Thanks for all the input. Ian.

[AlexC]

Stuart177

[/ They are basically surface combustion off cuts from products designed to radiate heat – not to provide high temperature gases to heat a boiler.quote]


So HOT gasses are the best way possible to heat boiler water?...... and that the radiated heat given off by a ceramic burner is of little use?...... care to explain this to us?

Regards.

AlexC 

[gondolier88]

Stuart,

Radiant heat is a far more efficient way of heating than convection- radiation is relatively impervious to factors that hugely affect convection- wind and damp being the main ones- also the burners on the link you give while giving a good flame picture would need a very long flue to work properly- horizontal if possible- because it is such a long flame and is not really a true radiant burner then flame inpingment could mean that it would be highly inneficient in a standard small vertical boiler firebox- the hottest part on a vertical boiler should be in the firebox, not in the tubes as this leaves cold spots and can lead to sooting and acidic condensation- really unwelcome in a copper boiler.

Greg

[stuart177]

Alex and Greg

Hot gases are the best way to heat a boiler. The generation of hot gasses can be either by solid fuels or gas.

As with all heat sources there is a mixture of hot gases and radiant heat. Getting the ratio right is the most important aim to achieve maximum thermal efficiency. Radiant heaters are designed to heat large spaces such as a room and those in it.

Burners for center flue boilers should have the generation of hot gas as their main aim.  As the gas passes through the boiler’s center flue and past the cross tubes it transfers its heat to these surfaces.  If the burner produced mainly radiant heat it would only heat those parts that are within sight of the heater and not heat the cross tubes out of sight of the burner.

The Miniature Steam boilers are designed with a center flue and multiple water filled cross tubes arranged in a spiral. The spiral cross tubes impart a spiral action to the gas causing maximum turbulence hence producing maximum heat transfer to the heating surfaces.

Use of a primarily radiant heat burner in this environment would be next to useless. The small quantity of hot gases produced would be absorbed in the first few cross tubes producing very little heat transfer.

The burner supplied with our boilers generates hot gases in sufficient quantity to fill the whole heating space relatively uniformly. This gives very efficient heat transfer to all heating surfaces as well as eliminating combustion condensation around the tubes. The cross tubes also provide good convection flow of water within the boiler

Stuart

[AlexC]

Stuart,

The’ radiant heat’ from any heat source is far more effective in transferring the ‘HEAT of COMBUSTION’ into the boiler water than any amount of hot gas.

A firebox having an exposed heated surface area only 1/8th of the total for the boiler would in fact be responsible for approx 60% of the total evaporation capability of the boiler, solely by exposure to RADIANT heat.

Hot gasses are an inevitable bye-product of any combustion process and in a simple boiler these would normally just pass to atmosphere, as a normal CONVECTION flow, taking the majority of any available heat with them.

Placing water filled cross tubes in the path of these gasses will, under certain conditions, permit the extraction of some of the available heat (but by no means all) by means of CONDUCTION to assist in the evaporation process.

The Main problem with extracting this heat from the gas is an effect known as the ‘boundary layer’ (a thin layer of stagnant gases which cling to the walls of the flue and tubes), which prevents/blocks the conduction of heat through the tube walls.
Whilst this layer is present, little or no heat can be extracted from the hot gas and it will pass on to atmosphere without any contribution.

This ‘boundary layer’ varies in thickness in direct proportion to the gas velocity and can be largely overcome by high gas velocity, which will scour the layer away, however, there is a trade off.

Too high a velocity will reduce the effective contact time, hence reducing the conduction time available.

A similar boundary layer exists on the ‘WATER’ side of the flue/tube and this further reduces the heat transfer.

Turbulence can also help to overcome the effects of these boundary layers, but in the case of the gasses, only to a point.

Too much turbulence on the gas side can disrupt the flow velocity of the gasses to the point where it effectively re-enforces the boundary layer.

Gas velocity is also effected by other external/internal influences, such as wind from the funnel top and other atmospheric conditions and/or changes in gas flow direction etc.

In short Stuart, if you are looking for best efficiency from a given boiler then make as much use of the available radiant heat as possible at the firebox end and then attempt to extract as much of the remaining heat of combustion from the gasses as they pass to atmosphere.

Note, this does not always mean directly into the boiler.

To rely on hot gasses as the majority heat source is certainly not the correct approach.

I am sure that Greg will add some more observations to this as between us I think we have around 60 – 80 years of experience with both full size and model size steam plant.


Best regards.


AlexC

[gondolier88]

Hi Stuart, Alex,

I'm afraid it looks like two to one here Stuart.

In a boiler as you describe your company makes then the burner would most likely be ok- probably a sealed firebox/combustion chamber big enough for full combustion with 95%-98% of air for combustion being primary air (ie. entrained with the gas before combustion via the injector pre-mix holes) am I right?

In the MAJORITY of existing model steam boilers, of which again the MAJORITY are probably vertical multitubular designs with large firebox areas acting as much as combustion chambers, having secondary air in the firebox combining with the pre-mixed gas/air coming into the burner, with ceramic burners then your burner would not be as efficient- you would have to slow the gas velocity right down to have anywhere near as much effect as a ceramic burner of the same output, but that would render your burner output redundant.

As I said a horizontal boiler with a long flue and a standard cross-tube, preferably with a wetback return tube design would suit your blowlamp burner down tho the ground- don't believe me- try it?

Greg


PS, solid fuels are 100% radiant heat - only the air used for combustion reveals any sort of convection heat- all of which then has to overcome the aforementioned boundary layer, preferably with turbulance made by the exhaust through the funnel or a forced-draught fan on a condensing plant- which of course is conduction through the material of the tubes whatever that may be- copper preferably as it has better thermal properties- however steel is made stronger than copper so can be made thinner- hence improving conduction properties a debate which still goes on between mofdern steam builders.