Grate Spacing

[JonRiley56]

All,

I decided to start a separate topic on this since it was imbedded in the conversation about ash pans etc. To set the stage:

The grate bars in my water tube boiler are 3" apart which is resulting in all of my coals falling though into the ashpan. The way the boiler is built involves the grate bars sitting in v shaped notches. The current grate bars are square cast iron bars.

It has been suggested that the preferred spacing between the bars is 1/2" and that I might be able to use angle iron to accomplish this, using the current mountin notches that I have. In order to get into the 1/2" spacing range, the angle iron would have to be 2". (thanks to Bart for this and others for the hippopotamus hide jokes).

My concern is this: If I actually go that route I will have ~85% of my horizontal space taken up by the grate angles and only 15% as air spaces.

I was studying the UK Steamboating Association Guide, ( I am one of the few members in the colonies)....and saw the following statement:

Design feature: The grate area should be as generous as possible and the
firebox/furnace volume adequate for the designed firing rate. Generous air
entry and distribution are essential for oil burners and coal. The latter is
dependent on good firing which in turn calls for a large firebox door so that
the coal can be seen to be distributed evenly and thinly.
Of the total grate area, approximately ⅓ should be devoted to firebars and ⅔ to
the gap to permit combustion air. For example, 6 mm wide firebars with a 12 mm
air space between them.

Does this apply only to a coal grate ? It would seem that if we stick to the 1/2" spacing guidelin and overlay the 1/3 grate bar idea, it mean that the width of oour grate bars would be 1/4".

I know I am not accounting for any free space at the front or rear of the firebox, but does that make sense ? I would likely have to use something similar to a barbecue grate to get that type of size.

Looking forward to your comments.

jon

[Mike Rometer]

The ideal shape for fire-bars in my opinion is wider at the top than the bottom, but not by much, i.e. enough to not allow any coals that do manage to fall through to stick part way. "T" shaped would also do.

You could still utilise the notches, but build your grate in sections to mount in them.

I find the air spacings really depends largely on what you burn and its propensity to either clinker or just disappear to ash. I find 50:50 works well with steam coal.

[JonRiley56]

Thanks Mike,

I see you are in middle earth, give my regards to the Baggins clan. What if I am burning wood ?

jon

[gondolier88]

Spacing depends on quite a lot, it goes without saying that getting it right is important- a properly combusting furnace is imperative to a well behaved plant- and while it is true that it is the boiler that powers your boat, it is your fire that powers your boiler!

Points to consider;
- what fuel you are burning- burning true steam coal that has a high carbon content, little ash and low volatile content requires less air to combust properly than high volatile content/high ash/medium carbon content (house coal). In fact, with some house coal top air draught is required to completely burn all the smoke away. Depending on what type of coal you have I would suggest 50:50 air:grate spacing for steam coal, with around 70:30 air:grate for house coal types.

- draught- if I remember rightly, please forgive me if I'm wrong, but you are using a three drum watertube boiler, which requires a grate full of glowing coal to steam properly, even tiny holes have a detrimental effect on the temperature of the boiler. In order for the coal to get enough air then a draught is an absolute must for WT boilers as natural draught is all but non-existent. Of course, best option is to use the engine exhaust, properly seperated from condensate, as this is linked directly to engine speed and steam consumption. A number of condensing launches in the 1900's used forced draught fans, with a lever from the fire door to stop the draught when open so that the fire didn't blow back at the engineer. Numerous condensing launches today use a blower and manage perfectly well, however constant adjustment is required to match boiler output.

What has this to do with spacing? Lets assume a situation of you steaming along at cruising speed, engine at around half rev.s. You put four shovelfuls of coal on 5 mins ago, well spread and covering the grate, the funnel has fairly dense smoke emitting from it, although not opaque and not black. This is assuming the bottom air draught is around 1/3 to 1/2 open. You decide that you're going to take her up to flank speed and increase rev.s to around 90% max. speed. You know that the grate is covered, and the volatiles are still burning off. You open the bottom air draught to around 3/4 and the funnel gasses change to clear from the funnel, changing to slight blue tinge around 4-6ft from the funnel- this is perfect combustion. However, if the spacing in the grate is not allowing enough air in, in the same example above at cruising speed you would be emitting thick black smoke, and at flank speed you would have thick blue smoke.

It would certainly be worth your while trying to come up with some way of experimenting with different spacings, to suit best your cruising speed and your flank speed.

At flank speed your furnace should be a bright yellow, filling the combustion space, showing a bright glow in the ash pan and producing just a light haze from the funnel- anything else is just not working properly.

Greg

[Albert]

This grate showed a very satisfying performance during the last season. I got the idea from my friend Klaus (S.L. Lagom) who found it somewhere in a discussion forum.

After welding I had to saw it apart in two halves so I could mount it through the fire door. It is especially suitable for burning dry wood.

Former grate designs (mainly flame cut from a thick steel plate) with different slot widths showed strong clogging after some operating hours.





This is one of the grates for a former boiler. Oxy-fuel cut, extremely wide slots.




Albert

[Lopez Mike]

That first one made of angle iron is the best design I've seen in a good while. Gaps open to the bottom; retention of a small bit of ash for good protection of the steel from full heat.

When my current one, made of solid bars, eventually dies, I will build one like that if for no better reason than to save weight.

Mike

[Albert]

Mike,

The angle bar grate was extensively used during the last season. It showed no kind of erosion, warping or the like. I know of your praiseworthy attitude... but by spending a couple of bucks in stainless angle bar you will have a grate which will last for the rest of your steamboating career.

Albert